Closure time in the East Qilian Ocean and Early Paleozoic ocean–continent configuration in the Helan Mountains and adjacent regions, NW China

Abstract

Lithofacies and faunal assemblage affinities of the Helan Mountains and its adjacent blocks are explored using field investigations and analysis of previous results. The tectonic affinity between the Alxa Block and the North China Block is analyzed by means of LA-ICP-MS detrital U–Pb zircon age spectrums. The Early Paleozoic stratigraphic framework is constructed and stratigraphic contact relationships between the study area and its adjacent regions are assessed. The ocean–continent configuration during the Caledonian period is discussed and the closure time in the east of North Qilian Ocean is determined. Finally, the basin type and its evolutionary process during the Early Paleozoic are analyzed. The carbonate platform facies of the Alax Block and the abyssal flysch deposition on the margin of the Central Qilian Block, established the coexistence of ocean and continent configuration during the Cambrian. During the early Ordovician, the stable shallow water platform-shelf facies was distributed over all the study area and the biofacies was also unified. The east of the North Qilian Ocean closed at that time. The North Qilian Ocean and the North Qinling Ocean did not affect the study area, but the North China Sea was connected intermittently to the research area. The east Central Qilian Block, the Alax Block, the North China Block and the Qinling Microblock collided together to form a unified megablock, which resulted in the formation of a complex foreland depression during the early Ordovician. The eastern boundary of the basin is the Aselang–Chedao Transform Fault, which lies between the Alax Block and the North China Block. The southern boundary of the basin is marked by the Guyuan–Qingtongxia Fault which is located between the Alax Block and the Central Qilian Block. The western boundary of the basin is an overlapping boundary toward the Alax Block. During the late Ordovician, when the Yangtze Block approached toward the North China Block, the Shangdan Ocean began to influence the study area and the turbidite fans were formed because of the large-scale transgression. Thus, the basin type is a complicated foreland depression influenced by transgression during the late Ordovician.