Geochemical and chronological studies of the alkaline basalt in Saga along the Yarlung Zangbo suture zone, Tibet

Abstract: Located in the central part of Yarlung Zangbo Suture Zone (YZSZ), the Saga ophiolite is mainly composed of peridotites, gabbro, diabase and lava. This paper reports a suite of alkaline basalts to the south of the Saga ophiolite, which extends in NWW direction and crops out 1~3km in width. This suite of basaltic lavas has vesicular/ amygdaloid structure and porphyric texture with phenocrysts of plagioclase (15 modal %-20 modal.% in content) and clinopyroxene (~5 modal % in content). The plagioclase is albite and the clinopyroxene is titaniferous diopside (TiO2=1.46wt.％). Based on the composition and TAS diagram, the authors hold that the lava belongs to alkaline basalt. The lava shows the characteristics of low SiO2, high TiO2 ,K2O and Al2O3 content as well as enrichment of large ion lithophile elements ( LILE ) (Rb, Ba, Th ) and high field strength elements ( HFSE ) ( Zr, Hf, Nb, Ta ). The rare earth elements (REE) content of the basalt is higher (∑REE=266.93×10-6), with a strong fractionation between LREE and HREE （( Ce/Yb )N = 7.30-14.18）. The chemical characteristics are similar to features of continental rift volcanic rocks. This study shows that garnets exist as residual minerals in the source. In addition, at the early stage of magmatism, there existed crystallization fractionation of olivine, clinopyroxene and other mafic minerals. U-Pb dating of zircons by LA-ICP-MS yielded a wide range of ages. The youngest group has average age of (54.2±1.4)Ma，with εHf（t） being -11-+1.7, and has the Mesoproterozoic two-stage model age, indicating that these zircons were captured from the crust and the lava had not formed until early Eocene.