| The Qinghai-Tibet plateau, the world’s "third pole", whose influence on global climate is not allowed to be ignored, has been in the public focus.The Yarlung Zangbo suture zone is located in the southern margin of Qinghai-Tibet plateau, which is the youngest suture zone in Qinghai-Tibet plateau and formed during the India and Eurasia collision process. The study object, Zhongba basalts, belongs to the accretionary wedge in the western segment of the Yarlung Zangbo suture zone. The Zhongba basalts expose in the southern melange belt of Yarlung Zangbo suture. The basalts from Zhongba Melange are in coexistence with radiolarian chert, siliceous shale and mudstone, which show a pillow structure.Based on the large scale mapping in study area, profile measured work and systematically sampling test, through the research on the basalt petrology and whole rock geochemical characteristics, we analyses the tectonic environment and the nature of the source area of magma formation, discuss the passive continental margin formation mechanism of the early cretaceous volcanic events.In this paper, the study object, Zhongba basalt, is the matrix of Tanwa Rock Group ((J3-K1)MT). Tanwa Rock Group ((J3-K1)MT) is a set of ophiolite melange, whose main petrography composition in the study area are basalt, siliceous rocks and siliceous shale. The matrixes are mainly basalt, siliceous rocks and siliceous shale, and the rock mass are mainly basalt rock and siliceous rocks. The weathered basalt surface is deep purple or fresh green. The basalts have relatively high P2O5 (0.34%-0.87%), lower A12O3 (13.48%～15.38%) and Mg# (15-35). The samples are characterized by high iron (Fe2O3=7.62%-13.16%) and titanium (TiO2=1.76%-3.11%), and are inadequate for silicon (SiO2=43.57%-51.99%), and can be classified as Fe-Ti basalts (ferrobasalts). And the basalts are alkaline. The Fe-Ti basalts are produced through high degree of high iron and poor silicon trend differentiation. The REE distribution patterns are similar to those of OIB, with LREE enrichment ((La/Yb)N=8.48～32.04), no obvious Ce and Eu anomaly. The basalts have typical geochemical characteristics to those of OIB, with large ion lithophile elements (LILE) (Ba, Th) and high field-strength elements (HFSE) (Nb, Ta, Zr, Hf)enriched, and no obvious Nb and Ta anomaly.Radiolarian stratigraphy study results show the main formation age of siliceous rocks in Tanwa Rock Group ((J3-K1)MT) is the early Cretaceous Berriasian-Aptian, which is accompanying Zhongba basalt. Therefore the age of Zhongba basalt is constraint to the early cretaceous.The geochemical characteristics suggest that the basalts were formed in an ocean island within an extensional tectonic setting, and the magma originated from enriched mantle composed mainly of garnet peridotite with 6%～10% partial melting. Geological signatures of whole-rock trace element indicate little or no appreciable crustal contamination for the samples. The Zhongba basalts could be derived from the mantle plume, which suggests that there could exist hotspot in the Neo-Tethys in early Cretaceous. |
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