On exhumation velocities of high-pressure units based on insights from chemical zoning in garnet (Tianshan, NW China)

Abstract

Exhumed high-pressure rocks formed during subduction provide insight into global geodynamic and plate tectonic processes. Their metamorphic history is partially encoded in their mineralogy and mineral chemistry, which can record the P-T evolution and rates of subduction and exhumation. Here, we study a garnet-glaucophane-albite schist from the SW Tianshan in China in which complexly zoned garnet displays a distinct and sharp oscillatory Mn-zonation and irregular, jagged Ca distributions with peak widths of ∼5 μm. Combined Raman quartz-in-garnet barometry and carbonaceous matter thermometry suggests a P-T path that reached ∼2.5 GPa and was followed by ∼0.9 GPa of nearly-isothermal decompression at temperatures of 530–540°C, associated with final garnet growth. This P-T path is largely confirmed by thermodynamic modeling. Three different approaches to modeling coupled Ca, Mn, Mg and Fe diffusion in garnet were used to estimate the duration between establishment of high-resolution Mn, Fe and Ca zoning during growth and cooling below nominal closure conditions upon exhumation. All models agree best with conditions of ∼530°C maintained for around 100 kyr, though slight differences are found depending on published diffusion coefficients. For the constrained exhumation P-T path, we infer that the studied sample experienced exhumation velocities in the range of 40–45 cm/yr from ∼80 km to ∼50 km depth. We propose an exhumation model involving diapiric Stokes flow along the plate interface and low viscosities around the exhumed unit that may facilitate such high exhumation velocities.