Oxygen isotope composition of granulites from Dabieshan in eastern China and its implications for geodynamics of Yangtze plate subduction

The oxygen isotope composition of whole-rock and mineral separates was meas ured for granulite rocks from Dabieshan. According to their whole-rock delt a O-18 values relative to the normal mantle delta O-18 values of 5.7 +/-0.5 parts per thousand, two groups are classified., (1) mafic granulite which exhibits lower delta O-18 values of 3.5 to 4.7 parts per thousand, and (2) felsic granulite which shows higher delta O-18 values of 7.6 to 7.8 parts p er thousand. Consistent isotope temperatures of 800 to 900 degreesC are obt ained for mineral pairs containing such refractory minerals as pyroxene, ga rnet, hornblende and iron oxides, suggesting the achievement and preservati on of oxygen isotope equilibrium at the conditions of the peak granulite-fa cies metamorphism. This not only points to a rapid cooling and ascent for t he granulite rocks in the early stage of exhumation, but also precludes the infiltration of external fluids during exhumation as the cause for the O-1 8-depletion in the mafic; granulite. It is evident that the granulite rocks acquired the low delta O-18 values before the granulite-facies metamorphis m by interaction with a certain O-depleted surface fluid. The surface fluid is assumed to exchange oxygen isotopes with the granulite protoliths prior to plate subduction. Fluid-absent metamorphism is suggested for the format ion of the granulites on local scales. It is likely that the granulites tog ether with the ultrahigh pressure eclogites and gneisses in Dabieshan were part of a single tectonic entity in the processes of subduction and Triassi c metamorphism but experienced differential two-stage uplifts prior to amph ibolite-facies retrogression. (C) 2001 Elsevier Science Ltd. All rights res erved.