Geochronology and geochemistry of the high magnesium and high potassium ultrabasic leucite basanite in northern Tibetan Plateau
Yang Di, Ding Lin
1. Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101;
2. University of Chinese Academy of Sciences, Beijing 100049
The high magnesium and high potassium ultrabasic leucite basanites were first found in the Silver Mountain region of northern Tibet.Leucite basanites are located in the south of A'nyemaqen-Kunlun-Muztag suture zone,covering an area of about 0.2 km2 and the age of the volcano eruption is about 5～4 Ma dated by Ar-Ar method.The phenocrysts are mainly composed of olivine(10%～12%),leucite(21%～27%),nepheline(13%～16%) and diopside(29%～31%),and the groundmass(～30%) is composed of fine grain of diopside,biotite,Fe-Ti oxide and glass.The composition of the olivine phenocrysts are Fo74-87Fa13-26,without obvious kink band,which suggests that the olivine phenocrysts were crystallized from magma rather than xenoliths captured from mantle.The estimated crystallization temperature ranges from 1 226 ℃ to 1 234 ℃.Leucite basanites contain SiO2<45%,MgO>10%,Na2O+K2O>8%,higher Mg#(68.84～70.80),high concentration of some compatible elements,such as Cr(149×10-6～156×10-6) and Ni(138×10-6～151×10-6).The ratios of 87Sr/86Sr(～0.708 6) and 143Nd/144Nd(～0.512 4) are within a typical EMII source area.Therefore,we argued that these ultrabasic leucite basanites were originated from the partial melting of the mantle rocks at the top of the asthenosphere,and probably contaminated by the subducted crust material.