Chemical Geology 244 (2007) 1 – 24
www.elsevier.com/locate/chemgeo
The tectonic significance of (U,Th)/Pb ages of monazite inclusions
in garnet from the Himalaya of central Nepal
Aaron J. Martin a,⁎, George E. Gehrels b , Peter G. DeCelles b
a
Department of Geology, University of Maryland, College Park, MD, 20742, USA
b
Department of Geosciences, University of Arizona, Tucson, AZ, 85721, USA
Received 1 August 2006; received in revised form 26 March 2007; accepted 4 May 2007
Editor: S.L. Goldstein
Abstract
Textural observations, chemical zoning, and 196 in situ LA-ICP-MS 232Th/208Pb and 238U/206Pb dates of monazite inclusions in
Greater Himalayan garnets demonstrate that inclusions are frequently composite grains composed of multiple generations of
monazite. Cenozoic prograde and retrograde monazite are commonly intergrown, and both of these components are frequently
intergrown with lower Paleozoic monazite. Because zones in most inclusions are too small to date, and because some inclusions
show little zoning of Y and Th, chemical characterization alone is often insufficient to guide interpretation of the crystallization
significance of Th/Pb dates. Comparison of 238U/206Pb and 232Th/208Pb dates thus is critical for interpreting the tectonic
significance of dates derived from these composite inclusions. Most Cenozoic monazite crystallized between 42–29 and 22–
12 Ma, and these age ranges may record prograde and retrograde metamorphism of Greater Himalayan rocks, respectively. Dates
younger than ∼ 12 Ma are discordant, without exception. The b12 Ma dates come from the eastern side of the Annapurna Range
and record limited monazite growth during retrograde metamorphism and metasomatism of these Greater Himalayan rocks.
Microcracks allow communication between the interior and exterior of garnet, affecting monazite inclusions by facilitating Pb loss,
dissolution, recrystallization, and intergrowth of younger monazite.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Monazite; Garnet; (U,Th)/Pb dating; Himalaya; Microcrack; Retrograde metamorphism
1. Introduction
Over the past ten years, 232Th/208Pb dating of
monazite has emerged as the tool of choice for
constraining the timing of mid-high temperature Cenozoic metamorphism and metasomatism of Himalayan
metapelitic and metapsammitic rocks (e.g. Harrison
⁎ Corresponding author. Tel.: +1 301 405 5352; fax: +1 301 405
3597.
E-mail address: martinaj@geol.umd.edu (A.J. Martin).
0009-2541/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemgeo.2007.05.003
et al., 1995, 1997; Foster et al., 2000; Catlos et al., 2001,
2002b, 2004; Kohn et al., 2004). Monazite is a
monoclinic light rare earth phosphate that usually
incorporates high concentrations of Y and Th into its
crystal structure (Catlos et al., 2002a; Harrison et al.,
2002; Spear and Pyle, 2002). Monazite 232 Th/208Pb
dating offers several advantages over other dating
methods. First, although the 232 Th–208Pb system has a
half-life of 14 billion years, it can be used to date even
small parts of very young crystals because monazite
typically contains 3–9 wt.% ThO2 (Overstreet, 1967;
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A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
Van Emden et al., 1997). This high abundance of Th
allows precisely and accurately measurable quantities of
radiogenic 208 Pb to accumulate in only a few million
years in most grains. In contrast, UO2 concentrations in
monazite are normally b 0.01 to 0.3 wt.% (Van Emden
et al., 1997). This relatively low abundance makes U/Pb
dating of small portions of some young monazite
crystals more difficult than Th/Pb dating because
insufficient radiogenic 206 Pb and 207Pb accumulate in
a few tens of M.y. to allow accurate isotopic measurements using conventional in situ techniques. Using
newer high-sensitivity techniques, acquisition of precise
238
U/206Pb dates for young monazites is feasible,
although the common presence of excess 206Pb derived
from 230Th incorporated during crystallization complicates interpretation of the 238U/206 Pb dates of some
monazite grains (Mattinson, 1973; Scharer, 1984;
Parrish, 1990). Errors on 235U/207Pb dates remain
prohibitively high for young monazite grains. Second,
because the half-lives of the intermediate daughter
products in the decay chain between 232Th and 208Pb are
short, the system attains secular equilibrium in only
∼ 30 years (Harrison et al., 2002). Third, compared to
the maximum abundance, concentrations of Th and Y
commonly vary substantially within individual monazite crystals, and the mineral frequently displays
conspicuous zoning of Th and Y (e.g. Zhu and O'Nions,
1999; Gibson et al., 2004; Kohn et al., 2004). Analysis
of the zoning patterns of these elements often yields
valuable insight into the growth history of the monazite
grain. Fourth, monazite is common in Himalayan
medium to high grade metapelitic and metapsammitic
rocks. Finally, although metamorphic zircon could be
used to constrain the timing of rocks that reached high
temperatures, many medium grade Himalayan rocks
likely did not experience high enough temperatures for
long enough times to either grow significant quantities
of metamorphic zircon or to reset the (U,Th)/Pb system
in existing grains. Garnet-bearing Lesser Himalayan
rocks record peak temperatures of ∼500–650 °C (e.g.
Catlos et al., 2001; Beyssac et al., 2004; Bollinger et al.,
2004; Kohn et al., 2004; Martin, 2005), but the closure
temperature of most zircons is N800 °C (Cherniak and
Watson, 2001) and abundant zircon growth in pelitic
rocks metamorphosed on a typical Barrovian path
commences near 700 °C with the formation of partial
melt (Rubatto et al., 2001; Williams, 2001; Rubatto,
2002). Large amounts of Cenozoic metamorphic
monazite grew in both medium and high temperature
rocks, however.
As with any geochronometer, understanding a
monazite (U,Th)/Pb date in paragenetic context is
critical for understanding its tectonic significance.
Matrix monazite crystals, i.e., grains that are not
included in garnet, quartz, plagioclase feldspar, or
some other phase, are exposed to the ambient fluids in
the rock and thus are expected to lose some Pb and/or
partially recrystallize even at fairly low temperatures.
The closure temperature for Pb loss in monazite is
∼ 500–800 °C according to Smith and Giletti (1997),
but Cherniak et al. (2004) obtain a closure temperature
of ∼ 900–1100 °C, depending on cooling rate and grain
size (see also McFarlane and Harrison, 2006). Probably
more importantly for interpreting the tectonic significance of (U,Th)/Pb dates, dissolution, recrystallization,
and intergrowth of different generations of monazite are
all common and can occur at temperatures well below
the closure temperature (DeWolf et al., 1993; Crowley
and Ghent, 1999; Kohn et al., 2004). Indeed, monazite
grows at temperatures as low as 260–400 °C during
metasomatism and retrograde metamorphism (Poitrasson et al., 2000; Rasmussen et al., 2001; Townsend
et al., 2001; Harlov et al., 2002; Harlov and Forster,
2003; Bollinger and Janots, 2006).
In the late 1990s, geologists began producing in situ
(U,Th)/Pb dates of monazite inclusions in garnet from
the Himalaya because these dates promised to yield
information about the age of growth of the surrounding
garnet (Harrison et al., 1997; Foster et al., 2000; Catlos
et al., 2001, 2002b). Because garnet incorporates only
very small amounts of Pb, and diffusion of Pb in garnet
is slow, growth of garnet around a monazite grain was
expected to shield the monazite crystal from both Pb
loss and growth of younger monazite (DeWolf et al.,
1993; Zhu et al., 1997; Foster et al., 2000). Thus,
according to this early work, (U,Th)/Pb ages of
monazite inclusions in garnet should represent the
time at which the garnet grew around the monazite
crystal, even if the rock experienced high temperatures
for long times subsequent to growth of the garnet.
Application of this technique in several places in eastern
Asia led to numerous important and surprising results
(Harrison et al., 1997; Foster et al., 2000; Hacker et al.,
2000; Catlos et al., 2001, 2002b; Gilley et al., 2003;
Foster et al., 2004; Catlos et al., 2004). For example,
using Th/Pb ages of both monazite inclusions in garnet
and matrix monazite crystals, Catlos et al. (2001, 2002b)
concluded that the Main Central thrust (MCT), a major
fault now exposed in the interior of the Himalayan thrust
belt and thought by most workers to be inactive since the
early Miocene, was active throughout the late Miocene
and perhaps as recently as 3 Ma (Fig. 1). The presence of
∼ 45 Ma Th/Pb ages of monazite inclusions in garnet
from the hanging wall of the MCT further led Catlos
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
3
Fig. 1. Generalized geologic map of Nepal modified from Amatya and Jnawali (1994). K—Kathmandu, P—Pokhara. The rectangle north of Pokhara
locates the map shown in Fig. 2. Inset: Highly generalized geologic map of the Himalayan orogen from Sorkhabi and Macfarlane (1999). IYSZ—
Indus–Yarlung Suture Zone, STDS—South Tibetan Detachment System, MCT—Main Central Thrust, DT—Dadeldhura Thrust, MT—Mahabarat
Thrust, RT—Ramgarh Thrust, MBT—Main Boundary Thrust, MFT—Main Frontal Thrust.
et al. (2002b) to conclude that these rocks attained
metamorphic conditions sufficient for monazite growth
during the middle Eocene, earlier than previously
accepted by most Himalayan geologists.
Although the pioneering work by these groups led to
some exciting conclusions, the monazite (U,Th)/Pb ages
reported in Harrison et al. (1997) and Catlos et al. (2001,
2002b) raise additional unanswered questions. These
authors treated most or all of the Cenozoic ages from an
individual sample as a single population, which allowed
them to obtain a single crystallization age for each
sample by averaging multiple ages from the same
sample. Single crystallization ages for each sample, in
turn, allowed the authors to compare ages between
samples and to place the ages in geographic context.
However, many of the samples yielded a large range of
Cenozoic ages, sometimes an order of magnitude larger
than the errors on the ages, with multi-million year time
gaps between analyses. This pattern suggests multiple
age populations in many samples. More recently,
Bollinger and Janots (2006) used monazite 208 Th/232Pb
ages and hornblende, biotite, and muscovite 40Ar/39Ar
ages from Lesser Himalayan rocks to show that many
monazites grew during late Cenozoic retrograde metamorphism, not during prograde metamorphism as
previously assumed by most workers. Working on
rocks outside the Himalaya, several groups found that
ages of monazite inclusions near cracks in their host
garnet are significantly different than ages of inclusions
far removed from cracks (DeWolf et al., 1993; Zhu et al.,
1997; Montel et al., 2000). These workers caution that
garnet only completely shields monazite inclusions from
Pb loss and/or intergrowth of younger monazite if the
inclusions are not intersected by cracks in the garnet.
These results suggest that kinematic interpretations in the
Himalaya should be reconsidered if they are based on
any of the following assumptions or interpretations: 1)
Garnet completely shields monazite from intergrowth of
younger monazite, 2) Monazite dates represent ages of
prograde or peak metamorphism, or 3) Most or all
Cenozoic ages from an individual sample define a single
population of ages.
In this paper we examine the tectonic significance of
(U,Th)/Pb dates of monazite inclusions in garnet using a
combination of textural characterization, analysis of
element zoning patterns, and in situ 238U/206 Pb and
232
Th/ 208Pb isotopic dating of monazite inclusions
using an inductively coupled plasma mass spectrometer
equipped with a laser ablation source (LA-ICP-MS).
Throughout the paper we use the term “date” to refer to a
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A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
relatively uninterpreted radiometric time determination
that might or might not have crystallization significance
and the term “age” to refer to a date which we interpret
to possess crystallization significance. The garnets come
from Greater Himalayan metapelitic rocks collected
from five transects across the Annapurna Range of
central Nepal (Fig. 1). We address only Greater
Himalayan garnets because we found monazite inclusions in garnets from just two Lesser Himalayan
samples, both from the Nayu Ridge transect (Table
ESM11). Building on the work of Foster et al. (2000,
2004), we illustrate how comparison of 238 U/206Pb and
232
Th/208 Pb dates lends important insight into interpretations of the tectonic significance of monazite ages.
We show that garnet crystals only partially shield
monazite inclusions from Pb loss and younger monazite
intergrowth; thus most monazite inclusions yield
discordant 238U/206 Pb and 232 Th/208Pb dates.
2. Geologic setting
Greater Himalayan series rocks are the highest-grade
rocks that are widely exposed in the Himalaya;
thermobarometric estimates usually peg peak conditions
at ∼ 10–14 kbar at ∼ 700–800 °C (e.g. Ganguly et al.,
2000; Stephenson et al., 2000; Daniel et al., 2003; Kohn
et al., 2004; Martin, 2005). In central Nepal, geologists
usually divide the Greater Himalayan series into three
informal units (Fig. 2; LeFort, 1975; Searle and Godin,
2003). The structurally lowest unit, Formation I,
consists of schists and gneisses produced by the
metamorphism of a sedimentary succession dominated
by mudstones and interbedded minor sandstones.
Formation II consists of marbles and layered calcsilicates and sits structurally above Formation I (Fig. 2).
Granitic rocks known as Unit III intrude near the
structural top of Formation II. Formation I rocks yield
detrital zircons as young as ∼900–590 Ma, and Unit III
yields a crystallization age of ∼ 500 Ma, constraining
the depositional age of the Greater Himalayan series
protoliths to the Neoproterozoic–Cambrian (Hodges
et al., 1996; DeCelles et al., 2000; Godin et al., 2001;
Gehrels et al., 2003; Martin et al., 2005). Ductile thrust
and normal faults internally shortened and extended
Greater Himalayan rocks (Reddy et al., 1993; Hodges
et al., 1996; Kohn et al., 2004; Martin, 2005).
The South Tibetan Detachment System (STDS)
marks the top of the Greater Himalayan series. The
STDS is a series of north-dipping normal faults that
1
All tables and figures beginning with ESM are available as
electronic Supplementary material.
accommodated tens of kilometers of slip during the
Miocene (Burg and Chen, 1984; Hodges et al., 1998;
Searle et al., 2003). The STDS places less-metamorphosed Tethyan series metasedimentary rocks on moremetamorphosed Greater Himalayan series metasedimentary rocks. The Tethyan series is a thick succession
of Neoproterozoic to Eocene sedimentary rocks that
extend from the STDS north to the Indus–Yarlung
suture zone, which marks the former subduction zone
between the Indian and Eurasian plates (Fig. 1;
Ratschbacher et al., 1994; Garzanti, 1999; Murphy
and Yin, 2003). Numerous thrust faults cut the Tethyan
rocks (Ratschbacher et al., 1994; Murphy and Yin,
2003). Lowermost Tethyan strata may be the protoliths
for Greater Himalayan metasedimentary rocks LeFort,
1975; Gehrels et al., 2003; Myrow et al., 2003; Richards
et al., 2005).
The MCT, the structure that bounds the base of the
Greater Himalayan series, places higher-grade Greater
Himalayan metasedimentary rocks on lower-grade
Lesser Himalayan metasedimentary rocks (Fig. 2). In
Nepal, stratigraphers divide the Lesser Himalayan series
into two informal units (Upreti, 1996). The lower and
thicker part of the Lesser Himalayan series, the Nawakot
unit, is dominated by metamorphosed mudstones and
several clean quartzite units with thicknesses of a few
hundred meters (Schelling, 1992; Upreti, 1996; DeCelles at al., 2000, 2001; Dhital et al., 2002; Robinson et al.,
2006). Detrital zircon U/Pb ages and the age of a granitic
intrusive indicate that the basal part of the Nawakot unit
was deposited between ∼ 1845 and 1831 Ma and that
the upper part was deposited after ∼ 1500 Ma (DeCelles
et al., 2000; Gehrels et al., 2003; Martin et al., 2005).
The upper, thinner part of the Lesser Himalayan series is
called the Tansen unit. Permian–Paleocene Gondwanan
sedimentary rocks comprise the lower part of the Tansen
unit, and the upper part consists of strata deposited in the
Himalayan foreland basin between the Eocene and the
Early Miocene (Sakai, 1983; Upreti, 1996; DeCelles
et al., 1998a; 2004). Numerous thrust faults, most
importantly the Ramgarh thrust, repeat Lesser Himalayan stratigraphy (Schelling, 1992; Srivastava and Mitra,
1996; DeCelles et al., 2001; Pearson and DeCelles,
2005; Robinson et al., 2006).
The Lesser Himalayan series was thrust southward
along the Main Boundary thrust onto the Sub-Himalayan series (Fig. 2). Middle-Miocene to Quaternary
foreland basin deposits, the Siwalik Group, comprise the
Sub-Himalayan series (Harrison et al., 1993; Quade
et al., 1995; DeCelles et al., 1998b). Several thrusts
repeat parts of the Siwalik Group, and Sub-Himalayan
rocks are thrust onto Quaternary and modern foreland
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
5
Fig. 2. Geologic map of the southern Annapurna range between the Modi and Marsyangdi rivers. Sample locations shown with filled circles. Some
sample locations omitted for clarity as follows. Sample 402089 is located between 402088 and 402091. 402092 is located directly below 402091.
502152 is located between 502151 and 502149. 502123 is located between 502122 and 502124. 502125 is located between 502124 and 502126.
502104 is located directly below 502103. BF—Bhanuwa Fault (Martin, 2005), CT—Chandrakot Thrust (Hodges et al., 1996), DD—Deorali
Detachment, GT—Ghandruk Thrust, HD—Hiunchuli Detachment, MCT—Main Central Thrust, MD—Machapuchre Detachment, NT—Nayapur
Thrust, RF—Romi Fault, RT—Ramgarh Thrust, STDS—South Tibetan Detachment System. Both isotopic and microstructural data constrain the
location of the MCT (Martin et al., 2005). Hodges et al. (1996) assign the rocks between the DD and the MD to the Tethyan series whereas Searle and
Godin (2003) assign these rocks to the Greater Himalayan series. Geology north of MD from Hodges et al. (1996). Geology in Khudi Khola from
Pearson and DeCelles (2005). Greater Himalayan units from LeFort (1975). Contours: 1000 m.
basin deposits along the Main Frontal thrust (Schelling,
1992; Powers et al., 1998; Lave and Avouac, 2001).
3. Methods
Garnet grains were separated by hand from 0.5–
1.0 kg of each garnet-bearing Greater or Lesser
Himalayan sample from the Annapurna Range. All
Greater Himalayan samples come from Formation I.
Garnet grains were mounted in epoxy and polished to
reveal the interiors of the grains. Monazite inclusions
were identified using a scanning electron microscope
equipped with an energy dispersive spectrometer.
Compositional zoning in monazite inclusions, in large
areas of thin sections, and in garnet was imaged using a
Cameca SX50 electron microprobe operating at an
accelerating voltage of 15 or 25 kV and a Faraday cup
current of 200 or 250 nA.
Monazite 232 Th/208Pb and 238U/206 Pb dates were
obtained using the Micromass Isoprobe LA-ICP-MS at
the University of Arizona. Monazite was ablated using a
New Wave DUV193 Excimer laser with an emission
wavelength of 193 nm operating at ∼ 50 mJ at 22 kV.
The laser spot had a diameter of 10 μm and the firing
rate was 4 Hz. For analyses that begin with the prefix
‘T4’ or ‘T5,’ the ablated material was carried into the
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A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
plasma source in 100% He gas; for the remaining
analyses the carrier gas was 70% Ar and 30% He. The
mass spectrometer was equipped with a flight tube of
sufficient width that U, Th, and Pb isotopes were
measured simultaneously. All measurements were made
in static mode, using Faraday detectors for 238 U, 232 Th,
208
Pb, 207Pb, and 206Pb and an ion-counting channel for
204
Pb. Each analysis consisted of one 20-second
integration on peaks with the laser off to measure
backgrounds, twenty 1-second integrations on peaks
with the laser firing, and a 30-second delay to purge the
previous sample and prepare for the next analysis. Any
Hg contribution to the 204 Pb voltage was removed by
subtracting background values. Very little Hg was
detected in the Ar and He gasses.
Th and U concentrations were estimated by normalization of the 232Th and 238U voltages obtained from
unknown grains with the voltages obtained by ablation of
NBS610 trace element glass, which was embedded in
epoxy near the garnet grains. The glass was analyzed at
the beginning and end of the analysis of each Greater or
Lesser Himalayan sample. Analyses were corrected for
common Pb content using the measured 204Pb and
assuming an initial Pb composition according to Stacey
and Kramers (1975) with an uncertainty of 2.0 for
208
Pb/204Pb and 1.0 for 206Pb/204Pb. Fractionation of Pb
isotopes of ∼5% and U/Pb and Th/Pb fractionation of
∼20% were corrected by normalization to a monazite
standard with a concordant U/Pb age of 424 ± 2 Ma (2sigma error). The standard, from the Wissahickon
Formation, Delaware, USA, was provided by J. Aleinikoff and dated using isotope dilution thermal ionization
mass spectrometry (ID-TIMS) by S. Kamo. This standard
was embedded in epoxy near the garnet grains and
analyzed after every 3 analyses of unknown monazite. In
order to assess the validity of this normalization technique, we treated ∼46% of the standards as unknowns
and compared computed 232Th/208Pb and 238U/206Pb
ages with the ID-TIMS age (Figs. ESM1, ESM2). Although removal of standards degrades the accuracy of the
mass fractionation correction, all but two of the Th/Pb
analyses and all but six of the U/Pb analyses agree within
error with the ID-TIMS age of the standard.
Mass fractionation also increased with depth into the
laser pit by up to 10%. During most analyses, signal
intensity decreased after 6 to 10 s. To mitigate both
effects, accepted isotopic ratios were taken as the
weighted mean of the ratios from the integrations from
the first 2 to 5 s within the 20-second analysis. Most
analyses exhibit significant within-run complexity,
probably caused by mixing of different amounts of
lower Paleozoic, Cenozoic prograde, and Cenozoic
retrograde monazite as the laser penetrates the monazite
crystal. The resulting within-run variability in isotopic
ratios and the uncertainty in selecting appropriate
isotopic ratios contribute most of the error to each
analysis; measurement errors and systematic errors are
almost always significantly less than these uncertainties.
4. Monazite dates and Th and U concentrations
Table ESM1 provides isotopic data for 213 (U,Th)/Pb
dates of monazite inclusions in garnet from the southern
Annapurna Range. 17 of these dates come from an equal
number of inclusions that grew in two Lesser Himalayan
rocks, both from the Nayu Ridge transect. The
remaining 196 dates come from 186 monazite inclusions
found in garnets from Greater Himalayan rocks
(multiple spots were dated in several inclusions).
4.1. Th/208 Pb dates
Fig. 3 and ESM3 show all of our Greater Himalayan
monazite Th/Pb dates. There is a continuous distribution
of Th/Pb dates from 505 ± 46 Ma to 6 ± 1 Ma. There are
also several bends in the temporal trend of the dates,
including a bend at ∼30 Ma (Fig. 3). The most prominent
bend occurs at ∼50 Ma because 69% of the Th/Pb dates
are between ∼6 and 50 Ma whereas only 31% of the dates
fall between ∼51 and 505 Ma. The large percentage of
inclusions that yield Cenozoic dates demonstrates that late
Cenozoic metamorphism strongly overprints early Paleozoic metamorphism, although remnants of the early
Paleozoic metamorphism clearly remain. Fig. 4 shows
that inclusions with Th concentrations greater than ∼12%
uniformly yield Cenozoic dates, but below ∼12% there is
no correlation between Th concentration and age.
Monazite Th/Pb dates from three adjacent Greater
Himalayan samples are shown in Fig. 5. Samples such
as 502122 that contain garnets with many monazite
inclusions show a large range of monazite dates
stretching from the middle to late Cenozoic to the
Mesozoic or Paleozoic (Fig. 5A, B, Table ESM1). Such
a large range of dates is rarely observed for samples such
as 502123 and 502124 that contain garnets with fewer
monazite inclusions (Fig. 5C, D). Instead, these samples
yield only a few dates scattered throughout the
Cenozoic, Mesozoic, and Paleozoic.
Sample 502122 is located 150 m structurally above
sample 502123, which is located 150 m structurally
above sample 502124, and no major faults are known to
exist between the samples. Monazite inclusions in these
closely-spaced samples yield radically different Th/Pb
dates, however, and the Cenozoic dates from an
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
7
Fig. 3. 232Th/208Pb dates obtained from all 196 analyses of monazite inclusions in Greater Himalayan garnets. Error bars show total error at the 2sigma level. The analyses are arranged with the youngest dates on the left and the oldest dates on the right. 69% of the dates fall between ∼ 6 and
50 Ma, indicating that these rocks were strongly overprinted during Cenozoic metamorphism. With decreasing age, an increasing number of analyses
come from inclusions that are intersected by obvious microcracks in their host garnet. The dates between ∼ 400 and 46 Ma and younger than ∼12 Ma
are not crystallization ages; instead they are mixed dates or dates produced by significant Pb loss.
individual sample clearly do not define a single
population. If we wrongly treat the post-55 Ma dates
from each sample as a single population and calculate
weighted averages of the dates, the upper sample yields
an estimate of 23 ± 6 Ma, the middle sample an estimate
of 18 ± 120 Ma, and the lower sample an estimate of 30 ±
3 Ma (from one date). Given the very close spacing of
these samples and the lack of known structures between
them, it is unlikely that they experienced metamorphism
at such different times. We conclude that it is incorrect to
treat these ages as a single population. Examination of
Table ESM1 shows that most closely-spaced samples,
even those with numerous inclusions, do not yield a
single population of Cenozoic Th/Pb dates. Because the
Cenozoic dates from most samples do not define a single
population, we cannot obtain a single crystallization age
for each of these samples. It is thus difficult to compare
Cenozoic dates between individual samples or to place
dates from individual samples in geographic context.
Instead, in the remainder of the paper we pool the
Greater Himalayan ages in order to obtain useful
information about the metamorphism of Greater
Fig. 4. 232Th/208Pb date versus Th concentration for all 196 analyses of monazite inclusions in Greater Himalayan garnets. No correlation exists
between monazite Th/Pb date and Th concentration for concentrations less than ∼ 12%, but monazite inclusions with Th concentrations greater than
∼ 12% yield Cenozoic dates.
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A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
4.2. U/206Pb dates
Fig. 5. Th/Pb dates of monazite inclusions from three samples from the
Madi Nadi transect. A) All dates from sample 502122. B) The post55 Ma dates from sample 502122. C) Both analyses from sample
502123. D) The only analysis from sample 502124. MSWDs and errors
of the weighted means are large because we treated the post-55 Ma
dates from each sample as a single population in order to show that such
treatment is incorrect. Thick lines show the weighted mean of the
dates, obtained by weighting all of the dates only by data point errors.
MSWD—Mean Squares of Weighted Deviates. Error bars show total
error at the 2-sigma level. The horizontal scale is the same for B, C, and D.
Fig. 6 and ESM4 show all of the Greater Himalayan
monazite 238 U/206Pb dates. As for the 232Th/208 Pb
dates, there is a nearly continuous distribution of
238
U/206 Pb dates from 497 ± 21 Ma to 7 ± 2 Ma. There
are several bends in the temporal trend of these dates,
including a subtle bend at ∼ 25 Ma. Similar to the
pattern of Th/Pb dates, the most prominent bend occurs
at ∼ 50 Ma because 69% of the 238U/206 Pb dates are
between ∼7 and 50 Ma whereas only 31% of the dates
fall between ∼ 51 and 497 Ma. The large percentage of
inclusions that yield Cenozoic 238U/206Pb dates shows
again that Cenozoic metamorphism strongly overprints
early Paleozoic metamorphism, though vestiges of the
early Paleozoic metamorphism clearly remain.
Fig. 7 shows a plot of Th/Pb date versus 232Th/238U
ratio for all Greater Himalayan monazite analyses. No
correlation exists between Th/Pb date and 232Th/238U for
most of the analyses except that the two analyses with the
highest 232Th/238 U yield Cenozoic dates. However,
consideration of only the concordant analyses (analyses
whose Th/Pb and U/Pb dates agree within 10%) shows
that the concordant lower Paleozoic monazites have a
higher 232Th/238U ratio than the concordant Cenozoic
monazites. The mean 232Th/238 U for the four concordant
lower Paleozoic monazites is 14.7 whereas the mean for
the 57 concordant Cenozoic monazites is 5.9.
Fig. 7 also shows that the 10 analyses with the
highest 232 Th/238U come from monazites taken from the
Marsyangdi Nadi transect and that 18 of the 32 analyses
with 232Th/238 U ratio greater than 10 come from this
transect. Most of the analyses from the Marsyangdi
transect with high 232Th/238 U yield Cenozoic dates,
though a few yield Mesozoic or Paleozoic dates.
Although most of these high 232Th/238U analyses from
the Marsyangdi transect are not concordant, the large
number of analyses that yield high 232Th/238 U suggests
that either during early Paleozoic metamorphism or
during Cenozoic metamorphism, or both, conditions in
Greater Himalayan rocks in the Marsyangdi transect
allowed growth of monazite with higher 232Th/238U
than in Greater Himalayan rocks in other transects.
5. Textural observations and zoning patterns
5.1. Monazite
Himalayan rocks in general. Overall, the Th/Pb results
suggest that the significance of Th/Pb dates of monazite
inclusions in garnet alone as estimates of the timing of
Cenozoic metamorphism is highly uncertain and
requires validation using another isotopic system.
Throughout our discussion of (U,Th)/Pb dates of
Greater Himalayan monazite inclusions and their
tectonic significance, we divide the monazites into
three groups based on Th/Pb date: 505–400 Ma, 399–
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
9
Fig. 6. 238U/206Pb dates obtained from all 196 analyses of monazite inclusions in Greater Himalayan garnets. Error bars show total error at the 2-sigma
level. The analyses are arranged with the youngest dates on the left and the oldest dates on the right. 69% of the dates fall between ∼ 7 and 50 Ma,
indicating that these rocks were strongly overprinted during Cenozoic metamorphism. With decreasing age, an increasing number of analyses come
from inclusions that are intersected by obvious microcracks in their host garnet. The dates between ∼ 400 and 46 Ma and younger than ∼ 12 Ma are
not crystallization ages; instead they are mixed dates or dates produced by significant Pb loss.
56 Ma, and 55–6 Ma. We use Th/Pb dates rather than
238
U/206Pb dates for these divisions for two reasons: 1)
The Th/Pb dates usually have smaller or similar errors,
and 2) Using Th/Pb dates alleviates any possible
complications to 238 U/ 206 Pb dates caused by the
presence of excess 206Pb. The upper limit for the first
group, 505 Ma, is the Th/Pb date of the oldest inclusion
dated, and the lower limit, 400 Ma, is near the Th/Pb
date of the youngest concordant lower Paleozoic
monazite. The boundary between the second and third
groups, 55 Ma, is near the upper limit of estimates for
the age of Cenozoic metamorphism of exposed
Himalayan rocks (de Sigoyer et al., 2000; Leech et al.,
2005). These estimates are based on radiometric dating
of minerals in eclogite facies metamorphic rocks of the
Tso Morari complex in the western Himalaya. The Tso
Morari complex is bounded to the north by the Indus–
Yarlung Suture Zone, and thus is located north of the
traditionally-defined outcrop belt of Greater Himalayan
rocks. Most of the Greater Himalayan series between the
Fig. 7. Th/Pb date versus 232Th/238U ratio for all analyses of Greater Himalayan monazite. The 4 concordant lower Paleozoic monazites have a mean
Th/238U ratio of 14.7 whereas the mean for the 57 concordant Cenozoic monazites is 5.9. The 10 analyses with the highest 232Th/238U come from
monazites taken from the Marsyangdi Nadi transect. Circles show concordant analyses, squares show discordant analyses. Open squares indicate
analyses from monazites from the Marsyangdi Nadi transect with 232Th/238U ratio greater than 10.
232
10
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
analyses L15 and L16 in sample 502126, the two grains
give very different Th/Pb and 238 U/206Pb dates, and
neither of the dates is concordant (Fig. 9A). The shape
of inclusion D5 in sample 502091 suggests that it
consists of a combination of two or more monazites
(Fig. 9B). Both the Th/Pb date and the 238U/206Pb date
from the center of the composite grain are intermediate
between corresponding dates from the left and right
sides of the grain. Only the analyses from the right and
the center of the grain yield concordant dates.
Large inclusions often show prominent zoning of Y
and Th that aids interpretation of (U,Th)/Pb dates. A
jagged low Y, high Th, high Si stripe cuts across
Fig. 8. Histogram showing the number of Th/Pb dates in three age
ranges that come from monazite inclusions that are or are not
intersected by microcracks in their host garnet. The proportion of
inclusions intersected by obvious microcracks increases with decreasing age.
MCT and the STDS experienced metamorphism after
∼ 40 Ma (Vannay and Hodges, 1996; Coleman and
Hodges, 1998; Guillot et al., 1999; Godin et al., 2001;
Harris et al., 2004; Kohn et al., 2004). The lower limit of
the third group, 6 Ma, is the Th/Pb date of the youngest
inclusion dated. The boundaries between the groups do
not correspond to breaks within the series of monazite
dates; a continuum of Th/Pb dates exists between 505 ±
46 and 6 ± 1 Ma (Fig. 3, ESM3).
Fig. ESM5A shows a backscattered electron image
(BEI) of a monazite inclusion intersected by a prominent
microcrack (a crack less than about 1 mm long; Engelder,
1987) in its host garnet, and Fig. ESM5B shows another
inclusion that is not intersected by a microcrack in the
garnet, at least in the plane of observation. 60% of the
196 analyses of monazite inclusions in Greater Himalayan garnets from the Annapurna Range come from
inclusions intersected by obvious microcracks in their
host garnet. A correlation exists between the location of
an inclusion relative to microcracks and the Th/Pb date
of the inclusion (Fig. 8). Obvious microcracks intersect
more of the inclusions that yield Cenozoic dates than
those that yield older dates.
Although most monazite inclusions appear to be
single grains, some inclusions are located adjacent to
other inclusions (Fig. 9). If both inclusions are
sufficiently large, each may be dated. In the case of
Fig. 9. BEIs of paired monazite inclusions in garnet from two Greater
Himalayan samples. A) Sample 502126, inclusions L15 and L16.
These inclusions are separated by ∼ 1 μm in this view, although they
may coalesce into a single grain out of the plane of observation. These
inclusions yield radically different Th/Pb and 238U/206Pb dates, and
neither of the dates is concordant. Neither date is a crystallization age,
thus no tectonic significance can be assigned to either date. B) Sample
502091, inclusion D5. The shape of this inclusion suggests that it is an
amalgam of at least 2 monazites. The distribution of dates within the
inclusion supports this interpretation. Grt—garnet. Th/Pb dates are
shown in upright font and 238U/206Pb dates in italic font.
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
11
Fig. 10. X-ray maps of inclusion T4–F2 in garnet from Greater Himalayan sample 502050 from the Modi Khola transect. Fonts as in Fig. 9.
inclusion T4_F2 in sample 502050 (Fig. 10). This stripe
also has low Ca concentrations relative to the rest of the
grain, particularly the right side of the stripe. The
correlations of Y, Th, Si, and Ca concentrations indicate
changing importance of Th substitution vectors in
monazite inside and outside the stripe. Outside the
stripe, paired substitution of Ca + Th for 2 REE, the
brabantite substitution, appears to dominate. The
huttonite paired substitution of Th + Si for REE + P is
more important inside than outside the stripe, however.
Although Th/Pb and 238U/206 Pb dates inside and outside
the stripe are the same within error, the element zoning
patterns in this inclusion indicate intergrowth of at least
two different generations of Cenozoic monazite.
Monazite inclusion T4_A1 in sample 502067 has at
least 3 distinct sectors defined by differing Y and Th
concentrations (Fig. 11). These areas also have radically
different Th/Pb and 238 U/206Pb dates, which demonstrate that multiple generations of monazite intergrew to
form this composite inclusion. It would not be possible
to reach this conclusion based on textural information
alone.
Individual zones within many inclusions are too
small to date without overlap with neighboring zones,
Fig. 11. X-ray maps of inclusion T4–A1 in garnet from Greater Himalayan sample 502067 from the Modi Khola transect. The right side of the
monazite inclusion touches a quartz inclusion in the garnet. Fonts as in Fig. 9.
12
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
Fig. 12. X-ray maps of inclusion T4–B2 in garnet from Greater Himalayan sample 502067 from the Modi Khola transect. Zoning at the left of the
grain is too small to allow dating of individual zones. Although most parts of the right of the grain show little zoning, the highly discordant analysis
and Mesozoic dates indicate that this sector of the inclusion consists of intergrown Paleozoic and Cenozoic monazite. Fonts as in Fig. 9.
however, and some whole inclusions and parts of
inclusions display little zoning of Y and Th (Fig. 12).
For these monazites, only comparison of Th/Pb and U/
Pb dates can help decipher which dates are likely to
represent crystallization ages and which result from
intergrowth of different generations of monazite.
Because some analyses from apparently unzoned sectors
of inclusions yield highly discordant dates (Fig. 12, right
analysis), even some of these monazites are composite
grains composed of several generations of monazite
intergrown at the sub-micron scale.
5.2. Garnet
Because monazites are included in garnets, the
growth and deformational history of the garnets can
yield insight into the significance of (U,Th)/Pb dates of
the included monazites. Except for extremely large
grains, garnets in Greater Himalayan rocks from the
Annapurna Range usually show flat major element
zoning in their cores and modification of this zoning near
their rims (Fig. 13, ESM6), although the scale of the
modification varies depending on cooling rate (Vannay
and Hodges, 1996; Catlos et al., 2001; Martin, 2005).
This pattern was produced by homogenization of major
element zoning during metamorphism to ∼ 700 °C
followed by retrograde modification at the rim and
diffusion of these modifications into the garnet. Thus
information about growth history is not preserved in
major element zoning patterns in most Greater Himalayan garnets.
Y zoning in some Greater Himalayan garnets is
subdued, including garnets from sample 502067 (Fig. 13).
We do not have absolute Y concentration information for
these garnets, but expect low concentrations based on the
conclusions of Pyle and Spear (1999) for xenotimebearing high grade metapelitic rocks from the northeast-
ern USA. The small difference in Y concentration
between garnet and biotite shown in the X-ray map
supports this inference because biotite generally contains
less than 1 or 2 ppm Y (Bea et al., 1994; Yang et al., 1999;
Yang and Rivers, 2000). Minimal addition of Y at
the rim during retrograde dissolution of the garnets
in this rock indicates that little Y was released during
breakdown of these garnets, again suggesting relatively
low Y concentrations.
Garnets in these rocks also show pervasive microcracks, including microcracks at the rims of the garnets
(Fig. 13, ESM6). Some of these microcracks clearly
control retrograde metamorphism and metasomatism
inside the garnet. Microcracks generally cut the major
element zoning patterns in garnet with no disruption of
zoning.
6. Interpretations
6.1. Comparisons between all
232
Th/208Pb dates
238
U/ 206 Pb and
Fig. 14 shows plots of all of the Greater Himalayan
U/206 Pb and 232 Th/208Pb dates. Several trends are
apparent on these plots. First, it is clear that only dates
between ∼ 400–500 Ma and ∼ 12–46 Ma are concordant within 10%. Analyses between ∼ 47–400 Ma and
younger than ∼ 12 Ma are discordant, without exception. All analyses that yield Th/Pb dates younger than
∼ 11 Ma are more than 35% discordant.
Second, 52% of the analyses yield 232Th/208 Pb dates
older than corresponding 238 U/206Pb dates, whereas
48% yield younger 2 32 Th/ 20 8 Pb dates. Older
232
Th/208Pb dates cannot result from excess 206Pb but
instead must result from partial Pb loss or, more likely,
from intergrowth of different generations of monazite.
Excess 206 Pb might explain some of the older
238
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
Fig. 13. BEI of, and X-ray maps of Mn and Y in, garnet from sample
502067 from the Modi Khola transect. Microcracks continue to the rim
of the garnet, and some microcracks clearly control retrograde
metamorphism inside the garnet. There is no change in density of
microcracks between the rim and core of the garnet. Y zoning is
subdued and Y concentrations appear to be low in this garnet,
consistent with growth continuing to high temperatures. Minimal
addition of Y at the rim during retrograde dissolution of this garnet
indicates that little Y was released during breakdown of the garnet, in
contrast to Mn. High Mn at the right-center of the garnet results from
exchange with an inclusion of chlorite + biotite. Ap—apatite, bt—
biotite, chl—chlorite, grt—garnet, ky—kyanite, mnz—monazite, ms
—muscovite, q—quartz, rt—rutile.
238
U/206Pb dates. However, we conclude that many of
the older 238U/206Pb dates likely result from partial Pb
loss or intergrowth of different generations of monazite
13
because these same processes explain the older
232
Th/208 Pb dates, commonly from monazites from the
same sample or nearby samples.
Third, monazites intersected by obvious microcracks
are as likely to yield concordant ages as monazites not
intersected by obvious microcracks. 61% of the 61
concordant ages come from monazite crystals intersected by obvious microcracks and 59% of the 135
discordant dates come from this type of monazite.
Further, the mean and the median discordance of
analyses from monazites intersected by obvious microcracks are nearly identical to the mean and median
discordance of analyses from monazites not intersected
by obvious microcracks. The median discordance for
grains intersected by obvious microcracks is 22%
whereas the median discordance for grains not intersected by obvious microcracks is 21%, and the mean
discordance for both types of grains is 47%. There are
two alternative, though not mutually exclusive, interpretations for the similarity of these numbers. First, it is
possible that some younger monazite intergrew with
older grains prior to inclusion in garnet, thus introducing
discordance prior to garnet growth. Second, it is also
possible that microcracks control the discordance of
most inclusions, though the microcracks may not be
obvious in a particular section through a host garnet.
Microcracks likely intersect many apparently nonintersected inclusions out of the plane of the section.
Additionally, garnets from poly-metamorphosed rocks
sometimes contain healed microcracks, and it is possible
that some of the inclusions were intersected by
microcracks that subsequently annealed, rendering
them very difficult to identify (e.g. Le Bayon et al.,
2006). We prefer the second interpretation because the
correspondence between the Th/Pb date of an inclusion
and the presence of an obvious microcrack indicates that
microcracks exerted some control on growth of most
Cenozoic monazite grains (Fig. 8).
Fourth, 86% of the 56 analyses that yield Th/Pb dates
between 47 and 399 Ma have a Th/Pb date that is older
than the corresponding 238U/206Pb date. This pattern
probably results from mixing during laser ablation of
different generations of monazite that have different
232
Th/238 U ratios. If lower Paleozoic monazite usually
has a higher 232 Th/238U ratio than Cenozoic monazite
(Fig. 7), then a Th/Pb date older than the corresponding
238
U/206 Pb date should result from mixture of the two
generations of monazite. The few analyses that yield
older 238U/206 Pb dates probably result from uncommon
mixture of lower 232 Th/238U lower Paleozoic monazite
with higher 232 Th/238U Cenozoic monazite during laser
ablation.
14
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
Fig. 14. Comparisons of Greater Himalayan 232Th/208Pb and 238U/206Pb dates. A) All Greater Himalayan analyses. B) Greater Himalayan analyses
that yield dates younger than 55 Ma. Squares indicate analyses from inclusions intersected by obvious microcracks, circles indicate analyses from
inclusions not intersected by obvious microcracks. The solid diagonal lines indicate equal 232Th/208Pb and 238U/206Pb dates. The dashed diagonal
lines indicate 10% difference between 232Th/208Pb and 238U/206Pb dates. Only analyses that yield dates between ∼500–400 Ma and ∼45–12 Ma are
concordant within 10%. 2-sigma errors average ∼ 12%, but are omitted for clarity.
6.2. Interpretations based on age groups
6.2.1. 505–400 Ma dates
Several lines of evidence, including early Paleozoic
monazite ages, indicate that Greater Himalayan rocks
experienced metamorphism accompanied by monazite
growth at ∼ 500 Ma (Stocklin and Bhattarai, 1977;
Stocklin, 1980; Garzanti et al., 1986; Hodges et al.,
1996; Argles et al., 1999; Marquer et al., 2000; Godin
et al., 2001; Viskupic and Hodges, 2001; Catlos et al.,
2002b; Gehrels et al., 2003; 2006; Myrow et al., 2006).
Accordingly, we interpret the 505–400 Ma dates from
the Annapurna Range samples to represent monazite
growth at this time. The strongest data that support this
interpretation are the 4 analyses that yield concordant
ages within this age range. The 505 ± 46 and 474 ± 29
concordant ages might record prograde metamorphism
whereas the 422 ± 18 and 403 ± 74 concordant ages
might record retrograde metamorphism of Greater
Himalayan rocks. Fig. 4 shows that 505–400 Ma
monazite generally has low to medium Th concentrations; the mean Th concentration from the four
concordant analyses that yield ages in this range is
3.1%. None of the nine inclusions that yielded Th/Pb
ages between 505 and 400 Ma was large enough to
allow dating of a second spot within the inclusion,
precluding direct comparison of Th and Y concentrations of lower Paleozoic and Cenozoic monazite within
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
a single inclusion. However, X-ray maps of monazites
that yielded Th/Pb dates between 399 and 56 Ma
suggest that Y and Th concentrations in 505–400 Ma
monazites are low relative to many Cenozoic grains. For
example, in Fig. 11, the sector of the inclusion that
yields a Th/Pb date of 318 ± 40 Ma has low Y and Th
concentrations relative to the parts that yield Cenozoic
dates. Because the 318 Ma date is not a crystallization
age but instead results from mixture of 505–400 Ma
monazite with Cenozoic monazite (see next section), Y
and Th concentrations in the 505–400 Ma monazite
must be low in order to maintain relatively low
concentrations after mixture with the high Y, high Th
Cenozoic monazite.
6.2.2. 399–56 Ma dates
The continuous distribution of dates between 505 ±
46 and 6 ± 1 Ma shown in Figs. 3, 6, ESM3, and ESM4,
with no date unrepresented between 505 and 6 Ma,
strongly suggests that the Th/Pb dates between 399 and
56 Ma are not crystallization ages but instead result from
intergrowth of middle or upper Cenozoic monazite with
pre-existing lower Paleozoic monazite and/or loss of Pb
from lower Paleozoic monazite. The recognition that
none of these analyses that yield dates between 399 and
56 Ma is concordant lends compelling support to this
interpretation. Textural information and the distribution
of major elements in these monazite inclusions also
substantiate this interpretation, because some grains
show clear evidence for intergrowth of Paleozoic and
Cenozoic monazite (Fig. 11). Although inclusions L15
and L16 in sample 502126 are separated by ∼ 1 μm in
the view shown in Fig. 9A, they might coalesce into one
grain out of the plane of observation. In this case,
an analysis of the grain would yield mixed, and thus
meaningless, Th/Pb and U/Pb dates. Coalescence of
grains and mixing of ages explains the texture of, and
dates from, inclusion D5 in sample 502091 (Fig. 9B).
The discordant date in the upper left of the composite
grain is not a crystallization age; instead it results from
intergrowth of younger monazite with older monazite. It
is likely that many inclusions are composed of monazite
intergrown in this way or on a finer scale, because large
grains that show no textural evidence for intergrowth
often show chemical evidence for such intergrowth
(Figs. 10 and 11). In most grains, however, zones are too
small to date or the grain shows neither textural nor
chemical evidence for intergrowth, leaving comparison
of Th/Pb and U/Pb dates the only available method for
discriminating between dates that likely represent crystallization ages and those that do not (Fig. 12). This
explanation for the 399–56 Ma dates is more likely than
15
one that calls for growth of monazite at this time, because metamorphism of this age is unknown for Greater
Himalayan rocks or any rocks exposed in the Himalaya.
The increased incidence of obvious microcracks that
intersect the inclusions that yield dates in this range
suggests that microcracks allow communication between the interior and exterior of garnets. That is,
microcracks permit mass exchange between a monazite
inclusion and the exterior of its host garnet, resulting in
Pb loss from and/or new monazite intergrowth with the
inclusion. The recognition that microcracks control
retrograde metamorphism in the interior of garnet supports this interpretation (Fig. 13).
6.2.3. 55–6 Ma dates
There are at least six possible explanations for the
55–6 Ma monazite dates: 1) Pb loss from monazite that
crystallized at ∼ 500 Ma. 2) Intergrowth of mid-upper
Cenozoic monazite with ∼ 500 Ma monazite, resulting
in a mixed date upon laser ablation. 3) Monazite growth
and inclusion in garnet during Cenozoic prograde
metamorphism with no older or younger monazite
component. 4) Monazite growth within garnet during
Cenozoic retrograde metamorphism and metasomatism
with no older monazite component. 5) Monazite growth
during Cenozoic prograde metamorphism followed by
intergrowth of Cenozoic retrograde monazite, resulting
in a mixed date. 6) Any other combination of two or
more of these possibilities. Only dates from monazites
that grew as described in explanations 3 and 4 are
crystallization ages, the remainder are mixed dates or
result from Pb loss. In the following paragraphs we
explore each of the first five explanations.
The continuous distribution of dates shown in
Figs. 3, 6, ESM3, and ESM4 suggests that some of
the 55–6 Ma dates likely result from Pb loss from and/or
mid-late Cenozoic intergrowth with lower Paleozoic
monazite crystals (explanations 1 and 2). The patterns of
concordance of these dates substantiate this interpretation. None of the analyses that yield Th/Pb dates
between ∼46 and 55 Ma is concordant, so we infer that
these analyses do not represent crystallization ages.
Likewise, we interpret the discordant analyses that yield
Th/Pb dates between ∼ 46 and 6 Ma to not represent
crystallization ages. We further discuss the dates
between 12 and 6 Ma below. Some of the discordant
dates between ∼ 46 and 6 Ma very likely were produced
by extensive mixing of middle or upper Cenozoic
monazite with lower Paleozoic monazite during laser
ablation of intergrown lower Paleozoic and upper
Cenozoic monazite crystals. For example, if the
monazite grains shown in Figs. 9 and 11 were slightly
16
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
smaller, the observed dates would commingle and
produce a date that is a mixture of these dates. Further,
it is likely that the discordant dates obtained for the
grains shown in these figures are actually mixed
Paleozoic and Cenozoic ages produced by intergrowth
of monazite on a scale too small to detect except by
testing for concordance.
Some of the inclusions that yield concordant mid-late
Cenozoic ages may consist of monazite that crystallized
only at this time, thus these grains might contain no
component of lower Paleozoic monazite (explanations
3, 4, and 5). Monazite probably crystallized during midCenozoic prograde metamorphism, and then garnet may
have grown around and completely included the newlyformed monazite crystal (explanation 3). Monazite may
grow during prograde metamorphism due to the
breakdown of a combination of allanite, garnet,
muscovite, biotite, ThSiO4, thorianite, apatite, and/or
other phosphates (Spear and Pyle, 2002; Wing et al.,
2003; Kohn and Malloy, 2004; McFarlane et al., 2005).
Monazite also likely grew inside garnet during midlate Cenozoic retrograde metamorphism and metasomatism (explanation 4). Most garnet contains high
enough concentrations of light REE, P, Y and Th to
crystallize monazite upon breakdown of the garnet
(Kohn and Malloy, 2004; McFarlane et al., 2005).
Retrograde breakdown of garnet alone may have
allowed the formation of some monazite inside some
garnet, but in many cases monazite growth inside garnet
probably was supplemented by mass exchange with the
exterior of the garnet via microcracks. Fig. 15 shows
that abundant P moved along mica foliation planes and
also through cracks and microcracks in this garnetbearing Greater Himalayan rock. The lack of obvious
crystallized melt along the foliation planes, the micro-
cracks, or elsewhere in the rock suggests that P was not
transported in a melt, but rather in an aqueous fluid. This
P probably was liberated by the breakdown of minerals
that also contain light REE, Y, and Th such as garnet,
muscovite, biotite, plagioclase feldspar, thorianite,
ThSiO4, monazite, apatite, and/or other phosphate
minerals. These elements, dissolved in an aqueous
fluid, probably entered garnet through microcracks and
precipitated as young monazite intergrowths with
monazite inclusions already in garnet or as entirely
new crystals. Whitney and Dilek (1998) propose a
similar mechanism to explain the growth of major
minerals such as biotite and sillimanite inside garnet.
Although the importance of aqueous fluids for monazite
crystallization previously has received little attention in
the Himalaya (Bollinger and Janots, 2006), crystallization of monazite assisted by aqueous fluids has been
inferred in numerous other places (e.g. DeWolf et al.,
1993; Hawkins and Bowring, 1997; Crowley and
Ghent, 1999; Poitrasson et al., 2000; Rasmussen et al.,
2001; Townsend et al., 2001).
It is also likely that Cenozoic retrograde monazite
grew on older Cenozoic prograde monazite, resulting in
an intergrown monazite grain (explanation 5). Most of
the Th/Pb dates between ∼ 12 and 6 Ma probably result
from such intergrowth, although small contributions
from lower Paleozoic monazite are possible. Analyses
from Greater Himalayan monazite inclusions that yield
dates younger than ∼ 12 Ma are discordant, without
exception, and all analyses that yield dates younger than
∼ 11 Ma are more than 35% discordant. The lone
monazite grain that yields a concordant age of 12 ± 2 Ma
comes from rocks from the Marsyangdi transect; in
other transects the youngest concordant ages are ∼13 or
14 Ma. Thus growth of monazite inclusions large
Fig. 15. BEI of, and X-ray map of P in, part of sample 502069 from the Modi Khola transect. Intensities on the P map inverted so that light areas
indicate low concentrations and dark areas high concentrations. Both mica foliation planes and cracks and microcracks controlled the movement of P
in this Greater Himalayan rock. P likely moved through the rock dissolved in an aqueous fluid. Arrows on the P map indicate cracks with moderate P
concentrations. Large patches of high P concentrations are apatite crystals.
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
enough to yield concordant ages ended at ∼ 14–12 Ma.
However, the presence of inclusions from the Marsyangdi and Nayu Ridge transects that yield extremely
discordant dates between 12 and 6 Ma indicates that
limited monazite growth or Pb loss continued in Greater
Himalayan rocks on the eastern side of the Annapurna
Range until at least 6 Ma, and probably later. Limited
monazite growth or Pb loss both suggest processes
active at relatively low temperatures or over short time
intervals. Because diffusion of Pb in monazite is slow
below ∼ 700 °C, limited monazite growth is a more
likely explanation for the 12–6 Ma dates than limited Pb
loss (Smith and Giletti, 1997; Cherniak et al., 2004;
McFarlane and Harrison, 2006). These considerations
lead us to conclude that all of the Th/Pb dates younger
than ∼ 12 Ma record limited monazite growth during
retrograde metamorphism at temperatures low enough
to grow only small quantities of monazite.
Published 40Ar/39Ar ages of muscovite from rocks
collected directly above the MCT in the Marsyangdi
valley support this interpretation. Edwards (1995)
obtained a muscovite 40Ar/39Ar age of 6.2 ± 0.2 Ma
from Greater Himalayan rocks very near our sample
502091, and thus at a similar structural position to our
samples 402086, 402088, 402089, 402091B, and
402092B (Fig. 2). The structurally lowest of these
samples, 402092B, yielded two very discordant monazite Th/Pb dates that overlap this 40Ar/39Ar age within
error: 8 ± 4 and 9 ± 3 Ma. The similarity to the muscovite
40
Ar/39Ar age suggests that these Th/Pb dates record
limited intergrowth of monazite at temperatures below
the muscovite closure temperature of ∼ 350 °C (Purdy
and Jager, 1976; Jager, 1979) after 8–9 Ma rather than
crystallization of monazite and subsequent inclusion by
garnet at temperatures above 500 °C at 8–9 Ma.
Fig. 4 shows that Th concentrations of spots that
yielded Cenozoic dates range from less than 1 wt.% to
more than 28 wt.%. However, all of the analyses with Th
concentrations greater than ∼12% yielded Cenozoic
dates. The mean Th concentration of the monazites that
yielded concordant Cenozoic ages is 8.1%, significantly
higher than the Th concentration of the monazites with
concordant early Paleozoic ages. X-ray maps of monazite
inclusions that preserve large remnants of lower Paleozoic
monazite intergrown with Cenozoic monazite also show
that the Cenozoic monazite in these obviously composite
grains usually has higher concentrations of Th and Y than
the lower Paleozoic monazite (Fig. 11). Although
different generations of Cenozoic prograde and retrograde
monazite probably have different concentrations of Y and
Th (c.f. Kohn et al., 2004), the presence of intergrowths of
lower Paleozoic monazite obscures these patterns in the
17
inclusions in garnet. Matrix monazite is more likely to
display clearer zoning patterns of Y and Th produced by
episodes of monazite growth during Cenozoic prograde
and retrograde metamorphism because the preservation
potential during later metamorphism of lower Paleozoic
monazite in the matrix is much less than its preservation
potential as inclusions in garnet (Catlos et al., 2001,
2002b, 2004; Kohn et al., 2004).
The recognition that microcracks intersect the
inclusions that yield 69% of the dates from 6–55 Ma
strongly suggests that microcracks play an important
role in the genesis of many of these dates. Some
monazite inclusions are not intersected by obvious
microcracks in the host garnet, however. Microcracks
could intersect these inclusions out of the plane of
observation. A 3-dimensional tomographic image of the
garnet prior to sectioning would be required to assess
this possibility. It is also possible that healed microcracks intersect many inclusions, but it is difficult to
recognize healed microcracks in garnet, particularly in
grain mount (Le Bayon et al., 2006). Either possibility
would allow Pb loss from and/or younger monazite
intergrowth with an inclusion.
7. Implications for geochronology and tectonics
7.1. Tectonic implications of the monazite age data
The concordant ∼ 500–400 Ma monazite inclusions
record metamorphism of Greater Himalayan rocks at
this time, supporting previous interpretations (Stocklin
and Bhattarai, 1977; Stocklin, 1980; Garzanti et al.,
1986; Hodges et al., 1996; Argles et al., 1999; Marquer
et al., 2000; Godin et al., 2001; Viskupic and Hodges,
2001; Catlos et al., 2002b; Gehrels et al., 2003, 2006;
Myrow et al., 2006; Cawood et al., 2007). Our work is
the first to document concordant lower Paleozoic
monazite in Greater Himalayan Formation I rocks. We
speculate that the ∼ 505–475 ages record prograde
metamorphism whereas the ∼ 425–405 ages record
retrograde metamorphism of Greater Himalayan rocks.
Only 9 of 196 monazite analyses yielded Th/Pb dates
within 20% of 500 Ma, however, which indicates that Pb
loss and/or monazite intergrowth during Cenozoic
metamorphism strongly affected lower Paleozoic monazite (Figs. 3, 6, 8, ESM3, ESM4).
A relative probability plot of all 57 concordant
Cenozoic ages draws attention to the temporal patterns
of Cenozoic metamorphism of the Greater Himalayan
rocks of the Annapurna Range (Fig. 16). Most of the
ages fall into two groups, 42–29 Ma and 22–12 Ma,
with very few ages between 29 and 22 Ma and only one
18
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
Fig. 16. Relative probability plot and histogram of all 57 concordant Cenozoic monazite inclusions in Greater Himalayan garnets. Relative
probability, shown by the black curve, incorporates both ages and errors on the ages. Each gray box shows the number of ages in a particular 1 million
year interval. The concordant ages define two groups, 42–29 Ma and 22–12 Ma, with few ages between 29 and 22 Ma and only one age older than
42 Ma. We suggest that the old group represents prograde metamorphism and the young group records retrograde metamorphism. We cannot
distinguish between prograde and retrograde growth for the very few monazites between ∼ 29 and 22 Ma, but Th concentrations appear to match
those of the retrograde group.
age older than 42 Ma. We suggest that the older age
group represents monazite growth during prograde
metamorphism and that the younger age group records
monazite growth during retrograde metamorphism. If
this interpretation is correct, we expect differences in the
chemical compositions of the two generations of
monazite because different reactions produced the
monazites (c.f. Kohn et al., 2004). There is a small but
systematic difference in Th concentrations: the 42–
29 Ma concordant monazites have a mean Th
concentration of 6.4 wt.% and the 22–12 Ma concordant
monazites have a mean Th concentration of 10.4 wt.%
(Fig. 17). The younger group also has a much greater
range of concentrations. Unfortunately, we have X-ray
maps of only a few inclusions composed of both 42–
29 Ma and 22–12 Ma monazite. One of these inclusions
comes from sample 502067 from the Modi Khola
transect (Fig. 11). The Y concentration of the 31 ± 4 Ma
sector of this composite grain is much higher than the Y
concentration of the 19 ± 5 Ma sector, whereas the Th
concentration of the 31 Ma part is somewhat lower than
the Th concentration of the 19 Ma part (see also Table
ESM1). Interaction with the relatively low Y garnets in
sample 502067 (Fig. 13) might explain the relative Y
concentrations of these two parts of the inclusion. If
garnets growing in this rock sequestered relatively little
Y, then enough Y might be available to grow relatively
high Y monazite during prograde metamorphism
at ∼ 31 Ma. In contrast, garnet breakdown during
retrograde metamorphism released relatively little Y
(Fig. 13), making little Y available to monazite growing
at ∼ 19 Ma. The difference in Th concentrations
between the two generations of monazite is more
difficult to explain, but might result from different
reactions involving high Th minerals such as ThSiO4,
thorianite, older generations of monazite, or perhaps
apatite during prograde and retrograde metamorphism.
Burial of the Greater Himalayan series by thrust sheets
Fig. 17. Th/Pb age versus Th concentration for all 57 concordant
Cenozoic monazite inclusions. Open circle: 46 Ma analysis, closed
circles: 42–29 Ma analyses, x symbols: 28–23 Ma analyses, squares:
22–12 Ma analyses. The mean Th concentration of the 42–29 Ma
group is 6.4% and the mean Th concentration of the 22–12 Ma group
is 10.4%. Vertical lines show the means for each of these two groups.
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
composed of Tethyan rocks likely caused prograde
metamorphism of the Greater Himalayan rocks between
42 and 29 Ma, a conclusion broadly consistent with
results from the western Himalaya (Vance and Harris,
1999; Foster et al., 2000; Wiesmayr and Grasemann,
2002). Exhumation caused by slip on the STDS, MCT,
Ramgarh thrust, and other nearby faults probably drove
retrograde metamorphism of the Greater Himalayan
series between 22 and 12 Ma, although continuous
activity on each of these faults throughout this time
period is unlikely.
Although the oldest concordant Cenozoic monazite
has a Th/Pb age of 46 ± 1 Ma, all but one of the
concordant Cenozoic analyses yield Th/Pb ages of
42 Ma or younger. Indeed, ages between ∼ 37 and
29 Ma dominate the largest peak on the relative
probability plot. We conclude that Cenozoic monazite
growth in some Greater Himalayan rocks in the
Annapurna Range might have begun by 46 Ma, but
abundant monazite growth in most Greater Himalayan
rocks began at ∼ 42 or even 37 Ma.
de Sigoyer et al. (2000) and Leech et al. (2005) show
that Cenozoic prograde metamorphism of the Tso
Morari complex in the western Himalaya to medium
or higher grades was ongoing at ∼55 Ma. Parrish et al.
(2006) demonstrate that rocks in a similar structural
position in the Kaghan Valley of northern Pakistan
reached temperatures of 720–770 °C and pressures of
N 27.5 kbar at 46.4 Ma. These rocks from directly south
of the Indus–Yarlung suture thus reached temperatures
conducive to monazite growth (perhaps 300 to 400 °C)
about 5 to 15 M.y. before Greater Himalayan rocks of
the Annapurna Range reached similar temperatures.
This temporal difference in the age of Cenozoic heating
probably results from a spatial difference in the
positions of the near-suture and Annapurna rocks prior
to collision. The age data indicate that the Tso Morari
and Kaghan rocks were located outboard of (farther
north than) the Annapurna Greater Himalayan rocks
prior to collision of India and Asia, and thus subduction
and metamorphism of the Tso Morari and Kaghan rocks
began before subduction and metamorphism of these
Greater Himalayan rocks.
The termination of growth of large monazite inclusions in Greater Himalayan garnets at ∼12–14 Ma may
record the cooling of Greater Himalayan rocks through
some critical temperature range above which large
crystals can grow and below which they cannot. This
critical temperature range could control either the
reactions that break down minerals that contain the
chemical constituents of monazite or the reactions that
produce monazite from these constituents, or both. The
19
discordant dates younger than ∼ 12 Ma from the
Marsyangdi and Nayu Ridge transects demonstrate that
conditions on the eastern side of the Annapurna Range
remained conducive to limited monazite growth until at
least 6 Ma, especially near the MCT. As discussed in the
Interpretations section, we consider partial Pb loss from
older grains an unlikely explanation for these young dates
because Pb diffusion in monazite is so slow at medium to
low temperatures that significant resetting of ages by Pb
loss is unlikely (Smith and Giletti, 1997; Cherniak et al.,
2004; McFarlane and Harrison, 2006). Because the
young dates result from mixing of very young monazite
with ≥12 Ma monazite, it is likely that growth of small
quantities of monazite continued after 6 Ma for an
unknown length of time. Monazite growth may have
continued during cooling to temperatures as low as 260–
400 °C (Poitrasson et al., 2000; Rasmussen et al., 2001;
Townsend et al., 2001; Harlov et al., 2002; Harlov and
Forster, 2003; Bollinger and Janots, 2006). The eastern
side of the Annapurna Range may have remained warm
enough for monazite growth later than the central and
western parts of the range, or other conditions in the
eastern part of the range might have allowed limited
monazite growth to continue at lower temperatures than
in other parts of the range. For example, the dependence
of monazite stability on Th/U ratio is unknown. Higher
Th/U ratios, such as those found in monazite inclusions
from the Marsyangdi transect, might allow monazite
growth at lower temperatures than necessary for monazite
growth with lower Th/U ratios.
7.2. Using monazite for geochronology
The utility of monazite inclusions in garnet from
Greater Himalayan rocks for geochronology depends on
which monazite growth episode is of interest. Inclusions
in garnet are the only monazite crystals that remain intact
from the early Paleozoic metamorphism of Greater
Himalayan rocks; matrix monazite grains partially or
completely recrystallized during Cenozoic metamorphism (Catlos et al., 2001, 2002b; Gehrels et al., 2003;
Catlos et al., 2004; Kohn et al., 2004; Gehrels et al.,
2006). Consequently, investigations of early Paleozoic
metamorphism of Greater Himalayan rocks using
monazite should focus on inclusions in garnet.
The presence of remnants of lower Paleozoic
monazite crystals complicates the use of monazite
inclusions for studying the details of Cenozoic metamorphism of Greater Himalayan rocks, however. Because
microcracks allow communication between the interior
and the exterior of a grain, garnet only partially shields
monazite from Pb loss and/or intergrowth of new
20
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
monazite. Microcracks are ubiquitous, so many inclusions are complex intergrowths of combinations of lower
Paleozoic monazite, Cenozoic prograde monazite, and
Cenozoic retrograde monazite. Mixed dates from such
intergrown monazites usually have no tectonic significance because they are not crystallization ages. The
multiple generations of monazite intergrown throughout
single composite inclusions complicate interpretations of
element zoning patterns and (U,Th)/Pb dates, making
interpretation of the details of Cenozoic metamorphism
more difficult.
It may sometimes be easier to interpret the growth
history of matrix monazite crystals (e.g. Kohn et al.,
2004). Because the preservation potential during later
high grade metamorphism of lower Paleozoic matrix
grains is less than the preservation potential of lower
Paleozoic crystals included in garnet, interpretations of
(U,Th)/Pb dates, element zoning patterns, and textural
relationships of matrix monazite need not necessarily
involve lower Paleozoic monazite, only Cenozoic
monazite. Interpretation of (U,Th)/Pb dates of matrix
monazite is thus often simpler than interpretation of dates
of monazite inclusions because there are fewer likely
explanations of matrix monazite dates. However, because
garnet still partially shields inclusions from Pb loss and
intergrowth of new monazite, inclusions in garnet contain
the records of the earliest Cenozoic metamorphism of
Greater Himalayan rocks. Later Cenozoic metamorphism
overprinted much of the early-formed Cenozoic monazite
that remained in the matrix.
garnet are dubious. Second, several authors make clear
that monazite can grow at temperatures as low as 260–
400 °C, especially during metasomatism (Poitrasson
et al., 2000; Rasmussen et al., 2001; Townsend et al.,
2001; Harlov et al., 2002; Harlov and Forster, 2003;
Bollinger and Janots, 2006). Since Greater Himalayan
rocks experienced retrograde metamorphism and metasomatism during cooling from ∼ 700 °C (e.g. Ganguly
et al., 2000; Stephenson et al., 2000; Daniel et al., 2003;
Kohn et al., 2004; Martin, 2005), it is likely that
significant monazite growth occurred on the retrograde
path. For example, all of the dates younger than ∼ 12 Ma
record limited monazite growth during retrograde
metamorphism. Therefore, conclusions based on the
assumption that most or all monazite growth occurred
during prograde metamorphism are also suspect (c.f.
Bollinger and Janots, 2006). Third, because Cenozoic
prograde and retrograde metamorphism produced multiple generations of monazite in Greater Himalayan
rocks, it is usually inappropriate to treat most or all
Cenozoic dates as a single population. Finally, because
matrix monazites are completely unshielded from
recrystallization mediated by aqueous fluids in the
rock, whereas inclusions in garnet are partially shielded,
interpretations of the significance of ages from the two
types of monazite should not necessarily proceed in the
same manner. It is not obvious that ages from the two
types of monazite can be compared directly or grouped a
priori, without justification.
Acknowledgments
7.3. Implications for previous studies
Our new data, combined with previous studies of low
temperature growth of monazite, shed additional light
on the data and conclusions of previous workers who
used in situ (U,Th)/Pb methods to date monazite
inclusions in Greater Himalayan garnets. Four points
seem most relevant. First, the discovery of inclusions
that demonstrably consist of intergrowths of different
generations of monazite indicates that the assumption
that garnet shields inclusions from recrystallization and
intergrowth of younger monazite is likely invalid for
many inclusions. Instead, microcracks probably allow
communication between the interior and the exterior of
garnet. Most monazite inclusions yield discordant
238
U/206 Pb and 232Th/208Pb dates, and these dates are
not crystallization ages. Discordant dates should not be
combined with concordant ages to draw conclusions
about thrust belt kinematics. Accordingly, conclusions
based on the assumption that all dates are crystallization
ages or on the assumption of complete shielding by
We thank Martin Herrmann, Steven Kidder, Tank
Ojha, and the staff of Himalayan Experience for field
support; Fairlee Fabrett, Jennifer Pullen, and Joel Saylor
for help with sample preparation; Kenneth Domanik for
assistance with the electron microprobe; and Jibamitra
Ganguly for numerous illuminating discussions.
Reviews by Tom Argles, James Crowley, Kip Hodges,
two anonymous reviewers, and editor Steven Goldstein
significantly improved the manuscript. Supported by
NSF grants EAR-0105339 and EAR-0207179. A.J.M.
was supported by GSA, AAPG, the Department of
Geosciences at the University of Arizona, and ExxonMobil as a donor to the Geostructure Partnership at the
University of Arizona.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.chemgeo.
2007.05.003.
A.J. Martin et al. / Chemical Geology 244 (2007) 1–24
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