Late Palaeozoic Tectonics and Crustal Growth in the Eastern Central Asian Orogenic Belt: Insights From Newly Discovered Volcanic Rocks on the Northern Margin of the North China Block

Abstract

Understanding the Late Palaeozoic accretionary processes responsible for the formation of the eastern Central Asian Orogenic Belt (CAOB) is crucial for unravelling continental growth mechanisms in this region. This study systematically investigates newly identified volcanic rocks in the northern margin of the North China Block (NCB) and Bainaimiao Arc Belt (BAB), aiming to elucidate their petrogenesis, tectonic setting and implications for the evolution of the Paleo-Asian Ocean (PAO). Zircon U–Pb dating reveals that these volcanic rocks were emplaced between 278 and 260 Ma. The intermediate-mafic volcanic rocks exhibit typical subduction-related geochemical signatures, including low TiO₂ contents, enrichment in Rb, Ba, U, K, Pb and Sr., and depletion in Nb and Ta, reflecting derivation from a subduction-modified lithospheric mantle. The acidic volcanic rocks, characterised by high SiO₂ and Al₂O₃ contents and pronounced negative Eu anomalies, are interpreted as products of partial melting of the lower continental crust with fractional crystallisation. The _εHf(t) values range from highly positive in the BAB region to lower values in the NCB, highlighting the incorporation of juvenile crustal material in the north and ancient crustal components in the south. Geochemical and isotopic evidence suggests that the volcanic rocks formed in an Andean-type continental arc during the southward subduction of the PAO beneath the northern margin of the NCB. After 260 Ma, the tectonic setting transitioned to an extensional environment, as reflected in the geochemistry of younger intraplate granites. These findings suggest that the PAO continued subduction until the Late Permian, followed by slab break-off and post-collisional extension. The crustal thickening (40–66 km) and widespread magmatism indicate significant juvenile crustal growth during the Middle to Late Permian. Combined with regional tectonic data, this study provides critical insights into the geodynamic processes driving crustal evolution in the eastern CAOB.