阿尔金山北东东向构造带内枕状玄武岩的发现及其大地构造意义

The Discovery of Pillow Basalts in the NEE-trending Altun Mountains Belt and Its Tectonic Implications

阿尔金山是青藏高原的北缘边界,控制了青藏高原北部乃至中国西部的大地构造-地貌格局。通过野外地质调查,在阿尔金山北东东向构造带中段发现了枕状玄武岩,这对研究和探讨阿尔金山乃至青藏高原北缘边界的形成及构造演化将提供重要线索。地球化学测试表明,枕状玄武岩表现为低Si O2、A12O3、P2O5和（Na2O＋K2O）,显示亚碱性玄武岩和安山岩/玄武岩之间的过渡类型,其中两个样品的Si含量较低,Fe、Ti含量较高（TFe O〉12%,Ti O2〉2%,TFe O/Mg O〉1.75）,显示出具有Fe-Ti玄武岩特征。稀土元素含量（37.34×10–6~42.14×10–6）、稀土配分模式和微量元素比值,以及Ti/100-Zr-Y×3、Ti/100-Zr-Sr/2、Zr/4-2×Nb-Y图解均表明该枕状玄武岩具有正常大洋中脊玄武岩（N-MORB）的特征;而与典型正常洋中脊玄武岩相比,样品低度富集大离子亲石元素,亏损Zr、Hf和Ti等高场强元素,在Th/Nb-Ce/Nb图中,所有样品落入弧后盆地玄武岩范围内,显示出弧后盆地构造环境。结合北东东向构造带内相关镁铁-超镁铁岩的特征,推测新发现的枕状玄武岩形成于远离海沟的弧后盆地内具有扩张脊的构造环境,其岩浆源区应为一个类似于N-MORB的亏损地幔。通过与阿尔金北缘东西向红柳沟—拉配泉构造带内出露枕状玄武岩的地球化学特征相对比,显示出两者具有完全不同的岩浆源区和构造环境,但与柴北缘地区的早古生代蛇绿岩具有相似的构造环境及岩浆源区,推测阿尔金山北东东向构造带内的枕状玄武岩可能是柴北缘古生代蛇绿岩套的一部分,后遭受阿尔金断裂带左旋走滑运动的构造肢解与拖拽,最终残留在北东东向阿尔金构造带内。

The Altun Mountains belt on the northern margin of the Tibetan Plateau plays a key role in the tectonic evolution and geomorphic formation of the Tibetan Plateau and western China. Pillow basalts were firstly recognized in the central segment of the NEE-trending Altun Mountains belt, which can furnish important clues to the interpretation of the structural evolution of the Altun Mountains belt and even the northern Tibetan Plateau.Pillow basalts are characterized by low SiO2, A12O3, P2O5 and （Na2O＋K2O） values which suggest a transitional type between sub-alkaline basalt and andestie/basalt. There are two samples with relatively lower silicon and enriched iron and titanium （TFeO〉12%, TiO2〉2%, and TFeO/MgO〉1.75）, which can be classified as Fe-Ti basalts. The chondrite-normalized REE patterns, trace element ratios and Ti/100-Zr-Y×3, Ti/100-Zr-Sr/2 and Zr/4-2×Nb-Y diagrams show N-MORB affinity. The Th/Nb-Ce/Nb diagram along with a slight enrichment of the Large Iron Lithosphile Elements and depletion of HFS elements Zr, Hf and Ti indicates a tectonic setting of back-arc basin. The coexistence of mafic-ultramafic rocks in surrounding areas implies that pillow basalts were derived from the N-MORB-like depleted mantle source in a spreading ridge of trench-distal back-arc basin. Further geochemical comparison shows that these basalts are significantly different from the basalts developed in the EW-trending Hongliugou-Lapeiquan belt in Northern Altun Mountains, indicating a different tectonic setting;nevertheless, these basalts are similar to basalts from the Early Paleozoic ophiolite belt in the North Qaidam Basin, suggesting that they were probably developed in similar Early Paleozoic back-arc basins. These newly-recognized pillow basalts in the central segment of the NEE-trending Altun Mountains belt are considered to be remnant parts of the ophiolite belt assembly in the North Qaidam Basin, resulting from the drag and destruction of the Altun Mountains strike-slip fault.