埃达克质岩与成矿:困惑与探索

根据起源于下地壳的中酸性岩浆岩的成分变化,可以约束其源区深度以及地壳最小厚度,为大陆板内成矿作用的深部过程研究提供重要信息。全球范围内,许多世界级斑岩铜矿和浅成热液矿化系统与同期的埃达克质岩存在密切的时空与成因联系,在国内主要成矿区带也识别出与金属成矿作用有关的埃达克(质)岩。与俯冲过程无关的埃达克质岩的识别,使我们有可能结合其他地质证据构建完全不同于Sillitoe(1972)B型俯冲环境的斑岩铜矿成矿模式的大陆板内斑岩(夕卡岩)型金属矿床成矿模式。对于规模巨大、矿质主要源自地幔的热液矿床的形成,埃达克(质)岩可能是必要条件,但不是充分条件。埃达克(质)岩的成矿潜力通过地幔物质加入而获取,埃达克(质)岩的成矿专属性由上地幔成矿元素分布特征决定。与俯冲有关的埃达克质岩浆之所以有利于成矿,重要的原因是存在大量来自俯冲板片的高压、高温流体以及俯冲板片熔融形成高氧逸度(fO2)的熔体,但产出在大陆板内背景、与俯冲无关的埃达克(质)岩的成矿机制还不清楚。根据现代资源勘查理念,结合综合地质信息分析,埃达克质岩具有实际找矿意义。

Adakite-like rocks and mineralization:confusion and inquiry

According to the variation in the components of intermediate-acid igneous rocks coming from the lower crust,it is possible to restrain the depth of the source area and the minimum thickness of the crust,and thus to provide important information for the study of the deep process of the continental intraplate mineralization.Globally,many world level porphyry copper deposits and metallogenic systems formed by hypabyssal hot liquid have their close relationship with the synchronous adakite in space-time and genesis;domestically,the adakite-like rocks have been identified to be related to the metallogenesis in the main metallogenic areas in China.The recognition of adakite-like rocks having no relationship with the process of subduction makes it possible to construct a metallogenic model of continental intraplate porphyry metal deposits by combining other geological evidences,and this model is totally different from the metallogenic model of porphyry copper deposits with B-subduction setting constructed by Sillitoe(1972).The existence of adakite-like rocks may be the necessary condition but not the sufficient one for forming the large-scale porphyry deposits and the hypabyssal hot-liquid deposits,whose metallogenic elements mainly came from the mantle.The metallogenic potential of adakite-like rocks is achieved by the entering of the mantle material,and the metallogenic specialization of adakite-like rocks is decided by the distribution characteristics of the metallogenic elements in the upper mantle.An important reason for the adakite-like magma related to subduction being advantageous to mineralization is that there were abundant high pressure and high temperature liquid coming from the subducted plate-pieces and the magma of high fO2 coming from the melting of subducted plate-pieces.However,for those adakite-like rocks,whose occurrence has continental plate background but does not relate to the subduction,their metallogenic mechanism is not clear yet.Based on the concept of modern mineral exploration and combined with the analysis of integrated geological information,we may better realize the practical prospecting significance of the adakite-like rocks.