195-11 The Timing of Leucogranite Emplacement Throughout the Himalayan-Tibetan Orogen

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

Presenting Author:

Authors:

Abstract:

Leucogranite generation and emplacement likely plays an important role in the tectonic evolution of the Himalayan-Tibetan Orogen. However, the spatiotemporal distributions, formation processes and relationships between leucogranites and major structures are debated. Many leucogranites are emplaced, typically during the Miocene, below - and in some cases intruding across - the South Tibetan Detachment system either as large plutons or as discrete sills and dikes. Plutons can be hundreds of kilometers in diameter, while individual dikes can be just a few centimeters wide. Others are found associated with the North Himalayan Gneiss Domes within the Tibetan zone. Constraining the timing (and variation in timing) of the emplacement of leucogranites across the Himalaya will greatly inform tectonic models for the region.

Zircon, monazite and xenotime U-Th/Pb chronometers have been used widely throughout the literature to constrain the age of leucogranites. However, constraining the emplacement age is often not a straightforward process. Leucogranites contain multiple generations of a given chronometer, with individual crystal dates only varying by a few millions of years. While the youngest subset of magmatic crystals logically represents the timing of emplacement, there are many ways to define the emplacement age from a such suite of chronometers and there is no consensus on the best way. Some studies choose the youngest crystal as representative of the emplacement age, while others calculate a weighted mean of either the concordant or the youngest dates. Zircon U/Pb, which has been commonly used, is typically more inherited than monazite or xenotime. The inherited crystals are likely scavenged from metamorphic rocks or slightly older leucogranites. To truly compare the timing of emplacement across the orogen, the age needs to be estimated consistently.

The Bayesian Emplacement Estimation with Bootstrapped Crystallization Distributions provides a robust method to reconstruct the emplacement age of leucogranites. This Bayesian Markov-Chain Monte-Carlo method was initially developed to model the eruption age from a similarly dispersed suite of zircon U/Pb dates in a volcanic rock, but has been modified and used to constrain the emplacement age of leucogranites in the Annapurna region. This study looks to produce similar model emplacement ages of leucogranites from pre-existing data to directly compare timing and duration of leucogranite emplacement across the Himalaya, and explore the implications for the evolution of the Himalayan-Tibetan Orogen.

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

doi: 10.1130/abs/2025AM-9577

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.