Late Mesozoic volcanism in the southern Xiong'er Mountains, eastern Qinling: Partial melting of the subducted Qinling crust underneath North China

Abstract

Volcanism along the continental margins is a powerful tool with which to probe the orogenic processes and crustal components underlying orogenic belts. In this study, we report the zircon ages and geochemical compositions of volcanic rocks exposed in the Luanchuan area, along the southern margin of the North China block. In previous studies, these volcanic rocks were reported to be products of Paleoproterozoic (ca. 1.75 Ga) volcanism related to rifting in the Xiong’er Mountains. Our zircon age data reveal that most volcanic rocks contain late Mesozoic zircon grains of magmatic origin, suggesting that Late Jurassic to Early Cretaceous (ca. 150−120 Ma) volcanism must have occurred in the southern Xiong’er Mountains, coeval with emplacement of large granitoid plutons along the southern margin of the North China block. These late Mesozoic volcanic rocks mainly consist of trachyandesite, andesite, dacite, and rhyolite. They vary in their major-element compositions but are relatively uniform in their normalized patterns of trace and rare earth elements and have consistently enriched Nd isotopic compositions. Numerous inherited and/or xenocrystic zircon grains of mostly Paleoproterozoic and occasionally Neoproterozoic to early Mesozoic ages were found in all of the volcanic rocks, indicating complex magma sources and/or diverse crustal rocks overlying the magma chambers. The Paleoproterozoic zircon grains originated from the Paleoproterozoic volcanic and basement rocks in the southern North China block. Notably, some volcanic rocks contained early Mesozoic, early Paleozoic, and Neoproterozoic zircon grains of both metamorphic and magmatic origin that indicate derivation from the North and South Qinling blocks, which recorded thermo-tectonic events during the closure of the Shangdan and Mianlue oceans in the early Paleozoic and early Mesozoic. Therefore, we propose that these late Mesozoic volcanic rocks originated from partial melting of crustal rocks and are composed partly of the subducted basement of the Qinling orogenic belt underneath the southern Xiong’er Mountains due to asthenospheric upwelling in an extensional setting.