Metamorphic Evolution And Tectonic Implications Of The Garnet-clinopyroxenite From Hujialin,Sulu UHP Orogenic Belt,Eastern China

Rizhao Hujialin is located in the middle of the Sulu UHP orogenic belt,and the garnet-clinopyroxenite is exposed at the upper part of the ultramafic rock complex which shows dunite or serpentinized peridotite in the lower part,the wall rock of which is granitic gneiss.Whole-rock geochemical analysis indicates that the garnet-clinopyroxenite exhibits convex chondrite-normalized REE patterns,and it shows strong Pb positive anoamalies and depletion of high field strength elements,such as Nb,Ti,suggesting a mantle-derived protolith that underwent fluid metasomatism.Based on textural criteria,the garnet-clinopyroxenites are divided into three types:equigranular garnet pyroxenite,porphyroclastic garnet pyroxenite and megacrystic garnet-clinopyroxenite.According to a combined study of petrography,mineral chemistry,phase equilibrium modeling and zircon U-Pb geochronology,the Hujialin garnet-clinopyroxenite recorded five magmatic and metamorphic stages.Stage ?:the formation of the magmatic protoliths;stage ?:the formation of megacrystic garnet-clinopyroxenite accompanying the subduction process;stage ?:the formation of porphyroclastic or equigranular garnet-clinopyroxenite with mineral assemblage of Grt+Cpx+Ilm+Chu accompanying the initial exhumation process(215 ±5.7 Ma);stage ?:An progressive cooling and decompression process with crystallization of water bearing mineral such as clinochlore and pargasite.Zircon U-Pb age shows that this stage probably occurs at 206 Ma;Stage V:later epidote-amphibolite facies retrograde stage with mineral assemblage of Grt+Cpx+Amp+Chi+Ep+Spl+Ilm(180-174 Ma).Generally this stage was occured due to the water fluid activity in shear-tensional fractures or in the connected pores.The mineral transformation from Cpx to Grt and then to Di record the general metamorphic evolution that occurred in the subduction and exhumation process,and the higher Al content of Cpx inclusions in megacrystic garnet play a key role in undersdanding the metamorphic event.The Zircon U-Pb age data analysis showed that the upper intersection of Hujialin garnet-clinopyroxenites samples was 1817 ± 40 Ma and the lower intersection was 215 ±5.7 Ma.The paleoproterozoic age of 1817±40 Ma is consistent with the extensive magma and metamorphic events in the North China Craton,and may represent the formation age of its original rocks.In addition,the age of 215±5.7 Ma recorded the metamorphic age of Hujialin garnet-clinopyroxenites.