FLUID-ROCK INTERACTION AND GEOCHEMICAL TRANSPORT DURING PROTOLITH EMPLACEMENT AND CONTINENTAL COLLISION: A TALE FROM QINGLONGSHAN ULTRAHIGH-PRESSURE METAMORPHIC ROCKS IN THE SULU OROGEN

摘要

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 δ~(18)O values, suggesting that the ~(18)O-depletion of UHP rocks was acquired from their igneous protolith due to high-T meteoric-hydrothermal alteration during the Neopro-lerozoic. The O isotope heterogeneity in the protohth 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 ~(87)Sr/~(87)Sr ratios at t_1 = 750 Ma but much radiogenic Sr isotopes at t_2 = 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 protohth 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 δ~(18)O values, indicating precipitation from negative δ~(18)O fluids that were likely generated by metamorphic dehydration of the hydrothermally altered negative δ~(18)O 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 ε_(Hf)(t) and negative ε_(Hf)(t) values, indicating that the protolith of UHP rocks formed by reworking of both juvenile and ancient crustal rocks.