秦岭松树沟岩体——一个遭受角闪岩相变质作用的超镁铁堆晶岩的实例  

The Songshugou Rock Body from Qinling——A Example of Ultramafic Cumulate Undergone Amphibolite Facies Metamorphism

作　　者：王希斌[1] 杨经绥[1] 史仁灯[1] 陈松永[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《地质学报》2005年第2期174-189,i001-i002,共18页Acta Geologica Sinica

基　　金：科技部 973项目 (编号 2 0 0 3 C13 7165 0 0 );国土资源部大调查项目 (编号 2 0 0 3 C13 0 0 0 0 5 8)资助的成果

摘　　要：研究表明,松树沟岩体为一受到角闪岩相变质作用改造再结晶的层状超镁铁岩体,并保留着某些原岩面貌痕迹。其变质作用不仅导致微观上矿物变形和宏观上岩体的构造变形,而且也引起岩体内部矿物重新组合和再造,形成以含角闪石(直闪石和透闪石)为特征的Anth- Ol- En和Tre- Ol- Di两个变质矿物共生组合。其形成温度大约为70 0～75 0℃,压力为0 .6～0 .7GPa以下。岩相构造分析表明,该岩体并不是一个简单的透镜体。实际是由三个呈对称分布的岩相带组成的层状杂岩体,遭受同构造变形形成具有紧密褶曲特征的同斜向斜岩体。该岩体为一蛇绿岩构造岩块,其原岩来自大洋岩石圈板块壳层下部单元——由纯橄榄岩-透辉橄榄岩-透辉岩组成的层状超镁铁堆晶岩。它在遭受角闪岩相变质作用之前,曾经历过低变质蛇纹石化阶段(形成蛇纹岩)。恢复其原岩的原始层序具有垂直分异和多旋回岩浆演化的特点。故松树沟岩体不具有阿尔卑斯型橄榄岩,或地幔橄榄岩的成因属性。Comprehensive studies show that the Songshugou rock body is a layered ultramafic rock recrystallized after having been subjected to amphibolite facies metamorphism, with some traces of its protolith preserved. The metamorphism is reflected not only in the microscopic deformation of minerals and macroscopic structural deformation of the rock body, but also in the re-association and re-construction of minerals within the rock body, forming two assemblages of metamorphic minerals characterized by containing amphibolite (anthophyllite and tremolite), i.e. Ath-Ol-En and Tre-Ol-Di. The formation temperature ranges 700～750℃ and the pressure is below 0.6～0.7 GPa.Litho-tectonic analyses indicate that the rock body is not a simple lens. It is actually a homoclinal-synclinal rock body with close folds, which is formed by a layered complex consisting of three symmetrically distributed lithofacies zones that has been syntectonically deformed.The rock body is an ophiolitic tectonite rock body, the protolith of which is derived from the lower unit of the oceanic lithosphere plate——the layered ultramafic cumulate composed of dunite-tilaite-diopside. Before it underwent metamorphism of amphibolite facies, it had experienced low-grade serpentinization and formed into serpentinite. The original sequence of its protolith is characterized by vertical differentiation and multi-cycle magmatic evolution. Therefore, the rock body is not genetically attributed to the Alpine peridotite, or mantle peridotite.

关 键 词：松树沟岩体 角闪岩相 变质作用 超镁铁堆晶岩 秦岭地区 橄榄岩 

分 类 号：P588.3[天文地球—岩石学]