攀西古裂谷内龙帚山玄武岩—碱玄响岩建造

Late Permian Basalt-Phonolite Suite from Longzhoushan in the Panxi Rift Zone

在川、滇、黔三省晚二叠世峨眉山玄武岩省的中部,产于攀西古裂谷内的龙帚山火山岩建造自下而上有4个喷发旋回:拉斑玄武岩、碱玄响岩、钾粗面玄武岩以及拉斑玄武岩。上下两个玄武岩旋回的岩性与该裂谷带以外大面积展布的高钛玄武岩岩性相同;而其碱玄响岩旋回主要由响碱玄岩—碱玄响岩—响岩岩系组成,夹粗安岩和安粗岩层,此旋回厚达千米,类似于非洲肯尼亚裂谷带的高原溢流响岩岩系。龙帚山晚二叠世火山岩建造的层序和岩石组合不同于古裂谷带外正常的大陆板内暗色岩喷出相的层序和岩浆演化规律,可能是地幔热柱和裂谷背景下岩石圈拉张复合作用的结果。

In the Panxi rift zone, the Late Permian volcanic formation in the Longzhoushan area is composed of four volcanic cycles; the lower tholeiitic basalt member, the tephri-phonolite member, the trachyt-basalt member and the upper tholeiitic basalt member. Both the lower and upper basalt members are similar to the Emeishan high-Ti flood basalts which occur outside the Panzi rift zone. On the other hand, the tephri-phonolite member with a thickness over 1000 m includes phono-tephrite, tephri-phonolite, phonolite, latite and trachyte, and is lithologically similar to the plateau flood phonolite in the Kenya rift zone, Africa. It is proposed that the lithological association and sequence observed at Longzhoushan may have resulted from the composite action of mantle plume and continental rifting.