Lithos 239 (2015) 97–113
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Lithos
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Across and along arc geochemical variations in altered volcanic rocks:
Evidence from mineral chemistry of Jurassic lavas in northern Chile, and
tectonic implications
Pablo Rossel a,1, Verónica Oliveros a,⁎, Mihai N. Ducea b,c, Laura Hernandez d
a
Departamento Ciencias de la Tierra, Universidad de Concepción, Casilla 160-C, Concepción, Chile
Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
c
Universitatea Bucuresti, Facultatea de Geologie Geofizica, Strada N. Balcescu Nr 1, Bucuresti, Romania
d
Instituto de Geología Económica Aplicada (GEA), Universidad de Concepción, Chile
b
a r t i c l e
i n f o
Article history:
Received 16 February 2015
Accepted 7 October 2015
Available online 23 October 2015
Keywords:
Back arc
Central Andes
Clinopyroxene
Mineral chemistry
Thermobarometry
a b s t r a c t
Postmagmatic processes mask the original whole-rock chemistry of most Mesozoic igneous rocks from the Andean arc and back-arc units preserved in Chile. Mineral assemblages corresponding to subgreenschist metamorphic
facies and/or propylitic hydrothermal alteration are ubiquitous in volcanic and plutonic rocks, suggesting
element mobility at macroscopic and microscopic scale. However, fresh primary phenocrysts of clinopyroxene
and plagioclase do occur in some of the altered rocks. We use major and trace element chemistry of such mineral
phases to infer the geochemical variations of four Jurassic arc and four back-arc units from northern Chile.
Clinopyroxene belonging to rocks of the main arc and two units of the bark-arc are augites with low contents of
HFSE and REE; they originated from melting of an asthenospheric mantle source. Clinopyroxenes from a third
back-arc unit show typical OIB affinities, with high Ti and trace element contents and low Si. Trace elemental
variations in clinopyroxenes from these arc and back-arc units suggest that olivine and clinopyroxene were
the main fractionating phases during early stages of magma evolution. The last back-arc unit shows a broad
spectrum of clinopyroxene compositions that includes depleted arc-like augite, high Al and high Sr–Ca diopside
(adakite-like signature). The origin of these lavas is the result of melting of a mixture of depleted mantle plus Srrich sediments and subsequent high pressure fractionation of garnet.
Thermobarometric calculations suggest that the Jurassic arc and back-arc magmatism had at least one crustal stagnation level where crystallization and fractionation took place, located at ca. ~8–15 km. The depth of this stagnation level is consistent with lower-middle crust boundary in extensional settings. Crystallization conditions
calculated for high Al diopsides suggest a deeper stagnation level that is not consistent with a thinned back-arc
continental crust. Thus minor garnet fractionation occurred before these magmas reached the base of the crust.
The presented data support the existence of a heterogeneous sub arc mantle and complex magmatic processes in
the early stages of the Andean subduction.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
All the volcanic rocks that record the early Andean subduction
system (Jurassic to Lower Cretaceous) exhibit some degree of hydrothermal alteration and or low grade metamorphism, part of which
may have taken place in contact with seawater under submarine
conditions, as well as subsequent contact and regional metamorphism
related to the emplacement of later arc magmas (Aguirre et al., 1989;
Levi et al., 1989; Losert, 1974; Oliveros et al., 2008; Robinson et al.,
2004). The secondary mineral assemblages observed in lavas, dikes
⁎ Corresponding author.
E-mail address: voliveros@udec.cl (V. Oliveros).
1
Present address: Facultad de Ingeniería, Geología, Universidad Andres Bello, Autopista
Concepción-Talcahuano 7100, Concepción, Chile.
http://dx.doi.org/10.1016/j.lithos.2015.10.002
0024-4937/© 2015 Elsevier B.V. All rights reserved.
and related volcaniclastic rocks, (chlorite–epidote–quartz–calcite–
titanite–sericite–actinolite–K-feldspar–zeolite–prehnite–pumpeyllite)
represent either propylitic hydrothermal alteration or greenschist or
sub-greenschist facies low-grade metamorphic events that took place
during the Mesozoic (Losert, 1974; Oliveros et al., 2006; Parada et al.,
2007; Robinson et al., 2004; Tristá-Aguilera et al., 2006). Considering
that under the above described metamorphic conditions large ion
litophile elements (LILE), low field strength elements (LFSE) and some
high field strength elements (HFS) can have a mobile behavior, the original composition of the rocks may have been obliterated, which limits
our ability to decipher their original elemental and isotopic characteristics, and prevents a robust petrogenetic interpretation for the Andean
magmatism during the Mesozoic.
Most of the work discussing the petrogenesis of the Jurassic to Lower
Cretaceous Andean volcanics has focused on the study of the less mobile
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P. Rossel et al. / Lithos 239 (2015) 97–113
elements (i.e.: the rare earth and high field strength elements), the
isotopic ratios of volcanic rocks and the comparison with the composition of co-genetic less altered intrusive bodies (Kramer et al., 2005;
Lucassen et al., 2006; Oliveros et al., 2006, 2007; Palacios, 1978;
Parada et al., 1999; Rogers and Hawkesworth, 1989; Vergara et al.,
1995). This geochemical comparison to contemporaneous unaltered
intrusions is not possible in the case of the Upper Jurassic back arc
volcanics (Rossel et al., 2013), since back arc plutonic rocks, if they
exist, are not exposed in this area.
The chemistry of primary magmatic mineral phases can be an
important tool for the characterization of magmatism. Ever since the
definition by Bowen (1928) of the continuous and discontinuous
crystallization series in igneous rocks, there are numerous examples of
successful methodologies that use mineral compositions to assess
petrogenetic conditions; for instance the tectonic discrimination
diagrams based on clinopyroxene composition in basalts (Leterrier
et al., 1982), the characterization of magmatism based on trace and
rare earth elements (REE) compositions of clinopyroxenes and other
primary mineral phases (Marks et al., 2004; Vannucci et al., 1993) and
several thermo-barometers based on melt(whole rock)–mineral
equilibrium (Putirka, 2005, 2008; Putirka et al., 2003). Therefore, if the
chemistry of some primary igneous mineral phases is not changed
during secondary processes, then their major and trace elemental
compositions represent indirect petrogenetic proxies.
Jurassic volcanic rocks cropping out in the present-day Coastal
Cordillera, Precordillera and High Andes of north-central Chile (26°–
31°S) bear phenocrysts that are fresh or much less altered than the
surrounding groundmass. Here we present in-situ mineral chemistry
data in order to gain knowledge on the petrogenesis of altered Jurassic
Andean magmatic rocks. We compare these results with data obtained
from a companion study that focused on whole-rock chemistry
(Rossel et al., 2013). In that paper, we showed that there is a progressive
change in the geochemistry of the lavas from arc to back-arc, and linked
it to variations in the magma sources. Those changes should be also
traceable with the geochemistry of major and trace elements of primary
minerals, such as clinopyroxene, allowing an improvement in the
reconstruction of the sources and primary magmatic processes of the
early stages of Andean subduction.
2. Geologic background
During the Jurassic and Early Cretaceous, western South America
was characterized by an extensional and transtensional tectonic regime
(Charrier et al., 2007; Creixell et al., 2006; Grocott and Taylor, 2002;
Scheuber and González, 1999) due to the oblique subduction of the
dense and cold Phoenix plate (Jaillard et al., 1990; Scheuber and
González, 1999). This geodynamic regime resulted in a roll-back of the
slab and retreat of the trench, as well as progressive thinning of the
upper plate (Charrier et al., 2007; Ramos, 1999). The convergent margin
developed an arc region in the present-day Coastal Cordillera of southern Perú and northern-central Chile, and a series of back arc basins to
the east of the arc, within the modern Chilean Precordillera and High
Andes. The arc magmatism was voluminous (Kramer et al., 2005;
Lucassen et al., 2006; Oliveros et al., 2006, 2007; Palacios, 1978;
Rogers and Hawkesworth, 1989; Vergara et al., 1995), and led to the
accumulation of volcanic sequences up to 7000 m in thickness with
minor sedimentary intercalations, and subsequent intrusion of large
plutonic bodies (Buchelt and Tellez, 1988; Muñoz et al., 1988). Among
the units that represent the Jurassic arc are the La Negra Formation
and its equivalents (Camarca and Oficina Viz Formations) in northern
Chile (18°–29°S), and the Agua Salada Volcanic Complex (30°–31°S).
The back arc region developed in response to the progressive thinning
of the continental crust (Charrier et al., 2007; Martínez et al., 2012),
and it is composed of up to 4000 m of marine and continental sedimentary rocks that were deposited during the Jurassic and Early Cretaceous
(Charrier et al., 2007; Martínez et al., 2012; Mpodozis and Ramos, 1989;
Vicente, 2006). In this context, the Upper Jurassic geological units
cropping out in the Precordillera and High Andes between 26° y and
31°S, the Quebrada Vicuñitas Beds and the Lagunillas, Picudo and
Algarrobal formations (Fig. 1), represent a particular event in the evolution of the back arc basins, in which basic and intermediate volcanism
and continental sedimentation dominated over protracted marine sedimentary conditions (Charrier et al., 2007).
Since the Late Cretaceous, the continuous shifting of the magmatic
front to its present position in the high Andes (Charrier et al., 2007;
Mpodozis and Ramos, 1989) resulted in the occurrence of numerous
events of hydrothermal alteration or regional low grade metamorphism
that modified to a greater or lesser extent the geochemical composition
of the host rocks. These events are represented in all the studied units by
burial metamorphism, in the Precordillera area by the intrusion of Paleocene and mainly Eocene plutons (Cornejo et al., 1998; Iriarte et al.,
1999; Mpodozis and Cornejo, 1986; Tomilson et al., 1999) while in the
Coastal Cordillera the main thermal event affecting the arc rocks was
the product of the intrusion of composite Lower Cretaceous plutons
(Creixell et al., 2012; Dallmeyer et al., 1996; Emparán and Calderón,
2010; Emparán and Pineda, 2000; Grocott et al., 1994; Welkner et al.,
2006). In addition, much of the Jurassic volcanism was developed in a
submarine environment (Vergara et al., 1995; Vicente, 2005) that
affected the geochemical composition of the host rocks (Kramer et al.,
2005; Oliveros et al., 2007), hindering the clear interpretation of the
genesis of the Jurassic Andean magmatism.
3. Sample selection and analytical methods
In order to properly characterize the magmatic sources and evolution
of the arc and back arc volcanism in northern Chile, we selected eleven
samples for chemical analyses of fresh pyroxene and plagioclase phenocrysts (Table 1). One sample of the Agua Salada Volcanic Complex
(ASVC) represents the arc domain and two samples of the Quebrada
Vicuñita Beds, two samples of the Lagunillas formation, four samples of
the Picudo formation and two samples of the Algarrobal formation represent the back arc domain. Although slightly altered, the selected samples are the most primitive and best preserved from the arc and back arc
volcanic units, they have either low contents of silica or were classified as
basalt, basaltic andesite or andesite based on their mineralogy and texture. Their bulk chemistry is presented in Rossel et al. (2013). Given
that the olivine crystals in these rocks, when present, are completely
replaced by iddingsite (mixture of Fe-rich phylosilicates), the most
primitive phases in equilibrium with the original melt that have their
chemistry unaltered are the fresh clinopyroxene and to a lesser extent
plagioclase.
Electron microprobe analyses were carried out using a CAMECA
SX100 and a JEOL JXA-8600M electron microprobe at the University of
Arizona and University of Concepción, respectively. Analytical conditions were 15 kV, 20 nA with a 1 um beam size. Na and K were measured
at 15 kV, 8 nA and the remaining elements at 15 kV, 20 nA, with typical
detection limits of WDS (wavelength-dispersive spectrometry) in the
range between 0.13% (SiO2) and 0.01% (K2O). Ion microprobe analyses
were performed at Arizona State University SIMS (Secondary Ion Mass
Spectrometry) facility, with a primary beam of 16O− at a current of 3
to 4 nA. For the measurements the approach of “conventional energy
filtering” was used, as described by Shimizu et al. (1978), Hinton
(1990) and Zinner and Crozaz (1986).
Major element composition for pyroxene and plagioclase of 20
samples from the volcanic rocks of the Jurassic arc cropping out
between 18°30′ and 24°00S, from Oliveros et al. (2007), are included
in the database for comparison.
4. Bulk rock petrography and geochemistry of the studied units
The Jurassic to Lower Cretaceous volcano-sedimentary units in
the Precordillera and high Andes of northern Chile, 26°–31°S
P. Rossel et al. / Lithos 239 (2015) 97–113
99
Fig. 1. Simplified geological map of the studied area including sample locations. QVB: Quebrada Vicuñita Beds. LF: Lagunillas Formation. PF: Picudo Formation. AF: Algarrobal Formation.
ASVC: Agua Salada Volcanic Complex. DZ Max. age: maximum depositional age obtained through U–Pb dating of detrital zircons. IZ: crystallization age of rocks obtained through U–Pb
dating of igneous zircons.
Modified after SERNAGEOMIN (2003).
(Fig. 1) are relatively well characterized in terms of their petrography and geochemistry. In particular, the volcanic rocks representative of the Jurassic back arc domain have been studied in detail by
Rossel et al. (2013). In spite of the variable alteration degree exhibited by these rocks, the presence of fresh clinopyroxene phenocrysts is
not uncommon.
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P. Rossel et al. / Lithos 239 (2015) 97–113
Table 1
Main features of the analyzed samples.
Sample
Unit
Lithology Coordinates
(UTM E/N)
wt.%SiO2 #Mg Main
mineralogy
PR-09-19
QVB
BA
457666/7036398 –
–
PR-10-32
QVB
BA
456968/7085732 52.33
49
PR-10-72
LF
AB
446046/6935452 45.26
53
PR-10-80
LF
AB
444163/6929847 48.11
43
PR-09-05
PF
BA
375402/6796136 57.95
45
PR-10-41
PF
BA
374711/6791142 55.96
47
PR-10-45
PF
B
373840/6791219 49.74
55
PR-11-168 PF
BA
375288/796083
–
–
M-14
AF
BA
352250/6681573 53.87
41
M-25
AF
A
345541/6681372 62.45
44
275629/6729590 54.68
56
PR-11-153 ASVC BA
Main
First Ca
alteration phase
Cpx
composition
87
Bulk-rock
Bulk-rock Analysis
Sr/86Sr(i)a εNd(i)a
Measured
minerals
Chl
Plg
Augite
–
–
EMPA
Cpx, Plg
Chl
Plg
Augite
0.7047
4.1
Cpx, Plg
Id-Cc
Cpx =
Ti-subsilic
diopside
0.704
2.8
Id-Cc
Plg
Plg?
EMPA,
SIMS
EMPA,
SIMS
0.7043
3.3
EMPA
Cpx
Chl
Plg
Ti-subsilic
diopside
Augite
0.7045
1.3
Cpx
Chl
Plg
Augite
0.7042
0.2
EMPA,
SIMS
EMPA
Chl
Cpx =
Diopside
0.7042
2.7
EMPA,
SIMS
Cpx, Plg
Plg/Cpx/Fe–Ti
oxides
Chl
Plg?
Cpx =
Diopside
0.7041
3.8
EMPA,
SIMS
Cpx
Plg/Cpx/Fe–Ti
oxides
Plg/Cpx/Fe–Ti
oxides
Plg/Cpx/Fe–Ti
oxides
Chl
Plg?
Plg
Augite
0.7045
2.7
EMPA
Cpx
Chl
Plg
Augite
0.7042
0.7
EMPA
Cpx
Chl
Plg
Augite
0.704
2.7
EMPA,
SIMS
Cpx
Plg, Cpx, Fe–Ti
oxides
Plg, Cpx, Fe–Ti
oxides
Ol, Plg, Cpx, Fe–Ti
oxides
Ol, Plg, Cpx, Fe–Ti
oxides
Plg, Cpx, Opx, Fe–Ti
oxides
Plg/Cpx/Fe–Ti
oxides
Plg/Cpx/Fe–Ti
oxides
Cpx
Cpx, Plg
Plg = plagioclase. Cpx = clinopyroxene. Ol = olivine. Fe–Ti = Fe–Ti oxide. Chl = Chlorite. Id–Cc = Iddingsite + calcite. B = basalt. BA = basaltic andesite. A = andesite. AB = alkaline
basalt. EMPA = Electronic Mircroprobe Analysis. SIMS = Secondary Ionization Mass Spectrometry
a
Rossel et al. (2013)
4.1. Algarrobal Formation
This Upper Jurassic back arc unit crops out between ~30 and 31°S.
The selected samples of this formation have porphyritic textures with
seriate euhedral phenocrysts of plagioclase (~ 25% of rock volume)
that can reach ~ 7 mm along their major axis, and minor amounts
(~7%) of euhedral to subhedral clinopyroxene (Fig. 2a), ranging in size
between 1 and 2 mm. Completely altered euhedral crystals of olivine
Fig. 2. Back-scattered electron (BSE) and second electron (SE) images of typical compositional and textural features observed in the studied rocks. BSE images of a) Large clinopyroxene
(Cpx) crystal with opaque inclusions (Fe–Ti Ox.) and b) clinopyroxene phenocryst and large plagioclase (Plg) crystal with poikilitic clinopyroxene inclusions from the Algarrobal Formation from sample M-14; c) SE image of clinopyroxene phenocryst from sample PR-09-19 from Quebrada Vicuñita Beds; d) BSE image of equigranular arrangement of plagioclase and
clinopyroxene with interstitial olivine (Ol) from sample PR-10-80 of the Lagunillas Formation.
P. Rossel et al. / Lithos 239 (2015) 97–113
are present but in small amounts (b 3%). They range in size from 0.5 to
1 mm and can occur either as phenocrysts or in the groundmass.
Plagioclase and clinopyroxene mostly occur as isolated phenocrysts,
but clinopyroxene can also occur as inclusions in large plagioclase
poikilocrysts (Fig. 2b). Alteration affecting plagioclase phenocrysts
results in the development of sericite and minor amounts of epidote,
but is common to find fresh crystals, or at least a fresh area inside the
phenocrysts. On the other hand, clinopyroxene phenocrysts can be
divided in almost completely fresh and completely chloritizated
crystals. No zonation is observed by optical microscope or backscattered
electron (BSE) images into feldspars or pyroxenes. The groundmass
usually has a pilotaxitic texture and it is composed of euhedral plagioclase, clinopyroxene, Fe–Ti oxides and minor amounts of olivine. A
low-grade metamorphism or hydrothermal alteration overprint is
evidenced by the greenish color of the groundmass under the optical
microscope, the albitization of plagioclase microcrystals and the
presence of completely chloritized clinopyroxenes. The most common
secondary phases in the matrix are chlorite and clays.
According to their mineralogy, with abundant of mafic constituents
(mostly clinopyroxene) and the absence of primary quartz, K-feldspars
or hydrated minerals, the Algarrobal Formation rocks can be classified
mainly in the range between basalts and andesites. However, due to
the intense silicic alteration affecting these lavas, the apparent compositional range indicated by whole-rock chemistry varies from basaltic
andesite to rhyodacite (Rossel et al., 2013). These rocks have typical
geochemical features of subduction-related magmatism: calc-alkaline
affinities, with no early enrichment of FeOt, Mg# ranging between 30
and 56, enrichment in LILE and light rare earth elements (LREE), depletion in HFSE and Nb–Ta troughs similar to those observed in the Jurassic
arc volcanism (Oliveros et al., 2007; Rossel et al., 2013). The whole rock
contents of heavy rare earth elements (HREE) with respect to chondritic
reservoir (Rossel et al., 2013) suggest that the magmas were generated
above the garnet stabilization zone (YbN N 10).
101
4.3. Quebrada Vicuñita Beds
This Upper Jurassic back-arc unit crops out between ~26° and 27°S.
The rocks of the Quebrada Vicuñita Beds are very homogeneous in composition, corresponding to porphyritic basaltic andesites, with plagioclase and clinopyroxene phenocrysts (Fig. 2c). In the selected samples
of this unit plagioclase phenocrysts are euhedral and up to 10 mm in
size, and represent approximately 80% of coarse fraction, whereas
clinopyroxenes are subhedral, usually less than 2 mm in size, and can
occur grouped as glomerochrysts up to 5 mm in diameter. The
pilotaxitic groundmass represents approximately 70–75% of the volume
of the rock and is composed of plagioclase, Fe–Ti oxides and minor
clinopyroxene microcrystals. Hydrothermal alteration overprint is
mostly recorded in the feldspars: large plagioclase phenocrysts can be
partially replaced by sericite, whereas groundmass microliths are either
albitized or replaced by brownish-greenish fine clay. Clinopyroxenes
may exhibit thin chloritized rims in phenocrysts or in the groundmass.
A particular alteration feature recognized in one of the selected samples
is the presence of amphibole crystals closely related to areas where the
replacement of the groundmass by brownish-greenish fine clay is more
intense. Ar–Ar dating of the altered groundmass yielded an age of
72.9 ± 0.3 Ma (Rossel et al., 2013), that probably reflects a thermal
event due to intrusion of Upper Cretaceous to Paleocene plutons
(Cornejo et al., 1998), affecting the Quebrada Vicuñita Beds rocks.
The geochemistry of Quebrada Vicuñita Beds lavas indicates a transitional affinity for the volcanism, between calc-alkaline and tholeiitic,
with a marked enrichment in FeOt, Mg# that range between 37–52
and incompatible element contents similar to those of the Jurassic arc
rocks (Rossel et al., 2013). Spider diagrams of these units show typical
features of subduction-related magmas with deep Nb–Ta troughs
(LaN/NbN = 2–11), REE patterns flatter than those of the other back
arc units (LaN/YbN between 3 and 10), high concentrations of YbN
(13–23) and a flat tail in the REE diagrams normalized to the chondritic
reservoir (DyN/YbN ≤ 2).
4.2. Picudo Formation
4.4. Lagunillas Formation
This Upper Jurassic back arc unit crops out at ~29°S and is composed
of a bimodal suite of volcanic rocks ranging from olivine and
clinopyroxene-bearing basalts to andesites, and rhyolitic vitric tuffs.
The selected rocks have abundant fresh zoned clinopyroxene and
plagioclase phenocrysts, up to 2 mm in size, which represent ~30% of
the volume of the rock. Olivine-phenocrysts, when present, are
completely replaced by iddingsite. The observed proportion of phenocrysts is variable from the clinopyroxene-rich (≈30% of phenocrysts)
sample PR-10-45 to the clinopyroxene-poor (b5% of phenocrysts)
sample PR-09-05. The groundmass has a pilotaxitic texture and is
composed of euhedral plagioclases, clinopyroxenes, and minor amounts
of olivine and Fe–Ti oxides. The selected samples show scarce alteration
evidences such as the greenish color of the groundmass observed under
the optic microscope, probably related to minor degrees of propylitic
alteration, albitization of some plagioclases microcrystals and the
presence of hematite micro veins.
The volcanic rocks of this formation have transitional affinities (calcalkaline to alkaline), with no early enrichment of FeOt, Mg# ranging
from 45 to 55 and geochemical features typical of subduction-related
magmatism (Rossel et al., 2013): they are more enriched in incompatible elements, have less pronounced Nb–Ta troughs (LaN/NbN ~ 2) and
higher concentrations of REE, especially LREE (LaN/YbN = 8–12), than
the rocks of the Jurassic arc (Oliveros et al., 2007; Rossel et al., 2013).
Two of the six lava flows of this unit analyzed by Rossel et al. (2013)
have extremely high contents of Sr (2350 and 1935 ppm) which was
interpreted as the result of hydrothermal alteration processes, however,
a more complex petrogenetic history cannot be ruled out since they also
have low HREE content compared to chondritic reservoir (YbN ≤ 7),
which may be due to the presence of garnet as residual phase in the
magma source.
This formation is the easternmost Upper Jurassic back arc unit and
crops out as a discontinuous belt between ~27° and 30°S. The volcanic
rocks of the Lagunillas Formation are exclusively olivine basalts lava
flows with a typical massive central part and a vesicular top; they
have a fine grained equigranular texture, although minor (b5%) plagioclase and olivine phenocrysts are observed. Plagioclase is the most
abundant mineral phase, representing approximately ~ 80% of the
rocks by volume (Fig. 2d). Olivine usually represents about ~ 10% of
the volume of the rock, it can reach 2 mm in size and is always
completely replaced by iddingsite (mixture of Fe-rich phyllosilicates).
Plagioclase crystals are weakly altered to sericite and range in size
from 0.5 to 1 mm; they can also occur as isolated phenocrysts (2 mm)
in sample PR-10-80. The clinopyroxene occurs as crystals intergrown
with plagioclase; it represents less than 5 to 10% of the volume of the
sample and shows no evidence of alteration. Finally Fe–Ti oxides occur
as interstitial crystals but commonly they do not constitute more than
1 or 2% of the volume of the rocks. Locally, the volcanic rocks of this
unit are pervasively altered to calcite, which replaces plagioclase and olivine phenocrysts and fills numerous amygdules. The strongly altered
samples were not included in this work.
The selected rocks of Lagunillas Formation have alkaline affinities,
with no early enrichment of FeOt, Mg# that ranges between 32 and 61
and the lowest concentrations of SiO2 of all the back-arc units, ranging
between 45 and 49 wt.%. Spider diagrams of these lavas show patterns
similar to those of the ocean island basalts (OIBs), with no Nb–Ta anomalies (LaN/NbN b 1). The REE diagrams exhibit a marked enrichment in
LREE relative to HREE (LaN/YbN = 5–17) and low concentration of
HREE respect to chondritic reservoir (YbN ≤ 10), suggesting the presence
of garnet as a residual phase in the magma source.
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P. Rossel et al. / Lithos 239 (2015) 97–113
Table 2
Mean and range of values for EMPA and SIMS analysis of clinopyroxene crystals from the studied units and the previously published Jurassic arc samples (Oliveros et al., 2007).
Lagunillas Formation
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
Si
Ti
Al
Cr
Fe2+
Mn
Mg
Ca
Na
Mg#
En
Fs
Wo
Quebrada Vicuñita Beds
Picudo Formation
PR-10-72
PR-10-80
PR-09-19
PR-10-32
PR-09-05
PR-10-41
PR-10-45
(N = 11)
(N = 11)
(N = 29)
(N = 32)
(N = 4)
(N = 20)
(N = 33)
Mean
Range
Mean
Range
Mean
Range
Mean
Range
Mean
Range
Mean
Range
Mean
45.3
4.40
6.64
0.03
8.22
0.14
12.1
21.9
0.64
1.72
0.13
0.3
0
0.26
0.01
0.68
0.89
0.05
72.4
37.2
14.2
48.6
43.4/46.4
3.8/5.1
5.6/7.8
0/0.05
7.9/8.7
0.1/0.2
11.5/12.5
22.2/21.6
0.6/0.7
1.66/1.75
1.1/1.5
0.25/0.35
0/0.002
0.25/0.27
±0.001
0.66/0.71
0.88/0.91
0.40/0.53
70.7/73.7
36.2/38.2
13.7/15.0
48.1/49.1
48.2
2.98
3.99
0.02
9.08
0.18
12.7
21.6
0.64
1.83
0.09
0.18
0.00
0.29
0.01
0.72
0.88
0.05
71.3
38
15.3
46.7
44.7/50.5
1.7/3.5
1.9/4.6
0/0.04
8.8/9.5
0.1/0.2
12.1/13.8
21.2/21.9
0.6/0.8
1.8/1.91
0.05/0.1
0.09/0.2
0/0.001
0.28/0.3
-0.002
0.68/0.78
0.86/0.93
0.03/0.06
69.7/72.9
37.1/40.4
14.7/16.1
44.6/47.5
50.8
0.71
3.02
0.11
10.2
0.24
15.1
18.1
0.3
1.92
0.02
0.1
0
0.33
0.01
0.88
0.74
0.02
72.5
44.9
17
38
49.9/51.6
0.6/0.9
1.7/2.7
0/0.24
9.5/15.8
0.2/0.4
12.6/16.1
16.8/19.2
0.26/0.36
1.90/1.93
0.2/0.3
0.07/0.12
0/0.007
0.3/0.51
0.006/0.02
0.73/0.91
0.7/0.77
0.02/0.03
58.7/74.4
37.6/46.3
15.5/26.4
36/39.9
51.7
0.6
2.18
0.13
9.78
0.29
15.8
18.5
0.3
1.93
0.02
0.1
0
0.31
0.01
0.88
0.74
0.02
74.2
45.7
15.9
38.5
50.3/52.4
0.45/0.8
1.8/3.4
0/0.24
8.5/11.9
0.2/0.4
15.7/16.5
16.7/19.4
0.26/0.34
1.90/1.95
0.1/0.2
0.08/0.15
0/0.007
0.26/0.38
0.007/0.01
0.83/0.92
0.67/0.77
0.02/0.025
68.8/77.2
42.8/47.6
13.7/19.6
34.8/40.0
50.2
0.86
3.86
0.03
9.65
0.26
15.4
19
0.36
1.88
0.02
0.17
0
0.3
0.01
0.86
0.76
0.03
73.9
44.7
15.7
39.6
48.2/52.8
0.6/1.2
2.1/5.4
0.01/0.07
8.8/10.3
0.2/0.3
13.9/17.4
16.8/20.2
0.3/0.5
1.83/1.94
0.02/0.03
0.09/0.24
0/0.002
0.27/0.32
0.006/0.01
0.78/0.95
0.66/0.82
0.02/0.03
71.2/76.9
40.8/49.4
14.1/16.
34.2/42.7
51.3
0.85
3.16
0.03
8.78
0.25
14.9
20.1
0.44
1.91
0.02
0.14
0
0.27
0.01
0.83
0.8
0.03
75.2
43.5
14.3
42.2
50.1/52.2
0.6/1.2
2.4/4.1
0/0.15
7.6/11.2
0.2/0.3
13.9/16.2
18.7/20.8
0.3/0.6
1.88/1.93
0.02/0.03
0.10/0.18
0/0.004
0.24/0.35
0.005/0.01
0.79/0.89
0.75/0.82
0.02/0.04
69.6/78.0
41.6/46.2
12.4/18.4
39.3/43.4
49.4
0.96
7.01
0.15
7.11
0.16
14.4
20.4
0.57
1.82
0.03
0.31
0
0.22
0.01
0.76
0.81
0.04
78.3
43.5
12.1
44.4
4.5. Agua Salada Volcanic Complex
This Upper Jurassic arc unit crops out between ~ 30° and 31°S in
Coastal Cordillera of northern Chile. The rocks of this unit are mostly
volcanic and subvolcanic basaltic andesites strongly affected by contact
metamorphism related to the intrusion of large granitoids of Lower Cretaceous age (Emparán and Pineda, 2000). The rocks have porphyritic
textures with up to 40% of phenocrysts. Plagioclase is the main primary
mineral phase representing up to 80% of the volume of the rocks; the
crystals are strongly but not completely altered to sericite and epidote.
Clinopyroxene is a subordinate phase representing no more than 5% of
the volume of the rock; the crystals are replaced by chlorite, although
fresh cores can be found. Amphibole and quartz also occur, especially
in fine-grained samples, but these phases likely formed during contact
metamorphism.
The volcanic rocks from this complex have calc-alkaline/transitional
to tholeiitic affinities, like most of the Jurassic arc rocks, but with no
clear early enrichment of FeOt. HREE contents respect to chondritic reservoir are generally lower (YbN ≈ 5–11) than other arc rocks. The #Mg
ranges from 41 to 68, suggesting that these samples do not represent
primary melts.
Mg#2 diagram (Fig. 3), with Ti contents over or very close to 0.1
a.p.f.u. Clinopyroxene Mg# ranges between 50 and 85, with those
from back arc samples having a more restricted range, mostly between
67 and 85 (Fig. 3). The highest Mg# are from sample PR-10-45 of the
Picudo Formation, the lowest are from crystals of the Jurassic arc
rocks, and the more restricted range is observed in clinopyroxenes of
Lagunillas Formation (69–74). Clinopyroxenes in basaltic samples PR10-45 from Picudo and PR-10-72 and PR-10-80 from Lagunillas formations, and PR-11-168 from Picudo Formation are diopsides, whereas in
the other arc and back arc samples the pyroxenes are augites (Fig. 4a).
Three low-Wo clinopyroxenes from the Jurassic arc rocks can be classified as pigeonites. Clinopyroxene have Mg# ranging between 65 and 78,
the most primitives are those from sample PR-10-41 (Figs. 3, 4a). A few
analyzed phenocrysts from the Agua Salada Volcanic Complex are
augites with Mg# that ranges between 67 and 70; their composition is
very similar to the clinopyroxene phenocrysts of the Jurassic arc rocks
(Oliveros et al., 2007).
Scarce orthopyroxene crystals were found in evolved samples of the
Picudo Formation. (PR-09-05 and PR-10-41) and in the Jurassic arc
samples (Oliveros et al., 2007) and they correspond to enstatites.
5.2. Trace element chemistry
5. Pyroxene chemistry
5.1. Major elements
Major element chemistry of the clinopyroxenes is listed in Table 2
and detailed in the Table A (Suppl. content). Pyroxene description and
classification are based on atoms per formula unit (a.p.f.u.) normalized
to 6 oxygens (Morimoto et al., 1988) and include data from this work
and previously analyzed pyroxenes from Oliveros et al. (2007).
Most of the clinopyroxenes from the Picudo and Lagunillas formations are aluminous. They have higher contents of Ca, Na, Ti and lower
contents of Si, Fe, Mg compared to pyroxenes from the Algarrobal Formation, Quebrada Vicuñita Beds and those of the Jurassic arc rocks. In
particular, clinopyroxenes from sample PR-10-72 (Lagunillas Formation) can be classified as subsilicic and titanian, defining a completely
different fractionation trend compared to the other units in the Ti vs
Trace element chemistry of the clinopyroxenes is listed in the
Table B (Suppl. content); REE concentration and ratios were normalized
to the chondrite composition of Sun and McDonough (1989). The REE
patterns display a convex-upward shape with the highest enrichments
between Nd and Eu. Unlike the clinopyroxenes from the Agua Salada
Volcanic Complex (arc domain), the minerals from back arc samples
show no negative Eu anomalies (Fig. 5), suggesting that there was no
major previous fractionation of plagioclase in those melts, even when
petrographic analysis suggest that plagioclase is commonly the first
phase to crystallize. The CeN/SmN (LREE) and GdN/YbN (HREE) ratios
vary significantly between 0.1–1.5 and 0.2–3.2, but most of the samples
are between 0.3 to 0.6 and 0.9 to 1.9. However, the ratios obtained in
one alkali basalt from Lagunillas Formation (0.7 to 0.9 and 2.5 to 3.2 in
2
Mg/(Mg + Fet) abundances in a.p.f.u.
P. Rossel et al. / Lithos 239 (2015) 97–113
Picudo Formation
103
Algarrobal Formation
Jurassic Arc
PR-10-45
PR-11-168
All Samples
M-14
M-24
PR-11-153
Oliveros et al., 2007
(N = 33)
(N = 17)
(N = 4*)
(N = 9)
(N = 8)
(N = 5)
(N = 179)
Range
Mean
Range
Mean
Range
Mean
Range
Mean
Range
Mean
Range
Mean
Range
48.3/51.9
0.5/1.5
4.6/8.1
0/0.51
5.1/9.2
0.1/0.24
13.4/16.1
19.0/21.5
0.5/0.7
1.79/1.89
0.01/0.04
0.20/0.35
0/0.014
0.15/0.28
0.003/0.01
0.74/0.87
0.76/0.85
0.03/0.05
72.6/85.0
41.3/47.1
8.3/15.7
42.1/46.6
49.7
1.07
3.97
0.01
7.43
0.26
14.9
21.8
0.49
1.86
0.03
0.18
0
0.23
0.01
0.83
0.87
0.04
78.1
42.9
12
45.1
48.1/51.9
0.7/1.7
1.7/5.7
0/0.03
6.2/8.4
0.2/0.5
14/16.7
20.6/22
0.4/0.6
1.82/1.94
0.02/0.05
0.06/0.23
0/0.001
0.19/0.26
0.01/0.014
0.78/0.92
0.82/0.9
0.03/0.04
74.8/82.4
40.5/47.6
9.9/13.7
42.2/45.8
50.1
0.94
4.5
0.06
8.24
0.23
14.9
20.3
0.47
1.87
0.02
0.14
0
0.26
0.01
0.83
0.81
0.03
76.4
43.7
13.5
42.8
49.4/51.3
0.85/1.1
3.16/4.6
0.001/0.15
7.1/9.65
0.16/0.26
14.4/15.43
18.98/21.81
0.36/0.57
1.82/1.91
0.024/0.03
0.14/0.31
0.01/0.15
0.22/0.302
0.005/0.008
0.76/0.86
0.76/0.874
0.026/0.041
73.9/78.3
42.9/44.7
12./15.7
39.6/45.1
52.1
0.71
1.83
0.04
11.9
0.32
15.4
17.6
0.26
1.94
0.02
0.08
0
0.37
####
0.86
0.7
0.02
69.7
44.3
19.3
36.5
50.7/53.2
0.6/0.8
1.5/2.0
0/0.07
11.0/14.0
0.3/0.5
14.0/15.9
17.2/18.0
0.2/0.3
1.92/1.96
0.015/0.02
0.07/0.09
0/0.002
0.34/0.45
0.008/0.01
0.80/0.88
0.69/0.72
0.017/0.02
63.9/71.7
40.8/45.5
45.5
17.7/23.0
52.6
0.80
1.67
0.04
12.1
0.35
14.9
18
0.31
1.95
0.02
0.07
0
0.38
0.01
0.82
0.72
0.02
68.7
43.0
19.6
37.5
51.9/53.3
0.7/0.9
1.4/2.2
0.004/0.1
11.4/12.8
0.3/0.4
14.6/15.1
17.6/18.6
0.3/0.4
1.93/1.97
0.02/0.03
0.06/0.10
0/0.003
0.35/0.40
0.01/0.013
0.81/0.84
0.70/0.74
0.02/0.03
67.1/69.9
42.1/43.7
18.5/20.6
36.6/38.6
51.2
0.91
1.93
0.04
11.6
0.33
15.5
18.3
0.3
1.95
0.02
0.07
0
0.38
0.01
0.82
0.72
0.02
68.7
43.0
19.6
37.5
50.7/51.9
0.65/1.83
1.86/2.04
0.02/0.05
11.2/11.91
0.27/0.4
15.4/15.7
18.16/18.4
0.26/0.31
1.93/1.97
0.02/0.03
0.06/0.10
0/0.003
0.35/0.40
0.01/0.013
0.81/0.84
0.70/0.74
0.02/0.03
67.1/69.9
42.1/43.7
18.5/20.6
36.6/38.6
####
0.74
2.20
0.08
####
0.34
####
####
0.30
####
####
####
####
####
####
####
####
####
69.4
42.9
18.9
38.1
44.9/53.4
0.3/2.6
1.0/8.1
0.0/0.7
6.1/16.5
0.2/0.6
6.0/17.0
13.0/21.3
0.18/0.93
1.78/1.96
0.01/0.08
0.04/0.38
0.00/0.02
0.19/0.54
0.01/0.02
0.35/0.94
0.51/0.84
0.01/0.07
39.4/83.0
20.8/48.1
9.6/32.1
30.0/47.0
sample PR-10-72) and a basaltic andesite from Picudo Formation (1.0 to
1.5 and 2.0 to 2.6 in sample PR-11-168) are much higher than those of
the other samples. The pyroxenes of Quebrada Vicuñita Beds and Agua
Salada Volcanic Complex have the flattest HREE patterns (GdN/
YbN ≈ 1), but the arc unit is slightly more enriched in these elements
(YbN ≈ 12 versus ≈ 22, respectively).
Sr concentrations for all clinopyroxenes are high compared to
clinopyroxenes from mid ocean ridge basalts (MORB, Fig. 6a), especially
in the clinopyroxenes from Lagunillas and Picudo Formations. A
particularly high enrichment in this element is observed in one basaltic
andesite of the Picudo Formation (PR-11-168; Fig. 6a), with values
ranging between 86 and 265 ppm, which is four to ten times higher
than the mean of the other subduction related clinopyroxenes included
in this work. Contents of Nb, Zr and Hf for most samples are in the range
of MORB clinopyroxenes and together with the analyzed samples form
an evolutionary trend similar to that observed in arc clinopyroxenes of
the Aleutians (Yogodzinski and Kelemen, 2007; Fig. 6b). The Lagunillas
Formation clinopyroxenes have Zr contents that are two to five times
greater than MORB (Fig. 6a).
The orthopyroxenes of Picudo Formation basaltic andesite (PR-0905) display a distinct pattern in the REE diagrams, with depletion in
LREE relative to HREE (LaN/YbN ≈ 0.04), and lack of a negative Eu
anomaly (Fig. 5e).
6. Feldspar chemistry
6.1. Major elements
Major element chemistry of feldspars is listed in Table 3 and detailed
in Table C (Suppl. content). Description and classification is in atoms per
formula unit (a.p.f.u.) normalized to eight oxygens. Previously analyzed
samples from the Camaraca, Oficina Viz, La Negra Formations (Jurassic
arc, Oliveros et al., 2007) are included in the calculations and diagrams.
Most of the studied feldspars are bytownite to andesine plagioclase
(Fig. 4b). Some anorthite-rich bytownites are found in lavas of the
Picudo Formation and these samples exhibit a rather continuous
fractionation trend from An85 to An37. The sample with the most anorthitic plagioclase (An85–64) is the Picudo Formation basalt (PR-10-45)
and that with the least anorthitic plagioclase is PR-11-168 (An70–37).
Feldspars from the other units show a restricted range of composition (An64–50), with a few isolated bytownite and andesine crystals
from Quebrada Vicuñita Beds and Algarrobal Formation lavas and
oligoclase from the Lagunillas Formation lavas. On the other hand, the
plagioclases in the Jurassic arc lavas (Camaraca, Oficina Viz, La Negra
Formations and Agua Salada Volcanic Complex) are mainly labradorite,
although they exhibit a broad range of composition, ranging from An74
to An01.
Alkali feldspars are minor constituents of the studied rocks and they
can be found as microlites in a few evolved arc andesites (VO175 and
VO169 from Camaraca and Oficina Viz formations respectively), a
sample of the Picudo Formation (PR-10-41; not shown) and in the alkali
basalts of the Lagunillas Formation (Fig. 4b). In the latter, they correspond mainly to anorthoclase crystals, with a few sanidines (Fig 4b).
7. Thermobarometry of clinopyroxenes and plagioclases
Thermobarometry calculations using various equations (Putirka,
2005, 2008; Putirka et al., 2003) for plagioclase/whole rock,
clinopyroxene/whole rock, clinopyroxene only and two pyroxenes
equilibrium were performed on minerals from the Jurassic arc and
back arc rocks (see Fig. 7a for location and unit names).
Estimated crystallization temperatures and depth (pressures) for
plagioclase/whole rock equilibrium in arc samples mostly fall (~ 50%)
within a relative narrow range of 1160 to 1200 °C (Fig. 7b) and 18 to
26 km (Fig. 7c), with mean values of 1183 °C and 21.6 km.
Clinopyroxene/whole rock equilibrium of the same units yielded
lower crystallization temperatures, mostly between 1110 and 1150 °C,
and depths between 8 and 18 km, with mean values of 1120 °C and
15.6 km. Otherwise, single clinopyroxene equilibration of arc samples,
suggest a narrow range of temperature mostly between 1150 and
1175 °C, with a mean value of 1162 °C, with the exception of pyroxenes
in Camaraca Formation rocks that show lower values generally between
1130 and 1155 °C, but with a clear overlap of the upper 25% of the values
of this unit respect to other arc units. Calculated crystallization depths
using this method range mostly between 6 and 12 km, with a mean
value of 9 km. Uncommon ortho/clinopyroxene equilibrium pairs
from lavas of the Camaraca Formation (Fig. 7b) shown equilibration
temperatures of ~1025 °C.
104
P. Rossel et al. / Lithos 239 (2015) 97–113
Fig. 3. Mg# versus major cation abundances for the studied pyroxenes. Pyroxene composition of the Jurassic arc samples are after Oliveros et al. (2007) and this work. Dashed lines group
different populations of measurements in Picudo and Lagunillas formations.
Crystallization temperatures estimated for back arc units (Fig. 7d,f)
show that most of the used algorithms overlap between 1150 and
1200 °C, with higher mean values for plagioclase/whole rock equilibrium (1172 °C), followed by single clinopyroxene equilibration
(1165 °C), and lower mean values for clinopyroxene/whole rock calculations (1150 °C). Anomalous behavior is observed in the plagioclase/
whole rock equilibrium of the Picudo Formation samples. The obtained
values range mainly between 1210 and 1240 °C, with the lower temperatures slightly overlapping the mean values calculated for the other
back-arc samples (Fig. 7f). The clinopyroxene/whole rock equilibrium
calculation yielded a wide range of temperatures following a bimodal
distribution (Fig. 7f) with a peak at ~ 1200 °C grouping the results
from the basaltic sample PR-10-45, and a lower peak at ~1080 °C grouping the rest of the analysis of this unit. The clinopyroxene only calculation for the Lagunillas Formation basalts yield temperatures (1050 to
1080 °C) which are anomalously lower than the rest of the back arc
samples (Fig. 4d). Finally, a few ortho/clinopyroxene pairs from a
basaltic andesite (PR-10-41) from Picudo Formation yielded a range of
equilibration temperatures between ~975 and 1030 °C (Fig. 4d).
Crystallization pressures calculated for minerals from the back arc
units (Fig. 7c,e) have a more complex behavior compared to temperature plots, but the results using the three different equations overlap
in a range of depths between ~8 and 20 km. The most consistent results
are those obtained on the basis of clinopyroxene composition solely,
P. Rossel et al. / Lithos 239 (2015) 97–113
105
Fig. 4. a) Pyroxene quadrilateral diagram after Morimoto et al. (1988) and b) feldspar classification diagram for the studied minerals. Jurassic arc data are from Oliveros et al. (2007). ASVC:
Agua Salada Volcanic Complex.
with a mean value of ~9.6 km (excluding the pyroxenes from the Picudo
Formation basaltic sample PR-10-45, see Section 8.3). On the other
hand, the plagioclase/whole rock barometer show higher values for
the Lagunillas and Picudo formations crystals, but for the latter there
is a large dispersion in the data (Fig. 7e,g). Particularly consistent are
the results obtained in the Picudo Formation for clinopyroxene/whole
rock and clinopyroxene only barometers, with matching range of
values, mean value and bimodal distribution of the data (Fig. 7d,f).
8. Discussion
8.1. Compositional variations in clinopyroxenes as proxies of magma sources
The major element composition of clinopyroxenes from the Jurassic
arc and back arc units can be divided in two groups that follow distinct
fractionation patterns. The first group, represented by pyroxenes from
two samples of Picudo Formation (PR-10-45, PR-11-168) and the
clinopyroxenes in Lagunillas formation lavas, is characterized by higher
contents of Al, Ti, Ca, Na and lower contents of Si (Fig. 3). The second
group, which is represented by pyroxene from samples of the Quebrada
Vicuñita Beds, Algarrobal Formation, Picudo Formation (PR-09-05) and
Jurassic arc rocks, has low Al, Ti, Ca, Na and higher contents of Si.
Clinopyroxenes in the remaining sample of Picudo Formation (PR-1041) show features that resemble the second group, but they are slightly
enriched in Al and Na.
According to the discrimination diagrams of Leterrier et al. (1982)
the clinopyroxenes show a broad range of compositions, typical of
subduction zones (Fig. 8). The Lagunillas Formation crystals define a
particular pattern due to their high Ti contents, with clear alkaline
affinities (Fig. 8a), while pyroxenes from the other units have compositions that resemble the clinopyroxenes from the MORB and the basalts
from spreading centers such as back-arc basins (Fig. 8b), but with a clear
fingerprint of subduction-related magmas (Fig. 8c). The clinopyroxenes
of the Picudo Formation exhibit a significant alkaline component
(Fig. 8a inset), but plot near to or within the field of calc-alkaline affinity
(Fig. 8c). Thus, there are slight differences in the genesis of this unit with
respect to the Algarrobal Formation, the Quebrada Vicuñita Beds and
the Jurassic arc rocks for which the alkaline component is negligible
(Fig. 8a). The likely factors that control the observed differences
between the groups of studied pyroxenes are: i) variations in the
pressure/temperature crystallization conditions and/or composition of
a common parental magma, ii) different degrees of melting of a
homogeneous mantle source, iii) compositional heterogeneities in the
magma source, iv) previous fractionation of mineral phases, or a combination of the above.
The Mg# of clinopyroxene increases with increasing crystallization
temperature (Trigila and DeBenedetti, 1993) and is independent of
pressure (Putirka et al., 1996). On the other hand, Si in clinopyroxene
is generally replaced by Al in the equilibrium exchange of Jd
(NaAlSi2O6)–DiHd (Ca(Mg,Fe)Si2O6) and CaTs (CaAlAlSiO6)–DiHd,
which is sensitive to pressure. Experimental results show that Al in
clinopyroxenes increases with decreasing temperature and increasing
pressure (Putirka et al., 1996; Trigila and DeBenedetti, 1993), whereas
Si increases with increasing temperature and decreasing pressure. The
Mg#, Al and Si contents in the clinopyroxenes of the first group (pyroxenes from samples of the Lagunillas and Picudo formations) suggest that
these minerals crystallized from a melt that was either colder or deeperstorage than that from which the second group of pyroxenes crystallized. However, equilibration conditions calculated for the basaltic sample from the Picudo Formation (PR-10-45; Fig. 7e,f) suggest that the
stagnation level of this back arc melt was at greater depths and higher
temperatures than that of the magmas represented by other samples
from arc and back arc (see Section 8.2). Thus, the high content of Al in
the pyroxenes from the Picudo Formation basalt could be the result of
higher pressure, not lower temperature, of crystallization.
Furthermore, the most evolved samples of the Picudo Formation
(PR-09-05 and PR-10-41) have low Al contents (Fig. 3) suggesting crystallization pressures similar to the pyroxenes from the Quebrada
Vicuñita Beds, the Algarrobal Formation and the Jurassic arc rocks (depleted second group). This is consistent with the two populations of
temperatures calculated for the Picudo clinopyroxenes (Fig. 7e) and
the crystallization order of clinopyroxene and plagioclase observed in
each sample, which suggest that the magmas of the Picudo Formation
may have experienced complex transport and storage histories prior
to their emplacement during late Jurassic.
The observed variations in Na and other incompatible elements in
the clinopyroxenes could be due to different degrees of melting of a
common magma source, such as a depleted sub-arc mantle. The high
contents of Na and REE, and the high Sm/Yb ratios in the clinopyroxenes
of the Picudo and Lagunillas formations, along with the restricted
volume of volcanism in these two units compared to the Jurassic arc
rocks and Algarrobal Formation, are in agreement with a lower degree
of partial melting as proposed by Rossel et al. (2013). The high contents
of Ti, Zr, Nb and Hf in pyroxene crystals from the Lagunillas Formation,
106
P. Rossel et al. / Lithos 239 (2015) 97–113
Fig. 5. Chondrite normalized REE patterns for the clinopyroxenes of the Picudo Formation, Quebrada Vicuñita Beds, Lagunillas Formation and Agua Salada Volcanic Complex. Two analyzed
orthopyroxenes in sample PR-09-05 (basalt from the Picudo Formation) show typical LREE depleted pattern. Normalizing values are from Sun and McDonough (1989).
compared to the other arc and back arc units studied in this work,
suggest that the parental magmas of this formation derive from an
OIB-type source (Figs. 3, 6a,b, 8a).
Clinopyroxene in OIBs (Bohrson and Clague, 1988; Clague, 1988;
Chen et al., 1992; Fedor and Galar, 1997; Tracy, 1980) and continental
alkali basalt (Aoki, 1968, 1970; Arai et al., 2000, 2001; Ho et al., 2000;
Kovács et al., 2004) usually have high contents of Na and Ti. For a
given concentration of SiO2, the continental alkali basalt clinopyroxenes
have lower Mg# (70–86) than the clinopyroxene in the OIB (79–89) or
volcanic arcs (N 85). The Mg# in the Lagunillas Formation pyroxenes
ranges between 69 and 74, suggesting a close relationship of these
lavas to a continental intraplate mantle source. This is consistent and
confirms the results of Rossel et al. (2013), who propose the existence
of an OIB-like mantle as the source of the Lagunillas Formation lavas
based on whole rock geochemistry and radiogenic isotopes.
The Zr vs Sr and Sr vs Yb variations among clinopyroxenes of the
second group (low Al, Ti, Ca, Na) can be explained by fractionation of
similar proportions of clinopyroxene and olivine and minor amounts of
plagioclase (Fig. 9). The compositional pattern of pyroxenes from the
Picudo Formation basalt (PR-10-45) suggests the same fractionation
assemblage as the second group, but the Na and Al enrichment in these
crystals must be related to differences in the source. The high-Sr pyroxenes from one basaltic andesite in the same unit (PR-11-168) suggest a
source that was either Sr-rich or lacked of plagioclase fractionation (see
discussion in Section 8.3). The compositional variation trends of pyroxenes from this sample suggest that fractionation of clinopyroxene and
minor plagioclase is the most likely process for magma evolution
(Fig. 9b).
8.2. Mineral–liquid (whole rock) equilibrium and thermobarometry
implications
The thermobarometric results on clinopyroxene for each studied
unit, calculated with the same equation, display a Gaussian distribution
P. Rossel et al. / Lithos 239 (2015) 97–113
Fig. 6. a) Sr versus Zr and b) Sr/Zr versus Nd/Yb diagrams for clinopyroxenes. Aleutian arc
trend after Yogodzinski and Kelemen (2007). MORB data are from Dick and Naslund
(1996).
107
with a well-defined peak for both P and T. This behavior can be
interpreted as indicative of preferred or protracted crystallization
conditions for the clinopyroxenes, suggesting the existence of levels of
stagnation for the magmas before eruption, which is consistent with
the porphyritic nature of most of the analyzed samples. The magma
stagnations levels could have been favored by compositional discontinuities in the mantle and crust during the Jurassic.
The inferred temperature and depth of these apparent stagnation
levels (i.e., the crystallization conditions for the clinopyroxene and
plagioclase) seem to be highly dependent on the equation used
(Fig. 10), especially in the arc samples. In general, calculations based on
plagioclase chemistry show higher values than those based on pyroxene,
which is consistent with petrographic observations that suggest an early
crystallization of olivine, when present, followed by plagioclase and finally pyroxene. Otherwise, the gap between the P–T conditions calculated
for plagioclase or pyroxene could be in part due to the porphyritic nature
of the lavas or to the alteration degree of the rocks. Given that the results
based on clinopyroxene only geothermobarometry show more narrow
and concordant ranges than the rock/mineral methods, it is likely that
the alteration processes affecting the bulk-rock chemistry biased the
thermobarometric calculations.
Furthermore, few measurements of the studied minerals plot on the
equilibrium line, which is the method proposed by Putirka et al. (2003)
for testing the validity of the thermobarometric results (Fig. 10a). Only
four samples plot on the equilibrium line which matches measured
and predicted composition for the clinopyroxene, and therefore the
calculated pressures and temperatures probably approach their crystallization conditions. These are two lavas from the La Negra Formation in
the arc domain, VO04 and VO69 (Oliveros et al., 2007), and two lavas
from the back arc domain, PR-10-45 (Picudo Formation) and PR-10-72
(Lagunillas formation). In order to evaluate the reliability of the remaining calculations, we performed a comparison between the measured
REE contents on back arc samples and the predicted REE chemistry of
these rocks based on clinopyroxene REE composition (Fig. 10b). The
basalt and andesite of the Picudo Formation (PR-10-45 and PR-09-05,
respectively) along with one sample of the Quebrada Vicuñita Beds (PR10-32) show a good fit between measured whole-rock and clinopyroxene
based REE contents, suggesting lower degrees of alteration for these
samples. For the other analyzed samples and for the plagioclase/whole
rock equation, which yielded systematically higher P–T values, there is
no further means to assess the reliability of the obtained results.
The obtained temperatures and depths of equilibrated samples from
the arc are ~1100 °C and ~6–18 km (Fig. 7b,c); the mean temperature
for all samples and measurements calculated using the clinopyroxene/
Table 3
Mean and range of values for EMP analysis of feldspar crystals from the studied units and the previously published Jurassic arc samples (Oliveros et al., 2007).
SiO2
Al2O3
FeO
CaO
Na2O
Ka2O
Si
Al
Fe2+
Ca
Na
K
Ab
An
Or
Lagunillas Formation
Quebrada Vicuñita Beds
Picudo Formation
Algarrobal Formation
Jurassic Arc
N=9
N = 19
N = 38
N=8
N = 220
Mean
Range
Mean
Range
Mean
Range
Mean
Range
Mean
Range
58.13
25.71
0.48
7.39
6.13
2.09
2.62
1.37
0.16
0.36
0.54
0.12
52.8
35.4
11.8
53.27/64.93
20.42/29.56
0.28/0.64
2.42/11.55
4.93/7.74
0.34/6.07
2.41/2.91
1.08/1.58
0.009/0.022
0.07/0.56
0.43/0.67
0.022/0.35
42.7/65.2
6.7/65.3
2.0/34.1
53.66
28.76
0.87
11.8
4.74
0.32
2.43
1.54
0.03
0.57
0.42
0.02
41.2
56.9
1.8
48.06/59.24
25.13/32.95
0.55/0.98
6.73/16.52
2.18/7.53
0.11/0.79
2.20/2.66
1.33/1.78
0.02/0.04
0.32/0.81
0.19/0.66
0.01/0.05
19.2/64.0
31.6/80.2
0.6/4.4
52.05
30.42
0.74
12.87
4.04
0.39
2.36
1.62
0.03
0.63
0.36
0.02
35.4
62.4
2.2
47.04/63.89
21.30/34.24
0.40/1.22
2.76/17.34
1.88/6.81
0.17/5.82
2.15/2.86
1.12/1.84
0.02/0.05
0.13/0.85
0.14/0.59
0.01/0.33
13.8/59.1
13.0/85.8
1.0/32.6
53.39
28.24
1.01
11.51
4.81
0.37
2.44
1.52
0.04
0.56
0.43
0.02
42.2
55.7
2.1
53.00/53.94
27.45/28.83
0.91.1.17
10.91/12.12
4.56/5.13
0.34/0.41
2.41/2.47
1.48/1.55
0.03/0.04
0.54/0.59
0.40/0.46
0.02/0.024
39.7/44.9
52.9/58.3
1.9/2.4
54.79
27.94
0.89
11.03
4.76
0.75
2.48
1.49
0.03
0.55
0.41
0.04
41.88
53.75
4.37
41.27/69.66
18.48/32.45
0.00/15.63
0.00/20.42
0.22/11.77
0.02/18.39
2.09/3.00
1.01/1.91
0.00/0.61
0.00/1.10
0.02/0.98
0.00/1.09
1.83/98.95
0.01/90.96
0.12/98.01
Structural formulae on the basis of 6 oxygens.
N = number of measurements per sample.
108
P. Rossel et al. / Lithos 239 (2015) 97–113
Fig. 7. Calculated crystallization conditions for plagioclases and pyroxenes. Transformation from pressures (kbar) to depth (Km) were made considering a mean density for the crust of
2.9 g/cm3, except for calculations in pyroxenes of sample PR-10-45, in which was used an upper mantle density of 3.3 g/cm3 which induces a variation in calculated crystallization
depth of approximately 2.7 to 3.8 km more for cpx/wr equilibrium, and 2.0 to 3.6 km more for cpx only equilibrium than predicted crust densities. a) Location of the studied units.
b) Equilibration temperature calculated in arc units. c) Equilibration pressures calculated in arc units. d) Equilibration temperatures calculated in back-arc units. e) Equilibration pressures
calculated in back-arc units. The heavy line represents the mean value for all samples using the clinopyroxene only equation, the dashed line represents the mean value of all samples using
the clinopyroxene/whole rock equation, the dashed/dotted line represents the mean value of all samples using the plagioclase/whole rock equation. Mean values of equilibrated measurements according to Putirka et al. (2003) are plotted as black lines in corresponding boxes. Heterogeneous results for the Picudo Formation are plotted additionally as histograms of
calculated: f) equilibration temperatures (dashed line represents the equilibration temperatures obtained with the clinopyroxene only equation), and g) equilibration pressures (dashed
line represents the equilibration pressure obtained with the clinopyroxene only equation). Small circles and stars above and under box plots represent anomalous and extreme measurements respectively, beyond the 95% of confidence. All calculations are after Putirka (2005, 2008) and Putirka et al. (2003).
whole rock equation is only slightly lower and the pressure ranges
calculated using the clinopyroxene/whole rock and clinopyroxene
only equations are very similar. The consistency among the calculations
using equilibrated and non-equilibrated samples suggests that the
obtained P–T conditions probably represent a real stagnation level
during magma ascent in the arc domain. Furthermore, the calculated
pressure conditions are in agreement with the estimated depths of the
lower-middle crust boundary in extensional convergent margins
(Christensen and Mooney, 1995; Greene et al., 2006; Rudnick and
Fountain, 1995) and similar to the emplacement depths for Jurassic
plutonic bodies in Coastal Cordillera of northern Chile (González,
1999) that are commonly proposed as feeders of the Jurassic arc volcanism (Lucassen and Franz, 1994).
For the back-arc domain, the results using the different equations in
equilibrated and non-equilibrated samples from the Quebrada Vicuñita
Beds and Algarrobal Formation show narrow and overlapping ranges
for crystallization temperatures and depths (~1150–1200 °C and 8 to
20 km) (Fig. 7d,e) which suggests that the data are valid. The Picudo
Formation measurements show a wide range of equilibration conditions (Fig. 7f,g). Crystals in equilibrium with whole rock composition
from the Picudo Formation basalt (PR-10-45) indicate equilibration
temperatures higher than 1200 °C and depths of 35–36 km. The same
results are obtained with clinopyroxene only equation. These conditions
are somewhat higher than expected for mantle–lower crust boundary
in the back arc region of extensional subduction systems. Thus, the
calculated depth of crystallization of ~ 35–36 km of this sample could
reflect another process.
Other samples from the Picudo Formation show no equilibration between predicted and measured clinopyroxene composition. They
yielded lower temperatures than the equilibrated sample (PR-10-45),
especially with the clinopyroxene/whole rock equation (Fig. 7f). The
estimated crystallization depths are also lower than the equilibrated
samples, whether using the clinopyroxene/whole rock or clinopyroxene
only equation. These results are in agreement with the more evolved
nature of the samples and overlap the calculated P–T conditions for
other arc and back arc samples. However, given the lack of evidence
for equilibrium in the samples, with the exception of PR-09-05 (good
match between measured and calculated REE, Fig. 10b), the data should
be taken with caution.
The Lagunillas Formation alkaline basalts are equigranular, fine
grained and have a restricted range of Mg#, suggesting a history of
fast cooling and very little fractionation for the magmas. However,
they show a wide range of thermobarometric data, with no concordance
between different equations. The calculations could then be geologically
meaningless, even though a few pyroxenes show apparent equilibrium
in measured versus predicted clinopyroxene composition (Fig. 10a).
Taking into account that the crystallization depths estimated
from different geothermobarometers are similar for samples of the
arc and back arc domains, it is possible to infer the existence of
magma stagnation levels (or protracted P–T conditions for the crystallization of the phenocrysts) consistent with the existence of a
thinned continental crust as result of the transtensional/extensional
regime that dominated the Andean margin during the Jurassic
(Creixell et al., 2011; Grocott and Taylor, 2002; Scheuber and
P. Rossel et al. / Lithos 239 (2015) 97–113
109
Fig. 9. a) Sr vs Zr and b) Yb vs Sr diagrams with the proposed fractionation patterns for the
analyzed clinopyroxenes. Gray areas show the distribution of the Picudo Formation samples. Black arrows indicate the crystallization patterns for the given fractionated mineral
phases. Cpx: clinopyroxenes. Plg: plagioclase. Ol: olivine. Gt: garnet.
back arc domains, which probably corresponded to the lowermiddle crust boundary. The existence of different reservoirs or
stagnation levels has also been proposed for subduction-related
middle Jurassic dyke swarms in central Chile (Creixell et al., 2009).
8.3. Petrogenesis of the Picudo Formation lavas and implications for the
tectonics of the early Andean subduction
Fig. 8. a) Ca + Na versus Ti, b) Ca versus Ti + Cr and c) AlT versus Ti discrimination diagrams after Leterrier et al. (1982). Dashed lines group different populations of measurements in Picudo Formation.
González, 1999). Furthermore, based on the obtained results we suggest that the architecture of the Jurassic arc crust had at least one
main stagnation level between ~ 8 to 15 km, in both the arc and
Conditions predicted for the Picudo Formation basalts seems
difficult to be explained by a buoyancy driven stagnation level in a
extensional back arc framework. In fact, the high diversity of geochemical compositions observed in this single unit over a restricted geographical area, which include typical arc related calc-alkaline
andesites (PR-09-05), slightly Nb-enriched basalts (PR-10-45) and
high-Sr basaltic andesites (PR-09-06 and PR-11-168), is surprising.
110
P. Rossel et al. / Lithos 239 (2015) 97–113
setting proposed for the Jurassic and Lower Cretaceous in northern
and central Chile.
Melting of a subducted eclogitized slab, which produces senso stricto
adakites (Defant and Drummond, 1990), is not a likely scenario given
that (a) the subducted slab was probably old and cold during late Jurassic (Charrier et al., 2007) and (b) the depth of magma generation for the
Picudo Formation volcanism was at least 100 km beneath the volcanic
centers, if they were located in the back-arc, and for normal thermal
conditions, melting of the slab is not possible at these depths (Mibe
et al., 2011). Equally, melting of eclogitized rocks under a thickened continental margin, which produces sensu lato adakites, is unrealistic given
the protracted extensional setting observed in the Andean margin since
the Early Triassic (Charrier et al., 2007; Mpodozis and Ramos, 1989).
Recent works of Goss and Kay (2009) and Goss et al. (2013) have
shown the existence of adakitic-like volcanics in the transition between
the Chilean flat-slab and the Central Volcanic Zone, apparently related
to the tectonic erosion and subsequent assimilation of fore-arc crust.
Since no flat slab is observed in the area during Jurassic times this
hypothesis could be hard to apply. MacPherson et al. (2006) have proposed another mechanism to produce adakite-like signature in
subduction-related magmas that may apply to the samples of the
Picudo Formation: high pressure fractionation of basaltic magmas
beneath an island arc. Such a mechanism can explain the high variation
in the La/Yb (HREE) ratios, without significant changes in the isotopic
composition, of the Picudo formation lavas, but fails to reproduce their
high Sr contents.
Based on the available data published by Rossel et al. (2013) and
Lucassen et al. (2001) we constructed isotope mixing curves between
MORB and two other end members: the Paleozoic granitic rocks and
Fig. 10. a) Schematic presentation of predicted versus measured clinopyroxene components, after Putirka et al. (2003). Black dots that plot “on the equiline” represent
clinopyroxene calculations from samples (VO04 and VO69 from the arc and PR-10-45
and PR-10-72 from the back-arc) that probably are closer to real crystallization conditions,
compared to measurements that fall in shaded area. b) Modeled REE composition of the
melts in equilibrium with the analyzed clinopyroxenes (full and dashed lines). Gray
lines are the result of the use of partition coefficients for clinopyroxenes in basaltic
melts and black lines are the result of the use of partition coefficients for clinopyroxenes
in andesitic melts. Dashed lines in sample PR-10-72 are extreme partition coefficients
(High and Low) for clinopyroxenes in alkaline basaltic systems. The gray squares represent
whole rock REE contents.
These heterogeneities were attributed to secondary alteration processes
and remobilization of LILE by Rossel et al. (2013). However, the high Sr
content in the fresh clinopyroxene phenocrysts is undoubtedly a
magmatic feature. Furthermore, the high Sr lavas also have low contents
of HREE and MgO, high Al2O3 concentrations and intermediate SiO2
contents (Rossel et al., 2013), that represent, with the exception of the
low silica content, a typical adakitic signature (Defant and Drummond,
1990), as shown in the Sr/Y vs Y diagram (Fig. 11). In order to explain
this geochemical signature, it is necessary to compare the geological
frameworks required to produce adakitic-like lavas to the geological
Fig. 11. Sr/Y vs Y diagram for the basaltic to andesitic samples from the arc and back-arc
included in this work. Bulk-rock data are from Rossel et al. (2013). A and B are partial melting curves of a mixture of MORB and subducted sediments in a proportion of 8/2 leaving a
residue with modal composition of eclogite (clinopyroxene:garnet:rutile = 85:14:1 in A
and 90:9:1 in B). C, equilibrium crystallization curve of a basaltic melt with the composition of sample PR-10-45(*) and fractionating minerals in these proportions
garnet:olivine:clinopyroxene = 5:55:40. D, partial melting curve of a mixture of depleted
mantle and subducted sediments in a proportion of 8/2 leaving a residue of harzburgite
(olivine:orthopyroxene:clinopyroxene:garnet = 59:30:9:2). Sr and Y values for MORB
and depleted mantle MORB are from Workman and Hart (2005) and for subducted sediments are from Plank and Langmuir (1998). Mineral melt partition coefficients are 0.073
(Sr) and 1.63 for (Y) for clinopyroxene, 0.03 (Sr) and 9.3 (Y) for garnet, 0.036 (Sr) and
0.0076 (Y) for rutile, 0.01 (Sr) and 0.036 (Y) for olivine and 0.03 (Sr) and 0.01 (Y) for
orthopyroxene. Clinopyroxene, orthopyroxene and garnet coefficients are from Barth
et al. (2002); rutile coefficients are from Foley et al. (2000) and olivine coefficients are
from Philpotts and Schnetzler (1970) and Nielsen et al. (1992). BADR: “normal” arc basalt–andesite–dacite–rhyolite field.
P. Rossel et al. / Lithos 239 (2015) 97–113
111
the Lagunillas Formation average (Fig. A, Supl. content). Our results
show that the best way to achieve the isotopic composition for most
of the Picudo Formation lavas is a mixture of 80% of MORB and 20% of
Paleozoic rocks.
We consider unlikely that the upper plate Paleozoic rocks were
introduced via crustal contamination of the magmas because there are
no significant variations in the Sr isotopic composition of the Picudo Formation in spite of their range of silica concentrations (SiO2 ~ 45 to 53%).
Sediment subduction is thus a more plausible mechanism to achieve the
composition of the Picudo Formation lavas, because it implies that the isotopic enrichment takes place before fractionation of the magmas.
Forward modeling was performed in order to explain the high Sr/Y
and low Y contents of the Picudo Formation samples (Fig. 11). Based
on the most likely magma sources for this unit derived from the isotopic
composition of its volcanic rocks (see above and Fig. A of the suppl.
content), two types of mixture were used in the modeling: MORB/sediments and Average Depleted Mantle/sediments. The best fit for the calculated curves is achieved using the composition of the present-day
high Sr sediments from the Peru trench (Plank and Langmuir, 1998).
Thus, the Sr and Y contents of the Picudo formation adakite-like lavas
can be attained through 1) one stage melting (3–5%) of a mixture of
sediments and oceanic basalts or 2) a two stage process that starts
with melting (~ 25%) of a mixture of depleted mantle and sediments
to achieve the composition of the enriched basaltic sample PR-10-45,
and subsequent high pressure fractionation of olivine, clinopyroxene
and minor garnet.
The one-stage model fails to explain the low SiO2 and Mg contents
(~ 52% and ~ 3% respectively) of the adakite-like lavas, and it is not
realistic considering the expected thermal conditions of the slab
below the back-arc domain (Mibe et al., 2011). The basaltic sample
(PR-10-45) follows the melting curve of MORB and sediments but the
contents of Ni and Cr are higher whereas SiO2 is significantly lower
than the average MORB, which make a genetic relationship with this
raw endmember questionable. The two stage modeling on the other
hand, can explain both the elemental heterogeneity and the isotopic homogeneity of the Picudo Formation lavas. Moreover, the crystallization
conditions calculated in clinopyroxenes of Picudo Basalt (PR-10-45)
are in agreement with the high pressure fractionation model proposed
by MacPherson et al. (2006).
Otherwise, if no sediments are involved, it is possible to infer the
subduction erosion of the Jurassic fore-arc as the source of the high-Sr
component, as proposed by Goss and Kay (2009) and Goss et al.
(2013) for Miocene adakites in the flat slab region at 27°–30°S. If this
is the case, a contractional pulse during late Jurassic is needed in order
to trigger subduction erosion. Scheubert and González (1999), Creixell
et al. (2011) and Ring et al. (2012), propose the existence of a
transpressional event, recorded in the Middle to Upper Jurassic arc
units from northern and central Chile. The transpression could have
caused a minor event of subduction erosion that fed the magmatic
system and mixed with mantle melts beneath the back-arc area, producing the adakite-like lavas of the Picudo Formation lavas.
This study was funded by the FONDECYT No. 11080040 (Verónica
Oliveros) and Romanian National Sciences Foundation (UEFISCDI)
grant PHII-ID-PCE-2011-3-0217, and a research grant from ExxonMobil to the University of Arizona (Mihai Ducea). G. Saldías and C.
Marquardt (Antofagasta Minerals), L. Serrano and M. Ramírez (Lumina
Copper), C. Jara (Barrick Exploraciones) and C. Cornejo (Goldcorp) are
thanked for granting access to outcrops and providing logistic support
that was very important for the development of this study. D. R. Hervig
of and K. Domanik are thanked for assistance in analytical work. R.
Merino and M. Labbé are thanked for assistance in the field work. P.
Vásquez and M. Roden are thanked for the thorough and detailed
reviews that significantly improved earlier versions of this manuscript.
9. Conclusions
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were studied in order to gain petrogenetic information otherwise difficult to obtain via a conventional whole-rock study. Clinopyroxenes are
classified in two groups: the first are augite phenocyrsts in rocks of
the main Jurassic arc, Algarrobal Formation and Quebrada Vicuñita
Beds. These pyroxenes are depleted in Ti, trace elements such as Zr, Hf
and have flat REE patterns, suggesting that the parental melts originated
from depleted mantle sources typical for the supra subduction asthenosphere. The second group comprises aluminous diopsides from the
Picudo Formation and aluminous to aluminous-subsilic-titanian diopsides from the Lagunillas Formation. These pyroxenes, especially those
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and steeper REE patterns compared to the former group. The compositional features observed in the Picudo Formation clinopyroxenes are indicative of complex petrogenetic processes that involve subduction/
erosion of enriched sediments or forearc rocks and subsequent high
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Trace element variations in the studied samples suggest that
clinopyroxene and olivine extraction are responsible for the fractionation patterns observed in the back-arc units Quebrada Vicuñita Beds
and Algarrobal Formation, whereas in the Picudo Formation magmas,
garnet is fractionated.
The petrographic analysis and the thermobarometry calculations
suggest that plagioclase was the first Ca phase to crystallize in most of
the studied samples. Clinopyroxene crystallization conditions indicate
slightly lower but overlapping temperatures and pressures for the
crystals belonging to arc units, relative to the back-arc ones. The results
suggest the presence of at least one main stagnation level for the
magmas, both in the arc and back arc domains, located at ~ 8–15 km
(and 1100 °C) below the surface. This depth could represent the
lower-middle crust boundary. A sub-group of analysis from the Picudo
Formation pyroxenes suggest deeper crystallization levels at ~ 35–
36 km (and ~1200 °C) below the back arc area, which could be related
to high pressure fractionation and resulted in the adakitic-like signature
observed in two samples. The interpretation of the thermobarometric
results in Lagunillas Formation lavas is not straightforward, but the
equigranular textures and limited range of Mg# suggest rapid ascent
of the magmas to the surface through the lithosphere and very little
fractionation.
The obtained geochemical and thermobarometric results are consistent with the main extensional–transtensional framework proposed for
the Andean margin during Jurassic and Early Cretaceous.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.lithos.2015.10.002.
Acknowledgments
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