Petrography,Geochemistry,Zircon Geochronology And Geological Implications Of Neoproterozoic Granites In Dahanshan Region,Northern Margin Of Yangtze Plate

A large area of Neoproterozoic basic-acid instrusive rocks are well exposed in the Dahanshan region, northern margin of Yangtze block, which we called "Hannan Complex". This place is geologically eastward to the Dabashan curvilinear structural belt, westward to the Longmengshan tectonic belt and the Songpan-Garze orogenic belt and southward to the South Qinling Orogenic Belt. Also, The Neoproterozoic magmatic activities widely occurred in the western margin(Panxi region), southern west margin(Guibei region) and interior part(Huangling region) of Yangtze block. So far, a controversy about Supercontinent tectonic model and island arc model caused by these Neoproterozoic magmatic activities has rose a researching wave and earned so many attentions by researchers. This study aim to do some research with the four different types of Neoproterozoic granite by means of field geological investigation, Petrography, geochemistry, zircon geochronology and Hf Isotope. Then we constrain the spatial-temporal distribution, multi-stage magmatic activity, genesis and origin characteristics of the Neoproterozoic granitoid. Finally discuss on the tectonic implication of the granitoid and put forward some development and insights as follows:(1)Our study samples include Xishenba pluton(granodiorite with mafic microgranular enclave), Huangguan pluton(K-feldspar porphyritic granite), Hongmiaozheng pluton(syenogranite), and Zushidian pluton(trondhjemite) collected from Dahanshan region. Field geological survey and petrography reveal that each kind of granite in the Dahanshan region has distinct characteristics. All of the samples are fresh without any metamorphism and deformation.(2)Detailed LA-ICP-MS zircon U-Pb dating results yield the formation ages of 886.5±6.8 Ma for the Xishenba pluton (granodiorite),814±25 Ma for the Huangguan pluton (K-feldspar porphyritic granite),802.1±4.3 Ma/790.5±5.7 Ma for the Hongmiaozheng pluton (syenogranite) and 728±3 Ma for the Zushidian pluton (trondhjemite). The ages of 802.1±4.3 Ma and 790.5±5.7 Ma for the Hongmiaozheng pluton are consistent within error.(3)Systematically geochemical study reveal that Xishenba granodiorite and Zushidian trondhjemite exhibit island arc features with obviously negative N、Ta、Ti anomalies. Therefore, these two granitic plutons formed under the subductional-extrusional setting; The Hongmiao syenogranite and Huangguan K-feldspar porphyritic granite formed under the extensional background.(4)Combining with previous study, the Neoproterozoic tectonic-magmatic activities of Dahanshan region can be divided into three stages of ca.880 Ma, ca.800 Ma and<790 Ma. ca.880 Ma Xishenba granodiorite represent arc-related magmatism; the ca.800 Ma Hongmiaozhen syenogranite and Huangguan K-feldspar porphyritic granite represent an extensional tectonic background; After 790Ma, there was another subduction-related arc magmatism, which can be supported by the Dahanshan gabbro (790-780 Ma) and Zushidian trondhjemite.(5)Zircon Hf Isotope compositions show that the Xishenba granodiorite has εHf(t) values range from -25.0- -3.0 with peak two-stage Hf model age (TDM2) of 2.3 Ga; Hongmiaozhen syenogranite has εHf(t) values range from -26- -4 with peak two-stage Hf model age (TDM2) of 2.2 Ga; Zushidian trondhjemite has εHf(t) values range from 5.0-12.2 with peak two-stage Hf model age (TDM2) of 1.1 Ga. Thus, the Xishenba granodiorite sourced from reworking of ancient continental crust under the subductional setting; the Hongmiaozhen syenogranite also sourced from reworking of ancient continental crust, while formed under an extensional background; the Zushidian trondhjemite sourced from partial melting of juvenile crust, represent crustal growth.(6)Intergrated with regional geological tectonics,it can be speculated that the >880 Ma magma tism occurred in the northern margin of the Yangtze block may be response to the global Gren ville orogeny; the 830-790 Ma magmatism was probably related to the break up of Rodinia; w hile the 790-720 Ma magmatism may represent the amalgamation of the northern margin of Y angtze block and the South Qinling Belt.Since then, Yangtz block and South Qinling Belt coll aged together and kept stable.