Document Type

Thesis

Date of Degree Completion

Spring 2017

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Chris Mattinson

Second Committee Member

Wendy Bohrson

Third Committee Member

Audrey Huerta

Abstract

The North Qaidam UHP terrane, in western China, exposes minor UHP eclogites hosted by ortho- and paragneisses. In the southeastern part of the terrane, the Dulan area exposes eclogite with the mineral assemblage Grt + Omp ± Ky ± Ph ± Zo ± Qz/Coe. LASS U-Pb analyses of zircons yield weighted mean ages of 463-425 Ma, and REE patterns indicate that these ages record the eclogite-facies metamorphism. Inherited zircon cores yield ages of 928-905 Ma, and REE patterns indicate that the eclogite´s protolith had a magmatic origin. Zr-in-rutile thermometry reveals that Dulan eclogites record peak temperatures of 647-711°C. Spatially distributed peak temperatures show an east-west temperature gradient of ~1 °C km-1 and a slow temperature increase of ~1 °C Myr-1. Our results indicate that retrogression can modify the original rutile´s Zr and Nb content, and therefore the obtained Zr-in-rutile temperatures. Phase equilibria modelling and Zr-in-rutile thermometry of four eclogites indicate they reached peak metamorphic iv conditions at ~26-32 kbar and ~630-690 °C. Coupling garnet mode isopleths with Grs isopleths appears to be a powerful approach to build P-T paths in eclogites where the original garnet composition has been modified by diffusion. The prograde path determined from one sample indicates that the eclogite followed a gradient of ~4-8 °C km-1, and underwent prograde metamorphism through the lawsonite stability field. The presence of retrograde amphibole and zoisite suggests that hydration happened during early retrogression, at ~23 kbar and <750 °C. Pressure fluctuations in the P-T path suggest that the Dulan area experienced subduction and exhumation cycles. Our results indicate that the Dulan area experienced slow heating, and a protracted eclogite-facies metamorphism (~38 Myr). Comparing predictions of thermal models to both the shape pf the P-T path and to the range of peak temperatures from spatially distributed samples suggests that the Dulan UHP area exhumed as part of a body at least 3-4 km thick.

Available for download on Tuesday, June 14, 2022

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