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

Abstract

The oxygen isotope composition of whole-rock and mineral separates was measured for granulite rocks from Dabieshan. According to their whole-rock δ¹⁸O values relative to the normal mantle δ¹⁸O values of 5.7±0.5‰, two groups are classified: (1) mafic granulite which exhibits lower δ¹⁸O values of 3.5 to 4.7‰, and (2) felsic granulite which shows higher δ¹⁸O values of 7.6 to 7.8‰. Consistent isotope temperatures of 800 to 900 °C are obtained for mineral pairs containing such refractory minerals as pyroxene, garnet, hornblende and iron oxides, suggesting the achievement and preservation of oxygen isotope equilibrium at the conditions of the peak granulite-facies metamorphism. This not only points to a rapid cooling and ascent for the granulite rocks in the early stage of exhumation, but also precludes the infiltration of external fluids during exhumation as the cause for the ¹⁸O-depletion in the mafic granulite. It is evident that the granulite rocks acquired the low δ¹⁸O values before the granulite-facies metamorphism by interaction with a certain ¹⁸O-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 formation of the granulites on local scales. It is likely that the granulites together with the ultrahigh pressure eclogites and gneisses in Dabieshan were part of a single tectonic entity in the processes of subduction and Triassic metamorphism but experienced differential two-stage uplifts prior to amphibolite-facies retrogression.