Estimating the Thickness of an Ultrahigh-Pressure (UHP) Terrane: Insights from Phase Equilibria Modelling and Zr-in-Rutile Thermometry, Dulan Area, North Qaidam Terrane, Northwest China
David Hernández Uribe May 2017
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 conditions at ~26-32 kbar and ~630-690 °C. Garnet mode isopleth coupled with Grs isopleth appear to be a powerful approach to build P-T paths in eclogites that 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 the 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, a protracted eclogite-facies metamorphism (~38 Myr), and exhumed as a 3-4 km body.