Geohistory of Tso-Morari Crystalline, Eastern Ladakh, India: a plausible model for ultra-high pressure rocks in the Himalaya

Abstract

The Tso-Morari Crystalline (TMC), eastern Ladakh, is located between the ophiolitic rocks of the Indus-Tsangpo Suture Zone (ITSZ) in the north and the Zanskar sedimentary unit (Tethyan sedimentary zone) in the south. The TMC represents the northern Himalayan Crystalline. It is composed of quartzo-feldspathic gneiss, mostly augen-gneiss, derived from granitoids. Associated with these gneisses are the biotite schist and sillimanite-kyanite gneiss, which occur as discontinuous metasedimentary layers concordant with the banding of the gneisses. The Tso-Morari gneiss complex is heterogeneously deformed and show magmatic to mylonitic texture with dominant S-surface that is domed by NW-SE antiform. The TMC gneisses form the core of this vast dome. The Tso-Morari dome plunges in the west under metasediments, mainly garnet-bearing metapelites of the Phe or Haimanta Formation (Precambrian to Cambrain age). The UPb isotope on zircons, single grain as well as small multigrain fractions from the gneiss (deformed granite) yielded magmatic age of 479±2 Ma (Girard and Bussy 1999). Similar age is also reported in the nearby located Polokongla granite. The transition between the Polokongla granite and the Tso-Morari gneiss is gradual, devoid of any intrusive contact with the gneiss. This field occurrence together with their geochemical similarities, identical zircon morphology and same age, suggest that the Polokongla granite is the undeformed facies of the Tso-Morari gneiss. Other granite in the terrain is the Rupshu metagranite which is metaluminous with U-Pb zircon age of 482.5±1 Ma (Girard and Bussy 1999). Strikingly, there are no aplite or pegmatite dykes related to the granite-gneiss association in the TMC terrain. The Tso-Morari gneiss complex encloses eclogitic lenses (1–13 m long and 0.5–4 m thick), aligned parallel to the gneissic fabric. The eclogites, mostly retrograded, are scattered throughout the complex. They are found mainly within the Tso-Morari gneiss and also in the first few hundred meters of metasediments above, which prompted Steck et al. (1998) to designate the Tso-Morari complex as Tso-Morari Nappe – a tectonic unit of high pressure metamorphism. Because of the eclogitic lenses within the TsoMorari dome, it is obvious that the high pressure metamorphism also affected the Tso-Morari gneisses. The garnet (45-55 vol.%) in the eclogites is rimmed by glaucophane (5-7 vol.%) paragonite (<10%), and omphacite (25-35 %). These minerals also form matrix in the eclogitic rock (Sachan et al., 1999). Glaucophane