桑干地区早元古代花岗岩长石Pb同位素组成和锆石U-Pb年龄:变质与地壳熔融作用及构造-热事件演化

Pb isotopic composition of feldspar and U Pb age of zircon from early Proterozoic granite in Sanggan area, North China craton: Metamorphism, crustal melting and tectono thermal event.

桑干地区大同－集宁一带孔兹岩系分布区有大量早元古代花岗岩发育。依据矿物组成和地质特征，这些花岗岩可分为两种类型：大规模的石榴石花岗岩和小规模的淡色花岗岩。花岗岩中长石Ｐｂ同位素组成显示，石榴石花岗岩是孔兹岩系部分熔融的直接产物，而淡色花岗岩不是孔兹岩系简单的部分熔融产物。石榴石花岗岩中锆石Ｕ－Ｐｂ一致年龄为１８３６±１８Ｍａ，代表石榴石花岗岩的形成年代。淡色花岗岩的锆石Ｕ－Ｐｂ一致线上交点年龄为１９１２±９８Ｍａ，形成略早。孔兹岩系２０７Ｐｂ／２０４Ｐｂ值整体上高于华北麻粒岩下地壳，具有上地壳的Ｐｂ同位素组成特征，其沉积原岩应该来自华北晚太古代形成的大陆地壳。根据深成岩浆作用和变质作用的年代学资料，可以确定桑干地区早元古代如下构造－热事件序列：小基性岩体侵入（２．２～２．３Ｇａ）、早期淡色花岗岩生成（２．１～１．９Ｇａ）、麻粒岩相变质作用、剪切作用和大规模石榴石花岗岩发育（１．８４Ｇａ）、伟晶岩的形成（１．８０Ｇａ）和基性岩墙群的出现（１．７７Ｇａ）。

In the Khondalite series in Sanggan area of the North China craton, granites are well developed and occupy about half part of outcrops. Two types of granites have been recognized, they are garnet granite and lueco granite. The large volume of garnet granite is possibly direct products of anatexis of the Khondalite series, which is indicated by their feldspar Pb composition consistence and some petrological evidences. The formation age of garnet granite has been given by zircon U Pb concordia age of 1836±18Ma. The lueco granites are not simple direct products of the Khondalite series indicated by Pb compositions. In addition, they were formed earlier than garnet granite. U Pb data of zircon create a concordia upper intercept age of 1912±98Ma. The Khondalite series as a whole have relatively high 207 Pb/ 204 Pb ratio, that is the character of upper crust. The material source of the Khondalite deposits should be the late Archean continents in north China. The early Proterozoic crustal evolution in Sanggan area can be described as the following successive episodes: intrusion of basic plutons(2 2￣2 3Ga), intrusion of lueco granite (2 1￣1 9Ga), granulite facies metamorphism and garnet granite formation(1 84Ga), pegmatite(1 80Ga), mafic dyke swarm(1 77Ga). The large volume of garnet granite formed at about the peak of the tectono thermal event, simultaneous to or a little later than the granulite facies metamorphism.