Geochemical Characteristics And Genesis Of Early Triassic Ultrapotassic Stronglyperaluminous Granites In Hainan Island

Now,there are still much debates on the perception of tectonic evolution at Triassic?Indo-China epoch?in South China?including Hainan Island?.Because of the ultrapotassic rocks are rare,and these rocks are key evidence for defining the tectonic setting at Triassic,therefore,the discover of the ultrapotassic rocks are very important for the tectonic setting study.Recently,the ultrapotassic strongly peraluminous granites were found in Hainan Island Maizhouling area when study team conducting geological survey,for these granites had special petrological geochemistry property,finally the team collect several groups granites samples,and analysis on the samples' petrological and geochemical property.Their zircon SHRIMP U-Pb ages are 247.5±6.1 Ma and 248.43±5.9 Ma,respectively,and these ages indicate the granites are formed in early Triassic Period.The ultrapotassic strongly peraluminous granites are mainly composed by granite-porphyry,and their SiO₂ content high?73.41%⁷5.04%?,alkali-rich?Na₂O+K₂O =7.41%¹0.44%?,K₂O content especially high?7.31%¹0.20%?,potassium?K₂O/Na₂O ? 40 >>3?,belong ultrapotassic rocks;the Aluminum saturation index?A/CNK>1.1?,belong?strongly?peraluminous rocks;in Harker diagram,it can be seen most oxide related with SiO₂ in correlation diagram,TiO₂,MnO,FeOT and Na₂O have showed a good negative correlation with SiO₂,Al₂O₃ showed positive correlation in diagram,the diagram showing that biotite,Ti-Fe oxide and plagioclase occur fractional crystallization.The granites samples REE ?REE=281.86×10^-6519.05×10^-6,??LREE?/??HREE?12.28²0.32,showing the REE-deficit and LREE-riched type of dipping steeply to right in REE distribution curve.?Eu =0.275⁰.498?average 0.31?,negative Eu anomaly is obvious,indicating that the plagioclases occur fractional crystallization.In the ratio spidergrams of trace elements,LILE elements Rb,Th,U and K obvious enriched,and Ta,Nb,Ti and P relatively deficit,the spidergram shows that apatite and ilmenite occur fractional crystallization,especially Ti significant depleted showed granites were related to subduction zone.Because of Rb/Sr was too high made the I_Sr could not be used Isotope geochemistry indicated the source rocks of granites were not composed by single substance,and the source magma was not mixed uniformity either,all the evidence indicated that the crust subducted in the mantle and mixed in it composed to mixing magma.