Insights Into the P–T Evolution Path of Tso Morari Eclogites of the North-Western Himalayas: Constraints on the Geodynamic Evolution of the Region
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Insights into the P–T evolution path of Tso Morari eclogites of the north-western Himalayas: Constraints on the geodynamic evolution of the region Preeti Singh, Ashima Saikia∗, Naresh Chandra Pant and Pramod Kumar Verma∗∗ Department of Geology, University of Delhi, Delhi 110 007, India. ∗Corresponding author. e-mail: [email protected] The present study is on the Ultra High Pressure Metamorphic rocks of the Tso Morari Crystalline Com- plex of the northwestern Himalayas. Five different mineral associations representative of five stages of P–T (pressure–temperature) evolution of these rocks have been established based on metamorphic tex- tures and mineral chemistry. The pre-UHP metamorphic association 1 of Na-Ca-amphibole + epidote ± paragonite ± rutile ± magnetite with T–P of ∼ 500◦C and 10 kbar. This is followed by UHP metamor- phic regime marked by association 2 and association 3. Association 2 (Fe>Mg>Ca-garnet + omphacite + coesite + phengite + rutile ± ilmenite) marks the peak metamorphic conditions of atleast 33 kbar and ∼ 750◦C. Association 3 (Fe>Mg>Ca-garnet + Na-Ca amphibole + phengite ± paragonite ± calcite ± ilmenite ± titanite) yields a P–T condition of ∼28 kbar and 700◦C. The post-UHP metamorphic regime is defined by associations 4 and 5. Association 4 (Fe>Ca>Mg-garnet + Ca-amphibole + plagioclase (An05) + biotite + epidote ± phengite yields a P–T estimate of ∼14 kbar and 800◦C) and association 5 (Chlorite + plagioclase (An05) + quartz + phengite + Ca-amphibole ± epidote ± biotite ± rutile ± titanite ± ilmenite) yields a P–T value of ∼7 kbar and 350◦C. 1. Introduction (UHPM) and their subsequent exhumation and preservation at surface conditions (e.g., UHPM Reported occurrence of coesite, the high pressure rocks from the Kokchetav massif, Kazakhstan; polymorph of quartz as inclusions in the garnets Dabie-Shan, China and western Gneiss Region, of eclogitic rocks from Norway and the Alps region Norway, Dora Maria Massif, W. Alps; Franciscan in the early 80’s (Chopin 1984;Smith1984)was Complex, California). Ultra high pressure meta- instrumental in renewing the interest in the study morphic eclogite rocks in the Himalayan terrain of eclogite-bearing terrains. The presence of coesite has been reported from the Tso Morari Crystalline established beyond doubt that the source of origin Complex (TMC) in India, the Kaghan and the of eclogite was quite deep, at least 100 km beneath Neelum valleys in Pakistan and the Ama Drime the earth’s surface. Thereafter, numerous studies region and the Arun river valley in Nepal (Guillot were carried out to understand the mechanism of et al. 1997; Sachan et al. 1999; Pognante and plate subduction leading to formation of eclogite Spencer 1991; O’Brien et al. 2001;Groppoet al. as a result of ultra high pressure metamorphism 2007). Keywords. UHPM (Ultra high pressure metamorphism); eclogites; Himalayas; Tso Morari Crystalline Complex (TMC). ∗∗ Since deceased. Supplementary data pertaining to this article are available on the Journal of Earth System Science Website at http://www. ias.ac.in/jess/jun2013/supp/Ashima.pdf J. Earth Syst. Sci. 122, No. 3, June 2013, pp. 677–698 c Indian Academy of Sciences 677 678 Preeti Singh et al. The Tso Morari Crystalline Complex was the first in this work. The Taglang La Formation comprises area from where the occurrence of eclogite rocks of metamorphosed calcareous, marly and argilla- in the Himalayan terrain was reported in mid 20th ceous sediments with concordant bands of amphi- century (Berthelsen 1953). This was corroborated bolites. The Rupshu and Polokong La granites, nearly half a century later by reporting of coesite the third lithostartigraphic unit of the Tso Morari inclusions in garnets (Mukherjee and Sachan area are intrusive in the Puga Formation and the 2001). The peak UHP assemblage and its subse- Taglang La Formation (figure 1b). quent exhumation through amphibolite facies to Structurally, the area forms a doubly plunging greenschist facies in the TMC is generally agreed anticline or a dome. The Puga Formation forms upon by most workers (de Sigoyer et al. 2004; the core of this dome followed by the Taglang La Guillot et al. 1997) but textural and mineralogical Formation on the outer periphery. The Rupshu constraints for the prograde path for attainment and Polokong La granites crosscut these forma- of UHP are lacking. Recently, Guillot et al. (2008) tions in the dome. Three generations of structures suggested a refined P–T–t path for the rocks where are revealed in the field (Guillot et al. 1997;Jain a pre-UHP path through blueschist facies regime et al. 2003). The first set of structures is verti- was envisaged but not documented in terms of cal rootless folds and the associated lineation and mineral chemistry or textural evidence. cleavage. The second generation of structures are The present work defines a new P–T evolution preserved as comparatively large recumbent folds path for the UHP rocks of the Tso Morari Crys- with near horizontal axial plane. The third gen- talline Complex. It works out a pre-UHPM blue- eration of deformation formed normal faults and schist facies mineral association (association 1) the Tso Morari dome that delineates eclogite facies based on textural and mineral chemical data. A rocks from the surrounding lower grade rocks. new transitional mineral association (association 3) gradational between eclogite and amphibolites 2.2 Current status of metamorphic studies facies is being reported for the first time for these rocks. We show that the rocks of TMC were not Focus so far in the metamorphic studies of the Tso exhumed in a single step. Our work refines the P–T Morari eclogites has been on establishing the UHP evolution path for the TMC eclogites which is used mineral assemblage. Based on metamorphic stud- to comment upon the geodynamic evolution of this ies the eclogite rocks of the Tso Morari area are region in terms of slab kinematics and available inferred to exhibit three mineral assemblages, viz., geochronological constraints. eclogite facies assemblage, blueschist-amphibolite facies assemblage and greenschist facies assemblage (de Sigoyer et al. 1997; Guillot et al. 1997; Sachan 2. Geological setting et al. 1999). For the eclogitic assemblage a pressure estimate varying from 14 to 16 kbar and temper- The Himalayan terrain is the most tectonically ature estimate of 550 ± 50◦C to 580 ± 60◦Chas active zone of the Indian plate. It is characterized been documented; for the retrograde assemblage of by four major tectonic elements that can be seen amphibolite facies a pressure estimate of ∼8 ± 3 throughout from the Nanga Parbat in the west to to 10 ± 3 kbar and temperature of ∼570 ± 70◦C Namcha Barwa in the east. The Himalayan rocks to 630 ± 30◦C and for the third retrograde assem- between these tectonic elements exhibit distinct blage of grenschist facies the P–T estimates of ∼5 geological characteristics. Based on it the Hima- to 8 ± 3 kbar and ∼500◦C to 590 ± 90◦C. As is layas are classified into four divisions (Gansser evident the variability of the reported data lies well 1964) from south to north: the Sub Himalayas, the outside the error ranges of the data. Lesser Himalayas, the Higher Himalayas and the However, with the report of coesite inclusion Tethyan or the Tibetan Himalayas (figure 1a). in garnet of the eclogites from Tso Morari area (Mukherjee and Sachan 2001;Sachanet al. 2004) 2.1 The Tso Morari dome the minimum pressure attained by these rocks were redefined to be atleast 28 kbar at 650◦C. The Tso Morari area is a NW–SE trending belt Following this report a lot of revisions went into lying between the Tethyan Himalayas in the south P–T estimates for rocks of this area. For the eclog- and the Zildat ophiolite m´elange of the Indus ite facies assemblage, the temperature estimates Suture zone to the north. Lithostratigraphically, vary between ∼600 and 800◦C(Jainet al. 2003; the area consists of three formations, viz., the Puga de Sigoyer et al. 2004). A carbonate bearing assem- Formation, the Taglang La Formation and the blage has also been reported from the TMC indi- Rupshu and the Polokong La granites. cating P–T as ∼39 kbar and ∼750◦C (Mukherjee The Puga Formation comprises of schists and et al. 2003). Geodynamic evolution models have gneisses and hosts the boudins of eclogites studied been proposed by de Sigoyer et al. (2000, 2004), P–T evolution path of Tso Morari eclogites of NW Himalayas 679 70o 80o 90o KAGHAN VALLEY o 35 K a ra Main Boundary Thrust Fault ko TSO MORARI r am CRYSTALLINE Main Central Thrust fault In d COMPLEX u s Orogenic sediments or ophiolites S u tu Sub Himalayas re T ra ns s H im u a d lay n a I s K r AMA DRIME ol B j el t Tsangpo tlu Su s Tibetan or Tethyan Himalayas laya DELHI ima Less er H er H igh imal H ayas tra G apu an m g rah o e B 25 s 0100 300 500 kms (a) 770 50’ 780 40’ 3 ’ 3 5 I n ° 2 LADAKH 2 ° d u 5 3 Nyimaling s BATHOLITH ’ 3 Chumantang 6150 6270 Tso K a r + PO + Sumdo LOK+ + + ONG Mahe LA + + + Z INDUS SUTURE ild a ZONE t D e ta TAGLANG LA c FORMATION hm Kiagar e n 6540 t Tso F a u lt 6100 PUGA FORMATION ++ + Karzok RU P 4550 SH U + + Ts o + Morari Mata TETHYAN HIMALAYAS P h i rs e f u 3 ’ 2 0 Scale: 10km ° 4 4 ° 0 2 ’ 3 770 50’ 780 40’ (b) Figure 1. (a) Geological map showing the sub-divisions of the Himalayas (modified after Gansser 1964).