Yi-Xiang Chen

Developing plate tectonics theory from oceanic subduction zones to collisional orogens

Crustal subduction and continental collision is the core of plate tectonics theory. Understanding the formation and evolution of continental collision orogens is a key to develop the theory of plate tectonics. Different types of subduction zones have been categorized based on the nature of subducted crust. Two types of collisional orogens, i.e. arc-continent and continent-continent collisional orogens, have been recognized based on the nature of collisional blocks and the composition of derivative rocks. Arc-continent collisional orogens contain both ancient and juvenile crustal rocks, and reworking of those rocks at the post-collisional stage generates magmatic rocks with different geochemical compositions. If an orogen is built by collision between two relatively old continental blocks, post-collisional magmatic rocks are only derived from reworking of the old crustal rocks. Collisional orogens undergo reactivation and reworking at action of lithosphere extension, with inheritance not only in the tectonic regime but also in the geochemical compositions of reworked products (i.e., magmatic rocks). In order to unravel basic principles for the evolution of continental tectonics at the post-collisional stages, it is necessary to investigate the reworking of orogenic belts in the post-collisional regime, to recognize physicochemical differences in deep continental collision zones, and to understand petrogenetic links between the nature of subducted crust and post-collisional magmatic rocks. Afterwards we are in a position to build the systematics of continental tectonics and thus to develop the plate tectonics theory.

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

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 εHf(t) and negative εHf(t) values, indicating that the protolith of UHP rocks formed by reworking of both juvenile and ancient crustal rocks.