Early-Cretaceous Syenites and Granites in the Northeastern Tengchong Block, SW China: Petrogenesis and Tectonic Implications

Whole-rock major and trace element and Sr-Nd isotopic data, together with zircon LA ICPMS in-situ U-Pb and Hf isotopic data of the syenites and granites in the Tengchong Block are reported in order to understand their petrogenesis and tectonic implications. Zircon U-Pb data gives the emplacement ages of ca. 115.3±0.9 Ma for syenites and 115.7±0.8 Ma for granites, respectively. The syenites are characterized by low SiO2 content（62.01–63.03 wt%） and notably high Na2O content（7.04–7.24 wt%） and Na2O/K2O ratios（2.02–2.10）, low MgO, Fe2O3 T and TiO2, enrichment of LILEs（large-ion lithophile element） such as Rb, Th, U, K, and Pb) and obvious depletion HFSE（high field strength element; e.g. Nb, Ta, P, and Ti） with clearly negative Eu anomalies（d Eu=0.53–0.56）. They also display significant negative whole-rock εNd（t） values of-6.8 and zircon εHf（t） values（-9.11 to-0.27, but one is +5.30） and high initial 87 Sr/86 Sr=0.713013. Based on the data obtained in this study, we suggest that the ca. 115.3 Ma syenites were possibly derived from a sodium-rich continental crustal source, and the fractionation of some ferro-magnesian mineral and plagioclase might occur during the evolution of magma. The granites have high SiO2 content（71.35–74.47 wt%）, metaluminous to peraluminous, low Rb/Ba, Rb/Sr, and Al2O3/（MgO+FeOT+TiO2） ratios and moderate（Al2O3+MgO+FeOT+TiO2） content. They show low initial 87 Sr/86 Sr（0.703408 to 0.704241） and εNd（t） values（-3.8 to-3.5）, plotted into the evolutionary trend between basalts and lower crust. Hence, we suggest that the granites were derived from the melting of mixing sources in the ancient continental crust involving some metabasaltic materials and predominated metasedimentary greywackes. Together with data in the literatures, we infer that the Early Cretaceous magmatism in the Tengchong block was dominated by magmas generated by the partial melting of ancient crustal material, which represent the products that associated to the closure of Bangong-Nujiang Meso-Tethys.