Petrological Study On The Neoproterozoic Bimodal Intrusive Complex In Fuyans,North Zhejiang

Fuyang bimodal intrusive complex is located at Fuyang Zhangcun,Daolinshan and Zhuji Ciwu. It’s in the east of Jiangshao Fault, belonging to the Jiangnan Orogen. The Neoproterozoic stratas are complete in Ciwu Area and the boundary of the intrusion is generally clear, it’s a significant area for studying the the tectonic evolution of Jiangshao Fault in Neoproterozoic. Based on a comprehensive collection of previous studies and sufficient field geological survey,through petrology, petrography, geochemistry, isotope geochemistry and LA-ICP-MS zircon U-Pb dating methods,we make a system research on the complex and then discuss the tectonic evolution process and continental dynamics implications of the Jiangshao Fault in Neoproterozoic. The conclusions are as follows:1、There’re clear mixing phenomenon between the diabase and granite near the contact zone in the field. The diabasic enclaves are kidney-shaped,subrounded, rounded and irregular. Some enclaves are ductile and some have chilled borders or granitic back-veins. All of above indicate that the magma of diabase and granite have ever been together by liquid, and final made a non-uniform mixing. The diabase is formed before or at the same time with the granite. Both of them form the complex.2、LA-ICP-MS zircon U-Pb dating results show that the age of Zhangcun diabase, Yiping diabase, Zhangcun granite and Yiping granite is respectively812.0±6.6Ma,812.0±5.2Ma,801.4±6.1Ma and814.0±6.2Ma.They are consistent in error range,and similar to the age of Daolinshan granite (814.4±2.4Ma) dated by the Regional Geological Survey Party of Zhejiag Province. Compared with the field phenomenon, we interpret that the age of812Ma-814Ma can represent the crystallization age of the Fuyang bimodal intrusive complex.3、Based on the composition, content and texture of the minerals on microscope, we devide the mafic rocks into diabase, monzonite diabase, allgovite et al. Microprobe analyses of the mafic rocks show that the An of the plagioclases varies with the kinds of the diabase. The plagioclase in the ordinary diabase is labradorite (An=70-55). Monzonite diabase and quartz diabase have a zonal textur (An=45-50in th center, An=30-20in the edge). The pyroxene is augite and the amphibole is edenite. The geochemistry results show that the diabase belongs to weakly alkaline and tholeiitic series and has low Mg#and SI, indicating the magma is not original. It’s rich in LILE(K、Rb、Ba), depleted in HFSE(Nb,Ta, Zr,Hf) and Sr. Compared with HREE, it’s also rich in LREE and have a plat Eu pattern, reflecting that the source may be influenced by the subduction fluids.4、The granite consists of quartz, potassium feldspar, plagioclase and a bit of biotite. The plagioclase is more euhedral than potassium feldspar and has Carlsbad-albite compound twin. Microprobe analyses of the granite show that the plagioclase is mainly oligoclase (An=15-25) and the potassium feldspar is mainly perthite. The geochemistry results show that the granite is metaluminous and high-K calc-alkaline series. It’s rich in LREE compared with HREE, depleted in Eu.It’s also rich in LILE(K,Rb,Ba) and Ce,Sm, Th, depleted in HFSE (Ti,Nb,Ta,Zr,Hf) and Sr, P,showing the fractional crystallization of plagioclase, apatite, rutile et al. On the (Zr+Ce+Nb+Y),Nb,Zr,Y-10000Ga/Al diagrams, the granite is A-type. Associated with the mineral texture, it should be aluminum A-type granite.5、On the Harker diagram, we can find that the content of SiO2is absent of56%～65%and has a clear Daily Gap.So the diabase and granite form a typical bimodal intrusive complex. The oxide-oxide, element-element, element ratio-element ratio with the same denominator are linear relationship.The element-element ratio and element ratio-element ratio with different denominators are hyperbolic relationship. All of these prove the mixing of the mafic and fesic magmas of Fuyang bimodal intrusive complex in geochemistry.6、Compared with the regional geological datas and results of previous studies, we tend to interpret the Neoproterozoic Fuyang bimodal intrusive complex as a product of continent-arc collisional orogenic belt. Its dynamic environment is similar to the post-orogenic extension condition of Zhejiang and Fujian in the late Mesozoic. After the collision of the Cathaysian and Yangze Block (ca.825-800Ma), the tectonic condition turn into post-orogenic extension condition. The extension environment is good for the partial melting of the lithosphere mantle and lead to the rapid rising of the magma. The basaltic magma forming the diabase of the Fuyang bimodal igneous complex rise and underplate the continental crust basement, making the partial melting of the basement into granitic magma. The mafic and felsic magmas are non-uniformly mixing and finally form the complex by condensation and crystallization. Similar pheno-menon of non-uniform mixing are seen at other areas in Jiangnan orogen, such as the ca.822Ma Gangbian mafic-felsic igneous complex in Northeast Jiangxi.The forming of Fuyang bimodal intrusive complex is a significant incident recording the Neoproterozoic magmatism in the eastern Jiangnan orogen. The study of the complex further help we to have a understanding of the geological progress of the Neoproterozoic crust-mantle interaction,magma mixing et al. It’s of important geological meaning for studying the tectonic evolution and magmatism of the eastern Jiangnan orogen (i.e.,Jiangshao Fault)...