Origin of two differentiation trends in the Emeishan flood basalts

Abstract

Both the Bowen and Fenner differentiation trends have been recognized in the Emeishan flood basalts. While the Longzhoushan lavas in Panxi paleorift evolved on a trend of silica enrichment (Bowen trend), the lavas from Guizhou evolved along the Fenner trend leading to the magmas with high Fe2O3 (23%) and low SiO2 (44%) contents. This provides evidence for the existence of Fe-rich and Si-poor magmas in nature. Such contrasting differentiation trends, marked by different timing of crystallization of Fe-Ti oxides, are correlated with the extent of crustal contamination of the magmas. Limited crustal contamination in the Guizhou lavas is coupled with the low oxygen fugacity which delayed the onset of magnetite crystallization. In contrast, significant involvement of crustal components in the Longzhoushan lavas increased ƒO2 which in turn triggered early crystallization of magnetite. The close spatial association between the Longzhoushan lavas and the synchronous V-Fe-Ti deposit-bearing layered intrusions suggests a potential relationship between them.