利用碎屑岩物源示踪恢复西准噶尔早石炭世大地构造演化过程

According to former research products, Early Carboniferous is the key period of tectonic transformation in the West Junggar, but geologists have proposed different ideas on the tectonic evlution process. The Early Carboniferous volcanic-sedimentary rock series are spread in the Dalabute region of Western Junggar. They are the direct objects for studying about the paleogeographic structure and tectonic evolution process of the Western Junggar in Early Carboniferous. Our preliminary work has revealed that the early Carboniferous strata sequence of Xibeikusi formation, Baogutu formation and Tailegula formation (from bottom to top) was defined in the Dalabute region. The stratum are conformable contact with continuous retrogradational characteristics. This project intends to regard the Early Carboniferous volcano sedimentary rocks as a continuous filling process. Based on analysis of clasts composition, heavy mineral, geochemistry and Zircon U-Pb dating, we research the systematic source tracing of various lithostratigraphic units in the basin. From provenance tracing standpoint, this project wants to analyses sedimentary basin in Early Carboniferous, recover the character of sediment basin, research material sources, discriminate the character and tectonic setting of source rocks.This method connects the development of the sedimentary basin and the tectonic evolution of the orogenic belt and help to reconstruct the tectonic-paleogeography and tectonic evolution process of the West Junggar in Early Carboniferous. Our study can provide a sedimentary evidence for confining the oceanic basin closure time of the West Junggar in Late Paleozoic.

前人研究表明，早石炭世是西准噶尔构造转换的关键时期，但学者们对该时期大地构造演化过程的认识存在严重分歧。西准噶尔达拉布特出露巨厚的早石炭世火山-沉积岩系，为研究早石炭世古地理构造格局，探讨早石炭世大地构造演化过程提供了直接载体。本研究前期已厘定达拉布特早石炭世地层层序由下到上为希贝库斯组、包古图组、太勒古拉组，之间为整合接触，且具连续退积型特征。本申请拟将该套早石炭世火山-沉积岩系看作一个连续充填过程，采用碎屑成分分析、重矿物分析、岩石地球化学、同位素和锆石U-Pb定年等方法，对盆地中各岩石地层单位进行系统的物源示踪。从物源示踪角度对早石炭世地层进行盆地分析，恢复沉积盆地性质，研究沉积物质来源，判别源岩性质和源区构造背景，将沉积盆地发展阶段和造山带构造演化过程动态的结合起来，从而实现恢复西准噶尔早石炭世古地理构造格局和大地构造演化过程的目的，为限定西准噶尔晚古生代洋盆的闭合时限提供证据。

石炭纪是西准噶尔构造转换的关键时期，但学者们对该时期大地构造演化过程的认识存在严重分歧。西准噶尔达拉布特出露巨厚的石炭纪火山-沉积岩系，为研究石炭纪古地理构造格局，探讨晚古生代大地构造演化过程提供了直接载体。详细的野外地质观察揭示，该套石炭纪火山-沉积岩系为一套连续沉积地层，沉积层序（由下到上）为希贝库拉斯组、包古图组、太勒古拉组，沉积粒度由粗变细，沉积相由浅海相到半深水相再到深水相的变化特征，显示其处于出伸展拉张背景区。碎屑岩岩性主要为岩屑长石砂岩、长石岩屑砂岩、沉凝灰岩，碎屑成分为次棱角状石英、长石和岩屑，岩屑以火山碎屑为主。重矿物组合以锆石 + 磷灰石 + 辉石 + 角闪石 + 含铁矿物 (赤褐铁矿, 钛铁矿, 磁铁矿, 黄铁矿)为主，暗示源区岩性主要为火成岩。岩石地球化学特征呈现出中等Zr/Sc（均值 15.47）和Th/Sc（均值 0.61），低La/Sc （均值 1.82）和Co/Th（均值 1.45），进一步指示源区岩性为中-酸性火成岩。元素Hf含量和La/Th比值较低，约束该套地层源区可能来自弧地体。碎屑锆石U-Pb定年结果和正的εHf(t)进一步锁定沉积物质可能来源于铁厂沟-哈拉阿拉特地区晚古生代岛弧岩浆作用。碎屑岩地球化学含量、比值有着明显不同于被动陆缘的元素特征，近似于与岛弧有关的大地构造背景区，结合地层沉积特征及岩浆岩研究成果，认为该套地层可能形成于大陆弧后伸展背景区。以上研究认为达拉布特石炭纪火山-沉积地层形成于弧后盆地环境，物质来源于铁厂沟-哈拉阿拉特地区晚古生代岛弧岩浆，该时期西准噶尔古地理构造格局为南陆北洋，约束了古亚洲洋闭合晚于石炭纪。