摘要
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。
As a result of protracted granitic magmatism （ - 150 myr） in the extinct Mesozoic Sierran continental arc, California, the metasedimentary framework rocks had undergone up to amphibolite facies intermediate- to high-grade metamorphism and partial melting of labile wall rocks within the proximity of invading plutons. These partial melting processes resulted in the formation of a number of migmatite complexes in the southern Sierra Nevada batholith. Such well-preserved migmatite complexes provide a natural laboratory to investigate the geochemistry of metasediment-derived melts. Rare earth element geochemistry data on the Gomt Ranch migmatite complex, one of the best preserved migmatite complexes, show that the leucosomes, migmatites, and metapelites have： （1） pronounced negative Ce anomalies; （2） subparallel chondrite-normalized REE distribution patterrks; （3） leucosomes have lower LREE contents than those in migmatites and pelites; and （4） contrary to the migmatites and pelites, leuccsomes have various Eu anomalies. The Isabella metapelites contain a fair amount of graphite as shown in whole rock dissolution experiments and petrographic observations, which suggest that they are strongly reduced metasediments. A similar conclusion was also reached from previous studies on mineral geochemistry of biotites from these rocks and adjacent granitoids. These data and previous Sr, Nd isotopic composition data on the leucosomes, migmatites and metapelites demonstrate that pronounced negative Ce anomalies in leucosomes were inherited from their source rocks, not due to differential dissolution or fractionation crystallization of accessory phases （e. g. apatite, monazite or zircon ） during partial melting of metapelites. The finds from this study also suggest that if sediments with Ce anomalies above a subduction zone are introduced into the island arc magma source regime via either subduction or subduction erosion, and participate in island arc magmatism, then sediment-derived melts can contribute
出处
《岩石矿物学杂志》
CAS
CSCD
北大核心
2005年第5期425-430,共6页
Acta Petrologica et Mineralogica
基金
美国自然基金项目(EAR-9815024
EAR-0087347)
