1. metamorphic Geol., 1989, 7, 95-110
Ceothermobarometry and development of inverted metamorphism in the Darjeeling-Sikkim region of the eastern Himalaya A. MOHAN Department of Geology, Banaras Hindu University, Varanasi 221005, India
B. F. WINDLEY AND M. P. SEARLE Department of Geology, The University, Leicester LE1 7RH, UK
ABSTRACT The Darjeeling-Sikkim region provides a classic example of inverted Himalayan metamorphism. The different parageneses of pelitic rocks containing chlorite, biotite, garnet, staurolite, kyanite, sillimanite, plagioclase and K-feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones. Microprobe data of coexisting minerals show that X,, varies in the order: garnet garnet(,, > staurolite > chlorite > biotite (Thompson, 1976).
Chlorite Prograde chlorites are ripidolites which change in composition with grade of metamorphism. They accommodate the maximum possible A,O, content (up to c. 22 wt%), where they coexist with Al-rich White mi-. T k primary Chlorite is Mg-rich.
x x x
x x x x x x
x x x x x x
x x x x x x x x x x
x x x x x x x x
x x x x x x
x x x x x x x x x x
x x x x
x x x x x x x x x x x x x x
x x x
White mica After quartz, white mica is the most common mineral. It is a solid solution between muscovite (sensu snia0) and phengite. A celadonite-poor white mica aystallized in higher grades and is related to the muscovite-phengite by the tschermak substitution, Sih(Mg. Fev')
Biotite Biotite has a homogeneous composition and shows extensive Fe-Mg solid solution. While the imxeaSt in TiO, wt % in biotite coexisting with ilmenite displays a progressive increase with grade of metamorphism, the spread in the Mg/Mg + Fe values shows the reverse trend. Biotite in immediate contact with garnet is Fe-rich compared with biotite in the matrix.
Garnet The stoichiometry of garnet corresponds closely to the ideal formula R:+R:+Si30r2. The analyses reveal definite zoning profiles as shown in Fig. 6. The bell-shaped Mn profile is characteristic of growth zoning without internal diffusion (Hollister, 1969). In lower grade zones Mg increases towards the rim, and thus the rim preserves the highest grade compositions that reflect the peak metamorphic conditions; the latttcr being in equilibrium with the coexisting matrix (Tracy, Robinson & Thompson, 1976). Ca increases from just off the core, and then declines towards the rim. This can be explained by changes in growth during the garnet-producing reaction (Crawford, 19n) or by garnet growth contemporaneous with increasing temperature (Cygan Bt Lasaga, 1982). However, this normal growth zoning pattern is reversed scar the rim in garnets from higher grade zones; this may result from the resorption of garnet through a prograde continuous reaction. Continuous, smooth zoning prohles and lack of distinct textural taw for most of the garnets suggest uninterrupted garnet growth during a single metamorphic event.
Staurolite The composition of analyscd staurolites deviates from the theoretical formula of (Fe, Mg)2A15Si,0,(OH), because the octahedral sum is always less than 2. Zn content is small, but detectable (up to c. 0.89 wt %) and its progressive increase can be correlated with an increase in metamorphic grade. Chemical zoning patterns reveal slightly higher X, towards the core. Constant composition at different analysed points in the rim suggests that the rim of staurolite attained equilibrium with the rest of the assemblage.
pb. 3. (a) Elongated garnet, highly sieved with inclusions of inequant quartz merging into S, foliation (staurolite zone). Plane polarized light. (b) Synkinematic rotational core and post-kinematic inclusion-free idioblastic rim of garnet (staurolite zone). Plane polarized light. (c) Retrograde chlorite from garnet with abundant opaques (staurolite zone). Plane polarized tight. (d) Two-stage garnet growth with inner core and euhedral rim. Note inequant quartz grains at the junction of the inner core with the outer rim (kyanite zone). Plane polarized light.
GEOTHERMOBAROMETRY, DARJEEllNC-SIKKIM REGION
-arnphibolite f a c i e s
Chlorite Biotite Ouartz Garnet K-feldspar Plagioclase Muscovite/ sericite Staurolite Kyanite Sillimanite
In most low-grade rocks plagioclase is almost pure, unzoned albite (An&, In higher temperature zones most plagioclase shows an increase in anorthite component (c. An,), and is oligoclase.
The reaction responsible for the formation of biotite cannot be discussed fully because the lower grade chlorite zone is not present in the investigated region. On the basis of the mineral assemblage and texture reported by Sinha Roy (1981), it is possible to suggest the following reactions for the incoming of the biotite zone.
wios ‘Ihe aluminosilicates are kyanite and sillimanite which contain minor Fe,03 and Mn,O,.
For K-feldspar-bearing assemblages: K-feldspar + chlorite- muscovite
CHEMOGRAPHICAL RELATIONSHIPS The textural evidence for the relevant metamorphic reactions combined with microprobe data can be portrayed in terms of chemographical relationships (Fig. 7). Some of the reactions discussed here were described by Lal et al. (1981), Sinha Roy (1981) and Banerjee et ul. (1983).
+ biotite +quartz + H,O.
For K-feldspar-absent chlorite-white mica phyllites, the possible continuous Fe-Mg reaction could be phengite + ripidolite-, muscovite
+ biotite + quartz + H,O.
GEOTHERMOBAROMETRY, DARJEELING-SIKKIM R E G I O N
Tabk 2. Representative microprobe mineral analyses of main minerals, arranged within successive zones
Garnet zone Analysed garnets are almandine-rich and have a (Fez++ Mg)/Al ratio lower than that of coexisting chlorite. Therefore, the growth of garnet from chlorite quartz through the reaction (3),
Mineral zones SiO,
35.11 1.07 17.05
35.63 1.63 17.92 0.05 20.00 0.16 10.08
34.52 2.48 18.69 0.02 22.63 0.08 7.89
35.26 2.27 18.66 0.01 21.01 0.05 8.87 0.02 0.29 8.76
NazO K20 ZnO
34.60 2.48 18.45 0.07 21.58 0.08 7.38 0.01 0.17 9.25
TO, Al,03 Cr,O FeO MnO MgO CaO
20.17 0.14 11.71
chlorite, + q;artz+
Al Cr Fez+ Mn Mt3
2.605 0.018 2.689
5.405 0.291 3.397
5.448 0.187 3.229 0.006 2.557 0.021 2.296
0.009 2.819 0.011 1.719
0.051 1.844 0.379
0.051 1.951 0.471
5.334 0.288 3.404 0.002 2.925 0.010 1.817
Textural relations indicate that the disappearance of prograde chlorite coincides with the appearance of staurolite. This factor, combined with the modal increase of biotite, the Nacontent of muscovite and the abundance of garnet explains the appearance of staurolite in relatively magnesianrich rocks through the discontinuous reaction (9,
0.078 1.707 0.383
TO, A1,03 cr203
FeO MnO MgO CaO Na20
37.15 0.05 20.96 0.02 33.9 3.28 1.92 3.07 0.03 0.01 -
36.94 0.02 20.62 0.01 29.17 6.02 1.46 4.62
36.80 0.20 21.60
36.42 0.01 20.83
35.88 0.04 21.06
28.64 0.59 54.24
34.80 1.60 3.00 1.50 0.50
36.28 1.13 3.56 1.52 0.01 -
11.70 0.43 1.32 0.02
35.26 2.64 2.62 1.71 -
28.09 0.58 54.11 0.03 12.29 0.39 1.62
34.20 2.70 2.90 1.90 0.60 0.02
36.83 20.77 0.03 37.29 1.83 3.06 0.98 0.04
35.91 1.67 2.80 2.21 0.02
Cations based on 12 oxygens Si Ti Al
Cr Fe2+ Mn Mg Ca Na K Zn XM'
3.005 0.003 1.998 0.001 2.259
0.225 0.232 0.266 0.005
3.015 0.001 1.983 0.001 1.991 0.416 0.178
0.404 0.006 0.001
2.959 0.015 2.046
0.106 0.359 0.131 0.078 0.010
0.348 0.167 0.102
0.133 0.130 0.201
+ H20. (4)
T.MC 2. (Continued.) Mineral zones
The X,, of the reactant phases in this reaction is higher than that of the product assemblage. Hence, as the reaction proceeds from left to right, all the participating phases would be enriched in Mg. This is consistent with the chemical data on garnet which indicate a higher X,, in rims than in cores.
5.400 0.261 3.368 0.001 2.691 0.006 2.025 0.003 0.086 1.711 0.429
+ chlorite, + H20,
chlorite + muscovite + quartz- garnet +biotite
Cations based on 22 oxygens 5.424 0.124 3.109
would require more alumina than can be supplied by muscovite through the continuous reaction (4),
2.959 0.001 1.995
2.440 0.115 0.339 0.192 0.003 0.002
1.974 0.002 2.515 0.125
0.368 0.085 0.006
0.122 0.128 0.211
2.017 2.403 0.182 0.318 0.149
0.002 2.022 2.412 0.078 0.432 0.133 0.002
0.117 0.149 0.203
97.23 97.28 Cations based on 23 oxygens 3.959 0.061 8.836
1.352 0.050 0.273 0.003 -
0.060 8.842 0.003 1.425 0.046 0.334
0.001 0.017 0.185
1U A. M O H A N , B. F. W I N D L E Y & M. P. SEARLE
Table 2. (COhUrd.) Muscovite
23.80 0.12 21.31 0.02 27.60 0.27 12.60
50.82 0.50 34.26
46.61 0.19 37.94 0.05 1.32
45.29 1.14 32.27 0.04 1.34
48.78 O.% 36.42 0.02 1.20 0.04
0.37 0.01 2.11 8.16
0.87 0.12 0.55 10.23
0.01 0.75 7.25
29.21 0.03 m.81 0.00 25.48 0.13 11.66 0.00 0.01 0.14
46.34 0.30 33.16 0.02 2.41 0.02
26.25 0.08 22.18 0.02 20.81 0.23 18.11 0.01 0.00 0.08
no2 A203 OZo3
2.54 0.13 1.50 0.02 0.29 8.n
5.383 0.013 5.361
5.m 0.W 5.509
3.570 0.041 5.538 0.002 0.002
Ti Al Cr Fez+
MnO Mg Ca
6.293 0.032 5.313
5.119 0.001 4.448 0.023 3.627 0.001 0.003 0.038 0.449
0.177 1.701 0.369
chlorite + muscovite+ garnet + staurolite
+ biotite + quartz + H,O.
The X,, of the reactant phases is higher than that of the product phases in this reaction. Hence as the reaction proceeds from left to right, all the phases are enriched in Mg. With the incorporation of Mn, reaction (5) becomes a Fe-Mg-Mn continuous reaction. The growth of garnet
98.83 96.76 %.a Cations based on 22 oxygens
Cations based 28 oxygens - . - on .. - .
0.270 0.014 0.284 0.003 0.070 1.419 0.500
6.048 0.018 5.802 0.006
6.260 0.119 5.257 0.004
0.178 0.018 0.146 1.803 0.535
0.001 0.531 1.350 0.334
5.530 0.m 0.129 0.004
0.125 0.001 0.187 1.191 0.491
results in a decrease in the b a n t e n t of chlorite. The chemical data indicate a fall in the Mn-content of chlorite from the garnet zone to the staurolite zone and a concomitant increase in the X, of the garnet. Another important Fe-Mg continuous reaction in this zone is chlorite + muscovite- staurolite + biotite + quartz + HzO. (6) (12oxygen
Rg. 7. AFM topology in the different zones and inferred continuous and discontinuous reactions.Fiied circles show the observed mineral assemblagu.
For every biotite formed through reactions ( 5 ) and (6), K+ must be obtained from muscovite and so the remaining muscovite &odd become Na-rich, which is attested by the rise in the Nacontent of muscovite from the staurolite zone in comparison with preceeding zones.
Kyanite zone There is no direct textural evidence that kyanite grew at the expense of staurolite and muscovite. But the AFM topology, the decrease in the modal content of staurolite and muscovite, and the association of aggregates of biotite flakes with kyanite suggest the continuous (Fe-Mg) reaction (7),
staurolite muscovite quartz-, kyanite +biotite
There is a marked zoning in staurolite with higher X, in the rim compared with the core. X, of the product phases is also higher than that in the reactant phases of reaction (7). These relations explain the shift of the staurolitebiotite-kyanite three-phase field to the higher X,. side, because the participating phases would be depleted in Mg with the progress of the reaction.
Sillimite zone Decrease in the modal content Of kyanite, and the absence of the K-feldspar association indicates that silhanite could have been derived by the polyphase transformation of kyanite (La1 cr al., 1981). kyanite --+ sillimanite.
The reaction may also be a result of the breakdown of garnet and staurolite-bearing assemblages, instead of the breakdown of muscovite in the presence of quartz (Banerjee & Bhattacharya, 1981). Garnet porphyroblasts showing replacement by fibrolite, which is intergrown with biotite, indicate the resorption of garnet through reaction (9): garnet + muscovite-, sillimanite + biotite + quartz.
Reverse growth zoning in garnet is also explained by this reaction with the higher X,. in garnet rims compared with cores, and the higher X,, of the product phases. The resorption of garnet would occuf due to the shift of the two-phase field towards the lower X, side of the AFM projection.
106 A. M O H A N , B. F. WINDLEY & M. P. SEARLE
In order to estimate the P-T conditions of metamorphism, suitable geothermometers and geobarometers were applied to the relevant assemblages from the different zones. Except for the zoned minerals, compositions of the minerals in mutual contact have been used in the thermobarometric calculations. For garnets which reveal normal growth zoning, rim composition has equilibrated with the adjoining matrix, and reflects the peak conditons of metamorphism. For inversely zoned garnets near-rim and intermediate compositions were taken. In some cases it was not possible to obtain the core composition of garnets because of the presence of numerous inclusions. The following reactions have been used to constrain the conditons of recrystallization of the pelitic rocks of the Sikkim-Darjeeling region: (a) garnet-biotite, (b) garnet-plagioclase-AIzSiO,-quartz, (c) garnet-muscovite-biotite-plagioclase. Ceothermomety Several models have been proposed for Fe-Mg exchange equilibria in garnet-biotite pairs, e.g. Thompson (1976), Goldman & Albee (1977), Ferry & Spear (1978), Hodges & Spear (1982), Pigage & Greenwood (1982). The Thompson (1976) and Goldman & Albee (1977) calibrations do not correct for the effect of pressure on the equilibrium. Ferry & Spear (1978), who proposed the first experimental calibration, also placed certain limitations on its application. The restrictions include: (Ca+ Mn)/(Ca + Mn + Fe +Mg) should be less than 0.2 for garnet, (AIvi+ Ti)/(Alvi+Ti + Mg + Fe) should be not more than 0.15 for biotite, and the precision is *SOo C. The .above values for most biotites and some garnets are higher. Complications also arise due to the unknown oxidation state of biotite, because total iron is calculated as Fe2+ in the structural formulae. Hodges & Spear (1982) and Pigage & Green-
wood (1982) considered the effect of Ca and Mn for non-ideal garnet solution. The latter gives a slightly higher temperature than the former. These models yield a temperature of 591°C for the garnet zone to a maximum of about 685" C at the sillimanite zone (Table 3).
Ceobarometry The pressure-sensitive assemblages garnet-AI,SiO,qua-plagioclase and garnet-muscovite-biotite-plagioclase are common in the study region and hence were employed to estimate the pressure conditions at which the pelites were metamorphosed. Several modifications for the garnet + plagiochse + AIzSiO5 + quartz barmeter (GPAQ) by Ghent, Robbins & Stout (1979) and Newton & Haselton (1981) involve different models for non-ideality of mixing of grossular in garnet and anorthite in plagioclase. However, a very low grossular content in the garnets induces a large uncertainty in the calculations. The Al-avoidance model was used to calculate the anorthite activity. For the widely accepted geobarometer by Newton & Haselton (1981), the corrected expression, as given in Ganguly & Saxena (1984), was used to estimate the P values. In the absence of aluminosilicate polymorphs in the rocks of the area (garnet and staurolite zone), pressures were estimated by using the recalibrated solution model of Hodges & Crowley (1985), modified after Ghent & Stout (1981) for garnet-biotite-muscovite-plagioclase (GBMP). The assemblages suitable for either the GBMP or the GPAQ models were accordingly solved simultaneously with the garnet-biotite model for temperature and pressure. The pressure values obtained from these models are summarized in Table 3. In order to maintain consistency and observe the change in pressure values from the garnet to the sillimanite zones, we have chosen the Hodges & Crowley (1985) model. The values obtained for the garnet to the sillimanite zones range from 5.8 kbar to 7.6 kbar (Table 3). We d o not find any clear relationship
Table 3. Temperatures and prcssurcs Zones Models
Garnet Staurolite Kyanite Sillimanite (7) (3) (3) (9) Geothennometer (" C)
Ferry & Spear (1978)
Hodges & Spear Pigage & Greenwood (1982)
calculated for four metamorphic zones
Geobarometer (kbar) Hodges & Crowley (1985) Newton & Haselton (1981)
Garnet-biotite -mwovite-plagioclase Garnet-aluminosilicate -plagioclase -quartz
Numbers within brackets are the number of analyses uscd for geothermobarometry. f5(p C and f 1.0 kbar as standard deviation.
The mean values given in the table have
GEOTHERMOBAROMETRY, DARJEELING-SIKKIM R E G I O N
Fig. 8. P-T diagram showing the range of P-T values obtained from geothennobarometry for the garnet, staurolite, kyanite and sillimanite zones. Triple point of aluminosilicates after Salje (1986).
between calculated pressure values and the zonal transition from garnet to sillimanite. The lower pressure values for the staurolite zone may be a result of the analytical uncertainties of the barometers. The pressure estimate of 7.6 kbar suggests that the rocks were metamorphosed at and derived from a maximum depth of about 27 km. The range of P-T values for the garnet to sillimanite zones are shown in Fig. 8.
DISCUSSION The inversion of metamorphic isograds in the DarjeelingSikkim Himalaya is confirmed and mapping of isograds has resulted in the construction of a tentative thermal profile across the area (Fig. 2). An increase of c. 200°C and 2 kbar can be documented in the Darjeeling Klippe as one progresses structurally and topographically higher. The flat-lying isograds in the Darjeeling area have been folded around the Ranjit window by subsequent lower thrust culmination of the Lesser Himalaya along the Main Boundary Thrust system. Models to explain inverted metamorphic zones include: (1) shear heating along thrust planes (Graham & England 1976) or (2) overthrusts of exceptionally hot rock (e.g. obducted ophiolite complexes) over colder rock. Overthrusting of hot granitic material or extensive migmatites may also be invoked where there is no apparent heat source available. The most reasonable explanation (Model 3) seems to be syn- or post-metamorphic folding of isograds (Harte & Dempster 1987) (Fig. 9). England & Richardson (1977) showed that during crustal thickening and the consequent uplift and .erosion, the geotherm will
Andalusite 02 500
600 T ("C)
constantly change with time. Certainly in the High Himalaya multi-stage thrusting models have been proposed (Searle, Windley, Coward, Cooper, Rex, Rex, Tmgdong, Xuchang, Jan, Thakur & Kumar. 1987) and the resulting P-T-t paths are bound to be complex. Inverted metamorphic gradients such as that observed in Darjeeling-Sikkim, would need to be cooled rapidly in order to preserve the inverted metamorphic zonation (e.g. Thompson & Ridley 1987). The uplift and erosion rates would therefore need to be excessively high. The Darjeeling-Sikkim Himalaya shows a set of inverted isograds which apparently increase smoothly upwards from the chlorite to the sillimanite zone. There are apparently no major jumps in P-T conditions which would define a major thrust plane. In the western Himalaya of Zanskar-Kishtwar, mapping of isograds shows that P-T conditions decrease upwards (northwards) at the top of the High Himalayan slab and have been telescoped by subsequent normal faulting at the northern margin of the Central Crystalline complex. Burg & Chen (1984) and Burg, Giraud, Chen & Li (1984) described similar large-scale north-dipping normal faults in south Tibet and north of Everest. These faults appear to continue eastwards to the northern side of Kanchenjunga along the Sikkim-Tibet border. Thus our studies in Darjeeling and Sikkim have only focused on the lower portion of the High Himalayan slab. The thermal affects of shear heating (Model 1) can readily be discounted as this would only dect a narrow zone along the fault and could not possibly account for such widespread high Ttmedium P metamorphism. The ductile shear zones in the High Himalaya are particularly
108 A. MOHAN, B. F . WINDLEY 81 M. P. SEARLE
(a) Model 2
(b) Model 3
Fig. 9. Thee models to explain the
(c) Model 4
- models 2 &/or 3 intermediate P-T (kyanite) M2 - high T intermediate-low P (sillimanite grade) MI
related to granite magmatism
rich in hydrous mineral phases such as micas and tourmaline, which suggest a high volatile and fluidduxing component and are therefore unlikely to generate any frictional heating (Searle & Fryer, 1986). The thermal affectsof granite magmatism (Model4) are likewise unlikely to extend far beyond the close vicinity of the Miocene leucogranite plutons. These leucogranite plutons and sheet intrusions, exemplified by Jannu and the south face of Kanchenjunga, are 50km north of the southern limit of sillimanite grade metamorphism and we therefore discount the leucogranites as being the thermal source for the Mz metamorphism. It seems that the most plausible explanations for the inverted metamorphism in the Darjeeling-Sikkim Himalaya must be either the hot slab thrust over cold (Model 2) or a two-stage model, in which the earlier metamorphic isograds are deformed by subsequent south-verging crustal-scale folds and thrusts (Model 3). The MCT zone is a ductile shear zone approximately 5 km wide. The sparse exposure in this region is not sufficient for detailed
inverted metamorphism associated with the MCT zone. Model 2 is the 'hot slab over cold' (a). Model 3 is the syn- or post-metamorphic folding of isograds (b). Model 4 is a two-stage evolution where the high ?'-intermediate P sillimanite grade metamorphism is related to heat generated from Miocene leucogranites (c). See text for discussion.
structural mapping and thus it is not possible to map out small-scale thrust structures. In the upper part of the M a zone sillimanite, kyanite, staurolite and garnet have grown under static interkinematic conditions with respect to D, and D2 giving rise to poorly or randomly oriented grains. Garnets show a reverse zoning pattern which records the cooling history of the upper part of the slab. In contrast, the normal zoning in garnets in the lower part of the MCT zone records the heating up of the overthrusted racks (Spear, Selverstone, Hickmott, Crowley & Hodges, 1984). Late fluid enhanced retrogression has altered garnet to chlorite (Fig. 3c). The metamorphic convergence indicated by garnet zoning profiles across the M a zone might indicate that some thermal reequilibration has taken place during synmetamorphic deformation. The minimum southward translation along the MCT zone must be 50km (the distance from the inverted metamorphic zones north of Gangtok to the'southern limit of the Darjeeling Klippe), but in reality may be over 100 km (Brunel, 1986).
CEOTHERMOBAROMETRY, DARJEELING-SIKKIM R E G I O N
M. would like to thank the Leverhulme Trust for a
Commonwealth Fellowship that enabled him to visit Leicester. We would like to thank R. K. Lal for the loan of some samples and M.Norry for useful discussions.
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