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Keywords:

  • leucogranite;
  • microstructure;
  • geochemistry;
  • granite emplacement;
  • eastern Himalaya, India

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. GEOLOGICAL SETTING
  5. PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG
  6. GEOCHEMISTRY AND MINERAL CHEMISTRY
  7. P–T ESTIMATIONS OF THE HHC
  8. DISCUSSION
  9. CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The Higher Himalayan Leucogranites (HHLG) intruded into the high grade rocks of the Higher Himalayan Crystallines (HHC) in Arunachal Himalaya of the Eastern Himalaya, yield distinctive field data, petrography, microstructures, geochemical and mineral chemistry data. The Arunachal HHLG are characterized by the presence of two micas; normative corundum; high contents of SiO2 (67–78 wt.%), Al2O3 (13–18 wt.%), A/CNK (0.98–1.44) and Rb (154–412 ppm); low contents of CaO (0.33–1.91 wt.%) and Sr (19–171 ppm), and a high ratio of FeO(tot)/MgO in biotite (2.54–4.82). These distinctive features, along with their strong depletion in high field strength elements (HFSE), suggest their affinity to peraluminous S-type granite generated by the partial melting of crustal material. Geothermobarometric estimations and mineral assemblages of the HHC metapelites confirm that the HHLG were probably generated in the middle crust (~20 km) and the produced melts intruded the HHC in the form of sills/dykes. Microstructurally, the HHLG shows high temperature deformation features including chessboard extinction in quartz and cuspate/lobate grain boundaries between quartz and feldspars (plagioclase and K-feldspar). The deformation microstructures suggest that the HHLG was deformed under early high temperature ductile deformation conditions. These fabrics were subsequently superimposed by later brittle deformation features associated with decreasing temperatures during the exhumation of the HHLG towards shallow structural levels at the time of Himalayan orogeny. Copyright © 2013 John Wiley & Sons, Ltd.

INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. GEOLOGICAL SETTING
  5. PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG
  6. GEOCHEMISTRY AND MINERAL CHEMISTRY
  7. P–T ESTIMATIONS OF THE HHC
  8. DISCUSSION
  9. CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The Himalayan mountain chain, a distinctive example of continental–continental collision between the Indian and Eurasian plates during the Cenozoic preserves the geodynamic evolutionary history of a continental collision mountain belt (Gansser, 1964; Le Fort, 1975; Valdiya, 1980). Five distinct episodes of granitoid magmatic activity at ~2100–1800 Ma, 1200–1000 Ma, 600–400 Ma, 100–50 Ma and 25–9 Ma have been established from different parts of this belt and only the Miocene magmatism is related to the development of Himalayan orogenic belt (Wadia, 1957; Frank et al., 1977; Thimm et al., 1999; Singh and Jain, 2003; Islam et al., 2005; Guo and Wilson, 2012). Only four of these events at ~1760–1745 Ma, 878–825 Ma, 520–480 Ma and 28–20 Ma have been recorded in the Arunachal Himalaya (Yin et al., 2010). During the last five decades, the youngest ~25–9 Ma Higher Himalayan Leucogranites (HHLG) have been thoroughly investigated for their petrological characteristics with several models proposed to explain their generation (Le Fort, 1975, 1981; Bird, 1978; Dietrich and Gansser, 1981; Molnar et al., 1983; Searle and Fryer, 1986; Le Fort et al., 1987; Castelli and Lombardo, 1988; Hodges and Silverberg, 1988; Hodges et al., 1988; England and Molnar, 1990; England et al., 1992; Harris and Inger, 1992; Harris et al., 1993; Inger and Harris, 1993; Searle et al., 1993, 1997, 2010; Harrison et al., 1999, Searle, 1999; Sachan et al., 2010; Guo and Wilson, 2012). However, to date, a detailed study of their microstructures is lacking.

The study and interpretation of microstructures developed in naturally deformed rocks, plays an important role in unravelling the history and dynamics of tectonic processes recorded in the Earth's crust (Means and Xia, 1981; Schmid, 1982). Microstructural studies can help to identify magmatic or solid state deformation fabrics of granites (Simpson, 1985). The deformation features recorded in the HHLG provide information about the processes and conditions of deformation at different crustal levels and also provide information about the depth of their emplacement. This paper focuses on the petrological characteristics of the HHLG exposed in Arunachal Pradesh, eastern Himalaya, with special emphasis on the deformational microstructures. These data provide constraints on the petrogenetic model for the emplacement of HHLG in the Eastern Himalaya.

GEOLOGICAL SETTING

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. GEOLOGICAL SETTING
  5. PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG
  6. GEOCHEMISTRY AND MINERAL CHEMISTRY
  7. P–T ESTIMATIONS OF THE HHC
  8. DISCUSSION
  9. CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The lithotectonic units

The western Arunachal Himalaya comprises three major tectonic units from south to north: the Sub Himalaya, the Lesser Himalaya and the Higher Himalaya (Fig. 1, Table 1; Das et al., 1975; Verma and Tandon, 1976; Bhushan et al., 1991; Yin et al., 2010; Bikramaditya Singh and Gururajan, 2011). The Sub Himalaya comprises sandstone, siltstone and conglomerate of the Siwalik Group, which has a faulted contact (Himalayan Frontal Thrust—HFT) with Brahmaputra alluvium in the south. The Lesser Himalaya is bounded by the Main Boundary Thrust (MBT) in the south and the Main Central Thrust (MCT) in the north and dominantly consists of low to medium grade metamorphic rocks of the Lesser Himalayan Crystallines (LHC) with a small section of Lesser Himalaya Sedimentary Sequence (LHSS) and Gondwana Group (Das et al., 1975; Verma and Tandon, 1976; Bhushan et al., 1991). The LHSS is characterized by interbedded sequences of phyllite, quartzite and carbonate rocks with thin bands of dark grey slate while the Gondwana Group is made up of sandstone, black shale and slate. Based on lithology, the LHC is subdivided into two units, viz. Lower unit and Upper unit. The Lower unit comprises dominantly quartzo-feldspathic gneisses, metagranite, quartzite, phyllite (±garnet) and amphibolite while the Upper unit consists of garnet–staurolite schist, quartzite, phyllite and tremolite–actinolite marble. The Dirang Thrust separates the Upper unit and the Lower unit of the LHC. The Higher Himalaya comprises high grade metamorphic rocks known as Higher Himalayan Crystallines (HHC), including kyanite–sillimanite gneiss/schist, migmatite, amphibolite, calc-silicate, leucogranite and pegmatite.

image

Figure 1. (a) Structural zones of the Himalaya (after Gansser, 1974) showing the location of the study area in Eastern Himalaya. (b) Generalized geological map of western Arunachal Himalaya (modified after Bhushan et al., 1991) showing the metamorphic isograd boundaries with location of the representative Higher Himalayan Leucogranite (HHLG) samples and selected metapelites for geothermobarometric studies. This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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Table 1. Generalized geological framework of the Western Arunachal Himalaya (modified after Das et al., 1975; Verma and Tandon, 1976; Bhushan et al., 1991)Thumbnail image of

The HHLG mostly occurs as sills of limited thickness and lateral extent within the HHC and is also observed as dykes (Fig. 2a, b). Pegmatites commonly occur as boudins (Fig. 2c). The HHLG intrude the middle/core part of the HHC, north of the MCT and they also occur in the upper part of the HHC. The leucogranite sills are less than tens of cm thick and a few metres long, however near to the Se La Pass area the size of the HHLG increases to 20–40 m thick and >100 m long (Yin et al., 2010). The common occurrence of sills and minor dykes suggests that the leucogranite magma was intruded laterally, as well as vertically.

image

Figure 2. Field photographs showing (a) and (b) intrusion of HHLG as sills in the high grade rocks of Higher Himalaya; (c) boudinage structure of pegmatite; (d) abundance of tourmaline in HHLG (e) tight isoclinal fold (F2) and the axial plane is parallel to the regional schistosity (S2), near Lumla village; (f) photomicrograph showing kyanite interleaved with muscovite and biotite along S2 foliation in kyanite bearing metapelite; (g) replacement of kyanite by sillimanite; (h) minor undulation of S2 schistosity developing F3 (open) fold in HHC, near Jang village. Mineral abbreviations are after Kretz (1983): quartz (Qtz), K-feldspar (Kfs), plagioclase (Pl), muscovite (Ms), biotite (Bt), garnet (Grt), kyanite (Ky), sillimanite (Sil), tourmaline (Tur) and zircon (Zr). This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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At mesoscopic scale, the HHLG is medium- to coarse-grained, leucocratic and massive. Two distinct facies, an earlier two mica granite and a later biotite-free tourmaline granite, have been reported in other parts of the Himalaya, such as Badrinath in Garhwal Himalaya and Manaslu and Langtang in Nepal (Scaillet et al., 1990; Inger and Harris, 1993). However, in the study area, the HHLG dominantly consists of two mica granite containing minor tourmaline (Fig. 2d). Associated pegmatites are observed at a few localities.

Deformation and metamorphism of the HHC

The deformation and metamorphism of the HHC is probably related to the generation of the HHLG. In the study area, the HHC comprises a structurally lower kyanite zone and a structurally higher sillimanite zone. At the higher structural levels near Tawang and further north, the grade of metamorphism decreases up to the Lumla Thrust (Fig. 1). The sillimanite zone is overlain by kyanite schists, followed structurally upwards by staurolite–garnet schists, garnet schists and garnetiferous micaceous quartzites associated with phyllites at Lumla.

The HHC in Arunachal have been affected by three phases of deformation and two events of prograde metamorphic events (Goswami et al., 2009). The second phase of deformation (D2) is the dominant phase. The structural features produced during the D1 deformation phase are only poorly preserved due to obliteration by later deformation. The axial plane of the F2 fold is parallel to the regional foliation (S2) and presents a trend of ENE–WSW with moderate dip mostly towards NW (Fig. 2e). The kinematic indicators show a top-to-SSW sense of shear that suggests the HHC is thrusted toward south over the LHC along the MCT (Srivastava et al., 2011).

The kyanite-bearing metapelites occur as a narrow zone above the MCT. They consist of quartz, muscovite, biotite, garnet, kyanite and plagioclase. The kyanite grew mainly parallel to S2 along with biotite, indicating their growth during syn-D2 (Fig. 2f).

The sillimanite-bearing metapelites are well foliated, and contain a mineral assemblage of quartz, biotite, garnet, sillimanite, plagioclase, K-feldspar with minor amount of muscovite, apatite, monazite and zircon. Both fibrolitic and prismatic sillimanite occur along the foliation with biotite.

Kyanite re-appears in the assemblage in the upper part of the sillimanite zone near Tawang where it is partially replaced by fibrolitic sillimanite (Fig. 2g). The transformation of kyanite by sillimanite indicates decompression and/or heating.

Detailed petrography, textural features and mineral paragenetic sequences in different rock types indicate that M1 metamorphism initiated during D1 deformation, but is dominantly related to syn-D2 deformation. It represents the early Barrovian kyanite grade metamorphism under high pressure and moderate to high temperature that was the result of burial and heating due to under thrusting of the sediments of the Indian margin to greater depths (Grujic et al., 2002; Daniel et al., 2003).

M2 metamorphism is related to the later stages of the D2 deformation event, which produced high temperature sillimanite grade metamorphism, anatexis and production of leucogranites under decompression regime during the exhumation of HHC along the MCT (Grujic et al., 2002; Daniel et al., 2003). The M1 and M2 equilibrium mineral assemblages are superimposed by retrograde metamorphic assemblages (M3) related to D3 deformation. This third deformation phase produced open folds (F3) and S3 crenulations in their axial zones. These show a general trend of NW–SE with 30–40° dip towards E–W. In places the F3 folds are represented by chevron to open folds on the minor scale with the fold axis plunging towards N with low angle of 10–20° (Fig. 2h).

PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. GEOLOGICAL SETTING
  5. PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG
  6. GEOCHEMISTRY AND MINERAL CHEMISTRY
  7. P–T ESTIMATIONS OF THE HHC
  8. DISCUSSION
  9. CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The HHLG is medium- to coarse-grained and composed of quartz, K-feldspar, plagioclase, muscovite and biotite with accessory monazite, apatite and zircon. Tourmaline and garnet are also observed in some of the samples. It exhibits a hypidiomorphic and equigranular texture (Fig. 3a). Textural relationships suggest that the crystallization sequence was biotite–muscovite/plagioclase-quartz-K-feldspar.

image

Figure 3. Photomicrographs of HHLG showing (a) hypidiomorphic texture containing quartz, K-feldspar, plagioclase, muscovite and biotite; (b) chessboard extinction in large quartz grain; (c) quartz amoeboid grains; (d) lobate phase boundary between plagioclase and quartz; (e) undulose extinction and bending in microcline; (f) presence of bending in orthoclase; (g) lobate phase boundary between quartz and microcline grain; (h) well developed myrmekite at the margin of K-feldspar porphyroclast; (i) plagioclase porphyroclast with aggregates of fine recrystallized grains; (j) carlsbad twins in plagioclase; (k) undulose extinction in plagioclase and suture grain boundary with fractures; (l) twin tapering towards the core of the plagioclase and presence of bending; (m) fractures filled by fine grains of quartz and muscovite in K-feldspar (marked by arrow); (n) fractured plagioclase grain with fractures filled by fine quartz grains and displaced twin lamellae along the fractures; (o) presence of tourmaline (Tur) associated with microcline and quartz; (p) subrounded zircon inclusion with K-feldspar. This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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Quartz occurs both as large phenocrysts, generally irregular grains and as finer equant grains. It exhibits deformation features such as undulose extinction, suture boundaries and developments of fractures. It also shows chessboard extinction and development of amoeboid grain shapes with lobate grain boundaries (Fig. 3b, c).

Lobate grain boundaries between quartz and feldspars (plagioclase and K-feldspar) are also observed (Fig. 3d). Both microcline and orthoclase show deformation bands (Fig. 3e, f). Plagioclase commonly contains inclusions of fine K-feldspar grains. The grain boundaries of K-feldspar and quartz are strongly curved showing cusps pointing from K-feldspar to quartz (Fig. 3g). Myrmekitic overgrowths on the margins of K-feldspar are observed (Fig. 3h). Development of recrystallized grains of plagioclase is observed in a few samples (Fig. 3i).

Plagioclase, mainly albite, is subhedral to anhedral in shape with suture boundaries. Twins show extinction angles ranging from 10° to 30°. Carlsbad twins in some of plagioclase grains are also observed (Fig. 3j). Deformation features such as undulose extinction, development of twin tapering and bending are also observed (Fig. 3k, l). Occasionally plagioclase is altered to sericite. Plagioclase grains show the presence of fractures that filled up by fine-grained muscovite and quartz (Fig. 3m, n).

Biotites are fine- to medium-grained and exhibit reddish brown to brownish green colour. Biotite is randomly distributed without any preferred orientation. Usually the biotite is unaltered but rarely shows the effects of alteration to chlorite in some samples. Well-developed flakes and large phenocrysts of randomly-distributed muscovite are common.

Garnets are medium- to coarse-grained and idioblastic in shape. Most of the garnets are inclusion-free but a few garnets show the presence of inclusions of fine-grained quartz in the core.

Tourmaline is more common in pegmatite but also occurs as small grains in some leucogranite samples. It is subhedral to anhedral and overgrowth on the groundmass suggests as last stage of crystallization (Fig. 3o). Fine subrounded zircons are also observed in most of the HHLG samples (Fig. 3p).

GEOCHEMISTRY AND MINERAL CHEMISTRY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. GEOLOGICAL SETTING
  5. PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG
  6. GEOCHEMISTRY AND MINERAL CHEMISTRY
  7. P–T ESTIMATIONS OF THE HHC
  8. DISCUSSION
  9. CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

Analytical techniques

Electron microprobe analyses of representative samples of the HHLG and metapelites of the HHC were determined using the CAMECA-SX100 Electron Probe Micro Analyzer at the Wadia Institute of Himalayan Geology (WIHG), Dehradun. The samples were analysed using a probe current of 20 nA, an acceleration voltage of 15 kV and beam diameter of 0.5 µm, Natural standards for calibration. Data from selected minerals with calculated end members and P-T estimates are presented in Tables 2 and 3 respectively.

Table 2. Representative electron microprobe analyses of garnet of metapelites from different zones
 Kyanite metapeliteSillimanite metapelite      
 BD14/3B421B720AB496TS19B4102      
 corerimcorerimcorerimcorerimcorerimcorerim      
SiO237.0637.1037.5136.7436.7236.6837.4036.8337.0136.3936.7836.38      
Al2O320.8421.1220.5320.6320.4320.4220.9320.8220.6120.6220.3820.26      
Cr2O30.100.030.000.050.030.060.140.020.000.030.000.00      
FeO33.8732.8930.7230.4036.5837.4630.3030.2934.3234.2037.9837.68      
MnO2.342.365.045.321.941.975.887.162.712.921.621.72      
MgO3.973.782.002.011.961.912.532.302.692.601.501.31      
CaO1.731.693.173.051.030.651.311.361.831.971.361.11      
Total99.9198.9798.9798.2098.6999.1598.4998.7899.1798.7399.6298.46      
Formula proportions of cations based on 12 O atoms              
Si2.983.003.053.013.023.013.043.013.012.983.013.02      
Al1.972.011.972.001.981.982.012.011.981.991.971.98      
Fe2.282.222.092.092.522.572.062.072.342.352.602.61      
Mn0.160.160.350.370.140.140.410.500.190.200.110.12      
Mg0.480.460.240.250.240.230.310.280.330.320.180.16      
Ca0.150.150.280.270.090.060.110.120.160.170.120.10      
Sum8.017.997.977.987.997.997.937.987.998.028.007.99      
Almandine0.740.740.710.700.840.860.710.700.770.770.860.87      
Pyrope0.160.150.080.080.080.080.110.090.110.100.060.05      
Spessartine0.050.050.120.120.050.050.140.170.060.070.040.04      
Grossular0.050.050.090.090.030.020.040.040.050.060.040.03      
Fe / ( Fe + Mg)0.830.830.900.890.910.920.870.880.880.880.940.94      
Mg / (Fe + Mg)0.170.170.100.110.090.080.130.120.120.120.060.06      
 
Representative electron microprobe analyses of K-feldspar of leucogranite      
 BD41BD48TS43TS51RK39RK40      
points121212121212      
SiO264.1163.8063.9663.6764.6765.5963.8063.9564.0264.0863.9763.61      
Al2O317.6317.8317.9317.9218.0818.3117.6918.0618.3218.2318.1518.17      
CaO0.000.020.010.030.050.000.000.000.0500.030.01      
Na2O1.261.131.241.181.422.090.850.931.411.451.191.13      
K2O15.5115.8515.0415.2215.3314.0116.1015.8814.8214.9415.415.47      
Total98.5198.6398.1898.0299.55100.0098.4498.8298.6298.7098.7498.39      
Formula proportions of cations based on 8 O atoms            
Si3.012.993.003.003.003.013.002.992.992.992.992.99      
Al0.970.990.990.990.990.990.981.001.011.001.001.01      
Ca0.000.000.000.000.000.000.000.000.000.000.000.00      
Na0.110.100.110.110.130.190.080.080.130.130.110.10      
K0.930.950.900.910.910.820.970.950.880.890.920.93      
Sum5.025.035.015.025.025.005.025.025.015.025.025.02      
Ab0.110.100.110.110.120.180.070.080.130.130.100.10      
An0.000.000.000.000.000.000.000.000.000.000.000.00      
Kfs0.890.900.890.890.870.820.930.920.870.870.890.90      
Representative electron microprobe analyses of biotite of leucogranite and metapelites
 LeucograniteKyanite metapeliteSillimanite metapelite
 BD41BD48TS43TS51RK39RK40BD14/3B421B720AB496TS19B4102
points121212121212111111
SiO233.9534.634.2534.2434.6134.9533.7834.0834.6634.3833.3833.9935.6035.8134.1034.6235.1133.27
TiO22.22.212.612.473.333.572.32.462.362.232.892.523.042.882.123.493.281.43
Al2O317.7217.9717.517.4617.2616.7617.3117.9117.5517.1717.2816.7218.8118.6518.2018.7918.0117.62
FeO23.7523.0324.2724.5821.4322.3324.2323.7120.4820.5323.623.5318.4420.3124.7420.0421.0928.56
MnO0.490.480.340.400.460.330.430.380.570.540.490.570.000.190.000.170.090.09
MgO5.525.915.165.106.986.725.555.647.618.095.575.719.336.825.556.577.264.43
CaO000.020.01000.040.030.010.040.0200.010.020.030.020.000.02
Na2O0.070.110.090.120.120.090.170.160.10.130.090.130.320.170.250.200.110.25
K2O9.49.719.459.249.89.599.589.569.979.79.589.318.408.958.879.279.088.80
Total93.194.0293.6993.6293.9994.3493.3993.9393.3192.8192.992.4893.9593.8093.8693.1794.0394.47
Formula proportions of cations based on 24 O atoms            
Si5.445.475.465.475.445.495.435.425.475.465.385.495.435.545.415.425.465.37
Ti0.270.270.320.300.400.430.280.300.280.270.350.310.350.340.260.410.390.18
Al3.353.353.293.293.203.103.283.363.263.213.293.193.383.403.413.473.303.35
Fe3.193.053.243.292.822.933.263.152.702.723.183.182.352.633.282.622.743.85
Mn0.070.060.050.050.060.040.060.050.080.070.070.080.000.020.000.020.010.01
Mg1.321.391.231.211.641.571.331.341.791.911.341.382.121.571.311.531.681.07
Ca0.000.000.000.000.000.000.010.010.000.010.000.000.000.000.010.000.000.00
Na0.020.030.030.040.040.030.050.050.030.040.030.040.090.050.080.060.030.08
K1.921.961.921.881.971.921.961.942.011.961.971.921.641.771.801.851.801.81
Sum15.5815.5815.5415.5415.5615.5115.6615.6015.6215.6615.6215.5815.3815.3215.5515.3915.4215.72
 
Representative electron microprobe analyses of plagioclase of leucogranite and metapelites
 LeucograniteKyanite metapeliteSillimanite metapelite 
 BD41BD48TS43TS51RK39RK40BD14/3B421B496TS19B4102 
points1212121212121111 
SiO262.4062.7365.7766.0562.3462.5564.4265.4263.163.563.4364.0665.4964.8063.6161.6762.47 
Al2O322.2922.0820.6720.5723.2423.3921.6521.6522.5422.322.5621.9722.8322.9822.9824.3823.00 
CaO4.164.302.212.015.155.283.473.363.873.733.593.093.444.074.236.024.67 
Na2O9.209.3110.6410.648.718.5810.1810.259.819.799.9710.159.239.178.908.049.08 
K2O0.320.340.290.250.290.240.120.150.370.280.160.320.110.220.240.190.05 
Total98.3798.7699.5899.5299.73100.0499.84100.8399.6999.6099.7199.59101.10101.2499.96100.3099.27 
Formula proportions of cations based on 8 O atoms            
Si2.812.812.912.922.772.772.852.862.812.822.812.842.842.822.812.732.78 
Al1.181.171.081.071.221.221.131.121.181.171.181.151.171.181.201.271.21 
Ca0.200.210.100.100.250.250.160.160.180.180.170.150.160.190.200.290.22 
Na0.800.810.910.910.750.740.870.870.850.840.860.870.780.770.760.690.78 
K0.020.020.020.010.020.010.010.010.020.020.010.020.010.010.010.010.00 
Sum5.015.025.015.015.004.995.025.015.045.035.035.034.964.984.984.985.00 
Ab0.790.780.880.890.740.740.840.840.800.810.830.840.820.790.780.700.78 
An0.200.200.100.090.240.250.160.150.180.170.160.140.170.190.210.290.22 
Kfs0.020.020.020.010.020.010.010.010.020.020.010.020.010.010.010.010.00 
Table 3. Temperatures (°C) and Pressures (kbar) for the metapelites calculated by Thermobarometry
 
 Kyanite metapeliteSillimanite metapelite
BD14/3B421B720AB4102B496TS19
CoreRimCoreRimCoreRimCoreRimCoreRimCoreRim
  1. —, not detectable.

Geothermometer ( Garnet–Biotite)
Holdaway and Lee, 1977650652604614656639669637677647646637
Ferry and Spear, 1978684687613628695669718665730682680665
Hodges and Spear, 1982705707651664708677734764746699702688
Average679682622635686662707688717676676663
 
Geobarometer (GRT–PLG–ALS–QTZ)
Hodges and Spear, 19828.077.998.478.605.744.266.656.085.855.91
Ganguly and Saxena, 19847.887.798.498.604.302.615.885.315.195.27
Hodges and Crowley, 19858.668.629.329.466.745.137.216.656.446.43
Average8.208.138.768.895.604.006.586.015.835.87
 

Twenty-nine representative samples were analysed for major and trace elements and seven of these samples were analysed for rare earth elements (REE). The samples were broken into small chips using a jaw-crusher and powdered to −230 mesh using ball mills. XRF analysis (on a Siemens SRS [3000]) at the Wadia Institute determined the major and trace elements. REEs were determined by ICP-MS (Perkin Elmer, ELAN DRC-e) at the Wadia Institute. Analytical precision for major elements is within ±2–3% and for trace elements is within ±5–6%. Accuracy of REE analyses ranges from 2 to 12% and precision varies from 1 to 8% (Khanna et al., 2009). Major and trace elements with CIPW norms and A/CNK {molar (Al2O3/CaO + Na2O + K2O)} for the representative samples of HHLG are presented in Table 4 whereas Table 5 shows REEs data.

Table 4. Major (wt.%) and trace (ppm) elements data of Higher Himalayan Leucogranite (HHLG) from western Arunachal Himalaya, India
 
 BD41BD42ABD42BBD48BD49TS17TS43TS48TS51Bi138/2RK5RK9RK35RK40A
  1. —, not detectable.

SiO272.9172.6172.0375.5675.1276.1075.5573.3174.6174.6975.0672.9270.8166.6271.93
TiO20.030.070.010.090.090.110.100.140.070.050.140.130.140.140.16
Al2O315.6015.6916.1713.1313.6413.9114.1614.9814.6414.3714.2615.9115.6717.4915.73
Fe2O30.370.890.212.362.411.531.011.510.910.651.461.381.741.361.83
MnO0.010.020.030.080.110.040.010.010.010.010.020.010.020.010.02
MgO0.160.210.120.180.220.210.140.170.020.020.130.380.410.30.21
CaO1.100.920.930.730.961.911.620.900.881.581.230.690.950.921.12
Na2O3.704.564.951.411.472.563.333.543.063.033.272.953.711.593.87
K2O5.494.815.066.676.335.334.885.436.005.885.334.965.737.224.69
P2O50.030.180.030.010.010.010.040.200.060.020.040.060.060.590.05
 
Total99.4099.9699.54100.22100.36101.71100.84100.19100.26100.30100.9499.3999.2496.2499.61
Rb223298259280275203175284222195193208261289217
Sr72774319391041717481961549610669100
Ba13918567877783503591201163312692562586227532
Pb119136199186410911213514746819111314789122
Zn8314283625195517223432492831
Zr534610147149144621207315613437291139
Y4181136326210315236434636
Nb41391294414625107410
Th65617372350372263109322
U91058833138194547
Rb/Sr3.103.876.0214.747.051.951.023.842.742.031.252.172.464.192.17
A/CNK1.111.091.061.211.241.031.031.121.111.011.071.391.121.441.17
 
CIPW norm
Or32.6328.4430.0339.3837.3230.9928.6132.0535.3734.6431.2229.2933.8442.6427.7
Ab31.538.6242.0811.9312.4221.3227.9629.9325.8425.5727.4424.9531.3913.4532.74
An5.293.394.443.554.699.267.723.163.977.695.793.034.320.715.23
Hy0.841.590.613.473.682.341.482.131.120.821.990.941.020.740.52
C1.651.740.942.292.650.470.552.121.650.20.964.581.786.812.32
Q27.8725.4721.7738.6838.5335.0633.1929.5531.5830.831.9234.9324.8529.0128.92
 
 RK41RK44ARK44BRK51RK53KK37CKK39KK40BKK41AB420AB447B493B4101B4106 
SiO269.8970.7368.6472.9372.4769.4872.5271.7568.7378.3375.6173.7673.5872.84 
TiO20.010.020.020.10.010.070.080.140.070.000.090.090.010.01 
Al2O315.5916.3517.7515.6915.6916.1916.4516.5916.6114.2615.1616.3516.4315.82 
Fe2O30.610.790.551.551.550.961.061.181.051.081.691.290.340.27 
MnO0.060.040.010.060.200.010.010.040.010.090.180.020.000.00 
MgO0.040.020.020.130.040.200.120.150.10.040.090.170.020.01 
CaO1.110.920.810.721.30.331.290.821.290.441.101.411.001.06 
Na2O2.654.023.483.824.472.053.803.434.661.843.713.593.083.93 
K2O7.555.285.455.154.097.573.524.665.386.424.015.697.207.17 
P2O50.140.030.040.290.010.040.220.060.070.180.190.090.120.02 
 
Total97.6598.2096.77100.4499.8396.9099.0798.8297.97102.68101.83102.46101.79101.12 
Rb295229250412178228154191194369396237225240 
Sr11149462641115859110246358815372 
Ba4162348089898294666625158113256859218 
Pb42128128351771968011111140.055129.2239.8263.1 
Zn89123314211521208.55432.58.510.0 
Zr32495456735831110481764349432 
Y49424149106373753407.61911.68.219.4 
Nb15716438955.1197.13.23.5 
Th3129141923421141.150.90.50.6 
U4888145610817.11812.39.811.8 
Rb/Sr2.664.675.4315.854.341.981.812.101.907.9511.392.691.473.34 
A/CNK1.071.171.361.191.111.331.331.361.051.321.221.121.120.98 
 
CIPW norm
Or44.5931.1832.1930.4124.1520.7920.7927.5231.7837.9123.2933.6142.5342.35 
Ab22.4234.0129.4432.3137.8132.1432.1429.0139.4315.5630.8630.3726.0633.25 
An4.594.373.761.686.384.964.963.675.941.014.146.414.184.35 
Hy0.10.050.050.320.10.30.30.370.250.12.530.420.05 
C1.382.424.753.221.574.314.574.560.943.923.121.942.04 
Q23.5625.2525.9130.1128.0231.6134.6532.1818.3442.5935.0928.126.320.51 
 
Table 5. Rare earth element (ppm) data of Higher Himalayan Leucogranite (HHLG) from western Arunachal Himalaya, India
 
ELEMENTBD-42ABD-48TS-17TS-43TS-48TS-5138/2
La8.8548.8322.7829.8314.0611.8432.70
Ce18.1693.9549.1568.0831.5826.5371.04
Pr2.3612.045.787.983.933.218.92
Nd8.1040.9319.7526.9513.7711.5431.08
Sm2.327.813.956.074.073.276.85
Eu0.510.811.191.480.700.891.73
Gd2.005.864.976.934.473.567.54
Tb0.410.970.430.570.630.440.64
Dy2.756.671.821.812.891.802.14
Ho0.501.380.320.190.420.230.22
Er1.003.101.000.480.820.480.62
Tm0.230.750.140.030.080.050.04
Yb1.003.710.960.210.490.310.30
Lu0.110.490.140.020.060.040.04

Mineral chemistry and geochemical characteristics of the HHLG

Plagioclase contains high Na content (Ab74–Ab89) and low Ca (An9–An25) (Table 2). Potassium feldspars have a composition of Or87–93 Ab7–18An0. Biotite is enriched in Fe, with a composition of XFe = 2.70–3.29, XMg = 1.21–1.91, XAl = 3.10–3.28 and XTi = 0.27–0.43. The biotite FeOtot/MgO ratio ranges from 2.54 to 4.82 (average of 3.68)—these are typical peraluminous (S-type) granites (Fig. 4a, b).

image

Figure 4. (a) Fet/Fet + Mg vs. Al(iv) diagram (after Deer et al., 1963); (b) FeO*–MgO–Al2O3 diagram of biotite of Higher Himalayan Leucogranite—HHLG. Fields are after Abdel-Rahman (1994).

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The whole rock chemistry of the HHLG samples shows that the HHLG have a high SiO2 content (67–78 wt.%) and Al2O3 (13–18 wt.%) with a low abundance in Fe2O3, MgO, MnO and CaO and a wide variation in trace element concentrations, particularly in Ba, Rb, Sr and Zr (Table 4). The low abundance and narrow range of CaO, TiO2, Fe2O3, MgO and MnO indicate a lower abundance of mafic minerals and their insignificant role in fractional crystallization. In the major oxides (Na2O–K2O–CaO) ternary classification diagram of Gilkson (1974), the samples are scattered in granite to quartz–monzonite field while the normative An–Ab–Or ternary diagram of O'Connor (1965) shows the majority of HHLG samples are plotted in the granite field (Fig. 5a, b). The high A/CNK ratio values range from 0.98 to 1.44 with normative corundum values (0.20–6.81) suggesting a peraluminous nature (Fig. 6a). A variation in Fe2O3, MgO, TiO2 and P2O5 with increasing SiO2 has been observed that may be produced by partial melting of metapelites. The HHLG shows high Rb (154–412 ppm) and low Sr (19–171 ppm) contents with high Rb/Sr ratios (1–16), again typical for crustal material. The total REE abundance of the HHLG ranges from 48 to 227 ppm and the chondrite-normalized REE patterns show the enrichment of LREE and depletion of HREE concentrations (Fig. 6b, Table 5). They exhibit lower LREE contents LaN (37–206) and highly depleted HREE of YbN (1–22) and LuN (1–19) with wide variation in HREE fractionation [(Gd/Yb)N = 1.26–26.30]. The negative Eu anomalies (Eu/Eu* values = 0.37–0.83) may be due to removal of plagioclase feldspar from the source magma by fractional crystallization or residual feldspar in the source region. The wide HREE range in HHLG is probably associated to the variable residual zircon retention during the partial melting process and different Zr saturation level of the magma (Watson and Harrison, 1983). Figure 6b also show the fields of chondrite-normalized REE normalized pattern of the Langtang valley leucogranite (after Inger and Harris, 1993), Gangotri and Manaslu leucogranite (after Vidal et al., 1982; France-Lanord and Le Fort, 1988; Scaillet et al., 1990) and the Southern Tibet leucogranite (after Guo and Wilson, 2012) are shown for comparison with the present investigated western Arunachal Himalaya leucogranite.

image

Figure 5. (a) Major oxides (Na2O–K2O–CaO) ternary classification diagram of HHLG (fields plotted after Gilkson, 1974); (b) Normative Anorthite–Albite–Orthoclase (An–Ab–Or) classification diagram of HHLG (fields from O'Connor, 1965 modified by Barker, 1979).

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image

Figure 6. (a) Mol. A/CNK versus A/NK [mol. Al2O3/(CaO + Na2O + K2O) versus Al2O3/(NaO2 + K2O)] diagram of HHLG (after Maniar and Piccoli, 1989); (b) Chondrite-normalized plots of rare earth elements of selected samples from HHLG (normalized after Sun and McDonough, 1989). Fields of chondrite-normalized REE normalized pattern of the Langtang valley leucogranite (after Inger and Harris, 1993), Gangotri and Manaslu leucogranite (after France-Lanord and Le Fort, 1988; Vidal et al., 1982; Scaillet et al., 1990) and the Southern Tibet leucogranite (after Guo and Wilson, 2012) are shown for comparison.

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Mineral chemistry of the HHC

Kyanite-bearing metapelites

The garnets under study are almandine rich garnet (Xalm = 70–74%, Xprp = 8–16%, Xgrs = 5–9%, Xsps = 5–12%). Garnet has similar composition from core to rim and does not show zoning pattern (Fig. 7a, b, Table 2). This is probably due to homogenization as a result of volume diffusion at the peak temperature (Tracy, 1982). Biotite has the chemical composition of XFe = 2.35–2.63, XMg = 1.57–2.12, XAl = 3.38–3.40 and Fe/(Fe + Mg) ratio of 0.53–0.63 with high Ti content (0.34–0.35). Plagioclases are Na-rich and Ca-poor as Ab79–82 and An17–19.

image

Figure 7. (a) Photomicrograph showing idioblast garnet with some inclusion at core in kyanite-bearing metapelite from HHC; (b) figure shows the garnet zoning profile of kyanite-bearing metapelite; (c) Back scattered SEM image showing the mineral assemblage of garnet, biotite, muscovite and quartz in sillimanite-bearing metapelite; (d) figure shows the garnet zoning profile of sillimanite-bearing metapelite; (e) P–T diagram showing the average P–T values of the representative samples obtained from geothermobarometry for the kyanite/sillimanite-bearing metapelites. Arrowed curve is the P–T path determined for the metapelites of HHC in Zanskar Himalaya and metamorphic reaction curves are based on the average pelites in Searle et al. (2010). This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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Sillimanite-bearing metapelites

Almandine (70–87%) is most dominant garnet of this metapelite and contains minor amount of pyrope (5–11%), grossular (2–6%) and spessartine (4–17%). Garnets show no core to rim zoning. However, slight decreasing Fe and Mg concentrations and an increase in Mn near to the garnet rim at the contact with biotite are observed in garnets (Fig. 7c, Table 2). This trend corresponds to a diffusion zoning related to a progressive re-equilibration of the garnet rim composition during the retrograde metamorphic evolution (Spear, 1993). The biotite has a chemical composition of XFe = 2.62–3.85 XMg = and Ti content of 0.18–0.41. Plagioclases are oligoclase (An21–An29) in composition.

P–T ESTIMATIONS OF THE HHC

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. GEOLOGICAL SETTING
  5. PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG
  6. GEOCHEMISTRY AND MINERAL CHEMISTRY
  7. P–T ESTIMATIONS OF THE HHC
  8. DISCUSSION
  9. CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

An estimation of the pressure–temperature conditions of the peak metamorphism of the HHC is important for understanding the petrogenetic processes of the HHLG, as the HHLG are thought to have been sourced from the HHC in other parts of the Himalayas (Castelli and Lombardo, 1988; Scaillet et al., 1990; Harris and Inger, 1992; Inger and Harris, 1993; Searle et al., 1993, 1997, 2010). The garnet–biotite (GB) Fe–Mg exchange thermometer and the garnet–aluminosilicate–quartz–plagioclase (GAQP) barometer were used.

The GB thermometer is likely to be reset during cooling and therefore probably gives temperatures lower than the peak values (Spear, 1991, 1992). Garnet compositions corresponding to the minimum in Fe/(Fe + Mg) ratio and Mn content were used because these probably correspond most closely to the composition present at the peak of the metamorphism (Spear, 1993).

The geothermobarometry calculations yielded nearly identical temperatures (650 ± 28 °C to 658 ± 24 °C) and pressures (8.48 ± 0.28 kbar to 8.51 ± 0.38 kbar) using the core and rim compositions of garnet in the kyanite-bearing metapelites. The peak metamorphic mineral assemblage of garnet–kyanite–biotite supports the P–T data.

The calculated P–T condition of the sillimanite-bearing metapelite samples show an increase in temperature and a decrease in pressure from the underlying kyanite-bearing metapelites. The calculated temperatures range from 676 to 717 °C and pressure ranges from 5.60 to 6.58 kbar using core composition of garnet porphyroblasts, while the rim compositions yielded lower P–T values (662–688 °C and 4.00–6.01 kbar) (Table 3). The difference between core and rim is interpreted to be the result of retrograde equilibrium of garnet porphyroblast towards the rim. The P–T diagram shows that the samples from the kyanite zone fall in the kyanite stability field while the sillimanite zone samples fall in the sillimanite stability fields which are in good agreement with the observed mineral assemblage and inferred a clockwise P–T loop (Fig. 7e).

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. GEOLOGICAL SETTING
  5. PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG
  6. GEOCHEMISTRY AND MINERAL CHEMISTRY
  7. P–T ESTIMATIONS OF THE HHC
  8. DISCUSSION
  9. CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

Crystallization history

In the normative Q–Ab–Or diagram (Fig. 8a), the HHLG samples do not cluster about a minimum melt composition, which is characteristic of a water saturated haplogranite system (Tuttle and Bowen, 1958). The scattering of the data is due to variable quartz/albite ratios and the orthoclase content that may be due to collection of samples from different sills/dykes with variable quartz–feldspar contents. However the compositions plot in and around the cotectic minimum of 1 to 5 kbar with water activity roughly between 0.3 to 0.5 PH2O and the crystallization temperature is mostly less than 700 °C (Fig. 5b).

image

Figure 8. (a) Phase relations and minimum melt compositions in the system Quartz–Albite–Orthoclase + H2O + Anorthite + F. Minimum melt compositions are from Winkler (1979); Ebadi and Johannes (1991), +: aH2O = 1; X: aH2O = 0.5; O: aH2O = 0.3; (b) Primordial mantle-normalized spider diagrams of HHLG (normalized after Sun and McDonough, 1989).

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The primordial mantle-normalized spider diagrams for the HHLG are characterized by negative Ba, Nb, Sr, P and Ti anomalies and relatively high levels of Rb, Th, and U (Fig. 8b). Strong negative Sr and Ba anomalies together with negative Eu anomalies suggest either the fractionation of plagioclase or retention of plagioclase in the source during partial melting. The negative Ba anomaly cannot be correlated with fractionation of K-feldspar, since it is the last phase to crystallize. The negative Nb and Ti anomalies can be attributed to fractionation of biotite while the negative P anomaly may be due to fractionation or retention of apatite in the source. The enrichment of Rb, Th, U along with K suggests that the leucogranite was generated from a crustal source (Inger and Harris, 1993).

Based on the absence of macroscopic restite, as well as the microscopic restite like resorbed core of calcic rich plagioclase which could be of restitic origin (Chappell et al., 1987), the restite unmixing model can be ruled out. The geochemical characteristics of the HHLG in the study area, such as enrichment of large ion lithophile elements (LILE) and depletion of high field strength elements are indicative of fractional melting of a metasedimentary source. The negative anomalies of Ba, Nb, Sr, P and Ti along with negative Eu anomaly suggest crystal fractionation of early-formed major phases such as plagioclase and biotite and accessory phases such as monazite, apatite and zircon, or their retention in the source.

Source and melting processes

The peraluminous nature of the HHLG can be shown by plotting the data in A/CNK-A/NK [mol. Al2O3/(CaO + Na2O + K2O) vs. Al2O3/(NaO2 + K2O)] diagram which is based upon the Shand's alkali lime index (Fig. 6a). It is well documented that peraluminous granites can be produced by partial melting of crustal rocks with dominant pelitic components (Clemens and Wall, 1981; Todd and Shaw, 1985; Stuckless, 1989). The HHLG contains muscovite and biotite, high SiO2, K2O/Na2O ratio, Rb, normative corundum, molar A/CNK ratio mostly >1 with low Nb/Th ratios, Sr content and high FeOtot/MgO ratios in biotite which are characteristic of their derivation from crustal materials (Chappell and White, 1992; Abdel-Rahman, 1994; Frost et al., 2001). The enrichment of LREE and depletion of HREE in the HHLG are more or less coincident with typical crustally derived granitoids (Holtz et al., 1989; Rogers and Greenberg, 1990). Harris and Inger (1992) have shown that the strong LREE enrichment and large LREE/HREE ratio is also a characterized feature of the melt derived from a pelitic source.

Different reactions have been proposed for melting of the continental crust: (a) vapour/fluid present incongruent melting of muscovite about 620–650 °C (Thompson, 1982); (b) vapour absent incongruent melting of muscovite at about 700–750 °C (Harris et al., 1995); and (c) fluid absent melting of biotite after dehydration of muscovite at temperatures >750 °C (Le Breton and Thompson, 1988). In the absence of free water, melting depends on the availability in the source region of hydrous minerals like muscovite or biotite which may release their water during anatexis. For fluid absent conditions small quantities of melt can be produced at muscovite breakdown temperature that will not be capable of intruding far from its source region, but larger volumes can only be produced at higher temperature (850 °C) corresponding to biotite breakdown reaction (Clemens and Vielzeuf, 1987; Vielzeuf and Holloway, 1988). Patiño Douce and Harris (1998) have conducted a melting experiment on metapelitic rocks from the HHC and suggest that the Himalayan leucogranites were generated by fluid-absent melting at temperatures around 750 °C and 6–8 kbars during adiabatic decompression, and are solely the result of the breakdown of muscovite.

Harris et al. (1993) suggested that about 10% leucogranite melts can be produced by decompression during uplift of fluid absent pelites from 10 to 5 kbar at a temperature of around 700 °C. The calculated peak temperature of the study area is around 700 °C which is not enough to cross the biotite breakdown reaction line. The P–T diagram in Figure 7(e) shows the relevant pressure and temperature with the involved reactions for melting the metapelites of HHC. The preferred interpretation of the observed temperature inversion with a decrease in pressure from the kyanite to the structurally higher sillimanite zone is that the early kyanite grade peak metamorphic condition (M1) of the HHC is overprinted by M2 sillimanite grade metamorphism with production of leucogranite during decompression. In the upper levels the replacement of kyanite by fibrolitic sillimanite, abundance of sillimanite near the leucogranite sills and dissolution of garnet for the formation of biotite and sillimanite suggests decompression played a dominant role during M2.

In Bhutan, decompression reaction textures have been observed and in the upper levels of the HHC the final emplacement of the leucogranite has been estimated at around 5 kbar (Davidson et al., 1997). Goswami et al. (2009) proposed that the leucosomes are formed with the starting of melting in the kyanite-bearing metapelites through water-saturated and water-undersaturated melting of paragonite while the granitic leucosomes was produced through water-undersaturated melting of muscovite in the sillimanite zone. Harris et al. (1995) have also suggested that the melts produced during anatexis involving a fluid present melting reaction of muscovite + plagioclase + quartz, would have Rb/Sr ratios <1.5. Since the Rb/Sr ratios of the HHLG of the study area is very high (1.02–15.85), it must have produced by a fluid absent reaction involving dehydration of muscovite as was suggested for the Langtang leucogranite (Inger and Harris, 1993). Bhalla et al. (1994) have carried out Rb–Sr analysis of five samples of HHLG of the study area, which define a linear array corresponding to an age of 29.4 ± 7 Ma with an initial Sr ratio of 0.7949 ± 0.0019. The high initial Sr ratio along with high Rb/Sr ratios further suggests that the HHLG were derived from melting of crustal rocks.

The high concentration of LILE and strong depletions of HFSE in the HHLG suggests fractional melting from a metasedimentary source (Vidal et al., 1982; Harris et al., 1995). Based on petrography, geochemistry and initial Sr ratios it has been observed that the leucogranites from different sectors of the Himalaya are of two types, namely two mica luecogranite and biotite free muscovite–tourmaline leucogranite. These two types of leucogranite have been recorded from Nepal Himalaya and suggested these leucogranites are originated from different sources which are identified as the Formation-I consisting of metamorphosed pelites that are occurred at the lower part of the HHC and quartzo-feldspathic sediments (Daniel et al., 1987; France-Lanord and Le Fort, 1988; Reddy et al., 1993; Guillot and Le Fort, 1995).

In the study area, the probable equivalent of the Formation-I metasediments of Nepal is represented by the kyanite-bearing metapelites that occur at the base of the HHC. The kyanite zone is overlain by biotite gneisses which form interleaved with pelitic schists. About 2–3 km away from the above MCT, the pelitic schists become more gneissic in nature and contain biotite–garnet–sillimanite (fibrolite). The sillimanite gneisses and the biotite gneisses (locally migmatitic) are widely exposed in the central or core part of the HHC, wherein leucogranite sills associated with veins of tourmaline-bearing pegmatites. Since the HHLG are closely associated with the sillimanite gneisses/schist, it is suggested that the sillimanite gneisses/schist might represent the melt source and even represent a zone of in-situ melting. Their common occurrence within the high-grade regional metamorphic terrains, the lack of spatial and temporal association with basaltic magmatism strongly suggests that the HHLG are the product of pure crustal melt and uncontaminated by mantle material. Considering the above facts the source rock for the HHLG was more likely the kyanite pelitic schists occurred at the lower part of the HHC and sillimanite gneisses/schist exposed in the middle part of the HHC, from which the leucogranite was generated by fluid absent melting of muscovite.

Deformation mechanisms and temperatures

In the HHLG, ductile deformation features are well preserved in quartz porphyroclasts, which shows undulose extinction, bulging of grain boundaries with moderate recrystallization, subgrain formation, chessboard extinction and amoeboid structures. The presence of bulging (BLG), subgrain rotation (SGR) and grain boundaries migration (GBM) mechanisms during deformation are dependent on temperature, strain rate, and fluids, with each mechanism dominant at different conditions (Hirth and Tullis, 1992; Stipp et al., 2002).

The results of this study suggest that temperature is one of the important controlling factors for the deformation mechanisms of the HHLG in Arunachal, since these rocks record high temperature deformation features without any preferential mineral alignment. The deformation temperature for quartz recrystallization by BLG ranges across ~200–400 °C while SGR and GBM occur at above ~400–500 °C (Stipp et al., 2002; Rosenberg and Stünitz, 2003). The presence of chessboard extinction in quartz by GBM suggests a minimum temperature of ~630 °C at 2.5–3.0 kbar (Mainprice et al., 1986; Kruhl, 1996; Stipp et al., 2002).

The present of cuspate/lobate grain boundaries between quartz and feldspars suggest the HHLG is deformed under diffusion creep at high-grade metamorphic conditions (Gower and Simpson, 1992). These microstructural features are compatible with high-temperature ductile deformation (Tullis, 1983).

Well-recrystallized feldspars that start to recrystallize by BLG at low temperature (~ 400–500 °C) and at >550 °C by SGR and GBM are generally not observed (Tullis et al., 2000; Rosenberg and Stünitz, 2003). The cuspate/lobate grain boundaries between different minerals recognized in the HHLG may be either as the product of diffusion creep or crystallization from interstitial melt and these microstructures can develop simultaneously if the melt is present during deformation (Garlick and Gromet, 2004).

Based on the observation of higher quartz c-axis fabric opening angle in leucogranite from surrounding rocks of the Greater Himalayan slab in the Everest Massif, Law et al. (2004, 2011) speculated that plastic deformation of the leucogranite occurred before the intrusion cooled to ambient temperature and may indicate that the intrusion occurred at a late stage during top-to-the-north penetrative deformation. Many low-temperature features such as deformation lamellae, undulose extinction and fractures are also recognized in quartz, implying that deformational was continuous through decreasing temperature conditions.

The deformation features such as undulose extinction, tapering deformation twins and deformation bands in feldspars are generally observed at low grade conditions (~300–400 °C; Passchier and Trouw, 2005). These microstructural analyses suggest that diffusion creep and dislocation creep were the main deformation mechanisms of the HHLG. Both the K-feldspar and plagioclase porphyroclasts show ductile deformation features overprinted by brittle deformation. Both feldspar types passed through their plastic–brittle transition during the later stages of deformation. In some cases the fractures in plagioclase porphyroclasts are developed cutting across the bent twin lamellae, suggesting that fracturing is later than the early crystal plastic deformation.

There is some evidence for retrogression of feldspars to secondary minerals such as sericite. The presence of these retrograde mineral assemblages is also an evidence for the initial high temperature deformation superimposed by retrogression under low temperature conditions due to exhumation of the HHLG towards shallow structure levels.

Tectonic setting and models for the generation of the HHLG

In the tectonic discrimination diagram of R1–R2 (De la Roche et al., 1980; Pitcher, 1982), the HHLG samples scatter in the syn-collision field (Fig. 9a). This reflects the restricted range of S-type granites (Chappell and White, 1974). The trace elements discrimination diagram (Nb vs. Y diagram) of Pearce et al. (1984) clearly depicts that most of the data scatter in the VAG + syn-COLG fields (Fig. 9b). On the basis of field setting and geochemical characteristics of the HHLG, it suggests that the involvement of subduction-related magmatism for the genesis of HHLG can be ruled out. There is clear cut evidence for collision, crustal thickening, deformation and anatexis during the Tertiary in the Himalaya.

image

Figure 9. Tectonic discrimination diagrams of HHLG (a) R1 vs. R2 diagram (after De la Roche et al., 1980; Pitcher, 1982); (b) Nb vs. Y diagram (after Pearce et al., 1984).

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Several models have been proposed to explain the generation of the HHLG. Le Fort (1975, 1981) and Le Fort et al. (1987) suggested that the origin of the HHLG was related to water-saturated melting. They proposed that the generation of the HHLG was triggered by a flux of fluid liberated in the hot HHS rocks by dehydration of the Lesser Himalaya low grade footwall rocks during thrusting along the MCT. There is however little evidence to support the notion of pervasive aqueous fluids during metamorphism of the mid-lower crust, and trace element studies have argued against fluid present melting in the formation of crustal melts (Harris and Inger, 1992; Harris et al., 1993; Inger and Harris, 1993). Post-collision crustal thickening on the order of 70 km may have been responsible for abnormal heat production. Along with circulating fluids, this could have caused partial melting in the middle and lower part of the thickened crust to form leucogranite melts (Dietrich and Gansser, 1981). ‘Ultra metamorphism’ caused by continued crustal thickening (Searle and Fryer, 1986), widespread crustal melting (Hodges and Silverberg, 1988; Hodges et al., 1988), and exceptionally high heat production rates from a high modal percentage of radioactive minerals present in the Indian continental crust (Rao et al., 1976) are further mechanisms proposed for partial melting and generation of the leucogranite. Jaupart and Provost (1985) proposed that radioactive heating in tectonically thickened upper crust (HHC), blanketed at the top by low conductive sediments (Tethyan sediments) could have increased the temperature to cause water undersaturated dehydration melting in metapelitic rocks of the over-thrust slab.

The peak early Barrovian kyanite grade metamorphism (M1) under high pressure and moderate to high temperature was the result of burial and heating due to underthrusting of the sediments of the Indian continental margin to greater depths, possibly below the initial MCT, which was accommodating the ongoing post-collision convergence at deeper structural levels. The early metamorphism was followed by high temperature sillimanite grade metamorphism, anatexis and production of leucogranites (M2) under decompression regime during activation of the MCT (Grujic et al., 2002; Daniel et al., 2003). In the study area, it has been calculated that the MCT was active at about 10.1 ± 1.4 Ma based on ion-microprobe dating of monazite inclusions in garnet from the MCT zone but its initiation and termination ages remain unknown (Yin et al., 2006). In the Bhutan Himalaya, the MCT was active at around 22 Ma and continued its motion during and after ~14 Ma based on U–Pb monazite dating (Daniel et al., 2003). The production of sillimanite and leucogranite is related to the back flow in the centre of the HHC under decompressional regime (Grujic et al., 1996, 2002; Daniel et al., 2003; Searle et al., 2010). Based on the presence of a high conductive layer, possibly molten material in the mid-crustal levels beneath Tibet (Nelson et al., 1996), the ductile channel flow model was proposed (Grujic et al., 1996, 2002; Beaumont et al., 2001, 2004; Jamieson et al., 2002, 2004, Daniel et al., 2003, Searle and Szulc, 2005). In the channel flow model, the HHC is considered as hot channel material bounded at the base by south-directed ductile shearing along MCT and at the top by a low angle north dipping normal fault (STD) and flows outward from the beneath the Tibetan plateau in response to topographically-induced differential pressure. At the plateau flank the channel material representing the HHC is exhumed by focused surface denudation and juxtaposed with cooler, newly accreted material corresponding to the Lesser Himalayan Sequence (Jamieson et al., 2004). The channel is driven by a pressure gradient that can be triggered when the viscosity of the rocks in the HHC attains a low threshold values at high temperature and partial melting. Extrusion of the channel towards the surface brings material out from underneath Tibet up to middle and upper crustal levels (Daniel et al., 2003). During exhumation, cooling of the high-grade gneisses, migmatites and leucogranites have occurred, probably during middle Miocene time, but the deformation was continuous. Mineral assemblages of sillimanite metapelites and thermobarometric estimations suggest that the HHLG was generated at the middle crust (~20 km) and the produced melt of HHLG was intruded in the sillimanite-grade gneisses in the form of sills/dykes. Based on metamorphic mineral assemblages and thermobarometry estimations, Guillot et al. (1995) suggested that the Manaslu leucogranite of central Nepal Himalaya was emplaced at 18–21 km depth at its base and 9–13 km for its roof. The emplacement of the HHLG at the depth of middle crust is also supported by the occurrences of high temperature deformation microstructures like quartz amoeboid, chessboard extinction in quartz and cuspate/lobate boundaries between quartz and feldspars (Fig. 10a). These high temperature deformation features would be developed before the HHLG cooled at lower temperature (Law et al., 2004, 2011). Langille et al. (2010) observed high deformation temperature of around 700 °C in the sillimanite zone of Mabja Dome, southern Tibet that coincides with the peak metamorphic temperature. As the HHLG extruded towards shallow structural levels, the initial high deformation temperature microstructures are overprinted by low deformation temperature features like tapering deformation twins, deformation bands in feldspars followed by fractures (Fig. 10b, c). The deformational microstructures present in the HHLG indicate that the early high temperature ductile deformation conditions the HHLG was deformed at deeper level and superimposed by brittle deformation conditions with lowering temperature during extrusion of the HHLG with the mid crustal rocks towards the shallow structural levels.

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Figure 10. Schematic diagram shows the changing of deformation microstructural features related to the extrusion of Higher Himalayan leucogranite in associate with the middle crustal rocks from middle crust towards the upper crust: (a) high deformation temperature features—chessboard extinction in quartz at lower level (~650 °C; Stipp et al., 2002); (b) at low grade conditions—tapering deformation twins and deformation bands in feldspar at middle level (~300–400 °C; Pryer, 1993); (c) very low deformation temperature features—brittle fracturing filled by muscovite at upper level (<300 °C; Passchier and Trouw, 2005). This figure is available in colour online at wileyonlinelibrary.com/journal/gj

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CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. GEOLOGICAL SETTING
  5. PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG
  6. GEOCHEMISTRY AND MINERAL CHEMISTRY
  7. P–T ESTIMATIONS OF THE HHC
  8. DISCUSSION
  9. CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The Higher Himalayan Leucogranites (HHLG) of western Arunachal Pradesh intrude the Higher Himalayan Crystalline (HHC) as sill-like bodies or disturbed dykes. Whole rock and mineral chemical data suggest that like the majority of previously reported occurrences of these granites, the HHLG in Arunachal are peraluminous S-type granites generated by partial melting of Indian crustal material in the collision zone. The mineral assemblages, deformational microstructures and geothermobarometry estimations suggest that the HHLG were generated in the middle crust as the source materials were deeply buried. The HHLG melts were subsequently intruded into the high grade rocks of HHC as sills/dykes. High temperature deformation features such as chessboard extinction in quartz and lobate grain boundaries in between quartz and feldspars confirm emplacement at high temperatures. The observed microstructural features further suggest that the HHLG was deformed under early high temperature ductile deformation conditions at deeper structural levels; these were later superimposed by low temperature brittle deformation structures during the extrusion of the HHC + HHLG package.

ACKNOWLEDGEMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. GEOLOGICAL SETTING
  5. PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG
  6. GEOCHEMISTRY AND MINERAL CHEMISTRY
  7. P–T ESTIMATIONS OF THE HHC
  8. DISCUSSION
  9. CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES

The Authors would like to thank Prof. A. K. Gupta, Director, Wadia Institute of Himalayan Geology (WIHG), India for providing the facilities. Scientists in charge of the analytical laboratories, WIGH are gratefully acknowledged for their help during analytical works. Dr Clare Warren (The Open University, UK) and the two anonymous reviewers are thanked for their constructive comments and suggestions which were helpful in improving the manuscript. We are also grateful to Dr Erdin Bozkurt for his efficient editorial handling.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. GEOLOGICAL SETTING
  5. PETROGRAPHY AND MICROSTRUCTURES OF THE HHLG
  6. GEOCHEMISTRY AND MINERAL CHEMISTRY
  7. P–T ESTIMATIONS OF THE HHC
  8. DISCUSSION
  9. CONCLUSIONS
  10. ACKNOWLEDGEMENTS
  11. REFERENCES
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