Journal of Earth Science  2018, Vol. 29 Issue (5): 1010-1025   PDF    
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High-Temperature Metamorphism, Anataxis and Tectonic Evolution of a Mafic Granulite from the Eastern Himalayan Orogen
Zeming Zhang1,2, Huixia Ding1, Xin Dong2, Zuolin Tian2, Dongyan Kang1, Hongchen Mu1, Shengkai Qin2, Yuanyuan Jiang1, Mengmei Li1    
1. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;
2. Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, China
ABSTRACT: The Himalayan Orogen, resulting from the Tertiary collision of Indian and Asian continents, is a natural laboratory for studying metamorphism, partial melting and granite formation of collisional orogens. However, metamorphic and anatectic conditions and timescales of meta-mafic rocks in the Greater Himalayan Sequences (GHS) in the east-central Himalaya remain controversial. In this paper, we conduct a study of petrology and geochronology of mafic granulite from the Eastern Himalayan Syntaxis (EHS). The mafic granulite with abundant leucosome bands occurs as layers within felsic granulites and is well deformed. The granulite consists of garnet, plagioclase, amphibole and quartz with minor clinopyroxene, orthopyroxene, biotite, rutile, titanite and ilmenite. The garnet has growth compositional zoning and contains abundant mineral inclusions in its core. Peak metamorphic mineral assemblage of the granulite is garnet, amphibole, plagioclase, quartz, clinopyroxene and rutile, recording a high-pressure (HP) and high-temperature (HT) peak-metamorphism under conditions of 14-15.5 kbar and 780-790℃ in the presence of melt. The reconstructed clockwise P-T path is characterized by an early heating and burial prograde metamorphism, and late isothermal and cooling decompression retrogression. The granulite witnessed a long lasting HT metamorphism, partial melting and melt crystallization process which began at ca. 39 Ma and lasted to ca. 11 Ma. The present study shows that various high-grade rocks of the GHS in the EHS core experienced similar metamorphic conditions and P-T-t paths, indicating that they occurred as a coherent slab during the subduction and exhumation of Indian lithosphere. The significant melts generated during the prograde metamorphism of the GHS rocks not only contributed to the formation of the Himalayan leucogranite, but also resulted in the rheological weakening and ductile flow of the thickened lower crust of the Himalayan Orogen.
KEY WORDS: granulite    HT metamorphism    partial melting    timescale    collisional orogeny    

0 INTRODUTION

The Himalayan Orogen, formed by the Early Cenozoic collision of Indian and Asian continents (e.g., Ding et al., 2016; Najman et al., 2010; Guillot et al., 2008; Yin and Harrison, 2000), exposes a spectacular assemblage of metamorphic rocks from the mid- and deep crust that have fostered numerous models of how the crust responds to continental collisions (Ali et al., 2016; Huangfu et al., 2016; Kohn, 2014). In the east-central Himalayan Orogen, the metamorphism and partial melting of pelitic and felsic granulites have been extensively studied, whereas mafic granulites were rarely involved (Zhang et al., 2018, 2017a and references therein). Therefore, the metamorphic pressure (P) and temperature (T) conditions and P-T-t (time) path, and anatectic process of the mafic granulites were poorly constrained. In fact, the mafic metamorphic rocks, including granulite and amphibolite, are common rock types and occur as layers or lenses within gneisses and schists in the Himalayan orogenic core, and therefore are essential for revealing the metamorphism, partial melting and rheology of the thickened lower crust, formation of leucogranite and tectonic evolution of the Himalayan Orogen. Here, we conduct a petrological and geochronological study of the mafic granulite from the Eastern Himalayan Syntaxis (EHS). The aims are to reveal P-T conditions, timescales and mechanism of high-grade metamorphism and associated partial melting of the mafic granulite, and to discuss related tectonic implications.

1 GEOLOGICAL SETTING AND SAMPLE CHARACTERISTICS

The Himalaya is commonly described as a three layer-two fault stack. Namely, a high-grade metamorphic crystalline core, the Greater Himalayan Sequences (GHS), is separated from units above and below by shear zones. The Lesser Himalayan Sequences (LHS) underlie the GHS separated by the main central thrust, and the Tethyan Himalayan Sequences (THS) overlie it along the South Tibet detachment (Kohn, 2014 and references therein; Yin and Harrison, 2000).

The Eastern Himalayan Syntaxis (EHS), located at the eastern segment of the Himalayan Orogen, consists of three major tectonic units (Fig. 1). They are the Lhasa terrane, representing the southern segment of the Asian Continent, the Indus-Tsangpo suture zone (ITS), forming as the residual of the Neo-Tethyan Ocean between the Asian and Indian plates, and the Himalayan Sequences, the north margin of the Indian continent (Yin and Harrison, 2000). The Himalayan Sequences include the Tethyan Himalayan Sequences (THS) and the Greater Himalayan Sequences (GHS) (Fig. 1). The former consists of Paleozoic and Mesozoic sedimentary strata metamorphosed under greenschist to epidote-amphibolite facies conditions. The GHS, referred to the Namche Barwa complex, consists of high-grade metamorphic orthogneiss, paragneiss, amphibolite, schist, marble and migmatite; some felsic and mafic rocks are metamorphosed magmatic rocks with Late Paleoproterozoic to Early Paleozoic protolithic ages (Zhang et al., 2012). Available zircon U-Pb dating results show that the Namche Barwa complex has highly variable metamorphic and anatectic ages ranging from 40 to 7 Ma (Zhang et al., 2015, 2012, 2010; Liu and Zhang, 2014; Su et al., 2012; Xu et al., 2010; Booth et al., 2009, 2004; Liu et al., 2007; Ding et al., 2001; Burg et al., 1998; Liu and Zhong, 1997).

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Figure 1. (a) Sketch tectonic map of the Tibetan Plateau and southeastern Asia. ITS. Indus-Tsangpo suture zone. The blue square shows the locations of Fig. 1b; (b) sketch geological map of the Eastern Himalayan Syntaxis (modified after Zhang et al., 2015), showing the sampling location.

The studied mafic granulite is collected from the core of the EHS (Fig. 1). The granulite body occurs as a layer with a thickness of ca. 30 m within felsic granulites, shows steep to vertical deformation foliation, and contains abundant felsic leucosomes occurring as bands parallel to the foliation of hosting mafic granulite (Fig. 2). Garnet commonly occurs as residual grain in the mafic granulite, and shows a dark kelyphitic rim of very fine-grained amphibole and plagioclase (Fig. 2). The garnet-bearing mafic granulite has been completely transformed into garnet-free amphibolite along the marginal parts of the mafic granulite layer. The concordant bands of leucosomes contain plagioclase, K-feldspar and quartz with or without minor garnet, biotite or amphibole.

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Figure 2. Field photos showing the mafic granulites. (a) The retrograde mafic granulite consists of amphibole, garnet and plagioclase, and contains abundant felsic leucosomes; (b) the mafic granulite, consisting of garnet, clinopyroxene, amphibole and plagioclase, was partly retrograded to the amphibolite consisting of amphibole and plagioclase. Note that the coarse-grained red garnet grains are surrounded by dark kelyphitic rims of very fine-grained amphibole and plagioclase. The coins for scale are 1.5 cm across.
2 ANALYTICAL METHODS

Mineral chemical compositions were analyzed using a JEOL JXA 8900 electron microprobe (EPM) in the wavelength-dispersive detection mode with a 15 kV accelerating voltage, 20 nA beam current, 5 μm probe diameter, and counting time of 10 s for peak and background, at the Institute of Geology, Chinese Academy of Geological Science. Natural or synthetic standards were used for EPM analysis, and ZAF corrections were carried out. Major element compositions of the whole-rock were determined by X-ray fluorescence (XRF) (Rigaku-3080), with analytical uncertainties of < 0.5%, at the National Geological Analysis Center of China.

The U-Pb dating and trace element analysis of zircon were simultaneously conducted by LA-ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as description by Zong et al. (2017). Laser sampling was performed using a GeolasPro laser ablation system that consisted of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7700e ICP-MS instrument was applied to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. A "wire" signal smoothing device is included in this laser ablation system (Hu et al., 2015). The spot size and frequency of the laser were set to 24 μm and 5 Hz, respectively. Zircon 91500 and glass NIST610 were used as external standards for U-Pb dating and trace element calibration, respectively. Each analysis incorporated a background acquisition of approximately 20-30 s followed by 50 s of data acquisition from the sample. An Excel-based software ICPMSDataCal was used to perform off-line selection and integration of background and analyzed signals, time-drift correction and quantitative calibration for trace element analysis and U-Pb dating (Liu et al., 2010, 2008). Concordia diagrams were made using Isoplot/Ex_ver3 (Ludwig, 2003).

3 PETROLOGY

The studied mafic granulite shows a distinct foliation and banded structure, defined by alternating layers of light leucosomes and dark melanosomes (granulite) (Fig. 2). The granulite displays a porphyroblastic texture, and consists of porphyroblastic garnet, and matrix minerals amphibole, plagioclase, quartz, clinopyroxene, orthopyroxene, biotite, rutile, titanite and ilmenite (Figs. 2 and 3). The coarse-grained garnet porphyroblasts have a mineral inclusion-rich core, and a nearly inclusion-free rim (Figs. 3a, 3b), and are commonly replaced by symplectitic coronas of amphibole+plagioclase (Figs. 3a-3c) or orthopyroxene+quartz (Figs. 3d, 3e) along their rims. The mineral inclusions include amphibole, plagioclase, quartz, biotite and titanite. The clinopyroxene occurs as residue which is partly replaced by amphibole and symplectite of amphibole+plagioclase (Figs. 3d, 3e). These textural features show that the granulite probably witnessed four stages of metamorphism. The first stage of prograde metamorphism (M1) is represented by the core of porphyroblastic garnet and hosting inclusion minerals of amphibole, plagioclase, quartz, biotite and titanite. The peak-metamorphism (M2) is characterized by coexistence of the rim of porphyroblastic garnet and matrix minerals of amphibole, plagioclase, clinopyroxene, quartz, biotite and rutile. The retrograde stage (M3) is characterized by formation of symplectitic minerals, and the related mineral assemblage is garnet (outmost rim)+amphibole+plagioclase+ biotite+clinopyroxene+quartz+ilmenite. The presence of orthopyroxene+quartz in the corona around garnet is interpreted to occur if water in the local surrounding is lacking. Because the margins of the mafic granulite layer have completely transformed into garnet-free amphibolite, the latest stage of retrograde metamorphism (M4) is characterized by coexistence of amphibole, plagioclase, biotite, quartz, ilmenite and titanite.

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Figure 3. Photomicrographs (a), (b), (d) and BSE images (c), (e) of the mafic granulite. The granulite consists of porphyroblastic garnet and matrix of amphibole and plagioclase, with minor quartz, biotite and ilmenite (a), (b). The dashed line in (a) refers to the profile shown in Fig. 5. The garnet core is rich in mineral inclusions while the rim is nearly inclusion-free. At the outermost rim, garnet is replaced by amphibole, plagioclase and ilmenite, or plagioclase and biotite (b), (c), forming kelyphitic coronas. Occasionally, garnet is replaced by symplectitic coronas of orthopyroxene and quartz (d), (e). Clinopyroxene is replaced by symplectite of amphibole or amphibole+plagioclase (d), (e). Mineral abbreviations used in this paper are as follows: Ab. albite; Alm. almandine; Am. amphibolite; An. anorthite; And. andalusite; Anr. andradite; Bt. biotite; Cpx. clinopyroxene; Grs. grossular; Gt. garnet; Ilm. ilmenite; Ky. kyanite; Ms. muscovite; Opx. orthopyroxene; Or. orthoclase; Pl. plagioclase; Prp. pyrope; Qz. quartz; Rt. rutile; Sil. sillimanite; Spe. spessartine; Ttn. titanite; L. liquid (melt).

X-ray mapping and profile analysis show that the garnet porphyroblast has distinct compositional zoning, characterized by increasing in Mg, and decreasing in Mn and Ca from the core to rim (Table 1; Fig. 4), and therefore pyrope component (XMg) increases from 0.05 to 0.18, whereas grossular (XCa) and spessartine (XMn) components decrease from 0.30 to 0.25 and from 0.09 to 0.01, respectively (Table 1; Fig. 5). The gradual increase of pyrope and gradual decrease of spessartine are typical of growth zoning, indicating that the garnet porphyroblast grew during prograde metamorphism (Spear, 1991; Spear et al., 1990).

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Figure 4. X-ray mapping of Ca (a), Fe (b), Mg (c) and Mn (d) of garnet porphyroblast, showing that the garnet has distinct compositional zoning, characterized by an increase of Mg and a decrease of Mn from the core to the rim. Warm colors indicate higher concentrations of element. The dashed line in (c) refers to the location of compositional profile shown in Fig. 5 (see also Fig. 3a).
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Figure 5. Compositional profile of garnet porphyroblast. XMg=Mg/(Ca+Mg+Fe+Mn), XCa=Ca/(Ca+Mg+Fe+Mn), XFe=Fe/(Ca+Mg+Fe+Mn), and XMn=Mn/(Ca+Mg+Fe+ Mn). The profile location is marked in Figs. 3a and 4c.
Table 1 Chemical compositions (in wt.%) of representative garnet of the mafic granulite

Plagioclase in the different textural domains (inclusion within garnet, corona around garnet, or matrix) has variable compositions (Table 2). The plagioclase inclusion has low CaO and high Na2O contents, with albite and anorthite components of 0.40-0.66 and 0.28-0.59, respectively. The plagioclase in the corona enveloping garnet has relatively high CaO and low Na2O contents with albite and anorthite components of 0.25-0.38 and 0.61-0.74, respectively. The matrix plagioclase has moderate CaO and Na2O contents with albite and anorthite components of 0.40-0.44 and 0.55-0.59, respectively (Table 2).

Table 2 Chemical compositions (in wt.%) of representative plagioclase of the mafic granulite (Incl. inclusion)

Amphibole in the corona and matrix has similar compositions with Mg# (=Mg/(Mg+Fe)) of 0.37-0.45 (Table 3), belonging to calcic amphibole. Clinopyroxene has relatively low Na2O contents of 0.65 wt.%-0.75 wt.%, belonging to diopside (Table 4). Orthopyroxene in the symplectitic corona after garnet has FeO=32.60 wt.%-33.39 wt.% and MgO=13.43 wt.%-14.08 wt.% (Table 4). Biotite in the matrix and corona after garnet has variable but high TiO2 contents (3.99 wt.%-6.02 wt.%; Table 4). By comparison, TiO2 contents (5.64 wt.%-6.02 wt.%) of the matrix biotite are higher than those (3.99 wt.%-4.79 wt.%) of the corona biotite.

Table 3 Chemical compositions (in wt.%) of representative amphibole of the mafic granulite (Sym. symplectite; Mg#=Mg2+/(Mg2++Fe2+))
Table 4 Chemical compositions (in wt.%) of clinopyroxene, orthopyroxene and biotite of the mafic granulite (bd. below the detection limit)
4 METAMORPHIC CONDITION ESTIMATIONS 4.1 Phase Equilibrium Modeling

Metamorphic P-T conditions of the mafic granulite are constrained by phase equilibrium modeling using THERMOCALC 3.45 (Powell and Holland, 1988), the internally consistent dataset (ds62) of Holland and Powell (2011), and a new set of thermodynamic models of Green et al. (2016) for meta-mafic rocks in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCKFMASHTO). The activity-composition relations used are the same as Green et al. (2016).

The P-T pseudosection constructed using the measured bulk composition in the range of 4-18 kbar and 600-900 ℃ are shown in Fig. 6. The bulk composition (in wt.%) is SiO2=49.27, TiO2=3.59, Al2O3=13.70, CaO=9.11, MgO=5.00, FeO=10.95, Fe2O3=2.62, K2O=1.19, Na2O=2.20, H2O=1.32. In the calculated P-T range, garnet appears above 7-10 kbar, rutile appears above ca. 10-14 kbar, muscovite appears in upper left corner under the conditions of 10.6-18 kbar and 600-810 ℃, orthopyroxene appears in the lower right corner at ca. 4-10 kbar and 780-900 ℃; amphibole disappears in the upper right corner at ca. 10.2-18 kbar and 740-900 ℃, plagioclase disappears in the upper left corner above ca. 12.5 kbar, and biotite appears at ca. 750-825 ℃ below 14 kbar. The system solidus is at 665-720 ℃ below 14 kbar.

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Figure 6. (a) P-T pseudosections of the mafic granulite. The white ovals labelled with M1, M2, M3 and M4 refer to approximate P-T conditions of the prograde, peak, early and late retrograde metamorphism. The mineral assemblages that refer to stages M1, M2, M3 and M4 are given in bold fonts. The appearance (in) or disappearance (out) lines of various minerals are shown by the thick lines with different colors. (b) Isomodes of melt and garnet (Gt). (c) Isopleths of XMg of garnet and XCa of plagioclase. The red lines with arrow refer to the metamorphic P-T path of the mafic granulite.

The most basic constraints on the prograde, peak and retrograde metamorphic P-T conditions are provided by the locations of the stability field of the respective assemblages on the pseudosection. The observed prograde M1 assemblage of Am+Gt+ Pl+Qz+Bt+Ttn is stable in the P-T range of 9-11.3 kbar and 600-680 ℃ in the calculated P-T range (Fig. 6). The observed peak metamorphic M2 assemblage Am+Cpx+Gt+Pl+Qz+Rt is stable in the P-T range of 11.5-15.8 kbar and 760-860 ℃ in the presence of melt (Fig. 6). The retrograde M3 assemblage Am+ Gt+Pl+Cpx+Bt+Qz+Ilm is stable in a small P-T field of 770- 825 ℃ at 8.0-10.5 kbar. The late M4 assemblage Am+Pl+Bt+ Qz+Ilm+Ttn±H2O is stable at 600-700 ℃ and below 7 kbar in the calculated P-T range.

The metamorphic conditions are further constrained by mineral compositional isopleths. As described above, the garnet of the mafic granulite has distinct growth compositional zoning, characterized by XMg increasing from 0.05 at the core to 0.18 at the rim. The modeling result shows that isopleths of XMg have a steep negative slope and are basically independent of pressure, indicating that the granulite underwent distinct heating process from ca. 600 ℃ at 10 kbar (XMg=0.05) to ca. 790 ℃ at 15 kbar (XMg=0.18; Fig. 6c). The maximum XMg of 0.18 in the rim of garnet intersects the upper part of the stability field of the peak mineral assemblage, indicating a peak metamorphic condition of 780-790 ℃ and 14.0-15.5 kbar and a corresponding melt content of ca. 14%-16% of rock volume (Fig. 6b and 6c). The minimum XCa (0.28) of plagioclase inclusion within the garnet intersects the lowest part of the stability field of the peak mineral assemblage (Fig. 6c), providing a rough constraint on a lower limit of peak metamorphic pressure.

4.2 Conventional Geothermobarometry

The metamorphic conditions of the mafic granulite are also estimated by conventional thermobarometry. For the peak- metamorphic stage (M2), using compositions of clinopyroxene, garnet rim and plagioclase inclusions within the garnet, the combination of Gt-Cpx-Pl-Qz barometer of Eckert et al. (1991) with Gt-Cpx thermometer of Ai (1994) yielded P-T conditions of ca. 17 kbar and ca. 800 ℃, which are close to the upper stability field of the peak mineral assemblage documented by P-T pseudosection modeling (Fig. 6a). For the retrograde stage M3, using compositions of the garnet outermost rim and adjacent coronas of amphibole and plagioclase, the combination of Gt-Am-Pl-Qz barometer (Dale et al., 2000) with Am-Pl thermometer (Holland and Blundy, 1994) yielded conditions of 7.1-8.2 kbar and 775-820 ℃, which are close to the stability field of the retrograde M3 mineral assemblage (Fig. 6a). In addition, the presence of Ti-rich biotite also indicates the mafic granulite underwent high-temperature (HT) granulite-facies metamorphism during the early decompression.

5 ZIRCON U-Pb DATING AND TRACE ELEMENTS

Zircon of the granulite is subhedral and short prismatic shape with length of 100-150 μm, and commonly shows a core-rim structure, with a core of patchy zoning, and an unzoned rim in cathodoluminescence (CL) images (Fig. 7). All the analyzed spots of zircon have concordant or near concordant but varying U-Pb ages ranging from 39 to 11 Ma (Fig. 8a; Table 5), low HREE contents, fractionated HREE patterns with negative Eu anomalies for most analytical spots, and low Th/U values (mostly < 0.2) (Fig. 8; Table 6). Note that the zircon core domains with patchy zoning have relatively old ages of 39-22 Ma, relative low HREE contents and flat to slightly fractionated HREE patterns and variable Th/U ratios; whereas the unzoned rim domains of zircon have young ages of 22-11 Ma, relative high and variable HREE contents and fractionated HREE patterns, relative low and constant Th/U ratios (except for one spot with high Th/U of 0.271) (Figs. 7 and 8; Tables 5 and 6).

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Figure 7. Cathodoluminescence (CL) images of zircon, showing the analyzed spot locations and relevant ages (in Ma).
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Figure 8. Diagrams of U-Pb concordia (a), chondrite-normalized REE patterns (b), HREE versus ages (c), and Th/U ratios versus ages (d) of zircons in the mafic granulite.
Table 5 Zircon U-Pb dating data of the mafic granulite
Table 6 Trace element compositions (in ppm) of zircon of the mafic granulite (bd. below the detection limit)
6 DISCUSSIONS 6.1 Metamorphic P-T Path of the Mafic Granulite

Metamorphic evolution of pelitic and felsic granulites in the Eastern Himalayan Syntaxis has been well constrained by recent petrological and phase equilibrium modeling studies, which demonstrated that these rocks experienced high-pressure (HP) granulite-facies metamorphism with peak-metamorphic conditions of 800-850 ℃ and 15-16 kbar, and recorded clockwise P-T paths (Tian et al., 2016; Zhang et al., 2015; Xiang et al., 2013; Guilmette et al., 2011). Although similar clockwise P-T paths were also reconstructed for the mafic granulites, distinct peak-metamorphic conditions were estimated by previous studies (Fig. 9). For example, Liu and Zhong (1997), Ding et al. (2001), Liu and Zhang (2014) and Tian et al. (2017) reported that the peak metamorphic conditions of 17-18 kbar and 890 ℃, 14-15 kbar and 800 ℃, 14 kbar and 904 ℃, and 11.5 kbar and 790 ℃, respectively (Fig. 9).

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Figure 9. Metamorphic P-T-t paths of the mafic granulites in the Eastern Himalayan Syntaxis and predicated by channel flow model. All numbers refer to metamorphic ages in Ma.

The metamorphic evolution of the studied mafic granulite is assessed by phase equilibria modeling using the new thermodynamic data for meta-mafic rocks (Green et al., 2016; Palin et al., 2016) and conventional geothermobarometry. The relevant results show that the mafic granulite underwent peak metamorphism (M2) of HP and HT granulite-facies under condition of 14.0-15.5 kbar and 780-790 ℃, the prograde metamorphism (M1) of amphibolite-facies of 9-11.3 kbar and 600-680 ℃, the early isothermal decompression overprint (M3) of granulite-facies of 7.1-8.2 kbar and 775-820 ℃, and late retrograde metamorphism (M4) of amphibolite-facies under condition of < 7 kbar and 600-700 ℃ (Fig. 6).

Connecting the prograde, peak, and retrograde metamorphic conditions, a clockwise P-T path is reconstructed for the studied mafic granulite (Figs. 6 and 9). It is characterized by heating and burial during the prograde stage, and, after peak metamorphic conditions in lower crustal levels are reached, an isothermal decompression stage first, which is followed by cooling during the retrograde stage in mid crustal to shallow crustal levels, a typical feature of metamorphic development in the collisional orogenic belts (e.g., Thompson and England, 1984). This indicates that the granulite has been buried into the lower crust of 50-km-depth, and subsequent exhumed to the upper crustal level. Considering the uncertainties of phase equilibria modeling and geothermobarometry, the present study and some previous works show that the pelitic, felsic and mafic rocks from the EHS show similar peak metamorphic conditions and P-T paths, indicating that various high-grade metamorphic rocks of the GHS in the EHS core occur as a coherent slab during the subduction and exhumation of the Indian continent. Possibly the P-T conditions of 17-18 kbar and 890 ℃ derived on mafic granulites by Liu and Zhong (1997) are slightly overestimated.

6.2 Metamorphic and Anatectic Timescales of the Mafic Granulite

Zircon in the mafic granulite shows short prismatic form, patchy zoning or unzoned structures, low Th/U ratios, and flat or slightly fractionated HREE patterns mostly with negative Eu anomalies (Figs. 7 and 8; Table 6). These features are typical of metamorphic and anatectic zircons that grew during high-grade metamorphism and partial melting (Rubatto et al., 2013; Rubatto and Hermann, 2007; Wu and Zheng, 2004; Corfu et al., 2003; Hoskin and Schaltegger, 2003; Rubatto, 2002). Therefore, we consider that the zircon grains with varying ages (39-11 Ma) grew at different episodes of a single granulite-facies metamorphic process, indicating that the granulite witnessed a long-lived metamorphic process over ca. 30 Ma. The zircon core domains with relatively old ages ranging from 39 to 22 Ma have relative low HREE contents, flat to slightly fractionated HREE patterns and variable Th/U ratios (Fig. 8), indicating that they grew at the same time as garnet. The obtained ages of 39-22 Ma represent the time and duration of the heating and burial metamorphism and associated partial melting of the mafic granulite (Fig. 9) because the garnet mode distinctly increases during the prograde metamorphism (Fig. 6b). In contrast, the rim domains of most zircons have relatively young ages of 22-11 Ma, relative high HREE contents, more fractionated HREE patterns and low Th/U ratios (Fig. 8), indicating that the growth of the zircon rims was associated with the breakdown of garnet. The obtained ages of 22-11 Ma constrain the time and duration of the isothermal decompression, cooling retrograde and melt crystallization of the mafic granulite (Fig. 9) because the garnet mode distinctly decreases during the early decompression (Fig. 6b). The petrographic observation also shows that the garnet was partly replaced by symplectitic corona of amphibole+plagioclase during the decompression. In this case, we consider that the peak metamorphism of the mafic granulite occurred probably at ca. 22 Ma (Fig. 9). Noted that the rims of minor zircon grains have low HREE contents and slightly fractionated HREE patterns. We still consider that the zircon rims were formed during the retrograde metamorphism because they have the same age range as the rims with high HREE contents and fractionated HREE patterns. In fact, a considerable amount of garnets are preserved in metastable form during overprint of granulite-to amphibolite-facies metamorphism of the hosting granulite (Figs. 2 and 3). This is probably the reason for the rims of minor zircon grains having low HREE contents and slightly fractionated HREE patterns.

Available studies demonstrated that zircon can be formed during the HT metamorphism, partial melting and subsequent melt crystallization of the high-grade metamorphic rocks from the Himalayan Orogen (Wang et al., 2017, 2016, 2015, 2013; Zhang et al., 2017b, 2015; Rubatto et al., 2013), and that zircon U-Pb ages obtained from the anatectic pelitic and felsic granulites in the EHS vary in a wide range of 40-8 Ma (Zhang et al., 2015, 2012, 2010; Xu et al., 2010; Liu et al., 2007; Ding et al., 2001), and with a peak-metamorphic age of ~25 (Su et al., 2012), ~24 (Zhang et al., 2015) or ~21 Ma (Liu and Zhang, 2014). The present study obtains similar zircon U-Pb age which range from 39 to 11 Ma and peak age of ~22 Ma for the mafic granulite, indicating that the HT metamorphism and associated partial melting of the mafic granulite in the EHS probably initiated at ca. 39 Ma, and the late cooling retrogression and melt crystallization lasted to ca. 11 Ma. In summary, the granulites in the EHS witnessed a long-lasting process of HT metamorphism, anataxis and melt crystallization. Recent studies also demonstrated that the high-grade metamorphic rocks from the east-central Himalayan Orogen witnessed a protracted metamorphic, anatectic and melt crystallization process which initiated at ca. 45-36 and lasting to ca. of 18-9 Ma (Wang et al., 2017, 2016, 2015, 2013; Zhang et al., 2017b, 2015; Ambrose et al., 2015; Iaccarino et al., 2015; Zeiger et al., 2015; Anczkiewicz et al., 2014; Rubatto et al., 2013; Imayama et al., 2012; Kohn and Corrie, 2011; Kali et al., 2010; Streule et al., 2010; Cottle et al., 2009; Viskupic et al., 2005; Searle et al., 2003).

6.3 Partial Melting and Tectonic Evolution of the Himalayan Orogen

Available studies indicated that the pelitic and felsic rocks of the GHS in the east-central Himalayan Orogen underwent intensive partial melting during the prograde to peak metamorphism (e.g., Zhang Z M et al., 2017c, 2015; Groppo et al., 2012, 2010; Guilmette et al., 2011; Cottle et al., 2009; Harris et al., 2004; Zhang H F et al., 2004), and the generated melts are up to 20%-30% of the rock volume (e.g., Zhang et al., 2017b, c, 2015; Groppo et al., 2012; Guilmette et al., 2011). Although previous geochemical studies proposed that the Eocene Himalayan leucogranites were derived from partial melting of the meta-mafic rocks of the subducted Indian continent (Hou et al., 2012; Zeng et al., 2011), the anatectic condition, degree and timescale of the mafic granulites remain unclear. The studied mafic granulite contains abundant and concordant felsic leucosomes (Fig. 2), providing direct evidence for the partial melting of the mafic granulite. The present study demonstrates that the mafic granulite underwent intensive partial melting during the heating and burial metamorphism and the melt content is up to 14 vol%-16 vol% at the peak-metamorphic stage (Fig. 6).

Previous studies commonly argued that the Himalayan leucogranites were derived from the partial melting of pelitic and felsic rocks of the GHS (e.g., Gao et al., 2017; Zeng and Gao, 2017; Zhang et al., 2017c; Weinberg, 2016; Wu et al., 2015; Gao and Zeng, 2014; Guo and Wilson, 2012; Knesel and Davidson, 2002; Harris and Massey, 1994). However, the present study reveals that the partial melting of mafic rocks in the GHS significantly contributed to the formation of Himalayan leucogranites. In addition, this work indicates that the mafic granulites in the eastern Himalayan Orogen witnessed a prolonged melting and melt crystallization process which initiated at ca. 40 Ma and lasted to ca. 11 Ma. This is generally accord with the documented crystallization ages (ca. 45-7 Ma) of Himalayan leucogranites (e.g., Wu et al., 2015; Liu et al., 2014; Hou et al., 2012; Zeng et al., 2011).

The widespread occurrence of the HT metamorphic and anatectic rocks in the Himalayan orogenic core demonstrates that the Himalayan-Tibetan Orogen has a molten and thickened lower crust, which is consistent with the orogen forming as a large and hot orogen (Beaumont et al., 2010, 2006). Moreover, the intensive partial melting has undoubtedly resulted in the rheological weakening of the subducted crust of Indian continent. This also provides favorable evidence for the channel flow model, which proposed a partially molten low-viscosity layer extruding southward between the rigid upper crust and the lowest crust or upper mantle (Beaumont et al., 2004; Jamieson et al., 2004). Moreover, the metamorphic P-T-t path of the mafic granulite reconstracted by the present study is similar to that predicted by channel flow mode (Fig. 9), providing further support for the presence of channel flow in the thickened lower crust of the Himalayan Orogen.

7 CONCLUSIONS

(1) The mafic granulite in the Eastern Himalayan Syntaxis has a peak metamorphic mineral assemblage of garnet, clinopyroxene, amphibole, plagioclase, quartz and rutile, and recorded a peak-metamorphism of HP and HT granulite-facies, and a clockwise P-T path, characterized by an early heating and burial, and late isothermal decompression and cooling retrogression.

(2) The mafic granulite witnessed a long-lasting HT metamorphic process over ca. 30 Ma. The prograde metamorphism and associated partial melting began at ca. 39 Ma and lasted to ca. 22 Ma, and the isothermal decompression, cooling retrogression and melt crystallization occurred between 22 and 11 Ma.

(3) Various high-grade metamorphic rocks in the EHS core experienced similar metamorphic conditions and P-T-t paths, indicating that they occurred as a coherent slab during the subduction and exhumation of Indian Continent.

(4) The Greater Himalayan Sequences in the Eastern Himalayan Syntaxis underwent intensive partial melting during the prograde metamorphism, and the generated significant melts contributed to the formation of Himalayan leucogranites, and resulted in the rheological weakening and ductile flow of the thickened lower crust of the Himalayan Orogen.

ACKNOWLEDGMENTS

This paper is dedicated to Prof. Zhendong You for his outstanding contributions in the teaching and research field of metamorphic geology in China. I am very grateful to Prof. You for his guidance and selfless help in decades of study and work. We thank Prof. Hans-Peter Schertl and two anonymous reviewers for constructive and critical reviews, which greatly improved the manuscript. This research was co-supported by the National Key Research and Development Project of China (No. 2016YFC0600310), the National Natural Science Foundation of China (Nos. 41230205 and 41602062), and the China Geological Survey (No. DD20160122). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0852-y.


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