北京四合堂花岗质杂岩的两种演化趋势

北京密怀隆起西部的石城片麻岩、皂树片麻岩和云蒙山北部的片麻状花岗闪长岩是太古代同源岩浆演化形成的古深成侵入体,本文称之为四合堂花岗质杂岩。该杂岩中的石城片麻岩,皂树片麻岩的闪长质、石英门长质片麻岩—英云间长质片麻—脉状的奥长花岗岩构成了英云间长岩—奥长花岗岩演化趋势,片麻岩中的石英闪长质片麻岩与花岗间长岩—二长花岗岩构成了钙碱性演化趋势。这两个演化趋势是在不同的条件下分离的矿物相不同所致。

THE TWO DIFFERENT EVOLUTIONAL TRENDS OF SIHETANG GRANITOID COMPLEX, BEIJING

Sihetang granitoid complex is distributed in west part of Mihuai up-warping region and consists mainly of Zaoshu gneisses, Shicheng gneisses and the gneisso-granodiorites in north part of Yunmeng mountain. The three parts constructing the complex are similar in main mineral and accessory mineral associations and geochemical characters in which biotites or amphiboles are of the same chemical composition. All of these prove they are the crystalized products originated from a comagma in different conditions. The dioritic and qartz-dioritic gneisses-tona litic gneisses-trondhjemitic rocks belonging to Zaoshu and Shicheng gneisses show a tonalite-trondhjemite evolutional trend, and gneissose granodiorites together with the qartz-dioritic gneisses show a cal-alkline trend. The former was formed mainly by enriching plagioclases from fraction of biotites and amphiboles. The later was produced by fractionating a great deal of plagioclases and relatively riching or component in the magma. A synthetical study of whole complex indicates that the original magma of the two trends came from about 25% partial melting of mafic eclogitic and granulitic rocks in deep crust.