Geochemical Study Of Metamorphic Rocks Of The Zhangbaling Group In The Northeastern Margin Of The Yangtze Block

The Yangtze Block retain perfect records of middle Neoproterozoic magmatic and sedimentary rocks related to breakup of the Rodinia supercontinent,and takes important role in the reconstruction of this supercontinent.Over the past decades,petrogenesis and tectonic evolution of the extensive Neoproterozoic igneous rocks within the Yangtze Block has received considerable attentions.However,there are still debates,especially on the coeval tectonic setting and position of the South China Block in the Rodinia reconstruction.On the other hand,studies of the Meso-to Neoproterozoic lowgrade metamorphic rocks along the northeastern margin of the Yangtze Block are sparse.In this study,we report U-Pb ages and Hf isotopic composition of zircon grains,petrologic and geochemical data of the low-grade metamorphic rocks of the Zhangbaling Group exposed in different areas along the northeastern margin of the Yangtze Block.Including reported data from the previous studies,time of late Precambrian magmatic events in the region,nature of the magmas from which zircon minerals formed,magmatic and crustal evolution of the northeastern edge of the Yangtze Block and its relationship with the supercontinent Rodinia are evaluated.Zircon U-Pb dating results indicate that the Xileng Formation metamorphic volcanic protoliths can be divided into two stages:the early-stage volcanic rocks,only locally exposed,formed at ca.790 Ma.They contain numberous Archean to Early Paleoproterozoic inherited zircon grains.Volcanic rocks of the late stage are widely exposed and the magmatism lasted for a long time from 760 Ma to 700 Ma.The peak of the volcanism in the study area shows a temporal-spatial trend,i.e.,old volcanism in north to young one in south,ranging from 747 to 731 Ma.Ages of the inherited zircon grains are consistent with the magmatic events of volcanic rocks of the early stage.The maximum depositional time of the metasedimentary rocks from the Xileng Formation and overlying the Zhougang Formation can be constrained at about 698 Ma and 663 Ma,respectively,while the depositional time of the metasedimentary rocks of the Beijangjun Formation varies from 688 Ma to 633 Ma.The volcanic protoliths of the Xileng Formation of two magmatic stages are dominated by rhyolite and dacite and most of them belong to calc-alkaline series.The metamorphosed volcanic rocks show geochemical characteristics of arc rocks.They have similar trends in the chondrite-normalized REE patterns,exhibiting low REE contents(FREE contents of 77.27～194.17 ppm,LREE contents of 68.80～173.57 ppm),highly fractionated REE patterns,and negative to weak positive Eu-anomalies(Eu/Eu*=0.56 to 1.32).In the primitive mantle-normalized trace elements diagram,they are significantly enriched in large ion lithospheric elements(LILE)and Pb but depleted in P and Ti,and show slightly negative Nb-Ta anomalies but positive Zr-Hf anomalies.However,many differences can be observed from geochemical characteristics of trace elements and Sr-Nd-Pb-Hf isotopes of the volcanic rocks between these two stages:(1)Trace element characteristics of the early-stage volcanic rocks are similar to low-Mg adakitic rocks,and the magmatic process is dominated by partial melting.Besides,they also have low radiogenic Pb isotopic composition close to those of the lower crust,low initial εNd values(-11.0)and older two-stage Nd model ages(TDM2 of 2.35 Ga).In addition,crustal Hf model ages of the Archean-Paleoproterozoic inherited zircon grains range from 4.07 Ga to 3.30 Ga,while crustal Hf model ages of the Neoproterozoic magmatic zircon grains range from 2.95 Ga to 2.31 Ga.These features likely indicate that the early-stage volcanic rocks may have been formed by partial melting of ancient basement rocks underneath the Yangtze Block.(2)The late-stage volcanic rocks have trace element compositions similar to continental arc andesite.They have enrichment of radiogenic Pb isotopes,negative to slightly positive initial εNd values(-9.5 to 0.5),but slightly negative to positive initialεHf values of zircon grains(-0.4～8.8),corresponding to Hf model ages of 1.32 Ga to 1.08 Ga.These data possibly indicate that the magma(s)were derived from the enriched mantle due to metasomatism of the crustal subduction and,the magma sources contain both juvenile crustal rocks and recycling of old crustal material.The sedimentary sources of the protoliths of the Xileng Formation is mainly derived from the denudation and close transportation of the late-stage volcanic rocks of the Xileng Formation.The presence of a small amount of Paleoproterozoic detrital zircon grains indicates minor contribution of ancient basement rocks from the sedimentary source area.The protoliths of the metasedimentary rocks of the Zhougang Formation mainly sourced from volcanic-sedimentary rocks of the Xileng Formation with a small amount of andesitic island arc rock.The complexity in trace element compositions,Sr-Nd-Pb isotopic characteristics,and higher initial εHf values of zircon grains(6.7 to 15.9)likely point to more complex material sources for the sedimentary rocks of the Beijiangjun Formation relative to the Xileng Formation.The volcanic rocks of both stages in the Xileng Formation should have similar tectonic environment,i.e.,volcanic arc environment of an active continental margin.They might be magmatic products at different stage of the oceanic subduction.The sedimentary rocks of the Xileng Formation were also deposited in active continental margin.But the sedimentary environment of the Zhougang Formation and the Beijangjun Formation was more complex,possibly underwent change of depositional environment along the northeastern margin of the Yangtze Block at 688 Ma to 633 Ma.The protolith ages of the Zhangbaling Group correspond well with those of the Wudang,Suizhou,and Suixian Groups distributing along the northern margin of the Yangtze Block.The early-stage magmatism is close to the formation time of diorite and granite in the Feidong complex and the Daleishan pluton in the Suiying area.However,the magmatism of different stages and in different areas of the northern margin of the Yangtze Block is complex and variances in elemental and isotopic geochemistry.Zircon U-Pb dating evidence from metamorphic volcanic rocks and metasedimentary rocks indicates the existence of multiple periods of magmatism in the middle to late Neoproterozoic period on the northeastern margin of the Yangtze block.The associated island arc subduction may have continued until around 700 Ma,and during this time,the Yangtze plate should have been located on the edge of the Rodinia supercontinent.