Abstract:

The trachy basalt was discovered in Lower-Middle Jurassic Sewa Formation in Renacuo area of Gaize, Tibet. The contents of SiO₂, Al₂O₃, Na₂O and K₂O are 45.62%-48.46%, 14.09%-16.51%, 2.68%-4.12%, 1.35%-4.73%, respectively, and the ratio of K₂O to Na₂O (K₂O/Na₂O) is 0.34-1.77. The rocks are rich in LREE, and poor in HREE with a weak positive Eu anomaly, as well as an enrichment of elements such as Rb, Ba, U and a depletion of elements like Sr, Yb, Th, Y as a whole. The geochemical characteristics indicate that they are generated in oceanic island environment within the oceanic slab, and that the magma is derived from enriched mantle without or with a little contamination of crust materials and sub-continental lithosphere. The oceanic island rock combination consists of basalt, basaltic gravel and limestone. Combined with the idea that Sewa Formation is seafloor fan sedimentary environment, it can be inferred that the Bangong-cuo-Nujiang ocean was mature oceanic crust in Early-Middle Jurassic.

Key words: Renacuo area, Sewa Formation, volcanic rock, geochemistry, tectonic implication