195-10 Tectonic history among different metasedimentary units in deep arc crust: P-T-t data from the Mid to Deep Crust of the Gangdese Orogen

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism (Posters)

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Abstract:

The Gangdese Batholith, Tibet is a continental magmatic arc that records the transition from a subducting plate margin to continent-continent collision. The deep crust of the now-tilted Gangdese arc exposes multiple sediment packages, interpreted to represent underthrusted forearc sediment, basement host rock, and/or Indian plate sediments, that each may record different part of the Gangdese tectonic history. Here, we investigate the pressure-temperature-time history of these metasedimentary units using Zr-in-rutile and Ti-in-biotite thermometry and quartz-in-garnet barometry to better understand the tectonic history of the arc crust and its relation to the neighboring eastern Himalaya syntaxis (EHS) throughout the plate-margin transition. Previous studies have shown that the deep Gangdese crust underwent upper amphibolite- to locally granulite-facies metamorphism. Preliminary LA-ICP-MS detrital zircon (DZ) results reveal interfingering between rocks sourced from the Asian and India plate along part of the suture zone. A kyanite-bearing metapelite from the EHS and a muscovite-bearing gneiss from the mapped granulite-facies portion of the Gangdese crust have a Tethyan Himalaya DZ pattern. The EHS sample reached conditions of ~9.5 kbar and ~680°C from ca. 30–35 Ma. In comparison, the muscovite-bearing gneiss collected along the suture zone underwent metamorphism at ca. 22 Ma achieving lower temperatures of ~500 °C. Just to the northeast (~10 km) and still within the suture zone, a garnet–biotite gneiss was potentially sourced from the mélange or forearc and reached peak conditions of ~660 °C and ~12 kbar. Zircon from this sample have multiple rim populations, which suggest either two metamorphic events or a metamorphic event at ca. 40–50 Ma followed by a zircon growth due to likely partial melting from ca. 25–35 Ma. Detrital zircon data from a biotite-bearing schist farther (~30 km) south along the suture matches best with the Indian-derived Greater Himalya unit, suggesting yet another metasedimentary unit within the region. This sample reached temperatures of ~560 °C but lacked metamorphic rim growth. The various sediment sources and timing of metamorphic events among these closely spaced metasedimentary packages demonstrate the tectonic complexity of this section of the Gangdese arc and EHS. Defining the relationship among each unit and its P-T-t path will aid in the understanding of deep crustal changes during and after the India-Asian collision.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9669

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