新元古界铁建造的成因与构造背景：以中天山沙垄铁矿为例

The Neoproterozoic iron formations embrace occurrences from all continents,they are spatialy and temporally associated with glacial rocks or volcanic rocks, considered to be one of the important indicators for rise of the oxygen level in the ocean. So it is of great sciencitific significance to study Neoproterozoic iron formations for understanding the Neoproterozioc snowball Earth event and the break-up of Rodinia supercontinent and revealing Neoproterozoic crustal accretion process, the global climate, environment and life evolution. Previous studies have shown that, the Northern Xinjiang occurs three set of Cryogenian glacial rocks and numerous Neoproterozoic bimodal magmatic rocks and preserves significant imprints of the Neoproterozoic Snowball Earth and breakup of Rodinia supercontinent. However,till now there is no study on the Neoproterozoic iron formations in the Northern Xinjiang.This applying project chooses the Shalong iron deposit in the Central Tianshan,Northern Xinjiang as research object which lies in the key preriod of the Neoproterozoic break-up of Rodinia supercontinent and snowball Earth event. We plan on studying high-quality zircon U-Pb geochronology, geochemistry,Sr-Nd-Hf-O isotope and ore deposit geology, to exactly constrain the epoch, origin, depostional environment and tectonic setting of this deposit. Then combined with the comparative study of the global Neoproterozoic iron formations, the study will discuss and reveal the Neoproterozoic tectonic affility, the tectonic evolution and the relationship with the evolution of Rodinia supercontinent of the Central Tianshan as well as the Northern Xinjiang.

新元古代铁建造在全球各古大陆均有不同程度的分布，常与冰碛岩或裂谷火山岩系在时空上密切相关，其形成是大洋氧逸度显著升高的重要标志之一。因而，新元古界铁建造已成为理解新古元代雪球地球事件和Rodinia超大陆裂解过程的重要研究对象，同时对于揭示新元古代地壳增生过程、气候、环境和生命演化等具有重要研究意义。前人研究表明，北疆地区发育三套成冰纪冰碛岩和大量双峰式岩浆岩，雪球地球和Rodinia超大陆裂解事件在该区响应强烈。然而，目前尚无针对该区新元古界铁建造的详细研究。本项目选择处于新元古代Rodinia超大陆裂解和雪球地球事件关健时期的中天山沙垄铁矿为研究对象，开展精细的锆石年代学、地球化学、同位素和矿床学综合研究，在确定沙垄铁矿的精确年龄、沉积环境和大地构造背景的基础上，与邻区及全球新元古宙铁建造进行综合对比，探讨和揭示中天山和北疆地区新元古代的构造属性、构造背景及其与全球超大陆演化的关系。

位于中天山的沙垄铁矿具有条带状铁建造地质特征，本项目对沙垄铁矿的成矿时代、成矿物质来源、成因机制以及成矿动力学背景进行了全面研究，提供了一个新元古界铁建造的实例研究，同时也为揭示中天山新元古代构造背景及与Rodinia超大陆的关系提供了新证据。通过对铁矿密切相关的变质火山岩的锆石LA-ICP MS 测年，限定了沙垄铁矿的成矿时代为新元古代（760.4±6.7 Ma）。通过含矿变质岩系主微量元素特征及Nd-Hf同位素组成研究，揭示了沙垄铁矿含矿变质岩系原岩为一套双峰式火山岩，进而揭示了沙垄铁矿的大地构造背景应为裂谷环境。矿床地质、铁矿石主微量元素特征及Fe同位素研究表明,沙垄铁矿的Al2O3和TiO2含量较低、稀土总量较低、轻稀土亏损而重稀土富集、Eu的正异常、较低的Y/Ho比值，铁的重同位素富集，这些特征均表明成矿物质主要来自深海热液。沙垄铁矿的含矿建造未见冰碛岩，而是以变质双峰式火山岩为主，其稀土元素配分图解与新元古代Algoma-type铁矿相类似，铁的重同位素富集，同时含矿岩系全岩εNd(t)和锆石Hf同位素εHf(t)值都为正值，这些特征均指示沙垄铁矿属于与火山作用有关的新元古代Algoma-type铁矿，表明铁矿的形成过程与火山热液密切相关。含矿岩系的构造背景研究表明沙垄铁矿产于新元古代裂谷背景，是全球Rodinia超大陆裂解的产物。上述研究也表明在中亚地区不仅塔里木陆块与Rodinia 超大陆裂解有密切关系，而且其他的陆块比如中天山同样对于Rodinia 超大陆裂解也有着明显的响应。