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Inverted Metamorphic Sequence in the Sikkim Himalayas: Crystallization History, P–T Gradient and Implications

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Inverted Metamorphic Sequence in the Sikkim Himalayas: Crystallization History, P–T Gradient and Implications J. metamorphic Geol., 2004, 22, 395–412 doi:10.1111/j.1525-1314.2004.00522.x Inverted metamorphic sequence in the Sikkim Himalayas: crystallization history, P–T gradient and implications S. DASGUPTA1 ,J.GANGULY2 AND S. NEOGI3 1Department of Geological Sciences, Jadavpur University, Kolkata 700 032, India 2Department of Geosciences, University of Arizona, Tucson 85721, AZ, USA ([email protected]) 3Central Petrology Laboratory, Geological Survey of India, Kolkata 700 016, India ABSTRACT The metapelitic rocks of the Sikkim Himalayas show an inverted metamorphic sequence (IMS) of the complete Barrovian zones from chlorite to sillimanite + K-feldspar, with the higher grade rocks appearing at progressively higher structural levels. Within the IMS, four groups of major planar structures, S1,S2 and S3 were recognised. The S2 structures are pervasive throughout the Barrovian sequence, and are sub-parallel to the metamorphic isograds. The mineral growth in all zones is dominantly syn-S2. The disposition of the metamorphic zones and structural features show that the zones were folded as a northerly plunging antiform. Significant bulk compositional variation, with consequent changes of mineralogy, occurs even at the scale of a thin section in some garnet zone rocks. The results of detailed petrographic and thermobarometric studies of the metapelites along a roughly E–W transect show progressive increase of both pressure and temperature with increasing structural levels in the entire IMS. This is contrary to all models that call for thermal inversion as a possible reason for the origin of the IMS. Also, the observation of the temporal relation between crystallization and S2 structures is problematic for models of post-/late-metamorphic tectonic inversion by recumbent folding or thrusting. A successful model of the IMS should explain the petrological coherence of the Barrovian zones and the close relationship of crystallization in each zone with S2 planar structures along with the observed trend(s) of P–T variation in Sikkim and in other sections. A discussion is presented of some of the available models that, with some modifications, seem to be capable of explaining these observations. Key words: Himalayas; inverted Barrovian zones; main central thrust; thermobarometry; zoning in garnet. of the P–T gradients in different sections, and some- INTRODUCTION times also in the same section. The latter observation Over the entire length of the Himalayas, the Barrovian suggests that some of the variations of P–T gradients sequence of metamorphic isograds has been inverted in are artifacts of the method of estimation of P–T con- that the higher grade rocks appear at progressively ditions from the mineral chemistry and parageneses. higher structural levels. The origin of this inverted As recently noted by Stephenson et al. (2000) and metamorphic sequence (IMS) has been one of the most Kohn & Spear (2000), many published P–T estimates debated topics in Himalayan geology as discussed, for in the IMS lack enough documentation to convince example, by Hodges (2000) and Vannay & Grasemann that appropriate care has been exercised in the choice (2001). The resolution of this problem is of funda- of mineral compositions that reflect peak metamorphic mental importance to the understanding of the tectono- conditions. Stephenson et al. (2000) also noted that metamorphic evolution of the Himalayas. Of critical many thermobarometric studies of the Himalayan IMS importance in this respect is the nature of P–T gradient have used rim compositions of phases in mutual con- in the IMS since any thermo-tectonic model has a tact, and have consequently estimated the diffusion predictable consequence about the peak metamorphic closure conditions, which do not necessarily reflect the P–T gradient or metamorphic field gradient (i.e. peak peak metamorphic conditions, at least for the high- temperature v. pressure at the peak temperature). grade rocks. Numerous thermobarometric studies have been We have undertaken a systematic structural and undertaken in different sections of the IMS (e.g. petrological study of the IMS in the Sikkim Himalayas Hodges & Silverberg, 1988; Hubbard, 1989; Inger & (Neogi et al., 1998; Ganguly et al., 2000), which show Harris, 1992; McFarlane, 1995; Vannay & Hodges, the full spectrum of the Barrovian zones. In this work, 1996; Vannay & Grasemann, 1998; Vannay et al., we report the mineral parageneses within different 1999; Fraser et al., 2000; Stephenson et al., 2000; domains of IMS along a transect in East Sikkim, and Catlos et al., 2001; Kohn et al., 2001), but these have estimates of peak metamorphic P–T conditions on led to diverse pictures about even the qualitative nature the basis of detailed thermobarometric calculations Ó 2004 Blackwell Publishing Ltd 395 396 S. DASGUPTA ET AL. coupled with phase equilibrium constraints, and con- Formation) and a thick metasedimentary sequence of siderations relating to the kinetic response of mineral dominantly pelites with subordinate psammite and compositions to changing P–T conditions. Finally, the wacke (Daling Group). The overlying higher Hima- potential implications of the inferred P–T gradient on layan domain (HHD) is composed of medium to high- the models of origin of the Himalayan IMS are dis- grade crystalline rocks, commonly referred to as the cussed. No attempt is made to exhaustively evaluate higher Himalayan crystallines (HHC), which are earlier studies on the determination of P–T condition dominantly of pelitic composition, with sporadic of the Himalayan rocks, nor is it possible to do so quartzites, calc-silicate rocks, metabasites and small without detailed knowledge of the textural relations bodies of granite. The HHC is separated from the and the spatial variation of mineral compositions. lesser Himalayas by the Main Central Thrust (MCT). However, we comment on selected studies and meth- The exact location of this thrust has been controversial ods to emphasize some of the problems attending the in many areas, including Sikkim (Lal et al., 1981; determination of P–T condition. Sinha-Roy, 1982). In the far north, a thick pile of fossiliferous Cambrian to Eocene sedimentary rocks belonging to the Tethyan Belt (Tethyan Sedimentary REGIONAL GEOLOGICAL SETTING AND Sequence) overlie the HHC on the hanging wall side of STRUCTURAL HISTORY a series of north-dipping normal faults constituting the Figure 1 shows an overview of the geology of the South Tibetan Detachment System (STDS). The Himalayas, and location of Sikkim in the eastern deformation zone related to the MCT is often referred Himalayas. Like the other sectors, the Sikkim Hima- to as the Main Central Thrust Zone (MCTZ). Over the laya has been subdivided into distinct geotectonic entire length of the Himalayas, the inverted Barrovian domains, which are separated from one another by sequence is contained essentially within the MCTZ. thrust faults (Acharya & Shastry, 1979; Ray; 1976; Figure 2 shows a regional geological map of Sikkim Sinha-Roy, 1982). The Sub-Himalayan domain in the in the Eastern Himalayas. It has been developed on the south consists of mollase type deposits of the Siwaliks basis of mapping conducted by us and available data (Mio-Pliocene), and is separated from the lesser from other sources (Ray, 1976; Acharya & Shastry, Himalayan domain (LHD) in the north by the Main 1979; Sinha-Roy, 1982; Neogi et al., 1998). The Rongli Boundary Thrust (MBT). Proceeding northwards, the transect, in the eastern part of Sikkim, is the focus of LHD consists of a thin strip of Gondwana rocks the present study. This area was chosen primarily (Carboniferous-Permian), carbonate rocks (Buxa because of the reported presence of a sequence of all Transhimalayan batholith Indus-Tsangpo & Northern suture zones NANGA PARBAT Kohistan-Ladakh complex K2 Tethyan Sedimentary Sequence Higher Himalayan Crystallines (HHC) N Lesser Himalayas Islamabad Molasse (including Siwaliks) NANDA DEVI EVEREST LHASA M INDIAN BT Subcontinent GANGTOK Delhi STDS MC T KATHMANDU 200 km KANGCHENDZONGA 1000 km Fig. 1. An overview of some of the major lithological and structural units of the Himalayas, and location of the study area. MBT, Main boundary thrust; MCT, Main central thrust; STDS, South Tibetan detachment system. Ó 2004 Blackwell Publishing Ltd INVERTED METAMORPHISM IN SIKKIM HIMALAYA 397 Fig. 2. Simplified geological map of Sikkim showing the known locations of the inverted Barrovian zones, different tectonic domains, foliation attitudes and lithological units. The primary study area is highlighted by a polygon. The kyanite and staurolite isograd boundaries are not shown within the study for the sake of clarity. The stippled area indicates the Main Central Thrust Zone (MCTZ). The sample locations selected for thermobarometric studies are shown in Fig. 3, which is a cross-section approximately along the line within the polygon. the Barrovian metamorphic zones within a distance of c. 12 km wide, involving both the so-called lesser about 20 km. Along this transect, metamorphism and higher Himalayan rocks, and extending from the progressed in an easterly direction from chloritic, biotite zone to beyond the muscovite-out isograd. micaceous phyllite and quartzite to different varieties Rocks below the MCTZ are referred to as the LHD of micaschist, culminating in the appearance of mig- and those above it as the HHD. Figure 3 shows a matitic pelites. A dominant eastward dipping pervasive schematic E–W cross-section, along the line (approxi- planar structure indicates
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