南秦岭横丹浊积岩系——晚古生代发育于扬子板块被动陆缘上的弧前盆地充填物

南秦岭横丹浊积岩系是一套巨厚的浊流沉积,以向南或南东倾的单斜构造产出。由下而上,该沉积层序包括深水盆地、深水浊积扇和斜坡水道3个相序。相应地,沉积物粒度变粗,厚度变大,火山质组分含量增加,凝灰层大量发育,表明横丹浊积岩系为活动型浊积岩;其古水流方向为NNW—NNE向,物源区为南侧的碧口火山岩系。另外,横丹浊积岩系内还见石英岩、重结晶大理岩成分的砾石,说明其物源还包括被动陆缘环境的沉积物。相序、组构、沉积特征和物源区综合分析表明,横丹浊积岩系为弧前盆地充填物。构筑这一弧前盆地的动力学机制是洋壳板块向南俯冲于扬子板块被动陆缘之下,时代可能晚于中晚泥盆世。

Hengdan Turbidite terrane:Fillings in a Late Paleozoic forearc basin developed on the passive margin of the Yangtze plate

The Hengdan turbidite sequence is composed of very thick,normal turbidity current deposits.It is marked by a monocline dipping south or southeast.From above downward this sedimentary sequence consists of abyssal basin facies,bathyal turbidity fan facies and slope channel facies.Correspondingly the grain size,thickness,content of volcanic components and tuffs of the sediments increase,which suggests that the Hengdan turbidite sequence is an active-type one.The paleocurrents were oriented in a NNW-NNE direction,which indicates that the provenance was the Bikou volcanic series to the south.In addi-tion,gravels of quartzite and crinoid-and coral-bearing recrystallized marble are also found in the tur-bidite sequence,which suggests that the provenance also includes sediments of the passive continental mar-gin.An integrated analysis of the facies sequence,fabrics,sedimentary characteristics and provenance indi-cates that the Hengdan turbidite sequence is turbidity sediments fillings.The dynamic mechanism forming this forearc basin is the southward subduction of the oceanic crustal plate beneath the passive continental margin of the Yangtze plate.The age of the forearc basin is Late Paleozoic(after the Mid-Late Devoni-an).The active Hengdan turbidite sequence represents a Late Paleozoic forearc basin built on the Devonian passive margin of the Yangtze plate.