Petrogeochemistry And Geochronology Of Marbles And Coexisting Amphibolites From The Beihuaiyang Zone In The Dabie Orogen

In comparison with the Triassic ultrahigh-pressure metamorphic belt in the southern Dabie orogen,the Beihuaiyang zone(BZ)is generally regarded to be a relatively low-grade metamorphic unit located in the northern part of the orogen.The BZ can be subdivided into eastern and western segments separated by Shangcheng-Macheng fault zone,respectively.Among of them,the eastern segment of the BZ has no record related to Paleozoic oceanic subduction similar to those in the western segment.Also,there are rare reports on the Paleozoic magmatism and metamorphism in the region.As a result,these greatly have impeded the deeply understanding and precisely recognition on the formation and evolution of the Dabie orogen.The eastern segment of the BZ is mainly composed of three groups of rocks,i.e.(1)metamorphic rocks,e.g.,meta-granitoid(granitic gneiss),(garnet)amphibolite,marble,mica-quartz schist and meta-flysch;(2)igneous rocks mainly including Mesozoic granite,syenite and volcanics;(3)basin sediments mainly composed of Mesozoic-Cenozoic and subordinate Carboniferous rocks.However,up to now no petrological record of the Paleozoic metamorphism has been reported in the region.Therefore,this study carried out systematical petrology,element geochemistry,zircon geochronology study and field geological investigaition on the marble and amphibolite coexisting with marble from BZ.The results suggest that the studied rocks in Tiechong experienced a multistage metamorphic evolution with the peak at 355±5 Ma,and retrograde metamorphism at?330 and?310 Ma,respectively.The rocks have mineral assemblages of four metamorphic stage including Grt+Cpx+Pl+Ts+Qz,Hbl+P1+Grt+Ttn,Hbl+Pl+Mag and Chl+Act.The corresponding temperatures and pressures for the former three stages are 648?719?/1.0?1.2GPa,660?690?/0.71-0.90GPa and 607?646?C/0.28?0.50GPa,respectively.The studied rocks in Waimiao also underwent a multistage metamorphic evolution with the peak at 420 Ma,and retrograde metamorphism at?330 Ma.The rocks have mineral assemblages of four metamorphic stage including Cpx+Pl+Qz±Hbl,Hbl+Pl+Ap+Ttn,Hbl+Pl+Ttn+Ilm and Act+Cal+Mag.They all suggest a clockwise P-T path with near-isothermal decompressional processes.It is for the first time documented that the high-pressure metamorphism occurred at the eastern segment of the Beihuaiyang zone and the amphibolites here formed in the late Paleozoic subduction-collisional setting.A combined study of whole-rock major and trace elements and Sr-Nd-Pb isotopes has been carried out for the rocks from the BZ,the results provide constraints on the origin of sub_alkaline basalt for the(garnet)amphibolites.They exhibit arc-like trace element distribution patterns,with enrichment in K/Sr/Ba/Rb/T but depletion in high field strength elements(IFSE),like Ti/Nb/Ta/Zr/Hf.The amphibolites have enriched Sr-Nd-Pb isotope compositions,with high initial 87Sr/86Sr ratios of 0.706208-0.708142,positive ??d(t)values and elevated 207Pb/204Pb ratios at given 206Pb/ZO4Pb ratios,but The 208Pb/204Pb value hardly changed with the increase of 206Pb/204Pb value.These elements and isotopes indicate that the protoliths of Tiechong and Waimiao amphibolites are mainly subalkaline basalts formed in island arc or back-arc environments related to Paleozoic oceanic subduction,with the addition of melt/fluid during metamorphism,resulting in obvious changes in some elements and isotope compositions.