Zircon U-Pb-Hf Isotopes and Whole-Rock Geochemistry of the “Kulumudi Formation” from the Laofengkou Area (West Junggar): Implications of the Construction of a Juvenile Arc in the Junggar–Balkhash Ocean

Abstract

The properties of ancient magmatic arcs are crucial for understanding the tectonic evolution of the Central Asian Orogenic Belt. The Middle Devonian Kulumudi Formation in the Laofengkou area of West Junggar lacks accurate chronological data constraints, which hampers the knowledge of the nature of the Late Paleozoic magmatic arcs in the West Junggar and circum-Balkhash areas. In this contribution, samples of pyroclastic rocks and sedimentary rocks were collected from the volcano–sedimentary strata of the Kulumudi Formation. Petrography, zircon U-Pb-Hf isotopic analysis and whole-rock geochemistry were carried out to constrain the age and the tectonic setting of the Kulumudi Formation. The zircon U-Pb age of the lithic crystal tuff from the Kulumudi Formation on the northeast side of the Alemale Mountains was 386 ± 2 Ma, accurately indicating that this rock unit formed during the Middle Devonian. However, the fine sandstone near the Huojierte Mongolian Township, originally assigned as the “Kulumudi Formation”, yielded a maximum depositional age of 341 ± 3 Ma. Combined with the stratigraphic contact, this rock unit was redefined to belong to the Lower Carboniferous Jiangbasitao Formation. According to the whole-rock geochemistry study, the lithic crystal tuff of the Kulumudi Formation was characterized as medium potassium–calc–alkaline series rock, which is relatively enriched in light rare earth elements and large ion lithophile elements (i.e., Rb, Ba, K) and depleted in high-field-strength elements (i.e., Nb, Ta, Ti), showing similar geochemical characteristics to the volcanic arc rocks. By contrast, the fine sandstone from the Jiangbasitao Formation had Al2O3/SiO2 (0.25–0.29) and K2O/Na2O (1.29–1.72) ratios close to those derived from the continental arc and active continental margin and was characterized as part of the continental arc field in the La-Th-Sc and Th-Sc-Zr/10 tectonic discrimination diagrams. Zircon Hf isotope analysis showed that the εHf(t) values of the Kulumudi Formation were +5.6–+12.8, and those of the Jiangbasitao Formation were +11.43–+15.48, both of which show highly positive juvenile characteristics. The above data indicate that the Kulumudi Formation and Jiangbasitao Formation both formed in a juvenile arc setting with ocean–continent subduction. Combined with the previous work, it was concluded that the southward subduction of the ocean basin represented by the Darbut–Karamay ophiolitic mélanges beneath the newly accreted arc crustal segments produced a juvenile arc with positive Hf isotope characteristics.