Discovery of a Permian quartz syenitic porphyritic dyke from the Tarim basin and its tectonic implications.

Huge Permian basalts and igneous rocks related(260～292 Ma),occurred in the Tarim Basin,Xinjiang are mainly composed of basalts,diabases,basaltic andesites,uhramafic rocks and syenites.This study is the first report of the quartz syenitie porphyritic dyke occurred in Shuigongtuan of Bachu County,Tarim Basin.The quartz syenitic porphyry in this dyke is metaluminous (A/CNK＜1),and has SiO_2 of 66～67%,enriched in K_2O+Na_2O(10～11% )and K_2O/Na_2O(0.8～0.9),and low Mg/(Mg+ Fe)ratios.The rocks are enriched in LILE(Ba、Rb)and HSFE(Zr、Nb、Y),Ga/Al ratios and total rare earth elements(631～734×10~(-6)),high LREE/HREE ratios,and having negative Eu anomalies.The chemical characteristics and tectonic discriminative diagrams show that the rocks have geochemical affinity with A- type granites.Low Y/Nb(0.4)＜1.2,and enriched in LILE and Nb evenness -enrichment in the spider diagram indicates typical within-plate environment and a source from mantle,which may be derived from hot basic magma by fractional crystallization.Compared with the Xiaohaizi syenites,the quartz syenitic porphyry has almost the same geochemical characteristics,and they may he derived from the same source.The eoncordia age of 277Ma 4 Ma was obtained from the Xiaohaizi syenitic body by the SHRIMP U-Pb zircon age dating in this study,indicating that the intrusive age of the Xiaohaizi syenitic body is approximately 277Ma.Based on the present study,the Shuigongtuan quartz syenitie porphyritic dyke and Xiaohaizi syenitic body probably formed during Early Permian(277Ma),and represented the products of within- plate environments,indicating the end of the last large magmatic thermal event happened in the Tarim Basin.