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 40 Ar/ 39 Ar dating yielding the same ages.

Zircon growth in (U)HP quartzo-feldspathic host gneisses exhumed in the Woodlark Rift of Papua New Guinea

“…A similar result and interpretation is reported for zircons examined by Leech et al . [] from the Tso Morari (U)HP complex, where zircons extracted from quartzo‐feldspathic gneisses are characterized by inherited cores spanning the Proterozoic and Paleozoic with metamorphic overgrowths ranging from 53.3 ± 0.7 to 45.2 ± 0.7 Ma. In this case, the range of ages for the overgrowths is interpreted as reflecting ∼8 Myr of continuous zircon overgrowth crystallization beginning during (U)HP conditions and ending at amphibolite facies conditions, at which time the 40 Ar/ 39 Ar system also closed in the same rocks.…”

Early–Middle Eocene Exhumation of the Trans-Himalayan Ladakh Batholith, and the India–Asia Convergence

“…Narrow strips of mylonite, volcanics, sediments and serpentinite characterize the dextral transpressional Karakoram Shear Zone (KSZ) between SSZ and Karakoram batholithmetamorphics complex along the southern Asian Plate margin with initial thrust movements (Figure 1 b) 34 . In the south, ITSZ contains tectonically imbricated ophiolite, Triassic to Upper Cretaceous Lamayuru Formation, Jurassic-Lower Cretaceous (164-95 Ma) volcano-sedimentary Nindam Formation, deposited on an intra-oceanic 43,44), and southward ductile shearing with older pre-Main Central Thrust (MCT) discontinuities 8 . MCT resulted in thrusting HMB over the Proterozoic Lesser Himalayan belt, which overrides the Cenozoic Sub-Himalayan belt, and the latter on to the Indo-Gangetic Plains 14,32,39 .…”

“…Tie lines for co-existing minerals in same samples. 43,56 (Figure 8). Close perusal of exhumation rates from LB, located on southern margin of the Asia Plate and adjoining eclogitized Indian continental lithosphere in Tso Morari reveals that both the terrains underwent fast exhumation during Eocene, though the latter exhumed much faster.…”

Integrated garnet and zircon–titanite geochronology constrains the evolution of ultra‐high–pressure terranes: An example from the Sulu orogen

“…In contrast to these large UHP terranes (e.g. Dabie–Sulu and Western Gneiss Region), smaller UHP terranes, such as those in the western Alps (Dora Maira massif, Rubatto & Hermann, ), Himalaya–Tibet (Kaghan Valley, Parrish, Gough, Searle, & Waters, ; Tso Morari, Leech, Singh, & Jain, ) and Papua New Guinea (Baldwin, Fitzgerald, & Webb, ) have relatively short total durations and faster rates of subduction and exhumation (e.g. Kylander‐Clark et al., ).…”