Petrogenesis and tectonic significance of early Indosinian A-type granites in the Xinxing pluton, southern South China

The Xinxing granitic pluton is a large batholith cropping out in southern South China with its long axis parallel to the regional NE-trending faults. We conduct the first comprehensive study of zircon laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb chronology and trace element chemistry, rock-forming mineral chemistry and whole-rock element and Sr-Nd isotope geochemistry for this pluton. Our new data indicate that the pluton was emplaced during the interval 234-232 Ma, i.e. early Indosinian rather than late Mesozoic as previously considered. The southern pluton consists of syenogranite and monzogranite, which are composed of K-feldspar, oligoclase, quartz and ferri-biotite. The northern pluton consists of monzogranite and granodiorite, which are composed of andesine, K-feldspar, quartz and magnesio-biotite. The southern granites have high SiO2 contents (72.5-79.2 wt%) and high FeOT/(FeOT + MgO) (T refers to total) ratios with low Mg# [atomic Mg/(Mg + Fe-T)]. They are enriched in rare earth elements and depleted in Sr and Ba with Zr + Y + Ce + Nb >350 ppm and 10,000 x Ga/Al >2.6. Their initial magma temperatures are unusually high (>886 degrees C). All those features strongly suggest an affinity with A-type granites. The northern granites have lower SiO2 contents (63.4-72.5 wt%) and lower FeOT/(FeOT + MgO) ratios as well as higher Mg# than the southern granites. They are also enriched in rare earth elements and depleted in Sr and Ba, with high Ga/Al ratios and high Zr + Y + Ce + Nb contents. Geochemical data and major element modeling confirm that the southern A-type granites were generated by the moderate degree (~49%) shallow (ca. 15-20 km depth) dehydration melting of the early Paleozoic granitoids triggered by intraplating of basaltic magmas. Subsequent moderate degree (~57%) fractionation of such crustal melts produced the more felsic granites in the southern pluton, whereas subsequent mixing of such crustal melts with about 8 to 48% intraplated mafic magmas generated the less felsic granites in the northern pluton. Interaction of mafic magmas with A-type crustal melts causes the northern granites to lose some diagnostic geochemical features of A-type granites such as high FeOT/(FeOT + MgO) ratios and to be transitional towards I-type granites. We further suggest their origin to have been related to the transtension of the regional NE-trending strike-slip faults coupled with the early Indosinian continental collisions.