大陆碰撞造山带镁铁质岩浆岩记录俯冲古洋壳物质再循环

Mafic Igneous Rocks in Continental Collision Orogen Record Recycling of Subducted Paleo-Oceanic Crust

在大陆碰撞造山带中寻找消失的古洋壳再循环及其壳幔相互作用的证据,对理解从洋壳俯冲到陆壳俯冲化学地球动力学过程的转变,以及板块构造理论的发展具有重要意义.通过对桐柏-红安造山带晚古生代和晚中生代镁铁质岩浆岩的岩石地球化学特征进行总结,可以识别出俯冲古洋壳再循环的岩石学和地球化学记录.晚古生代岛弧型镁铁质岩石具有弧型微量元素特征和相对亏损的放射成因同位素组成,记录了俯冲古洋壳在弧下深度(80~160 km)的流体交代作用;而晚中生代洋岛型镁铁质岩石OIB型微量元素特征和亏损-弱富集的放射成因同位素组成,记录了俯冲古洋壳在弧后深度(>200 km)的熔体交代作用.这一定性的解释也进一步得到了定量计算的证实,其结果表明镁铁质岩浆岩中的不相容元素的含量以及放射性成因同位素的富集程度,主要受控于地幔源区中所加入的地壳组分的性质和比例.因此,碰撞造山带中的岛弧型和洋岛型镁铁质岩浆岩,分别记录了弧下和弧后深度的俯冲古洋壳物质再循环.

It is of great significance to search for the evidence of paleo-oceanic crust recycling in collisional orogens to understand the geodynamic transition from oceanic subduction to continental subduction,and also the development of plate tectonics.This is illustrated by the petrology and geochemistry of Late Paleozoic and Late Mesozoic mafic magmatic rocks in the Tongbai-Hong’an orogens.The fluid metasomatism of subducted oceanic crust at sub-arc depth(80-160 km)was recorded by the Late Paleozoic mafic rocks,which are characterized by the arc-like trace element features and depleted radiogenic isotopes,while the melt metasomatism of subducted oceanic crust at post-arc depth(>200 km)was recorded by Late Mesozoic mafic rocks,which are characterized by the OIB-like trace element features and depleted-weakly enriched radiogenic isotopes.These qualitative interpretations are further confirmed by quantitative calculations,which indicates that the content of incompatible elements and the enrichment degree of radiogenic isotopes in mafic igneous rocks are mainly controlled by the nature and proportion of crustal components in the mantle sources.Therefore,the recycling of the subducted paleo-oceanic crust at sub-arc and post-arc depths,are confirmed by arc-like and OIB-like mafic igneous rocks in the collisional orogenic belt,respectively.