Geochemistry, Geochronology And Its Geological Significances Of Early Cretaceous Magmatic Rocks Of Comei District In Tibet

A suite of bimodal volcanics assemblage composed of dacite and dolerite are distributed in the Sangxiu formation in the Longzi area, south Tibet. SHRIMP U-Pb zircon age of 134.9±2.3 Ma has been obtained from dacite, the geological age of the dacite is Early Cretaceous. Geochemical analysis indicates that the dolerites are enriched in Ti O2, TFe O and P2O5, HFSE loss is not obvious, and the dacite are characterized by Th、Nb、Zr、Y enrichment and P、Ti trough, obvious loss of Eu. These dolerites could be the products of interaction between lithosphere and mantle plume, and the dacite originated from the anatexis ensialic continental crust.The nearly EW-trending mafic dyke swarms are widely distributed in South Tibet, and the layered pyroxenite is located in the gabbro. Two weighted average ages of gabbro and pyroxenite(131.71±2.7Ma,2σ, MSDW=2.8; 135.0±2.4Ma,2σ, MSDW=2.1)based on SHRIMP zircon U-Pb dating have been obtained in this paper. The crystallization of the ultramafic rocks is 4 Ma earlier than the mafic ones. Element geochemical study shows that, in gabbro, Si O2 is 50.85%; Ti O2 is 1.45%; Mg O is 10.05%; TFe2O3 is 8.57%; Mg# is 71.33。In pyroxenite, Si O2 is 48.28%; Ti O2 is 2.02%; Mg O is 18.68%; TFe2O3 is 13.08%; Mg# is 78.76。Their K2 O content is rich(0.68 and 0.81), Eu loss is not obvious, the Nb, Ta and other elements are depleted, the patterns of REE distribution are similar to those of OIB. The formation of the mafic-ultramafic rocks is related to the mantle thermal events occurring in the same period, and may be contaminated by crustal materials in the rising process.Widely distributed in Longzi and Gudui area, south Tibet, the nearly EW-trending mafic rocks have not yet be constrained, this paper reported systematic research on geochronology and geochemistry. The SHRIMP U-Pb dating of zircons from three mafic samples yielded an age of 133-140.9Ma, representing the crystallization age of gabbro. According to the element geochemisty, the mafic rocks are calc-alkaline and alkaline series, and they have high Ti O2 with the average content of 3.08%. They are characterized by LREE enrichment and high ratios of LREE/HREE, and the LILE is slightly enriched,while the loss of HFSE is not obvious. All above have proved that the tectonic environment of the rocks is WPB.