ABSTRACT

Early Palaeozoic evolution of the Kazakhstan orocline in the Central Asian Orogenic Belt (CAOB) is key to reconstructing the Palaeozoic tectonic framework in Central Asia. This study presents field mapping, geochemistry, and geochronological data from the Wulasitai area of northern West Junggar to constrain the subduction polarity and major tectonic events in the Chingiz arc of the Kazakhstan orocline during the Early Palaeozoic. The mapping area outcrops imbricated coherent turbidite slices and mélanges that consist of chert, limestone, silt, sandstone, and conglomerate blocks in mudstone matrix, representing an OPS (Ocean Plate Stratigraphy) mélange in an accretionary complex. Existing fossil ages from these blocks range from the Ordovician to Silurian, while our new detrital zircon U-Pb samples from the turbiditic matrix yield maximum depositional ages (MDTs) from 428 ± 2 Ma to 450 ± 1 Ma. Detrital zircon ages of ~510 to 430 Ma in the mélange suggest the Chingiz arc to the south of the study area as a major sediment source, providing an indirect constraint on the polarity of subduction. We suggest that the northern part of the Chingiz arc may be underlain by southward subduction, with the accretionary complex located on the Wulasitai area of the northern West Junggar. The Wulasitai mélange is overlain depostionally by volcanic rocks carrying depleted HFSE and enriched LILE, LREE. The andesite and tuff are dated to 415 ± 5 Ma, 414 ± 3 Ma, and 402 ± 6 Ma, which we interpret as reflecting the development of the volcanic arc onto the previous subduction complex. This model implies the generation of the arc (accretion arc) as the slab rolling back and trench retreating, which may play an important role in the evolutionary history of CAOB.

摘要

中亚造山带（CAOB）中哈萨克斯坦山坡的早古生代演化是重建中亚古生代构造框架的关键。本研究提供了西准噶尔北部乌拉斯台地区的野外测绘、地球化学和地质年代学数据，以限制早古生代期间哈萨克斯坦斜斜成吉兹弧的俯冲极性和主要构造事件。测绘区露出由泥岩基质中的燧石、石灰岩、粉砂、砂岩和砾岩块组成的叠瓦状连贯浊积岩片和混杂岩，代表增生杂岩中的 OPS（海洋板块地层）混杂岩。这些地块的现有化石年龄范围从奥陶纪到志留纪，而我们来自浊质基质的新碎屑锆石 U-Pb 样品产生的最大沉积年龄 (MDT) 从 428 ± 2 Ma 到 450 ± 1 Ma。混杂岩中~510 至 430 Ma 的碎屑锆石年龄表明研究区南部的 Chingiz 弧是主要沉积物来源，间接限制了俯冲的极性。我们认为，成吉思弧北部可能处于南向俯冲之下，增生杂岩位于西准噶尔北部的乌拉斯台地区。乌拉斯台混杂岩被火山岩沉积地覆盖，其中携带着贫化的 HFSE 和富集的 LILE、LREE。安山岩和凝灰岩的年代为 415±5 Ma、414±3 Ma 和 402±6 Ma，我们认为这反映了火山弧在先前俯冲复合体上的发展。