| Abstract:Caledonian granitoids of southern China is the first large-scale magmatic activity record in south China mainland since the Phanerozoic, a detailed study of these Granitoids can provide scientific constraints on the regional tectonic evolution. In this paper, the author took Baimashan, Xuehuading, Hongxiaqiao granitoids as examples, then discussed the scientific problems such as rock types, sources, tectonic-magmatism evolutions from their mineralogy, petrology, geochemistry, isotope geochemistry features.Baimashan, Xuehuading, Hongxiaqiao granitoids are biotite monzonitic granite, biotite granite and biotite granodiorite, biotite granodiorite respectively. TAS diagrams show that they are mainly granite and granodiorite, SiO2-K2O and A/NK-A/CNK diagrams show that these granites mostly are weakly peraluminous and high-K calc-alkaline series,(Na2O+K2O)-10000Ga/Al and A-C-F diagrams show that these granites are mostly S-type granites, with a small amount of I-type granites. Trace elements spider patterns show that they are enriched in large ion lithopile elements(LILE) and depleted in high field strength elements(HFSE).CaO/Na2O-Al2O3/TiO2diagrams show that all these plutons were produced by partial melting of the arenite. The δEu of Baimashan, Hongxiaqiao granitoids were0.60-0.88, higher than that of Xuehuading (0.47～0.49), shows that their regions had more plagioclase remained. The Zr/Hf(20.27～35.76) shows that their regions were mainly derived from the continental curst. Rb/Sr of Hongxiaqiao granitoids were0.45-0.50, shows its region was immature continental crust, while the Rb/Sr of Baimashan and Xuehuading granitoids were1.21～2.35, shows their regions were mature continental crust. Nb/Ta of Hongxiaqiao granitoids were8.77～12.07, shows that mantle materials were added into its region, while the Nb/Ta of Baimashan and Xuehuading granitoids were6.30～10.14, shows there was nothing added into their regions. Zircon U-Pb dating by LA-ICPMS show that Hongxiaqiao, Baimashan, Xuehuading plutons were intruded in449.5±4.5Ma,422.0±5.8Ma～411.8±9.5Ma,420.9±1.2Ma～402.6±4.6Ma respectively. There were some inherited zircons in Xuehuading granitoid, there ages were630Ma and590Ma. The model ages of two stages ranges of Xuehuading zircon Hf isotope (TDM2) from1.57to1.91Ga(average value is1.77Ga), This feature indicates that Xuehuading granite bodies originated from melting of early crustal materials formed in mesoproterozoic.The field geological characteristics and geochemical characteristics indicate that Hongxiaqiao pluton was derived from lower crustal partial melting, which cuased by A-type subduction collision between Yangtze and China plate, with basaltic magma underplating; while Baimashan and Xuehuading plutons were derived from crustal partial melting under a stress relaxation tectonic environment, which caused by collision between Yangtze and China plate.The features of inherited zircons and Hf isotope show that:From Neoproterozoic to Caledonian, there were two extensional stages (760-690Ma and590-533Ma) and two crustal micro-accretion stages (830-760Ma and690-590Ma). According to crustal micro-accretion scope, the boundary between Yangtze and China plate in Southern Hunan should be through the Xuehuading area and Yuechengling area. |
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