青海省布青山早古生代末期埃达克岩的发现及其构造意义
Discovery of the end-Early Paleozoic adakite in the Buqingshan area, Qinghai province, and its tectonic implications.
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摘要: 在青藏高原北部阿尼玛卿蛇绿岩带布青山段首次发现了埃达克岩,它们多呈不规则岩脉状侵入于布青山地区北部早古生代蛇绿混杂岩中.其岩性为花岗-英云闪长岩,主矿物有斜长石、石英、黑云母、角闪石、少量钾长石和辉石;副矿物有榍石、磷灰石、锆石和磁铁矿等.属钙碱系列,具典型（Ⅰ类、O型、HSA）埃达克岩的地球化学特征,表现为：高SiO：（63.65%～71.59%）,高Al2O3（14.81%～16.11%）,高Na2O（4.68%～5.33%）和Na2O/K2O（1.48～3.08,平均2.39）,高Sr（444.08×10^-6～560.41×10^-6）和Sr/Y（50.8～105.3）,HREE强烈亏损,低Yb（0.38×10^-6～0.74×10^-6）,高La/Yb（44.93～90.97）,弱负Eu异常（δEu=0.74～0.90）,富集大离子亲石元素（LILE）,亏损高场强元素（HFSE）,Ba、Pb正异常而Nb、P负异常.获得布青山埃达克岩的锆石TIMS U-Pb年龄为402±24Ma.布青山埃达克岩熔体来自俯冲的昆祁秦洋壳（含海洋沉积物）在石榴石角闪岩相条件下部分熔融,主要残留相为石榴石（±角闪石）.布青山埃达克岩的发现指示早古生代末期洋壳（昆祁秦多岛洋的一部分）俯冲作用的存在,俯冲方向朝北（现在方位）,此前,俯冲带发生长时间的向洋迁移,早古生代末期俯冲带已后退到阿尼玛卿蛇绿混杂岩带以南;暗示典型埃达克岩也可以形成于洋壳俯冲晚期;提示阿尼玛卿蛇绿岩带可能有与典型埃达克岩有关的Au、Ag、Cu、Mo等热液和斑岩矿床.
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