A rift system in southeastern Central Asian Orogenic Belt: Constraint from sedimentological, geochronological and geochemical investigations of the Late Carboniferous-Early Permian strata in northern Inner Mongolia (China)

Abstract

The Late Carboniferous-Early Permian is the most controversial period for the tectonic evolution of the southeastern Central Asian Orogenic Belt (CAOB). In this study, we performed sedimentological and detrital zircon dating on the clastic sediments, as well as zircon U-Pb age and geochemical analysis on the rhyolitic rocks of the Late Carboniferous-Early Permian strata from northern Inner Mongolia (China). The sedimentological analysis showed that the Late Carboniferous-Early Permian sediments are composed of continental clastic and volcanic rocks with basal conglomerate overlying the Devonian strata. The detrital zircon study revealed three age peaks at ~300Ma, ~440Ma and ~530Ma, indicating that the clastic rocks derived from Paleozoic magmatic rocks and pre-Carboniferous sediments from northern Inner Mongolia. The age data revealed that rhyolitic rocks from northern Inner Mongolia extruded in two stages at Late Carboniferous and latest Carboniferous-Early Permian. The Late Carboniferous rhyolites show I- to A-type granitic affinities whereas the latest Carboniferous-Early Permian rhyolites have A-type granitic characteristics. The difference suggests a transition of tectonic setting from post-collisional extension to intraplate rift. The Late Carboniferous-Early Permian strata from northern Inner Mongolia and South Mongolia show similar sedimentary facies, detrital zircon age distribution and geochemistry features, hence they were probably deposited in the same tectonic setting. A unified block was finally proposed in northern Inner Mongolia and South Mongolia, where a rift system provided an intra-continental sedimentary basin and induced intense syn-rift magmatic activities. The intra-continental basin and syn-rift magmatism indicate that the Xing'an-Airgin Sum, Songliao-Hunshandake and South Mongolia Blocks in southeastern CAOB have been welded into a single continental block before the Late Carboniferous.