新疆喀喇昆仑阿然保泰二叠纪OIB型玄武岩地球化学特征及其地质意义

位于喀喇昆仑山喀喇昆仑断裂(塔什库尔干断裂)西侧的阿然保泰一带发育一套中二叠统灰岩-凝灰岩-枕状玄武岩地层。枕状玄武岩分布在北西向长约12km,宽约4.5km范围内。该套玄武岩枕状构造十分典型,岩石具气孔、杏仁状构造。玄武岩SiO2含量为44.14%～48.81%、TiO2为1.11%～1.83%,在Si2O-(Na2O+K2O)图中落入苦橄玄武岩、玄武岩和碱玄岩交界区,属于碱性岩石。稀土元素含量较高(54.40×10-6～139.9×10-6),Eu、Ce无异常,(La/Yb)N比值为2.87～6.29,配分模式为右倾型。大离子亲石元素富集(K、Rb、Ba等),但含量变化较大,高场强元素(Nb、Ta、Zr、Hf和P)相对亏损,Ti出现弱的负异常。玄武岩的地球化学特征显示阿然保泰玄武岩具洋岛玄武岩特征,源区为尖晶石二辉橄榄岩,其形成构造环境为板内拉张环境。阿然保泰OIB型玄武岩的发现证实了喀喇昆仑阿然保泰地区属于古特提斯主洋盆一部分。

Geochemistry and tectonic significance of Permian OIB-type basalts from Aranbaotai in Karakorum Mts., Xinjiang, NW China.

A suits of Middle Permian strata consists of limestone, tuff and pillow basalts outcrop in Aranbaotai area, locating the west of Karakorum fault (Taxkorgan fault) in Karakorum Mts. The basalts distribution in NW strike with 12km long and 4.5km wide in west Aranbaotai. The basalt has typical pillow structure, with pillow size from 0.5m to 0.8m, with stoma and amygdaloidal structure. The rock contents SiO₂ from 44.14% to 48.81%, TiO₂ from 1.11% to 1.83%, plotting in the boundary area of the picrobasalt, basalt, and tephrite basanite, belong to alkaline series. The rare earth elements (REE) contents vary from 54.40×10^-6 to 139.89×10^-6, with no Eu and Ce anomaly and (La/Yb)_N of 2.87 to 6.29. In the chondrite- normalized diagram, the patterns of the basalts are slight right dip type. The basalts contents variable large ion lithophile elements (LILE) (e.g. K, Rb and Ba), and relatively deplete in the high field-strength elements(eg. Nb, Ta, Zr, Hf and P), show Ti slight negative anomaly. The geochemical characteristics of the basalts show they have ocean island basalts, derived from the partial melting of spinel lherzolite. These Middle Permian basalts from Aranbaotai area in Karakorum Mts. maybe present the location of the main ocean basin of the Paleo-Tethys Ocean.