Episodic Magmatism And Continental Crust Growth And Reworking In Cathaysia Block

The South China is tectonically divided into two major blocks:the Yangtze Block to the northwest and the Cathaysia Block to the southeast, which amalgamated during Neoproterozoic time. Prior to the Neoproterozoic, the Cathaysia Block and Yangtze Block experienced independent histories. Unlike the Yangtze Block with its Archean-Paleoproterozoic crystalline basement, no reliable Neoarchean basement is found in the Cathaysia Block, and the Paleoproterozoic basement rocks only crop out in the NW Zhejiang and NE Fujian provinces. Since the Neoproterozoic, the Cathaysia Block and Yangtze Block underwent multiple tectono-magmatic activities such as in the early Paleozoic (Caledonian), early Mesozoic (Indosinian) and late Mesozoic (Yanshanian) as a unified South China entity. The episodic magmatism in Cathaysia Block led to different forms of crust-mantle interaction together with episodic growth and reworking of the continental crust.Paleoproterozoic granites and metamorphic rocks of the Badu complex are the oldest rocks found in Cathaysia Block. SHRIMP and LA-ICP-MS zircon U-Pb ages of five representative granite samples from Jinluohou (Lizhuang) and Jingju granitic complexes and Chatan plagioclase amphibolite indicate that the magmatism took place during Paleoproterozoic, and overprinted by early Paleozoic and early Mesozoic thermal events. The Jinluohou garnet-bearing biotite granite (1878±28 Ma) has high A/CNK (1.28-1.42) and contains a large amount of biotites and garnets, showing the S-type granite affinity. It was formed at low water fugacity and high Zr saturation temperature (792～809℃). The Jingju medium-coarse grained K-feldspar granite (1861±Ma), Jingju medium-fine grained K-feldspar granite (1849±30 Ma) and Jinluohou gneissic granodiorite (1877±10 Ma) can reach strongly peraluminous (A/CNK=0.97～1.26), are enriched in alkalis and HFSEs and have high Ga/Al and FeOT/(FeOT+MgO) ratios. Their high Zr saturation temperatures (840～854℃) together with high Rb/Nb and Y/Nb ratios suggest that they belong to the A2 subgroup of A-type granites. The ages of inherited zircon cores from Jinluohou garnet-bearing biotite granite (2088～2929 Ma) reveal the existence of the Archean basement, and the Hf and Nd isotopes and chemical compositions suggest that these S- and A-type granites were originated from the Paleoproterozoic and Archean crustal sources with different proportions of input from juvenile materials. The Chatan plagioclase amphibolite (1879±42 Ma) and Paleoproterozoic S-and A-type granites intruded simultaneously, and formed under intraplate extensional geodynamic setting, which is an important event of episodic crustal growth and reworking. The lower intercept age of Jinluohou gneissic granodiorite and Jingju K-feldspar granite (224～231 Ma) together with Chatan plagioclase amphibolite (418±23 Ma) may indicate other two important thermal overprinting events in this region.The accepted geodynamic scheme of the amalgamation between Cathaysia and Yangtze Blocks may be described as the following:the closing of an ocean that existed in the Neoproterozoic through northward seafloor subduction beneath an active continental margin of the Yangtze Block and the subsequent continent-continent collision between the Yangtze and Cathaysia Blocks, giving rise to the Jiangnan Orogeny. However, previous view that northwest Cathaysia was a passive continental margin during Neoproterozoic time has been challenged by recent discoveries of mafic and felsic igneous rocks with geochemical affinities to continental arc or back-arc volcanic rocks, including the ca.830 Ma Miaohou and Shanhou arc complexes (gabbro, diorite, and granite) with a similar geochemistry to continental arc rocks. According to recent studies on the temporal-spatial distribution of Neoproterozoic igneous rocks and related rocks in South China, we also recognize the presence of a Neoproterozoic intra-oceanic arc, continent-arc-continent collision and three tectono-magmatic belts between the Yangtze Block and Cathaysia Block. At present, the intra-oceanic arc assembly is located between the active continental margin of both Yangtze and Cathaysia Block along the Jiangshan-Shaoxing-Pingxiang-Shuangpai-Xinyi deep faults on the southeast, and along the Jingdezhen-Yifeng-Wanzai deep faults on the northwest.We have synthesized the recently available geochronological, geochemical and isotopic data on Neoproterozoic igneous rocks in the South China Block, which allow us to unravel the amalgamation and tectono-magmatic histories between the Yangtze and Cathaysia Blocks:At ca. 1000-860 Ma, northwestward ocean-ocean subduction and southeastward ocean-continent subduction with slab dehydration melting resulted in the intra-oceanic arc magmatism and active continental margin magmatism in the Cathaysia Block respectively. At ca.860～825 Ma, the steepening subdution caused back-arc basin developing in the intra-oceanic arc zone and the slab rollback induced the arc and back-arc magmitism in the Cathaysia Block. Meanwhile, a shallow dip northwestward ocean-continent subduction formed active continental margin magmatism in the Yangtze Block. At ca.825～805 Ma, the continent-arc-continent collision and final amalgamation between the Yangtze and Cathaysia Blocks yielded the Jiangnan Orogen. At ca. 805～750 Ma, the Jiangnan Orogen collapsed, and the Nanhua rift basin formed between the proto-active continental margin of the Yangtze Block and the intra-oceanic arc zone in the west. Our study also rules out any Grenvillian Orogenic event and mantle plume activity in South China and indicates a marginal position of South China in the Rodinia supercontinent.The early Paleozoic orogen in South China Block is an intracontinental orogen, and synchronous magmatism (440～390 Ma) is mainly acidic with minor intermediate-mafic magmatism. Previous studies suggest that most of the early Paleozoic granites in South China belong to peraluminous S-type genesis while amphibole-bearing I-type granites are subordinate. However, our results indicate that considerable amounts of these early Paleozoic granites have characteristics of both S- and I-type granites. Thus, we propose to divide these granites into two groups:fewer of them are Group A with relatively high εHf(t) values (clustering within -3.0 to +9.0) and εNd(t) values (-5.2 to+1.3) as well as higher initial temperatures at 810～850℃, and most of them are Group B with relatively low εHf(t) values (clustering within -16.0 to -1.0) and εNd(t) values (-13.2 to -4.1) as well as relatively low initial temperatures at 700～830℃. The Xiawan monzogranite and Duntou granodiorite are typical Group A granitoids and yield zircon U-Pb ages of ca.410 Ma. Geochemical and isotopic analyses on early Paleozoic granites, mafic enclaves and mafic to intermediate rocks demonstrate that the Group A granitoids including Xiawan and Duntou granites may be generated by AFC processes with interactions between asthenosphere-derived magma and metasedimentary rocks, and the Group B granitoids may be formed by AFC processes with interactions between synchronous basaltic magma and metamorphic basement. The post-collisional delamination and asthenospheric upwelling directly participate in the generation of Group A granitoids but indirectly induce the formation of Group B granitoids.Previous studies of early Mesozoic magmatism in the South china only focused on S-type and I-type granites. The syenites-monzonites-A-type granites association used to be neglected, which may indicate an extensional tectonic environment on a local or regional scale and crust-mantle interaction. The syenites are characterized by enrichment in LILE and LREE, and depletion in HFSE as well as enriched Sr-Nd-Hf isotopic compositions, which is interpret to be originated from a metasomatized lithospheric mantle inherited from an ancient subduction event. The monzonites is represented by Jingju porphyritic quartz monzonite (226±1 Ma), these monzonites show relatively lower LILE and LREE contents than syenites but similar enriched Sr-Nd-Hf isotopic compositions, indicating they may be derived by high-level partial melting of metasomatized lithospheric mantle. The A-type granites is represented by Wengshan porphyritic biotite monzogranites (ca.230 Ma). The Hf and Nd isotopes and chemical compositions suggest that the early Mesozoic A-type granites were originated from the mixing of basement sediments-derived and underplating mantle-derived melts. The early Mesozoic syenites-monzonites-A-type granites association may be genetically related to the activity of strike-slip faults which resulted from far-field effect of collision between Indochina Block and South China Block.The late Mesozoic magmatisms in South China mainly consist of felsic volcanic and intrusive rocks. Different from the coastal area, the dyke swarms in the inland area emplaced much earlier, lacking detailed research. The Miaohou and Shanhou dyke swarms, comprise granodioritic porphyry, granitic porphyry and diabasic dykes, intruded into Neoproterozoic plutons. The granodioritic porphyry (127±2 Ma) exhibits similar geochemical characteristics of "adakitic" rocks, implying the inland "adakitic" rocks last from ca.175 Ma to ca.130 Ma. Most of the "adakitic" rocks are calc-alkaline, and their Sr-Nd-Hf-O isotopic compositions indicate that their protolith should be the Neoproterozoic magmatic arc rocks in the lower crust. The granitic porphyry with A-type affinity and diabasic dykes are coevally emplaced at ca.130 Ma. The widespread ca.130 Ma A-type granites and coeval diabasic dykes compose the NE-SW-trending inland bimodal magmatic belt in South China. The occurrence of mafic enclaves in some A-type granites and the Sr-Nd-Hf isotopic compositions imply an inadequate mixing between basement sediment-derived melts and coeval mantle-derived basaltic melts to produce the bimodal magmatisms. The transitition from "adakitic" rocks to bimodal magmatisms in the inland SE China indicates the change of tectonic regime. From Middle Jurassic to Early Cretaceous, with the increasing dip angle of northwestward subducting paleo-Pacific plate beneath South China, a local intra-plate extensional tectonic regime induced by far-distance stress at plate margins converts to a back-arc extensional regime induced by rollback of subducted slab in the inland area.