Permian doleritic dikes in the Beishan Orogenic Belt, NW China: Asthenosphere–lithosphere interaction in response to slab break-off

Abstract

Extensive Permian mafic–ultramafic intrusions crop out within the Central Asian Orogenic Belt (CAOB). Because the magmatic activity is spatially and temporally associated with the Tarim Large Igneous Province (LIP), a mantle plume has been invoked to explain the petrogenesis of these mafic–ultramafic rocks. In order to test this hypothesis, we present new geochronological and geochemical data for doleritic dike swarms in the Beishan area, southern CAOB. Zircon U–Pb dating of the Yinaoxia and Gubaoquan doleritic dikes indicates that they were emplaced during the Early Permian (ca. 282Ma). The Yinaoxia doleritic dikes have higher Mg# (52–74) and εNd(t) values (9.0–9.1) than the Gubaoquan dikes (Mg#=35–56; εNd(t)=5.8–7.4), indicating that the former are more primitive than the latter. The Gubaoquan dikes are characterized by higher Ba/Nb (15–276), Ba/La (6–103), Th/Yb (0.12–0.24) and Th/Nb (0.20–0.36) ratios, suggesting that their parental magmas contained more subduction-related material. Calculated parental magmatic compositions in equilibrium with Cr-rich clinopyroxenes within the dikes have LREE-depleted to flat REE patterns ((La/Yb)N=0.57–1.28) and negative to slightly positive Nb anomalies ((Nb/La)PM=0.33–0.91). These data suggest that their parental magmas may have been generated by interaction between depleted asthenospheric mantle and metasomatized lithospheric mantle. Using clinopyroxene–liquid thermobarometers, we estimate a temperature range of 1142°C–1276°C, indicative of an upper asthenospheric mantle source. Although the doleritic dikes in Beishan are coeval with the Tarim LIP, their petrological and geochemical compositions do not suggest the abnormally high mantle temperatures or OIB-like mantle signatures that characterize the mantle source of the Tarim LIP magmas. This means that the Permian Tarim mantle plume cannot account for the formation of the doleritic dikes in the Beishan area, and that the Tarim LIP does not extend to the Beishan area. Instead, the doleritic dikes were probably derived from magmas generated by partial melting of upwelling depleted asthenospheric mantle that was subsequently contaminated to various degrees by metasomatized lithospheric mantle in a slab break-off scenario.