NEW U-PB SHRIMP AGES FOR THE UHP TSO-MORARI CRYSTALLINES, EASTERN LADAKH, INDIA

The Tso Morari Crystallines are a 100 x 50 km NW-SE-trending ultrahigh-pressure eclogitic subduction zone complex in the eastern Ladakh area in the western Himalaya, south of the Indus-Tsangpo suture zone. Estimates for the initiation of the Himalayan collision between India and Asia range from the Late Cretaceous (>65 Ma) and the latest Eocene (<40 Ma); the continent-continent collision is thought to have begun in the northwest part of the Himalaya <52 Ma based on collision-related sediments. U-Pb zircon SHRIMP data better defines three stages in the development of the Tso Morari complex and helps to constrain the timing of collision and subduction in the western Himalaya: the protolith age in zircon cores; the timing of Pan-African magmatism in the Indian sub-continent from analysis of zircon cores and mantles; and the timing of peak eclogite-facies metamorphism from analysis of metamorphic rims. Zircons for this study are from two samples of the country rock to eclogite that were metamorphosed at ultrahigh-pressure eclogite-facies based on P-T calculations (>560 ± 50° C, ~2.2 GPa) and the presence of coesite in related rocks. Precise new single zircon U-Pb ages for host rocks to eclogite in the Tso Morari crystallines indicate that peak eclogite-facies metamorphism occurred at 48 ± 1 Ma. Zircons include both euhedral and fairly rounded grains that display clear core/rim zoning relationships under CL. Zircons that yielded ca. 48 Ma rim ages occurred in rounded grains with dark cores and light-colored rims; these metamorphic rims had very low Th/U ratios (<<0.1). Many zircon mantles and cores gave ages ca. 400 Ma and have much higher Th/U ratios (>>0.1); these ages likely reflect a magmatic event that is recorded in many other areas throughout the Himalaya. A few zircon cores indicate a minor inherited component with Proterozoic ages (700 ± 6 Ma to 1668 ± 14 Ma). These new data show that eclogitization was ~7 m.y. later than other workers have reported from Sm-Nd, Lu-Hf, and U-Pb (alanite) dating; the 48 Ma ages correspond well with other Sm-Nd, Rb-Sr, and Ar/Ar dating which may indicate fast exhumation.