Metamorphism and 40Ar/39Ar dating of garnet-bearing epidote mica schist from the Yuka terrane,Northern Qaidam
The metapelites are the mainly country rocks of eclogites in the Yuka terrene,north Qaidam high-pressure/ultrahigh-pressure metamorphic belt.They are commonly composed of garnet,epidote,phengite,plagioclase,quartz,and a small amount of chlorite and titanite.They exhibit strong schistose structure and are characterized by middle to high-grade metamorphism.To evaluate their p-T metamorphic conditions,the age of retrograde metamorphism was constrained,and to estimate the uplift/exhumation rates,detailed petrographic observation,mineral chemistry,p-T pseudosections,and phengite 40Ar/39Ar dating were performed on a garnet-bearing epidote mica schist from this terrane.Microstructural observations,scanning electron microscope(SEM)X-ray elemental mapping,and electron probe micro-analyses(EPMA)compositional profiling showed that most garnet grains occur as"atoll-shaped"with mineral mixtures of phengite,quartz,and titanite encompassed inside a garnet ring,and only a few big grains exhibit weak chemical zonation.Phase equilibrium modeling and composition isopleths of garnet and phengite constrain a metamorphic p-T conditions of 1.39-1.59 GPa,515-530℃.However,according to the garnet zonal and mineral inclusion assemblage features,as well as the matrix phengite zoning patterns of Si,Al,and Mg,it is inferred that it might have experienced UHP metamorphism,while the present mineral assemblage only recorded a retrograde metamorphic p-T condition related to the exhumation.40Ar/39Ar stepwise heating analyses for oriented phengite from the matrix yielded consistent plateau and isochron ages of ca.415.2±3.8 Ma,and an initial 40Ar/36Ar ratio of 265±32,precluding the existence of extraneous 40Ar.Our new 40Ar/39Ar results,together with the reported ages of zircon and monazite from the Yuka metapelites,indicate that they might have experienced two stages of exhumation.During the first stage,from 89±9 km to 50±3 km depth,these rocks were exhumed with a relatively rapid velocity of ca.3.6 km/Ma,resulting from buoyancy forces with the assistance of channel flow due to slab rollback or oceanic crust breakdown.Subsequently,the rate of exhumation decreased to ca.0.3 km/Ma from 50±3 km to 43±3 km depth.This later gentle exhumation was probably governed by regional collision/extrusion,or post-orogenic extension and granite activities.
Yuka terranegarnet-bearing mica schistphase equilibrium modelingphengite40Ar/39Ar dating
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