中国东部埃达克岩及成矿作用

Adakites or Adakitic Rocks and Associated Metallogenesis in Eastern China

埃达克岩(adakite)最初是指与俯冲洋壳部分熔融有关的一类高铝的中酸性侵入岩或火山岩。主要地球化学特点有:高Sr、低Y和Yb,轻重稀土元素强烈分异,不具有明显的Eu负异常,K2O/Na2O比值低。埃达克岩从发现、命名以来,一直是国际地学界关注的前沿课题之一。埃达克岩研究之所以方兴未艾,一则是其特殊的地球化学性质,二则是其成矿专属性。中国东部是目前国内发现埃达克岩最多的地区,主要为中生代侵入岩和火山岩,分布在长江中下游区、胶东地区及晋京辽区(或京冀区、东北区)。其中,长江中下游是重要的铜金多金属成矿带,胶东是重要的金矿带。目前,对中国东部埃达克岩成因的解释有四种模式:加厚古老下地壳的直接部分熔融、底侵玄武质下地壳部分熔融、拆沉下地壳部分熔融以及洋中脊俯冲。埃达克岩与铜金成矿的机制目前的研究还不是很透彻。一般认为埃达克质岩浆富流体、高氧逸度和基性源岩,这些特点均有利于Cu、Au等深源金属元素的萃取与富集成矿。

Adakites were proposed over a decade ago to be products of the melting of young subducted oceanic crust, belonging to the intermediate-acid volcanic and intrusive rock, which have distinct characteristics of high Al2O3 , Sr contents and Sr/Y ratios, but low Y, Yb contents and K2O/Na2O ratios, without obvious Eu anomalies. Since discovered and named 30 years ago, adakite is universally acknowledged as one of the frontier topics in earth science. There are two main reasons why the study on adakites is not forwarding rapidly. One is their special geo- chemical nature, the other is metallogenesis. Eastern China is the most abundant adakites concentrated area, mostly distributed in the Lower Yangtze Region and Eastern Shandong and Shanxi-Beijing-Liaoning （subdivided into Beijing-Hebei and northeast region） three districts. Among them except Beijing-Hebei, other three regions are all important copper-gold metallogenic belts. Four genetic model proposed before by other geologists are as follows： ancient thickening lower continental crust melting, underplated basaltic lower crust melting, melting of delaminated lower continental crust and mid-ocean ridge subduction. However, the relationship between copper-gold metallogen- esis and adakites is still controversial. Generally, adakitic magmas are thought to be enriched in fluid, with high oxygen fugacity and mafic source rock, which are favored by Cu, Au and other deeply-source metals to be extracted, enriched, and finally mineralized.