Provenance and tectonic settings of the Late Paleozoic sandstones in central Inner Mongolia, NE China: Constraints on the evolution of the southeastern Central Asian Orogenic Belt

Abstract

Late Paleozoic sedimentary strata outcrop extensively in central Inner Mongolia, and are a key to understanding the tectonic evolution of the southeastern Central Orogenic Belt. A combined analysis of petrography, whole-rock major and trace element, and Nd isotope is carried out on representative sandstones from the Late Paleozoic sedimentary strata (420–270 Ma). The sandstones are mainly wackes and litharenites in lithology, with low SiO2/Al2O3 of 2.85–9.47 (averagely 5.22) and poor textural and compositional maturities, implying short sediment transportation between the depositional basins and provenances. The trace element compositions are generally comparable to that of the average upper continent crust (UCC), with negatively-sloping chondrite-normalized rare earth element distribution patterns ((La/Yb)N = 3.43–11; averagely 6.94) and flat UCC-normalized trace element distribution patterns. The Nd isotopic compositions show great variation (ԐNd(t) = −5.01 to 5.35) with depositional time of the sandstones, and coincide well with the arc magmatic phases in central Inner Mongolia. The geochemical signatures of the sandstones indicate that the dominant provenances are intermediate to felsic arc magmatic rocks that have ages approximating the deposition, although old, recycled sediments may have made a minor contribution. An active continental arc setting during the Late Paleozoic in central Inner Mongolia, controlled by the northward subduction of the Paleo-Asian oceanic slab, was the most likely depositional tectonic setting of the sandstones. This active continental arc setting continued to at least 270 Ma, implying that the final closure of the Paleo-Asian Ocean along the Solonker suture zone most likely occurred sometime during the Late Permian to Early Triassic. The northward subduction of the Paleo-Asian Ocean is likely of West Pacific-style, in which the present-day Baolidao arc has a close genetic link with the South Mongolian microcontinent and, likely, the former originally formed as the arc margin of the latter.