The Geochemical Characteristics And Tectonic Significance Of Oceanic Island Basalts In Mayer Kangri, Northern Qiangtang

Mayer Kangri oceanic island basalt is exposed in the middle of LongMuCuo-ShuangHu suture zone：Mayer Kangri area in Qiangtang of Tibet, the region’s mainlithology is the Carboniferous-Permian accretionary complex, and Rocks symbiosisedwith basalt rocks are metamorphosed sandy, silty phyllite, metamorphosed quartzsandstone and crystalline limestone. Basalt affected by the tectonic stress was east towest striped, a strong internal deformation and with plate processing or schistose.With a detailed analysis to basalt petrology, mineralogy, geochemistry andgeochronology of the area, the results showed that:Basalt in this article have the characteristics of high content of TiO₂、TFeO、P₂O₅（average respectively3.40%,10.62%,0.44%）, high total amount of REE,strongfractionation of LREE and HREE（the total amount of REE is177.72x10-6-425.20x10-6, averaged （La/Yb）Nis13.44）, it enriche in LILE（Rb, Ba）, and HFSE（Zr, Hf, Th）,the geochemical characteristics and trace element patterns is similar to the standardoceanic island basalt （OIB）.Trace element tectonic discrimination diagrams also show that the Mayer Kangribasalts was formed in the oceanic island environment within the oceanic slab. Theratio of trace elements La/Nb, Zr/Nb, Nb/U, Ce/Pb and the graphic of（La/Nb）PM-（Th/Ta）PMdisplay Magma source contaminated by crustal material indifferent degrees, but the degree of contamination is not deep; Diagram ofCe/Y-Zr/Nb shows original magma may come from the enriched mantle and it may berelated with the melting of garnet lherzolite.Dating results of zircon SHRIMP U-Pb of boudin-output plagiogranite that wasfound in basalt sandwich is465±2Ma, it shows that ocean island basalts should beformed in the Middle Ordovician period at least.Tectonic discrimination diagrams and dating results show that The Longmucuo-Shuanghu suture zone is already a mature ocean environment in the early Paleozoic（Ordovician） at least, basalt in the Mayer Kangri should be the product ofProto-Tethys subductionthe, and on behalf of the remnants of the Proto-Tethys oceanic crust. Along The Longmucuo-Shuanghu suture zone is intermittentlyexposed ophiolite of Late Paleozoic, gabbro of Carboniferous-Permian and eclogiteand blueschis in Late Triassic, combined with the oceanic island basalts of the earlyPaleozoic, it indicate that the Longmucuo-Shuanghu suture zone had already be amature ocean basin in the Middle Ordovician period at least, and may experiencethe subduction and evolution of Proto-Tethys and Paleo-Tethys.