Reworked early Precambrian basement of the Dunhuang Block (NW China) as revealed by titanite U–Pb dating

Abstract

The Dunhuang Block in the Central Asian Orogenic Belt (CAOB) consists of Archean to Paleoproterozoic crystalline basement, along with Paleozoic complexes that are related to the evolution of the CAOB. A current main controversy concerns whether the whole Dunhuang Block has experienced the Paleozoic tectonothermal events. Titanite, a common U–bearing accessory mineral in igneous and metamorphic rocks, is susceptible to magmatic and metamorphic processes, and thus tends to record relatively late tectonothermal events for metamorphic rocks. In this study, we conducted LA–ICPMS U−Pb dating on titanites from the Archean to Paleoproterozoic rocks in the Dunhuang Block. The titanites are likely metamorphic in origin and yielded U–Pb ages of 409−309 Ma. These ages can be divided into two major groups at 409−390 Ma and 354−309 Ma, corresponding to an early-stage retrograde of high–grade metamorphism and a later-stage magmatism, respectively. Overall data and regional geology indicate that the whole Dunhuang Block has been intensively reworked by the Paleozoic tectonothermal events, leading to metamorphic and magmatic overprints of the early Precambrian basement rocks during the evolution of the CAOB in the Paleozoic. Therefore, the Dunhuang Block should not be considered as an independent Paleozoic orogenic belt. More importantly, our study demonstrates that titanite U–Pb dating can be used as a potential geochronologic tool to reveal late tectonothermal events of the reworked Precambrian metamorphic terranes.