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

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.