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

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 similar to 410Ma, and 370 similar to 360Ma. The discordant epsilon(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.