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对美国加州南Sierra Nevada岩基中一个典型的中生代变质表壳岩及其混合岩带进行了详细的野外观察和元素地球化学研究。研究发现:在持久(约为150Ma)的花岗岩侵位作用下,早白垩世变泥质岩发生达角闪岩相的中高级变质作用和部分熔融,导致Isabella混合岩的形成;浅色体具有和变泥质岩及混合岩近平行的REE分布模式,但浅色体的LREE含量相对较低;和变泥质岩相似,混合岩中的浅色体具有显著的Ce负异常。野外观测、岩相观察及元素地球化学特征表明,浅色体显著的Ce负异常是继承了原岩的Ce负异常特征,而不是由于副矿物(磷灰石、独居石或锆石)的差异溶解或结晶分异作用造成的。早白垩世变泥质岩(浅色体的原岩)主要由泥质及沙质海相沉积物组成,局部夹基性火山灰和火山碎屑,形成于与大陆岛弧密切相关的浅海环境。原岩的Ce负异常反映了较还原的浅海沉积环境。具有Ce负异常浅色体的产出表明,如果俯冲带上的沉积岩在俯冲过程中发生部分熔融作用并且所产生的熔体参与大洋型岛弧岩浆作用,最终可以导致具有Ce异常的基性岩浆生成。
A typical Mesozoic metamorphic crust and its mixed rocks in the Sierra Nevada, South California, USA, were studied in detail by field observation and elemental geochemistry. It was found that the metamorphic and partial melting of the amphibolite facies during the Early Cretaceous argillaceous resulted in the formation of Isabella mixed rocks under long-lasting (about 150 Ma) granite emplacement. The pale-colored bodies had And REE distribution patterns of nearly variable argillaceous rocks and mixed rocks, but the LREE content of light-colored bodies is relatively low. Similar to the variable argillaceous rocks, the light-colored bodies in the mixed rocks have significant negative Ce anomalies. Field observations, facies observations and elemental geochemical characteristics show that the significant Ce anomaly of the light-colored body inherits the negative Ce anomaly characteristic of the original rock, not due to the difference of the accessory minerals (apatite, monazite or zircon) Dissolution or crystallization caused by differentiation. Early Cretaceous metamorphic argillaceous rock (paleochromic original rock) is mainly composed of argillaceous and sandy marine sediments, and local clayey volcanic ash and volcanic debris formed in the shallow sea environment closely related to the continental island arc. The negative Ce anomaly of the original rock reflects the more reduced shallow sea sedimentary environment. The output of the ce-light anomalies with Ce shows that if the sedimentary rocks on the subduction zone are partially melted during the subduction and the resulting melt participates in the oceanic island arc magma, it can eventually lead to the mafic magma with Ce anomalies generate.