Continuous Metamorphic Zircon Growth and Interpretation of U-Pb SHRIMP Dating: An Example from the Western Himalaya

Abstract

Ultrahigh-pressure (UHP) rocks in the northwest Himalaya are some of the youngest on earth, and allow testing of critical questions of UHP metamorphism and exhumation and the India-Asia collision. The Tso Morari Complex (TMC) is a UHP subduction-zone complex in eastern Ladakh in the western Himalaya, south of the Indus-Yarlung suture zone. U-Pb SHRIMP dating of zircon shows the TMC has a Proterozoic protolith, preserves a Pan-African magmatic history, and shows continuous metamorphic zircon growth during the Early to Middle Eocene, hence constraining the timing of collision, subduction, and exhumation in the western Himalaya. Zircon dating indicates that UHP metamorphism occurred at 53.3 ± 0.7 Ma, followed by 8 m.y. of continual zircon crystallization to amphibolite-facies metamorphic conditions at 45.2 ± 0.7 Ma. Similar continuous zircon growth during UHP metamorphism and through early exhumation to amphibolite-facies conditions occurs in other UHP subduction complexes, including the Sulu terrane, where coesite-bearing inclusions within dated zircon prove conclusively that zircon dates record UHP metamorphism. U-Pb SHRIMP dating of zircon for both the TMC and the Sulu belt demonstrate that zircon continues to crystallize at temperatures 400° ± 50°C based on 40Ar/39Ar dating yielding the same ages.