151-4 Constant 50 Ma Lu-Hf garnet ages across the 100 km Tso Morari complex indicate the exhumation of a coherent UHP thrust sheet

Session: Chronology of Orogenesis: Unlocking the Timelines of Mountain Building (Posters)

Poster Booth No.: 288

Presenting Author:

Sajjad Shah

Authors:

Shah, Sajjad Ahmad¹, Kohn, Matthew J.², Long, Sean Patrick³, Ionescu, Adelie⁴, Vervoort, Jeff David⁵, Baker, Peter Louis⁶, Sachan, Himanshu K.⁷

(1) Geosciences, Boise State University, Department of Geosciences, Boise, ID, USA, (2) mattkohn@boisestate.edu, Boise State University, Geosciences, Boise, ID, USA, (3) School of the Environment, Washington State University, Pullman, WA, USA, (4) School of the Environment, Washington State University, Pullman, WA, USA, (5) School of the Environment, School of the Environment, Pullman, WA, USA, (6) School of the Environment, Washington State University, Pullman, WA, USA, (7) Wadia Institute of Himalayan Geology, Dehra Dun, Uttarakhand, India,

Abstract:

A key question for tectonics is how ultrahigh-pressure (UHP) terranes are subducted and returned to Earth’s surface. The large areal extent of the Tso Morari complex and relative youth make it particularly amenable to testing tectonic models, especially whether large UHP terranes can be exhumed as coherent sheets. In this study, we collected high-precision garnet Lu-Hf ages, and attempted Sm-Nd dating for eight eclogites distributed along a ~85 km strike length of the Tso Morari complex. Previous Lu-Hf and Sm-Nd garnet ages are 55±12 and 55±7 Ma, respectively, and do not resolve either prograde or peak metamorphic ages precisely. Zircon U-Pb ages ranging from 44±1 to 53±1 Ma  have been interpreted as dating the peak of metamorphism. All prior garnet and zircon ages are from the central part of the complex, and do not address spatial variability.

 

Ganets preserve prograde zoning with Mn and HREE enriched cores and Mg rich rims. Lu-Hf garnet ages are 50.8±0.3, 49.4±0.6 (core), 47±1 (rim) Ma (NW part of the complex), 48.2±0.1, 49.1±1.1, 49.4±0.2 Ma, and 50.0±0.2 (central part of the complex), and 48.6±0.6, and 73.0±1.9 Ma (SE part of the complex). Two Sm-Nd garnet ages from the NW and SE part of the complex are 48.5±2.1 and 78.8±5.3 Ma. Excepting one sample, all ages are 48 to 50 Ma, and show no clear systematic differences in age across the complex.

 

These ages much more precisely date the timing of garnet growth, which corresponds to prograde to peak metamorphism. Core (pre-UHP) contributions to total Lu range from 40-85%, so UHP metamorphism must be later than ~48 Ma. Core (prograde) and rim (UHP) data from one sample (49±0.6 Ma and 47±1 Ma, respectively) suggest rapid subduction to UHP conditions. We do not know how to interpret the single pre-Himalayan age of ~74 Ma for both Lu-Hf and Sm-Nd. The constant garnet ages across the complex imply there are no major structural breaks and that the entire complex was exhumed as a structurally coherent sheet. A previously published phengite ⁴⁰Ar/³⁹Ar age of 48±2 Ma suggests that large UHP sheets can be rapidly (within 1-2 Myr) subducted, detached, and exhumed to mid-crustal levels.

 

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

doi: 10.1130/abs/2025AM-8231

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