中亚造山带中新生代壳幔相互作用特征与过程——新疆北部幔源岩浆岩系对比研究

Mesozoic and Cenozoic crust-mantle interaction in the Central Asian Orogenic Belt:A comparative study of mantle-derived magmatic rocks in northern Xinjiang

新疆北部中新生代幔源岩浆岩多位于大型走滑断裂带或者裂谷带附近,岩石系列均具有高的TiO_2含量,富碱、高度富集不相容元素和轻稀土(LREE),ε_(Nd)(t)值低正值到负值,(^(87)Sr/^(86)Sr)_i值较高,不相容元素比值La/Nb和Nb/U显示中新生代玄武岩浆在形成过程中的壳幔作用明显不同于古生代造山过程,岩浆来源于类似OIB源的富集地幔。而新疆北部地区的二叠纪幔源镁铁-超镁铁岩来源于亏损地幔,陆壳物质参与了岩浆的形成,源区可能混入了早期的弧组分或俯冲的洋壳。由于古生代多个洋盆闭合和增生造山作用致使中亚造山带的岩石圈不断增厚,于古生代晚期发生拆沉去根作用,软流圈亏损地幔上涌形成二叠纪镁铁-超镁铁岩系,而拆沉的岩石圈以及俯冲下沉的洋壳及其沉积物,在地幔环境熔融交代改造原始地幔,使中亚地区的地幔自古生代晚期以后逐渐转换为整体富碱、富含轻稀土(LREE)和不相容元素的富集地幔。被改造了的富集地幔在中新生代沿裂谷带或大型走滑断裂薄弱带上涌,形成了中新生代幔源岩浆的喷发和侵位。

Meso-Cenozoic mantle-derived magmatic rocks mainly occur near rift belts and major strike slip faults in northern Xinjiang.They are characterized by high TiO_2 alkali contents and enrichment in incompatible elements and light rare earth elements (LREE),and their La/Nb and Nb/U ratios show that the Meso-Cenozoic basaltic magma underwent typical crust-mantle interaction as ascending to the surface,while low positive to negativeε_(Nd)(t) and high(^(87)Sr/^(86)Sr)+i values indicate a source in the enriched mantle similar to that for OIB.Permian mafic-ultramafic rocks in northern Xinjiang were originated from partial melting of depleted mantle contaminated by arc and continental crust as well as subducted oceanic crust In the central Asian Orogenic Belt,the Late Paleozoic orogenies caused by nearly concurrent closure,of branches of the Paleo-Asian Ocean between terranes resulted in the thickened lithosphere,and then delamination of lithosphere and upwelling of asthenosphere generated Permian mafic-ultramafic magma. Therefore,the mantle in Central Asian Orogenic Belt was strongly modified by metasomatic melts from the delaminated lithosphere or ancient subducted oceanic crust and overlying sediments during the Paleozoic,making the mantle enriched in alkali,light rare earth elements(LREE) and incompatible elements.As a result,the Meso-Cenozoic mantle-derived magmatic rocks intruded or erupted along rift belts and major strike slip faults exhibit the geochemical signatures of the enriched mantle.