伊朗莫克兰造山带西段增生过程重建：对新特提斯洋构造演化的制约

Accretion is an important geological process leading to continental growth. However, following the superposition of terranes during continental-continental collision, details of the process become difficult to evaluate. Accretion of the Makran orogenic belt began in the Cretaceous and continues to the present day, but without continental-continental collision. It is therefore an ideal target area to study accretionary processes. Studies of the Makran are still very rudimentary due to the international and domestic political situation of Iran and the Middle East in recent decades. To further investigate accretionary processes, this project proposes to carry out systematic field-work in the west region of the Makran orogenic belt, supported by geochemical, isotopic and geochronological analyses. It is anticipated, that this project reconstructing the evolution of the Makran orogenic belt will constrain the timing of Neotethys opening and the initial subduction of oceanic lithosphere. Further, it will contribute new insights into the understanding of accretion models and the tectonic evolution of the entire Neotethyan ocean. It will also provide a baseline reference for research of other large orogenic belts, especially those collisional orogenic belts, which have undergone multiple accretionary phase.

增生作用是大陆生长的重要方式，但经历了陆－陆碰撞造山作用的叠加之后，对其详细过程的研究变得棘手。莫克兰造山带的增生造山作用始于白垩纪，并持续至今仍未发生陆-陆碰撞，是研究增生造山过程的最佳研究区之一。同时由于伊朗和中东近几十年的国际国内政治形势，对莫克兰的研究还非常薄弱。本项目以解剖增生造山过程为主线，拟综合野外地质考察、岩石地球化学、同位素、高精度年代学等多种研究手段，对伊朗莫克兰增生造山带西段系统深入地开展野外工作，进而由点及面的探讨莫克兰造山带的生长历史，约束新特提斯洋的打开和大洋岩石圈开始向欧亚大陆俯冲的时间，并争取在增生构造理论或模式上取得一些创新。此项研究有助于我们认识和理解整个新特提斯洋的构造演化，同时对研究全球其他大型造山带特别是经历过多重增生造山的碰撞造山带也具有一定的借鉴意义。

莫克兰增生杂岩（MAC）是地球上最大的增生体之一，宽400-600公里，堆积巨厚的沉积岩和混杂岩。莫克兰蛇绿混杂岩由地幔橄榄岩、蛇纹岩、镁铁质岩到长英质岩包括侏罗纪-白垩纪辉长岩，早-晚白垩世玄武质到安山质枕状熔岩，安山凝灰岩、流纹英安岩和晚白垩世远洋灰岩、早-晚白垩世放射虫岩和古近系浊积砂岩组成。除了覆盖蛇绿熔岩之上的古近系浊积沉积物和/或各类火山岩外，其内部还有丰富的不同年代的外来沉积岩块。.我们对伊朗东南部莫克兰蛇绿混杂岩中8个中生代-新生代砂岩和1个凝灰岩中的碎屑锆石，进行U-Pb年龄和Hf同位素分析，来揭示MAC的成因和生长。新的碎屑锆石研究结果表明莫克兰增生杂岩由三叠纪演化至始新世，在其演化周期中整合了包括大陆岩石圈、洋壳及其附近大陆弧在内的所有造山要素。此外，结合碎屑锆石U-Pb年龄和Lu-Hf同位素数据得出如下认识：1. MAC中大量晚白垩世锆石可能来自大洋残片碎屑；2. MAC经历了从中侏罗世裂谷到晚白垩世-始新世弧岩浆作用的漫长演化历史；3. MAC中存在中侏罗世至晚白垩世的洋壳碎屑；4. 新生代（古新世-始新世）碎屑锆石的源区主要为Urumieh-Dokhtar Magmatic Belt和可能的莫克兰岩浆岩带；5. MAC碎屑锆石其它来源包括伊朗Sanandaj-Sirjan Zone的侏罗纪-白垩纪岩浆岩，中部和NE-NW段伊朗的三叠纪-石炭纪岩石，NE伊朗和伊朗-安纳托利亚的Cadomian地壳的奥陶系岩石；6. Arabian-Nubian 地盾以及非洲-阿拉伯克拉通和撒哈拉Metacraton是MAC中的Pre-Cadomian锆石的潜在来源。