Estimating the thickness of an ultrahigh-pressure (UHP) terrane: insights from phase equilibria modelling and Zr-in-rutile thermometry, Dulan area, North Qaidam terrane, China

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.

Name thesis:degree_name

Master of Science (MS)

Discipline thesis:degree_discipline

Geological Sciences

Year dc:date.available

2017

Author dc:creator

• Hernández Uribe, David

Contributors dc:contributor

• Chris Mattinson
• Wendy A. Bohrson
• Audrey Huerta

• North Qaidam
• ultrahigh-pressure metamorphism
• phase equilibria modelling
• Zr-in-rutile
• Earth Sciences
• Geology
• Physical Sciences and Mathematics

Language dc:language

English

Repository record dc:identifier

https://digitalcommons.cwu.edu/etd/631

OAI identifier oai:identifier

oai:digitalcommons.cwu.edu:etd-1647