Geological characteristics and age determination of the Palaeoproterozoic Gudongjing Group complex in the Beishan Mountain, Gansu Province

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There are abundant outcrops of Gudongjing Group in Beishan region without paleontological fossil, therefore its strati-graphic age was previously determined by regional stratigraphic correlation and the Sm-Nd isochron age obtained from the overlying strata and underlying layer, resulting in no precise age for Gudongjing Group. In this study, the authors applied LA-ICP-MS and SHRIMP methods to determine isotopic age of detrital zircon in Gudongjing Group with the purpose of obtaining its age. The youngest age of detrital zircon is 1213±24Ma and it is considered to be the lower age limit. The detrital zircon age of 3110±6Ma and 2786±7Ma may indicate ancient remnants of the Earth's crust in Beishan micro-block, and Gudongjing Group may be one of the metamorphosed basements in Beishan. Three continuous peak ages (1750Ma, 1580Ma and 1470Ma) show that the provenance of most quartzite in Gudongjing Group went through the Luliang movement. Therefore, the results of this study can serve as powerful evidence for completing the stratigraphic framework in Beishan.

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The Precambrian metasedimentary rocks that are extensively distributed in the Beishan Orogenic Belt (BOB) can provide crucial geological information to constrain its tectonic affinity and geological evolution. Field investigation, zircon U–Pb dating and Lu–Hf isotopic analyses were conducted in combination for investigating them. Detrital zircon U–Pb dating shows that a quartzite sample of the Beishan Complex yields the youngest age at ca. 1710 Ma, with a major age peak at 1828–1778 Ma. Five quartzite samples from the Gudongjing Group and Pingtoushan Formation exhibit similar detrital zircon age distributions mainly from the late Paleoproterozoic to Mesoproterozoic, with the youngest ages at ca. 953 Ma, ca. 1000 Ma, ca. 978 Ma, ca. 960 Ma and ca. 985 Ma. Considering the 1.4 Ga granitoids and age peak of the early Neoproterozoic metasedimentary rocks, the supracrustal rocks within the Beishan Complex were probably deposited during the late Paleoproterozoic to early Mesoproterozoic (ca. 1.7–1.4 Ga). Moreover, owing to the presence of ca. 933–871 Ma granitoids, the deposition ages of the Gudongjing Group and Pingtoushan Formation can be constrained during the early Neoproterozoic (ca. 953–933 Ma). The metasedimentary rocks, the same as the Meso-Neoproterozoic granitic magmatism, support the existence of a Precambrian microcontinent in the BOB. The early Neoproterozoic metasedimentary rocks have continuous detrital zircon ages during the Mesoproterozoic, mainly with positive εHf(t) values. Integrated with the magmatic records of ca. 0.9 Ga and ca. 1.4 Ga, we believe that the Precambrian basement of the BOB could be a fragment drifted from the Fennoscandia of the Baltica Craton. Field investigation revealed that carbonate rocks, mature sandstones and siltstones primarily comprise the protoliths of the early Neoproterozoic metasedimentary rocks, implying the sedimentation in a passive continental margin. However, an almost contemporaneous granitic magmatism was formed in a continental arc setting associated with the assembly of Rodinia. Therefore, it can be inferred that the BOB could be located in the periphery of the Rodinia supercontinent, where the sedimentation of the passive continental margin occurred at its outboard position.
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