Fluid-rock interaction and geochemical transport during protolith emplacement and continental collision: A tale from Qinglongshan ultrahigh-pressure metamorphic rocks in the Sulu orogen

Abstract

Ultrahigh-pressure (UHP) metamorphic rocks from the Qinglongshan region of the Sulu orogen are comprehensively studied for their whole-rock geochemistry, mineral O isotopes and zirconology. The metamorphic minerals, which experienced eclogite- to amphibolite-facies metamorphism, exhibit low to negative δ18O values, suggesting that the 18O-depletion of UHP rocks was acquired from their igneous protolith due to high-T meteoric-hydrothermal alteration during the Neoproterozoic. The O isotope heterogeneity in the protolith was not homogenized during the Triassic UHP metamorphism, indicating very limited fluid flow during orogenesis. However, the fluid flow is locally significant during exhumation of the UHP rocks, resulting in the formation of quartz veins, symplectites and coronas. Geochemical transport due to fluid action is evident in whole-rock geochemistry and mineralogical composition. The UHP rocks exhibit unreasonably low 87Sr/86Sr ratios at t1 = 750 Ma but much radiogenic Sr isotopes at t2 = 230 Ma, suggesting the mobility of water-soluble elements due to hydrothermal alteration during protolith emplacement and metamorphic dehydration during continental collision. Fluid-rock interaction during exhumation would also have mobilized Al, Si, Ca and LREE, resulting in the formation of high-pressure veins in the UHP eclogites. The protolith zircon of magmatic origin underwent different types of metamorphic recrystallization in response to fluid-mineral interaction, leading to differential redistribution of trace elements and O-Hf isotopes. Newly grown zircons of metamorphic origin exhibit negative δ18O values, indicating precipitation from negative δ18O fluids that were likely generated by metamorphic dehydration of the hydrothermally altered negative δ18O rock-forming minerals during the Triassic. The metamorphic zircons exhibit relatively homogeneous Hf isotope compositions, suggesting that fluid Hf isotopes originated from the same Hf isotope composition of the protolith. Relict zircon domains of magmatic origin exhibit both positive eHf(t) and negative eHf(t) values, indicating that the protolith of UHP rocks formed by reworking of both juvenile and ancient crustal rocks.