梵净山群沉积地质特征与原型盆地分析

梵净山群出露于江南造山带西北端,为一套浅变质火山、碎屑岩系,其上亚群主要由深海、半深海浊积岩系组成,自下而上划分为铜厂组、洼溪组和独岩塘组。铜厂组主体由浊积扇相和盆地相组成,洼溪组则主要由浊积扇扇中、扇根亚相组成。进一步的沉积相序研究揭示,自回香坪组至铜厂组、洼溪组,沉积环境总体呈一先海进、后海退的沉积旋回,显示裂陷扩张过程。铜厂组和洼溪组浊流成因的小型斜层理位态研究揭示,前积层优选方位指向北、北东,证实沉积物来自其南,向北扩散。浊积砂岩骨架颗粒统计结果表明,石英含量占23%~34%,且主要为单晶石英,而长石含量占14%~23%,岩屑含量占39%~50%,岩屑以火山岩屑为主。骨架颗粒统计图解揭示其物源为岩浆弧。基于上述,并结合回香坪组火山岩大地构造属性研究,提出并确定梵净山群上部碎屑岩系的原型盆地形成于弧后裂陷,板块构造环境为初始边缘海。

Sedimentary Features and Basin Prototype of Fanjingshan Group

The Fanjingshan Group, outcropped at the northwest end of the Jiangnan orogenic belt, is a set of low-grade metamorphic volcanic rocks and clastic rocks. The upper part of the Fanjingshan Group could be recognized as turbidite facies and deep marine (bathyal) basin facies. It was divided into Tongchang, Waxi and Duyantang formations. The Tongchang Formation mainly consists of turbidite facies and basin facies, the Waxi Formation is mostly composed of middle fan and outer fan of turbidite facies. The present study of sedimentary facies sequence has revealed that the sedimentary environments from the Huixiangping Formation to Tongchang Formation and to Waxi Formation are of a sedimentary cycle from marine transgression to marine regression. The sediment diffuse of the Tongchang and Waxi formations is in north or northeast direction, suggesting that its source might have been from the south. The particle statistical results show that quartz makes up 23%~34%, consisting mainly of monocrystal quartz. Feldspar makes up 14%~23%, with clasts up to 39%~50%, including sedimentary and volcanic clasts. The source of the Fanjingshan Group was magma arc. In combination with the results obtained and the geochemical analysis of the volcanic tectonic environment of the Huixiangping Formation, it could be concluded that the basin prototype of the clastic rocks of the upper Fanjingshan Group was a initial marginal sea, formed in a back-arc extension.