Geochemistry and petrogenesis of the Permian mafic dykes in the Panxi region, SW China

Abstract

Numerous intrusive bodies of ultramafic–mafic to felsic compositions are exposed in association with volcanic rocks in the Emeishan large igneous province (LIP), southwestern China. In this paper, we present new elemental and isotopic data for the Permian mafic dykes from the Panxi region which is located in the western part of the Emeishan LIP. The characteristics of major and trace elements and Sr–Nd isotopes, in combination with isotopic ages, suggest that these mafic dykes were originated from a similar mantle source related to an upwelling mantle plume and their formation was involved in a process of variable degrees of fractional crystallisation coupled with assimilation of enriched crustal materials. The mafic dyke samples from the Panxi region show some systematic variations in geochemical compositions. The gabbros in Yanyuan contain the lowest REE (e.g., La = 5–12 ppm), the highest MgO (8.38–21.33%) and compatible elements (e.g. Cr = 104–2610 ppm, Ni = 135–496 ppm), and display depleted Sr–Nd isotopic ratios (initial 87Sr/86Sr = 0.703752–0.703844; eNd(t) = + 4.87 to + 5.13). These gabbros likely represent an undifferentiated composition similar to primary melts derived from a depleted mantle source. In contrast, the gabbro samples from Panzhihua and Huili have more enriched LREE abundances and incompatible elements, and display depleted to slightly enriched Sr and Nd isotopic signatures (initial 87Sr/86Sr = 0.704354–0.705436; eNd(t) = - 0.29 to + 4.58), though their trace element spidergram patterns are similar to those of the Yanyuan samples. These gabbros were formed probably through the removal of a few percent of olivine, clinopyroxene, plagioclase and apatite from the melts that formed the Yanyuan gabbros. All the mafic intrusives in the region exhibit elemental and Sr–Nd isotopic features generally comparable with those of the nearby volcanic rocks. In addition, their emplacement immediately preceded or was synchronous with the main pulse of volcanism that formed lavas over a large area along the western margin of the Yangtze Craton. These lines of evidence suggest that there is a genetic link between the mafic dykes and flood basalts in the Emeishan LIP, and the both share a common mantle source related to the Emeishan plume.