5 - Creation, preservation, and exhumation of UHPM rocks

Summary

Abstract

Coesite-bearing eclogites exposed in the Western Alps and China record peak metamorphic temperatures of 550–900°C at pressures ≥ 2.5 GPa (≥25 kbar). Their presence as regionally metamorphosed rocks requires subduction of >400 km² of upper crustal material to depths ≥90 km and subsequent exhumation. Existing experimental kinetic data suggest that pure coesite rock should not survive exhumation in the presence of a fluid, and this is corroborated by field observations. Partial survival of coesite is linked to its occurrence as inclusions in porphyroblasts that can maintain high internal pressures and prevent ingress of fluids. In the absence of coesite, reliable indicators of its former presence include subparallel, sometimes curving quartz subgrains and sets of quartz subgrains that truncate other sets. Polycrystalline quartz aggregates and cracks radiating outward from inclusions are not a priori evidence of ultrahigh pressure (UHP) metamorphism.

Petrologic constraints demand that coesite-bearing regional metamorphic rocks cooled during exhumation. Cooling during exhumation requires either (1) continued subduction beneath the eclogites that effectively chills the overlying UHP rocks during exhumation, or (2) transport toward the surface in the lower plate of an extensional structure such as a low-angle normal fault or shear zone. Both processes may occur; neither process demands rapid exhumation rates, although radiometric dating of different portions of the Dora-Maira pressure-temperature path indicate average long-term exhumation rates of 3.00 km Ma^–1.

Introduction

Coesite is a high-pressure polymorph of SiO₂ stable at pressures ≥ 2.5 GPa for metamorphic temperatures ≥500°C (Fig 5.1).