湘东北燕山晚期花岗岩构造环境判别

TECTONIC SETTING DISCRIMINATION OF LATE YANSHANIAN GRANITES IN NORTHEAST HUNAN PROVINCE

在湘东北中生代多期陆内花岗岩浆活动中 ,燕山晚期花岗岩占有重要地位 ,燕山晚期大规模花岗岩侵入代表一种特殊的构造环境。本文通过湘东北主要燕山晚期花岗岩的岩石化学和微量元素来判别成岩构造环境 ,认为该期花岗岩属于富碱质的钙碱性花岗岩 ,为陆内拉张环境下的产物 ,进一步进行构造环境分类 ,判别其属于后造山PA型花岗岩 ,形成于造山作用后期的拉张环境 ,与陆内裂谷长期拉张的构造环境明显不同。在区域构造演化分析的基础上 ,提出湘东北燕山晚期花岗岩形成于印支—燕山早期陆内挤压 走滑剪切造山作用后期的构造松弛阶段拉张环境 ,燕山晚期PA型花岗岩的出现标志着陆内活化由挤压向拉张的构造转折 ,同时也反映湘东北的陆内活化至少经历了陆内挤压和陆内拉张两个演化阶段。

The Late Yanshanian granites play an important role in multi periodic granitic magma evolution within Northeast Hunan Province.The Late Yanshanian large scale granitic magma intrusion represents a special tectonic setting.In this paper,according to the contents of major and trace elements of Late Yanshanian granites in Northeast Hunan Province,the lithogenetic tectonic setting is distinguished.The granites belong to alkali rich calc alkaline granitic series,which were produced in an extensional tectonic setting.In the further classification of granitic rocks within the continental extensional tectonic setting,the Late Yanshanian granites belong to PA type granitic rocks,which represents post orogenic extensional tectonic setting.They were different from the AA type,produced in an extensional tectonic setting within continental rift.The PA type granitic rocks produced in a tectonic relax setting followed by Indo Chinese Early Yanshanian compresso shear orogenesis.The occurrence of Late Yanshanian a post orogenic PA type granites marked at least two stages of tectonic evolution (compression shear and extension) within Northeast Hunan Province.The Late Yanshanian (140-128Ma) was an epoch of conversion of tectonic setting.