对北祁连造山带前寒武纪基底物性、火山岩的 源区性质及找矿问题的分析和综述

摘 要]本文主要在先前一些研究者对北祁连山加里东造山带的构造、海相火山岩和块状硫化矿床研究所获成果和认识的基础上,重点从造山带的前寒武纪基底的物性特征,来分析和讨论所见不同时代地层中的变火山岩类的物源区及形成机制,进而提出一些区域找矿思路。所得结论主要包括:(1) 北祁连山加里东造山的真正基底应是从阿拉善或华北地块分裂出来的一套绿岩类型火山沉积建造,并以古元古宙的北大河群和龙首山群为代表;(2)北祁连造山带的基底从克拉通向大陆裂谷构造体制转换大致发生在早-中元古宙的分界时限(约1777Ma),并普遍以一套大陆裂谷型双峰式火山岩组合为典型特征;(3)不同时代或不同构造体制下形成的火山岩,各有其不同的物源区和成岩模式。具体说,在元古宙 绿岩类型的火山岩的源区可能是克拉通早期由原始玄武质下地壳转变来榴辉岩或基性麻粒岩相,进入大陆裂谷构造体阶段,便转换为一种幔-壳双层岩源区模型,而晚期的次火山岩类的源岩明显是已经过先期大量改造或改变了中上地壳物质作为物源;(4) 按大陆裂谷成矿体系并结合以往一些区域调查资料,提出托莱山西段的小龙口-九个青羊地段可能是个与海相火山岩有关的块状硫化物矿床潜在远景找矿地段。

An Analytical Overview of the Precambrian Basement Properties,Source Area Characteristics of Volcanic Rocks and Some Prospecting Problems in the North Qilian Orogenic Belt

Abstract:Based on the previous results about structure,marine volcanic rocks and massive sulfide ore deposits in the Caledonian Orogenic belt in North Qilian,focusing on the material characters of Precambrian basement,this study analyzed the source and formation mechanism of metamorphic volcanic rocks in strata of different ages,then suggested some prospects of this region. The conclusions are as follows. The actual basement of North Qilian Caledonian should be a set of green volcanic sedimentary rocks,which was separated from the Alxa splinter or the North China block,with Beidahe Complex- Group and Longshoushan Group in Paleoproterozoic being typical. Transformation of the basement of the North Qilian orogenic belt from craton to continental rift tectonics occurred in the boundaries of Early-Middle Proterozoic (about 1777Ma),with the typical feature of continental rift-type bimodal volcanic rocks. The volcanic rocks formed in different times or different tectonic systems can be explained by different source and diagenism-metallogenesis models. Specifically,the source of Proterozoic greenstone types volcanic rocks may be converted from the original basaltic lower crust to eclogite or granulite facies in early craton,then it was converted to a mantle-crust bilayer source model in continental rift stage,but late subvolcanic rocks was obviously modified by the middle and upper crust material that was reformed earlier or changed as a source or contamination. Finally,based on the continental rift system and in combination with previously regional survey data,we proposed that the Xiaolongkou-Jiuge Qingyang section is a potential ore prospecting area,which is associated with massive sulfide deposits related to marine volcanic rocks.