Journal of Earth Science  2018, Vol. 29 Issue (5): 1040-1048   PDF    
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Water in the Thickened Lower Crust of the Eastern Himalayan Orogen
Li Zhang1,2, Yu Ye2, Shan Qin1, Zhenmin Jin2,3    
1. School of Earth and Space Sciences, Peking University, Beijing 100871, China;
2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;
3. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
ABSTRACT: Water content in nominally anhydrous minerals (NAMs) of the high-pressure (HP) metamorphic rocks controls the thermal structure, rheology and partial melting of orogenic belts. This paper conducts a systematic analysis of water in NAMs of the HP granulites from the Greater Himalayan Sequence (GHS), representing the thickened lower crust of the eastern Himalayan Orogen. The present result shows that the garnet, clinopyroxene, feldspar, quartz and kyanite contain 188 ppm-432 ppm, 193 ppm-547 ppm, 335 ppm-1 053 ppm, 125 ppm-185 ppm and 89 ppm H2O, respectively, and indicates that the thickened lower crust of the Himalayan Orogen is relatively wet rather than dry. The considerable concentrations of water in the HP granulites are expected to promote the rheological weakening of the metamorphic core of the Himalayan Orogen, providing a favorable evidence for the channel flow model of the exhumation of thickened lower crust.
KEY WORDS: micro-FTIR (Fourier transform infrared spectroscopy)    water content    granulite    Himalayan Orogen    

0 INTRODUCTION

The Himalayan Orogen, typical of collisional orogenic belts in the world, was formed during the Cenozoic collision between the Indian and Asian continents (e.g., Leech et al., 2005; Yin and Harrison, 2000; Rowley, 1996). The orogen consists of three tectonic units from south to north: the Lesser Himalayan Sequence (LHS), the Greater Himalayan Sequence (GHS) and the Tethyan-Himalayan Sequence (THS), which are separated by the main central thrust (MCT) and the South Tibetan detachment system (STD) (e.g., Ma et al., 2017; Kohn, 2014; Yin and Harrison, 2000).

The Greater Himalayan Sequence (GHS), consisting mainly of high-grade metamorphic rocks and leucogranites, forms the metamorphic core of the Himalayan Orogen. Thus, the GHS plays a significant role in revealing the formation and evolution history of the most spectacular collisional orogen on the globe (e.g., Kohn, 2014). Previous studies demonstrated that the GHS represents the deeply subducted continental crust, and underwent high-temperature (HT) and high-pressure (HP) metamorphism and intensive anatexis (e.g., Wang Y H et al., 2017; Wang J M et al., 2015; Kohn, 2014; Groppo et al., 2012; Guilmette et al., 2011; Ding et al., 2001).

Water in nominally anhydrous minerals (NAMs) of high-temperature (HT) and high-pressure (HP) rocks were studied extensively (e.g., Zhang and Jin, 2016; Zhang L et al., 2016; Schmödicke et al., 2015; Wang et al., 2014; Seaman et al., 2013; Yang et al., 2008; Gong et al., 2007; Sheng et al., 2007; Katayama et al., 2006; Xia et al., 2006, 2005; Bell et al., 2004; Su et al., 2004; Zhang J F et al., 2001; Nakashima et al., 1995; Rossman and Aines, 1991; Beran and Gotzinger, 1987). Numerous workers have demonstrated that trace amounts of water affect the viscosity and deformational behavior, melting temperature, seismic wave velocity, and electrical conductivity of the lower crust and mantle (e.g., Seaman et al., 2013; Johnson, 2006; Kohlstedt, 2006; Kronenberg, 1994). However, water contents of NAMs in the HT and HP granulites from the eastern Himalayan Orogen remained unclear. This paper reports a systematic analysis of water in NAMs of the HP ed results imalayan syntaxiscertain er contents of the NAMs of the HP granulitestes granulites from the eastern Himalayan Orogen. The present results show that the NAMs contain considerable amounts of water, indicating that the thickened lower crust of the eastern Himalayan Orogen is relatively wet. The significant amounts of water in NAMs are expected to promote the rheological weakening and ductile flow of the thickened lower crust.

1 GEOLOGICAL BACKGROUND AND SAMPLES

The Eastern Himalayan Syntaxis (EHS) consists of the Tethyan-Himalayan Sequence and the Greater Himalayan Sequence (Fig. 1; e.g., Zhang Z M et al., 2012; Booth et al., 2009, 2004; Zhang J J et al., 2004; Yin and Harrison, 2000). The former consists of low-grade metamorphic sedimentary rocks. The GHS consists mainly of meta-magmatic rocks with minor meta- sedimentary rock (e.g., Zhang et al., 2012; Geng et al., 2006). The GHS rocks underwent HT and HP granulite-facies peak- metamorphism and partial melting under conditions of > 16 kbar and 800 ℃ (Tian et al., 2016; Zhang Z M et al., 2015; Liu and Zhang, 2014; Su et al., 2012; Guilmette et al., 2011; 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). Therefore, the GHS rocks represent the thickened lower crust of the Himalayan- Tibetan Orogen.

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Figure 1. Sketch geological map of the Eastern Himalayan Syntaxis (modified after Zhang et al., 2015).

In this study, four HP granulite samples, including one pelitic (LZ06-19-3, GPS position: 29°41′31″N and 94°53′35″E), two felsic (T12-43-2, GPS position: 29°38′04″N and 94°52′43″E; LZ06-19-2, GPS position: 29°41′31″N and 94°53′35″E) and one mafic granulite (T12-45-1, GPS position: 29°36′29″N and 94°56′23″E) from the GHS were studied by electron microprobe for mineral chemical compositions and microscopic Fourier transform infrared spectroscopy (micro-FTIR) for water contents of NAMs. Field observation shows that the felsic granulite commonly occurs as thick layers within the GHS, whereas the mafic and pelitic granulites occur as lenses or thin-layers within the felsic granulites (Fig. 2).

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Figure 2. Field photos of the granulites, showing the anatectic and ductile flow structures. (a) Folded and necked lenses of mafic granulites occur within the felsic gneisses. The gneisses contain leucosome bands, suggesting intensive anataxis, and the mafic granulites have minor felsic veins; (b) anatectic mafic granulite with the fresh and dark red garnet grains, the felsic leucosomes parallel to the foliations of the host rock; (c) coarse-grained garnet-bearing pelitic granulite (schist) occurs as thin-layer within fine-grained garnet-bearing gneisses, both of them containing bands or lenses of leucosomes parallel to the foliation of host rocks. Notably the red garnet grains within the schist show a white-eye socket structure, suggesting a retrograde reaction; (d) coarse-grained garnet- bearing mafic granulite occurs as a thin-layer within fine-grained garnet-bearing gneisses, both of them containing bands of leucosomes parallel to the foliation of host rocks. Notably the red garnet grains in the mafic granulite show a white-eye socket structure, suggesting a retrograde reaction.
2 ANALYTICAL METHODS

Mineral chemical compositions were analyzed with the JEOL 8900 electron microprobe (EPM) at the Chinese Academy of Geological Science, Beijing. Test conditions of 15 kV accelerating voltage, 20 nA beam current, and 5 μm beam size were used in the wavelength-dispersive detection mode. Natural minerals or synthetic oxides were used as standards.

For micro-FTIR measurements, thin sections of four granulite samples were double polished to a thickness of 0.15 to 0.20 mm. Unpolarized infrared absorbance spectra were obtained within the range of wave number from 650 to 4 000 cm-1, at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, using a Nicolet 6700 spectrometer with a MCT-A detector. Each spectrum was accumulated over 128 scans with a 4 cm-1 resolution at room temperature. Apertures were about 30 μm×30 μm to 50 μm×50 μm. Analytical areas were selected under a microscope that coupled with the spectrometer to avoid interferences of cracks, grain boundaries, and inclusions. To define the average water content of the corresponding mineral in each sample, 4-13 grains for each mineral were analyzed. The calculation of the water contents was referred to Li et al. (2017), Yang et al. (2008), Sheng et al. (2007), Katayama et al. (2006), and Xia et al. (2006), using a modified form of the Beer-Lambert Law

$ \Delta = I \times c \times t \times \gamma $

where Δ is the integrated absorption area (cm-1) of bands, I is the integral specific absorption coefficient of corresponding mineral (1/ppm·cm2). In this paper, the integral area was accumulated in a range between 3 000 and 3 800 cm-1, using specific coefficient of 7.09 for clinopyroxene from Bell et al. (1995), 2.38 for garnet from Rossman and Aines (1991), 13.50 for quartz from Thomas et al. (2009), 6.80 for kyanite from Bell et al. (2004), and 15.30 for feldspar from Johnson and Rossman (2003). c is the water content (ppm H2O by weight), t is the thickness of the double-polished sample section (cm), γ is the orientation factor suggested by Paterson (1982). The orientation factor of 1/3 for anisotropic clinopyroxene, quartz, feldspar and kyanite and of 1 for isotropic garnet were selected according to the discussion of Paterson (1982).

3 PETROGRAPHY AND MIERNAL COMPOSITIONS

The pelitic granulite (sample LZ06-19-3) consists of garnet (33%), kyanite (14%), K-feldspar (8%), quartz (34%), biotite (8%) and plagioclase (2%) (Fig. 3a). The aligned oval garnet, prismatic and bent kyanite, biotite flake, and feldspar and quartz ribbons define the marked deformation foliation. The felsic granulite (T12-43-2) consists of garnet (28%), plagioclase (25%), biotite (13%), clinopyroxene (10%) and quartz (23%) (Fig. 3b). The garnet was partly replaced by a symplectitic corona of plagioclase and biotite, indicating decompression retrograde metamorphism. Another felsic granulite (LZ06-19-2) containing plagioclase (20%), quartz (54%) and garnet (21%), with minor biotite (4%), shows marked deformation foliation defined by oval plagioclase porphyroblasts, and feldspar and quartz ribbons (Fig. 3c). The mafic granulite (T12-45-1) consists of garnet (30%) and clinopyroxene (29%), plagioclase (12%), quartz (11%), amphibole (12%), biotite (5%) and with minor rutile and ilmenite (Fig. 3d). The garnet was partly replaced by symplectitic corona of plagioclase and amphibole, suggesting a significant decompression retrograde metamorphism (e.g., Zhang et al., 2008). The matrix amphiboles occur commonly along the margin of clinopyroxene. The mineral chemical compositions of the granulites are listed in Table S1.

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Figure 3. Photomicrographs of the granulites. (a) Pelitic granulite (LZ06-19-3), the aligned oval garnet, prismatic kyanite, feldspar and quartz bands or ribbons and biotite flakes form a deformation foliation under HP granulite-facies conditions. Note the bended kyanite grain in the central part of view. (b) Felsic granulite (T12-43-2), consisting of garnet, plagioclase, biotite, quartz and rutile. The aligned biotite flakes define the deformation foliation. The garnets are rimmed by symplectitic corona of biotite and plagioclase. (c) Felsic granulite (LZ06-19-2), the oval plagioclase porphyroblast, feldspar and quartz bands or ribbons and biotite flakes form a deformation foliation. (d) Mafic granulite (T12-45-1), consisting of garnet, clinopyroxene, plagioclase, amphibole, quartz, rutile and ilmenite. The porphyroblastic garnet is partly replaced by symplectitic corona of amphibole and plagioclase along its margin, and clinopyroxene is rimmed by amphibole.
4 WATER CONTENTS OF NOMINALLY ANHYDROUS MIERNALS

The representative spectra in OH stretching vibration region (3 000-3 800 cm-1) of the NAMs are shown in Fig. 4. The calculated water contents are given in Table 1.

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Figure 4. Representative FTIR spectra of NAMs in the granulites. (a) Clinopyroxene; (b) garnet; (c) quartz; (d) kyanite; (e) and (f) feldspar. All the spectra were normalized to 1 cm thickness but were vertically offset.
Table 1 Water contents (ppm H2O) of NAMs in the HP granulites from the eastern Himalayan Orogen

The FTIR spectra of the clinopyroxenes from the felsic and mafic granulites show three bands at wave number between 3 000 and 3 800, ~3 620, ~3 510 and ~3 460 cm-1 (Fig. 4a), which are consistent with the reported bands for clinopyroxene (e.g., Zhang and Jin, 2016; Yang et al., 2008; Xia et al., 2006; Skogby et al., 1990). The average water contents of clinopyroxenes from two granulite samples are 193 ppm and 547 ppm H2O, respectively (Table 1).

The FTIR spectra of the garnets are characterized by three bands for OH stretching vibration modes at ~3 630, ~3 580, and ~3 430 cm-1 (Fig. 4b). The first two bands agree with the reported typical OH bands in garnet (e.g., Schmödicke et al., 2015; Katayama et al., 2006). However, the last one is commonly ascribed to sub-microscopic fluid inclusions (Langer et al., 1993; Rossman and Aines, 1991), and it is therefore removed from the calculation using the Peakfit V4.12 program. The average water contents range from 188 ppm to 432 ppm H2O for the garnets from the four samples (Table 1).

The FTIR spectra of the quartz show a series of bands at wave number between 3 000 and 3 800 cm-1, including ~3 470, ~3 420, ~3 380, ~3 320 and ~3 210 cm-1 (Fig. 4c). Previous studies demonstrated that these band absorptions are all likely due to OH groups in the quartz structure (e.g., Zhang and Jin, 2016; Johnson, 2006). The average water contents of quartz from four granulite samples range from 125 ppm to 185 ppm H2O (Table 1).

The FTIR spectra of the kyanite in the pelitic granulite are divided in five bands in the OH absorption range: ~3 700, ~3 630, ~3 550, ~3 380 and ~3 280 cm-1 (Fig. 4d), which are similar to the reported bands in previous studies (e.g., Bell et al., 2004; Beran and Gotzinger, 1987). Since the spectral features of layer-silicates can be identified in the 3 700-3 450 cm-1 region (Beran and Gotzinger, 1987), we ascribe the three bands at 3 700, 3 630 and 3 550 cm-1 to the alteration of layer silicates. While the two bands at 3 380 and 3 280 cm-1 can be attributed to OH in the kyanite structure (Bell et al., 2004). Therefore, we estimated water contents on the basis of the integrated area of these two bands. The average value of seven analytical points is 89 ppm H2O (Table 1).

In the OH stretching vibration region, the feldspars from the four samples show different FTIR absorption bands: 3 680, 3 650, 3 530, 3 480, and 3 400-3 410 cm-1 (Figs. 4e and 4f). The two bands at 3 680 and 3 650 cm-1 are due to clay minerals (Potter and Rossman, 1977). The three bands at 3 530, 3 480, and 3 400-3 410 cm-1 are commonly OH absorption bands (e.g., Yang et al., 2008; Johnson and Rossman, 2004, 2003; Potter and Rossman, 1977). Although, for feldspar, bands of H2O may also appear at about 3 550 and 3 440 cm-1, hydrogen in plagioclase generally exists as OH (Xia et al., 2006; Johnson and Rossman, 2004; Rossman, 1996). Considering that the feldspars in this study are mostly plagioclase, we ascribe the three bands at 3 530, 3 480, and 3 400-3 410 cm-1 to OH. Therefore, we estimated water contents on the basis of the integrated area of these three bands. As shown in Table 1, the average water contents range from 335 ppm to 1 053 ppm H2O.

5 DISCUSSION

Available studies demonstrated that the NAMs in UHP metamorphic rocks from continental collisional orogens contain significant contents of water (up to several thousands ppm H2O), indicating that considerable amounts of water can be recycled into the mantle by the deeply subduction of supracrustal rocks (e.g., Zheng, 2009; Gong et al., 2007; Katayama et al., 2006; Xia et al., 2005; Su et al., 2004; Zhang et al., 2001). However, many previous studies argued that NAMs of granulites are commonly dry (Table 2). For example, garnet in the mafic granulite from the Kaapvaal Craton contains only 2 ppm-29 ppm H2O (Schmödicke et al., 2015). Kyanite in the pelitic granulite from the Bohemian massif contain only 3 ppm H2O (Beran and Gotzinger, 1987). Pyroxene and feldspar of the mafic granulites from the central part of the Carpathian-Pannonian region, Hungary, and pyroxene of the granulite-facies marble from Adirondack Mountain, Grenville Province, have the water contents of about 100 ppm H2O (Kovács et al., 2015; Johnson et al., 2002). The water contents of quartz in the pelitic and mafic granulites from the Napier complex, Antarctica, are less than 40 ppm H2O (Nakashima et al., 1995). In addition, Johnson (2006) and Kronenberg and Wolf (1990) indicated that the OH concentrations of quartz and kyanite in crustal rocks commonly range from < 1 ppm to ~40 ppm and from < 3 ppm to 44 ppm H2O, respectively.

Table 2 Water contents (ppm H2O) of NAMs in granulites

Recent studies revealed that NAMs in some granulites are relatively wet (Table 2; Zhang and Jin, 2016; Zhang et al., 2016; Yang et al., 2008; Xia et al., 2006). For example, Yang et al. (2008) and Xia et al. (2006) showed that NAMs in the granulites from the North China and Cathaysia cratons are able to incorporate significant amounts of water (up to 2 330 ppm) at lower crustal conditions (Table 2). The present study shows that the garnet, clinopyroxene, quartz, feldspar and kyanite of the HP granulites from the eastern Himalayan Orogen also contain considerable amounts of water, with 188 ppm-432 ppm H2O for garnet, 193 ppm-547 ppm H2O for clinopyroxene, 335 ppm- 1 053 ppm H2O for feldspar, 125 ppm-185 ppm H2O for quartz, and 89 ppm H2O for kyanite. The water contents of NAMs are similar to those of the previously reported wet NAMs of the granulites (Table 2).

For NAMs in lower-crustal rocks, the formation environment and/or the evolution history of the rocks are the main factors controlling the water contents (e.g., Schmödicke et al., 2015; Yang et al., 2008). For example, the early hydrous Precambrian continental lower crust resulted in the relatively high water contents in the NAMs from the Hannuoba granulites in eastern China (Yang et al., 2008). Therefore, the relatively high water contents of the NAMs in this study are likely due to the host rocks which contained considerable water. In addition, according to Shen et al. (2010), the HP granulites from eastern Himalayan Orogen contain dominantly H2O-CO2 fluid inclusions, rather than dry CO2 fluid inclusions in granulite terrains worldwide. These facts, combining with the presence of minor hydrous mineral in the HP granulites, indicate that the thickened lower crust of the eastern Himalayan Orogen is relatively wet.

Tens to hundreds ppm water in NAMs of HP and UHP rocks significantly affects the rheologic properties of the lower crust and the upper mantle (e.g., Seaman et al., 2013; Johnson, 2006; Kohlstedt, 2006; Kronenberg, 1994; Griggs, 1974). For example, Seaman et al. (2013) demonstrated that several hundred ppm water in NAMs can lower the melting temperature of the host rock in the lower crust, and trigger original partial melting, resulting in rock weakening (Seaman et al., 2013; Handy et al., 2001). Therefore, except for HT and HP conditions, the considerable amounts of water in NAMs may play a significant role in the partial melting and ductile deformation of the granulites in the thickened lower crust of the eastern Himalayan Orogen, where hydrous minerals are sparse or locally absent.

Previous studies suggested a channel flow model to explain the evolution of the Himalayan Orogen. This model is based on the hypothesis that the middle to lower crust of the Himalayan Orogen is partially molten and flows between the rigid upper crust and the lowest crust or upper mantle, along pressure gradients (e.g., Parsons et al., 2016; Groppo et al., 2012; Kohn, 2008; Jamieson et al., 2006, 2004; Beaumont et al., 2004). Considering that the certain amounts of water in the HP granulites may promote the rheological weakening of the thickened lower crust (e.g., Seaman et al., 2013), this study provides further support for the channel flow model.

6 CONCLUSION

The granulites from the Eastern Himalayan Syntaxis consist of nominally anhydrous minerals, such as garnet, feldspar, clinopyroxene, quartz and kyanite, and witnessed HP and HT metamorphism, and represent the thickened lower crust of the Himalayan Orogen. All the NAMs in the pelitic, felsic and mafic HP granulites contain significant amounts of water, with 188 ppm- 432 ppm H2O for garnet, 193 ppm-547 ppm H2O for clinopyroxene, 335 ppm-1 053 ppm H2O for feldspar, 125 ppm-185 ppm H2O for quartz, and 89 ppm H2O for kyanite. We suggest that the relatively high water concentration in NAMs of the HP granulites is likely to promote 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 important contributions to the research of petrology in China. This research was funded by the National Natural Science Foundation of China (Nos. 41772034, 41174076 and 41672041) and the China Postdoctoral Science Foundation (No. 2017M620508). Li Zhang acknowledges support from Peking University Boya Postdoctoral Fellowship. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0880-7.

Electronic Supplementary Materials: Supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-018-0880-7.


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