Acc
epte
d A
rtic
le
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which may
lead to differences between this version and the Version of Record. Please cite this article as
doi: 10.1111/jmg.12457
This article is protected by copyright. All rights reserved.
Petrology and zircon U-Pb dating of well-preserved eclogites from the Thongmön area
in central Himalaya and their tectonic implications
Q.Y. LI 1, L.F. ZHANG
1*, B. FU
2, T. BADER
1and H.L. Yu
1
1MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space
Science, Peking University, Beijing 100871, P.R. China
2Research School of Earth Sciences, the Australian National University, Canberra, ACT 0200,
Australia
* Corresponding author, e-mail address: [email protected]
ABSTRACT
The discovery of eclogites are reported within the Great Himalayan Crystalline Complex
(GHC) in the Thongmön area, central Himalaya, and their metamorphic evolution is
deciphered by petrographic studies, pseudosection modelling and zircon dating. For the first
time, omphacite has been found in the matrix of eclogites taken from a metamorphic mafic
lens. Two groups of garnet have been identified in the Thongmön eclogites on the basis of
major and rare earth elements and mineral inclusions. Core and intermediate sections of
garnet represent Grt I, in which the major elements (Ca, Mg, and Fe) show a nearly
homogenous distribution with little or weak zonation. This Grt I displays an almost flat
chondrite-normalized HREE pattern, and the main inclusions are amphibole, apatite, quartz,
and abundant omphacite. Grt II, forms thin rims on large garnet grains, and is characterized
by rimward Ca decrease and Mg increase and MREE enrichment relative to HREE and
http://crossmark.crossref.org/dialog/?doi=10.1111%2Fjmg.12457&domain=pdf&date_stamp=2018-10-08
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
LREE. No amphibole inclusions are found in Grt II, indicating the decomposition of
amphibole contributed to its MREE enrichment. Two metamorphic stages, recorded by
matrix minerals and inclusions in garnet and zircon, outline the burial of the Thongmön
eclogites and progressive metamorphic processes to the pressure peak: (1) the assemblage of
amphibole-garnet-omphacite-phengite-rutile-quartz, with the phengite interpreted as having
been replaced by Bt+Pl symplectites, represents the prograde amphibole eclogite facies stage
M1(1), (2) in the peak eclogite facies [stage M1(2)], amphibole was lost and melting started.
Based on the compositions of garnet and omphacite inclusions, M1(1) is constrained to 19–20
kbar and 640–660 °C and M1(2) occurred at > 21 kbar, > 750 °C, with appearance of melt and
its entrapment in metamorphic zircon. SHRIMP U–Pb dating of zircon from two eclogite
samples yielded consistent metamorphic ages of 16.7 ± 0.6 Ma, and 17.1 ± 0.4 Ma,
respectively. The metamorphic zircon grew concurrently with Grt II in the peak eclogite
facies. Thongmön eclogites characterized by the prograde metamorphism from amphibolite
facies to eclogite facies were formed by the continuing continental subduction of Indian plate
beneath the Euro-Asian continent in the Miocene.
Key Words: Eclogite; Zircon U–Pb dating; phase equilibria; Central Himalayan orogeny
1 | INTRODUCTION
Eclogites in general have recorded a wealth of important information on the tectonic
evolution of an orogenic belt. Yet, only a few eclogite occurrences have been reported in the
Great Himalayan Crystalline Complex (GHC). Ultrahigh-pressure (UHP) eclogites
containing coesite inclusions have been documented in two areas in the northwestern
Himalayas: the Kaghan Valley in northwest Pakistan (Spencer et al., 1990; Pognante &
Spencer, 1991; Tonarini et al., 1993; O'Brien, Zotov, Law, Khan, & Jan, 2001; Kaneko et al.,
2003; Parrish, Gough, Searle, & Waters, 2006) and the Tso Morari Dome of India (de
Sigoyer, Guillot, Laudeaux, & Mascle, 1997; de Sigoyer et al., 2000; Mukherjee & Sachan,
2001). Granulitized eclogites have been found in four localities in the central Himalayas: the
Ama Drime Massif (ADM) of China (Lombardo, Pertusati, Rolfo, & Visoná, 1998; Groppo,
Lombardo, Rolfo, & Pertusati, 2007; Liu, Siebel, Massonne, & Xiao, 2007; Cottle et al.,
2009a; Cottle, Searle, Horstwood, & Waters, 2009b; Wang, Zhang, Zhang, & Wei, 2017),
northwestern Bhutan (Chakungal, 2006; Chakungal, Dostal, Grujic, Duchêne, & Ghalley,
2010; Warren et al., 2011), Sikkim in India (Kellett, Cottle, & Smit, 2014), and the Arun
Valley in Nepal (Corrie, Kohn, & Vervoort, 2010). However, the omphacite inclusions in
zircon and garnet reported by Wang et al. (2017) constitute the only direct evidence that an
eclogitization event has been overprinted strongly by a granulite facies metamorphic event. In
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
contrast with the UHP eclogites from the northwestern Himalayas, the granulitized eclogites
do not occur immediately south of the Indus-Yurlung-Tsangpo Suture Zone (IYSZ), although
all occur within the GHC (Corrie et al., 2010; Grujic, Warren, & Wooden, 2011; Kellett et
al., 2014). Given that all eclogites from the central Himalayas have been strongly overprinted
during the process of exhumation, few eclogite facies minerals have been preserved (Liu,
Zhang, Shu, & Li, 2005; Groppo et al., 2007). The only known omphacite inclusions in
garnet and zircon that have been discovered in the Dinggye eclogite, have low jadeite of
22.1–27.9%, and thus cannot be used to satisfactorily identify eclogite facies metamorphic
P–T conditions in the central Himalayas (Wang et al., 2017). Moreover, sodic clinopyroxene
containing up to 15% jadeite has been found in eclogite garnet from the Arun River Valley,
but it may not represent peak eclogite facies clinopyroxene (Groppo et al., 2007; Corrie et al.,
2010). Therefore, the metamorphic evolution and peak P–T conditions of eclogites are still
unconstrained in the central Himalayas.
The metamorphic ages of eclogite facies metamorphism for the granulitized eclogites
from the central Himalayas are another hotly debated topic. On the basis of published data,
three different age groups can be distinguished (Table S1) ― Eocene (39–33 Ma; Liu et al.,
2007; Cottle et al., 2009b; Kellett et al., 2014), late Oligocene (26–23 Ma; Corrie et al.,
2010), and Middle Miocene (15–14 Ma; Grujic et al., 2011; Wang et al., 2017). Eclogites that
we have discovered in the Thongmön area, central Himalayas, constitute the basis of this
study. Given their pristine nature ― omphacite has well been preserved in the rock matrix,
garnet, and zircon ― they provide the opportunity to resolve the ambiguity regarding the
eclogite facies P–T evolution and ages outlined above. These aims are achieved by
combining detailed petrological study with pseudosection modelling and SHRIMP zircon
U–Pb analyses.
2 | GEOLOGICAL SETTING
The thrust sheet of the central Himalayas is subdivided into three principal, parallel
litho-tectonic units by tectonic contacts. The northern of which, the Tethyan Himalayan
Sequence (THS), is a weakly metamorphosed boundary zone southerly abutting the IYSZ.
South of it, the GHC mainly consists of high-grade gneisses and overlies the Lesser
Himalayan Sequence (LHS), which generally consists of phyllites and quartzites. The GHC is
separated from the two lower-grade metamorphosed units, THS and LHS, by the Southern
Tibetan Detachment System (STDS) and the Main Central Thrust (MCT), respectively
(Figure 1a).
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
The THS extends for more than 1500 km along the Himalayan Orogen, comprising a
deformed collage of low-grade metamorphic Proterozoic to Eocene siliciclastic and carbonate
sedimentary rocks, which were deposited along the northern margin of the Indian Continent
(Yin, 2006).
The GHC, showing a much higher degree of metamorphism relative to the THS and
LHS, mainly consists of granites and gneisses (Kali et al., 2010). The granites in the GHC are
divided into three stages of the Eohimalayan (44–26 Ma), the Neohimalayan (26–13 Ma) and
the Posthimalayan (13–7 Ma), according to age and geological geochemical characteristics
(Wu, Liu, Liu, & Ji, 2015). To the south of ADM, in the Arun area, the GHC can be divided
into two principal litho-tectonic units: the Upper and Lower Great Himalayan Crystalline
Complex (UGHC and LGHC; Kali et al., 2010). The UGHC is mostly composed of
paragneisses, which have widely been intruded by Miocene leucogranites (Borghi, Castelli,
Lombardo, & Visoná, 2003). The LGHC consists mostly of strongly deformed rocks, with
metapelites overlying the Num/Ulleri orthogneiss (Meier & Hiltner, 1993; Goscombe, Gray,
& Hand, 2006).
The ADM (Figure 1b) is a prominent north–south striking elongated dome located at the
transition between the High Himalaya and the Tibetan Plateau (Kali et al., 2010). It is
separated from the GHC by two opposite-dipping normal-sense shear zones, the Ama Drime
Detachment (ADD) on the west and the Nyönno Ri Detachment (NRD) on the east (Figure
1b). The ADD is defined by a 100–300 m thick normal-sense detachment system linked to
young brittle faults, consisting of leucogranite, quartzite, marble, and calcsilicate rock
(Jessup, Newell, Cottle, Berger, & Spotila, 2008). The NRD offsets the position of the STDS
right-laterally (Burchfiel et al., 1992) and likely is connected with the western margin of the
Xainza–Dinggye rift (Zhang & Guo, 2007; Jessup et al., 2008; Jessup & Cottle, 2010;
Langille, Jessup, Cottle, Newell, & Seward, 2010; Figure 1b). The core of the ADM is
composed of orthogneiss to the south and a small unit of paragneiss to the north (Jessup et al.,
2008; Jessup & Cottle, 2010; Kali et al., 2010; Langille et al., 2010). The orthogneiss unit is
mainly made up of migmatitic gneiss with little paragneiss, while the paragneiss unit is
mainly composed of mica schist and paragneiss. Mafic metamorphic lenses, often surrounded
by migmatitic gneisses, are common within the core of the ADM. They are commonly
lenticular or boudinaged, and are generally subparallel to the main tectonic foliation. The
long axis of mafic lenses accommodated the ductile fabric developed in the migmatitic
gneisses.
The previous studies show that almost all the granulitized eclogites in the central
Himalayas in China were from the ADM (Lombardo & Rolfo, 2000; Liu et al., 2005; Groppo
et al., 2007). Recently, omphacite-bearing eclogites have been identified in Riwu, ADM
(Wang et al., 2017), implying that vertical and horizontal movement of both STDS and
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
north-south striking normal-sense shear zones may contribute to the exhumation of eclogites
from great depths (Kali et al., 2010). The STDS has been reported as extending on both sides
of the ADM — to the east near Saer (Burchfiel et al., 1992) and across the Dzakar Chu river
to the west (Burg, Brunel, Gapais, Chen, & Liu, 1984; Cottle et al., 2007, 2009a; Figure 1b).
The STDS is deflected northward around the ADM and cut and offset by ADD and NRD
normal faults (Burchfiel et al., 1992; Zhang & Guo, 2007; Jessup et al., 2008; Kali et al.,
2010; Leloup et al., 2010). The motion of the STDS started at c. 24Ma and stopped between
13.6 and 11 Ma; it was directly followed by E-W extension (Zhang, 2007; Zhang & Guo,
2007; Jessup et al., 2008; Jessup & Cottle, 2010; Leloup et al., 2010). The activity of the
STDS may not have ended at the same time along strike ― movements stopped at c. 17 Ma
in Zanskar, western Himalayas, but only at c. 12–11 Ma east of Gurla Mandhata (Leloup et
al., 2010; Kellett, Grujic, Coutand, Cottle, & Mukul, 2013).
The eclogites in this study have been collected in the Thongmön area outside of the
ADM, southern reaches of the Dzakar Chu River, southern Tibet (Figure 1b). The outcrops
show that the overlying STD and the regional foliation have been folded by ADD-related
tectonism (Jessup et al. 2008). Constituting the first eclogite found in the central Himalayas,
they represent boudinaged dykes and lenses within well-foliated metapelites (Figure 2).
Contrastingly, all previously reported granulitized eclogites are enclosed in the granitic
orthogneiss of the ADM and were first found in outcrops southeast of Kharta (Lombardo et
al., 1998). The mineral assemblages of the studied eclogites are similar to those from the
previously reported granulitized eclogites from the ADM, except for good preservation of
omphacite in the matrix.
3 | ANALYTICAL METHODS
3.1 | Whole Rock major elements
The whole-rock major element analyses were conducted by X-ray fluorescence (XRF) and
titration (ferric Fe only) at the Institute of Geology and Mineral Resources, Regional Survey
of Hebei Province, Langfang city, China. Whole rock major elements are given in Table S2.
3.2 | Major elements of minerals
The major element contents of minerals were determined on polished thin sections and zircon
grain mounts with a JEOL JAX-8100 electron microprobe (EMP) at the Key Laboratory of
Orogenic Belts and Crustal Evolution, Peking University, Ministry of Education, China. The
operating conditions were 15 kV acceleration voltage, 10 nA beam current, and 2μm beam
size. SPI standards were utilized and raw data were reduced with the Phi-Rho-Z (PRZ)
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
correction (Li, Zhang, Wei, Slabunov, & Bader, 2018). Representative garnet data are given
in Tables S3 and S4.
3.3 | Trace elements of garnet
Trace elements of garnet were analyzed with an Agilent 7500ce ICP-MS coupled to a
COMPexPro102 193 nm Excimer Laser System at Peking University. Detailed techniques are
similar to those described in Yuan et al. (2004). The diameter of the laser spot was 60 µm,
and each analysis comprised approximately 15 s background acquisition followed by 60 s
data acquisition. Table S5 shows the trace elements data of garnet.
3.4 | Zircon U–Pb ages and trace elements
Zircon grains were separated from two crushed samples (DR1509 and DR1528) using
conventional density-based and magnetic separation techniques and handpicked under a
binocular microscope. Selected grains were mounted in epoxy resin and polished to expose
their equatorial sections. Zircon was imaged with 2-minute scanning time under
cathodoluminescence (CL) at Peking University (PKU) utilizing a FEI Philips XL30
Schottky field-effect scanning electron microscope (SEM) operated at 15 kV accelerating
voltage and 120 nA emission current.
SHRIMP U-Th-Pb zircon analysis was carried out with the SHRIMP II ion microprobe at
the Australian National University (ANU), Canberra. The analytical data are shown in Table
S6.
Zircon REE abundances have been obtained by LA-ICP-MS, using a 193 nm excimer
laser-based HELEX ablation system equipped with a Varian 820 quadrupole inductively
coupled plasma mass spectrometer at ANU. The laser spot size was 30µm with ablation times
of up to 60 s. Data were reduced offline using the software package Iolite (Hellström, Gradin,
& Carlberg, 2008; Paton et al., 2010; Paton, Hellstrom, Paul, Woodhead, & Hergt, 2011).
Synthetic glass NIST 612 was utilized with Si as internal standard. Representative data are
given in Table S7.
4 | PETROGRAPHY AND MINERAL CHEMISTRY
Omphacite is well-preserved in both matrix and as inclusions in garnet, beyond that, the other
petrological features of the studied eclogite samples are similar to the previously described
granulitized eclogites from central Himalayan (Lombardo & Rolfo, 2000; Groppo et al.,
2007; Cottle et al., 2009a, b; Corrie et al., 2010; Wang et al., 2017). Thongmön eclogites
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
mainly consist of garnet (25–30%), omphacite & clinopyroxene (20–25%), orthopyroxene
(2–3%), biotite (5–8%), amphibole (15%–20%), plagioclase (20–25%), and quartz (3–5%),
with accessory rutile, ilmenite, apatite, zircon, and Fe-oxides (Figure 3). Different types of
some of these minerals have been identified from textural and chemical features, and
although mentioned initially below, the differences of each type are documented in the
sections describing each mineral type.
The garnet has patchy microstructures. Grt I is represented by garnet cores and
intermediate sections, which contain abundant inclusions, such as omphacite (Cpx I),
amphibole (Amp I), quartz, rutile and ilmenite (Figure 3a,e); while the rims on large garnet
grains represent Grt II, which only contain a few inclusions of omphacite. Omphacite (Cpx I)
occurs as inclusion in garnet (Figure 3a,e) and in the matrix (Figure 3b,c). Omphacite
inclusions in garnet often coexist with quartz and/or amphibole (Figure 3a,e). In the matrix,
omphacite is well-preserved in central sections of extensive Cpx II+Pl I(1) symplectites,
which replaced it from the rim. Both inclusion-type and matrix-type omphacite contain
abundant quartz rods. Symplectites of Bt I+Pl I(2) are widespread in the matrix (Figure 3d),
and as in other occurrences these are interpreted as having formed after phengite (Lombardo
& Rolfo, 2000; Groppo et al., 2007). The Opx+Pl II±Cpx III assemblages occur around
garnet porphyroblasts as coronas (Figure 3e,f), and also formed at microcracks in the outer
sections of some garnet grains (Figure 3a). Amphibole occurs as inclusions in garnet (Amp I),
in coronas around garnet (Amp II, Figure 3a), and in the matrix (Amp III) coexisting with Bt
II+Pl IV (Figure 3b,d). Rutile only occurs as inclusions in garnet, and ilmenite grows in the
matrix and around rutile in garnet (Figure 3d,e).
4.1 | Garnet
Almandine (45–57 mol. %), pyrope (19–26 mol. %), and grossular (21–32 mol. %) dominate
the garnet solid solution; spessartine (< 3 mol. %) is a minor component (Table S3). Garnet
in the studied eclogites can be grouped into two types: Grt I and Grt II, according to the
major and minor element profiles zoned in Ca, Mg, and Fe (Figures 4-6). Grt I is nearly
homogeneous with indistinct compositional zoning. The outermost sections of Grt I show the
maximum grossular (30–32 mol. %) and minimal pyrope (21–23 mol. %). Grt II is
characterized by decrease of grossular (22–28 mol. %) and increase of pyrope (22–26 mol.
%), relative to Grt I, and especially at its outer sections.
Trace element analyses also reveal distinctness of Grt I and Grt II domains: Whereas the
Grt I domains display almost flat HREE relative to chondrite, with slight rim-ward
steepening, the Grt II domains are enriched in MREE relative to HREE and LREE (Table S5;
Figure 6).
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
Grt I contains many inclusions of amphibole, rutile, ilmenite and apatite in the core, but
abundant omphacite was found in its intermediate and outer sections. Conversely, Grt II
encompasses only a few inclusions of omphacite.
4.2 | Pyroxene
Clinopyroxene in Thongmön eclogites has been grouped into three types, according to its
microstructural association with different minerals as described above: (1) omphacite occurs
as inclusions in garnet and as a matrix constituent (Cpx I); (2) clinopyroxene forms
symplectites with plagioclase after omphacite in the matrix (Cpx II); (3) clinopyroxene is part
of the Opx+Pl II+Cpx III assemblage in coronas around and as inclusions in garnet
porphyroblasts (Cpx III). Omphacite included in garnet has the highest XNa [Na at the M2
site] of up to 0.35; its XMg [Mg / (Mg + Fe2+
)] varies from ~0.74 to 0.98, and aegirine is
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
(Pl I(2)) has a similar composition as Pl I(1) (An22–27); (3) Plagioclase (Pl II) occurring with
Opx+Cpx III in the coronas around and as inclusions in garnet shows a wider range of
compositions (An25–42); (4) Plagioclase associated with amphibole in coronas around garnet
(Pl III) varies from andesine to labradorite composition (An45–65); (5) Plagioclase in
equilibrium with brown amphibole and biotite (Pl IV) in the matrix is characterized by an
anorthite range from An14 to An33.
On the basis of microstructures and mineral chemistry, a sequence of four mineral
assemblages is recognized (Figure 8): M1: an eclogite facies, interpreted to consist of garnet,
omphacite, phengite (which is interpreted to have been completely replaced by Bt I+Pl I(2)
symplectites, Lombardo & Rolfo, 2000; Groppo et al., 2007), rutile, quartz, and/ or
amphibole; M2: high-pressure granulite-facies, represented by Cpx II +Pl I(1) symplectites
after omphacite and Bt I+Pl I(2) symplectites after phengite in the matrix; M3: granulite-facies,
represented by development of Opx+Pl II±Cpx III coronas occur around garnet
porphyroblasts and along widened microcracks in the outer sections of some garnet grains;
M(4); retrograde amphibolite facies, represented by Amp III+Pl IV+Bt II assemblages is
widespread in the matrix. M1 has been further subdivided into two stages: the amphibole
eclogite facies stage M1(1) and the peak eclogite facies stage M1(2). M1(1) is represented by
Grt I and its inclusions of omphacite, amphibole, rutile, quartz, and phengite. M1(2)
characterized by growth of Grt II and omphacite from the decomposition of amphibole.
5 | ZIRCON GEOCHRONOLOGY, REE AND MINERAL INCLUSIONS
Zircon was collected from two samples directly (DR1509, and DR1528), which can not be
identified in thin sections. Zircon from sample DR1509 is subhedral to euhedral, with long
axes ranging from 70 to 200 μm. In CL images, most of grains show blurred zoning or none
at all and overgrowth textures; < 15% of the grains contain light, irregular inherited cores
(Figure 9). Zircon from DR1528 is ovoid, subhedral, and smaller given with long axes
varying from 50 to 100 μm.
5.1 | U–Pb dating
Abundant analyses of zircon from the two eclogites provide information on metamorphic
grains and domains and the inheritance preserved in some cores. These inherited cores have
yielded discordant 206
Pb/238
U ages ranging from 931 to 63.5 Ma (Table S6), with high Th/U
values mostly ranging from 0.124 to 0.908. The metamorphic zircon domains from both
samples show extremely low Th/U values of 0.003–0.028 and all yield similar Miocene ages
on the concordia diagrams (Figure 10). For sample DR1509, 15 analyses yielded concordant
U–Pb data (Figure 10a; Table S6), with a lower intercept age of 16.7 ± 0.6 Ma (MSWD =
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
1.0) and a weighted mean 206
Pb/238
U age of 16.6 ± 0.4 Ma (MSWD = 0.2). For sample
DR1528, 18 analyses yielded concordant U–Pb data (Table S6; Figure 10c), with a lower
intercept age of 17.1 ± 0.4 Ma (MSWD = 0.6) and a weighted mean 206
Pb/238
U age of 17.0 ±
0.4 Ma (MSWD = 0.2). Most of the metamorphic zircon yields 206
Pb/238
U ages ranging from
17.3 to 16.1 Ma (Table S6).
5.2 | Trace elements
The REE dates were obtained from zircon with middle Miocene ages; both samples have
similar characteristics (Figure 10b,d) with low Th/U ratios and a weighted average of 0.012.
The HREE patterns are flat, consistent with concurrent growth of garnet (Rubatto, 2002) and
obvious Eu anomalies do not exist (Eu/Eu* = 0.93–1.39; weighted average 1.13).
5.3 | Mineral inclusions
The Miocene zircon mainly contains garnet, omphacite, rutile, quartz, and glass/melt (Table
S8; Figure 9). Most of the garnet inclusions show similar compositions to Grt II in the matrix,
with 20–25 mol. % grossular, 23–25 mol. % pyrope and ~2 mol. % spessartine. Some garnet
inclusions are characterized by much higher spessartine (6–7 mol. %) and lower grossular
(15–18 mol. %). Sodic clinopyroxene is preserved well in the zircon with XNa up to 0.24,
similar to the omphacite in the matrix. The glass/melt is cryptocrystalline with very high SiO2
(66–84 wt. %); elements other than SiO2, Al2O3, CaO, Na2O, K2O, and little FeO have not
been detected through EMPA.
6 | PHASE EQUILIBRIA MODELLING AND P–T EVOLUTION
P–T and T–H2O pseudosections have been calculated for the eclogite sample DR1608 in the
NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) system. The
input bulk compositions based on the whole-rock composition analyzed by XRF (Table S2).
All calculations were performed with the software THERMOCALC, version 3.40i (Powell &
Holland, 1988; updated February, 2012), and the following mineral activity-composition
relationships: garnet, orthopyroxene, and biotite: White, Powell, Holland, Johnson, and Green
(2014a); mafic melt, augite, and hornblende: Green et al. (2016); plagioclase: Holland and
Powell (2003); muscovite–paragonite: White, Powell, and Johnson (2014b); epidote: Holland
and Powell (2011) and talc: Holland and Powell (1998). Pure phases included rutile, quartz,
and aqueous fluid (H2O). H2O is considered as being in excess for the modelling of eclogite
facies conditions. Mineral abbreviations are after Whitney and Evans (2010), alongside L for
mafic melt.
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
The P–T pseudosections of the Thongmön eclogite DR1608 are presented in Figure 11.
Also shown are the evolution of the mineral assemblages and the isopleths of the Ca and Mg
content in garnet, the Na content in omphacite, and the modal amounts (mol. %) of garnet
and melt. Based on the maximum grossular content (30–32 mol.%) of Grt I (Figure 11c) and
the maximum XNa (up to 0.35) of an omphacite (Figure 11b), inclusion in the intermediate
section of a big garnet, prograde metamorphic conditions are constrained at 640–660 °C and
19–20 kbar in the field garnet+omphacite+amphibole+phengite+quartz+rutile [M1(1)]. This
assemblage is consistent with the observed inclusions of omphacite+amphibole+quartz+rutile
in Grt I, except that phengite has neither been found in the matrix nor is it included in garnet
― it is interpreted to have broken down to biotite+plagioclase symplectites during late
decompression (Groppo et al., 2007; Wang et al., 2017). However, the composition of Grt II
cannot be plotted into the P–T pseudosection. Nevertheless, it still preserves prograde zoning,
with grossular decreasing and pyrope increasing from Grt I to Grt II (O’Brien, 1997; Zhao,
Cawood, Wilde, & Lu, 2001; Štípská & Powell, 2005; Medaris, Ghent, Wang, Fournelle, &
Jelínek, 2006; Groppo et al., 2007; Groppo, Rolfo, & Indares, 2012; Cruciani, Franceschelli,
& Groppo, 2011; Cruciani, Franceschelli, Groppo, & Spano, 2012). The modal amounts of
garnet should have increased slightly when Grt II formed. Given coexistence of Grt II and
zircon hosting melt inclusions (the coexistence of metamorphic zircon and Grt II will be
discussed in detail below), at least a little melt should have been present when Grt II grew
(Figure 11d). Taken also into account the texturally implied absence of amphibole — it is not
included in Grt II and all amphibole in the matrix occurs in post-eclogite facies reaction
textures — the peak eclogite facies should be represented by the field garnet
+omphacite+phengite+quartz+rutile+melt [M1(2)], indicating conditions of > 750 °C and > 21
kbar. Therefore, the prograde P–T path of Thongmön eclogites is characterized by the near
isobaric heating, i.e., from 640–660 °C and 19–20 kbar to 750 °C and 21 kbar (Figure 11).
To investigate the effects of the H2O content on prograde metamorphism, a T–H2O
pseudosection was calculated at 20 kbar (Figure 12). During prograde metamorphism, the
H2O saturation level decreases quickly in the field of the M1(1) assemblage, and tends to
stabilize at a low H2O content (about 0.9 mol. %). Thus, H2O should be in excess during
prograde metamorphism and the H2O content does not affect the conditions of M1(1) and the
solidus.
7 | DISCUSSION
7.1 | Element zoning of garnet
Previous studies have shown that core and rim domains of the large garnet of granulitized
eclogites from the central Himalayas are generally preserved (Groppo et al., 2007; Corrie et
al., 2010; Kellett et al., 2014). The cores have been considered as record of eclogite facies
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
metamorphism, while the thin rims were explained as granulite-facies overgrowths (Groppo
et al., 2007; Kellett et al., 2014). Corrie et al. (2010) found sparse sodic clinopyroxene (up to
15% jadeite) inclusions in garnet, which may be prograde relicts rather than eclogite facies
clinopyroxene (Groppo et al., 2007; Corrie et al., 2010). Furthermore, for the first time, Wang
et al. (2017) reported omphacite (up to 27.9% jadeite) inclusions in garnet from granulitized
eclogites in Dinggye ― it represents a high-pressure relict, which formed at eclogite facies,
and substantiates the notion that the garnet cores grew in the eclogite facies.
The major elements in Thongmön eclogite garnet typically show concentric zoning, with
grossular decreasing from Ca-richer cores (Grs 30–32 mol. %) to Ca-poorer rims (Grs 21–26
mol. %) and little rimward increase of Fe, Mg. In some big garnet crystals, the outermost
rims have the lowest grossular and highest spessartine, reflecting possibly a diffusional
re-equilibration and/ or net-transfer reactions at elevated temperatures (Zhao et al, 2001;
Groppo et al., 2007). Principally, the major element zoning of garnet would be reset at
temperatures higher than 700 °C (Caddick, Konopásek, & Thompson, 2010). However, Ca
diffuses slower than Mg, Fe, and Mn (Chakraborty & Ganguly, 1992; Chernoff & Carlson,
1999; Florence & Spear, 1995; Spear & Daniel, 2001) ― its mobility is controlled by
intra-grain diffusion while that of Mg, Fe, and Mn is controlled by inter-grain diffusion
(Spear & Daniel, 2001). This is evident from our Thongmön eclogite garnet (Figures 4 and
5), as well as from that of other samples reported in the central Himalayas (Groppo et al.,
2007; Corrie et al., 2010; Kellett et al., 2014).
From plagioclase+orthopyroxene+clinopyroxene symplectites in the rock matrixes or in
coronas around garnet, peak metamorphic temperatures of central Himalayan granulitized
eclogites were estimated as being higher than 775℃ (Liu et al., 2007; Groppo et al., 2007;
Warren et al., 2011). Alternatively, the plagioclase+ orthopyroxene symplectites could have
formed under water-undersaturated conditions without heating (Štípská & Powell, 2005;
Wang et al., 2017), but the peak temperatures still were as high as 750℃ (Wang et al., 2017).
In spite of the high-temperature granulite-facies events overprint, prograde garnet zoning is
still preserved ― the rimward grossular decrease and pyrope increase are consistent with
prograde growth at increasing pressures and temperatures. Similar prograde garnet zoning
that has been preserved at high temperatures has been reported from granulitized eclogites
worldwide (O’Brien, 1997; Zhao et al., 2001; Štípská & Powell, 2005; Medaris et al., 2006;
Groppo et al., 2007, 2012; Cruciani et al., 2011, 2012). Likewise, the REE patterns (Figure 6)
indicate that Grt I and Grt II grew at two different stages. Trace element diffusion is
controlled by many factors, including transport mechanisms, atomic radius, heating duration,
and fluid influence (Spear, 1991; Spear & Daniel, 2001; Wang & Liou, 1991; Yardley, 1977).
Having large ionic radii, REE show obvious core to rim differences.
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
In addition, Grt I contains inclusions of amphibole, omphacite, quartz, apatite, and
zircon, while Grt II mainly contains inclusions of omphacite and rutile, without amphibole.
This may indicate that amphibole had been decomposed at the time of, or before garnet rim
growth.
Overall, based on major and trace elements as well as mineral inclusions, the core-rim
zoning of garnet has been preserved. Garnet core (Grt I) is nearly homogenous with Ca, Mg,
and Fe zonation lacking or weak, and an even distribution of Mn. Chondrite-normalized REE
of Grt I are characterized by almost flat HREE pattern with little rim-ward steepening. Grt I
contains inclusions of amphibole, apatite, and zircon and abundant omphacite. Garnet rims
(Grt II) are very thin and characterized by decrease of Ca, increase of Mg, and MREE
enrichment relative to the cores. No amphibole inclusions have been found in garnet rims,
indicating the decomposition of amphibole contributed to the MREE enrichment in Grt II.
7.2 | Time of garnet growth
Garnet can grow from the blueschist-facies onwards and it can be stable over a very wide
P–T range (Zhou, Xia, Zheng, & Chen, 2011). Its major and trace element contents are
mirrors of the garnet growth environment. Consumption and/or breakdown of concomitant
minerals contribute most to the trace element characteristics of garnet, thus allowing the
temporal evolution of the mineral assemblages during metamorphism to be deduced
(Chernoff & Carlson, 1999; Hickmott, Shimizu, Spear, & Selverstone, 1987; Hickmott,
Sorensen, & Rogers, 1992; Jung et al., 2009; Konrad-Schmolke, Zack, O'Brien, & Jacob,
2008; Schwandt, Papike, & Shearer, 1996; Spear & Kohn, 1996; Wang, Pan, Chen, Li, &
Chen, 2009; Zhou et al., 2011). The core–intermediate (Grt I) and rim (Grt II) sections of
garnet from the samples detailed here are distinct with respect to chemical elements and
inclusions and, consequently, they should have grown at different metamorphic stages.
Grt I is rather homogenous, and lacks or has weak major element zonation, and its Ca
content reaches a maximum in the outermost sections. The REE contents continuously
increase from LREE to MREE and flatten out toward HREE. This indicates that garnet cores
grew during prograde metamorphism. Grt II is characterized by rim-ward decrease of Ca and
increase of Mg, also indicating prograde metamorphism. Moreover, the garnet rims have high
MREE relative to HREE and LREE (Figure 6), suggesting they grew when concomitant
processes resulted either in MREE-influx from coexisting minerals or in HREE-release from
the garnet itself (Zhou et al., 2011). MREE enrichment has also been reported from eclogite
veins from Monviso in the Western Alps (Rubatto & Hermann, 2003), UHP eclogites from
the Western Gneiss Region, Norway (Konrad-Schmolke et al., 2008), eclogites from the
Chinese Continental Scientific Drilling (CCSD) main hole (Zong, Liu, Liu, & Zhang, 2007),
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
and eclogites from the Dabie Orogen (Zhou et al., 2011). This kind of garnet zoning was
attributed to amphibole and/ or epidote breakdown (Konrad-Schmolke et al., 2008; Zhou et
al., 2011), or to HREE release from preexisting garnet during increasing temperatures (Klein,
Stosch, Seck, & Shimizu, 2000; Zong et al., 2007). The garnet rims in DR1508 have lower
MREE than the cores, while garnet rims of the other three samples have higher MREE
contents than the cores. Hence, the garnet rim REE pattern should not result from HREE
release from preexisting garnet cores. According to the mineral inclusions in it, Grt I grew in
the stability field of amphibole, omphacite, quartz, apatite and zircon. In contrast, Grt II
mainly contains omphacite and rutile inclusions. This difference in inclusion type indicates
that amphibole breakdown contributed to the MREE enrichment in garnet rims.
The time of garnet growth can be directly determined by Lu–Hf and Sm–Nd isochron
dating of garnet (Schmidt et al., 2008; Thöni, 2003) or by dating of paragenetic mineral
inclusions in garnet, such as zircon and monazite (Cutts et al., 2010). We identify the relative
ages of specific zones in garnet from the composition of metamorphic zircon and the trace
element distribution coefficients between zircon and garnet (TE
DZrn/Grt values; Rubatto, 2002;
Rubatto & Hermann, 2003, 2007; Rubatto, Hermann, & Buick, 2006), which provide a direct
link between datable zircon and garnet (Rubatto, 2002). All metamorphic zircon exhibits flat
HREE patterns and lacks Eu anomalies, implying it grew in the presence of garnet, and
absence of feldspar (Rubatto, 2002; Whitehouse & Platt, 2003). The TE
DZrn/Grt values were
calculated from the mean compositions of zircon and the two endmenber garnet
compositions, from core to rim (Table S9; Figure 13). The TE
DZrn/Grt of all samples are
similar, but only the TE
DZrn/Grt values calculated for the garnet rims are consistent with the
trends and distribution coefficients of eclogites (Rubatto, 2002; Rubatto & Hermann, 2003;
Zhou et al., 2011), while the TE
DZrn/Grt values calculated for the garnet cores show that zircon
did not coexist with them (Rubatto, 2002). Therefore, the metamorphic zircon and garnet
rims coexisted in the eclogite facies stage, and garnet cores grew during the prograde
metamorphism.
7.3 | P–T conditions of the Thongmön eclogites
The eclogite facies mineral assemblages of the central Himalayan eclogites have been
strongly overprinted by granulite-facies metamorphism (Lombardo et al., 1990; Groppo et al.,
2007; Liu et al., 2007; Warren et al., 2011; Chakungal et al., 2010; Corrie et al., 2010; Kellett
et al., 2014; Wang et al., 2017). Many well-preserved disequilibrium textures, such as
symplectites, coronas, kelyphytes, and compositional zoning, reflect widespread post-eclogite
facies metamorphism in these rocks (O'Brien, 1997, 1999; Tsai & Liou, 2000; Santos et al.,
2009; Cruciani et al., 2012). Correspondingly, the prograde mineral assemblages have not
been completely destroyed by re-equilibration, but the decompression processes created local
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
equilibria, which provide important constraints on the retrograde evolution (Elvevold &
Gilotti, 2000; Groppo et al., 2007; Cruciani et al., 2012). The P–T conditions of multiple
post-eclogite stages could be quantified by using effective compositions for pseudosections
modelling. These effective compositions could be identified at the microscale, or attained by
detailed interpretation of reaction textures (O'Brien & Ziemann, 2008; Powell & Holland,
2008; Cruciani et al., 2012) as both reactants and products can be identified from
micro-structures. However, calculating balanced reactions between reactants and products of
microdomains, in which symplectites and coronae grew, has many challenges (Cruciani et al.,
2012) and the calculation of pseudosections and the determination of P–T conditions on the
basis of effective compositions possesses many uncertainties. Thus, in this paper, we focus on
the prograde and the peak eclogite facies metamorphism, the information of which is
well-preserved by garnet itself and inclusions in it as well as by inclusions in zircon.
The peak eclogite facies P–T conditions are still controversial for the granulitized
eclogites of the central Himalayas. Lombardo et al. (1998) reported granulitized eclogites
from the Kharta region and estimated conditions of 12–14 kbar and 600–650 °C for the
eclogitization event based on mineral assemblages, reaction textures, and
geothermobarometry (Lombardo & Rolfo, 2000). Combining conventional geothermometers
with geobarometers, Liu et al. (2005) suggested the HP granulite-facies P–T conditions of
13.5–14.8 kbar and 625–675 °C. Eclogite facies mineral assemblages combined with
compositional isopleths of garnet and pseudosection modelling indicated peak metamorphic
P–T conditions of > 15 kbar and > 580 °C (Groppo et al., 2007). Grujic et al. (2011) inferred
the eclogite facies zircon crystallized at ~760 °C and > 15 kbar based on the analyses of
zircon trace-elements, Ti-in-zircon temperatures, and their textural association in the
granulitized eclogites. Wang et al. (2017) found omphacite inclusions in garnet and zircon,
and speculated about peak eclogite facies temperatures of between 720–760 °C at a pressure
of 20–21 kbar based on geothermometry and quantitative P–T pseudosections. Overall, the
peak ecologite-facies mineral assemblage should be garnet, omphacite, muscovite (biotite
pseudomorph), rutile, and quartz (Groppo et al., 2007; Wang et al., 2017); it is stable across a
wide range of pressures and temperatures.
Based on garnet variations and inclusions coupled with pseudosection modelling, two
stages were recognised, i.e, the amphibole eclogite facies M1(1) and the peak eclogite facies
M1(2). The amphibole eclogite facies stage M1(1) is characterized by the assemblage
garnet+omphacite+amphibole+phengite (biotite+plagioclase psedomorph) +quartz+rutile.
The P–T conditions of this stage are well-constrained as 19–20 kbar and 640–660 °C by the
pseudosection calculations. Yet, phengite predicted by the modelling has neither been found
in the matrix nor in garnet; it is interpreted to have been replaced by biotite+plagioclase
symplectites on the basis of it being recorded in other Himalayan areas (Groppo et al., 2007;
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
Wang et al., 2017). The calculation using Grt–Cpx geothermometer (Ravna, 2000) for the
omphacite inclusions and their host garnet core obtained similar temperatures (630–660 °C)
at pressures of 19–20 kbar which are similar to the above phase relations. Garnet cores have
previously been used to constrain a peak eclogite facies assemblage without amphibole
(Groppo et al., 2007; Wang et al., 2017), but it does not match the observed coexistence of
omphacite +amphibole+quartz as mineral inclusions in the outer core of garnet. We interpret
the core–intermediate section of garnet (Grt I) as representing the amphibole eclogite facies
M1(1), and the garnet rim (Grt II) belong to the peak eclogite facies on the basis of major and
rare earth elements, mineral inclusions and pseudosection modelling, which can be
comparable to eclogites of the European Variscan Belt (Cruciani et al., 2012). The garnet rim
coexisted with metamorphic zircon given that TE
DZrn/Grt values calculated from the mean
composition of zircon and garnet rim are consistent with eclogite facies trends and
distribution coefficients (Rubatto, 2002; Rubatto & Hermann, 2003; Zhou et al., 2011).
Moreover, the garnet rim and the metamorphic zircon contain similar inclusions of
omphacite, rutile, and quartz (melt inclusions have only been found in zircon). The
decomposition of amphibole contributed to the MREE enrichment in the garnet rim.
However, the major composition of garnet rim no longer reflects the P–T conditions of peak
eclogite facies M1(2) due to the strong overprint by post-eclogite facies events, but the trend
of a prograde metamorphism can still be identified in its major elements given rim-ward Ca
decrease and Mg increase. Based on the mineral assemblage and the modal amount of garnet,
we conclude the peak eclogite facies metamorphism M1(2) proceeded at > 750 °C and > 21
kbar.
Nevertheless, the metamorphic assemblage constrained not only the P–T conditions, but
also the H2O content in the rock (Guiraud, Powell, & Cottin, 1996; Carson, Clarke, & Powell,
2000). During the prograde metamorphism, from M1(1) to M1(2), H2O was liberated by the
decomposition of amphibole, which is why the H2O-saturation level of M1(2) is much lower
than that of M1(1). The water content does not affect the P–T conditions of M1(1), and the
solidus [the lower temperature limit of M1(2)].
The widespread Cpx II+Pl I(1) and Bt I+Pl I(2) symplectites, Opx+Pl II±Cpx III coronas,
and Amp III+Pl IV+Bt II assemblages in the matrix, indicate successive retrograde
metamorphism through HP granulite facies, granulite facies and amphibolite facies,
respectively, similar to the retrogression described for granulitized eclogites in the central
Himalayas (Lombardo et al., 1998; Groppo et al., 2007; Cottle et al., 2009a, b; Corrie et al.,
2010; Grujic et al., 2011; Warren et al., 2011; Kellett et al., 2014; Wang et al., 2017).
Therefore, the Thongmön eclogites possibly experienced a similar near-isothermal
decompression path (Groppo et al., 2007; Corrie et al., 2010; Grujic et al., 2011; Wang et al.,
2017, Figure 14).
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
7.4 | Age of eclogitization
As mentioned before, two different groups of metamorphic ages have been obtained from
central Himalayan eclogites. (1) Eocene (39–33 Ma): Hodges et al. (1994) determined a 40
Ar/39
Ar hornblende age of 25 Ma for an amphibolite from the Dinggye area, indicating the
eclogite facies metamorphism should be older than 25 Ma. The Barrovian metamorphism
related to crustal thickening in the Himalayas began at > 32 Ma on the basis of research on
pelitic gneiss from the GHC (Simpson, Parrish, Searle, & Waters, 2000; Stübner et al., 2014).
Liu et al. (2007) obtained a 206
Pb/238
U weighted mean zircon age of 33 ± 2 Ma for a pelitic
gneiss from the Kharta area. Cottle et al. (2009b) obtained 38.9 ± 0.9 Ma from monazite in a
pelitic gneiss from the GHC. The Barrovian metamorphism of the GHC may have happened
as early as 39 Ma and continued till 16 Ma (Cottle et al., 2009b). Belonging to this Eocene
age group, Lu–Hf garnet-whole rock isochron dating implies that the eclogite facies
metamorphism of granulitized eclogites from ADM proceeded at c. 38 Ma (Kellett et al.,
2014). (2) late Oligocene (26–23 Ma): Lu–Hf ages of 20.7 ± 0.4 Ma (granulitized eclogite,
GHC) and 15–14 Ma (LHS amphibolite, Arun River Valley) suggest the eclogite facies
metamorphic event dates to 26–23 Ma (Corrie et al., 2010). (3) Middle Miocene (15–14 Ma):
This age group is testified by U–Pb zircon ages of 15.3 ± 0.3–14.4 ± 0.3 Ma (granulitized
eclogites, NW Bhutan; Grujic et al., 2011) and 14.9 ± 0.7–13.9 ± 1.2 Ma (Dinggye, China;
Wang et al., 2017). In the latter study, the discovery of relict omphacite inclusions in zircon
and garnet has been reported, Wang et al. (2017) proposed that the eclogite facies
metamorphism may have continued till Miocene time (c. 14 Ma).
The Miocene ages (17.1–16.7 Ma) obtained from the Thongmön eclogites can be
interpreted as reflecting the eclogite-facies peak metamorphic stage considering the
inclusions and REE characteristics of zircon together with the trace element distribution
coefficients between garnet and zircon:
(1) We have found eclogitic mineral inclusions such as garnet, omphacite and rutile in
several metamorphic zircons. Flat HREE pattern and slightly positive Eu anomalies
indicate the metamorphic zircon and zircon domains (re)crystallized under eclogite facies
conditions.
(2) The REE distribution coefficients between garnet and zircon indicate equilibrium of
zircon with garnet rim (Grt II), both coexisting in the eclogite facies stage (Rubatto, 2002;
Rubatto & Hermann, 2003; Figure 13).
Our data do not coincide with earlier reported eclogite facies ages from the central
Himalayas. Older Eocene (39–33 Ma; Kellett et al., 2014) and late Oligocene (26–23 Ma;
Corrie et al., 2010) ages were all obtained by Lu–Hf dating of garnet separated from
granulitized eclogites. This garnet shows similar major element characteristics to the garnet
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
from both eclogite and granulitized eclogites in this study. From the detailed study of garnet
in terms of major elements, trace elements, and mineral inclusions, we conclude that the
cores, constituting the largest portion of an individual crystal, grew during prograde
metamorphism until the amphibole eclogite facies M1(1). Consequently, the garnet Lu–Hf age
could be the age of prograde metamorphism, mostly that of the M1(1) stage. Obviously, the
majority of the zircon U–Pb ages obtained from the central Himalayas ― most are Middle
Miocene (15–14 Ma; Grujic et al., 2011; Wang et al., 2017) ― are much younger than the
garnet Lu–Hf ages. We obtained a metamorphic zircon U–Pb age of 17.1–16.7 Ma for the
peak eclogite facies metamorphic stage M1(2), which is a little older than the zircon data from
NW Bhutan (Grujic et al., 2011) and Dinggye of China (Wang et al., 2017). This
metamorphic zircon coexisted with garnet rims at the peak eclogite facies. Overall, the garnet
Lu–Hf age may have kept track of the beginning prograde metamorphism, while the zircon
recorded the peak eclogite facies age, indicating the eclogites from the central Himalayas
may have experienced a long nearly isobaric heating journey due to the flat subduction in
central Himalayas (maybe from as early as 38 Ma to about 14 Ma). These ages further
suggest that the eclogites in Thongmön from central Himalayia should be formed by the
prograde metamorphism during the continental subduction of Indian plate beneath
Euro-Asian plate.
7.5 | The formation and exhumation of Thongmön eclogites
In contrast to the UHP eclogites from the northwestern Himalayas, the eclogites from the
central Himalayas do not preserve any UHP minerals and have younger ages (Groppo et al.,
2007; Cottle et al., 2009b; Corrie et al., 2010; Grujic et al., 2011; Kellett et al., 2014; Wang et
al., 2017). The eclogites from the central Himalayas are usually considered to have been
formed by crustal thickening of the Himalayan-South Tibetan (Grujic et al., 2011). In this
paper, we interpret this crustal thickening to be the result of the continental subduction of the
Indian plate followed the collision between Indian and Euro-Asian plates after 55 Ma. The
main evidence can be summarized as following: 1) The eclogites experienced progressive
metamorphism from the amphibole eclogite facies (640–660 °C and 19–20 kbar) to the peak
eclogite facies (750 °C and 21 kbar) with a P–T path of near isobaric heating, which suggests
flat subduction characterized by increasing temperature, rather than crustal thickening and
pressure increase. 2) According to geochemical data and the outcrop characteristics, the
central Himalayan eclogites are more likely rift-related basaltic rock protoliths of Indian
continental origin (Lombardo et al., 2016; Wang et al., 2017), and the occurrences of these
eclogites far away from the IYSZ, is not coincident with the hypothesis of Grujic et al. (2011)
that the protolith of the central Himalayan eclogites was middle Miocene mafic intrusions
into the lower crust of southern Tibet. 3) We distinguish two groups of garnet, Grt I and Grt
II, in the Thongmön eclogites, which mainly grew during the prograde metamorphic stage
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
and peak eclogite facies stage, respectively. Lu–Hf garnet-whole rock isochron aegs of the
granulitized eclogites from ADM (c. 38Ma, Kellett et al., 2014) may record the beginning of
the subduction of Indian continent. Therefore, we suggest that the central Himalayan
eclogites were formed by the continuing subduction of the Indian plate after the initial
collision between the Indian and the Euro-Asian plate at c. 55Ma. Despite the low subduction
angle, the plate reached a depth of ~70 km in the Miocene.
The STDS is crosscut by N-S normal faults at Yadong, ADM, Thakhola, and Gurla
Mandhata (Jessup et al., 2008; Kali et al., 2010; Leloup et al., 2010). The activity of STDS
ended asynchronously along strike, at c. 17 Ma at Zanskar in the west but at c. 12–11 Ma east
of Gurla Mandhata (Leloup et al., 2010) and Sikkim (Kellett et al., 2013). On both sides of
the ADM, the STDS is cut and offset by ADD and NRD normal faults (Burchfiel et al., 1992;
Zhang & Guo, 2007; Jessup et al., 2008; Kali et al., 2010; Leloup et al., 2010). The activity of
the STDS is synchronous around the ADM, and it ended at c. 13 Ma (Zhang & Guo, 2007;
Cottle et al., 2009b; Leloup et al., 2010; Kali et al., 2010 ; Kellett et al., 2013).
Previously granulitized eclogites were mainly reported within the ADM (Lombardo &
Rolfo, 2000; Groop et al., 2007; Cottle et al., 2009a, b; Kali et al., 2010; Wang et al., 2017).
Movement on N-S trending normal faults contributed to the exhumation by < 6 kbar (~22
km) prior to 11 Ma (Kali et al., 2010). Tomographic imaging shows that the N-S normal
faults around the ADM do not cut through the crust (Huang, Wu, Roecker, & Sheehan, 2009),
and the discovery of Thongmön eclogites which are located outside of the ADM confirms the
dominant role of the STDS for the exhumation of eclogites both inside and outside of the
ADM. Different mechanisms have been used to explain the uplift of eclogites in the central
Himalayas (Jessup et al., 2008; Kali et al., 2010; Corrie et al., 2010; Grujic et al., 2011), we
believe that their uplift are mainly controlled by the STDS.
We recognized one prograde and a peak metamorphic stage based on garnet, inclusions,
and quantitative P–T pseudosections: the amphibole eclogite facies M1(1) and the peak
eclogite facies M1(2). The peak eclogite facies M1(2) occurred at > 750 °C and > 21 kbar in
the Miocene at 17.1–16.7 Ma . The formation of Opx+Pl±Cpx symplectites has been
recognized at 750 °C and 7–9 kbar (Wang et al., 2017) or lower depths (~0.4 kbar; Groppo et
al., 2007). Therefore, we speculate that the Indian continental crust has been subducted to
depths of at least ~70 km beneath the Asian continental crust and the vertical exhumation of
the Thongmön eclogites from the eclogite facies peak pressures (17.1–16.7 Ma) to the end of
the STDS motion (c. 13 Ma), should have proceeded in c. 4 Ma at a rate of ~12–14 mm/y.
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
8 | CONCLUSIONS
(1) Two groups of garnet, Grt I and Grt II, are recognized from the Thongmön eclogites
in central Himalayas. Grt I mainly grew during the prograde metamorphism, while Grt II
formed at the peak eclogite facies stage, and coexisted with metamorphic zircon with a
metamorphic age of 17.1–16.7 Ma.
(2) Detailed petrological studies and pseudosection analyses outline how the Thongmön
eclogites reached the peak pressure: 1) The prograde amphibole eclogite facies is represented
by the assemblage amphibole, garnet (Grt I), omphacite, phengite, rutile and quartz, and
constrained to the P–T conditions of 19–20 kbar and 640–660 °C; 2) During the peak eclogite
facies, amphibole disappeared and small amounts of Grt II and melt appeared. The peak
eclogite facies proceeded at > 21 kbar, >750 °C in the middle Miocene.
(3) The new data demonstrate that Thongmön eclogites were formed by the continuing
continental subduction of Indian plate beneath Euro-Asian plate in the Miocene.
ACKNOWLEDGMENTS
This study was financialy supported by the National Natural Science Foundation of China
(grant 91755206, 41121062). We are grateful to D. Robinson, D. Grujic, H. Ur Rehman for
their critical and constructive comments which improved this paper significantly. We express
our gratitude to Profs C. Wei, S. Song and X. Zhang for their constructively discussing on the
early draft. The great thanks also go to engineer X. Li for assistance with microprobe
analyses, Z. Duan for help in P–T pseudosection calculation, and Y. Wang, F. Liu for
assistance during fieldwork.
REFERENCES
Borghi, A., Castelli, D., Lombardo, B., & Visoná, D. (2003). Thermal and baric evolution of
garnet granulites from the Kharta region of S Tibet, E Himalaya. European Journal of
Mineralogy, 15, 401–418.
Burchfiel, B. C., Chen, Z. L., Hodges, K. V., Liu, Y. P., Royden, L. H., & Deng, C. R.
(1992). The South Tibetan detachment system, Himalayan orogen: Extension
contemporaneous with and parallel to shortening in a collisional mountain belt. Geological
Society of America Special Papers, 269, 1–41.
Burg, J. P., Brunel, M., Gapais, D., Chen, G. M., & Liu, G. H. (1984). Deformation of
leucogranites in the crystalline Main Central Sheet in southern Tibet (China). Journal of
Structural Geology, 6, 535–542.
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
Caddick, M. J., Konopásek, J., & Thompson, A. B. (2010). Preservation of garnet growth
zoning and the duration of prograde metamorphism. Journal of Petrology, 51, 2327–2347.
Carson, C. J., Clarke, G. L., & Powell, R. (2000). Hydration of eclogite, Pam Peninsula, New
Caledonia. Journal of Metamorphic Geology, 18, 79–90.
Chakraborty, S., & Ganguly, J. (1992). Cation diffusion in aluminosilicate garnets:
experimental determination in spessartine–almandine diffusion couples, evaluation of
effective binary diffusion coefficients, and applications. Contributions to Mineralogy and
Petrology, 111, 74–86.
Chakungal, J. (2006). Geochemistry and metamorphism of metabasites, and spatial variation
of P–T paths across the Bhutan Himalaya: Implications for the exhumation of the Greater
Himalayan Sequence. Dissertation Abstracts International, 68-09, 5811.
Chakungal, J., Dostal, J., Grujic, D., Duchêne, S., & Ghalley, S. K. (2010). Provenance of the
Greater Himalayan Sequence: evidence from mafic eclogite–granulites and amphibolites in
NW Bhutan. Tectonophysics, 480, 198–212.
Chernoff, C. B. & Carlson, W. D. (1999). Trace element zoning as a record of chemical
disequilibrium during garnet growth. Geology, 27, 555–558.
Corrie, S. L., Kohn, M. J., & Vervoort, J. D. (2010). Young eclogite from the Greater
Himalayan Sequence, Arun Valley, eastern Nepal: P–T–t path and tectonic implications.
Earth and Planetary Science Letters, 289, 406–416.
Cottle, J. M., Jessup, M. J., Newell, D. L., Searle, M. P., Law, R. D., & Horstwood, M. S. A.
(2007). Structural insights into the early stages of exhumation along an orogen-scale
detachment: The South Tibetan Detachment System, Dzakaa Chu section, Eastern
Himalaya. Journal of Structural Geology, 29, 1781–1797.
Cottle, J. M., Jessup, M. J., Newell, D. L., Horstwood, M. S., Noble, S. R., Parrish, R. R., ...
Searle, M. P. (2009a). Geochronology of granulitized eclogite from the Ama Drime Massif:
implications for the tectonic evolution of the South Tibetan Himalaya. Tectonics, 28(1).
Cottle, J. M., Searle, M. P., Horstwood, M.S.A., & Waters, D.J. (2009b). Timing of
midcrustal metamorphism, melting, and deformation in the Mount Everest Region of
southern Tibet revealed by U(-Th) -Pb geochronology. The Journal of Geology, 117,
643–664.
Cruciani, G., Franceschelli, M., & Groppo, C. (2011). P-T evolution of eclogite-facies
metabasite from NE Sardinia, Italy: insights into the prograde evolution of Variscan
eclogites. Lithos, 121, 135–150.
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
Cruciani, G., Franceschelli, M., Groppo, C., & Spano, M.E. (2012). Metamorphic evolution
of non-equilibrated granulitized eclogite from Punta de li Tulchi (Variscan Sardinia)
determined through texturally controlled thermodynamic modelling. Journal of
Metamorphic Geology, 30, 667–685.
Cutts, K. A., Kinny, P. D., Strachan, R. A., Hand, M., Kelsey, D. E., Emery, M., ... Leslie, A.
G. (2010). Three metamorphic events recorded in a single garnet: integrated phase
modelling, in situ LA-ICPMS and SIMS geochronology from the Moine Supergroup, NW
Scotland. Journal of Metamorphic Geology, 28, 249–267.
de Sigoyer, J., Chavagnac, V., Blichert-Toft, J., Villa, I. M., Luais, B., Guillot, S., ... Mascle,
G. (2000). Dating the Indian continental subduction and collisional thickening in the
northwest Himalaya: Multichronology of the Tso Morari eclogites. Geology, 28, 487–490.
de Sigoyer, J., Guillot, S., Laudeaux, J. M., & Mascle, G. (1997). Glaucophane-bearing
eclogites in the Tso Morari dome (eastern Ladakh, NW Himalaya). European Journal of
Mineralogy, 9, 1073–1083.
Droop, G. T. R. (1987). A general equation for estimating Fe3+
concentration in
ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric
criteria. Mineralogical Magazine, 51, 431–435.
Elvevold, S., & Gilotti, J.A. (2000). Pressure–temperature evolution of retrogressed kyanite
eclogites, Weinschenk Island, North-East Greenland Caledonides. Lithos, 53, 127–147.
Florence, F. P. & Spear, F. S. (1995). Intergranular diffusion kinetics of Fe and Mg during
retrograde metamorphism of a pelitic gneiss from the Adirondack Mountains. Earth and
Planetary Science Letters, 134, 329–340.
Goscombe, B., Gray, D., & Hand, M. (2006). Crustal architecture of the Himalayan
metamorphic front in eastern Nepal. Gondwana Research, 10, 232–255.
Green, E. C. R., White, R. W., Diener, J. F. A., Powell, R., Holland, T. J. B., & Palin, R. M.
(2016). Activity–composition relations for the calculation of partial melting equilibria for
metabasic rocks. Journal of Metamorphic Geology, 34, 845-869.
Groppo, C., Lombardo, B., Rolfo, F., & Pertusati, P. (2007). Clockwise exhumation path of
granulitized eclogites from the Ama Drime range (eastern Himalayas). Journal of
Metamorphic Geology, 25, 51–75.
Groppo, C., Rolfo, F., & Indares, A. (2012). Partial melting in the Higher Himalayan
Crystallines of Eastern Nepal: the effect of decompression and implications for the
‘channel flow’ model. Journal of Petrology, 53, 1057–1088.
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
Grujic, D., Warren, C. J., & Wooden, J. L. (2011). Rapid synconvergent exhumation of
Mioceneaged lower orogenic crust in the eastern Himalaya. Lithosphere, 3, 346–366.
Guiraud, M., Powell, R., & Cottin, J.Y. (1996). Hydration of
orthopyroxene–cordierite-bearing assemblages at Laouni, Central Hoggar, Algeria. Journal
of Metamorphic Geology, 14, 467–476.
Hellström, L. M., Gradin, P. A., & Carlberg, T. (2008). A method for experimental
investigation of the wood chipping process. Nordic Pulp & Paper Research Journal, 23,
339-342.
Hickmott, D. D., Shimizu, N., Spear, F. S., & Selverstone, J. (1987). Trace-element zoning in
a metamorphic garnet. Geology, 15, 573-576.
Hickmott, D. D., Sorensen, S. S., & Rogers, P. S. Z. (1992). Metasomatism in a subduction
complex: Constraints from microanalysis of trace elements in minerals from garnet
amphibolite from the Catalina Schist. Geology, 20, 347-350.
Hodges, K. V., Hames, W. E., Olszewski, W., Burchfiel, B. C., Royden, L. H., & Chen, Z.
(1994). Thermobarometric and 40
Ar/39
Ar geochronologic constraints on Eohimalayan
metamorphism in the Dinggyê area, southern Tibet. Contributions to Mineralogy and
Petrology, 117, 151-163.
Holland, T. J. B., & Powell, R. (1998). An internally consistent thermodynamic dataset for
phases of petrological interest. Journal of Metamorphic Geology, 16, 309–343.
Holland, T. J. B. & Powell, R. (2003). Activity-composition relations for phases in
petrological calculations: an asymmetric multicomponent formulation. Contributions to
Mineralogy and Petrology, 145, 492–501.
Holland, T. J. B. & Powell, R. (2011). An improved and extended internally consistent
thermodynamic dataset for phases of petrological interest, involving a new equation of
state for solids. Journal of Metamorphic Geology, 29, 333–383.
Huang, G. C. D., Wu, F. T., Roecker, S. W., & Sheehan, A. F. (2009). Lithospheric structure
of the central Himalaya from 3-D tomographic imaging. Tectonophysics, 475, 524-543.
Jessup, M. J. & Cottle, J. M. (2010). Progression from south-directed extrusion to
orogen-parallel extension in the southern margin of the Tibetan Plateau, Mount Everest
region, Tibet. The Journal of Geology, 118, 467-486.
Jessup, M. J., Newell, D. L., Cottle, J. M., Berger, A. L., & Spotila, J. A. (2008).
Orogen-parallel extension and exhumation enhanced by denudation in the trans-Himalayan
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
Arun River gorge, Ama Drime Massif, Tibet-Nepal. Geology, 36, 587-590.
Ji, J. Q., Zhong, D. L., Song, B., Zhu, M. F., & Wen, D. R. (2004). Metamorphism,
geochemistry and geochronology of high-pressure granulite in the central Himalayas. Acta
Petrologica Sinica, 20, 1283–1300 (in Chinese with English abstract).
Jung, C., Jung, S., Nebel, O., Hellebrand, E., Masberg, P., & Hoffer, E. 2009. Fluid-present
melting of meta-igneous rocks and the generation of leucogranites—Constraints from
garnet major-and trace element data, Lu–Hf whole rock–garnet ages and whole rock
Nd–Sr–Hf–O isotope data. Lithos, 111, 220-235.
Kali, E., Leloup, P., Arnaud, N., Mahéo, G., Liu, D., Boutonnet, E., ... Li, H. (2010).
Exhumation history of the deepest central Himalayan rocks, Ama Drime range: Key
pressure-temperature-deformation-time constraints on orogenic models. Tectonics, 29(2).
Kaneko, Y., Katayama, I., Yamamoto, H., Misawa, K., Ishikawa, M., Rehman, H. U., ...
Shiraishi, K. (2003). Timing of himalayan ultrahigh-pressure metamorphism: sinking rate
and subduction angle of the indian continental crust beneath asia. Journal of Metamorphic
Geology, 21, 589–599.
Kellett, D. A., Cottle, J. M., & Smit, M. (2014). Eocene deep crust at Ama Drime, Tibet:
early evolution of the Himalayan orogen. Lithosphere, 6, 220–229.
Kellett, D. A., Grujic, D., Coutand, I., Cottle, J., & Mukul, M. (2013). The south tibetan
detachment system facilitates ultra rapid cooling of granulite‐facies rocks in sikkim
himalaya. Tectonics, 32, 252-270.
Klein, M., Stosch, H. G., Seck, H. A., & Shimizu, N. (2000). Experimental partitioning of
high field strength and rare earth elements between clinopyroxene and garnet in andesitic
to tonalitic systems. Geochimica et Cosmochimica Acta, 64, 99–115.
Konrad-Schmolke, M., Zack, T., O'Brien, P. J., & Jacob, D. E. (2008). Combined
thermodynamic and rare earth element modelling of garnet growth during subduction:
examples from ultrahigh-pressure eclogite of the Western Gneiss Region, Norway. Earth
and Planetary Science Letters, 272, 488–498.
Langille, J. M., Jessup, M. J., Cottle, J. M., Newell, D., & Seward, G. (2010). Kinematic
evolution of the Ama Drime detachment: insights into orogen-parallel extension and
exhumation of the Ama Drime Massif, Tibet—Nepal. Journal of Structural Geology, 32,
900–919.
Leloup, P. H., Mahéo, G., Arnaud, N., Kali, E., Boutonnet, E., Liu, D., ... Haibing, L. (2010).
The South Tibet detachment shear zone in the Dinggye area: time constraints on extrusion
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
models of the Himalayas. Earth and Planetary Science Letters, 292, 1–16.
Li, X. L, Zhang, L. F, Wei, C. J, Slabunov, A. I., & Bader, T. (2018). Quartz and
orthopyroxene exsolution lamellae in clinopyroxene and the metamorphic p–t path of
Belomorian eclogites. Journal of Metamorphic Geology, 36, 1-22.
Liu, S. W., Zhang, J. J., Shu, G. M., & Li, Q. G. (2005). Mineral chemistry, P–T–t paths
andexhumation processes of mafic granulite in Dinggye, Southern Tibet. Science in China
Series D: Earth Sciences, 48, 1870–1881 (in Chinese with English abstract).
Liu, Y., Siebel, W., Massonne, H. J., & Xiao, X. C. (2007). Geochronological and
petrological constraints for tectonic evolution of the Central Greater Himalayan Sequence
in the Kharta Area, Southern Tibet. The Journal of Geology, 115, 215–230.
Lombardo, B., Pertusati, P., Rolfo, F., & Visoná, D. (1998). First report of eclogites from the
E Himalaya: implications for the Himalayan orogeny. Memorie di Scienze Geologiche, 50,
67–68.
Lombardo, B. & Rolfo, F. (2000). Two contrasting eclogite types in the Himalayas:
implicationsfor the Himalayan orogeny. Journal of Geodynamics, 30, 37–60.
Medaris, L. G., Ghent, E. D., Wang, H. F., Fournelle, J. H., & Jelínek, E. (2006). The
Spaciche eclogite: constraints on the P-T-t history of the Gföhl granulite terrane,
Moldanubian Zone, Bohemian Massif. Mineralogy and Petrology, 86, 203–220.
Meier, K., & Hiltner, E. (1993). Deformation and metamorphism within the Main Central
Thrust zone, Arun Tectonic Window, eastern Nepal. Geological Society Special
Publication, 74, 511-523.
Morimoto, N. (1988). Nomenclature of Pyroxenes. Mineralogy and Petrology, 39, 55–76.
Mukherjee, B. K. & Sachan, H. K. (2001). Discovery of coesite from Indian Himalaya: a
record of ultra-high pressure metamorphism in Indian continental crust. Current Science,
81, 1358–1361.
O'Brien, P. J. (1997). Garnet zoning and reaction textures in overprinted eclogites, Bohemian
Massif, European Variscides: A record of their thermal history during exhumation. Lithos,
41, 119–133.
O'Brien, P. J. (1999). Asymmetric zoning pro files in garnet fromHP–HT granulite and
implications for volume and grain-boundary diffusion. Mineralogical Magazine, 63,
227–238.
O'Brien, P. J. & Ziemann, M. A. (2008). Preservation of coesite in exhumed eclogite: insights
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
from Raman mapping. European Journal of Mineralogy, 20, 827–834.
O'Brien, P. J., Zotov, N., Law, R., Khan, M. A., & Jan, M. Q. (2001). Coesite in Himalayan
eclogite and implications for models of India-Asia collision. Geology, 29, 435-438.
Parrish, R. R., Gough, S. J., Searle, M. P., & Waters, D. J. (2006). Plate velocity exhumation
of ultrahigh-pressure eclogites in the Pakistan Himalaya. Geology, 34, 989–992.
Paton, C., Woodhead, J. D., Hellstrom, J. C., Hergt, J. M., Greig, A., & Maas, R. (2010).
Improved laser ablation U-Pb zircon geochronology through robust downhole fractionation
correction. Geochemistry, Geophysics, Geosystems, 11(3).
Paton, C., Hellstrom, J., Paul, B., Woodhead, J., & Hergt, J. (2011). Iolite: Freeware for the
visualisation and processing of mass spectrometric data. Journal of Analytical Atomic
Spectrometry, 26, 2508-2518.
Pognante, U. & Spencer, D. A. (1991). First report of eclogites from the Himalayan belt,
Kaghan Valley (northern Pakistan). European Journal of Mineralogy, 3, 613–618.
Powell, R. & Holland, T. J. B. (1988). An internally consistent dataset with uncertainties and
correlations: 3. Applications to geobarometry, worked examples and a computer program.
Journal of metamorphic Geology, 6, 173-204.
Powell, R. & Holland, T. J. B. (2008). On thermobarometry. Journal of Metamorphic
Geology, 26, 155–179.
Ravna, E. K. (2000). The garnet–clinopyroxene Fe2+
–Mg geothermometer: an updated
calibration. Journal of Metamorphic Geology, 18, 211–219.
Rubatto, D. (2002). Zircon trace element geochemistry: partitioning with garnet and the link
between U–Pb ages and metamorphism. Chemical Geology, 184, 123–138.
Rubatto, D. & Hermann, J. (2003). Zircon formation during fluid circulation in eclogites
(Monviso, Western Alps): implications for Zr and Hf budget in subduction zones.
Geochimica et Cosmochimica Acta, 67, 2173–2187.
Rubatto, D., Hermann, J., & Buick, I.S. (2006). Temperature and bulk composition control on
the growth of monazite and zircon during low-pressure anatexis (Mount Stafford, central
Australia). Journal of Petrology, 47, 1973–1996.
Rubatto, D. & Hermann, J. (2007). Experimental zircon/melt and zircon/garnet traceelement
partitioning and implications for the geochronology of crustal rocks. Chemical Geology,
241, 38–61.
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
Santos, T.J.S., Garcia, M.G.M., Amaral, W.S., Caby, R., Wernick, E., Arthaud, M. H., ...
Santosh, M. (2009). Relics of eclogite facies assemblages in the Ceará Central Domain,
NW Borborema Province, NE Brazil: implications for the assembly of West Gondwana.
Gondwana Research, 15, 454-470.
Schmidt, A., Weyer, S., Mezger, K., Scherer, E. E., Xiao, Y., Hoefs, J., Brey, G. P. (2008).
Rapid eclogitisation of the Dabie-Sulu UHP terrane: constraints from Lu-Hf garnet
geochronology. Earth and Planetary Science Letters, 273, 203–213.
Schwandt, C. S., Papike, J. J., & Shearer, C. K. (1996). Trace element zoning in pelitic garnet
of the Black Hills, South Dakota. American Mineralogist, 81, 1195–1207.
Simpson, R. L., Parrish, R. R., Searle, M. P., & Waters, D. J. (2000). Two episodes of
monazite crystallization during metamorphism and crustal melting in the Everest region of
the Nepalese Himalaya. Geology, 28, 403–406.
Spear, F. S. (1991). On the interpretation of peak metamorphic temperatures in light of garnet
diffusion during cooling. Journal of Metamorphic Geology, 9, 379–388.
Spear, F. S. & Kohn, M. J. (1996). Trace element zoning in garnet as a monitor of crustal
melting. Geology, 24, 1099–1102.
Spear, F. S., & Daniel, C. G. (2001). Diffusion control of garnet growth, Harpswell Neck,
Maine, USA. Journal of Metamorphic Geology, 19, 179–195.
Spencer, D.A., Ramsay, J.G., Spencer-Cervato, C., Pognante, U., Chaudhry, M.N., &
Ghazanfar, M. (1990). High pressure (eclogite facies) metamorphism in the Indian plate,
NW Himalaya, Pakistan. Geological Bulletin of the University of Peshawar, 23, 87–100.
Štípská, P. & Powell, R. (2005). Constraining the P–T path of a MORB-type eclogite using
pseudosections, garnet zoning and garnet–clinopyroxene thermometry: an example from
the Bohemian Massif. Journal of Metamorphic Geology, 23, 725–743.
Stübner, K., Grujic, D., Parrish, R. R., Roberts, N. M., Kronz, A., Wooden, J., Ahmad, T.
(2014). Monazite geochronology unravels the timing of crustal thickening in NW
Himalaya. Lithos, 210, 111-128.
Sun, S. S. & McDonough, W.F. (1989). Chemical and isotopic systematics of oceanic basalts:
implications for mantle composition and processes. Geological Society, London, Special
Publications, 42, 313-345.
Thöni, M. (2003). Sm–Nd isotope systematics in garnet from different lithologies (Eastern
Alps): age results, and an evaluation of potential problems for garnet Sm–Nd chronometry
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
[Chem. Geol. 185 (2002) 255–281]. Chemical Geology, 194, 353-379.
Tonarini, S., Villa, I. M., Oberli, F., Meier, M., Spencer, D. A., Pognante, U., Ramsay, J. G.
(1993). Eocene age of eclogite metamorphism in Pakistan Himalaya: implications for
India-Eurasia collision. Terra Nova, 5, 13–20.
Tsai, C. H. & Liou, J. G. (2000). Eclogite facies relics and inferred ultrahigh-pressure
metamorphism in the North Dabie Complex, central-eastern China. American Mineralogist,
85, 1–8.
Wang, Y. H., Zhang, L. F., Zhang, J. J. & Wei, C. J. (2017). The youngest eclogite in central
Himalaya: P–T path, U–Pb zircon age and its tectonic implication. Gondwana Research,
41, 188-206.
Wang, Q. Y., Pan, Y. M., Chen, N. S., Li, X. Y., & Chen, H. H. (2009). Proterozoic
polymetamorphism in the Quanji Block, northwestern China: evidence from microtextures,
garnet compositions and monazite CHIME ages. Journal of Asian Earth Sciences, 34,
686–698.
Wang, X. M. & Liou, J. G. (1991). Regional ultrahigh-pressure coesite-bearing eclogitic
terrane in central China: evidence from country rocks, gneiss, marble, and metapelite.
Geology, 19, 933–936.
Warren, C. J., Grujic, D., Kellett, D. A., Cottle, J., Jamieson, R. A., & Ghalley, K.S. (2011).
Probing the depths of the India–Asia collision: U–Th–Pb monazite chronology of granulites
from NW Bhutan. Tectonics, 30(2).
White, R. W., Powell, R., Holland, T., Johnson, T. E., & Green, E. (2014a). New mineral
activity–composition relations for thermodynamic calculations in metapelitic systems.
Journal of Metamorphic Geology, 32, 261–286.
White, R. W., Powell, R., & Johnson, T. E. (2014b). The effect of Mn on mineral stability in
metapelites revisited: new a–x relations for manganese-bearing minerals. Journal of
Metamorphic Geology, 32, 809–828.
Whitehouse, M. J., & Platt, J. P. (2003). Dating high-grade metamorphism: constraints from
rare-earth elements in zircon and garnet. Contributions to Mineralogy and Petrology, 145,
61–74.
Whitney, D. L. & Evans, B.W. (2010). Abbreviations for names of rock-forming minerals.
American Mineralogist, 95, 185-187.
Winter, J. D. (2001). An introduction to igneous and metamorphic petrology. New York:
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
Prentice Hall. 1-697.
Wu, F. Y., Liu, Z. C., Liu, X. C., & Ji, W. Q. (2015). Himalayan leucogranite: Petrogenesis
and implications to orogenesis and plateau uplift. Acta Petrologica Sinica, 31, 1-36.
Xu, Z. Q., Wang, Q., Pêcher, A., Liang, F. H., Qi, X. X., Cai, Z. H., ... Cao, H. (2013).
Orogen‐parallel ductile extension and extrusion of the Greater Himalaya in the late
Oligocene and Miocene. Tectonics, 32, 191-215.
Yardley, B. W. (1977). An empirical study of diffusion in garnet. American Mineralogist, 62,
793–800.
Yin A. (2006). Cenozoic tectonic evolution of the Himalayan orogen as constrained by
along-strike variation of structural geometry, exhumation history, and foreland
sedimentation. Earth-Science Reviews, 76, 1–131.
Yuan, H. L., Gao, S., Liu, X. M., Li, H. M., Günther, D., & Wu, F. Y. (2004). Accurate U–Pb
age and trace element determinations of zircon by laser ablation-inductively coupled
plasma mass spectrometry. Geostandards and Geoanalytical Research, 28, 353–370.
Zhang, J. J. (2007). A review on the extensional structures in the northern Himalaya and
southern Tibet. Geological Bulletin of China, 26, 639-649.
Zhang, J. J. & Guo, L. (2007). Structure and geochronology of the southern Xainza–Dinggye
rift and its relationship to the South Tibetan Detachment System. Journal of Asian Earth
Sciences, 29, 722–736.
Zhao, G. C., Cawood, P. A., Wilde, S. A., & Lu, L. Z. (2001). High-pressure granulites
(retrograded eclogites) from the Hengshan Complex, North China Craton: Petrology and
tectonic implications. Journal of Petrology, 42, 1141–1170.
Zhou, L. G., Xia, Q. X., Zheng, Y. F., & Chen, R. X. (2011). Multistage growth of garnet in
ultrahigh-pressure eclogite during continental collision in the Dabie orogen: constrained by
trace elements and U–Pb ages. Lithos, 127, 101-127.
Zong, K. Q., Liu, Y. S., Liu, X. M., & Zhang, B. H. (2007). Trace elemental records of
short-lived heating during exhumation of the CCSD eclogites. Chinese Science Bulletin,
52, 813–824.
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
SUPPORTING INFORMATION
Additional Supporting Information may be found online in the supporting information tab for
this article.
Figure S1. Classification diagram for different plagioclase in the Thongmön eclogites.
Plagioclase occurring in different textures are characterized by different Ca contents.
Table S1. Published eclogitization ages of the granulitized eclogites from the central
Himalaya.
Table S2. Bulk compositions of Thongmön eclogites.
Table S3. Major elements of garnet in Thongmön eclogites analyzed by EMPA.
Table S4. Representative microprobe analyses of clinopyroxene, orthopyroxene, plagioclase,
amphibole from Thongmön eclogites.
Table S5. Trace element contents of garnet in Thongmön eclogites analyzed by LA-ICP-MS.
Table S6. SHRIMP U–Pb isotope data of zircon from the Thongmön eclogites (DR1509 &
DR1528).
Table S7. Trace element compositions of zircon from the Thongmön eclogites (DR1509 &
DR1528).
Table S8. Representative microprobe analyses of mineral inclusions in zircon.
Table S9. Distribution coefficients between zircon and garnet from the Thongmön eclogites.
FIGURE CAPTIONS
Figure 1. (a) Geological sketch map of the Himalayan orogeny, and locations of Himalayan
eclogites (modified from Yin, 2006;Xu et al., 2013); (b) Geological map of the Ama
Drim Massif (modified from Kali et al., 2010), showing the sample location.
Figure 2. Outcrops of the (granulitized) eclogites in Thongmön area. (a) & (b) granulitized
eclogite lenses surrounded by paragneiss; (c) local parasitic folding of granulitized eclogite
layers; (d) well preserved eclogite.
Figure 3. Photomicrographs and BSE images of the Thongmön eclogites. (a) A
porphyroblastic garnet contains abundant mineral inclusions — omphacite (Cpx I),
amphibole (Amp I) and quartz coexist as inclusions in the garnet mantle, and
clinopyroxene (Cpx III) + orthopyroxene + plagioclase (Pl II) coexist in a sealed
microcrack; plane-polarized light and BSE images; (b) Omphacite (Cpx I) surrounded by
Acc
epte
d A
rtic
le
This article is protected by copyright. All rights reserved.
diopside + plagioclase symplectite (Cpx II + Pl I(1)) in the matrix; plane- and
cross-polarized light. (c) Relict omp