活动大陆边缘岩浆作用及构造演化——以敦煌地块为例

大陆边缘弧是汇聚板块边界与俯冲有关的岩浆作用产物, 通常记录了造山带弧岩浆作用、造山过程和大陆地壳形成与演化等诸多重要的地质过程。作为中亚造山带中段最南部的构造单元, 敦煌地块为传统定义上具有早前寒武纪变质结晶基底的微陆块, 其后在古生代时期经历了多期次、多阶段的与中亚造山带造山相关的构造热事件并使其发生再活化, 进而产生了一系列广泛的弧岩浆-变质作用事件。然而, 对于该陆块古生代弧岩浆作用机制和地壳构造演化历史缺乏系统的研究。本文综合近十多年来对敦煌地块的诸多最新研究进展, 系统梳理了古生代岩浆岩岩石组合类型、年代格架、地球化学组成以及同时期变质-沉积构造热事件演化特征, 得到以下认识: (1)敦煌地块古生代岩浆作用过程呈现阶段性特征, 即幕式岩浆作用, 构造位置上从东北部逐渐迁移到南部再折返到中部, 大致可划分为五期: ~510Ma、440~410Ma、390~360Ma、~330Ma和280~245Ma, 而变质作用事件主要集中在450~360Ma; (2)古生代岩浆岩类型主要以钙碱性Ⅰ型花岗岩、埃达克质岩石、少量S型花岗岩和高钾花岗岩为主, 且岩石成分从寒武纪低钾拉斑系列逐渐转变为二叠纪高钾、富硅特征; (3)同位素地球化学特征表明, 敦煌地块中-北部寒武纪-泥盆纪发育的与俯冲相关的弧岩浆对新生地壳的生长起了重要贡献, 并伴随古老地壳再造事件; 而南部泥盆纪-石炭纪岩浆作用则以古老地壳物质重熔为主; (4)基于埃达克质岩石的证据, 敦煌地块在古生代时期经历了两次显著的地壳增厚事件, 早期可能是与北山南部石板山地体/弧碰撞以及幔源岩浆底侵有关, 而晚期可能是与俯冲板片后撤或回卷相关, 地壳厚度可达~55km; (5)敦煌地块属于中亚造山带中段最南部一个具有前寒武纪基底的古老微陆块, 后期卷入了古亚洲洋南向俯冲相关的造山事件使其被强烈破坏与改造。该微陆块作为古亚洲洋南部的活动大陆边缘弧, 被与俯冲作用有关的阶段性弧岩浆底侵、部分熔融、增厚地壳和区域性变质作用等机制改造与活化, 产生了阶段性侵位的陆缘弧岩浆作用。这些认识为探究中亚造山带微陆块的起源和造山带的构造演化提供新的约束。

Magmatism and tectonic evolution at an active continental margin: A case study of the Dunhuang Block

Continental margin arcs are the products of magmatic activity associated with subduction at convergent plate boundaries, which generally record many essential geological processes, i.e., arc magmatism, orogenic process, and the formation and evolution of continental crust. As the southernmost tectonic unit in the middle part of the Central Asian Orogenic Belt (CAOB), the Dunhuang Block has traditionally been defined as a microcontinent/terrane with the Early Precambrian metamorphic crystalline basement. It subsequently underwent multiple phases of tectonothermal events related to the CAOB and was reactivated in the Paleozoic, resulting in a series of extensive arc magmatism and metamorphism events. However, the mechanism of Paleozoic arc magmatism and the tectonic evolution history of the block are not systematically studied. Based on the most recent research progress of the Dunhuang Block in the past decade, we comprehensively complied the type of rock association, chronology framework, and geochemical composition of Paleozoic igneous rocks, as well as the characteristics of contemporaneous metamorphism and sedimentary tectonthermal events, culminating in following findings. (1) The Paleozoic magmatic processes in Dunhuang Block show multi-stages intrusion characteristic, i.e., episodic patterns. The Paleozoic magmatism gradually migrated from the northeast to the south and then back to the middle Dunhuang area, which can be approximately divided into five episodes: ~510Ma, 440~410Ma, 390~360Ma, ~330Ma and 280~245Ma. Whereas the metamorphism events are mainly concentrated at 450~360Ma. (2) The types of Paleozoic igneous rocks mainly include calc-alkaline Ⅰ-type granites, adakitic rocks, a small amount of S-type and high-potassium granitoids. The compositions of Paleozoic granitoids have gradually transformed from the low-potassium tholeiite in the Cambrian to the high-potassium and rich-silicon in the Permian. (3) Isotopic geochemical characteristics indicate that the development of subduction-related arc magmatism during the Cambrian-Devonian in the central and north part of the Dunhuang area significantly contributed to juvenile crust growth, accompanied by ancient crustal reworking events. In contrast, magmatism events in the southern part during the Devonian-Carboniferous period were primarily dominated by remelting of ancient crustal material. (4) Based on evidence from adakitic rocks, the Dunhuang Block experienced two significant crustal thickening events in the whole Paleozoic. The early stage may be caused by the collision between the Dunhuang and Shibanshan terrane/arc in the southern Beishan and mantle-derived magma underplating. While the later stage may have been related to the slab rollback or retreat. The thickness of the crust for two thickening events reached ~55km. (5) The Dunhuang Block is an ancient microcontinent with the Precambrian crystalline basement in the southern CAOB, which was strongly remodified and destroyed by the orogenic events related to the subduction southward of the Paleo-Asian Ocean. As an active continental margin arc in the southern Paleo-Asian Ocean, the Dunhuang Block was modified and reactivated by the geological processes such as subduction-related magmatism, partial melting, crustal thickening and regional metamorphism, and that generated widely continental arc magmatism characterized by shows episode features. In conclusion, these insights provide new constraint for exploring the origin of the microcontinents within the CAOB and the tectonic evolution of orogen.