Study On The Longquan Subduction Accretionary Complex In The Cathaysian Orogen

Cathaysia is an important part of South China,but whether it is an old land or an orogeny has been controversial for a long time.Metamorphic rocks of Longquan Group were regard as Precambrian metamorphic basement,which is developed in south Zhejiang province,north part of Cathaysia.Recent geological survey show that Longquan group may be formed by the metamorphism of igneous rocks and sedimentary rocks of Neoproterozoic-early Paleozoic,which were formed in different times and tectonic settings.In this article,based on previous study,we perform zircon U-Pb dating on the rocks of the “Longquan group” and rocks intruding in,analyze these rocks petrography and geochemistry.From zircon U-Pb dating,we find:1.The youngest age of four sedimentary rocks is618 Ma,620Ma,570 Ma and 520 Ma,respectively,suggesting original rocks of metasedimentary were formed in Neoproterozoic-early Paleozoic.Their age spectrums show multiple peaks,reflecting the complexity of the source area.Especially,the peaks in the Pan-Africa and the Grenville period reflect their affinity with the Gondwana paleocontinent.2.Original rocks of meta-mafic rocks were formed in 529~480Ma,and metamorphosed in 400Ma;3.Longquan pluton,which intruded in Longquan group,was formed in ~400Ma.According to the analysis of petrographic and geochemical characteristics,the original rocks of the meta-sedimentary rocks of "Longquan rock group" are sandy or argillaceous rocks,similar to flysch.According to the identification of the tectonic environment,these meta-sedimentary rocks were formed in the continental arc-active continental margin environment.Meta-mafic rocks are divided to three kinds,have similar characteristics with IAB,MORB and OIB respectively.And,Longquan pluton,including tonalite,trondhjemite and granodiorite,has similar characteristic with TTG,showing that the pluton was formed in environment related to subduction.Main body of "Longquan group" is meta-sedimentary matrix,subjected to regional ductile shear function,belonging to like-flysch formation.In the matrix,meta-basite,siliceous rocks,marble and other rock blocks were mixed.Among that,meta-mafic has similar characteristic with island arc basalt,mid-ocean ridge basalt and oceanic island basalt.Due to subduction and accretion,these fragments from different tectonic environment were mixed in the matrix.The tectonic property of Longquan Group should be subducted accretionary complex.Existence of the TTG suite associated with subduction indicates that there was oceanic and continental subduction in the Early Paleozoic,and the Cathaysia block did not form a unified continental block until the Early Paleozoic.