苏鲁超高压变质带西北缘五莲断裂外侧浅变质花岗片麻岩的地球化学特征与原岩成因研究  

Geochemical characteristics of weakly metamorphosed orthogneisses outside the Wulian boundary fault along the northwestern margin of the Sulu UHP metamorphic belt and their protolith petrogenesis

作　　者：薛怀民[1] 刘福来[1] 许志琴[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《中国地质》2005年第2期249-258,共10页Geology in China

基　　金：国家重点基础研究发展规划项目(2003CB716502);国家自然科学基金重大项目(40399143);中国地质调查局地质大调查项目(121201056606)资助。

摘　　要：五莲杂岩中的正片麻岩主要为正长花岗质片麻岩,少数石英正长质片麻岩,具有适度碱性的演化趋势。以贫CaO、低MgO、高Na2O为特征,岩石中虽然碱度较高,但仍属较强过铝型。这套正片麻岩还表现为较强的轻、重稀土元素分馏程度和强的负Eu异常,尤以强烈富集大离子亲石元素(Rb、Ba、Th、U、K)和亏损高场强元素Ti、Zr、Hf、Nb、Ta(部分样品中Zr、Hf除外),明显的Ba、P负异常为显著特征。岩石地球化学的总体特征与苏鲁造山带内的花岗质片麻岩有一定的可比性,而与华北克拉通内的前寒武纪正片麻岩明显不同。推测五莲地区正片麻岩的原岩是晚元古代由大陆下地壳富钾的变质玄武质组分部分熔融形成的,母岩浆的演化主要表现为以长石(奥长石和钾长石)的分离结晶为主,其中可能还有少量黑云母和角闪石,以及副矿物锆石和磷灰石的分离结晶。Orthogneisses in the Wulian area are mainly syenogranitic gneiss and less commonly quartz syenitic gneiss, showing a mildly alkaline trend. The rocks are characterized by low CaO and MgO contents, and high Na2O contents, belonging to the peraluminous series though the rocks have relatively alkaline contents. These orthogneisses are also characterized by relatively strong fractionation between LREE and HREE, a strong negative Eu anomaly and particular strong enrichment in large-ion lithophile elements (Rb, Ba, Th, U and K) and depletion in high-field-strength elements (Ti, Zr, Hf, Nb and Ta), as well as pronounced negative Ba and P anomalies. The general geochemical characteristics of the rocks are comparable to those of granitic gneisses in the Sulu orogenic belt in some degrees but notably different from those of Precambrian orthogneisses in the North China craton. It is inferred that the protolith of the orthogneisses was derived by partial melting of K-rich metabasaltic sources of the continental lower crust in the Neoproterozoic and that the parental magmas mainly evolved by fractional crystallization of feldspar (oligoclase and K-feldspar), accompanied by fractional crystallization of small amounts of amphibole, biotite and accessory zircon, apatite and allanite.

关 键 词：花岗片麻岩 地球化学 分离结晶 五莲 苏鲁造山带 

分 类 号：P588.3[天文地球—岩石学] P595[天文地球—地质学]