The Dunhuang block is a Paleozoic orogenic belt and part of the Central Asian Orogenic Belt (CAOB), NW China

Abstract

The Dunhuang region is located in the east of the Tarim Craton and has long been considered as part of a Precambrian terrane of the Tarim or North China Craton. Aiming to further understand the tectonic evolution of the Dunhuang region and its relations with the adjacent tectonic units, this study is to investigate the metamafic rocks in the Sanweishan area, Dunhuang region, and in turn to summarize the tectono-thermal events and re-examine the tectonic characteristics of the region.The metamafic rocks in the Sanweishan area include three types of amphibolites. They are garnet–clinopyroxene amphibolites, garnet-free amphibolites and garnet–mica amphibolites. The whole-rock major and trace element compositions suggest that their protoliths probably are E-MORB type tholeiitic basalts with some crustal addition. The LA-ICP-MS zircon U–Pb data of the garnet–clinopyroxene amphibolite and the garnet-free amphibolite yielded lower intercept ages of 410±11Ma and 403±8Ma, respectively, while the garnet–mica amphibolite gave three metamorphic ages of 441±3Ma, 408±12Ma and 370±2Ma. Three suites of metamorphic conditions have been estimated at 1.14–1.55Gpa and 458–548°C, 0.67–0.82Gpa and 590–600°C, and 0.47–0.66Gpa and 557–586°C for garnet–clinopyroxene amphibolite, using different mineral thermobarometers, which illustrate a collisional orogeny-related clockwise P–T path. Integrated with the study on the high-pressure granulite in the Mogutai area, it can be inferred that the metamafic rocks in the Sanweishan area obviously experienced amphibolite–facies metamorphism at ca. 410–400Ma, and they could also underwent high-grade metamorphism at ca. 440–430Ma. Therefore, we suggest that the E–W-trending collisional orogeny of ca. 440–400Ma could sweep cross the Dunhuang region.The Dunhuang region records multiple tectono-thermal events occurring at ca. 3.1–2.5Ga, ca. 2.3–1.6Ga, ca. 440–400Ma and ca. 370–310Ma. However, compared with the limited exposure of the Precambrian complex, the Early Silurian–Late Carboniferous (ca. 440–310Ma) orogeny-related metamorphic and magmatic rocks are widely distributed in the Dunhuang region, making up nearly 70% of the total exposure. Thus, it is proposed that the geological bodies in the Dunhuang region were mainly produced by a Paleozoic orogeny and represent a Paleozoic orogen, while the limited Precambrian complex probably is a microcontinental fragment or part of the margin of the Tarim or North China Cratons that was involved in this Paleozoic orogeny during the Early Silurian–Late Carboniferous. We infer that the Dunhuang orogenic belt is part of the Central Asian Orogenic Belt (CAOB), and thus the CAOB probably extends beyond the Beishan orogenic collage and southward into the Dunhuang region.