Thermobarometry and geochronology in the Dulan region, North Qaidam ultrahigh-pressure terrane: resolving spatial variation of ages, temperatures, and pressures
Megan Eileen Regel
The Dulan ultrahigh-pressure (UHP) terrane exposes 449-422 Ma eclogites in the east and 449-418 Ma high-pressure granulites to the west, but the relationship between the units is poorly understood. Zirconium-in-rutile temperatures show an east-west 3- 5°C/km gradient in four UHP eclogites: 662-716±10°C; garnet-omphacite-phengite thermobarometry of peak conditions yields 659-730±82°C, 25.3-27.1±3.2 kbars. Zircon U-Pb ages of four eclogites are the same within uncertainty, averaging 434±2.9 Ma. Ti- in-zircon thermometry and rare earth element patterns indicate these ages represent eclogite facies metamorphism. One high-pressure/high-temperature (HP/HT) granulite yields a minimum rutile temperature of 773±4°C. Previous thermobarometry of high- pressure granulites yields ≥800°C and 14-17 kbars. Both eclogites and high-pressure granulites share an amphibolite facies retrogression texture. The evidence presented here supports either of two pressure-temperature models: 1) a melt-weakened slab exhumes, placing HP/HT granulites in a UHP terrane or 2) that HP/HT granulites formed in thickened overriding crust and were tectonically juxtaposed with UHP eclogites later.
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