177 Chronology of Orogenesis: Unlocking the Timelines of Mountain Building

The focus of this session is the use of geochronological methods (including but not limited to U-Pb, Ar/Ar, FT, (U/Th)-He) to establish the timing of key events in mountain building, including different orogenic phases, exhumation, and uplift rates.

Tuesday, 21 October 2025
8:00 AM - 12:00 PM
HBGCC, 304C

Constraining the Timing of the India-Asia Collision by Laser-Ablation (U-Th)/He Dating of Chromite and Magnetite

Paper number: 177-11
Program: Technical Sessions
Time: 10:19 AM – 10:34 AM

Authors

Abstract

Chromium spinel (incl. chromite) and magnetite, abundant in ultramafic rocks and serpentinites that lack mineral phases commonly used for for geo- and thermochronometry, offer a promising archive for reconstructing the timing and thermal conditions of ophiolite emplacement and tectonic activity along continental suture zones. While conventional magnetite and chromite (U-Th)/He chronometry has been successful applied to dating of basalts, serpentinites, peridotites, and kimberlites, these attempts encountered significant obstacles, such as U-rich inclusions or He implantation. More importantly, however, the employed methodologies lacked the spatial resolution to distinguish between multiple growth phases within individual grains. In order to overcome such intra-grain mineralogical or textural complexities, we have developed novel methodologies for laser-ablation magnetite and chromium spinel (U-Th)/He dating at UT Austin’s UTChron Laboratory, using in-vacuo laser-ablation and ultra-low-blank magnetic-sector noble gas mass spectrometry for He, a laser-ablation ICP-MS Element2 system for U and Th, and a Bruker white light interferometer for ablation pit volume measurements. The method enables ~100 µm resolution and ~10% analytical precision, allowing targeted analysis of discrete growth zones and complex intra-grain textures.

We applied this methodology to serpentinites within the Indus and Shyok Suture Zones (ISZ and SSZ) in Ladakh, India. Constraining tectonic activity along these sutures is crucial for robustly determining the timing and sequencing of the final collision between India and Asia. Electron microprobe mapping reveals complex chromite–magnetite intergrowths, including ferritchromitization of chromite rims and magnetite fracture infilling and veining.  Preliminary in-situ (U-Th)/He analyses of chromite capture distinct phases of cooling, while magnetite records the timing of late-stage brittle back-thrusting. Simultaneously acquired mayor and trace element data provide additional information about origin and formation temperature of the dated grains. Our results demonstrate the significant potential of laser-ablation (U-Th)/He dating of chromite and magnetite to resolve complex tectonic histories and improve our understanding of continental collision dynamics, particularly in settings where conventional geochronometers are absent.

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

doi: 10.1130/abs/2025AM-414734

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