The Lopu Kangri High-Pressure Metamorphic Complex: A Tso Morari Analog in Southern Tibet

Abstract:

The Lopu Range, located along the Yarlung-Tsangpo suture ~600 km west of Lhasa city in southern Tibet, exposes a high-pressure metamorphic complex composed of Indian passive margin (Tethyan) rocks. An integrated approach involving geologic mapping, kinematic analysis, phengite geobarometry, Zr-in-rutile geothermometry, garnet-phengite Fe-Mg exchange geothermometry and pseudosection modeling reveals that Lopu Range meta-Tethyan rocks reached peak pressures of 20-25 kbar (2.0-2.5 GPa) at temperatures <550-630 ºC along a clockwise P-T path. These data indicate subduction to mantle depths (~75 km) at eclogite facies conditions followed by exhumation to mid-crustal depths and retrogression at upper greenschist to amphibolite facies conditions. The structural geometry and interpreted P-T-t history of Lopu Kangri rocks is similar to the Tso Morari complex, located ~700 km along-strike to the northwest. Therefore, we interpret that these two localities formed in a similar manner following the onset of Tethyan Himalaya—Eurasia collision ca. 58-52 Ma. A previously published Ar-Ar date from Lopu Kangri suggests that exhumation to mid-crustal levels occurred by ~41 Ma.

Two key differences exist between the Lopu Kangri and Tso-Morari complexes. 1) the high-grade nappe in the Lopu Kangri complex is composed entirely of Cambrian-Ordovician metasedimentary rocks whereas the high-grade nappe in the Tso Morari complex is composed of the Tso Morari orthogneiss, eclogite boudins (meta-mafic enclaves) and Cambrian-Ordovician metasediments. We interpret that the lack of eclogite boudins at Lopu Kangri resulted from the absence of a basic protolith. 2) Lopu Kangri is located along the Yarlung-Tsangpo segment of the Indus-Yarlung (India-Asia) suture whereas Tso Morari and nearby Kaghan Valley are located along the Indus suture. Prior to this study, no continental high-pressure metamorphic complexes were known along the Yarlung-Tsangpo suture. Previously formulated tectonic models sought to explain this discrepancy through variations in exhumation and subduction zone dip. Our results indicate that continental margin subduction and rapid, synconvergent exhumation was not restricted to the Indus suture in the northwestern Himalaya.