冀西北太古宙条带状麻粒岩的岩石学和地球化学特征

The Petrological and Geochemical Characteristics of Archean Banded Granulites in Northwestern Hebei Province

条带状麻粒岩是冀西北太古宙麻粒岩相带中特殊而重要的一种岩类，它由浅色条带和暗色条带组成，条带含量的比例变化不大。浅色条带由较多的斜长石、紫苏辉石、透辉石和一定量的石英组成，伴生少量小而细的磁铁矿；暗色条带由较多的紫苏辉石、透辉石和普通角闪石以及一定量的斜长石组成，伴生有较多较大的磁铁矿。少量黑云母在两种条带中均有所见。两种条带中的同种矿物的光性特征基本相同，而化学成分和稀土元素特征只有很少的差别。暗色条带为基性麻粒岩属ＴＨ２型拉斑玄武岩，而浅色条带的岩石为中酸性麻粒岩，相当英云闪长质类型。这种条带状的岩石形成于不同时期不同性质的岩浆作用和特定的构造环境。暗色条带的物源来自较早的（约２．８Ｇａ）亏损地幔，而浅色条带的物源来自较晚（约２．５Ｇａ）基性岩的部分熔融，与周围的大片区域性英云闪长岩的侵位大致同时。

Banded granulite occurring at some places in the granulite facies belt of northwestern Hebei Province is a typical and dominant metamorphic rock type. The metamorphic rock consists mainly of light bands and dark bands intercalated each other in constent proportion. The light bands contain more plagioclase, moderate hypersthene, diopside, quartz, and minor fine grained magnetite. The dark bands is are mainly composed of more hypersthenes, diopside, hornblende, more coarse grained magnetite, moderate plagioclase. Minor biotite is present in two bands. The optical features of same minerals form two bands are basically similar, but their chemical composition and REE are slightly different in amount. Based on these characteristics of petrochemical and geochemical compositions, the dark bands (basic granulite) correspond to the TH 2 type of the tholeiite, while the light bands belong to tonalitic rock type. Preliminary study suggests that the genesis of this metamorphic rock is neither vocanics nor metamorphic differentiation. A multistage petrogenesis is proposed for this banded granulite. Original material for the dark bands were derived from depleted mantle in early stage (2.8Ga±), while the light bands are formed from later partial melting of another basic rocks with regional tonalitic intrution in same time.