Sedimentary Record of the Proterozoic Changchengian Volcanic Events in Beijing and Its Neighbouring Area
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摘要: 北京及邻区的中元古代大红峪组中发育着与火山事件有关的沉积相,主要包括两种基本类型:火山源硅质—陆源砂—碳酸盐岩混积相和火山碎屑重力流沉积,后者又可分为火山碎屑基浪沉积相和火山—沉积角砾碳酸盐岩混积相。初步分析表明,大红峪期大量的硅质沉积主要来自同期的水下火山活动;火山碎屑基浪沉积与火山口内残余热气冲破熔岩封堵而爆发泄出,造成一定范围的海水涌浪作用有关。Abstract: The volcanic rocks of the Proterozoic Dahongyu Formation mainly distribute in Pinggu County of Beijing and Jixian county of Tianjin about 600 km 2 with 718 m and 490 m of the maximum thickness in the both areas. More than 40 paleo carters were survived in the areas.The types of volcanic rocks mainly are potassic basalts and trachyte. Sedimentary facies related to volcanic event in the Middle Proterozoic Dahongyu Formation in Beijing and its neighboring area mainly include two basic types: siliceous sand carbonate mixed facies and pyroclastic gravity flow deposit, in which the later can be divided into pyroclastic basic surge and volcanic breccia-carbonate mixed deposit. Siliceous sand carbonate mixed facies: the rock types of this facies include white and light blue siliceous bearing quazites, light blue sandy silicalites, blue grey tuff, siliceous dolomites, intraclast shaped silicalites and geltexture silicalites. Siliceous sand carbonate mixed facies widely distribute in the NE basin axis inclined to the NW side. Preliminary results of analysis, according to characteristics of rocks, geochemistry and sedimentation, show that enormous siliceous sediments of the Dahongyu Formation mainly originated from the submarine volcanic eruption in the temporary time. Pyroclastic basic surges: Basic surges directly cover on basalts and occur around the paleo craters. They are composed of coarse sand grade pyroclastic rocks with large scale dune like cross bedding and poly grade bedding, fine sand silt grade pyroclastic rocks and tuff shales. The pyroclastic basic surge was caused by sea water surging in small range, which was related to release of remnant steam bursting to seal of volcanic craters. Volcanic breccia carbonate mixed facies: Volcanic breccia carbonate generally is single bed placed in between dolomites. In the beds breccia components are composed of sedimentary rocks (40～60%) and of basalts and tractytes (30～40%). According to analysis of sedimentary breccia content more than volcanic breccia, this facies probably formed under the condition bursting and returnning breccia mixed with submarine surface carbonates and water in the basin, as submarine gravity flow deposited around the places of the volcanic craters.
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