The Significance of Ultrahigh-Pressure Metamorphism in the Tso Morari Complex, NW Himalaya

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Recently discovered, Eocene ultrahigh-pressure (UHP) metamorphic assemblages in thenorthwestern Himalayas may indicate the limited subduction of Indian continental crust beneath Eurasia during Himalayan-Tibetan orogenesis. At present, the tectonic significance of this observation is controversial because the distribution of Himalayan UHP metamorphic assemblages has not been mapped in detail, because the age of metamorphism is imprecisely known, and because the structural relationships among UHP rock packages and their surroundings are largely unconstrained. We propose an integrated geochronologic (U-Pb, Sm-Nd, and 40 Ar/39 Ar), Nd isotopic, petrologic, and structural study of the most recently discovered UHP terrain in the Himalayas: the Tso Morari complex of Ladakh, India. Thus far, the UHP-diagnostic mineral coesite has been found at Tso Morari only in mafic eclogite that may or may not be of Indian plate affinity; an important part of our proposed work will be to determine if UHP assemblages occur in lithologies that can be traced unequivocally to a provenance in the Indian plate. If so, our aim of establishing high-precision dates for UHP metamorphism will determine whether or not continental subduction occurred subsequent to the initiation of India-Eurasia collision - well-constrained by independent means - or prior to collision in an intraoceanic environment, as has been proposed recently. We also intend to develop a more comprehensive knowledge of the pressure-temperature- time evolution of this UHP terrain over the Eocene-Miocene interval, one of the most poorly understood intervals in the history of the orogen. The integration of such data with detailed maps and kinematic analysis of major structures should result in much better constraints on the processes responsible for the exhumation of UHP rocks in the Himalayan-Tibetan system.