A non-plume model for the Permian protracted (266–286 Ma) basaltic magmatism in the Beishan–Tianshan region, Xinjiang, Western China

Abstract

The convenient mantle plume model for the Permian protracted mafic–ultramafic intrusions and mafic dykes (266–286Ma) in the Beishan–Tianshan region, northern Xinjiang, western China can be rejected, because their temporal–spatial distribution does not show a hotspot track predicted by such model. New zircon U–Pb ages reveal that two small mafic dyke clusters (Podong, 280.5±2Ma; Luodong, 266.2±3.2Ma) that are separated by only ~20km in the Pobei area, the southernmost part of the Beishan–Tianshan region, have a large age difference of ~18Ma. The older mafic dykes are characterized by nearly flat mantle-normalized rare-earth-element patterns, pronounced negative Nb–Ta anomalies and positive εNd(t) values from 5.5 to 7.5, similar to the majority of the Permian mafic–ultramafic intrusions in the region. The younger mafic dykes are characterized by significant light rare-earth-element enrichments as well as pronounced negative Nb–Ta anomalies, plus lower εNd(t) (−1.2 to 2.6) values and higher initial 87Sr/86Sr ratios than the older mafic dykes. The observed compositional variations can be explained by source mantle heterogeneity plus different degrees of crustal contamination. Overall, the Permian mafic–ultramafic rocks in the Beishan–Tianshan region are geochemically consistent with the products of basaltic magmatism induced by lithospheric delamination and asthenosphere upwelling in a convergent tectonic zone.