Age of the Early Cretaceous Longjiang Formation in the Central Great Xing’an Range, Discovery of Its Basal Unconformity, and Implications for Tectonic Transition

The geochronological framework of the Late Mesozoic volcanic succession in the Great Xing’an Range is crucial for understanding the tectonic regime transition in Northeast Asia. However, the ages and stratigraphic relationships of key volcanic units remain poorly constrained. This study presents zircon LA-ICP-MS U–Pb geochronological data from volcanic rocks above and below the basal unconformity of the Longjiang Formation in the Zhalantun–Jalaid Banner area, central Great Xing’an Range, aiming to determine the timing of volcanic activity, constrain the formation age of the unconformity, and explore its regional tectonic implications. The volcanic–stratigraphic succession in the study area, from base to top, comprises the Baiyingaolao Formation, the basal andesitic conglomerate of the Longjiang Formation, and the Longjiang Formation andesites. Geochronological results indicate that the underlying rhyolitic tuff of the Baiyingaolao Formation yields an age of 130.0 ± 0.1 Ma. Within the andesitic conglomerate overlying the unconformity, andesitic clasts yield an age of 135.8 ± 1.1 Ma, whereas the matrix provides a youngest detrital zircon population age of 130.7 ± 1.0 Ma, constraining the maximum depositional age of the conglomerate. The overlying andesite of the Longjiang Formation gives an eruption age of 125.6 ± 0.8 Ma. These data indicate that the main phase of Longjiang Formation volcanism occurred at ~125.6 Ma, and the basal conglomerate was deposited after ~130.7 Ma. Combined with the ~130 Ma age of the underlying Baiyingaolao Formation and the presence of weathering crusts and erosional surfaces between the two formations, the sedimentary hiatus and exhumation event represented by this unconformity are precisely constrained to have occurred between ~130 Ma and 125.6 Ma. The timing of this unconformity closely coincides with the regional transition in magmatic assemblages from bimodal to andesitic compositions, suggesting that it records a significant tectonic adjustment event in the Great Xing’an Range during the middle to late Early Cretaceous. This finding provides key chronological evidence for understanding the episodic tectonic evolution of Northeast Asia during the Late Mesozoic.