西天山下石炭统大哈拉军山组底部角度不整合特征及其对天山古生代洋陆转换时限的约束

Angular unconformity at the bottom of the Lower Carboniferous Dahalajunshan Formation: constraints on the Palaeozoic ocean-continent transition in the Tianshan Mountains

西天山广泛出露下石炭统大哈拉军山组火山-沉积岩系,其与下伏地层(前寒武纪结晶基底或前石炭纪褶皱基底)之间呈广泛的区域性角度不整合接触。通过对这些角度不整合面及大哈拉军山组底部冲洪积相碎屑岩或陆相火山岩特征的研究,认为该不整合面代表了一次强烈的褶皱、隆升造山事件;不整合面之上初始沉积物地层序列是天山石炭纪后碰撞裂谷盆地新一轮沉积旋回的起点。取自大哈拉军山组底部粗碎屑岩中夹层安山岩样品的锆石LA-ICP-MS U-Pb同位素年龄为359±2.3Ma,这一年龄值不但限定了这一区域性角度不整合的形成时代,而且代表天山后碰撞裂谷盆地的开启时间。因此,天山古生代洋陆转换时限在晚泥盆世—早石炭世之交,随后,天山造山带进入后碰撞裂谷演化阶段。

The volcanic-sedimentary rocks from the Lower Carboniferous Dahalajunshan Formation are widely exposed in the western Tianshan Mountains. There is a wide regional angular unconformity between the Dahalajunshan Formation and its underlying strata, i. e. , the Precambrian crystalline basement or pre-Carboniferous folded basement. The regional angular unconformity represents a strong fold and uplift orogenic event judged from the angular unconformities and the alluvial-proluvial clastic rocks or continental volcanic rocks at the bottom of the Lower Carboniferous Dahalajunshan Formation. The stratagraphic sequences of initial deposits overlying the unconformity surface represent the onset of a new round of sedimentary cycles in the Carboniferous post-collision rift basin. The interbedded andesite samples collected from the bottom of the Lower Carboniferous Dahalajunshan Formation gives a LA-ICP-MS zircon U-Pb age of 359 ± 2. 3 Ma, which not only defines the age of the above-mentioneol regional angular unconformity, but also represents an initial age of the Tianshan post-collision rift basin. Therefore, we can draw the conclusion that the time duration of the Palaeozoic ocean-continent transition should be limited at the boundary of the Late Devonian to the Early Carboniferous, followed by the post-collision rift evolutionary stage of the Tianshan orogenic belt.