西秦岭甘加地区OIB型钾质拉斑玄武岩的发现:对西秦岭晚中生代大陆裂谷作用的启示

西秦岭晚中生代火山岩出露于青海省泽库县多福屯地区、甘肃省夏河县红墙和甘加地区。初步研究表明,甘加火山岩属于一套钾质拉斑玄武岩。该玄武岩富集REE、LILE及HFSE,但轻、重稀土元素分馏程度及不相容元素含量均略低于典型OIB和西秦岭晚中生代钠质碱性玄武岩。岩浆起源于软流圈释放的小体积富挥发份硅酸盐熔体交代形成的富集岩石圈地幔,并在上升中经历了较大程度的镁铁质矿物的分离结晶作用。岩石具有典型的陆内OIB成因特点,既不同于前人提出的甘加海山玄武岩,也不属于“二叠纪隆务峡-甘加蛇绿岩”组成部分,而与西秦岭晚中生代钠质碱性玄武岩均为大陆裂谷系OIB型岩浆作用的产物。甘加玄武岩可能具有比较复杂的岩石组合,跨越了晚古生代到晚中生代的一个较长时间范围。西秦岭晚中生代的大陆裂谷作用夭折于岩石圈拉张的早期阶段,它的出现及研究区广泛发育的近SN向或NW向断裂,可能是贺兰-川滇南北构造带与大型走滑断裂系复杂叠加并相互影响与改造的表现。

The discovery of OIB-type potassic tholeiitic basalts from the Ganjia Area in West Qinling: Implications for the Late Mesozoic continental rift of West Qinling

The Late Mesozoic volcanic rocks of West Qinling Area exposed in the Duofutun Area of Zeku County,Qinghai Province and the Hongqiang and Ganjia areas of Xiahe County,Gansu Province.Preliminary study showed that the Ganjia volcanic rocks belong to potassic tholeiitic basalts.The rocks are enriched in REE,LILE and HFSE,but the fractionation degree of LREE and HREE and the incompatible element contents are slightly lower than those of typical OIB and the Late Mesozoic sodic alkaline basalts of West Qinling.The magma of these basaltic rocks were originated from the lithosphere metasomatized by small-volume,volatiles-enriched,asthenosphere-derived silicate melts and had undergone a large degree of fractional crystallization of mafic minerals.These K-tholeiitic basalts,bearing the typical petrogenetic characteristics of intracontinental OIB,not only differ from the Ganjia seamount basalts suggested by previous authors,but also are not the member of the "Permian Longwuxia-Ganjia ophiolite".These rocks together with the Late Mesozoic sodic alkaline basalts of West Qinling are both the products of the OIB-type magmatism in the Late Mesozoic continental rift system of West Qinling.The Ganjia basalts may have a more complex assemblage of different basaltic rocks,and have a longer time span from the Late Paleozoic to the Late Mesozoic.The Late Mesozoic continental rift of West Qinling died young in the early stage of the lithospheric extension,whose occurrence and the widespread development of the near-SN or NW faults in the studied area,may be the performance of the interaction and reworking of the complexly superimposed Helan-Sichuan-Yunnan N-S trending tectonic belt with the large scale strike-slip fault system.