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    秦岭印支期构造背景、岩浆活动及成矿作用

    Indosinian tectonic setting, magmatism and metallogenesis in Qinling Orogen, central China

    • 摘要: 提要:秦岭造山带以其独特的大地构造位置、复杂的地质演化和丰富的矿产资源而成为地质科学研究的焦点,科学家已经基本清楚了其大地构造格局和地质演化轮廓,共识其在印支期(三叠纪:251~199.6 Ma)彻底实现了由海盆向大陆造山带的转变。但是,盆山转变的过程细节、洋盆闭合的时间、三叠纪大地构造属性以及相关的岩浆作用和成矿作用研究薄弱,认识分歧较多。笔者通过综合分析地质、地球物理、地球化学、矿产资源等方面的研究成果,认为三叠纪的秦岭恰似现今地中海,并存着洋陆俯冲和陆陆碰撞,并逐渐由洋陆俯冲转变为陆陆碰撞体制;秦岭古特提斯洋于230~200 Ma期间自东向西拉链式缝合,扬子陆块与华北-秦岭联合大陆之间的碰撞造山作用接踵而至;三叠纪的秦岭构造背景并非单一的陆陆碰撞,更非过去认为的造山后或碰撞后。秦岭印支期岩浆作用强烈,形成了埃达克岩、钙碱性花岗岩、高钾钙碱性花岗岩、碱性岩、疑似奥长环斑花岗岩、碳酸岩等多种岩浆岩;它们自勉略缝合带向北显示分带性,依次是:阳山—胭脂坝过铝质S型或改造型花岗岩带、南秦岭高镁埃达克质的钙碱性花岗岩带、北秦岭高钾钙碱性花岗岩带、华北克拉通南缘碱性岩-碳酸岩带;印支期岩浆作用的复杂性、多样性、空间分带性和成分极性等特点无法用陆陆碰撞或碰撞后构造体制来解释,而应是勉略洋板块向北俯冲的结果。秦岭印支期成矿作用长期被忽视,但最近已发现有重要经济价值的印支期矿床类型有碳酸岩脉型、造山型和斑岩型钼矿床,卡林型-类卡林型、造山型和斑岩-爆破角砾岩型金矿床,造山型银多金属矿床,表明在洋陆俯冲向陆陆碰撞转变体制的成矿作用强烈、成矿类型多样,印支期矿床的找矿潜力较大。

       

      Abstract: Abstract:The Qinling Orogen in central China has been a long-term geologicalyl studied focus for its unique tectonic location, complicated geological history and abundant mineral resources. In previous studies geologists have clarified its tectonic framework, outlined its geological evolution, and reached a consensus that the final transition from marine basin to intracontinental mountains occurred in Indosinian (Triassic:251~199.6 Ma). However, the details of basin-to-mountain transition process, the age of final oceanic closure, the nature of Triassic tectonic setting, and the associated magmatism and mineralization in the area have been poorly constrained and hotly debated. Based on a comprehensive re-assessment of results from geological, geophysical, geochemical and ore deposit studies, the author compares the Triassic Qinling with present Mediterranean Sea, which contemporaneously accommodates oceanic slab subduction and intercontinental collision as well as gradual transition from oceanic subduction to intercontinental collision. The Qinling paleo-Tethys finally closed in a westward zipper-like way during the period of 230~200 Ma, and instantly followed by intercontinental collision between the Yangtze block and Qinling-North China united continents. Therefore, the Triassic tectonic setting in Qinling was neither a simple intercontinental collision, nor a post-orogenic or post-collisional regime. Indosinian magmatism was intensive in Qinling, forming igneous rocks including at least the adakites, calc-alkaline granitoids, high-K calc-alkaline granitoids, alkaline intrusions, Rapakivi-like granites, and carbonatites. These rocks show zoning spatial distribution, namely, northward from the Mian-Lue suture, the Yangshan-Yanzhiba peraluminous S-type granite belt, southern Qinling high-Mg and adakitic calc-alkaline granite belt, northern Qinling high-K calc-alkaline granite belt, and the carbonatite-alkaline intrusion belt at the southern margin of North China craton. Such Indosinian magmatites with distinctive petrologic complexity, lithologic diversity, spatial zonation and compositional polarity cannot be formed in syn- and/or post-collisional tectonic settings, but could have resulted from a northward paleo-Tethysan slab subduction along the Mian-Lue geosuture. Although the Indosinan mineralization has been ignored for a long time, a great number of Triassic ore deposits of economic significance have been recently discovered, including Molybdenum-containing carbonatite dykes, porphyries and orogenic-type quartz veins; orogenic-type, porphyry-/breccia pipe-type, and Carlin-type and Carlin-like gold deposits; and orogenic-type silver-dominant poly-metal deposits. This shows that mineralization of various genetic types strongly occurred in the transition regime from oceanic slab subduction to intercontinental collision, and that the Indosinian ore deposits are of exploration potential in Qinling Orogen.

       

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