Late Permian–Early Triassic mafic dikes in the southwestern margin of the South China block: Evidence for Paleo-Pacific subduction

Abstract

The link of the widespread Late Permian–Triassic magmatism in the southwestern corner of the South China block to a subduction zone or the Emeishan mantle plume is debated. We have carried out an integrated geochronological-petrological-geochemical study of four mafic dikes in the Gulinjing–Nanxi area of this region to shed new light on the issue. Zircon grains from the Gulinjing–Nanxi mafic dikes yield UPb ages from 254 ± 2 to 250 ± 2 Ma, indicating that they are coeval with abundant felsic intrusive-extrusive rocks in the region, but are 6–7 myr younger than the nearby Emeishan flood basalts and associated mafic-ultramafic intrusions. The Gulinjing–Nanxi mafic dikes are characterized by light rare-earth element enrichments and pronounced negative NbTa anomalies, similar to typical arc basalts worldwide, plus enriched Sr-Nd-Hf isotope compositions (initial 87Sr/86Sr from 0.7088 to 0.7103, εNd(t) from −6.6 to −4.6, and zircon εHf(t) from −3.7 to −0.3). The isotopes and incompatible trace element ratios indicate that the parental magmas for the Gulinjing–Nanxi mafic dikes are consistent with contaminated arc mafic magmas, but not contaminated Emeishan flood basalts in the region. This, together with a significant age difference between the mafic dikes and the Emeishan flood basalts and associated mafic-ultramafic intrusions in the nearby areas, support the interpretation that the Gulinjing–Nanxi mafic dikes are unlikely to be parts of the Emeishan mantle plume. Based on regional igneous association and the distribution of known paleo subduction zones in the surrounding regions, we conclude that the Gulinjing–Nanxi mafic dikes are linked to Paleo-Pacific subduction to the south, and not Paleo-Tethys subduction to the west.