新疆北部地区二叠系幔源碳酸岩质喷积岩研究*
Permian mantle-derived carbonatite originated exhalative sedimentary rocks in North Xinjiang

新疆北部地区的二叠纪芦草沟期发育陆内裂谷型欠补偿湖盆,形成了中二叠统芦草沟组的重要烃源岩系。该烃源岩系中夹有一套以深源碎屑为特征的新型沉积组合,即薄层、纹层状地幔岩浆—热液喷流型沉积岩——喷积岩。本文报道的碳酸岩质喷积岩包括喷爆岩和喷溢岩2种类型。碳酸岩质喷爆岩厚0.5~4 cm,岩石以微粒晶质方解石矿物为碎屑,方解石碎屑颗粒呈大小不一、形态各异的角砾状,星散分布于黑色基质中,并显示粒序特征;碳酸岩质喷溢岩多呈2~4 mm厚的纹层和2~3 cm厚的薄层被夹于黑色含油沉凝灰岩、含油凝灰质泥晶白云岩等纹层岩中。纹层中的方解石具有典型的镶嵌粒状结构、环带结构。喷爆岩中方解石的流体包裹体测温高达435 ℃以上;C-O、Sr-Nd同位素显示碳酸岩质喷积岩中的方解石源自地幔,但受到湖水的较大影响。研究表明,本区的碳酸岩质喷积岩是一种源自地幔的碳酸岩质岩浆—热物质流体喷发、溢流入湖底,与湖水混合沉积而成的一类新型沉积岩。
In the Permian Lucaogou epoch, an underfilled intracontinental rift basin was developed in the North Xinjiang, which yields an important source rock of the Lucaogou Formation. A new series of sedimentary rocks that are laminated, thin bedded, carbonate rick and mantle-derived were observed in the source rock and termed as Exhalative Sedimentary Rocks (ESR). The carbonatite ESR in this paper include two types, Effusive Rocks and Explosive Rocks. The thickness of carbonatite Explosive Rocks ranges from 0.5~4 cm. Fine-grained calcite clasts distribute dispersedly in black matrix with various sizes and shapes and display normal grading. The calcite carbonatite originated Effusive rocks are characterized by 2~4 mm laminae and 2~3 cm thin bed, and are intercalated within oil-bearing sedimentary tuffs and tuffaceous dolostones. The calcites represent granular mosaic and zonal structure; homogenization temperatures of fluid inclusions reaches to 435℃. C-O and Sr-Nd isotope compositions indicate the calcite in canbonatite ESR was originated from mantle yet has been largely influenced by interaction with lake water. This study shows that the carbonatite ESR are the product of interaction between mantle-derived carbonatite melt/fluid and fresh lake water