Paper No. 49-6
Presentation Time: 9:00 AM-5:30 PM
GEOCHEMISTRY AND GEOCHRONOLOGY OF THE SAGA ALKALINE BASALT IN SOUTHERN TIBET
The Yarlung Zangbo Suture Zone (YZSZ) west of Saga, Tibet is divided into southern and northern branches. The Saga ophiolite, in the southern branch, is mainly composed of peridotite, gabbro, diabase and lava. Here, we report a suite of alkaline basalts south of the Saga ophiolite, which form a WNW-striking belt, 1-3 km wide. These basaltic lavas have a vesicular/amygdaloidal structure and a porphyritic texture with phenocrysts of plagioclase (15-20 modal %) and clinopyroxene (~5 modal %). The plagioclase is albitic and the clinopyroxene is titaniferous diopside (TiO2=1.5 wt. %). Based on their compositions these rocks are classified as alkaline basalts. They have SiO2 contents of 44.-53 wt.%, Al2O3 contents of 15-18 wt.%, MgO contents of 5-8 wt.%, and TiO2 contents of 2.6-3.5 wt.%. Overall, the lavas are characterized low SiO2 and high TiO2 and Al2O3, as well as enrichment in large ion lithophile elements (Rb, Ba, Th) and high field strength elements (Zr, Hf, Nb, Ta). Total rare earth elements average about 265 ppm, and chondrite-normalized REE patterns show strong enrichment in LREE [( Ce/Yb)N = 7.3 - 14.2],* similar to typical OIB. The geochemistry of these rocks suggests that garnet existed as a residual mineral in the source of the Saga alkaline basalts. In addition, the early stage of magmatism was marked by fractional crystallization of olivine, clinopyroxene and other mafic minerals. Based on incompatible element ratios and tectonic discrimination diagrams, the Saga basalts plot in the oceanic intraplate field. U-Pb dating of zircons by LA-ICP-MS yielded a wide range of ages. The minimum average age is 54.2±1.4 Ma, which can provide some important information about the evolution of the Tibetan plateau.
* YANG JingSui. E-mail:firstname.lastname@example.org