The Post-orogenic Mantle Derived Magmatism In The Eastern Jiangnan Orogen

The Zhejiang section,located in the east of Jiangnan Orogen,lies on the boundary of the Cathaysia and Yangtze blocks.Plenty of post-orogenic mantle derived igneous rocks occur as flow,pluton,dyke,sheet in Zhejiang section,including volcanic rocks like Mengshan spilite in Pujiang,Daqiao basalt in Xiaoshan,Gaoling andisite basalt in Kaihua and intrusive rocks like Dalong/Lingli diabase pluton in Kaihua,Yecun/Shijingyu diabase sheet in Pujiang,diabase sheets in Fuyang and Ciwu diabase pluton in Zhuji.This paper systematically researched the field geological,mineralogical,petrographical,chronological,geochemical and isotopic features on the above rocks.The results are as follows:Zircon LA-ICP-MS U-Pb analyse results indicate that the post-orogenic mantle derived igneous rocks in Zhejiang section mainly formed during 836?774 Ma:the Mengshan spilite at?836 Ma,the Fuyang diabase at?808 Ma,the Ciwu diabase at 809?807 Ma,the Yecun diabase at?803 Ma,the Daqiao basalt at?801 Ma,the Gaoling andisite basalt at 797?794 Ma and the Dalong diabase at?774 Ma.Major and trace element analyse results indicate that the post-orogenic mantle derived igneous rocks in Zhejiang section mostly belong to tholeiitic series.The Mengshan spilites have trace element content similar to MORB.They are slightly rich in LREE,LILE and not depleted in Nb,Ta,Zr,Hf,showing E-MORB features.The Lingli diabases have trace element content similar to OIB.They are rich in LREE,LILE and HFSE,showing OIB features.Daqiao basalts,Gaoling andisite basalts and Fuyang,Pujiang,Kaihua,Zhuji diabases have trace element content between arc basalt and OIB.They are rich in LREE,LILE and depleted in Nd,Ta in various degree.Mengshan spilites,Daqiao basalts and Fuyang,Pujiang,Zhuji diabases have depleted Nd-Hf isotopic features,reflecting depleted mantle source.Gaoling andisite basalts and Dalong diabases have enriched geochemical and Nd isotopic features,reflecting enriched mantle source.Lingli diabases were originated from mixed mantle source.The mantle derived magma experienced crystallization and crust-mantle interaction during ascent.The crust-mantle interaction is mostly remarkable in the magma mixing,while Ciwu-Daolinshan complex provides field geological,petrographical,chronological,geochemical and isotopic evidences for the magma mixing.The post-orogenic mantle derived igneous rocks in Zhejiang section are the typical products of the Neoproterozoic magmatism in eastern Jiangnan Orogen.Together with the rocks in Jiangxi and Anhui sections,the evolution of the eastern Jiangnan Orogen may be as follows:The South China oceanic crust subducted into the Yangtze Block during 1000?890 Ma.Then the collision of the Cathaysia and Yangtze blocks took place during 890?860 Ma.Finally,the united continent started to break up after 860 Ma.The magmatism during 860?750 Ma can be divided into two stages.During 860?820 Ma,the eastern Jiangnan Orogen wan in post-orogenic environment.The lithospheric mantle is the main source of the mantle derived igneous rocks and the asthenospheric mantle took a mimor part in the magma activities.After 820 Ma,the eastern Jiangnan Orogen turned into within plate rifting environment.Most of the mantle derived igneous rocks at this stage reflect a depleted asthenospheric mantle source.The magma with high temperature from the asthenospheric mantle remelted the lithospheric mantle in partial area during ascent.This study researched on the post-orogenic mantle derived igneous rocks in Zhejiang section,to get a better understanding of the temporal-spatial distribution,associated rock suites and mantle features of the post-orogenic mantle derived igneous rocks in the eastern Jiangnan Orogen.This study offers plenty of materials about the magmatism and tectonic evolution of the eastern Jiangnan Orogen.It has significant meaning in the research of the eastern Jiangnan Orogen.