陕西西乡群火山-沉积岩系形成构造环境:火山岩地球化学约束

Tectonic settings of the volcano-sedimentary rock association from Xixiang Group, Shaanxi province: volcanic rock geochemistry constraints.

详细的地球化学解析表明,西乡群火山岩系具有Nb、Ta亏损等弧岩浆系列特征,其中白勉峡组火山岩与一个部分亏损的地幔源区有关,形成于洋内岛弧(或初始岛弧)的大地构造环境。而局限于柳树店-孙家河-三郎铺一带分布的孙家河组、三郎铺组和大石沟组火山-沉积岩系可能为一外来构造移置体,形成于泥盆-石炭纪,与一个活动大陆边缘及加积增生岛弧型地壳裂陷形成的初始弧间盆地环境有关,并在区域上与勉略缝合带相关联,原应为该缝合带的一个组成部分。

The detail research indicates that the volcanic rock association of Xixiang group exhibits distinctive geochemical characteristics of volcanic-arc origin. Basalts,andesites and rhyolites in the studied area show remarkable depletion of elements Nb and Ta. Among the Xixang group, the Baimianxia basalt association might be related with a partial depleted mantle source and formed in a within ocean island-arc ( immature island-arc) tectonic setting. And the Sunjiahe, Sanlangpu, Dashigou volcanic-sedimentary rock series that distributed in Liushudian-Sunjiahe-Sanlangpu area should be a tectonic allochthon. It was formed during Devonian-Carboniferous period and developed in an active continental margin, which was the result of the oceanic crust subduction of Mianlue limited oceanic basin and /or developed in a special tectonic setting which belongs to a rift valley within an island-arc type crust. This tectonic allochthon should have close relationship and initially connected with the Mianlue suture zone, south Qinling.