40Ar/39Ar geochronology and exhumation of high-pressure to ultrahigh-pressure metamorphic rocks in east-central China

Abstract

New 40 Ar/ 39 Ar ages from rocks in the high-pressure and ultra-high-pressure (HP-UHP) metamorphic complex in the Hong9an and Dabie Mountains areas, east- Central China, document a two-phase cooling and exhumation history following Triassic continental collision and metamorphism. Phengite ages from blueschist through kyanite-bearing eclogite facies rocks in Hong9an record initial exhumation from the collision zone between 230 and 195 Ma. Biotite and hornblende ages from migmatites and eclogite-bearing gneisses from the Dabie Mountains record a cooling event between 128 and 117 Ma, corresponding to a regional episode of crustal anatexis and emplacement of granitic plutons. The Triassic through Early Jurassic 40 Ar/ 39 Ar cooling ages corroborate U-Pb and Sm-Nd metamorphic ages from previous studies and suggest that initial exhumation of these rocks was rapid. Emplacement of granitic melts within the HP-UHP sequences ∼80 m.y. after metamorphism suggests that the metamorphic rocks were either exhumed at slower rates or became arrested at depth subsequent to the initial, rapid exhumation episode.