柴北缘斜长角闪岩的地球化学特征及其构造背景

分布在柴北缘超高压变质带中的斜长角闪岩主要有两种类型,一种由榴辉岩退变而成,一种只经历角闪岩相变质作用。它们的原岩属于拉斑玄武质岩石,轻稀土富集,Nd同位素组成亏损,这些玄武岩浆分别来自不同的地幔源区,地壳混染不明显,形成的环境可能为大陆裂谷或初始洋盆,这可能是柴北缘早古生代洋盆打开的前兆。随着早古生代洋盆的关闭,这些基性火成岩部分经历了超高压变质作用,即发生了深俯冲,部分只经历角闪岩相变质。无论哪种情况,它们在遭受变质作用之前,就与陆壳岩石共生在一起,支持柴北缘榴辉岩、斜长角闪岩与片麻岩的关系为原地关系(in situ)?

Tectonic setting and geochemistry of amphibolites in the North Qaidam

Amphibolites from HP/UHP metamorphic belt in the North Qaidam Mountains can be subdivided into two types. One type was resulted from retrograded eclogite, whereas another type underwent only amphibolite-facies metamorphism. Their protoliths may be tholeiites with characteristics of enriched-LREE and depleted Nd isotope. The basic magma was derived from different mantle sourse volume without obvious crust contamination, and formed in a continental rift or an incipient oceanic basin setting representing precursor of the early Paleozoic ocean. Following the closure of the early Paleozoic ocean basin, one part of the basic igneous rocks underwent HP/UHP metamorphism related to deep subduction, another part experienced only amphibolite-facies metamorphism. However, their igneous precursors are associated with continental rocks, supporting the model of the "in situ" relationship between eclogites, amphibolites and country rocks (gneisses).