阿拉善地块早古生代岩浆作用成因及构造意义

阿拉善地块早古生代岩浆作用的成因研究，对理解阿拉善地块与古亚洲洋相互作用过程至关重要。本文在阿拉善地块中部新识别出一中志留世花岗岩体（噶顺花岗岩），其锆石U-Pb年龄为432Ma，以高Sr低Y为特征，属弱过铝质、中钾-高钾钙碱性花岗岩，ε_Hf（t）=-8.8~-19.4，形成于古老下地壳岩石的部分熔融。本文同时总结了阿拉善地块其他晚奥陶世-早泥盆世岩浆岩的成分特征，发现阿拉善地块早古生代岩浆岩在成因上可分为两类：类型Ⅰ，侵位于晚奥陶世-早中志留世，为典型幔源弧岩浆岩，形成于俯冲流体交代地幔楔的部分熔融；类型Ⅱ，侵位于中晚志留世-早泥盆世，普遍高Sr低Y，形成于古老中基性地壳岩石的部分熔融。纵观阿拉善地块整个早古生代的岩浆作用，在晚奥陶世-早中志留世→中晚志留世-早泥盆世→中晚泥盆世期间，阿拉善地块的岩浆作用从幔源弧岩浆岩过渡到壳源高Sr低Y型花岗岩再到岩浆作用逐渐消失，反映了阿拉善地块陆缘弧从相对伸展环境向挤压弧的转变。这一岩浆作用演化记录了区域构造作用从典型洋陆俯冲到俯冲作用逐渐减弱（直至停止）或者俯冲角度（从陡俯冲向平俯冲）的转变过程；总体上，阿拉善地块早古生代处于越来越挤压的动力学背景之中。

Origin and tectonic significance of the Early Paleozoic magmatism in the Alxa block

Genetic studies on the Early Paleozoic magmatism in the Alxa block is crucial for understanding the interaction processes between the Alxa block and the Paleo-Asian Ocean. We have newly identified a granite intrusion (the Gashun granite) emplaced in Middle Silurian, with a zircon U-Pb age of 432Ma. The samples collected from this granite are weakly peraluminous, belonging to the medium-high-K calc-alkaline series, with high Sr/Y ratios and εHf(t)=-8.8~-19.4, which suggests that the Gashun granite should be formed by melting of ancient lower crustal rocks. Based on the major and trace element compositions of Late Ordovician to Early Devonian magmatic rocks in the Alxa block, two genetic types of magmatic rocks have been identified: Type Ⅰ, emplaced during Late Ordovician to Early and Middle Silurian, are typical arc magmatic rocks formed by partial melting of mantle wedge metasomatized by fluids from subducting slab; Type Ⅱ, emplaced during Middle and Late Silurian to Early Devonian, are characterized by high Sr/Y ratios resulted from partial melting of ancient intermediate to mafic crustal materials. The magmatism in the Alxa block transitioned from mantle-derived typical arc magmatic rocks during Late Ordovician-Early and Middle Silurian to crust-derived high Sr/Y-type granites during Middle and Late Silurian-Early Devonian, and then gradually disappeared, reflecting that the Alxa block transformed from a relatively extensional continental arc to a compressional arc. This magmatic evolutionary process recorded that either a subduction of Paleo-Asian Ocean beneath the Alxa block were gradually weakening and ceasing, or a transition of the subduction angle from steep to flat happened during the Late Ordovician to Early Devonian. In general, the Alxa block was under an overall increasingly compressional dynamic background in Early Paleozoic.