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笔者近期在甘肃黑石山地区早古生代火成岩的研究中发现了埃达克质岩,埃达克质岩的岩石类型为奥长花岗岩、花岗闪长岩及花岗癍岩,侵入于北祁连东段的白银陆缘弧火山岩中。岩石具高Sr低Y,Sr／Y>40;富钠贫钾(Na2O／K2O>2), 富SiO,但含量变化不大;富集LREE,亏损重稀土元素,La／Yb>20,无Eu负异常或轻微正异常(δEu=1．0-1．19)。与典型的俯冲板块熔融的埃达克岩比较,黑石山埃达克质岩具有更高的SiO2含量和更低的MgO、Mg#(-46)、Cr(均11．6μg／g)、Ni(均 6．52μg／g),未显示出初始埃达克质熔体与地幔橄榄岩明显的交代过程,因而不大可能由俯冲板块直接熔融形成的;与中国东部燕山期高钾钙碱性埃达克岩相比,具有较高的Na2O(Na2O>4．0％;Na2O／K2O=1．97-2．59),但类似Cordilera Blanca岩基的(非板片熔融成因的埃达克质岩),推测形成于加厚的陆缘弧中基性下地壳的部分熔融。黑石山埃达克质岩的发现表明, 在早古生代晚期,北祁连东段经历了洋盆闭合、板块碰撞、陆缘弧地壳加厚、玄武质岩浆底侵作用及下地壳再造作用的构造演化过程。同时指出,在本区寻找与埃达克质岩有关的斑岩型铜、金矿可能是一个新的有希望的找矿方向。
The author recently found that the adakitic rocks were studied in the Early Paleozoic igneous rocks in the Heishishan area of Gansu. The types of rocks in the adakitic rocks are ophiolites, granodiorites and granodiorites, which intrude into the eastern part of the northern Qilian Mountains Of the silver continental arc volcanic rocks. The rocks are characterized by high Sr and low Y, with Sr / Y> 40; Na + / K2O> 2, rich in SiO, but little change in content; LREE enrichment, depleted heavy rare earth elements, La / Yb> Eu Negative or Slight Positive Anomalies (δEu = 1.0-1.19). Compared with typical adakite melted by subducting slates, the Adisite rocks in Heishishan have higher SiO2 contents and lower MgO, Mg # (- 46), Cr (11.6 μg / g), Ni (Both 6.52μg / g). The initial adakitic melt and mantle peridotite did not show obvious process of metasomatism, so it is unlikely to be formed by the direct melting of the subducted slab. Adakite rocks have higher Na2O (Na2O> 4.0%; Na2O / K2O = 1.97-2.59) but similar to Cordilera Blanca (non-plate melting origin) It is speculated that the part of the underlying crust formed in the thickened continental margin arc melts. The discovery of the ADKQ rocks in Heishishan shows that in the late Early Paleozoic, the eastern Qilian Mountains experienced tectonic evolution of ocean basin closure, plate collision, continental marginal crustal thickening, basaltic magma underplating and the crustal reworking . At the same time, it is pointed out that looking for porphyry copper and gold related to adakitic rocks in this area may be a new promising prospecting direction.