Geochronology,Geochemistry And Tectonic Attributes Of The Gneisses And Amphibolites From The Chencai Group,Zhejiang Province

The Chencai Group is an important geological unit in the northeast Cathaysia Block.The formation age and structural affinity of the Chencai Group is essential for understanding the Neoproterozoic-early Paleozoic tectonic evolution of the Cathaysia Block.We carried out an integrated study of petrology,zircon U-Pb geochronology,Hf isotopes and geochemistry on the gneisses and amphibolites from the Chencai area.The detrital zircon ages of four gneiss samples were determined.A total of 342 detrital zircon ages have been obtained ranging from 2974 Ma to 501 Ma with the main peak at ca.820 Ma.The youngest age indicates that the protoliths of the gneisses were formed in the middle early Paleozoic.The detrital materials with Neoproterozoic age may derive from the Jiangnan Orogen.In addition,a small subset of Paleo-Mesoproterozoic detrital materials have also been recognized,which may come from the Wuyi terrane.The?_Hf?t?values?-20.6-+20.9?of the detrital zircons suggest that the detrital materials of the protoliths are composed of old crustal and juvenile materials.The geochemical characteristics suggest that the protoliths of the gneisses are graywacks,which formed in an active continental margin or continental island arc environment.The metamorphic age of the three amphibolite samples are 428±4 Ma,430±4 Ma and 438±3 Ma,indicating the metamorphism happened during Caledonian.The protoliths of the amphibolites are tholeiite,which can be divided into two groups.Group 1 is characterized by lowcontents,LREE/HREE and?La/Yb?_N ratios showing left-inclined REE pattern and Nb,Ta depletion which resembles N-MORB.Group 2 has higher?REE contents,LREE/HREE and?La/Yb?_N ratios,with Eu negative anomalies and right-inclined REE pattern showing affinity to E-MORB.The geochronology and geochemical characteristics of the gneisses and amphibolites show that the Chencai Group is composed of both oceanic crust remnants and continental crust materials,indicating it should be a subduction accretionary complex rather than metasedimentary sequence and thus we believe that there was no unified Precambrian basement in the Cathaysia Block.