EXHUMATION RATE AND REACTION HISTORY OF PLEISTOCENE MIGMATITES FROM THE WESTERN HIMALAYA: INSIGHTS FROM CHEMICAL ZONATION IN GARNET

The feedback relationships between surficial dynamics and deep crustal metamorphic processes during orogenesis are not well understood. Exceptionally young (Pleistocene) high-grade metamorphic rocks found at the surface of the western Himalayan syntaxis – the Nanga Parbat Massif (NPM) – provide a unique opportunity to assess this relationship. The primary driving force of the rapid exhumation (5-13 mm/yr) of the massif is debated. Under the “Tectonic aneurysm” model, rapid surface erosion, driven by the Indus River, drives rock uplift and exhumation and establishes a positive feedback with tectonic processes. Other studies have challenged this paradigm, instead showing support for tectonic/lower-crustal processes (e.g. partial melting, orogen-parallel channel flow, crustal scale folding, or unique plate geometries) being the primary control on exhumation rates in the NPM. Central to evaluating these hypotheses are robust constraints on exhumation rates of the NPM, changes in exhumation rate through time, and how these changes relate to partial melting reactions.

Our study aims to quantify exhumation and cooling rates of a garnet-sillimanite-K-feldspar migmatite from the core of NPM using diffusion modeling of major elements in zoned garnet crystals. Compositional X-ray maps of garnet porphyroblasts show well-preserved zoning of major and trace elements. High Ca rims coincident with a step-like change to higher Cr concentrations are suggestive of garnet growth during biotite and plagioclase breakdown melting. Multiple compositional thermobarometers will be used to quantify the pressure-temperature (P-T) conditions of the growth of different zones in garnet. The P-T conditions of garnet growth, as well as the preserved trace element zonation in garnet, will inform our major element diffusion models, which will provide constraints on the exhumation and cooling rates of the NPM, and how these rates relate to the reaction history of garnet growth and partial melting.