新疆阿合奇地区喀拉治尔加组砂岩地球化学特征及大地构造意义

Geochemical Characteristics of the Sandstone of the Kalazhierjia Formation in the Aheqi Area of Xinjiang and their Tectonic Significances

南天山是古亚洲洋闭合造山的关键地区,为探讨古亚洲洋闭合时限及前陆盆地的演化过程,对晚古生代喀拉治尔加组砂岩进行了岩石主、微量元素分析。结果显示,砂岩的主量元素具有低SiO2(平均含量65.99%)、高MgO(平均含量3.45%)的特点;稀土元素表现出中等的轻重稀土分异,具弱Eu正异常(δEu平均值为1.06);CIA和ICV值表明其经历了中等风化作用,成熟度较低,为近源沉积的产物;主、微量元素整体特点与大陆弧背景下的砂岩相似。研究表明,其物源区构造背景为南天山洋古生代的大陆岛弧,主要为长英质火山岩,少部分来自再旋回造山带。对喀拉治尔加组构造背景和物源的分析结果表明,南天山洋在晚石炭世已经闭合,中天山地块与塔里木陆块于早二叠世之后开始碰撞造山。

The South Tianshan is a key area for orogeny of the closed paleo-Asian ocean. In order to discuss the closure time of paleo-Asian ocean and the evolution of foreland basins, the petro-geochemical study of the late Paleozoic Kalazhierjia Formation sandstone has been carried out in this paper. The results show that the sandstone is characterized with low SiO2(average 65.99%) and high MgO(average 3.45%) contents, intermediate differentiation of LREE and HREE, with weak positive Eu anomalies(average δEu of 1.06). The CIA and ICV values indicate that the sandstone was moderately weathered, has relatively low maturity, implying a depositional product of proximal materials. Its overall characteristics of major and trace elements are similar to those of the sandstone formed in the continental arc setting. The research indicates that the sandstone was mainly sourced from felsic volcanic rocks of the Paleozoic continental arc in the South Tianshan Ocean, with a little from recycle orogenic belt. Comprehensive research on the material sources and tectonic setting of the Kalazhierjia Formation sandstone indicates that South Tianshan ocean had been closed in the Late Carboniferous and the collisional orogeny between the Central Tianshan and Tarim blocks initiated after the Early Permian.