Fluid events and exhumation history of the main central thrust zone
Garhwal Himalaya (India)
H.K. Sachan
a,*, R. Sharma
a, A. Sahai
b, N.S. Gururajan
aaWadia Institute of Himalayan Geology, Dehra Dun 248 001, India bIndian Institute of Remote Sensing, Dehra Dun 248 001, India
Received 18 November 1998; revised 16 August 1999; accepted 16 June 2000
Abstract
Mineral thermobarometric and ¯uid inclusion studies were carried out on the low to medium grade metamorphics which occur at different tectonic levels of the main central thrust (MCT) zone in Garhwal Himalaya (MCT-I in north, MCT-II in the centre and the southern most MCT-III). The augen gneiss and pelitic schists present in this zone show an increase in the grade of metamorphism from chlorite to kyanite towards the north.
The pressure calculated from garnet±biotite±muscovite±plagioclase phase equilibria, increases from 1.9 to 8 kbar and the temperature obtained through garnet±biotite and chlorite geothermometer varies between 360 and 5628C. The mineral assemblages and these thermo-barometric estimates reveal that the grade of metamorphism in the MCT zone increases from south to north. Considering the inclusion types, it is apparent that the carbonic ¯uid increases towards the MCT-I, which is related to increase in P±T conditions whereas the aqueous phase is more pronounced near the MCT-III re¯ecting ¯uid assisted retrogression. This observation is also substantiated by the systematic increase in CO2density from 0.75 g/cm3in CO2±H2O inclusions near MCT-III to 1.01 g/cm3in pure CO2inclusions near MCT-I. The pressure and
temperature estimated using ¯uid isochores varies from 1.9 to 4.8 kbar and 360 to 5628C. When compared with mineral phase thermo-barometry, these P±T conditions suggest a post peak-metamorphic nature of the ¯uids. A smooth trend of mineral and ¯uid phase P±T conditions is observed in the MCT zone. Based on combined mineral P±T data and ¯uid isochores, a decompressional uplift path is suggested. The decrease in P±T and the evidence of movement along well distributed shear fabric across the MCT zone indicate that exhumation from north to south occurred under decreasing P±T conditions.q2001 Elsevier Science Ltd. All rights reserved.
Keywords: Metamorphics; Garhwal Himalaya; Thrust zone; Fluid inclusions; Thermobarometry; Exhumation
1. Introduction
The Himalayan fold belt is marked by major thrust faults such as the Main Frontal Thrust (MFT), Main Boundary Thrust (MBT) and Main Central Thrust (MCT). The MCT, ®rst recognized by Gansser (1964), is a zone of deep intracrustal fractures separating rocks of low to moder-ate metamorphic grade in the south from higher grade central crystalline rocks in the north. The relationship between thrust movements, metamorphism and magmatism in the different tectonic levels of the MCT zone have been discussed by Le Fort (1975), Valdiya (1980), Hodges and Silverberg (1988), Pecher (1989) and Metcalfe (1993). The early Precambrian crystalline rocks form the highly deformed hinterland has been thrust over the younger sequences within the southward-thrusting system in the
Himalayas, and younger thrusts appear towards the south overlying undeformed foreland (Siwaliks). The Vaikrita thrust (Valdiya, 1988) is considered to be a roof thrust, the middle branchline thrust is locally known as Muns-siari/Jutogh thrust and further in the south the second branchline or ¯oor thrust is locally referred to as Ramgarh/Chail Thrust (Fig. 1). These three thrust planes are emergent thrusts separated from one another and exposed at the present topographic/erosional level. The area bounded by these thrusts is referred as MCT zone (Fig. 1).
On the basis of K±Ar and Ar±Ar dating of mica and hornblende in the MCT zone, Metcalfe (1993) and Oliver et al. (1995) suggested a younging of the thrust sheets towards the foreland basin to the south. They suggested an age of 19:8^2:6 Ma for the initiation of the thrusting movement i.e. for northernmost MCT-I. Further, an age of 8.0 Ma for the southernmost zone of the MCT infers that the MCT-III has remained active until this time.
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The tectonometamorphic history of the central crystal-lines have been dealt by previous workers such as: Maruo and Kiazaki (1983), Sauniac and Touret (1983), Brunel and Kienast (1986), Hodges and Silverberg (1988), Pecher (1989), Hubbard (1989), Metcalfe (1993) and Morrison and Oliver (1993). However, a detailed microthermometric and thermobarometric study of the rocks across the MCT zone in this region has not been carried out. In the present investigation, we performed mineral phase thermobarome-try and ¯uid inclusion microthermomethermobarome-try at different tectonic levels of the MCT zone along the two principal valleys of the Garhwal Himalaya. This study provides some new constraints on the metamorphic and exhumation process for the MCT zone.
2. Geological setting
The crystalline rocks generally known as central crystal-lines exposed along the Yamuna and Bhagirathi valleys of Garhwal Himalaya can be divided into three major lithotec-tonic units. These crystalline formations are termed as upper
crystalline (Vaikrita Group), middle crystalline (Jutogh or Munssiari Group) and the lower crystalline (Chail or Ramgarh Group). The lower Ramgarh, the middle Muns-siari and upper Vaikrita Group are limited by thrust faults (Valdiya, 1980). The upper crystalline is separated from middle crystalline by MCT-I (Vaikrita Thrust) whereas the middle crystalline is separated from lower crystalline by MCT-II (Munssiari Thrust or Jutogh Thrust). In the south, rocks of lower crystalline are separated from Lesser Himalayan sedimentaries by MCT-III (Ramgarh Thrust or Chail Thrust). Thus in the present paper Vaikrita Thrust is referred as MCT-I, Munssiari Thrust as MCT-II and Ramgarh Thrust as MCT-III.
The highly deformed crystallines between MCT-I and MCT-III lie in a 10 km thick NE dipping shear zone. This zone consists of mylonitic augen gneiss, chlorite±biotite± garnet bearing schist, quartzite, amphibolite and calc silicate bands. The grade of metamorphism increases from south to north i.e. towards MCT-I at higher structure level. Four metamorphic isograds are demarcated in the area from south to north i.e. chlorite, biotite, garnet and kyanite (Fig. 1). To the south of MCT-III, the Lesser Himalayan H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
Formation (Garhwal Group) includes rocks of chlorite grade i.e. chlorite phyllite, chlorite schists and metabasic rock. Between MCT-III and II, biotite grade rocks with augen gneiss and biotite-bearing metapelite are exposed. The rocks between the MCT-II and I show garnet and staurolite grade metamorphism. Kyanite appears just below the MCT-I and is uncommon.
Three deformational episodes are recorded in the MCT zone (Pati and Rao, 1983). The D1 deformation phase is represented by S1 schistosity and associated with M1 phase of Barrovian metamorphism in the MCT zone. D2 deformation phase is evident by the presence of main pene-trative shear fabric S2, related with the M2 peak meta-morphism. The regional fabric is parallel to the attitude of thrusts. D3 is a widely present brittle phase of deformation. This last deformation episode is associated with the retro-gressive phase of metamorphism (M3).
3. Petrography
Samples were collected from two transects along the Yamuna and Bhagirathi Valleys (Fig. 1). The rocks around
MCT-III are characterized by a mineral assemblage of quartz1chlorite1muscovite1albite. Graphite and carbonates are found in subordinate amount in this zone. Evidence for prograde metamorphism is observed in quart-zite and phyllite from preferred orientations and alignments of grains and the development of chlorite, as well as from the crystalloblastic textures in phyllites. The metapelites (pelitic schists and augen gneisses) occur between the MCT-III and the MCT-I. The characteristic mineral assem-blages found from the south of the MCT-III to the MCT-I are as follows:
(South of MCT-III)2Chlorite1Muscovite1Biotite1 Quartz^Plagioclase
(Between MCT-III and II)2Biotite1Chlorite1 Quartz1K-Feldspar1Garnet1Plagioclase
(Between MCT-II and I)2Garnet1Biotite1 Musco-vite 1 Quartz 1 Plagioclase 1 Staurolite ^ Kyanite^ Graphite.
Staurolite, garnet and kyanite are not universally found and their percentage increases northward. Bands of graphite occur in some places across this zone. Garnet, feldspar and quartz grains show porphyroblastic textures. Two genera-tions of chlorite occur in the MCT zone, chlorite-I was H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
formed during the prograde stage related to D2 deformation. Chlorite-I occurs in the rock matrix south of MCT-III, while biotite, garnet, staurolite and kyanite progressively appears towards MCT-I (Fig. 1). This metamorphic progression is also evident by the granoblastic texture in the augen gneisses (Fig. 2a). Chlorite-II is found around garnet rims indicating their formation during a retrogressive event synchronous with the D3 phase of deformation.
Garnet shows variation in deformational features and effects of retrogression in this zone. Garnet grains around MCT-II are highly stretched and are aligned along the meta-morphic foliations. Cracks in these garnets are ®lled with secondary quartz. To the north of MCT-II, euhedral to subhe-dral garnet (mainly almandine) with a size ranging from,1 to 4 mm exhibit synkinematic growth and contain inclusions of quartz, staurolite and kyanite (Fig. 2b). The pressure shadows observed around these garnets indicate a post-meta-morphic deformation event (presumably thrusting) (Fig. 2c). The garnet between MCT-II and III occur as well developed porphyroblasts with the development of chlorite ¯akes along the rims, suggesting retrogressive conditions (Fig. 2d).
The amphibolites, migmatites and calc-silicates are present near MCT-I. The mineral assemblage of these amphibolites includes hornblende±clinozoisite±epidote± biotite and quartz. In the migmatites, the leucosome consists of coarse grained feldspar±quartz±muscovite and biotite while the mesosome contains quartz±plagioclase and biotite. The melanosome is characterized by biotite±hornblende± epidote and garnet. The calc-silicates consist of tremolite± calcite±hornblende±diopside±scapolite and quartz.
The textures of metamorphic rocks in the MCT zone along Yamuna and Bhagirathi valley transects do not show major dissimilarities, although they vary with
lithol-ogy and tectonic level. As a whole, the MCT zone represents a distinct mylonitic zone with brittle and ductile shears, revealed by the presence of microstructures such as pressure shadows and penetrative S±C or shear band fabrics. The textures observed in these rocks show an increase in grade of metamorphism from south to north, with an overprinted retrogressive event. The rocks show chlorite grade meta-morphism south of MCT-III which gradually increases northward to biotite, garnet±staurolite and ®nally to kyanite grade near MCT-I (Fig. 1).
4. Thermobarometry
For chlorite thermometry, ®ve samples were chosen from the chlorite grade of rocks present around MCT-III. Ten samples were selected from south of MCT-II to MCT-I for garnet±biotite thermometry and garnet±biotite±plagio-clase±muscovite±quartz geobarometry. The locations of these samples are given in Fig. 1. The mineralogy of the studied samples around MCT-III includes chlorite±biotite± muscovite and quartz. Only primary chlorite (i.e. chlorite-I) was analysed in these samples. Plagioclase was noticed in few of them. The studied samples between MCT-II and III contain biotite±garnet±plagioclase±muscovite and quartz with some amount of chlorite-II. While the minerals H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
Table 1
Analysis of chlorite mineral around MCT-III
Sample 1 2 3 4 5
SiO2 24.50 24.77 24.9 25.68 24.86
Al2O3 23.13 22.85 23.18 20.18 22.86
FeO 25.25 26.85 27.51 25.34 23.86
MgO 12.22 12.07 12.08 16.8 15.47
MnO 0 0.06 0.13 0.08 ±
CaO 0 0.03 ± 0.12 ±
K2O 0 ± ± ± ±
Na2O 0 0.019 ± ± 0.081
TiO2 0 0 0 0 0
Total 85.13 86.67 87.90 88.27 87.17
Cation on the basis of 14 oxygen
Si 2.654 2.658 2.648 2.691 2.614
AIIV 1.346 1.342 1.36 1.309 1.386
AIVI 1.6 1.547 1.535 1.184 1.446
Fe 2.28 2.409 2.438 2.2213 2.098
Mg 1.97 1.931 1.909 2.6337 2.424
Mn ± ± 0.0118 0.0076 ±
Ca ± 0.004 ± 0.0014 ±
K ± ± ± ± ±
Na 0.003 0.004 ± ± ±
Table 2
Representative microprobe analysis of main minerals across MCT zone between MCT-II and MCT-I
Garnet Biotite Muscovite Plagioclase
Core Rim
SiO2 37.88 37.42 35.61 45.39 62.58
Al2O3 20.62 20.84 17.79 31.65 22.98
FeO 36.24 35.7 22.14 1.95 0.01
MgO 1.984 1.84 9.05 0.82 0.003
MnO 1.16 1.09 0.01 0.01 0.01
CaO 2.54 2.18 ± 0.01 4.98
K2O ± ± 8.77 8.92 0.06
Na2O ± ± 0.01 1.51 9.35
TiO2 0.014 0.06 1.54 0.03 0.01
Si 3.047 3.045 2.745 3.184 2.778
Al 1.95 1.99 1.616 2.617 1.202
assemblage of the samples between MCT-II and I include garnet±biotite±staurolite±muscovite±plagioclase and quartz. Here the development of kyanite was noticed, though its predominance was found above MCT-I. The representative analyses of the minerals are given in Tables 1±31. The analyses were performed on a Jeol JCXA-733 superprobe at Roorkee University, Roorkee. The operating conditions for Na, K, Fe, Si, Ti, Mn, Ca and Mg were 15 kV accelerating voltage, 15 nA sample current and 2mm beam size.
The rim analyses were made (,3±6mm) as close to the grain boundary as possible. Five analyses were carried out for each spot to check the reproducibility. A Monte Carlo technique (Hodges and McKenna, 1987) was used to esti-mate standard deviations of mineral compositions in order to determine the precision of calculated temperature and pressure.
4.1. Chlorite thermometry
The chlorite geothermometry was ®rst developed by Cathelineau and Nieva (1985) on the basis of correlation of Aliv with the temperatures. Kranidiotis and MacLean (1987) modi®ed the expression of Cathelineau and Nieva (1985) by taking into account the variation in Fe/(Fe1Mg) in chlorite. A revised calibration was presented by
Cathelineau (1988). Jowett (1991) suggested a correction derived from isothermal Fe/(Fe1Mg) normalization based on Salton sea and Los Azufres chlorite composition by placing the restriction to the applicability of his empirical calibrations to only where chlorites contain Fe/(Fe1Mg) values ,0.6. However, Caritat et al. (1993) cautioned the use of chlorite geothermometry and suggested the use of other thermometric methods to constrain the data of chlorite geothermometry.
The application of the chlorite thermometer by Kranidio-tis and Mac Lean (1987) empirical calibration gave the temperature range of 177±2238C for the rocks around MCT-III. While for the same rocks, calibration of Catheli-neau (1988) estimates the temperature range of 360±3778C. Jowett (1991) model estimated the temperature (Table 4), which are in agreement with the temperatures of chlorite grade of metamorphism suggested by Weaver (1984) and Kisch (1987).
4.2. Garnet±biotite thermometry
Garnet±biotite thermometry (Table 4) is based on Fe±Mg partitioning between garnet and biotite (Rameshwar Rao, 1995). Perchuk and Laver'enteva (1983) used natural samples for their calibration while Ferry and Spear (1978) used synthetic binary phase for the calibration assuming ideal Fe±Mg mixing. Application of Ferry and Spear (1978) model gave very low temperatures. One of the reasons could be the limitations of their method to (Ca1Mn)/(Ca1Mn1Fe1Mg) ratio and (Alvi1Ti)/ (Alvi1Ti1Mg1Fe) ratio to less than 0.2 and 0.15 in garnet and biotite respectively. The temperature estimates using Perchuk and Laver'enteva (1983) method is around 470±4808C for zones between MCT II and III and 530± 5368C for zones between MCT I and II.
The effect of non-ideal mixing in Himalayan garnet phases is expected to be important because grossular and spessartite contents are same as pyrope (Table 2). Hodges and Spear (1982) and Pigage and Greenwood (1982) consid-ered the Ca and Mn for non-ideal garnet solution. Their calibration gave higher range of temperature than that of Ferry and Spear (1978).
Inderas and Martignole (1985) had taken care of Ti and Alvi content in biotite for the application of garnet±biotite thermometry. The application of this thermometry gave a much lower temperature than the Ferry and Spear (1978) and Hodges and Spear (1982) calibration. The correction for non-ideal mixing of quaternary garnets (Fe±Mg±Ca±Mn) was done using model of Bhattacharya et al. (1992) and Applegate and Hodges (1994). The above mentioned cali-bration gave temperature in the range of 470±4808C for the samples between MCT II and III. Holdoway et al. (1997) have taken care of best possible use of Fe31in their recent calibration and redetermined the Ferry and Spear (1978) exchange equations, Perchuk and Laver'enteva (1983) experiments and used Mukhopadhyay et al. (1997) H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
Table 3
Representative microprobe mineral analyses across MCT zone in between MCT-II and III
Garnet Biotite Muscovite Plagioclase
Rim Core
SiO2 37.51 37.25 35.29 45.97 62.34
Al2O3 20.70 20.86 17.31 33.85 22.94
FeO 34.88 35.05 17.80 0.79 0.00
MgO 1.84 2.26 11.05 0.41 0.00
MnO 2.22 0.99 0.31 0.01 0.00
CaO 3.20 3.10 0.02 0.01 5.25
K2O ± ± 9.39 8.86 0.06
Na2O ± ± 0.05 1.65 9.2
TiO2 0.040 0.03 2.81 0.25 0.01
Si 3.02 3.01 2.78 3.08 2.77
Al 1.967 1.967 1.52 2.80 1.20
Fe 2.35 2.352 1.113 0.046 ±
Ti 0.00 0.00 0.160 0.013 ±
Mn 0.151 0.068 0.02 0.001 ±
Mg .0221 0.272 1.31 0.043 0.250
Ca 0.276 0.277 0.02 0.001 0.794
Na 0.01 0.007 0.225 0.003
K 0.00 0.00 0.896 0.793 ±
garnet margules parameters. The temperature estimates using Holdoway et al. (1997) method are 507±5178C for the zone between MCT II and III. From the above discussion one can see that the temperature estimates between MCT II and III using Perchuk and Laver'en-teva (1983), Bhattacharya et al. (1992), Applegate and Hodges (1994) and Holdoway et al. (1997) fall within a narrow range of 470±5178C. The above methods between MCT I and II give temperature of 530± 5648C. These values are in agreement with the tempera-ture estimated by Hodges and Silverberg (1988) and Metcalfe (1993).
5. Geobarometry
Pressures are estimated using Ghent and Stout (1981), Hodges and Crowley (1985), Hoisch (1990) and Apple-gate and Hodges (1994) methods which are based on the cation exchange in the garnet±biotite±muscovite± plagioclase assemblages, Fe in between garnet and biotite, and Ca in garnet and plagioclase (Ghent, 1976). The values of calculated pressure are summarised in Table 4. All four models for the rocks between MCT-II and III suggest the pressure range from 3.2 to 6.7 kbar with a minimum 0.3 to maximum 1.1 standard deviation. Whereas for the rocks between MCT II and I, the pressure values range from 5.1 to 8 kbar with standard deviation of 0.3±0.96 kbar. These pres-sure values show increasing trend from south to north. The maximum values of estimated pressure between MCT-I and II closely match with the observations of Hodges and Silverberg (1988) and Metcalfe (1993).
6. Fluid inclusion study
The same samples were used for ¯uid phase and mineral phase thermobarometry in order to best constrain the P±T path. Double polished wafers with a thickness of about 0:4^0:2 mm were prepared from these quartz samples. The microthermometry was conducted on a SGE gas ¯ow heating±freezing stage and Linkam THMSG 600 stage mounted on a Zeiss microscope and Olympus microscope respectively at Wadia Institute Fluid inclusion Lab. The density estimation and isochores calculations were made using the equations of Bowers and Helgeson (1983), Saxena and Fei (1987) and Brown and Lamb (1989), in the ª flin-corº computer program of Brown (1989). The ¯uid inclu-sions found in the MCT zone are of three types, namely carbonic inclusions, carbonic-aqueous inclusions and aqueous inclusions.
7. Distribution of inclusions
Of the three types of inclusions observed, the carbonic-aqueous and carbonic-aqueous inclusions are found in all of the studied samples, whereas carbonic inclusions occur in the MCT zone and have not been observed south of the MCT III. The inclusion distribution trend is not very well de®ned, however, their features such as mode of occurrence, length of trails and their relation with grain boundaries are helpful in establishing order of entrapment (Fig. 4). The monophase carbonic inclusions and biphase CO2±H2O inclusions occur in isolated distribution and in small irregular groups of about 3±7 inclusions. Their occurrence in planer arrays is common. These arrays are seen both pinching within the grain boundaries as well as transecting them. Two H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
Table 4
Average temperature and pressure estimates for metamorphic P±T conditions between MCT-I and III (Chail to Vaikrita Thrust) (temperature estimates in8C and pressure in kbar)
Location Around MCT-III Between MCT-III and II Between MCT-II and I
Model Range X SD Range X SD Range X SD
Chlorite thermometry
Cathelineau (1988) 360±377 369 7
Jowett (1991) 363±442 404 33
Garnet±biotite thermometry Perchuk and Laver'enteva (1983)
470±480 476 4 530±536 534 2
Pigage and Greenwood (1982) 444±456 450 5 520±530 526 4
Bhattacharya et al. (1992) 466±474 470 3 528±532 530 2
Holdoway et al. (1997) 507±517 511 5 554±562 558 3
Applegate and Hodges (1994) 470±490 482 9 516±546 530 11
Garnet±biotite±plagioclase±muscovite barometry
Hodges and Crowley (1985) 3.9±4.5 4.2 0.3 6±0±6.8 6.4 0.32
Ghent and Stout (1981) 3.9±6.7 5.2 1.1 6.1±6.2 6.1 0.86
Hoisch (1990) 3.2±4.8 4.1 0.67 5.1±7.5 6.4 0.96
generations of trails can be distinguished. The early trails are smaller, less predominant and consist of carbonic aqueous and monophase carbonic ¯uid. The later trails occur along healed fractures transecting the earlier trails and at times the clusters of carbonic ¯uid inclusions. The inclusions trapped along them are aqueous containing a
liquid and a vapour bubble at room temperature. Near MCT III, aqueous inclusions also occur in small clusters and are abundant. These observations suggest that the carbonic and carbonic aqueous inclusions are earlier and might have been trapped simultaneously. The aqueous inclusions are later and are secondary. They are considered to have trapped during later stage of deformational episode.
8. Microthermometric results
8.1. Carbonic inclusions
These inclusions consist of one phase at room tempera-ture (ca. 248C) and nucleate a gas CO2phase upon cooling. They do not contain visible water. CO2inclusions are sub-rounded to regular in shape and vary from about 5±15mm in size. Large numbers of carbonic inclusions are found in samples near MCT-I (Fig. 3a). These inclusions melt between257.5 to256.68C with a frequency maximum at around2578C (Fig. 5) indicating a near pure CO2 composi-tion with,8 mol% CH4. The homogenization temperature of CO2inclusions between MCT-III and II ranges from22 to 288C, corresponding to a density of 0.94±0.97 g/cm3. CO2inclusions from samples between MCT-II and I have lower homogenization temperatures of 26 to 2168C, revealing a CO2density of 0.96±1.01 g/cm3(Fig. 6). The highest density (1.01 g/cm3) was determined from samples near the MCT-I.
8.2. Carbonic-aqueous inclusions
The carbonic-aqueous inclusions are abundant in all the samples from chlorite through kyanite grade. These inclu-sions are commonly sub-rounded and vary in size from 10 to 20mm (Fig. 3b). They are biphase at room temperature with a CO2volume proportion varying between 50 to 80%.
Their melting temperature range (257.2 to 256.88C) indicate that the carbonic phase is near pure CO2(Fig. 5). Homogenization temperature of CO2in the CO2±H2O inclu-sions across the MCT zone are in the range of 5±228C (Fig. 7), consistent with a CO2density range of 0.75±0.89 g/cm3. The average degree of ®ll in these inclusions is about 0.3 and bulk density of the carbonic-aqueous ¯uid varies between 0.83 and 0.93 g/cm3. The total homogenization temperature of CO2±H2O inclusions occurred between 260 and 2908C. The clathrate melting temperatures in these inclusions is above 188C, which suggest a salinity of ,4% wt. NaCl equiv. for aqueous ¯uid.
8.3. Aqueous inclusions
These inclusions are abundant in most of the samples from the MCT zone. The biphase inclusions have a small vapour bubble of about 5±15 vol.% (Fig. 3c). They are generally less than 15mm in size and occur in innumerable shapes like sub-regular, elongated and rounded.
H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
Fig. 3. Photomicrographs of ¯uid inclusions observed in syn MCT quartz veins: (a) nearly pure CO2inclusions; (b) CO2±H2O inclusions in trails;
Eutectic temperatures (Te) of the aqueous inclusions range from 220 to 2238C, suggesting a NaCl±KCl±H2O composition of the aqueous ¯uid. The Tm (ice) of the aqueous inclusions in samples between the MCT-III and II vary from 23 to 288C corresponding to salinities in the range of 5±12 wt% NaCl equiv. Their homogenization temperature varies from 300 to 4508C. At the level of the MCT-III, it is in the range of21 to248C (Fig. 8) which is equivalent to a salinity of 1.7±6.4 wt% NaCl. Homogeniza-tion of these inclusions occurred at a temperature range of 250±3758C. Inclusions from samples north of the MCT-II show a low range of Tm (ice), between 26 and 2108C, equiv. to salinities of 9.2 to 13.9 wt% NaCl. These inclu-sions homogenized in a temperature range of 300±4758C (Figs.8 and 9). The densities of aqueous ¯uid are calculated considering the homogenization and melting temperatures through the equation of Brown and Lamb (1989). This shows a marginal decrease in density of aqueous ¯uid from 0.62 to 0.8 g/cm3 around MCT-III, 0.6±0.8 g/cm3 between MCT-III and II and 0.6±0.7 g/cm3 north of MCT-II.
9. Discussion
9.1. P±T conditions
In the MCT zone a metamorphism from chlorite to kyanite grade is observed from MCT-III to MCT-I. Around MCT-III, the rock underwent chlorite grade metamorphism (Fig. 1), which is inferred through the mineral assemblage as well as from chlorite geothermometry. The estimated metamorphic temperature is around 360±4408C. The rocks
of this zone preserve the mixed carbonic-aqueous and low saline aqueous ¯uids. Their isochores are based on CO2 density and total density of inclusion ¯uid and are calculated using the ThCO2, average degree of ®ll and the clathrate melting temperature of mixed ¯uid. They are estimated through equation of Bowers and Helgeson (1983). This CO2density range from 0.75 to 0.84 g/cm3and is consistent with the chlorite grade of metamorphism. The isochore of histogram peak (CO2 density 0.77 g/cm3, total density 0.84 g/cm3) is drawn in Fig. 10, which together with the temperature derived from chlorite thermometer suggest a pressure of 1.9±2.6 kbar.
Barometric estimates of mineral phase (Table 4) between MCT-III and MCT-I suggest a pressure variation from the lowest of 3.2 kbar at just above the MCT-III to about 8.0 kbar near MCT-I. The estimated temperature for this zone varies from a lowest value of 4448C at south of MCT II to maximum value of 5628C near MCT-I. Mineral thermobarometric estimations are in accordance with the estimated CO2 densities in the MCT zone as discussed below. Hodges and Silverberg (1988) has reported a pressure of 8.8 kbar and temperature of 5868C for MCT-I (Vaikrita Thrust) while Metcalfe (1993) estimates a P±T condition of 8 kbar: 6008C near MCT-I. The little difference observed in present and earlier studies is insigni®cant. This may be because Hodges and Silverberg (1988) concentrated mainly on the samples just on and above the MCT-I while this study present maximum estimation of the sample south of MCT-I. A palaeodepth of 29 km during peak metamorphism in MCT Zone is estimated, considering the maximum pressure obtained (cf. 8.0 kbar) and a lithostatic gradient of 0.27 kbar/km.
H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
Fig. 5. Histogram of melting temperatures of CO2in CO2and CO±H2O inclusions across the MCT zone.
H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
Fig. 7. Histogram of homogenization temperature of CO2in CO2±H2O inclusions.
9.2. Nature of ¯uids in relation to metamorphism
The ¯uids in the MCT zone belong to the C±O±H system. CO2-rich and mixed CO2±H2O ¯uids are dominant towards the northern part of MCT zone associated with amphibolitic grade rocks while aqueous ¯uids are abundant at chlorite grade in the south. Hollister and Crawford (1986) and Andersen et al. (1991) also reported change in nature of ¯uid with grade of metamorphism. The calculated isochores for the ¯uids trapped in rocks of various grads are plotted in Fig. 10, along with the mineral P±T estimates. This P±T presentation of different tectonic levels of MCT zone is based on the consideration that there is no break in the metamorphism as indicated by our mineral and ¯uid data. The observations of Valdiya (1979) and Hodges and Silver-berg (1988) also substantiate that the metamorphic isograds in the MCT zone are not disturbed. The data for the rocks between MCT-II and I reveal that the ¯uid entrapment occurred at 4.4±4.8 kbar and 548±5628C. This data show that isochore of histogram peak transect the P±T box at lower edge. The estimated P±T values of ¯uid entrapment are also not in good agreement with the garnet1biotite1 staurolite^kyanite assemblage present in this zone. These features together suggest that CO2and CO2±H2O ¯uid was trapped during waning stage of metamorphism or after attain-ing the peak metamorphism in this zone (Lamb et al., 1987).
The ¯uid entrapment conditions inferred for the rocks between MCT-III and II are at a relatively lower range of
3.4±3.9 kbar and about 444±5178C (Table 5). The isochores of their histogram peak transect the mineral P±T box at lower end indicating retrograde nature of ¯uid. Relation of isochores drawn for CO2±H2O ¯uids with mineral P±T boxes reveals that these ¯uids are post peak metamorphic. For south of MCT-III, CO2±H2O isochore of histogram peak together with chlorite solid solution geothermometry suggest a pressure of 1.9±2.6 kbar. The retrogression is less pronounced around MCT-III as evident by absence of chlorite II and also further substantiated by closely matching P±T estimates of mineral and ¯uid phase. However, repre-sentative isochores of H2O±NaCl ¯uid plotted in Fig. 10 are at low level indicating a retrometamorphic nature of aqueous ¯uid. Hence, the retrogressive nature of the ¯uids associated with MCT-II and MCT-III suggest that these thrusts are post peak metamorphic.
9.3. Source of CO2
A number of sources have been proposed for the presence of CO2inclusions in shear zones in metamorphic belts. Most workers support a combined metamorphic and deep-seated origin of the ¯uids ascending through shear conduits (Pecher, 1979, 1989 and Sauniac and Touret, 1983; Casquet, 1986 etc). In the MCT zone, the CO2and CO2±H2O inclu-sions are present in boudins and interfoliated quartz segre-gations. They are common in healed microcracks and their trails are parallel to foliation direction, which infer H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
migration of carbonic ¯uid through microcracks. These features suggest that ¯uid in®ltration through deep-seated fractures occurred during metamorphism related to thrusting and uplift. However, their relation with estimated mineral P±T suggest a post-peak metamorphic nature of the ¯uid (Lamb et al., 1987). In the MCT zone, the progression of metamorphism towards north is well correlated with the increasing CO2 contents. Higher concentration of primary CO2¯uids towards north is attributed to early stage of meta-morphism. Pecher (1979) also found the abundance of CO2 inclusions in MCT-I in Anapurna and Manaslu area of Nepal Himalaya. Similar observations suggesting that the presence of CO2increases with the increase of metamorphic grade have been made by various workers (Crawford, 1981; Santosh, 1987; Touret, 1988). The increase in densities of CO2along with metamorphic grade is also well known in isothermal decompression path. In addition to these, other possible sources which might have also added to the increasing CO2 concentration may include the oxidation of graphite intercala-tions present in the area and the decarbonation of calc-silicate rocks (Glassley, 1983) as these rocks are present near to the MCT-I.
9.4. P±T Path and exhumation history
The understanding of the mineral P±T events, ¯uid isochores and the entrapment history of the ¯uid help in de®n-ing the exhumation path. The combined mineral P±T data and representative isochores are drawn in Fig.10 for different tectonic levels of the MCT zone. It is evident that the ¯uids are post-peak metamorphic and the presence of late H2O domi-nated phase further demonstrate for its role in late stage meta-morphic/retrograde events (Lamb et al., 1991). The presence of CO2 rich ¯uids in the rocks around MCT-I, the boudin quartz in the MCT-II and III consisting primary inclusions of post-peak metamorphic CO2±H2O ¯uids, and the ubiqui-tous presence of H2O rich late ¯uid inclusions in the studied samples from the MCT-II and III and the greenschist facies mineral assemblages suggest that these thrust represent the splays of the MCT-I which climb up the structural levels during lowering of P±T conditions. The earlier ductile fabrics in the mylonitic augen gneisses are superimposed by the brittle deformation structures, thus substantiate the development and reactivation of MCT-II and III occurred at shallow depth.
The area of study from north to south show a smooth trend of decrease in metamorphic grade wherein a pressure drop from 8.0 to 1.9 kbar and the temperature decrease from 562 to 3608C is seen in the MCT zone. We ®nd a decrease of 6.1 kbar in pressure and 2028C in temperature, which account for dP/dT as 28 bars/8C suggesting isothermal decompression uplift path (Harley, 1988; Nicoles and Berry, 1991). Formation of garnet at the expense of staur-olite and kyanite seen near MCT-I also support isothermal decompression for MCT zone rocks. An uplift path such as the present one applies to continental±continental collision or continental±magmatic arc environment (Harley, 1989) whereas the isobaric cooling indicate rift tectonic type environment (Sandiford and Powell, 1986; Sandiford, 1989). The observed decompressional uplift path (Fig. 10) is clearly related to collision tectonics building Himalaya.
An increase in metamorphism towards north re¯ects that rocks in the north were subjected to higher P±T conditions, this is also attested by the presence of ductile deformational structures towards MCT-I. It appears they were metamor-phosed at deeper crustal level than the rocks around MCT-III. In the MCT zone, the well-distributed ductile shear fabric is mainly de®ned by muscovite, biotite and ¯uid inclusion trails and the occurrence of porphyroblasts with pressure shadows suggest that the shear fabric developed during post metamorphic period or at the waning stage of metamorphism. This criterion suggests that the metamorph-ism is prior to the reactivation along MCT-I. The movement along MCT-I was further transferred to MCT-II and III under lowering of P±T conditions, supported by the H2O rich ¯uid ¯ux under retrograde regime. The reverse meta-morphic character observed across the MCT zone along with the presence of pervasive shear fabrics indicate that the MCT zone represents a broad shear zone cut by MCT-II and MCT-III, which further localized strain along thrusts. The relation between metamorphism and shear fabrics suggests that the movement along numerous shear planes caused the inverted metamorphism in this part of Himalaya as also suggested by Hubbard (1996) for eastern Nepal.
Acknowledgements
Authors are thankful to Director, Wadia Institute of H.K. Sachan et al. / Journal of Asian Earth Sciences 19 (2001) 207±221
Table 5
Temperature and pressure conditions of ¯uid entrapment and its composition between MCT-III to MCT-I (i.e. Chail to Vaikrita Thrust)
Location Around MCT-III Between MCT-II and III Between MCT-I and II
Fluid composition CO2±H2O, H2O±NaCl CO2, CO2±H2O±H2O±
NaCl, a few occurrence of pure CO2inclusion
CO2, CO2±H2O, H2O±
NaCl, abundance of pure CO2inclusion near MCT-I
Density (g/cm3) (a) Bulk density
0.82±0.89 0.86±0.92 0.90±0.92
CO2±H2O (b) CO2density0.75±0.84 0.81±0.86 0.86±0.89
CO2 0.94±0.97 0.96±1.01
H2O±NaCl 0.62±0.80 0.56±0.76 0.55±0.73
Himalayan Geology, Dehra Dun for the encouragement received and permission to publish this paper. AS thanks Dean, Indian Institute of Remote Sensing, Dehra Dun and Prof. A.K. Roy for the permission to carry out this work. Authors are very much indebted to Prof. K.V. Hodges, MIT, USA, Prof. R.J. Bodnar and Prof. R.J. Tracy of VPI and SU, USA, for offering constructive reviews on an earlier version of this manuscript. Authors are also thankful to the referees: Dr M. Hubbard, USA and Prof. D. Craw, New Zealand for giving constructive suggestions for the improvement in the manuscript.
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