银根-额济纳旗盆地苏红图坳陷早白垩世火山岩地球化学特征与成因

Geochemistry Characteristics and Origin of Early Cretaceous Volcanic Rocks in Suhongtu Depression, Inner Mongolia, China

银根-额济纳旗盆地苏红图坳陷早白垩世火山活动强烈。火山岩的40Ar／39Ar年龄值为106．48±1．32～113．12±1．38Ma；里特曼指数、K20和K2O＋Na2O与Si02的关系表明该区火山岩为钾质碱性玄武岩；微量元素Nb、Ta富集的特点显示出苏红图火山岩具有洋岛玄武岩（OIB）特征，说明其源区可能主要是对流的软流圈地幔。火山岩的87Sr／86Sr和143Nd／144Na比值分别变化在O．705477-0．707098及0．512318～0．512480之间，其Nd、Sr同位素组成靠近第一类富集地幔（EMI）端元，并有向EMII延伸的趋势，显示本区火山岩的源区很大程度上受EMI的影响，并受EMII的微弱影响。此外，随着εNd的减少，P205、Rb、Sr、Ba及Zr逐渐富集，暗示苏红图火山岩没有经历明显的地壳物质混染。所以，苏红图火山岩是来自对流软流圈地幔的熔融产物。岩浆在快速上升过程中受到了EMI型地幔的混染，但是没有受到明显的地壳物质混染，经分离结晶形成苏红图钾质碱性玄武岩。

In the Early Cretaceous the volcanic activity was strong in the depression of Suhongtu in Inggen-Ejin Qi basin. The 440Ar/39Ar age was 106.48±1.32～113.12±1.38 Ma of the volcanic rocks through analysis test of the field samples. We analyzed the volcanic rocks＇ experimental data oxides, trace, REE and isotopic. Content of silica is 47.58%-56.28%, and the volcanic rocks are belong to potassic alkali basalt with Rittmann index value. Trace elements Ba, Nb, Ta, La and other large ion lithophile elements （LILE） and LREE are concentrated, with HREE losses and minor Sr loss. Under the conditions of MgO content is more than 4% incompatible elements Rb, St, Y, Zr, Nb, Ba, etc. With the abundance of a decrease in MgO content increased, while the compatible elements Sc, Ni, Co, Cr has a positive correlation with MgO, indicate that the early evolution of magma through a certain amount of olivine or clinopyroxene crystallization resulted in the separation of rising melt process or a high magma chamber has undergone a certain amount of mineral crystallization differentiation. The isotope 87Sr/86Sr and 143Nd/144Nd ratios are changed in 0.7054774-0.707098 and 0.512318-0.512480. In the Nd, Sr correlation diagram, the volcanic rocks of the Nd, Sr isotopic compositions close to the first category enriched mantle （EMI） end-member, and has the trend of extended to EM II shows the source region of volcanic rocks in this area mainly affected by EMI control, and the impact of the weak by the EMII, in addition, with the εNd reduction, P205, Rb, Sr, Ba, and Zr gradual accumulation, suggest that the volcanic rocks have not experienced significant crustal contamination of the material. In this period, the research area lithosphere was thinning period, with the lithospheric mantle thinning, account for the formation of pre-enrichment mantle asthenosphere upwelling quickly, in the upwelling process, the crustal material mixed dye rarely occurring, resulting in some decompression melting occurs. So the volcanic rocks in this area are the products of lithospheric thinning during early Cretaceous.