扬子地台西缘新生代富碱斑岩中的深源包体及其意义

DISCOVERY OF DEEP-DERIVED XENOLITHS IN CENOZOIC ALKALI-RICH PROPHRIES ALONG THE MARGIN OF THE YANGTZE PLATFORM AND ITS SIGNIFICANCE

在扬子地台西缘的碱性正长斑岩中发现由镁铝榴石聚晶,榴辉岩,榴闪岩,麻粒岩等组成的下地壳包体。推测杨子地台下部存在麻粒岩—榴辉岩高级变质相系的基底地层,它们可能是比早元古代大红山群更老的基底地层,也可能是构造机制造成的高级相转变。广泛分布在杨子地台西缘的喜山期富碱斑岩的原始岩浆可能有统一的形成机制,岩浆源区在壳幔边界,深部壳幔物质混染。随着杨子板块向西俯冲,岩浆源区深度加大,其原始岩浆的碱质也随之增加,所以西部的斑岩更偏基富碱。斑岩体的分布西界已到达澜沧江附近,这也可能是扬子板块地壳消失的边界。

The eclogite xenoliths and other deep-derived ones are first discovered in a ring pipe of alkali syenite-prophyries in Heqing County,yunnan Province.This suggests the existence of granutite-eclogit facies under the cover of the Yangtze Platform.Either the rocks represented by these xenoliths may be elder than the Early Proterozoic Dahong Shah group,or they may be similar to the Dahong shah group,but suffered higher-rank metamorphism.The alkali-rich porphyries,widely distributing both inside and outside the western margin of the Yangtze Plat- form,may be of the same origin: their original magma was to derive from somewhere of The crust-mantle boundary,and it was subjected to crust-mautle mixing.Such a process is assumed that with the westward underthrusting of the Yangtze Plate,magma source would gradually deepen westwards,and alkalinitf of original magma would increase in the same direction.