扬子地块西北缘侏罗纪重磁化及其大地构造意义

Jurassic remagnetization in the northwestern margin of the Yangtze Block and its tectonic implications

造山过程通常伴随着重磁化的广泛发生,因而造山带的重磁化研究有助于厘定其构造定型的时间.为了确定扬子地块西北缘逆冲推覆构造的定型时间,本文对大巴山弧形逆冲带西北端的西乡地区和龙门山逆冲带北段的宁强至广元地区成冰纪—寒武纪多个岩石单元开展了古地磁学、岩石磁学和岩相学研究,揭示出广泛的侏罗纪重磁化作用.样品的退磁特征、岩石磁学和扫描电镜观察结果显示,该重磁化分量为化学剩磁,载磁矿物为自生磁铁矿和赤铁矿,其形成与造山作用引发的流体迁移有关.结合研究区已发表的侏罗纪重磁化数据,大巴山弧形逆冲带和西乡—宁强地区记录了一致的侏罗纪重磁化方向,表明这一带逆冲构造格架的形成不晚于侏罗纪,而广元地区青林乡和陈家坝剖面的重磁化方向相对发生了~60°的逆时针旋转,表明该地区逆冲构造格架的形成晚于侏罗纪,但该区灌县—安县断层以东的近四川盆地区域未受影响,没有发生旋转.

The study of remagnetization in orogenic zones is conducive to determine the timing of their formation,since the orogenesis is usually accompanied by ubiquitous remagnetization.To constrain the age of the thrust framework of the northwestern margin of the Yangtze Block,an integrated paleomagnetic,rock magnetic and petrographic study has been carried out on the Cryogenian-Cambrian strata,including the Xixiang area to the northwest of the Dabashan thrust belt and the Ningqiang-Guangyuan area in the northern part of the Longmenshan thrust belt.Extensive Jurassic remagnetization is revealed.Demagnetization characteristics,rock magnetic results and scanning electron microscopy observations of these rock units suggested a chemical remagnetization carried by authigenic magnetite and hematite,whose growth was related to fluid migration resulting from the Jurassic orogenesis.Together with the published Jurassic remagnetization data of the studied area,the consistent Jurassic remagnetization direction was discovered from the Dabashan thrust belt and the Xixiang-Ningqiang area and this implied that the thrust framework formed no later than Jurassic.Whereas,the remagnetization component of the Guangyuan area of the northern part of the Longmenshan thrust belt anticlockwise rotated~60°relatively and this suggested that the thrust framework formed after Jurassic.But the area close to the Sichuan basin in the Guangyuan area east to the Guanxian-Anxian fault was not affected,with no obvious rotation being observed.