黔、桂中三叠世浊流沉积的形成环境

黔、桂中三叠世浊流沉积的物源非西非东,而是来自桂东南岛弧,根据在于:(1)浊积岩的时代由东南向西北渐次变新;(2)东南发育近源相,西北发育远源相;(3)桂东南火山岩发育,近火山口特征明显,而桂西北却只有较薄的夹层,远火山口特征显著;(4)近源槽模 NNE 向,远源近于东西;(5)槽模规模由东南向西北明显变小。浊积岩石英含量43—59%,SiO 70.46—72.04,K₂O/Na₂O<1,正属于 Crook(1974)所划分的中等石英含量杂砂岩范畴,作为安第斯型大陆边缘海沟型的代表。其实,此一区间范围较广,应进一步细分,含量偏低者与洋壳关系密切,应属海沟型;偏高者与陆壳关系亲近,应属弧后盆地型。此区浊流沉积正属后者而非前者。广布70000km²,厚达3000—5000m 的浊流沉积,北为边缘礁堤所限,南受火山岛弧所控,沉积物中既有中、酸性岩浆岩的踪迹,又有微型扩张地幔上溢的基性、超基性侵入岩体,正是弧后盆地的沉积特征。

FORMATION ENVIRONMENT OF TURBIDITY CURRENT DEPOSITS IN THE MIDDLE TRIASSIC OF GUIZHOU AND GUANGXI

Turbidity current deposits developed widely in Guizhou and Guangxi du-ring the Middle Triassic.Terrigenous materials came from island are in thesoutheast part of Guangxi,this is based on that:1.The turbidites in the southeast occured anterior to that in the northwest;2.The proximal facies in the southeast changes into distal facies in the northwest;3.Volcanic rocks decreasein thickness and even pinch out from southeast to northwest;4.The flow direc-tion of the flute castes point to NNE but not point to EW in the proximal facies;5.The scale of the flute castes changes gradually small from southeast to northwest.The quartz contentes are from 43 to 59%,SiO