Compositional variations of Cretaceous pumpellyites along the western margin of South America and their relation to an extensional geodynamic setting

Abstract Pumpellyites in pervasively altered basic to intermediate rocks from three Cretaceous Andean volcanic sequences, the Colombian Diabasic Group, the Ecuadorian-Peruvian Celica Formation/Casma Group and the Chilean Ocoite Group, were studied to test their compositional behaviour in relation to changes in geodynamic setting. They occur mostly in assemblages of the prehnite-pumpellyite facies filling amygdules and inside plagioclase phenocrysts.
(a) Pumpellyites from the three geodynamic settings define three distinct compositional fields in AFM space: (i) those in the Diabasic Group plot closer to the Fe corner; (ii) those in the Ocoite Group plot closer to the Al corner; (iii) those in the Celica Formation/Casma group plot between, and partly overlapping, fields (i) and (ii).
(b) Pumpellyites in group (i) formed in T-MORB like, K-poor tholeiites (high Fe/Al), affected by ocean-floor metamorphism in an oceanic back-arc basin. Pumpellyites in group (ii) generated in K-rich, calc-alkaline (low Fe/Al) to shoshonitic metabasites affected by burial metamorphism in an ensialic, aborted, marginal basin with moderate attenuation of the continental crust. Pumpellyites in group (iii) formed in basic and intermediate, calc-alkaline to tholeiitic rocks, metamorphosed in ensialic marginal basins with various degrees of continental crust thinning, from splitting to moderate attenuation. A correlation between pumpellyite and host-rock composition is suggested by these characteristics.
(c) Pumpellyites in prehnite-pumpellyite facies assemblages of some of the units studied plot inside higher (and lower) grade reference fields (AFM space) corresponding to different geodynamic settings elsewhere. This anomaly is attributed to the changing characteristics of the marginal basins at the South American margin and emphasizes the need to compare equal facies referred to equal settings.