Petrogenesis And Tectonic Implications Of Late Jurassic S-type Granites In The Southeast Of The Shi-Hang Zone,SE China

The tectonic settings in southeast China is continental arc-back-arc extraction related to subduction of paleo-Pacific plate,however,the location of the continental arc and the nature of those arc magmas are in dispute.Although various kinds of means to diagnose the genetic types and the nature of their origin,the common ways can't be sufficient in many situations,thus,comprehensive analyzing methods are need.On the basis of those issues,we conducted a combined study of field geology,petrography,zircon U-Pb ages and Lu-Hf isotopes,whole-rock major-trace elements and Sr-Nd-O isotopes for the late Jurassic Xinfengjie,Jiangbei and Dabu granitic intrusions in the southeast of the Shi-Hang zone and in the central to south Jiangxi province,Southeast China.The results not only place constraints on the origin of granites but also provide insights into the tectonic evolution of southeast China.The results show that the plutons we studied consist of two-mica granites and biotite granites,and the petrosal type assemblage is monzogranitesyenogranite-alkali-feldspar granite.All of those granites are intruded in late Jurassic:respective ages of two-mica granites and biotite granites are 148.2±1.4 Ma and 148.4±1.4 Ma for the Xinfengjie pluton,157.1±3.1 Ma and 159.1±2.3 Ma for the Jiangbei pluton;the age of two-mica granites of Dabu pluton is 158.2±2.5 Ma.All these granites are peraluminous and belong to high-K calc-alkaline series,and their Mg#are similar to the melts from pure crust.The two-mica granites in the three plutons have SiO2 contens of 74.73?78.22 wt.%,and are mostly strongly peraluminous.They have low Ga/Al ratios(mostly<2.6×1-4)and low contents of Zr+Nb+Ce+Y(<350×10-6).They have whole rock ?Nd(t)of-10?-4.5,whole rock ?18O more than 9.5‰,and zircon in situ ?Hf(t)of-15.7?-9.4.The biotite granites in the three plutons have SiO2 contents of 70.31?74.41 wt.%,and are mostly weakly peraluminous.They also have low Ga/Al ratios(mostly<2.6×10-4)and low contents of Zr+Nb+Ce+Y(<350×10-6).They have whole rock initial 87Sr/86Sr ratios of 0.7136?0.7166,whole rock ?Nd(t)of-10.0?-7.2,whole rock ?18O more than 9.5‰,and zircon in situ ?Hf(t)of-17.7?-10.9.In comparison with the biotite granites,the two-mica granites have higher contents of SiO2 and higher ratios of Al2O3/TiO2,Rb/Ba,Rb/Sr ratios,but lower contents of FeOT+MgO+TiO2 and lower ratios of CaO/Na2O,in association with lower zircon saturation temperatures.Our research indicate that the two-mica granites belong to S-type granites and were formed at lower temperature by partial melting of the meta-pelites in normal lower crust(<40km),without strongly fractional crystallization and mixing of crust-derived and mantle-derived magmas during their magmatism process;while the biotite granites also belong to S-type granites and were formed at higher temperature by partial melting of the meta-greywackes in normal lower crust(<40km),without strongly fractional crystallization and mixing of crust-derived and mantle-derived magmas during their magmatism process.These plutons intruded in a continental-arc tectonic regime,induced by the subduction of the paleo-pacific plate.