Preliminary discussion on tectonic setting of the magmatic-metamorphic complex in the northern Dunhuang region: Insight into southward extension style of the southern Central Asian Orogenic Belt

Abstract

中亚造山带南缘如何向南扩展，对深入理解增生型造山作用和大陆地壳生长机制以及中亚构造域与特提斯构造域的衔接具有重要科学意义。作为中亚造山带南缘的关键构造单元，敦煌构造带大地构造属性长期备受关注且颇有争议。传统观点认为敦煌构造带是古亚洲洋南侧的前寒武纪稳定大陆地块，以刚性块体的形式参与了中亚造山带南缘的最终拼贴过程。然而，近年来研究认为敦煌构造带卷入了古亚洲洋南部的俯冲增生造山过程，属于中亚造山带南缘的增生系统。显然，这一争议限制了对中亚造山带南缘向南扩展方式及增生造山过程的理解。敦煌北部三危山地区出露一套古生代岩浆-变质杂岩，是解开这一争论的关键。本文综合前人研究基础及新的资料，归纳了这套岩浆-变质杂岩的野外岩石-构造组合、地球化学和年代学等方面特征：该岩浆-变质杂岩整体显示二元结构特征，即较老的增生杂岩为基底，弧岩浆岩侵入或不整合覆盖其上；其中岩浆岩属于中钾-高钾钙碱性系列中酸性岩浆岩，富集大离子亲石元素（LILE）和轻稀土元素（LREE），亏损高场强元素（HFSE），与典型的弧岩浆岩类似，并且微量元素组成特征反映中酸性岩浆的源区与俯冲沉积物部分熔融有关；岩浆作用大致归为510Ma、460~410Ma和370~360Ma三期。岩浆岩中结晶锆石不一致的ε_Hf（t）值（既有正值，又有负值）以及继承锆石的存在表明，岩浆源区既有古老地壳物质的加入，也有新生地壳物质的形成。以上这些特征与发育在增生杂岩之上的增生弧十分类似，因此本文提出敦煌北部岩浆-变质杂岩的属性为古生代增生弧，并且该增生弧与其南部的红柳峡俯冲增生杂岩共同勾勒出敦煌构造带自北向南增生弧-增生杂岩的基本构造格架，即敦煌构造带的大地构造属性实为造山带而非稳定地块。结合区域地质背景及敦煌地区与北山地区古生代至早中生代构造-热事件的对应关系，认为敦煌造山带属于中亚造山带中段南缘的增生系统，中亚造山带中段以增生弧-增生杂岩的形式向南扩展至敦煌地区。;The southward extension style of the southern Central Asian Orogenic Belt (CAOB) is significant for understanding the process of accretionary orogeny and growth mechanism of continental crust, as well as the connection between the Central Asian tectonic domain and Tethys tectonic domain. As a key tectonic unit located in the southern CAOB, the tectonic attribution of Dunhuang tectonic belt has long been concerned and controversial. Traditionally, the Dunhuang tectonic belt is considered to be a Precambrian stable continental block on the southern side of the Paleo-Asian Ocean (PAO), which participated in the final collage process of the southern margin of the CAOB in the form of rigid plates. However, in recent years, studies have suggested that the Dunhuang tectonic belt is involved in the subduction-orogenic process in the southern PAO, and belongs to the accretionary system of the southern CAOB. This controversy has limited the understanding of the southward expansion mode and accretive orogenic process of the southern margin of the CAOB. The focus of the debate is whether the Dunhuang tectonic belt is a Precambrian block or a Phanerozoic orogenic belt. The Paleozoic magmatic-metamorphic complex in the Sanweishan area is the key to reveal the tectonic attribution of Dunhuang tectonic belt. Integrating the published and new data, this paper sums up the field rock-structure association, geochemistry, and geochronology of the magmatic-metamorphic complex. In the field, the magmatic-metamorphic complex show dual structure, namely the older accretionary complex as the basement, and the arc magmatic rocks intrude or unconformity overlie on it. The magmatic rocks belong to medium potassium-high potassium calc-alkaline series, enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), depleted in high field strength elements (HFSE), which resembles the arc magmatic rocks. And the trace elements compositions indicate that the magma sources are perhaps related to partial melting of subducted sediment. The magmatism can be roughly classified into three periods: 510Ma, 460~410Ma, and 370~360Ma. The discordant ε_Hf(t) values (both positive and negative) of crystalline zircons in magmatic rocks and the existence of inherited zircons indicate that both the reworking of ancient crust and juvenile material were added to the magmatic sources. These characteristics are very similar to the accretionary arcs, so we propose that the magmatic-metamorphic complex in the northern Dunhuang region represent a Paleozoic accretionary arc. This accretionary arc and the southside Hongliuxia subduction-accretion complex collectively outline the basic tectonic framework of the Dunhuang orogenic belt. In combination with the relations of tectono-thermal events between Dunhuang and Beishan regions during Paleozoic to Early Mesozoic eras, it is considered that the Dunhuang orogenic belt belongs to the accretionary system of the southern margin of the middle section of CAOB. The middle section of CAOB extends southward to Dunhuang region in the form of accretionary arc-accretionary complex.