北山造山带古生代浊积岩构造环境探讨及其大地构造意义

Tectonic settings of Paleozoic turbidites in the Beishan orogen and its implications for regional accretionary tectonics

北山造山带是研究中亚造山带增生造山的关键地区之一,浊积岩是增生造山带的重要组成部分。北山古生代浊积岩主要出露于营毛沱、柳园和黑山口地区。营毛沱浊积岩发育于下奥陶统,古水流方向由南向北,内部砂岩具中高等风化程度的长英质源区,构造背景为被动陆缘。早二叠世柳园浊积岩内部砂岩具低到中等风化程度的中基性源区,构造背景为大洋岛弧。早二叠世黑山口浊积岩中的砂岩源区具中等风化程度,环境相对柳园砂岩较为稳定,和长英质源区的沉积岩具相似性,构造环境可能为活动陆缘弧。对北山古生代浊积岩的解剖揭示北山古生代经历了复杂的俯冲增生过程。早古生代花牛山—火石山一带发育向北的俯冲,火石山南部被动陆缘形成营毛沱浊积岩,之后的俯冲带局部后撤形成泥盆纪墩墩山岛弧。柳园地区晚古生代洋壳向花牛山和石板山岛弧带俯冲分别形成了柳园和黑山口浊积岩。本研究支持北山增生时间持续到早二叠世的观点,对认识天山、索伦缝合带的衔接对比研究具有重要的意义。

The Beishan orogen is one of the key parts to study the accretionary orogenesis of the Altaids, and turbidites are an important composition of the accretionary orogenic belt. The Paleozoic turbidites in the Beishan are mainly distributed in the Yingmaotuo, Liuyuan and Heishankou areas. The turbidites in the Yingmaotuo area is Early Ordovician in age and were derived from felsic sources by medium-high weathering degree,indicating a passive continental margin tectonic setting. The turbidites in the Liuyuan area is Early Permian in age, and were derived from intermediate-basic sources by low-medium weathering degree, indicating an oceanic arc tectonic setting. The Heishankou turbidites are a little younger than the Liuyuan turbidites,and were derived from felsic sources by medium weathering degree, indicating a continental arc tectonic setting. Together with other data, we conclude that a complex oceanic subduction process took place in the Beishan orogen in the Paleozoic. There was a northward subduction zone in Huaniushan-Huoshishan area, and to the south of this subduction zone the Yingmaotuo turbidites formed in a passive margin in the Early Paleozoic. In the Late Paleozoic, the subduction zone was retreated southward,forming the Dundunshan arc and the Liuyuan and Heishankou turbidites were formed by two way subductions. Our work supports the Late Paleozoic active margin model and indicates that complicated accretionary orogenesis might have happened until the end Permian or even to the Middle Triassic in the Beishan. Therefore the termination of the CAOB was at least to the end-Permian.