北阿尔金红柳泉早古生代枕状玄武岩及其大地构造意义

The Early Paleozoic Pillow Basalt in Northern Altyn,Western China and it Tectonic Implications

对北阿尔金蛇绿混杂岩带中红柳泉剖面的枕状玄武岩进行了岩石地球化学研究,主元素具有高钛高钠、低铝低钾的特点。其中SiO2含量为43.34%～52.55%,Na2O含量为3.97%～7.36%,K2O含量为0.27%～0.94%。TiO2含量为2.13%～5.49%,Al2O3含量为9.02%～13.77%,显示出细碧岩(Spilite)的地球化学特征。微量元素配分模式类似于洋岛或海山玄武岩(OIB/Seamount),不同于洋中脊玄武岩(MORB)。87Sr/86Sr比值为0.7043～0.7052,143Nd/144Nd比值为0.51253～0.51265,εNd(448Ma)为2.19～3.88,玄武岩浆主要来自于上涌的含有富集组分的软流圈地幔。综合考虑剖面上出现的沉积岩石组合,推测玄武岩形成于弧后盆地的海山环境。在阿尔金走滑断裂形成以前,北阿尔金与北祁连构成一个统一的弧后盆地系统,是原特提斯洋在中国境内的最北部边界,这对认识青藏高原和中国西部大地构造格局的演化具有重要意义。

This paper reports petrologic and geochemical study results of Early Paleozoic pillow basalt from ophiolite melange of the Hongliuquan section in the Northern Altyn Tagh,western China.The content of SiO2ranges from 43.34% to 52.55%;Mg-index vary from 39 to 58.They are high in TiO2（2.13%~5.49%）and Na2O（3.97%~7.36%）,low in Al2O3（9.02%~13.77%）and K2O（0.27%~0.94%）,which resembles to those of spilite.Their trace elements are different from MORB but comparable to the OIB or seamount basalt.They display varied 87Sr/86Sr（0.7043~0.7052）,143Nd/144Nd（0.51253~0.51265）.Our study interprets the pillow basalt originated from upwelling asthenospheric mantle with enriched components.Considering comprehensively the sedimentary rock assemblage on field profile,it is assumed that the magma extruded into the back-arc basin.System of back-arc basin in the northern Altyn Tagh was in the same unit as that in the northern Qilian Mountain,and likely it was the northernmost margin of proto-Tethys in China before the formation of the Altyn strike-slip fault.This is of great importance for understanding the evolution of the Qinghai-Tibetan Plateau and tectonic patterns of western China.