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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

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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: lfzhang@pku.edu.cn

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

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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been replaced by Bt+Pl symplectites, represents the prograde amphibole eclogite facies stage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

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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).

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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

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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)

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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

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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).

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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

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(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 =

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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.

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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

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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.

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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),

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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

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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;

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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).

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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

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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

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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.

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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.

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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

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diopside + plagioclase symplectite (Cpx II + Pl I(1)) in the matrix; plane- and

cross-polarized light. (c) Relict omp