T386 Challenges in Tectonics: Beyond Steady State: New Developments in Understanding Strain Localization Processes and the Rheology of the Lithosphere (Posters)

Wednesday, 25 October 2017
9:00 AM - 6:30 PM
Halls 4EF

ON THE SURVIVAL OF INTERGRANULAR COESITE IN UHP ECLOGITE

Paper number: T386-1
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

The survival of intergranular coesite in UHP eclogite at Yangkou (Sulu belt, eastern China) is surprising and implies low a(H2O) and minimal interstitial fluid throughout exhumation. The dominant structures in the eclogite are a strong D2 foliation associated with tight-to-isoclinal F2 folds overprinted by close-to-tight F3 folds. The coesite-bearing eclogite occurs as rootless isoclinal F1 fold noses wrapped by a composite S1–S2 foliation in interlayered Ph-bearing quartz-rich schist. Samples from coesite eclogite, Ph-bearing coesite eclogite (2–3 vol.% Ph) and quartz eclogite (6–7 vol.% Ph) were selected to evaluate controls on the survival of intergranular coesite at Yangkou. Since the variation in modal Ph implies higher a(H2O) and higher structural water (SW) concentrations from coesite to quartz eclogite, we determined the number density of intergranular coesite grains per cm2 and measured the amount of SW in Grt and Omp. As the coesite decreases in amount the SW in eclogite increases from 116/197 ppm H2O in the coesite eclogite to 174–290 ppm in the Ph-bearing coesite eclogite to 386/432 ppm in the quartz eclogite. If the SW contents reflect original differences locked in during the late prograde evolution, this trend implies that the amount of fluid stored in the NAMs was higher outside of the F1 fold noses. During exhumation, dehydroxylation of NAMs may generate a fluid phase. Strong D2 deformation likely promoted interconnection of the fluid and migration along the developing S2 foliation, enabling conversion of the intergranular coesite into quartz outside the F1 fold noses. By contrast, the eclogite forming the F1 fold noses behaved as independent rigid bodies within the composite S1–S2 foliation of the surrounding schist. Thus, any exsolved fluid remained isolated in pores and the fold noses remained immune to fluid penetration from outside, and dehydroxylation during exhumation did not produce sufficient fluid to convert coesite to quartz. This inherited drier environment in the F1 fold noses was maintained during exhumation by deformation partitioning and strain localization in the schist, allowing coesite to survive. Thus, variations in the amount of inherited SW and the effects of strain partitioning are important in the survival of coesite during exhumation of deeply subducted continental crust.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-307882

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.