COMPOSITIONAL VARIATIONS OF CRETACEOUS PUMPELLYITES ALONG THE WESTERNMARGIN OF SOUTH-AMERICA AND THEIR RELATION TO AN EXTENSIONAL GEODYNAMIC SETTING

Pumpellyites in pervasively altered basic to intermediate rocks from t hree Cretaceous Andean volcanic sequences, the Colombian Diabasic Grou p, the Ecuadorian-Peruvian Celica Formation/Casma Group and the Chilea n Ocoite Group, were studied to test their compositional behaviour in relation to changes in geodynamic setting. They occur mostly in assemb lages of the prehnite-pumpellyite facies filling amygdules and inside plagioclase phenocrysts. (a) Pumpellyites from the three geodynamic se ttings define three distinct compositional fields in AFM space: (i) th ose 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 Celi ca Formation/Casma group plot between, and partly overlapping, fields (i) and (ii). (b) Pumpellyites in group (i) formed in T-MORB like, K-p oor tholeiites (high Fe/Al), affected by ocean-floor metamorphism in a n oceanic back-arc basin. Pumpellyites in group (ii) generated in K-ri ch, calc-alkaline (low Fe/Al) to shoshonitic metabasites affected by b ufial metamorphism in an ensialic, aborted, marginal basin with modera te attenuation of the continental crust. Pumpellyites in group (iii) f ormed in basic and intermediate, calc-alkaline to tholeiitic rocks, me tamorphosed in ensialic marginal basins with various degrees of contin ental crust thinning, from splitting to moderate attenuation. A correl ation between pumpellyite and host-rock composition is suggested by th ese characteristics. (c) Pumpellyites in prehnite-pumpellyite facies a ssemblages of some of the units studied plot inside higher (and lower) grade reference fields (AFM space) corresponding to different geodyna mic settings elsewhere. This anomaly is attributed to the changing cha racteristics of the marginal basins at the South American margin and e mphasizes the need to compare equal facies referred to equal settings.