Geochemical Characteristics and Genesis of Permian OIB-type Volcanic Rocks in Saga Region,Tibet

The main types of Permian volcanic rocks are strongly altered basalt and altered stomatal-amygdaloid andesite. The study on petrology, lithochemistry, and geochemistryshows that these volcanic rocks have high Ti(basalt and andesite average in 3.16 and 2.15, respectively), FeOT(average in 12.21and 9.45, respectively) and low Mg (with the average values of 4.33 and 4.72, respectively) contents, which belong to alkaline series.The ΣREE, LREE/HREE and La_N/Yb_N of basalts are 210×10^-6~444.7×10^-6, 1.77~12.18 and 1.14~29.66, while the ones of andesites are 94×10^-6~386.1×10^-6, 4.02~24.4 and 4.19~57.16, respectively. The REE distribution curve shows a dipped-right pattern, while the trace element spidergram shows "uplift" type, with thecharacteristics of alkaline basalt and OIB. The tectonic environment discriminant diagram also indicates that these rocks belong to the oceanic island basalt (OIB), which areserved as the products of mantle plume or hotspot material differentiation,without any crustal contamination. The volcanic rocks are derived from the same source of Permian Tethys rift as the other Permian volcanics in this belt, testifying that the Tethys Himalaya began to rift before Permian.