摘要:
西天山伊犁地区广泛出露早石炭世大哈拉军山组火山—沉积岩系, 其内赋存了金、铁、铜等重要的金属矿床。但迄今为止, 关于大哈拉军山组形成构造环境还存在很大的争议。通过区域地层对比, 研究区大哈拉军山组与下伏地层之间存在区域性角度不整合接触关系。其底部自下而上依次发育砾岩、粗砂岩、砂岩、粉砂岩、泥岩、碳酸盐岩、安山岩。沉积学和层序序列分析显示, 该组底部的陆源碎屑岩建造组合形成于扇三角洲沉积环境, 进一步可划分出扇三角洲平原、扇三角洲前缘和前扇三角洲3个亚相, 向上出现的碳酸盐岩形成于浅海碳酸盐岩台地环境。这种沉积物由粗到细、水体由浅到深, 沉积环境由陆相变为海相的地层充填序列, 反映了一种裂陷拉伸的动力学背景。因此, 西天山早石炭世大哈拉军山组火山—沉积建造形成于后碰撞裂谷环境。
Abstract:
Volcanic-sedimentary rocks from the Dahalajunshan Formation of the Lower Carboniferous are widely exposed in the Yili area, western Tianshan. Large amounts of mineral deposits have been found in this association. However, there is still a considerable controversy on the tectonic setting where the Dahalajunshan Formation deposited. Through regional stratigraphic correlations and field observations, we discovered a regional angular unconformity between the Dahalajunshan Formation and its underlying strata in the study area. The lower Dahalajunshan Formation consists of conglomerate, coarse sandstone, sandstone, siltstone, mudstones, carbonate and andesite in an ascending order. In combination with sedimentological and stratigraphical analysis, we suggest that the terrestrial clastic rocks at the bottom of the Dahalajunshan Formation were formed in a fan-delta environment, which can be further divided into three subfacies:fan-delta plain, fan-delta front and pro-fan delta, and the carbonates were formed in carbonate platform. The stratigraphical sequences, in an ascending order, have creasing sediment sizes from coarse to fine, enlarging water depths from shallow to deep, but varying depositional environment from land to marine, and thus reflect an extensional setting. Therefore, we believe that the early Carboniferous Dahalajunshan Formation in western Tianshan could have formed in a post-collision rift environment.
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