北祁连山南侧阿拉斯加型岩体的发现及地质意义

在北祁连山南侧祁连县境内发现侵位于寒武-奥陶系火山-沉积岩系中的阿拉斯加型岩体。岩体以辉长岩为主,超基性岩分布于其中并发育堆晶结构,遵循钙碱性演化趋势,是由同一岩浆经不同程度分离结晶形成的。扎麻什东沟岩体的ε_Nd(t)≈0,初始锶比值高,暗示形成于活动大陆边缘环境,来自受到俯冲作用影响的,EMII富集地幔源区。阿拉斯加型岩体与其大致同时代蛇绿岩的空间分布关系表明,在北祁连山古洋壳存在向南的消减事件。

THE FOUNDING OF ALASKA-TYPE MAFIC-ULTRAMAFIC COMPLEX FROM QILIAN COUNTY AND CONSTRAINTS ON QILIAN Mt. TECTONIC EVOLUTION

Several mafic-ultramafic complexes have been found from Qilian county, which intruded into Cambrian-Ordovician volcanic-sedimentary rocks. The Zhamashe gully and Bin gully pefidofite-gabbro complexes are concentrically zoned. The center of the complex is peridofite, with amphibole-clinopyroxene peridofite, amphibolite, and gabbro occurring progressively outward. The ultmmafic rocks have cumulative textures. Field, petrographic, and geochemical data indicate that the complexes are originated by crystallization differentiation of a calc-alkalic magma, which belongs to Alaska-type mafic-ultramafic complexes. Their ε_Nd(t) values are proximally equal to 0, while their (⁸⁷Sr/⁸⁶Sr)_i iratios are high, varying between 0.7067 to 0.7095. These isotopic data imply that the complexes were formed at active continental margin and derived from EMII-type mantle soures which enriched by ancient subduction material. The relative distribution places between these Alaska-type complexes and same period opholites suggested the existence of southward subduction in ancient Qilian Ocean during Caledonian period.