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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 40 Ar/ 39 Ar dating yielding the same ages.

The crustal-scale Karakoram shear zone structurally distinguishes the western Himalaya from-and provides an opportunity to compare tothe central and eastern portions of the orogen. To evaluate the tectonic evolution of the western Himalaya, this paper presents granite U/Th-Pb ages and zircon Hf isotopic signatures along the two major structures in northern India: the Karakoram shear zone and the Zanskar shear zone, the westernmost limb of the South Tibetan detachment system. Leucogranites in Zanskar crystallized 27-20 Ma and exhibit Precambrian to Paleozoic inheritance and predominantly negative ε Hf (t) values typical of the Greater Himalayan Sequence. Karakoram shear zone leucogranites have igneous crystallization ages over a prolonged period from 22 Ma to <13 Ma, contain Late Cretaceous through Paleocene inherited cores, and have ε Hf (t) values from +1 to +9. These inherited ages and mostly positive ε Hf (t) values compare closely to the adjacent Ladakh batholith, but low ε Hf (t) values along the Karakoram shear zone suggest an input of older crustal material from the proximal Karakoram terrane or subducted Indian crust. The Zanskar Greater Himalayan Sequence contains two suites of Paleozoic granites: (1) Pan-African Cambrian-Ordovician granites at the cores of gneiss domes and (2) Mississippian-Permian granites related to magmatism associated with the Panjal Traps. Monazite ages record peak through retrograde metamorphic conditions from 27.3 ± 1.2 Ma to 17.2 ± 0.9 Ma concurrent with anatectic leucogranite crystallization. Cenozoic partial melting in the Greater Himalayan Sequence occurred contemporaneously across the Himalayan orogen, but lower degrees of partial melting and ubiquitous doming distinguish the westernmost Greater Himalayan Sequence in Zanskar.

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