Contrasting zircon and garnet behaviors during metamorphic transformation from eclogite to granulite facies: Constraints from orogenic metabasites from North Qaidam in northern Tibet

Abstract

A combined study of garnet and zircon from the same growth stages can provide effective constraints on the pressure–temperature-time path of metamorphic processes. This is illustrated by distinguishing different episodes of garnet and zircon in metabasites from North Qaidam. Two different garnet domains grown under eclogite- and granulite-facies conditions were recognized to have different trace elements and inclusions. The variable trace elements are ascribed to the coexisting MREE-rich minerals and whole-rock compositions under eclogite-facies conditions, and to variable growth of feldspar and consumption of precursor garnet at the continuous granulite facies metamorphism. The majority of zircon cores with ages of 430–441 Ma and zircon rims with ages of 419–424 Ma record metamorphism at eclogite-facies and granulite-facies, respectively. They exhibit different trace elements and Hf-O isotopes, indicating zircon growth via different mechanisms with differential involvement of precursor zircon and other minerals as well as anatectic melts under eclogite- and granulite-facies conditions. The metabasites experienced an increase of metamorphic thermal gradients with time and thus the tectonic transition from deeply continental subduction for Alpine-type HP to UHP eclogite-facies metamorphism at 2.6–3.1 GPa and 827–864 °C through decompressional exhumation to the lower crust for Barrovian-type HP granulite-facies overprinting at 1.05–1.70 GPa and 725–872 °C, eventually to extensional exhumation to the mid-crustal level for Buchan-type low-pressure amphibolite-facies overprinting at 0.34–0.56 GPa and 528–667 °C in the collisional orogen. While HREE disequilibrium between the two minerals is common, the equilibrium HREE partitioning may be achieved between zircon and garnet rims during crustal anatexis at the late HP granulite-facies metamorphism.