The Petrogenesis And Metamorphic Evolution Of The Early Precambrian Metamorphic Basement In The Sulu UHP Metamorphic Belt

Haiyangsuo is located at the northeastern part of the Sulu UHP metamorphic belt.Metamorphic complex in this area formed at late Neoarchean to Palaeoproterozoic,which is comparable to the Precambrian basement of North China Craton.Detailed studies on their petrography,geochemistry,metamorphism and isotopic geochronology are significant in recognizing the formation,evolution and continental geodynamics of the Paleoproterozoic Jiao-Liao-Ji tectonic belt and the Sulu-Dabie UHP metamorphic belt.The metamorphic rocks within the studied area are mainly composed of meta-mafic rocks,granitic gneisses and paragneisses.The meta-mafic rocks,represented by the garnet pyroxene granulites,occur as irregular lenses or layers in the gneisses.Based on geochemical characteristics,all the meta-mafic rocks are tholeiites series,some of which may be related to island arc setting and others are associated with mid-ocean ridge setting.In the An-Ab-Or diagram,protoliths of most granitc gneisses are plotted in the trondhjemite region with similar geochemical affinities to the TTGs in Jiaobei.Two periods of metamorphic events have been recognized in the meta-mafic rocks.The first period of metamorphism(M1)shows a middle to high-pressure granulite-facies metamorphic event,and the second period of metamorphism include a peak metamorphic stage(M21)and a late amphibolite-facies retrograde stage(M22).P-T conditions for each stage were estimated using thermo-dynamical and experimental geothemometers and geobarometers.Corase-grained mineral assemblage of garnet(Grt1)+clinopyroxene(Cpx1)+ plagioclase(Pl1)±quartz is regarded as the typical mineral assemblage of peak metamorphic stage of M1 and yields P-T conditions of 725?845 ? and 9.5?12.4 kbar.In contrast,the peak metamorphic stage of the second period of metamorphism(M21)is characterized by "red-eye cycle"texture and fine-grained mineral assemblage of garnet(Grt21)+clinopyroxene(Cpx21)+plagioclase(P121)± quartz,which formed at 765?845 ? and 14.8?17.5 kbar.The mineral assemblage of green amphibole(Amp22)+ plagioclase(Pl22)±quartz ± garnet(Grt22)as matrix minerals and symplectites indicate that the late amphibolite-facies retrograde stage(M22)occurred at P-T condition of 575?680?and 6.0?8.0 kbar.These data suggest that the meta-mafic rocks in the Haiyangsuo area record a clockwise P-T-t path including near-isothemal decompression(ITD)to decompression cooling evolutional process.According to zircon U-Pb dating results,protolith ages of the meta-mafic rocks are 2600?2540 Ma and protolith ages of the granitic gneisses are 2570?2540 Ma and 2450?2425 Ma,which are consistent with those in the Jiaobei area.The age range of inherited detrital zircon from the paragneisses are 2560?2450 Ma.Metamorphic zircons from these metamorphic rocks record two major periods of metamorphic ages.The first period of metamorphism took place at 1880?1840 Ma,which is identical to that of the HP granulite facies metamorphism in the Jiao-Liao-Ji belt.The second period occurred at 248?231 Ma,which is close to the time of the UHP metamorphism occurred within the Sulu UHP metamorphic belt.Based on the lines of evidences of petrography,geochemistry,genetic mineralogy,metamorphic evolution and geochronology,we have concluded that the meta-mafic rocks and their country rocks in the Haiyangsuo area are from the Precambrian basement in Jiaobei terrane of North China Craton.They were subjected to the Paleoproterozoic thermal event and involved in the Dabie-Sulu Triassic orogenic event during the middle to upper Triassic.This study not only help us to reveal the petrogenesis and metamorphic evolution of meta-mafic rocks and their country rocks in both Haiyangsuo and its adjacent area,but also be useful in revealing the evolution history of the basement of Jiaobei area and Sulu-Dabie ultrahigh-pressure metamorphic belt.