Precambrian Research 267 (2015) 285–310
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Precambrian Research
journal homepage: www.elsevier.com/locate/precamres
Unraveling crustal growth and reworking processes in complex
zircons from orogenic lower-crust: The Proterozoic Putumayo
Orogen of Amazonia
Mauricio Ibanez-Mejia a,∗ , Alex Pullen a,b , Jesse Arenstein c , George E. Gehrels a ,
John Valley d , Mihai N. Ducea a,e , Andres R. Mora f , Mark Pecha a , Joaquin Ruiz a
a
Department of Geosciences, The University of Arizona, Tucson, AZ 85721, USA
Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA
c
Alicat Scientific Inc., Tucson, AZ 85743, USA
d
WiscSIMS, Department of Geoscience, University of Wisconsin, Madison, WI 53706, USA
e
Universitatea Bucuresti, Facultatea de Geologie Geofizica, Str. N. Balcescu Nr 1, Bucuresti 010041, Romania
f
Instituto Colombiano del Petroleo ICP-ECOPETROL, Piedecuesta, Santander, Colombia
b
a r t i c l e
i n f o
Article history:
Received 6 March 2015
Received in revised form 3 June 2015
Accepted 23 June 2015
Available online 2 July 2015
Keywords:
U–Pb–Hf–O
Amazonia
Rodinia
Putumayo Orogen
Crustal evolution
a b s t r a c t
High-grade basement massifs exposed in the northern Andes and the buried basement of the adjacent
Putumayo foreland basin contain a record of Amazonia’s involvement in the supercontinent Rodinia.
Metasedimentary granulites and orthogneisses, strongly deformed during at least one metamorphic
episode dated at ca. 0.99 Ga, provide critical information on the pre-collisional history of the Mesoproterozoic continental margin. Here, new U–Pb, Lu–Hf, Sm–Nd and O isotopic data from outcrop samples of
the Garzón and Las Minas Cordilleran basement massifs as well as fragments of drill-core recovered from
the Putumayo basin basement are reported. We explore the application of a dual-ICP-MS approach to
obtain concurrent U–Pb and Lu–Yb–Hf information on complexly zoned zircon from orogenic lower-crust,
and demonstrate its use to retrieve reliable pre-metamorphic information despite possible complexities
introduced by mixed-domain ablation and isotopic disturbance of the U–Pb system by thermally induced
recrystallization. In combination with ␦18 O compositions from the same zircon growth domains, and
bulk-rock Nd isotope information, we reconstruct segments of the tectonic and crustal evolution of a longlived accretionary orogen that developed along the (modern) NW margin of Amazonia during most of the
Mesoproterozoic. Inherited zircons in metaigneous samples from the Cordilleran massifs, with protolith
crystallization ages in the range from ca. 1.47 to 1.15 Ga, have Hf–O compositions that indicate significant
crustal reworking in their source region, but denote a trend of increasing 176 Hf/177 Hf with decreasing age
that can be attributed to rejuvenation by progressive addition of radiogenic components during this time
interval. Detrital zircons within this same age range found in metasedimentary granulites of the Garzón
massif also follow this trend, further supporting previous inferences that their protoliths were deposited
in arc-proximal basins with little to no coarse-grained detritus delivered from an older cratonic domain. A
shift in orogenic deformation style starting at ∼1.15–1.10 Ga, inferred to be associated with the accretion
of fringing-arc terranes against the continental margin, triggered an early amphibolite-grade metamorphic episode; this was accompanied by pervasive partial melting and migmatite development in fertile
metasedimentary units and is interpreted to be responsible for enhanced crustal reworking evidenced
from the shallowing of 176 Hf/177 Hf vs. age trends in detrital and metamorphic zircons from the Garzón
and Las Minas massifs. Convergent tectonism along the Putumayo margin came to an end during the final
incorporation of Amazonia to the core of the Rodinia supercontinent, possibly during collision against
the Sveconorwegian segment of Baltica at 0.99 Ga. Although the position and role of Amazonia within
Rodinia remains controversial, the new Nd and Hf isotope data provide additional evidence to link the
∗ Corresponding author at: Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Tel.: +1 6172532927.
E-mail address: ibanezm@mit.edu (M. Ibanez-Mejia).
http://dx.doi.org/10.1016/j.precamres.2015.06.014
0301-9268/© 2015 Elsevier B.V. All rights reserved.
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M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
evolution of this orogenic segment with the basement of Oaxaquia, as well as continue to draw fundamental differences with the timing and nature of the tectonic processes associated with the development
of the Sunsás-Aguapeí Orogen of SW Amazonia.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Zircon isotope geochemistry is fundamental to understanding
the formation and differentiation of continental crust on Earth
(e.g., Patchett et al., 1981; Valley et al., 2005; Belousova et al.,
2010; Dhuime et al., 2012; Cawood et al., 2013; Hawkesworth
et al., 2013), but becomes increasingly challenging when applied
to fragments of strongly modified continental lithosphere that
have undergone poly-phase tectonic-thermal histories and pervasive metamorphic recrystallization (e.g., Whitehouse et al., 1999;
Zeh et al., 2007; Kemp et al., 2009a). High-temperature crustal
reworking has the potential to induce multi-phase growth histories within single-zircon crystals, which complicate the age
assignment to complementary geochemical indicators. However,
despite their possible textural complexities, reliable primary
geochronologic and geochemical information can still be determined from reworked zircon crystals owing to the extremely
slow volume-diffusion properties of U–Pb–Hf–O in this mineral
(Cherniak et al., 1997; Watson and Cherniak, 1997; Cherniak
and Watson, 2000, 2003; Peck et al., 2003; Page et al., 2007;
Bowman et al., 2011). Therefore, the continuous improvement
in spatial resolution, precision, and accuracy of in situ radioisotopic methods for studying minerals with multi-phase growth
histories continues not only to revolutionize our understanding
of the geochemistry of zircon, but also to provide unprecedented insights into crust-forming and reworking processes at
increasingly finer resolutions in strongly deformed orogenic
crust.
Continental collision tectonism has been inferred to play a
crucial role in the shaping and stabilization of Earth’s preserved
continental lithosphere (Hawkesworth et al., 2009), and exerted a
primary control on the global detrital-zircon geochronologic pattern of ancient and modern sediments (Campbell and Allen, 2008;
Voice et al., 2011; Spencer et al., 2015). If major peaks and troughs
in the global detrital-zircon age distribution are indeed an artifact of selective preservation instead of arising from changes in
fundamental crust-generating processes (e.g., Hawkesworth et al.,
2009, 2013; Condie et al., 2011), then further scrutinizing the
geochemical and isotopic record of major pre-collisional orogens
through time should provide fundamental insights into understanding the composition of Earth’s preserved continental crust
(Rudnick and Gao, 2014). Furthermore, in addition to providing
clues for evaluating the mechanisms responsible for long-term
crustal differentiation, linkages between isotopic compositions and
particular tectonic processes can be used to impose robust temporal and geodynamic constraints on the developmental history
of complex and/or poorly represented orogens. This information
plays a critical role in the study of processes associated with the
supercontinent cycle, as any proposed topological model for the
global agglomeration of continental masses has to satisfy the temporal, tectonic, and structural constrains dictated by the geological
evolution of lithospheric fragments formed prior, during and after
continental collision (e.g., Dalziel, 1991; Pisarevsky et al., 2003;
Bogdanova et al., 2008; Li et al., 2008, 2009; Evans and Mitchell,
2011).
The recently defined Putumayo Orogen of northern South America holds critical information for understanding the participation of
Amazonia in the assembly of the supercontinent Rodinia (Cordani
et al., 2009; Ibanez-Mejia et al., 2011), and the role that Mesoproterozoic accretionary orogenesis had in the growth of one of Earth’s
largest Precambrian landmasses (Cordani and Teixeira, 2007). In
order to elucidate the Proterozoic crustal development history of
the NW Amazon Craton and to provide a more comprehensive
understanding of its role within Mesoproterozoic tectonics, we
applied a combination of texturally resolved U–Pb, Lu–Hf and O
isotopic techniques to investigate the record of complex polycyclic
meta-igneous and meta-sedimentary zircon crystals from highgrade rocks of the Putumayo Orogen. These results, complemented
by bulk-rock Nd isotopic data, are used to reconstruct segments
of a long-lived history of Mesoproterozoic convergent tectonism
along this segment of the Amazonian margin, as well as draw strong
geochronologic and isotopic correlations between the now dismembered fragments that have been proposed to once make an
integral part of this orogen. In an effort to assign the most reliable U–Pb ages to each Lu–Hf isotopic composition, a critical step
toward accurately interpreting such data, U–Pb and Lu–Yb–Hf isotopic data were simultaneously acquired from the same ablated
zircon volumes by a laser ablation dual-ICP-MS approach calibrated
at the University of Arizona during the course of this study. We
show that this analytical strategy offers the possibility to study a
variety of processes such as igneous and metamorphic petrogenesis, detrital-zircon provenance, and paleogeographic development
of orogenic crustal fragments where complex zircon growth histories are the end result of their poly-phase tectonothermal
evolutions.
2. Tectonic context of the Putumayo Orogen and sampling
locations
The Amazon Craton, commonly referred to as Amazonia
(Fig. 1A), is thought to be one of the major Precambrian continental
nuclei that occupied the core of the Rodinia Supercontinent during
the late Meso- to early Neoproterozoic (Hoffman, 1991; Li et al.,
2008; Cordani et al., 2009). The occurrence of a late Mesoproterozoic collisional metamorphic belt in the lowlands of eastern Bolivia
and western Brazil, the Sunsás-Aguapeí belt, has traditionally
provided support for this idea (Litherland and Bloomfield, 1981;
Teixeira et al., 1989, 2010). Although paleogeographic, geochronologic and paleomagnetic evidence indicate the Grenville margin of
Laurentia as a feasible hypothesis for a conjugate collisional margin
to the Sunsás, debate still persists over the specific segment of Laurentia against which Amazonia collided (Sadowski and Bettencourt,
1996; Tohver et al., 2002, 2004, 2006; Loewy et al., 2003; D’AgrellaFilho et al., 2008; Gower et al., 2008; Johansson, 2009, 2014). A
possible northern continuation of the Sunsás belt along the fringe
of the Amazon Craton has been contentious, as fragments of late
Mesoproterozoic crust east of the Andean deformation front in
modern-day Peru, Ecuador, and Colombia had not been previously
recognized (Fig. 1A). There is now, however, growing geological
and geochronological evidence that supports the existence of a
collisional Stenian–Tonian orogenic belt hidden under the north
Andean foreland basins in northwestern South America, herein
called the Putumayo Orogen, which provides important new constraints for the role of Amazonia prior and during the assembly of
the Rodinia supercontinent.
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
2.1. North Andean Precambrian basement massifs
High-grade Precambrian basement massifs of late Meso- to
Neoproterozoic age comprise the basement of the Eastern and portions of the Central Cordilleras of the Colombian Andes (Fig. 1A;
Kroonenberg, 1982; Priem et al., 1989; Restrepo-Pace et al., 1997;
Cordani et al., 2005; Jimenez Mejia et al., 2006; Cardona et al.,
2010; Ibanez-Mejia et al., 2011). Despite their modern geographic
location in the NW South American margin, intense segmentation
caused by Phanerozoic deformational events, including significant
strike-slip displacement along the Meso-Cenozoic proto-Andes
(Bayona et al., 2006, 2010), has made their interpretation a challenging task within the context of Amazonia and greater Rodinia
reconstructions (Cardona et al., 2010). Crustal development of the
North Andean Precambrian basement massifs prior to Neoproterozoic collisional deformation took place within an active margin
setting possibly along the western edge of Amazonia (Cardona
et al., 2010 and references therein). Although faunal assemblages in
Paleozoic sequences that overlie the Garzón, La Macarena, and Las
Minas massifs (Fig. 1) provide evidences to support a NW Gondwanan affinity of these basement domains by early Phanerozoic
times (Harrington and Kay, 1951; Forero-Suarez, 1990; OrdonezCarmona et al., 2006; Borrero et al., 2007; Moreno-Sanchez et al.,
2008a, 2008b), Proterozoic correlations with the proximal nonremobilized cratonic basement are hampered by the lack of isotopic
data from a coeval orogenic belt east of the Andean deformation
domains.
Compelling geochronologic and isotopic evidence has been
found to establish Proterozoic tectonic correlations and a shared
287
metamorphic evolution of the Andean massifs with the Precambrian basement exposures of Mexico, specifically those included
in what is known as the Oaxaquia terrane (Ortega-Gutierrez et al.,
1995; Restrepo-Pace et al., 1997; Ruiz et al., 1999; Weber et al.,
2010). Similarly to the north Andean Precambrian massifs, the Oaxaquian basement is also characterized by a protracted late Meso- to
early Neoproterozoic history of back-arc related magmatism, sedimentation, intrusion of intermediate to acidic volcanic-arc plutons,
massif-type anorthosite–mangerite–charnockite–rapakivi granite
(AMCG) magmatism, and Tonian granulite-facies metamorphism
(Patchett and Ruiz, 1987; Ruiz et al., 1988, 1999; Ortega-Gutierrez
et al., 1995; Lawlor et al., 1999; Weber and Köhler, 1999; Keppie
et al., 2001, 2003; Lopez et al., 2001; Solari et al., 2003, 2013; Weber
and Hecht, 2003; Cameron et al., 2004; Keppie and Dostal, 2007;
Keppie and Ortega-Gutierrez, 2010; Weber et al., 2010). These
various geochronological, isotopic and structural evidences could
provide the tectonic framework to reconstruct a long-lived Mesoproterozoic history of convergence along the NW portion of the
Amazonian margin, if evidences from an autochthonous belt in this
region of South America can be reconciled with the geologic evolution of the joint Oaxaquian-North Andean Precambrian basement.
In this study, we provide new data from metaigneous and
metasedimentary units exposed within the Garzón, Las Minas and
La Macarena massifs (Fig. 1). The western segment of the Garzón
massif is dominantly comprised by a banded sequence of metasedimentary migmatites and granulites of the “El Vergel” unit and
hornblende-biotite augen gneisses of the “Guapotón-Mancagua”
orthogneiss (Fig. 1C; Kroonenberg, 1982; Jimenez Mejia et al.,
2006). We studied one sample of a felsic orthogneiss from the
Fig. 1. Location of the study area within the context of the Amazon Craton. (A) Tectonic overview of the South American continent with emphasis on its Precambrian
architecture and highlighting one model for the provinces of Amazonia (after Ibanez-Mejia et al., 2011), modified from Tassinari and Macambira (1999), Cordani and Teixeira
(2007) and Fuck et al. (2008). Distribution of crustal fragments interpreted as associated to the Putumayo Orogen are shown as: AB – autochthonous belt, and RB – remobilized
belt. (B) Digital elevation model of SW Colombia and N Ecuador showing the location of drilling-sites in the north Andean foreland discussed in this study, and their position
with respect to the Garzón massif (DEM image and outline of major rivers from http://jules.unavco.org/Voyager/Earth). (C) Detailed geological map of the Upper Magdalena
Valley (UMV) area with emphasis on exposed Precambrian units and showing sampling locations from the Garzón and Las Minas massifs. Stratigraphic units shown in figure
are: (a) volcanic/plutonic rocks of Jurassic age, (b) Paleozoic clastic sedimentary rocks, (c) migmatites and granulites of the “El Vergel granulites” unit (hereafter only referred
to as “El Vergel” unit), (d) metaigneous rocks of the Guapotón-Mancagua orthogneiss, (e) Toro gneiss, (f) metasedimentary migmatites of the “El Zancudo migmatites” unit,
(g) metaigneous rocks of the Las Minas orthogneiss.
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M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
Guapotón-Mancagua unit (sample MIVS26), one sample from a leucosome of a stromatic migmatite within the El Vergel unit (sample
MIVS16A), a deformed granitic pegmatite spatially associated to
these migmatites (sample MIVS15A), and two massive metasedimentary granulites also from the El Vergel unit (samples MIVS12
and MIVS13). Located to the west of the Garzón massif, in the eastern flank of the Central Cordillera, the Las Minas massif consists
of a core of meso- and leucocratic augen gneisses denominated by
Ibanez-Mejia et al. (2011) as the “Las Minas” orthogneiss (Fig. 1C).
This unit is overlain by a series of stromatic metatexites of metasedimentary origin denominated by Ibanez-Mejia et al. (2011) as the
“El Zancudo” migmatites, best exposed in the eastern flank of the
Serrania de Las Minas range. We provide new data from one sample of the Las Minas orthogneiss (sample MIVS41) and one sample
from a melanosome of the Zancudo stromatic metatexites (sample
CB006). The Serrania de La Macarena range is a basement uplift
to the north of the Putumayo basin, exposed along a series of
basement-high structures that divide the Putumayo basin to the
south and the Llanos basin to the north (Fig. 1B and C). Due to the
extremely difficult access to this area in the Andean foothills jungle
the Serrania de La Macarena still remains very poorly mapped, but
its core is known to consist of a series of mylonitic gneisses and
schists crosscut by deformed syenogranitic dikes (Gansser, A., in
Trumpy, 1943). We analyzed one sample of a mylonitic gneiss unit
collected in the northeastern flank of the La Macarena uplift (sample Macarena2). For a more detailed description of these samples,
the interested reader is referred to the study of Ibanez-Mejia et al.
(2011) and the supplementary material to this article.
2.2. The Northwestern Amazon Craton
The Guyana Shield, exposed over a vast area in northern South
America, comprises the northern half of the larger Amazon Craton
and is offset from the southern half (the Guaporé shield) by the
E–W trending Amazon River graben (Fig. 1A; Teixeira et al., 1989;
Tassinari and Macambira, 1999; Cordani and Teixeira, 2007). Previous workers have shown that this shield exhibits a general pattern
of westward younging basement domains with Paleoproterozoic
high-grade gneiss, granite-greenstone belts and supracrustal volcanic sequences associated to the Transamazonian orogeny in the
east (e.g., Santos et al., 2000; Teixeira et al., 2002; Roever et al.,
2003; Delor et al., 2003; Fraga et al., 2009; Hildebrand et al., 2013)
and younger Paleo- to Mesoproterozoic granite-gneiss terranes and
rapakivi granites dominating in the west (Gaudette et al., 1978;
Tassinari et al., 1996; Ibanez-Mejia et al., 2011). In proximity to
the Andean fold and thrust belt, the shield flexurally subsided
under the load of this thickened active orogen, and a series of
large retroarc foreland basins – the Llanos and Putumayo basins
– have buried a sizable portion of the Precambrian basement in
this area (Fig. 1A and B; Milani and Thomaz Filho, 2000). In the
absence of basement outcrops, the only evidences available on
the age and nature of the craton beneath the Cenozoic cover are
provided by deep exploratory wells that reach the bottom of the
sometimes kilometer-thick sedimentary successions (e.g., Kovach
et al., 1976; Feo-Codecido et al., 1984). Deformed granitoids and
upper-amphibolite to granulite-facies metaigneous and metasedimentary rocks recently described from wells in the Putumayo
foreland basin of south-central Colombia (Fig. 1A and B), provided
the first direct evidence to support the occurrence of a sizable Mesoto early Neoproterozoic collisional belt hidden along this margin of
the Amazon Craton (Putumayo Autochthonous Belt – AB – Fig. 1A;
from Ibanez-Mejia et al., 2011). Furthering our understanding of the
geochronologic and isotopic characteristics of this belt can provide
geological constraints that might serve as piercing points to trace
the history of the Oaxaquian and North Andean terranes back to an
Amazonian ancestry, which is one of the objectives of the present
contribution.
This study provides new U–Pb geochronologic and Nd–Hf–O isotopic results on some of the basement drill-core samples previously
reported by Ibanez-Mejia et al. (2011), namely from the Mandur-2,
Payara-1, and La Rastra-1 exploratory wells. All these wells were
drilled in the Putumayo foreland basin just east of the Garzón
Cordilleran massif and south of the La Macarena uplift (Fig. 1A
and B), and pierced the crystalline basement at depths between
∼940 and 1700 m below the modern day surface east of the modern
Andean deformation front.
3. Analytical methods
3.1. Zircon oxygen isotopes by SIMS
Oxygen isotope measurements on zircon crystals were performed by secondary ion mass spectrometry (SIMS), using a
CAMECA IMS-1280 at the University of Wisconsin WiscSIMS Laboratory. Details on the sample preparation, analytical procedures
and data processing strategies are discussed in detail by Kita
et al. (2009) and Valley and Kita (2009). Zircons were mounted
in epoxy plugs within a 0.5 cm radius from the center, where
large fragments of the KIM5 zircon reference crystal were located
(␦18 O = 5.09‰ VSMOW; Valley, 2003). Fragments of the SL2, R33,
Mud Tank, Plešovice and FC-1 reference zircons were placed around
the margin of the unknowns, so U–Pb–Hf measurements on the
same crystals analyzed for oxygen isotopes could later be obtained
from these sample mounts. All samples were imaged by cathodoluminescence using a secondary electron microscope (SEM-CL)
prior to isotopic analysis in order to reveal their internal zoning
structures. SIMS data was acquired prior to laser ablation split
stream–inductively coupled plasma–mass spectrometry (LASSICP-MS) in order to avoid possible sample topography-induced
inaccuracies in the oxygen isotope measurements that could arise
due to the presence of laser ablation craters in the crystals (Kita
et al., 2009). Analyses were conducted using a 10 ␮m diameter Cs+
primary ion-beam with a 1.5–2.0 nA current and using a normally
incident electron gun for charge compensation. The secondary-ion
accelerating voltage was set to 10 kV, and measurements of 16 O
and 18 O were conducted simultaneously in two Faraday cup detectors. Corrections for instrumental mass fractionation (IMF) biases
on the raw measured ␦18 O values were conducted by reference
material (RM) sample bracketing using repeated analyses of the
KIM5 crystal; six analyses of KIM5 were conducted at the beginning
and at the end of each session, with another four measured every
12 unknowns. The average value of eight KIM5 analyses bracketing each set of unknowns was used to correct the latter for IMF,
and the individual spot uncertainties for all analyses are reported
as the external reproducibility of the particular set of RM analyses
used for IMF correction (i.e., 2 S.D. of the eight bracketing KIM5
analyses). All IMF-corrected values reported here and discussed
throughout the text are quoted in ␦18 O notation with respect to
VSMOW. After oxygen isotope analyses were performed, but before
U–Pb–Hf analysis, all ion-probe sputtering pits were carefully evaluated by back-scattered electron imaging (SEM-BSE) in order to
identify possible complexities that could compromise the accuracy
of individual ␦18 O measurements (Cavosie et al., 2005). Accordingly, pits that intersected obvious cracks and/or inclusions or that
overlapped zircon growth domains were excluded from calculations of the mean ␦18 O values. Analyses conducted in areas where
subsequent U–Pb analyses exhibited discordance of a magnitude
greater than 10% were also avoided for establishing primary zircon
␦18 O compositions.
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
289
3.2. Zircon U–Pb–Hf coupled isotopic analyses
Fig. 2. Schematic diagram showing the configuration of our laser ablation “splitstream” (LASS-ICP-MS) setup. Details of instrumental operating conditions and
balancing of gas flows are provided in Table 1 and in the analytical appendix.
Analyses of the U–Pb and Yb–Lu–Hf isotopic compositions of
zircon presented in this study were all conducted at the Arizona
Laserchron Center (ALC; www.laserchron.org) at the University of Arizona. Two plasma-source mass-spectrometers, a single
collector–inductively coupled plasma–mass spectrometer (SCICP-MS) and a multicollector–inductively coupled plasma–mass
spectrometer (MC-ICP-MS), were simultaneously connected to a
single ArF Excimer laser ablation (LA) system in order to obtain
concurrent U–Pb and Yb–Lu–Hf isotopic measurements from the
same volumes of ablated zircon material. A schematic diagram
illustrating our analytical configuration is shown in Fig. 2; the SCICP-MS used was a Thermo-Finnigan Element2 equipped with an
enhanced-sensitivity ‘Jet interface’, and the MC-ICP-MS a Nu Instruments Plasma I equipped with 12 fixed faraday detectors using
3 × 1011 resistors. Details on the instrumental parameters, collector configuration for Yb–Lu–Hf measurements, scanned masses
on the Element2 and their respective dwell-times, are summarized
in Table 1. Given that this is the first contribution from the ALC
lab using this laser ablation ‘split-stream’ approach (abbreviated
LASS-ICP-MS herein), only a brief description of the method is given
in this section whereas a more extensive explanation of the procedures is provided as an analytical appendix. A series of zircon
reference crystals with a wide range of U–Pb ages and U concentrations previously determined by ID-TIMS, 176 Hf/177 Hf compositions
obtained by solution-MC-ICP-MS, and HREE concentrations, were
analyzed during the course of this study in order to validate the precision and accuracy of the obtained results. A summary of the U–Pb
and Lu–Hf results determined from reference crystals during the
course of this study, showing the consistency with their respective
Table 1
Instrument operating parameters used in this study for: (a) mass-spectrometers and (b) laser-ablation system. (c) Detector configuration used in the multicollector and (d)
collected masses on the Element2 and measuring parameters.
(a) Instrument operating parameters – mass spectrometers
Manufacturer
Model
Forward power
Mass resolution
Cooling gas flow (SLPM Ar)
Auxiliary gas flow (SLPM Ar)
Sample gas flow (SLPM Ar)
Total flow on MFM1 to Nu Plasma (see Fig. 2)
Ion-counting deadtime (E2 only)
Analog calibration factor (E2 only)
(b) Instrument operating parameters – laser ablation
Manufacturer
Model
Type
Energy fluence
Energy attenuation
Repetition rate
Number of pulses
Spot diameter
Ablation depth
Mass Flow Controller 1 (Helex)
Mass Flow Controller 2 (Cell volume)
Nu instruments
Nu Plasma I
1300 W
Low
13.0
0.800
1.200
∼1.468
–
–
Thermo-Finnigan
Element2
1200 W
Low (400)
16.0
0.800
1.507
–
16 ± 3 ns
Auto
Photon Machines
Analyte G2
193 nm – ArF Excimer
7 J/cm2
8%
7 Hz
560
40-50 ␮m
∼30 ␮m (from optical interferometry)
0.050 SLPM He
0.340 SLPM He
(c) Collector configuration and measured masses in the Nu Plasma MC-ICP-MS
H2
Ax
180
179
Hf
L1
178
Hf
L2
177
Hf
L3
Hf
L4
L5
176
Hf
176
Lu
176
Yb
174
175
L6
L7
L8
Hf
Lu
173
172
Yb
171
Yb
Yb
(d) Measured masses and dwell-times in the Element-2 SC-ICP-MS
238
Dwell time (ms)
Collection mode
No. of samples/peak
U
5
Both
4
232
Th
1
Counting
4
208
Pb
1
Counting
4
207
Pb
9
Counting
4
206
Pb
6
Counting
4
204
(Pb + Hg)
4
Counting
4
202
Hg
2
Counting
4
290
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
Table 2
Summary of U–Pb and Lu–Hf isotopic compositions obtained for secondary zircon reference crystals during our LASS-ICP-MS session.
Hf/177 Hf(0) ± 2 S.D.
Zircon name
U–Pb age (Ma)a
Ref. ageb
176
49127
Plešovice
Temora
R33
91500
FC-1
FC-52
OG-1
134.1 ± 1.3/1.4 (n = 15, MSWD = 0.8)*
338.7 ± 3.1/3.4 (n = 15, MSWD = 0.5)*
417.0 ± 3.8/4.2 (n = 15, MSWD = 0.9)*
417.3 ± 3.6/4.0 (n = 13, MSWD = 0.9)*
1067 ± 12/13 (n = 13, MSWD = 1.8)**
1104 ± 5/7 (n = 15, MSWD = 0.8)**
1103 ± 5/7 (n = 15, MSWD = 1.1)**
3467 ± 3/14 (n = 15, MSWD = 0.6)**
ca. 136.4 (a)
337.13 ± 0.37 (b)
416.78 ± 0.33 (c)
419.26 ± 0.38 (c)
1066.4 ± 0.3 (d)
1098.8 ± 0.25 (e)
ca. 1099 (f)
3465.4 ± 0.6 (g)
0.282944 ± 47 (n = 15, MSWD = 0.9)
0.282498 ± 41 (n = 15, MSWD = 0.6)
0.282668 ± 56 (n = 15, MSWD = 1.2)
0.282764 ± 59 (n = 15, MSWD = 1.5)
0.282299 ± 66 (n = 15, MSWD = 1.4)
0.282182 ± 72 (n = 15, MSWD = 1.2)
0.282171 ± 93 (n = 15, MSWD = 2.3)
0.280630 ± 74 (n = 15, MSWD = 1.7)
Ref. 176 Hf/177 Hf(0) b
176
Yb interf (%)c
N.A.
0.282482 ± 13 (b)
0.282686 ± 8 (h)
0.282764 ± 14 (i)
0.282313 ± 12 (i)
0.282183 ± 12 (i)
ca. 0.282183 (f)
N.A.
5–15
1–3
10–22
7–58
3–4
2–35
5–21
6–25
a
Uncertainties are 2. * Reported age is the 206 Pb/238 U weighted-mean. ** Reported age is the 207 Pb/206 Pb weighted-mean.
Reference values are ID-TIMS for U–Pb and solution-MC-ICP-MS for 176 Hf/177 Hf. (a) Mattinson et al. (1986), (b) Slama et al. (2008), (c) Black et al. (2004), (d) Schoene
et al. (2006), (e) Hoaglund, 2010, (f) similar to FC-1, (g) Stern et al. (2009), (h) Woodhead et al. (2004), (i) Fisher et al. (2014a,b).
c
Isobaric interference of 176 Yb calculated as Interf. (%) = 100 * ((176 Yb/177 Hf)Corr /(176 Hf/177 Hf)Corr ).
b
benchmark values, is presented in Table 2. Uranium–Pb concordia
diagrams and 176 Hf/177 Hf vs. 176 Yb/177 Hf plots for these reference
crystals are provided as supplementary material.
One of the most critical corrections applied to LA-MC-ICP-MS
Lu–Hf data is the subtraction of the 176 Yb isobaric contribution to
the 176 Hf mass. This correction, whose magnitude is proportional to
the relative HREE content of the analyzed zircon, commonly represents an adjustment on the order of hundreds or even thousands of
equivalent ␧Hf units to the raw measured 176M/177M values (Chu
et al., 2002; Woodhead et al., 2004; Wu et al., 2006; Fisher et al.,
2011, 2014b). Therefore, it is extremely important that this correction be accurately applied for the reported 176 Hf/177 Hf to have
any meaningful geological significance. Ytterbium corrections were
performed using the isotopic compositions of Vervoort et al. (2004),
and the determination of an empirical instrumental bias factor
to the ‘natural’ 176 Yb/173 Yb composition (see analytical appendix
for details) achieved by repeated measurements of the Yb-doped
synthetic zircon crystals introduced by Fisher et al. (2011). The
mean measured value from 80 analyses of these synthetic crystals during our LASS-ICP-MS session was 0.282131 ± 61 (2 S.D.,
MSWD = 2.0; Fig. 3), which is in close agreement and within analytical uncertainty from the reference solution-MC-ICP-MS value
of 0.282135 ± 7 (Fisher et al., 2011), and similar to the original LAMC-ICP-MS values reported by Fisher et al. (2011) of 0.282139 ± 53
(2 S.D., MSWD = 2.5, n = 134). After Yb and Lu interference corrections were performed, all 176 Hf/177 Hf ratios were normalized with
respect to a Mud Tank value of 0.282507 (Woodhead and Hergt,
2005; reported relative to a JMC-475 value of 0.282160); measurements of this reference crystal were performed twice every
∼15 unknowns during the session. The mean measured value for
all analyses of Mud Tank zircon performed during the course of
this study was 0.282463 ± 50 (2 S.D., MSWD = 1.1, n = 107), which
is slightly lower than the accepted solution-MC-ICP-MS value
and indicates an average bias of ca. 4.4 × 10−5 in the measured
176 Hf/177 Hf ratios. Such deviations of laser data from reference
solution values have been observed by different laboratories and
instruments in numerous previous studies (e.g., Wu et al., 2006;
Iizuka et al., 2009; Fisher et al., 2014a; Kemp et al., 2015), and so
a normalization procedure using a well characterized low-Yb–Lu
reference zircon crystal has been proposed as an effective way
to reduce the magnitude of inter-laboratory bias (Fisher et al.,
2014b). Furthermore, in order to monitor the accuracy of the Yb
interference and Hf bias correction throughout the entire session,
fragments of the R33 and FC-1 reference zircon were frequently
analyzed along with the unknowns; these crystals are known to
have elevated Yb/Hf ratios and are therefore well suited for this purpose. The mean values obtained for R33 and FC1 during our session
were 0.282752 ± 46 (2 S.D., MSWD = 2.0, n = 118) and 0.282193 ± 61
(2 S.D., MSWD = 2.1, n = 63), respectively. These values are accurate with respect to solution-MC-ICP-MS results to within ±0.5
␧Hf and further validate the accuracy of our analytical routine
(also see results of secondary reference crystals presented in
Table 2).
For the U–Pb analyses, weighted mean ages for cogenetic suites
of zircons (i.e., igneous samples or metamorphic rims) reported
in the Figures and Tables are quoted with two increasing levels of uncertainty in the form ±(A)/[B], where (A) represents the
weighted-mean uncertainty calculated using the internal integration and within-session normalization uncertainties, while [B]
represents the total uncertainty on the mean including all other
sources of systematic error (i.e., decay constant errors and primary reference material calibration). Since all the data presented in
this study were acquired using the same analytical protocols and
normalized with respect to the same primary reference material
(SL2 zircon), internal comparisons between our U–Pb geochronologic results can be performed using the level of uncertainty (A).
However, the reader should be aware that for comparing these
results with respect to external data acquired using different a
geochronologic system (e.g., Ar–Ar), or normalized against a different reference material (e.g., Plešovice zircon), the uncertainty
level [B] should be preferred as it accounts for systematic sources
of uncertainty which, in our case, are dominated by the calibration
of the primary reference material used for fractionation correction.
For a detailed description of the procedures involved in these calculations the interested reader is referred to the extended analytical
appendix.
3.3. Whole-rock Sm–Nd isotopes
Fresh whole-rock samples were crushed to a fine powder using
a shatter-box, and then dissolved in Savilex® vials with a hot
concentrated mixture of HF-HNO3 . The dissolved samples were
spiked with a mixed 147 Sm–150 Nd isotopic tracer (Ducea and
Saleeby, 1998) and allowed to equilibrate on a hot-plate before
ion-exchange chromatography. The bulk REE’s were separated in
cation-exchange columns using AG50W-X4 resin, and then Sm and
Nd were separated using anion-exchange columns with LNSpec®
resin using ultra-pure 0.1 N and 0.5 N HCl. Sm and Nd were loaded
onto single Re filaments using platinized carbon and resin beads,
respectively. Isotopic analyses were carried out in a VG Sector multicollector thermal ionization mass spectrometer (TIMS) equipped
with adjustable 1011 Faraday collectors and a Daly photomultiplier. Concentrations of Sm and Nd were calculated by isotope
dilution, and isotopic compositions were determined from the
same spiked run. Each individual analysis consisted of 100 integrations of the isotopic ratios, and uncertainties were calculated as the
√
standard error of the mean (2 S.E. = 2 S.D./ 100). Instrumental mass
discrimination was corrected by normalization with respect to the
reference ratio 146 Nd/144 Nd = 0.7219, and repeat analyses of the
LaJolla standard were used to monitor the accuracy of the measured
composition; the average 143 Nd/144 Nd value for this standard was
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
0.511849 ± 4 (2SD). More details on the methods used are described
in Ducea and Saleeby (1998) and Otamendi et al. (2009).
4. U–Pb and Lu–Hf systematics of complex zircon: pitfalls,
visualization and interpretations
Various potential pitfalls on the interpretation of coupled
U–Pb–Hf isotopic data could be introduced by (a) complexities in
the U–Pb systematics of discordant zircon, (b) incorrect assignment
of ages to a particular Hf composition, or (c) from inadvertent analyses of mechanical mixtures caused by the simultaneous ablation
of different growth-zoning domains. These complexities, which
are especially prevalent when dealing with multi-phase zoned
zircons from tectonically complex terranes, impose important
analytical challenges that need to be considered for the accurate measurement and interpretation of U–Pb–Hf isotopic data.
Some of these issues can be circumvented by the use of quasisimultaneous U–Pb–Hf measurements (Woodhead et al., 2004;
Kemp et al., 2009a), or fully simultaneous dual-mass spectrometry
approaches (Xie et al., 2008; Yuan et al., 2008; Tollstrup et al., 2012;
Kylander-Clark et al., 2013; Fisher et al., 2014a, this study). These
analytical strategies can help prevent incorrect age assignments
to Hf isotopic compositions and to identify mixing complexities
during excavation of the ablation pit. Nevertheless, even when
assuming that an accurately determined U–Pb isotopic composition is adequately assigned to an accurate 176 Hf/177 Hf value, natural
complexities in the isotope systematics can introduce further variability. Post-crystallization thermal events affecting a zircon can
perturb its radiogenic Pb (Pb*) composition (Mezger and Krogstad,
1997; Hoskin and Black, 2000; Valley et al., 2014), specially if
the crystal has been partially amorphized by accumulated radiation damage (Cherniak et al., 1991; Murakami et al., 1991; Ewing
et al., 2003). Ancient events of Pb-remobilization cause the apparent 207 Pb/206 Pb dates of affected grains to yield values that are
measurably younger (if Pb* is locally lost) or older (if Pb* is locally
gained) than their true age of primary crystallization, an issue that
is specially problematic for the accurate determination of detrital zircon ages (Nemchin and Cawood, 2005; Valley et al., 2006)
Such complexities in the U–Pb system are relevant for interpreting
Hf isotope systematics, especially when the latter are evaluated
and visualized in terms of ␧Hf values. Given the time-dependent
nature of the reference frame utilized for calculating ␧Hf values
(i.e., the theoretical 176 Hf/177 Hf composition of the chondritic –
CHUR – or bulk silicate Earth – BSE – reference frames at a given
time t; Patchett et al., 1981), the magnitude of this parameter is
291
strongly dependent on the age that is assigned for a particular Hf
isotope composition, and therefore only minimum apparent ␧Hf
values for any given apparent single-crystal date t can be retrieved.
However, given the almost negligible changes in 176 Hf/177 Hf values
through time that occur within a zircon, the determination of the
initial isotopic ratio is almost independent of age. Consequently,
when dealing with complex zircons – particularly of detrital origin – that have undergone important post-crystallization thermal
events, our ability to retrieve accurate initial ␧Hf values for these
crystals might be severely impaired by complexities in their U–Pb
systematics, whereas establishing an accurate initial 176 Hf/177 Hf
value is readily achievable (see discussion in Amelin et al., 2000).
For this reason, all Hf isotope vs. age plots shown and discussed
below are constructed in terms of 176 Hf/177 Hf vs. time instead
of using ␧Hf. Mean ␧Hf values are only reported for groups of
cogenetic zircon (e.g., igneous protoliths) for which a robust crystallization age could be calculated, as it should be evident that in
such cases the calculated ␧Hf(t) are likely accurate and not just a
minimum estimate like in the case of single detrital zircon crystals. One of the samples analyzed in this study, MIVS-13, provides
a good natural example to illustrate the implications of the above
discussion and highlights the power of the LASS-ICP-MS technique
to filter out extremely perturbed analyses that would otherwise
introduce noise to the interpretation of detrital zircon U–Pb–Hf
data (Fig. 10).
Key to the application of Hf isotopic compositions in crustal evolution studies is the fact that different terrestrial reservoirs diverge
in terms of their 176 Hf/177 Hf signatures as a function of time, owing
to their contrasting parent/daughter compositions and at a rate
that is proportional to their relative Lu enrichments (Patchett et al.,
1981). Therefore, evaluating the rate of change in isotopic ratios (or,
graphically, the slope) of a particular reservoir or group of genetically related rocks – or zircon crystals – as a function of time, is
arguably as useful for the study of crustal evolution as accurately
determining their absolute 176 Hf/177 Hf ratio at the time of crystallization. The rate of these isotopic evolutionary trends (or slopes)
can be directly linked to an apparent 176 Lu/177 Hf composition, and
provide not only valuable insights into the source(s) from which
the samples were derived, but also the mechanisms responsible
for these secular changes (Fig. 4).
Following from the discussion on ancient Pb-remobilization
above, inaccurate determinations of crystallization ages on a given
suite of zircon crystals due to ancient Pb-loss will result in horizontal 176 Hf/177 Hf(t) vs. time arrays, or an apparent evolutionary trend
with a slope of 176 Lu/177 Hf ≈ 0 (Fig. 4). This horizontal reference line
Fig. 3. Yb–Hf isotopic results for synthetic zircon crystals (Fisher et al., 2011) analyzed several times during our analytical session using methods described in the text. The
retrieval of statistically undistinguishable 176 Hf/177 Hf values with increasing magnitudes of Yb interference (expressed as 176 Yb/177 Hf) demonstrates the adequacy of the
applied 176 Yb interference correction. Error bars for individual measurements are shown at 95% confidence; the interested reader is referred to the analytical appendix of
this article for further details on how this interference correction is addressed.
292
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
Table 3
Sample location coordinates and codes for those analyzed by LASS-ICP-MS used
throughout the manuscript and in data repository (see text for details).
Codea
(a)
(b)
(c)
(d)
(e)
(f), (g)
(h)
(i)
(j)
(k)
a
Sample name
MIVS-26
MIVS-41
Macarena-2
MIVS-15A
MIVS-16A
Mandur-2 well
Payara-1 well
MIVS-12
MIVS-13
CB-006
Solita-1 well
La Rastra-1 well
MIVS-37a
Latitudeb
◦
N2
N 2◦
N 3◦
N 2◦
N 2◦
N 0◦
N 2◦
N 2◦
N 2◦
N 2◦
N 0◦
N 1◦
N 2◦
03
13
01
08
08
55
07
09
08
13
52
09
15
Longitudeb
19.2
34.0 45.0 28.2 11.7 25 31 20.7 09.0 26.5 29 58 37.3 ◦
W 75
W 75◦
W 73◦
W 75◦
W 75◦
W 75◦
W 74◦
W 75◦
W 75◦
W 75◦
W 75◦
W 75◦
W 75◦
42
50
52
36
36
52
33
35
35
50
37
30
49
IGSNc
47.6
30.3 13.5 44.7 55.7 34 36 27.4 22.7 22.0 21 13 49.9 IEURI0001
IEURI0002
IEURI0003
IEURI0004
IEURI0005
IEURI0013 & 14
IEURI0012
IEURI0007
IEURI0008
IEURI0011
IEURI0015
IEURI0016
IEURI0009
Codes only used for samples analized by split-stream U-Pb-Hf.
All cordinates in WGS-84 system.
International Geo Sample Number, www.geosamples.org.
Fig. 4. Schematic diagram of the different mechanisms that can cause changes in
176
Hf/177 Hf compositions as a function of time for a particular crustal reservoir, and
the interpretations that can be derived from time-integrated apparent 176 Lu/177 Hf
compositions or trends in 176 Hf/177 Hf vs. time. See Section 4 in text for discussion.
(For interpretation of the references to color in text, the reader is referred to the
web version of the article.)
5. Results
also represents a divide between contrasting geological processes
that can result in trends of decreasing 176 Hf/177 Hf over time (field
of apparent time-integrated 176 Lu/177 Hf < 0; green area in Fig. 4)
or its increase (field of apparent time-integrated 176 Lu/177 Hf > 0;
red and blue areas in Fig. 4). Because both 176 Hf and 177 Hf are
stable and mass-dependent fractionation induced by typical geological processes is not analytically resolvable, the only way for
a negative trend in 176 Hf/177 Hf(t) over time to be achieved is by
significant progressive addition of less radiogenic material (e.g.,
older crust) to a mixed source. This scenario could conceivably
occur in areas were extensive underplating of supracrustal materials is a prevalent process induced by subduction erosion (Scholl
and von Huene, 2007; Roberts et al., 2012), or where underthrusting of continental lithosphere along accretionary and collisional
orogens transports large volumes of old lithospheric crust into
the melt generation zone (e.g., DeCelles et al., 2009; Hacker et al.,
2011). Recent datasets and global data compilations show systematic variations for the Lu/Hf compositions of different terrestrial
crustal reservoirs (Vervoort and Patchett, 1996; Vervoort et al.,
1999, 2000; Plank and Langmuir, 1998; Rudnick and Gao, 2003;
Hawkesworth et al., 2010); in general, it is observed that the mean
modern Lu/Hf compositions of typical crust lie between 0.052 and
0.224, or approximate 176 Lu/177 Hf between 0.0072 and 0.0312.
Therefore, positive 176 Hf/177 Hf vs. age trends (or apparent timeintegrated 176 Lu/177 Hf values >0) can be further subdivided in
two fields; one that is characterized by small positive slopes with
time-integrated 176 Lu/177 Hf values ≤0.0312 (red area in Fig. 4),
and the other one with steeper time-integrated slopes equivalent
to a 176 Lu/177 Hf ≥ 0.0312 (blue area in Fig. 4). Accordingly, timeintegrated 176 Lu/177 Hf values between 0 and ∼0.0312 could be
reasonably explained by radiogenic decay within a closed-system
crustal reservoir without the need of mixing with any external
sources, whereas steeper trends (176 Lu/177 Hf ≥ 0.0312) would most
likely involve the progressive addition of a radiogenic component
derived from a depleted reservoir (e.g., depleted mantle) to the
overlying crust. Lastly, vertical arrays in 176 Hf/177 Hf vs. age space, if
not derived from instrumental variations, incorrect 176 Yb interference corrections of LA data, or domain convolution during ablation,
would represent geologically short-lived events of mixing between
two compositionally contrasting sources that were not isotopically
homogenized. This simple geometrical construct serves as the basis
for some of our data interpretations discussed below.
A total of 13 samples collected from the Garzón, Las Minas and
Macarena massifs in the Northern Andes as well as from drill cores
that reached the basement of the Putumayo foreland basin were
analyzed in this study (Fig. 1, Table 3). For some of these samples,
U–Pb results obtained by LA-MC-ICP-MS have previously been published by Ibanez-Mejia et al. (2011). These existing data provide
excellent reference values for critically assessing the accuracy of the
new U–Pb results obtained using the LASS-ICP-MS approach. The
U–Pb–Hf and ␦18 O isotopic results obtained here are shown graphically in Figs. 5–10 and summarized in Table 4. Given the contrasting
spot sizes necessary for conducting the different types of analyses
(i.e., ∼10 ␮m for ␦18 O and 40–50 ␮m for simultaneous U–Pb–Hf),
not all the growth domains analyzed for ␦18 O were amenable to
be analyzed for U–Pb–Hf with the same textural resolution (see
Figs. 6 and 9). To simplify the discussion and the correlation of
analyses obtained from the same portion of a zircon grain between
the different methods, a simple nomenclature was adopted as
follows: each samples analyzed for U–Pb–Hf was assigned a letter code (first column of Table 3), which is then followed with a
sequentially assigned number in the same chronologic order as
spots were ablated during LASS-ICP-MS. When the LA spots coincide with domains previously analyzed by SIMS for ␦18 O, this LA
spot code is also shown next to the ␦18 O composition both in the
figures and in the supplementary material to facilitate their correlation. For the sake of simplicity, analyses that occur in tight U–Pb
and/or Hf compositional clusters are not labeled unless relevant to
the discussion, and only the calculated mean values for these clusters are quoted in the text. Results for all the individual U–Pb–Hf
and ␦18 O analyses are available as supplementary material to this
paper. Discordance values quoted in the text and data tables are
calculated as Disc. (%) = 100 − (100 * (206 Pb/238 U date)/(207 Pb/206 Pb
date)). All U–Pb figures were plotted and weighted mean ages calculated using U–Pb Redux (Bowring et al., 2011), by importing the
LASS-ICP-MS data as a legacy format. The individual redux files
for all the samples reported in this study are available from the
Geochron data repository and can be freely accessed through the
Geochron database (www.geochron.org). All Nd isotopic results are
summarized in Table 5.
Given the complexity of the obtained results and correlations
between the different isotopic systems, detailed descriptions of
sample data processing, outlier filtering and selection of analyses
used toward the reported mean values are provided as supplementary material to this communication. In addition to the detailed data
description presented in the supplementary material, comments
b
c
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
293
Fig. 5. Uranium–Pb–Hf–O isotopic results in zircons from metaigneous units included within cordilleran basement massifs. Sample codes are according to Table 2 and colorcoding of symbols is as follows: open symbols (circles for ␦18 O and squares for Hf) were rejected due to sputtering pits intersecting cracks and/or inclusions for ␦18 O (based on
SEM-BSE imaging), or identifiable zircon growth-domain mixing during sputtering or ablation for both ␦18 O and Hf (identified from SEM-CL imaging, or mixing evidenced in
changing 207 Pb/206 Pb time-resolved ratios; see analytical appendix for details of the latter). Solid black symbols are ␦18 O results that passed the crater morphology inspection
by post-analysis SEM-BSE imaging. However, some of these analyses were not considered toward the weighted mean calculations after subsequent U–Pb data from the same
domains showed evidences of partial age resetting by metamorphic overprint or discordance greater than 10%; these analyses lie outside of the gray boxes in the O panels
that denote the range of the mean ± 2 S.D. of ␦18 O values. Solid red squares (and filled concordia ellipses) are U–Pb–Hf analyses considered reliable and used toward the
interpretations (see supplementary material). Solid gray symbols – circles for ␦18 O and squares for Hf – were analyses conducted on rims and or older xenocrysts; these were
not considered toward the reported mean values of most samples, but are mentioned in the text when meaningful for the discussion. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of the article.)
294
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
Fig. 6. Detailed high-resolution cathodoluminescence imaging of representative zircons from metaigneous units included within the cordilleran basement massifs. Location
of SIMS and LASS-ICP-MS analyses are shown along with the individual results obtained for each one of the isotopic systems. For simplicity, values in this figure are quoted
without their associated uncertainties (refer to supplementary material for these values). Spots marked with an asterisk are analyses not included in the final calculations
for reasons explained thoroughly in the supplementary materials.
were annotated on a spot-by-spot basis in the tabulated analytical results (also provided as supplementary material) in order
to facilitate their identification and further data evaluation. If a
given analysis displayed complexities in its U–Pb systematics that
deemed it unusable for accurate age estimation, then the associated
Hf isotope composition was also not considered in the discussion.
Uranium-Pb results obtained from zircon of one particular sample,
MIVS-13 (Fig. 10), showed extreme perturbed isotopic systematics;
these results are still included here for illustrating the data interpretation complexities discussed in Section 4, but they were given
the least weight in the following discussion and conclusions.
to its amalgamation at the core of the Rodinia supercontinent. These
data also allow us to better establish tectonic correlations between
the NW Amazon Craton, high-grade basement massifs exposed in
the northern Andes, and the Proterozoic basement of Oaxaquia
in SW North America. In the following sections we explore the
implications of our data for evaluating the chronology of major
tectonic events within Putumayo Orogenic phase, place first order
constraints on the timing and possible nature of crust forming
and reworking processes, and discuss these interpretations in the
context of Amazonia’s Mesoproterozoic tectonics prior to Tonian
collision.
6. Discussion
6.1. Mesoproterozoic magmatism within the Putumayo Orogen
The U–Pb, Lu–Hf, O, and Sm–Nd isotopic data presented here
provide new insights into the long-term history of crustal evolution along the leading margin of Mesoproterozoic Amazonia prior
Uranium-Pb results by LASS-ICP-MS support previous observations indicating that magmatism occurred within crustal domains
represented by the north Andean Precambrian basement massifs
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
295
Fig. 7. Uranium–Pb–Hf isotopic results from a graphic-textured segregation of a pegmatitic dike (MIVS-15A) and a garnet-bearing leucosome from a metasedimentary
migmatite (MIVS-16A) in the Garzón massif. Sample codes are according to Table 2 and color-coding of symbols follows the description provided for Fig. 5. Cathodoluminescence images for these samples are included in the supplementary material. (For interpretation of the references to color in this figure legend, the reader is referred to the
web version of the article.)
from 1.33 to 1.15 Ga, and possibly as early as 1.47 Ga as implied
by igneous protolith crystallization ages of the Guapotón (MIVS26),
Las Minas (MIVS41) and La Macarena (Macarena-2) orthogneisses
(Figs. 5 and 11). In addition, U–Pb data from oscillatory-zoned
detrital zircon cores found in the Garzón and Las Minas massif
metasediments further support this inference and suggest that
magmatism may have extended to as young as 1005 Ma (Fig.
7 of Ibanez-Mejia et al., 2011), just prior to the early Neoproterozoic collisional metamorphism at 990 Ma. The lack of U–Pb
ages >1.47 Ga in detrital zircons from the Garzón and Las Minas
massif metasediments strengthens the hypothesis that coarsegrained detritus delivered to these late Mesoproterozoic basin(s)
were not derived from cratonic Amazonia; sediment derived from
the cratonic interior during late Mesoproterozoic time would have
abundant detrital zircon crystals >1.5 Ga (Ibanez-Mejia et al., 2011),
such as those observed in the Aguapeí Group metasediments in
Brazil (Geraldes et al., 2014). Consequently, our observations support a local sourcing of these detrital zircon crystals and lead us
to argue that they were deposited within intra-arc or arc-proximal
basins preserved in the Putumayo Orogen. This implies that the
Hf isotopic values from these detrital zircon crystals can be used
in conjunction with the data from the orthogneiss samples (i.e.,
MIVS26, MIVS41 and Macarena-2) to understand the magmatic evolution of this long-lived peri-Amazonian accretionary arc system.
6.2. A multi-phase tectono-metamorphic history during
protracted continental collision
Zircon U–Pb geochronological results obtained from a deformed
pegmatitic body included within the ‘El Vergel’ metasediments
(1022.3 ± 7.9 Ma, MIVS-15A, Fig. 7), as well as from a foliationcoherent leucosome band from stromatic metatexites within this
same unit (1001 ± 11 Ma, MIVS-16A, Fig. 8), indicate that they crystallized during periods that preceded the granulite-facies event
previously determined from this unit at ∼990 Ma (Ibanez-Mejia
et al., 2011). The age of emplacement for the MIVS-15A pegmatite
dike is similar to an age of migmatite development obtained from
nearby metasedimentary gneisses of the ‘Las Margaritas’ unit which
are exposed in the eastern Garzón massif and display upperamphibolite mineral assemblages (zircon U–Pb SHRIMP age of
1015 ± 7.8 Ma presented by Cordani et al. (2005) for their sample
Gr-15). These results indicate that some of the preserved structures and metamorphic textures observed within the Garzón group
must be associated with at least one, if not more, metamorphic
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M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
Fig. 8. Uranium–Pb–Hf–O isotopic results for zircons from drill-core samples of a syenogranitic sill (Mandur-2 Leuco) and metaigneous units (Mandur-2 Melano and Payara1 c7) from the basement of the Putumayo basin. Samples of the Mandur-2 and the Payara-1 wells come from depths of ca. 1700 m and 1400 m below the present day surface,
respectively. All other conventions follow the description provided for Fig. 5.
events that predate the widespread Neoproterozoic granulitegrade metamorphism at 990 Ma. Although still difficult to ascertain
given the analytical precision limitations and current coverage
of the geochronological dataset, it is permissible to suggest that
the observed deformation within the Garzón massif could be the
result of at least two distinct metamorphic phases: (1) an earlier
phase occurring between ∼1030 and 1000 Ma that resulted in the
widespread migmatization observed in metasedimentary units of
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
297
Fig. 9. Detailed high-resolution cathodoluminescence imaging of zircons from drill-core samples of a syenogranitic sill and metaigneous units from the basement of the
Putumayo basin. See legend in Fig. 8 for conventions.
the Garzón group (i.e., ‘Las Margaritas’ and ‘El Vergel’ migmatites);
and (2) the later 990 Ma event associated with the main collisional
tectonic burial of the Putumayo Orogen. Migmatite formation
during the first phase may have been facilitated by heating of meltfertile sedimentary protoliths, which would be prone to develop
migmatitic textures even under amphibolite-facies conditions due
to the relatively low melting temperatures of average wet pelites
(Thompson, 1982; Chen and Grapes, 2007). The second event at
ca. 990 Ma, which formed dominantly granoblastic metasedimentary and metabasic granulite-facies rocks with scarce evidence
of migmatization, suggests that peak metamorphic conditions in
our study area during this later event were likely attained under
low aH2 O conditions. Leucocratic granulites with granoblastic textures, characterized by abundant perthitic K-feldspar and quartz in
metasedimentary orthopyroxene-bearing assemblages (e.g., sample MIVS-12 in this study – see supplementary sample descriptions
– and MIVS-11 from Ibanez-Mejia et al. (2011); Jimenez Mejia et al.,
2006; Kroonenberg, 1982) support the observation that low or negligible degrees of water-assisted partial-melting may have occurred
in this unit during the 990 Ma metamorphic event, and that peak
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M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
Fig. 10. Uranium–Pb–Hf isotopic results from metasedimentary granulites (MIVS-12 and MIVS-13) of the Garzón massif and a metasedimentary migmatitic melanosome
(CB-006) of the Las Minas massif. Light red vs. solid red areas in the probability density diagrams correspond to calculations made considering the results of all analyzed
zircons, and the most reliable analyses kept after discordance filtering, respectively. Peak-ages labeled on top of the curves correspond to calculations made after discordance
filtering was applied (see supplementary material for detailed discussion). Green shaded areas in the 176 Hf/177 Hf vs. 207 Pb/206 Pb panels represent the estimated age range for
metamorphic zircon crystallization. Diagonally hatched area in MIVS-13 represents the range of significantly younger 207 Pb/206 Pb dates by ancient Pb-loss observed in this
sample – data points in this last range are excluded from further graphs and discussion. The goodness of fit for the highlighted regressions is described using the reduced
chi-squared value 2v , taking into consideration the 176 Hf/177 Hf uncertainty of each point at 95% confidence (the expectancy value of this test is unity when the scatter can
be explained by quoted uncertainties alone). Cathodoluminescence images for these samples are included in the supplementary material. See text for further details. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
metamorphic temperatures must have remained mostly below the
dry granitic solidus.
The occurrence of the two metamorphic pulses discussed above
is also supported by geochronological results obtained from basement samples of the Putumayo basement, which show a similar
two-stage late Meso- to early Neoproterozoic metamorphic history. The metamorphic evolution of this domain is characterized by
migmatite and granulite formation between ca. 1046 and 1020 Ma
observed in the Solita-1, Mandur-2 and La Rastra-1 wells (IbanezMejia et al., 2011, in preparation), which was shortly followed by a
second granulite-grade event recorded at ca. 990 Ma such as that
observed in the orthopyroxene–garnet migmatitic gneisses of the
Payara-1 well (Fig. 8; Ibanez-Mejia et al., 2011).
An analogous temporal relation between two closely spaced
metamorphic events of widespread migmatite formation followed
by dry granulite facies recrystallization has been proposed for the
Adirondack Mountains of the Grenville Orogen. In the Adirondack scenario, migmatites in metapelitic units have been shown
to develop during the 1180–1150 Ma heating associated with
the Shawinigan phase and intrusion of the AMCG suite, preceding the granulite-grade metamorphism of the ca. 1050 Ma
Ottawan orogeny (Heumann et al., 2006; Bickford et al., 2008;
McLelland et al., 2010). Pervasive dehydration occurring during the early melt-extraction event has been suggested to be
responsible for depleting the system of easily accessible water,
thus in part controlling the low aH2 O metamorphic assemblages
found in the younger Ottawan granulites of the Adirondack Highlands (Bohlen et al., 1985; Valley et al., 1990; Heumann et al.,
2006; Lancaster et al., 2009). More detailed petrological and
geochronological studies of the migmatites and granulite-facies
rocks of the Garzón massif, however, are yet necessary in order
to substantiate whether an Adirondack-style control on granulite
formation is possibly also the case within the Putumayo Orogen.
299
6.3. Petrologic implications of the Hf–O isotopic data
The Hf–O isotopic compositions obtained from igneous/
metaigneous units of the Cordilleran terranes and those from the
Putumayo basin basement display some systematic differences
that indicate the contrasting nature of these two crustal domains
(Fig. 11). Whereas zircon crystals from the igneous precursors of
orthogneiss units included in the Cordilleran basement massifs
show moderately elevated and progressively increasing ␦18 O values between 6.4 and 7.2‰ (squares in Fig. 11), a composition typical
of ‘I-type’ arc granitoids (Eiler, 2001; Valley et al., 2005; Kemp et al.,
2007), the basement of the Putumayo basin shows a markedly bimodal distribution in its ␦18 O values (circles in Fig. 11); mantle-like
compositions are found in igneous zircon from the Mandur-2 borehole (5.4–5.6‰), whereas the cores of zircon from the Payara-1 well
gneiss display an ‘S-type’ supracrustal signature (ca. 9.0–9.4‰).
The Hf isotopic ratios of the hornblende–biotite orthogneisses
from the Cordilleran massifs have relatively enriched compositions, with only small positive deviations from CHUR values at
their respective ages (Fig. 11). This evidences an important contribution of pre-existing crustal material in their source region,
an observation that is compatible with results obtained from the
classic hornblende-bearing I-type granites of the Lachlan Fold
Belt, where previous U–Pb–Hf–O studies have demonstrated the
crucial role that metasedimentary-source melting plays in the genesis of cordilleran-type calc-alkaline magmas (Kemp et al., 2007).
The Hf isotopic compositions obtained from the Paleoproterozoic
Putumayo basin basement, on the other hand, are also bi-modal
like their ␦18 O values; igneous-protolith zircon cores found in
the Mandur-2 mafic gneisses, which have mantle-like ␦18 O, also
display an elevated mean ␧Hf(t) of ca. 8.0 (Figs. 8 and 11) thus
indicating that portions of the north Andean foreland might be
underlain by true juvenile Amazonian Paleoproterozoic crust. In
contrast, the protolith zircon cores found in the Payara-1 gneiss
Table 4
Summary of U–Pb–Hf and O isotopic compositions of samples from the Putumayo Orogen.
Hf/177 Hf(t) ± 2 S.D.
␧Hf(t) ± 2 SDd
Code
Sample name
LASS age (Ma)a
Ref. ageb
Eventc
176
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
MIVS-26
MIVS-41
Macarena-2
MIVS-15A
MIVS-16A
Mandur-2 Leuco
Mandur-2 Melano
Payara-1 c7
Payara-1 c7
1154 ± 20/21 (n = 11, MSWD = 0.5)
1329 ± 14/15 (n = 13, MSWD = 1.1)
1467 ± 12/13 (n = 13, MSWD = 1.3)
1022.3 ± 7.9/8.8 (n = 23, MSWD = 0.4)
1001 ± 11/12 (n = 11, MSWD = 0.5)
1010 ± 19/20 (n = 12, MSWD = 0.4)
1602 ± 15/16 (n = 21, MSWD = 0.6)
1590 ± 8/10 (n = 11, MSWD = 1.7)
986 ± 27/27 (n = 4, MSWD = 0.1)
1135 ±
1325 ±
1461 ±
–
–
1017 ±
1592 ±
1606 ±
986 ±
Ign-P
Ign-P
Ign-P
Ign-C
Migm
Ign-C
Ign-P
Ign-P
Met
0.282087 ± 39 (n = 10, MSWD = 0.9)
0.282007 ± 43 (n = 12, MSWD = 1.1)
0.281868 ± 63 (n = 13, MSWD = 1.6)
0.282141 ± 40 (n = 23, MSWD = 1.2)
0.282099 ± 54 (n = 10, MSWD = 1.8)
0.282197 ± 45 (n = 12, MSWD = 1.1)
0.281974 ± 42 (n = 18, MSWD = 0.8)
0.281796 ± 70 (n = 11, MSWD = 2.1)
0.281981 ± 21 (n = 4, MSWD = 0.3)
a
b
c
d
6
5
10
4
8
6
17
+1.2
+2.4
+0.6
+0.1
−1.9
+2.0
+7.6
+0.8
−6.4
±
±
±
±
±
±
±
±
±
1.4
1.5
2.2
1.4
1.9
1.6
1.5
2.5
0.8
␦18 O ± 2 S.D.
7.16 ± 0.22 (n = 8)
6.55 ± 0.26 (n = 8)
6.36 ± 0.27 (n = 10)
–
–
5.60 ± 0.22 (n = 11)
5.43 ± 0.23 (n = 22)
ca. 9.0–9.4
7.94 ± 0.10 (n = 6)
Uncertainties are 2.
Reference ages are previous results obtained by LA-MC-ICP-MS on the same samples and reported in Ibanez-Mejia et al. (2011).
Ign-P: crystallization age of igneous protolith, Ign-C: age of igneous crystallization, Migm: migmatization age, Met: age of metamorphic zircon.
Calculation of uncertainties on the ␧Hf(t) value only consider errors on the 176 Hf/177 Hf(0) measurement, not on the 176 Lu/177 Hf used for age correction.
Table 5
Bulk-rock Sm–Nd and Lu–Hf isotopic data for metaigneous and metasedimentary rocks of the Putumayo Orogen.
Sample
Sm
(ppm)
Nd
(ppm)
147
Sm/144 Nd
0.121097
0.132086
0.136687
0.072948
0.164007
0.120298
0.123077
MIVS-26
MIVS-41
Mandur-2 (L)
Payara-1
MIVS-37a
Solita-1
La Rastra-1
15
11
8
12
12
7
3
75
48
37
96
44
34
14
Sample
Lu
(ppm)
Hf
(ppm)
176
0.23
1.90
0.017105
La Rastra-1
Lu/177 Hf
Nd/144 Nd ± 2
(0)
Age
(Ma)
143
(t)
␧Nd ± 2
(t)
1154
1325
1017
1606
1005
1046
1035
0.511151
0.510970
0.511285
0.510638
0.511232
0.511146
0.511209
0.06
0.87
−0.79
1.50
−2.13
−2.77
−1.82
Hf/177 Hf ± 2
(0)
Age
(Ma)
176
(t)
0.282420 ± 30
1035
0.282086
143
0.512065
0.512115
0.512194
0.511404
0.512310
0.511969
0.512042
±
±
±
±
±
±
±
12
12
19
8
15
11
15
176
Nd/144 Nd
Hf/177 Hf
fSm/Nd
Nd-T(DM)
(Ga)
−0.384
−0.328
−0.305
−0.629
−0.166
−0.388
−0.374
1.60
1.72
1.67
1.77
2.24
1.74
1.67
␧Hf ± 2
(t)
fLu/Hf
Hf-T(DM)
(Ga)
−1.53 ± 1.06
−0.491
1.99
±
±
±
±
±
±
±
0.24
0.24
0.38
0.16
0.30
0.22
0.30
300
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
with a 176 Hf/177 Hf composition near CHUR (Fig. 11), is in-line with
the interpretation drawn from the ␦18 O data that suggests this
gneiss formed via re-working of older Paleoproterozoic crustal
material. Given these evidences, a possible northward extension
of a Rio Negro-Juruena-like basement (sensu Tassinari et al., 1996)
under the Oriente-Putumayo foreland basins would be plausible (RNJ, Fig. 1A) judging by the similar bi-modal nature of this
belt characterized by Paleoproterozoic juvenile magmatism and
overlying supracrustal sequences of similar age (e.g., Jauru region
and Roosevelt supracrustals; Tassinari et al., 1996; Tassinari and
Macambira, 1999, and references therein).
The occurrence of evolved syenogranitic sills in the Mandur-2
well with mantle-like ␦18 O zircon values and enriched Hf isotope
compositions (sample Mandur-2 Leuco; Figs. 8 and 10) might be
explained in at least two simple ways: (1) these syenogranitic magmas were formed by melting of pre-existing crustal sources with
␦18 O values that were both higher and lower than mantle-like compositions, which were mixed during anatexis in such a proportion
that the resulting melt appears to be mantle-like by mere coincidence; or (2) the syenogranitic melts were formed by reworking
of an ancient mantle-derived mafic crust which remained unaltered for hundreds of millions of years such that mantle-like ␦18 O
values of the resulting partial melts are preserved, but their Hf isotope ratios had enough time to sufficiently diverge from a depleted
mantle-like composition. Following from the observation that
these deformed syenogranites are hosted within mafic gneisses
with mantle-like ␦18 O and juvenile 176 Hf/177 Hf(t) compositions at
ca. 1.6 Ga (i.e., Mandur-2 Melano sample; Figs. 8 and 11), and that
the host mafic gneisses show structures compatible with partial
melting (Ibanez-Mejia et al., 2011), we favor the second interpretation for the origin of the evolved syenogranitic intrusives retrieved
in the Mandur-2 well (sample Mandur-2 Leuco). If this was the
case, then the difference in 176 Hf/177 Hf(t) values between the host
gneiss and the syenogranite sills could be explained by closedsystem radiogenic ingrowth along a whole-rock 176 Lu/177 Hf ≈ 0.02,
which is in agreement with the average composition of mafic crust
(Rudnick and Gao, 2003; Pietranik et al., 2008).
The observation that metamorphic overgrowths in zircons from
the Guapotón and Las Minas orthogneiss have undistinguishable
␦18 O values with respect to their inherited igneous cores is also significant (Figs. 5 and 6), as it may provide further evidence to support
the interpretation that extensive dehydration melting of metasediments and subsequent rock–fluid interactions (e.g., volatile fluxing)
with the orthogneiss units probably had only a subdued role during the granulite-forming event at 990 Ma. The 176 Hf/177 Hf vs.
207 Pb/206 Pb date correlations in the bulk single-crystal data presented by Weber et al. (2010) for the Guapotón orthogneiss is
incompatible with metamorphic rims being developed by solidstate recrystallization, and indicates that the rims observed in Fig. 6
likely represent newly formed zircon precipitated from a metamorphic fluid or anatectic melt (e.g., Bowman et al., 2011). If this
is the case, then the similarity of ␦18 O compositions between the
cores and rims of zircons from this unit (Fig. 5) is likely an indication of closed-system behavior during metamorphism, and implies
that no externally-derived fluids with contrasting ␦18 O values –
as those that would be expected from dehydration of the host
metasediments to these plutons – participated in metamorphic
zircon crystallization of the Guapotón and Las Minas orthogneisses.
Fig. 11. U–Pb–Hf–O data from basement igneous and metaigneous rocks of
the Putumayo Orogen. (A) Age-corrected 176 Hf/177 Hf vs. age for granitoids,
orthogneisses and migmatites analyzed during this study. For reference, ␧Hf values with respect to CHUR (Bouvier et al., 2008) are shown as dashed light-gray lines
parallel to this reservoir, plotted at successive increments of +2 and −2␧ deviations.
Red dotted lines are apparent iso-TDM contours, showing values for the apparent
model ages that would be obtained by assuming a reservoir Lu/Hf composition of
‘average crust’ (i.e., 176 Lu/177 Hf = 0.015; Condie et al., 2005). Other reservoir slopes
shown in inset are for (a) Island arc crust (Hawkesworth et al., 2010), (b) bulk lowercrust (Rudnick and Gao, 2003), (c) global subducting sediments (GLOOS; Plank and
Langmuir, 1998), (d) bulk continental crust (Rudnick and Gao, 2003), (e) average
Precambrian granites (Vervoort and Patchett, 1996), (f) bulk upper-crust (Rudnick
and Gao, 2003). DM is the juvenile-crust depleted mantle model using the data of
Vervoort and Blichert-Toft (1999); NC is the ‘New Crust’ model of Dhuime et al.
(2011). Fields for different terranes within Oaxaquia are from Weber et al. (2010).
(B) ␦18 Ozircon compositions vs. age for granitoids and orthogneisses analyzed during
this study. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of the article.)
6.4. Secular changes in Hf isotopic compositions through time
The time-integrated compositional trends defined by the Hf
isotopic results of protolith igneous and detrital zircon crystals
obtained here provide a wealth of information on the modes
and mechanisms responsible for crustal development along this
pre-collisional Mesoproterozoic margin. Long-term changes in
176 Hf/177 Hf and 143 Nd/144 Nd ratios within a closed system are
expected to occur through time, simply owing to the in situ decay
of radiogenic 176 Lu and 147 Sm, and will occur at ratesthat directly
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
depend on the parent/daughter composition of the system. If a particular crustal reservoir experiences various episodes of partial melt
extraction over a sufficiently long time-span, then the isotopic compositions of those melts will track the long-term evolution of the
source or the various possibilities of mixing between the different
contributing components (Murphy and Nance, 2002; Hawkesworth
and Kemp, 2006; Kemp et al., 2009b; see discussion in Section
4 and Fig. 4). Secular changes preserved in the protolith zircon
from orthogneiss units and in metasediments of the Putumayo Orogen allow for the identification of two main intervals within its
protracted Mesoproterozoic tectonic evolution: Interval 1, which
occurred between ca. 1.47 Ga and 1.15 Ga, is characterized by a constant steep positive progression in 176 Hf/177 Hf(t) values versus age,
whereas Interval 2, defined between ca. 1.15 and 0.99 Ga, displays
an overall shallowing in the trends of time-changing 176 Hf/177 Hf(t)
compositions (Figs. 11 and 12). We interpret these two intervals
as representing contrasting phases within the tectonic evolution
of the orogen, reflecting the prevalence of different geological processes that exert control on the rates at which juvenile melts are
being extracted and added to the crust or the amount of older material being reworked at any given time. In this sense, Interval 1 is
suggested to be the result of a long-term progressive addition of
juvenile mantle melts at the base of the arc crust. The observed
overall trend is unlikely to simply represent radiogenic ingrowth
within a closed-system reservoir, or to be generated by older crustal
reworking (Fig. 4), and thus might be evidencing the cryptic role
that juvenile magmatic input had in the overall growth of this
long-lived active continental margin. Based on observations from
younger accretionary orogens that preserve a much more complete
magmatic record (e.g., Kemp et al., 2009b), it has been suggested
that rapid periods of crustal growth are intimately linked with
tectonic processes that enhance mantle melting, such as back-arc
spreading associated to slab roll-back events. Therefore, we propose that the long-term trend observed during this first interval
of the Putumayo margin took place in a dominantly retreating
accretionary orogen, thus resulting in a long-term net addition
of juvenile material to the overlying arc system. We speculate
that this process, however, is likely not the result of monotonous
steady-state juvenile input (as simplified in Fig. 11), but rather
could be the final apparent result of a series of cyclic episodes of
punctuated juvenile melt extraction and older crustal reworking,
controlled by shorter-lived tectonic pulses typical of active continental margins (DeCelles et al., 2009; Kemp et al., 2009b). Such an
interpretation could explain the apparent de-coupling in the longterm trends of the ␦18 O and Hf compositions from the orthogneiss
protoliths, which show increasing ␦18 O values through time – indicating increased supracrustal material incorporation (Fig. 11B) –
whereas our interpretation of the changing 176 Hf/177 Hf compositions is one of progressive rejuvenation. If the granitic precursors
to the Macarena, Las Minas, and Guapotón orthogneisses represent only a snapshot into individual shorter-lived cycles rather
than a single mixing of two end-member components, then the
Hf–O trends need not be directly linearly correlated. Nevertheless, it is possible to suggest, based on the existing data, that
although more surficially altered material is participating in arcmagma generation through time, the composition of the arc system
as a whole – including any underplated sediments from the arcproximal basins – were experiencing a rapid positive change in
their bulk 176 Hf/177 Hf compositions. This is interpreted as the result
of progressive rejuvenation of the system by the addition of mantlederived juvenile magmas to the base of the arc crust throughout this
interval.
Interval 2 (ca. 1150–990 Ma) is more difficult to define directly
in terms of the magmatic products being produced along the interpreted arc system as no orthogneisses with protolith age younger
than the Guapotón unit (i.e., ∼1.15 Ga) have yet been identified
301
Fig. 12. U–Pb–Hf–Nd data from metasedimentary and metaigneous units of the
Putumayo Orogen. (A) 176 Hf/177 Hf vs. apparent 207 Pb/206 Pb date of detrital zircons
from metasedimentary granulites and migmatites of the Garzón and Las Minas North
Andean Precambrian basement massifs. External reproducibility (at 95% confidence)
of low Yb (Mud Tank) and high Yb (R33) reference zircon crystals analyzed during
this analytical session are shown as reference for the typical uncertainty bars of
individual analyses. All other symbols are the same as Fig. 11. (B) 143 Nd/144 Nd vs.
age for metasedimentary (plotted at their age of metamorphism) and metaigneous
units of the North Andean basement massifs and the basement of the Putumayo
basin. Analogous to the Hf plots, the y-axis of this plot is in 143 Nd/144 Nd units and
␧Nd values are also shown as deviations in +2 and −2 increments around the CHUR
composition. Slopes for the evolution of different reservoirs as a function of their
Sm/Nd compositions (inset) follow the same nomenclature as Fig. 11. Fields for
the different units and lithologies within Oaxaquia are recalculated from: P&R87 –
Patchett and Ruiz (1987); R&P88 – Ruiz et al. (1988); W&K99 – Weber and Köhler
(1999). The composition of metasedimentary migmatites and granulites of the North
Andean Precambrian basement massifs are recalculated after Restrepo-Pace et al.
(1997) and Cordani et al. (2005).
302
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
in the study region. This interval appears to be a period of
more intense crustal reworking, an inference that is supported by
observations of the Hf isotope data as well as other lines of geological and geochronologic evidence that argue for the widespread
occurrence of regional metamorphic events within the orogen during this time interval. In addition to the U–Pb geochronological
results that demonstrate the occurrence of metamorphic events
in the Garzón massif and the Putumayo basin basement between
∼1.00 and 1.05 Ga as described earlier (Section 6.2), detrital zircon populations with clearly sector-zoned textures of interpreted
metamorphic derivation occur in metasediments of the Las Minas
massif and have ages mainly in the 1.10–1.17 Ga range (sample
CB-006; Fig. 10). Although it cannot be unequivocally interpreted
that these detrital populations are directly related to this segment
of the arc system (e.g., possible derivation from distant sources),
the limited age distribution of inherited components in this sample marked by a tight bi-modal age distribution with ages close
to those of sediment deposition favor a localized provenance of
these detritus (Cawood et al., 2012). Furthermore, thermobarometric modeling coupled with Sm–Nd and Lu–Hf age constraints from
garnet-bearing metasedimentary granulites in the Putumayo basin
basement (i.e., drilling cores retrieved from the La Rastra-1 well),
indicate that thermal pulses leading to regional metamorphism in
this area may have started as early as ∼1060 Ma (Ibanez-Mejia et al.,
in preparation). This further supports the interpretation that Interval 2 involved a significant component of contractional tectonism
and crustal reworking, which we tentatively interpret as reflecting
early terrane accretion during the protracted evolutionary history
of this orogen as initially proposed by Ibanez-Mejia et al. (2011).
6.5. The along-strike diachronic nature of continental suturing
Comparison of the detrital zircon populations found in metasediments from the Garzón group (i.e., sample MIVS-12 and MIVS-13
from the Garzón massif) and the Zancudo migmatites (i.e., sample
CB-006 from the Las Minas massif) reveals fundamental differences
in the zoning textures and Hf isotopic compositions of zircons that
crystallized within a similar age range. Whereas all post-1.2 Ga
inherited crystals found in the Garzón group metasediments show
textures indicative of igneous crystallization (see CL image repository) and define positive 176 Hf/177 Hf vs. time progressions similar
to those described above for Interval 1 (Fig. 10, Hf panel of sample
MIVS-12), zircon cores from the Zancudo migmatites show textures indicative of derivation from metamorphic sources (see CL
image repository) and have a sub-horizontal to slightly negative
slope in their 176 Hf/177 Hf vs. age variation (Fig. 10, Hf panel of
sample CB-006). These contrasting textural and isotopic characteristics indicate fundamental differences in the dominant processes
occurring in their respective sediment source areas during the
∼1.20–1.05 Ga time interval, which can be interpreted in terms of
their tectonic significance. As discussed in previous sections, steep
positive 176 Hf/177 Hf vs. time trends are indicative of juvenile crust
additions, whereas sub-horizontal or negative slopes – when not
due to complexities in the U–Pb system – are indicative of periods
of extensive older crustal reworking. The simultaneous occurrence
of these two trends in the detrital-zircon record of the Putumayo
orogen possibly indicates that these two processes were concurrent
in different segments of the orogenic system.
As observed within a modern continental collisional zone,
the early stages of interaction of irregularly shaped continental
fragments induce a marked along-strike variation in the deformation and magmatism that occurs within the overriding plate
(Malinverno and Ryan, 1986). In some instances these diachronous
processes lead to the widespread development of wide back-arc
basins induced by slab roll-back, which causes trench retreat within
the overriding collisional plate margin (Royden, 1993). Therefore,
if back-arc spreading centers constitute a major locus of juvenile crust generation (Kemp et al., 2009b), then rapid extraction
of juvenile continental crust should in many cases be expected
to occur in synchrony with the early stages of continental collision along these segmented retreating subducting boundaries.
Following these inferences, we speculate that the mixed evidences
of crustal growth and reworking observed for Interval 2, could
potentially be a result of the terrane accretions occurring within
a ‘Mediterranean-style’ tectonic setting (e.g., Pullen et al., 2008);
in such a scenario, segments of the arc-basin system could have
continued to experience subduction-related magmatism and rapid
additions of juvenile melts in extensional back-arc environments,
while other segments of the trench were probably chocked by
the underthrusting of buoyant continental lithosphere resulting in
older crustal reworking and the flattening of the 176 Hf/177 Hf vs.
age isotopic patterns (Figs. 5 and 11). The impingement of continental indentors causing localized deformation within the northern
segment of the Grenville Orogen (Gower et al., 2008), might be a
reflection that this process is more common in ancient orogens than
usually recognized.
6.6. Paleo-tectonic linkages with the Precambrian basement of
Oaxaquia
Despite previously available geochronological and isotopic evidence that suggested correlations between the North Andean
basement massifs and the Mexican basement inliers (RestrepoPace et al., 1997; Ruiz et al., 1999; Weber et al., 2010), a possible
Amazonian connection for their ancestry remained speculative
due to the dearth of isotopic data from a coeval orogenic belt
east of the Andean deformation front in northern South America. The new whole rock Nd and Hf data presented here provide,
for the first time, direct evidence from cratonic Amazonia in support of these linkages. Neodymium isotopic values from bulk-rock
samples of a syenogranite from the Mandur-2 well, a migmatitic
gneiss from the Payara-1 well, and metasedimentary migmatites
of the Solita-1 and La Rastra-1 wells (also bulk-rock Hf from this
last one) provide a reference composition for the NW Amazonian basement and the bulk sedimentary load it delivered to its
peripheral Mesoproterozoic basins. Fig. 12B shows a comparison
of the Nd isotopic compositions from Oaxaquia, the north Andean
basement massifs, and the basement of the Putumayo basin. The
compositional fields from the different lithologic components of
Oaxaquia were delineated by recalculating the Nd isotopic compositions of Patchett and Ruiz (1987), Ruiz et al. (1988) and Weber
and Köhler (1999), using the chronology for the different magmatic and sedimentation events as discussed by Weber and Köhler
(1999), Weber and Hecht (2003), Cameron et al. (2004), Keppie and
Ortega-Gutierrez (2010) and Weber et al. (2010). Compositional
fields for metasedimentary units of the Colombian basement massifs use the Nd data of Restrepo-Pace et al. (1997) and Cordani
et al. (2005) recalculated using the inferred chronology of sedimentation discussed in Cordani et al. (2005) and Ibanez-Mejia
et al. (2011). To a first order, the 143 Nd/144 Nd(t) values of the
metasediments from cratonic Amazonia (i.e., Solita-1 and La Rastra1 wells) are in excellent agreement with the compositional field
defined by metasediments of the Garzón group, Las Minas massif
and Oaxaquia (Fig. 12). These results support the interpretation
that, although not clearly represented in the detrital zircon record
as evidenced by the scarcity of Amazonian-like U–Pb age components in the north Andean and Oaxaquian metasediments (Cordani
et al., 2005; Ibanez-Mejia et al., 2011; Solari et al., 2013; this
study), the basement and bulk-sediment isotopic composition of
Mesoproterozoic NW Amazonia provide a viable match for the
older crustal inheritance found in metasedimentary units of these
remobilized basement domains. The compositions of basic and
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
intermediate meta-igneous units from the Oaxaquian basement
(basic granulites and amphibolites in the Guichicovi complex and
metabasic rocks found elsewhere in Oaxaquia; Fig. 12B), which
display more radiogenic 143 Nd/144 Nd values with respect to the
basement of the Putumayo basin basement and the compositions
of the Andean orthogneisses (i.e., MIVS-26 and MIVS-41) and Oaxaquian orthogneisses (i.e., felsic orthogneisses of the Guichicovi
complex; Fig. 12B), are consistent with a mixture between an inherited crustal component of possible Amazonian descent and younger
juvenile melt additions in an arc environment. The bulk-rock Hf
data from the La Rastra-1 well is also compositionally similar, but
slightly less radiogenic, than the 176 Hf/177 Hf(t) values obtained from
zircons of the North Andean and Mexican terranes (Fig. 11A). This
result also shows that the addition of variable degrees of juvenile
radiogenic material to an NW Amazonian reworked component
is a feasible and simple model in order to explain the Hf isotopic compositions of the mid- to late Mesoproterozoic basement
blocks of the Northern Andes and Oaxaquia (Weber et al., 2010,
this study). Finally, we also note that recent detrital-zircon U–Pb
data from metasediments of the Oaxaquian Complex of Solari et al.
(2013) display strikingly similar patterns with respect to metasediments studied in this contribution (Fig. 10) and presented earlier
by Ibanez-Mejia et al. (2011). Furthermore, the most radiogenic
detrital-zircons from the Garzón group metasediments (apparent
␧Hf(t) values between 3 and 6), which have no known bedrock
counterpart (i.e., orthogneiss protolith) in the north Andean basement domains, are in excellent agreement with the compositions
of the igneous protoliths of the Oaxaca, Guichicovi and Novillo
orthogneisses (Fig. 12A). This interpretation is also in line with the
recent discovery of early Mesoproterozoic (∼1.4 Ga) protolith ages
found in orthogneisses and migmatites of the Huiznopala and Oaxaca complex (Weber and Schulze, 2014), which further support the
inference that the construction of Oaxaquia also involved reworking of older Mesoproterozoic crustal material. These new isotopic
and geochronologic observations, along with all of the previous
lines of evidence discussed above, continue to reinforce interpretations that postulate a close tectonic, magmatic, and metamorphic
evolution of the north Andean and Oaxaquia basement domains.
We propose that evidence now exists in order to support linkages
between these blocks and the basement of NW South America, and
that combined evidences of their mineralogical and isotopic inheritance store abundant information to identify a long-lived history
of convergent margin tectonism along the (modern) NW Amazonia
during most of the Mesoproterozoic.
6.7. Relations with the Sunsás-Aguapeí belt and implications for
Rodinia reconstructions
In addition to providing new compelling evidence to link the
Meso- to Neoproterozoic evolution of the Putumayo basin basement with the North Andean basement massifs and the basement
of Oaxaquia (see discussion is Section 6.6), the new data obtained
in this study allow important distinctions to be drawn between
the Putumayo orogen and the late Mesoproterozoic collisional belt
of modern SW Amazonia, the Sunsás-Aguapeí orogen (Fig. 1A). In
contrast to the late Mesoproterozoic magmatic history inferred
herein for the Putumayo margin, which likely lasted until the final
collisional closure of the hypothesized Putumayo-Sveconorwegian
orogen at ca. 990 Ma (Cardona et al., 2010; Ibanez-Mejia et al.,
2011), no clear geochronologic evidence exists to infer that active
margin tectonism occurred along the Sunsás-Aguapeí belt after the
docking of the Paraguá block in the mid Mesoproterozoic (marking the end of the Rondonia-San Ignacio orogeny; Bettencourt
et al., 2010; Teixeira et al., 2010). Instead, the geological evolution
of the Sunsás-Vibosi and Aguapeí-Huanchaca (meta)-sedimentary
belts is associated with the development of passive-margin and
303
intra-continental rift basins that developed along the fringes of
the Paraguá block, which were later inverted and variably metamorphosed during the Sunsás collision at ∼1.1 Ga and intruded by
un-deformed post-collisional granites starting at ∼1.08 Ga (Boger
et al., 2005; Santos et al., 2008; Teixeira et al., 2010). Recent detritalzircon U–Pb data from sedimentary units of the Aguapeí group by
Geraldes et al. (2014) indicate that the sedimentary infill of this
basin was dominantly derived from the cratonic interior to the ENE (e.g., Rio Negro-Juruena province, RNJ in Fig. 1) and the Paraguá
block to the W (rifted fragment of the Rondonia-San Ignacio composite orogen, RSI in Fig. 1), but lacks zircon with ages younger than
∼1.25 Ga. If the age estimate proposed by Rizzotto et al. (2014)
for the opening of the Aguapeí-Huanchaca rift ca. 1.15 Ga is correct, then the absence of detrital zircon with ages between ∼1.25
and 1.15 Ga in sedimentary sequences of the Aguapeí group further supports an intra-cratonic rift setting for its deposition and
the absence of a nearby arc system such as the one we hypothesize
was concurrently active along the Putumayo margin. Based on the
contrasting geologic histories for the Sunsás-Aguapeí belt and the
Putumayo margin during the late Meso- to early Neoproterozoic,
paleogeographic models that propose a common accretionary margin to explain the development of both orogenic belts (e.g., SAMBA
reconstruction; Johansson, 2009) become unsupported as they fail
to reconcile these markedly differing tectonic scenarios.
Although paleomagnetic data are still somewhat equivocal with
respect to the exact positioning of Amazonia during mid to late
Mesoproterozoic times (see reviews in Evans, 2013; Pisarevsky
et al., 2014), reliable poles from basaltic rocks of the Nova Floresta Fm. (Tohver et al., 2002) and red beds of the Fortuna Fm. in
the Aguapeí group (D’Agrella-Filho et al., 2008) suggest a translation from lower to higher latitudes for Amazonia from a near
equatorial position during the Stenian. This is in contrast to the
near-polar paleolatitude of Baltica in the late Ectasian and its
subsequent clockwise rotation toward lower latitudes during the
Stenian (Pisarevsky et al., 2003; Cawood and Pisarevsky, 2006),
which, as recently pointed out by Pisarevsky et al. (2014), also
make the SAMBA reconstruction paleomagnetically unsupported
for the mid to late Mesoproterozoic paleogeography. Alternatively,
the reconstructions proposed by Tohver et al. (2002) and Evans
(2013) for Amazonia’s incorporation to Rodinia – independently
from Baltica – using the post-1.3 Ga poles are both consistent with
the geological history of the Putumayo margin as described by
Ibanez-Mejia et al. (2011) and in this study. Although the alternative paleogeographic model of Evans (2013) would raise further
questions with respect to the evolution of the Sunsás-Aguapeí orogen, because a collision between this margin and the Llano segment
of Laurentia (Tohver et al., 2002) would no longer be a feasible
scenario to explain the coeval Sunsás and Llano deformation, it
could be an attractive alternative if a Paleoproterozoic connection between Amazonia and Baltica (e.g., SAMBA-like hypothesis of
Bispo-Santos et al., 2013) could be further substantiated. However,
as recently noted by Pisarevsky et al. (2014), although paleomagnetically viable the participation of northern Amazonia in Paleoto Mesoproterozoic Columbia (or Nuna) is still doubtful. Instead,
the reconstruction proposed by Pisarevsky et al. (2014) envisions
an independently drifting Amazonia with a long-lived active subduction margin along its (modern) western side for most of the
Mesoproterozoic, a model that is compatible with the protracted
history of convergence inferred from the geologic and geochronologic data (Bettencourt et al., 2010; Ibanez-Mejia et al., 2011).
7. Conclusions
We provide detailed U–Pb, Lu–Hf, Sm–Nd, and O isotopic data
from the Precambrian basement of NW South America to support
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M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
the hypothesis that a long-lived accretionary margin was developed along this segment of the Amazon Craton for almost the entire
duration of the Mesoproterozoic era. Based on isotope composition
vs. time trends from granitoids and detrital-zircons, this protracted
evolution is proposed to consist of at least two main intervals:
(a) an earlier one dominated by overall crustal growth associated
with the development of a long-lived accretionary arc system (or
a series of) between 1.45 (?) and ∼1.15 Ga, and (b) a later interval
between ∼1.15 and 0.99 Ga characterized by mixed geochemical
and geochronological evidence that indicates the simultaneous
occurrence of mechanisms that induce juvenile crust additions
and older crust reworking along the margin. This apparent contradiction is interpreted as resulting from a strong segmentation
of upper-plate deformation styles in the orogenic system, which
can be explained by the accretion of terranes along some segments of the trench while other portions continued to experience
ongoing subduction and possibly fast trench retreat. These new
geochronologic and isotopic data also allow us to postulate tectonic
linkages for the evolution of the Putumayo basin basement, basement blocks exposed in the northern Andes, and the Precambrian
basement of central Mexico, prior to the Tonian collisional climax at
ca. 990 Ma that resulted in the docking of Amazonia at the heart of
the Rodinia supercontinent. We provide the first direct geochemical
evidences from autochthonous NW Amazonia in support of these
connections, and observe that the isotopic compositions of the
different crustal domains that make up the northern Andean basement and the Oaxaquia composite terrane can be accommodated
by different stages of the long-lived history of convergence herein
proposed.
Augmenting the initial hypothesis put forward by Ibanez-Mejia
et al. (2011), the new data presented in this study continue to
draw fundamental differences between the tectonic history and
the timing of major crustal growth and deformation episodes that
characterize this margin of Amazonia and the Sunsás-Aguapeí orogen (Teixeira et al., 2010;Fig. 1A; Fig. 8 of Ibanez-Mejia et al.,
2011). This has important implications for models of Rodinia
reconstructions, as any paleogeographic scenario advocating either
for a long-lived connection between Amazonia and Baltica (e.g.,
SAMBA reconstruction of Johansson, 2009) or collisional interactions along oppositely facing Putumayo and Sveconorwegian
margins (e.g., Bingen et al., 2008; Cardona et al., 2010; Li et al.,
2008; Weber et al., 2010) would need to simultaneously satisfy
the geodynamic and timing constraints imposed by the pre- and
syn-collisional history of the contrasting Sunsás and Putumayo
orogens.
Analytical results presented in this Putumayo case study
highlight the potential that combined high-resolution imaging,
texturally resolved ␦18 O compositions, and concurrent U–Pb–Hf
isotopic analyses by LASS-ICP-MS methods have for retrieving
crucial information about the tectonics, crustal development and
sedimentation history of strongly modified orogenic crust. The
‘split-stream’ analytical method calibrated during the course of
this study, which allows for fine-tuning of the aerosol splitting proportions and takes advantage of the enhanced-sensitivity
interface of the Element2 SC-ICP-MS, shows that concomitant U–Pb and Lu–Hf isotopic information obtained by the
methods herein described can be acquired with little compromise to the precision and accuracy typically achieved for
these system using ‘single-stream’ data collection. This approach
represents a tremendous advantage for characterizing complexities in the isotope systematics or ablation-induced mechanical
mixtures of growth domains in complex poly-phase zircons
that, if unresolved, could significantly hinder the effective
use of these data to construct accurate geological interpretations.
Acknowledgements
The authors would like to thank the Instituto Colombiano del
Petroleo (ICP-ECOPETROL) and the Litoteca Nacional de Colombia for
allowing access to the core samples, which were taken with the
kind assistance of Ana Milena Rangel. Ariel Strickland tuned and
assisted MI with the analysis of oxygen isotope ratios at WiscSIMS.
Tom Milster and Melissa Zaverton are acknowledged for facilitating the optical interferometry measurements at the Department
of Optical Sciences, University of Arizona. We express our gratitude to journal editor Randy Parrish, as well as Bernard Bingen and
an anonymous reviewer for constructive comments that significantly improved the clarity and the quality of the final version of
this article. This work was partially supported by the U.S. National
Science Foundation EAR-1118525, EAR-1032156 and EAR-1338583
awards. WiscSIMS is partially supported by NSF-EAR-1053466. The
ICP-MS and SEM facilities at the Arizona Laserchron center are partially funded by NSF-EAR-1338583 and EAR-0732436 awards. The
ALC Element2 SC-ICP-MS was acquired thanks to the support of
ExxonMobil.
Appendix A. Analytical methods for concurrent U–Pb–Hf
measurements using laser-ablation ‘split-stream’
A.1. Analytical setup and data acquisition
All U–Pb–Hf isotopic analyses presented in this study were
simultaneously acquired using a SC-ICP-MS and a MC-ICP-MS in
the Arizona Laserchron Center (ALC) at the University of Arizona. This approach is generally known in the literature as laser
ablation–split stream (LASS), which throughout the text we refer
to as LASS-ICP-MS. For simultaneous acquisition of the U–Pb and
Lu–Yb–Hf data using magnetic-sector mass spectrometers, most
previous workers have utilized SC- and MC-ICP-MS instruments
built by the same manufacturer (e.g., Nu instruments Attom SCICP-MS and Nu Plasma MC-ICP-MS in Kylander-Clark et al., 2013;
Thermo-Finnigan Element2 SC-ICP-MS and Neptune MC-ICP-MS in
Fisher et al., 2014a; Tollstrup et al., 2012). For this study, we utilized a Thermo-Finnigan Element2 SC-ICP-MS equipped with an
enhanced sensitivity ‘Jet Interface’ for measuring U–Pb isotopes
and Yb–Lu–Hf were measured using a Nu Plasma MC-ICP-MS. These
two instruments have considerably different auxiliary and sample
gas pressurizations, which make splitting the aerosol generated
after laser-ablation slightly more difficult than when two instruments from the same manufacturer and with similar primary gas
line pressures are used. A schematic plumbing diagram for the
split-stream setup herein described is shown in Fig. 2 of the main
text.
Samples were ablated using a Photon Machines Analyte G2,
ArF Excimer LA system, equipped with a fast-washout two-volume
HelEx® cell and adapted with custom-made Au traps placed along
the carrier gas line upstream from the sample cell in order to
minimize Hg contributions to the 204 mass (Gehrels et al., 2008).
Ultra-high purity He was used as the sample carrier gas, flowing through the cell at a total rate of 0.390 SLPM (MFC1 = 0.050
SLPM, MFC2 = 0.340 SLPM; Fig. 2). After the exit of the ablation cell, Ar was added flowing at a rate of 1.200 SLPM before
the aerosol + He + Ar mix entered a mini cyclonic spray chamber
(Cinnabar spray-chamber by Glass Expansion® ), which served as a
mixing volume in which the gas-analyte blend was homogenized
before splitting. After exiting the cyclonic chamber the mixture was
split using a regular Y-connector. Since the gas-line pressure to the
Nu Plasma is lower than that of the Element2, if un-obstructed, the
total gas flow will tend to go toward the MC-ICP-MS. To overcome
this problem we placed a high-precision needle valve in the line of
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
least pressure (i.e., line going to the Nu Plasma), which allowed us
to adjust the relative proportion of the total aerosol + He + Ar flow
sent to the multicollector; the total mass flow in this line was monitored on a precision Mass Flow Monitor Whisper unit manufactured
by Alicat Scientific® (MFM1 in Fig. 2). By using this flow-balancing
setup, the relative proportion of aerosol sent to both instruments
at the split can be tuned to optimize signal intensity and obtain
ion beams that would result in accurate and precise analyses, and
attain long-term stability for the instrumental drift of LASS-ICP-MS
measurements. We found the optimal splitting proportions to be
close to 70–80% of the analyte directed toward the Nu Plasma and
20–30% toward the Element2. In addition to balancing the gas flows
such that the MFM1 unit reads a value of ∼80% of the total input gas
flow, the signal intensity measured on both mass-spectrometers
was also used to confirm that the desired split proportions had been
achieved. To do this, the measured signal resulting from the ablation of the primary Sri Lanka (SL2) zircon reference material was
frequently monitored. Under typical conditions, where the laser
is connected to only one of the mass-spectrometers and a 30 ␮m
diameter spot is being used, a 180 Hf beam intensity of ∼1.3 V in
the Nu Plasma and a 238 U intensity of ∼4 × 107 cps on the Element2 are generally observed; these intensities are reduced by
∼20% and ∼80%, respectively, when using our split-stream setup
describe here.
A.2. Yb–Lu–Hf data processing
The data reduction strategy for Yb–Lu–Hf analyses generally
follows that described by Ibanez-Mejia et al. (2014) for the LA-MCICP-MS analysis of baddeleyite. In this study, data was acquired
using time resolved acquisition (TRA) analyses on the Nu plasma
using a 1.5 s integration time, and was reduced using Iolite (Paton
et al., 2011) and an in-house data reduction scheme (DRS) adjusted
to process data from our collector-block configuration. The DRS file
is available from the authors upon request. Mass-bias factors to
correct for hafnium fractionation (ˇHf ) were calculated by measuring the 179 Hf/177 Hf value of each analysis and using an exponential
fractionation law (Russell et al., 2002) with respect to a reference
ratio of 0.7325 (Patchett and Tatsumoto, 1981). In the presence of a
measurable Yb signal (i.e., considered detectable when the ablation
signal mean of all masses of interest is above 3 S.D. from their baseline means), its own mass fractionation factor ˇYb can be calculated
by monitoring two non-interfered masses (e.g., 171 Yb and 173 Yb)
and normalization with respect to a reference ratio as proposed by
Woodhead et al. (2004). During this study the Yb model of Vervoort
et al. (2004) was adopted, and ˇYb factors were calculated using a
173 Yb/171 Yb reference value of 1.129197 for analyses with an average total-Yb beam greater than 200 mV. Alternatively, in samples
where the Yb signal intensities are lower than this value – as is
the case for many natural zircons – the uncertainties on the calculated ˇYb factors are large and consequently have a negative impact
on the accuracy of the estimated 176 Yb interferences. In this latter
scenario, the mass-fractionation of Yb was approximated from that
of Hf by establishing a robust ˇHf –ˇYb relation for each particular
session using only those analyses performed on high-Yb zircons
(e.g., R33 and FC-1) and Yb-doped synthetic zircons (Fisher et al.,
2011). The observed relationship can then be used to recast the
ˇHf of each analysis into a term xˇHf using the following expression: xˇHf = (ˇYb )meas /(ˇHf )meas , which was used in exchange of
the measured ˇYb values when correcting the moderate- to lowREE zircon analyses for Yb interference measured during that same
session. The validity of this approach is demonstrated by the results
obtained from analyses on several secondary reference zircons with
varying 176 Hf/177 Hf values and degrees of 176 Yb interference, as
summarized in Table 2 of the main text and shown graphically in
figures accompanying this supplementary material.
305
After this first step for Yb mass-bias and interference subtraction has been applied, a secondary Yb-bias correction (described
in detail by Fisher et al., 2014a,b; Ibanez-Mejia et al., 2014) was
applied by determining a bias factor to the reference ‘natural’
176 Yb/173 Yb composition such that the slope in 176 Hf/177 Hf vs.
176 Yb/177 Hf space for the Yb-doped zircons was minimized. This
approach is similar to that used previously by Vervoort et al. (2004)
to correct solution Hf data for Yb interference. Final mass-bias and
interference-corrected 176 Hf/177 Hf ratios are thus calculated using
the following expressions:
176 Hf
177 Hf
corr
=
176 (Hf + Yb + Lu)meas − 176 Yb
177 Hf
M176 ˇHf ·
M177
176
Ybcalc = 173 Ybmeas ·
Lucalc =
175
Lumeas ·
− 176 Lucalc
· BFHf
(A1)
(M176/M173)
176 Lu/175 Lu
meas
(176 Yb/173 Ybref ) · BFYb
176
calc
(ˇYb ) or (xˇHf )
(A2)
ref
(ˇYb )
(A3)
(M176/M175)
where M176/M177, M176/M173 and M176/M175 are the ratios of
the exact masses of these isotopes of Hf, Yb, and Lu, respectively.
The BFYb term is the secondary Yb correction factor derived from
the Yb-doped zircon measurements, and BFHf is the instrumentalbias factor for Hf. This last term was obtained by normalization
against repeated analyses of Mud Tank zircons and using the reference solution-MC-ICP-MS value of 0.282507 (Woodhead and Hergt,
2005) as recommended by Fisher et al. (2014b).
A.3. U–Pb data processing
U–Th–Pb isotopes were measured on an Element2 SC-ICP-MS
using the scanning parameters listed in Table 1 in the main body
of the text. By using the dwell-time values listed, each complete
scan across the designated mass range is of ∼110 ms in duration.
Each individual analysis consists of a 15 s gas baseline measurement, which is immediately followed by 80 s of ablation. After the
laser stops firing an additional 5 s of measurement were allowed in
order to capture the tail of the signal dropping back down to baseline values. Therefore, the total measurement time for each spot
is of 90 s in duration, during which the Element2 completes 810
scans over the defined mass range. The typical washout time, or the
time elapsed between the end of one analyses and the beginning
of baseline measurements for the next spot, was typically set to 20
or 30 s. Although 80 s of signal from the entire ablation pass were
always collected, only the first 30 s (270 on-peak scans) were used
to calculate the 206 Pb/238 U and 207 Pb/206 Pb compositions used for
age calculation; the remaining ∼40 s of data (ca. 440 scans) were
used to visually evaluate intra-grain compositional complexities
and potential zone-mixing that might have occurred deeper in the
ablation pit when the Hf isotopic analysis were still underway. The
main reasons for not using the entire collection time for U–Pb ratios
calculation are the fast drop in signal observed during the entire
ablation time and the deviation from linearity of the down-pit fractionation, which make the last 40 s of data an unnecessary source
of scatter in the calculation of the ratios due to the already reduced
count-rates in the detector during LASS-ICP-MS acquisitions. We
note that, although the LASS-ICP-MS technique described in this
contribution assigns U–Pb dates to particular Yb–Lu–Hf isotope
compositions on a whole-spot basis, future developments using
this split-stream approach have the potential to take further advantage of the time-resolved capabilities of LA-ICP-MS to scrutinize the
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M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
data on a time-slice basis rather than using whole-spot integrations
(e.g., Tollstrup et al., 2012).
The uncertainty propagation scheme adopted during this study
follows the procedure recommended by the PlasmAge workgroup
(www.plasmage.org). The scheme is as follows. (1) Average baseline values for each mass, calculated from the 15 s of data collection
prior to initiation of ablation, were subtracted line-by-line from
the time-resolved measured intensities. (2) Uncertainties in the
baseline estimation for each mass i were calculated using Poisson statistics as the square root of the baseline integrations mean
√ i
i
= x̄baseline
). (3) The baseline corrected 206 Pb/238 U values
(baseline
for the first 30 s of ablation were plotted against elapsed acquisition
time and a linear least-squares fit was performed as described previously in Cecil et al. (2011), Gehrels et al. (2008) and Ibanez-Mejia
et al. (2014) in order to correct for the laser-induced elemental fractionation (LIEF). (4) The first second of ablation data was discarded
to allow for the signal to stabilize (e.g., Frei and Gerdes, 2009), and
the LIEF-corrected 206 Pb/238 U value for each ablation pass was calculated as the intercept of the linear regression at ablation time
(t) = 1 s; the uncertainty of this ratio (206 Pb/238 U ) is estimated from
the uncertainty on the intercept of this parametric fit. (5) For analyses with a 204 Pb signal above detection level (i.e., considered
detectable when the Hg-corrected ablation signal mean is above
3 S.D. from the Hg-corrected baseline mean), common-Pb corrections were applied using the model of Stacey and Kramers (1975)
and additional uncertainties to the instrumentally-determined
206 Pb/204 Pb and 207 Pb/204 Pb of 1.0% and 0.3%, respectively, were
assigned. These uncertainties were propagated throughout the age
calculations as described by Gehrels et al. (2008). At this stage, the
total uncertainty of each spot (which will be henceforth referred
to as internal integration uncertainty) consists of the propagation
in quadrature of the uncertainties related to the gas blank subtraci
tion (baseline
), the LIEF correction (206 Pb/238 U ) and the common-Pb
correction (pbc-corr ) if the later was applied.
Elemental- and mass-dependent fractionation corrections,
induced by the ablation process and ICP-MS measurement, are best
corrected by normalization with respect to a matrix-matched reference material (Kosler et al., 2002; Kosler and Sylvester, 2003;
Chang et al., 2006; Gehrels et al., 2008; Cottle et al., 2009, 2012;
Slama et al., 2008; Frei and Gerdes, 2009; Thomson et al., 2012;
Ibanez-Mejia et al., 2014). To do this, we use fragments of the same
Sri Lanka (SL2) reference crystal of Gehrels et al. (2008), which
has an ID-TIMS age of 563.5 ± 3.2 Ma and an average U concentration of approximately 520 ␮g/g(Zr) . Each session was bracketed
by five SL measurements at the beginning, five at the end, and one
every five unknowns. A running mean fractionation factor for the
206 Pb/238 U and 207 Pb/206 Pb values at each point in the session was
calculated by averaging the measured values for the ratios in the
six neighboring SL measurements and dividing it over the reference
ID-TIMS ratios. This running-average factor can then be used to
correct the unknowns for fractionation at the same time as correcting for any instrumental drift that might occur during a particular
session (Gehrels et al., 2008; Paton et al., 2010; Ibanez-Mejia et al.,
2014). It has been argued by Horstwood (2008), Cottle et al. (2009),
and the PlasmAge community, that the minimum uncertainty of
any given measurement should incorporate the internal integration uncertainty and the excess variability observed on the primary
reference material used for fractionation correction. Therefore,
after applying the instrumental drift correction, a normalization
uncertainty factor (or over-dispersion factor, OD) was derived for
both the 206 Pb/238 U and 207 Pb/206 Pb ratios in order to make the
MSWD values of the reference material mean equal to unity. The
internal integration uncertainty and the normalization uncertainty
are regarded as independent sources of error, and therefore this
excess scatter factor is propagated in quadrature with the calculated uncertainty of each data point in order to obtain the minimum
A
B
Fig. 13. (A) Cross-sectional profile of an ablation pit in zircon, acquired using a Wyko
NT9800 optical interferometer. This crater was produced using 200 laser pulses and
other laser parameters listed in Table 1. (B) Summary of interferometry measurements from various craters excavated with varying numbers of pulses per burst;
from this experiment, a laser excavation rate of 0.053 ␮m/pulse (or 0.37 ␮m/s) was
determined and proven to be linear with time at least down to the total pit depths
excavated during this study. The LASS-ICP-MS results presented in this study were
conducted using 560 laser repetitions per spot (∼30 ␮m total crater depth).
uncertainty of each individual measurement. In the case when
the calculated MSWD values of the 206 Pb/238 U and/or 207 Pb/206 Pb
means of the primary reference material are unity or below, no
over-dispersion uncertainties need to be added. From this it follows
that the reported uncertainties for the apparent dates of LA-SC-ICPMS analyses are quoted at three levels of increasing uncertainty, in
the form ±[X][Y][Z] (columns ±1␴(a) , ±1␴(b) and ±1␴(c) in the U–Pb
columns of the supplementary material). The first level [X] refers
to the internal integration uncertainty of each individual analysis,
[Y] accounts for the propagation of the excess scatter (or normalization uncertainty) that is specific for each sample or session, and
[Z] is the final total uncertainty that includes the systematic errors
associated with the U decay constant and reference-material (SL2
zircon) calibration. Weighted mean ages for cogenetic suites of zircons (i.e., igneous samples) reported in the Figures and Tables are
quoted with two levels of uncertainty in the form ±(A)[B], where
(A) represents the weighted-mean uncertainty calculated using the
individual-spot uncertainties [Y] described above (thus incorporating over-dispersion), and [B] represents the total uncertainties
after systematic errors have been propagated (accounted for in the
uncertainty level [Z]).
A.4. Dimensions of the analytical pits
In order to fully quantify the dimensions – and most importantly the depth – of the generated analytical pits, the total depth
of several ablation craters were measured by means of optical interferometry. This quantification is important to estimate whether
U–Pb–Hf data collected for a particular ablation spot using our analytical approach remains within a relatively shallow depth and is
therefore likely to correspond to the zircon growth domain of interest. These measurements were performed using a Wyko NT9800
M. Ibanez-Mejia et al. / Precambrian Research 267 (2015) 285–310
interferometer at the College of Optical Sciences at the University
of Arizona. A series of craters with increasing depths were excavated on a Mud Tank zircon fragment, using successive increments
of 50 pulses from 50 to 450 total pulses. Additional craters where
excavated with 490 and 560 bursts, with the latter value being used
for the LASS-ICP-MS measurements presented here. Results of this
experiment are shown in Fig. 13; the upper panel is an example of
a profile obtained for a 200-pulse crater (all other lasing settings as
listed in Table 1 of the main text), where it can be noted that the
bottom of the ablation crater is slightly rounded and has a maximum topography of ca. 0.9 ␮m. We therefore adopted ±1 ␮m as
the uncertainty of the depth estimations. The lower panel of Fig. 13
shows a summary of all the interferometry results, which demonstrates that the drilling rate of the G2 laser in zircon is close to
linear up to at least 560 pulses and on the order of ∼0.053 ␮m/pulse
(equivalent to 0.37 ␮m/sec using a 7 Hz repetition rate). This rate
is slightly slower than what was estimated by Ibanez-Mejia et al.
(2014) for the ablation of baddeleyites using similar lasing parameters. The total analytical pit generated by the LASS-ICP-MS routine
herein described (i.e., 560 pulses) is of approximately 30 ␮m in
depth, thus never exceeding a 1:1 aspect ratio for the excavated
craters and likely mostly staying (in depth) within growth domains
that were wide enough to accommodate a 40 ␮m or 50 ␮m diameter ablation spot.
Appendix B. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.precamres.2015.
06.014
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