东昆仑南缘布青山构造混杂带哥日卓托洋岛玄武岩地球化学特征及构造意义

Geochemical characteristics of Gerizhuotuo OIB and its tectonic significance in Buqingshan tectonic mélange belt,southern margin of East Kunlun Orogen

东昆仑南缘布青山地区哥日卓托一带出露一套洋岛玄武岩,主要由枕状玄武岩与杏仁状玄武岩组成,空间上与厚层-块状碳酸盐岩及薄层状硅泥岩相伴产出。详细的地质、地球化学特征研究表明,哥日卓托玄武岩SiO2质量分数为44.51%～48.43%,MgO为4.67%～6.99%,Na2O为1.74%～3.72%,TiO2为1.29%～1.85%,与拉斑玄武岩平均值相似。主量元素特征表明其主体属于亚碱性系列拉斑玄武岩,与冰岛拉斑玄武岩或夏威夷群岛喷发早期的拉斑玄武岩特征相类似。哥日卓托玄武岩∑REE为（68.71～133.50）×10-6,LREE/HREE为3.55～5.17,（La/Yb）N为3.07～5.32,稀土元素球粒陨石标准化配分曲线图呈现轻稀土元素富集右倾型。原始地幔标准化微量元素蛛网图表现出向上大隆起的富集形态。玄武岩微量元素判别表明其形成于洋岛构造环境。岩石成因研究表明陆壳物质对其混染的可能性较小,岩石成分主要受熔融源区控制,并估算其为由地幔源区1%～3%的尖晶石二辉橄榄岩部分熔融的产物。结合前人资料认为该套洋岛玄武岩形成于早石炭世,进一步证明布青山古特提斯洋石炭纪时很可能是一个北缘发育复杂被动大陆边缘而布青山地区发育深水洋盆、洋岛/海山遍布的有限洋盆系统。

A suite of Oerizhuotuo basalt is exposed at Buqingshan tectonic mélange belt, southern margin of Eastern Kunlun Orogen, and is comprised mainly of pillow basalt and amygdaloidal basalt, which spatially associated with thick-massive carbonate rock and silicon mudstone. We have studied in detail the geological and geochemical characteristics of the Gerizhuotuo basalt and revealed that the SiO2 content of the basalt ranges from 44.51%- 48.43 %, MgO content ranges from 4.67%- 6.99 %, Na2O content ranges from 1. 74%- 3.72%, and TiO2 content ranges from 1.29%- 1.85%, which are similar to the average value of tholeiites. Moreover, the major elemental characteristics of the basalt can be attributed to tholeiitic basalt of subalkaline series, showing the same characteristics as the Iceland tholeiite hasalts or the earlier erupted tholeiites of Hawaii islands. For Gerizhuotuo basalt, the ∑REE ranges from （68. 71 - 133. 50）× 10-6, LREE/HREE ranges from 3. 55 - 5. 17, （La/Yb）N ranges from 3. 07 - 5.32, and the chondrite normalized REE patterns show dipped-right pattern suggesting enrichment of LREE. The primitive mantle normalized trace element spider diagram shows large uplift, which is also similar to the oceanic island basalt. In addition, some tectonic discrimination diagrams suggest that the Gerizhuotuo basalt was formed in oceanic island tectonic environment（OIB）. Petrogenesis research shows that the possibility of the continental crust contamination is small, by contrast, the rock composition was mainly controlled by the mantle source, which further proves that it is the product of partial melting of 1%- 3% lherzolite in mantle source. Combined with previously published data, the oceanic island basalt was formed in the Early Carboniferous, which further indicates that there was Paleotethyan oceanic basin developed in the Buqingshan area in Carboniferous time, in which many oceanic island/seamounts were developed.