Petrology and origin of the type monchiquites and associated lamprophyre dykes of Serra de Monchique, Portugal

Abstract

SynopsisThe term ‘monchiquite’ has suffered such conflicting usage that a redefinition is here proposed:monchiquitesare alkaline lamprophyres with essential brown amphibole and/or biotite but not necessarily with olivine, possessing a near-isotropic feldspar-free base of glass (hyalomonchiquites), analcime (analcime-monchiquites) or nepheline (nepheline-monchiquites). This effectively reverts to Rosenbusch's original definition, whilst accommodating established practices in the subsequent literature. Notwithstanding certain published interpretations, the type monchiquites from Monchique and Rio de Janeiro are hyalomonchiquites.The Monchique monchiquites are associated with camptonites and sannaites in a dyke swarm invading the Monchique foyaitic intrusion. Field, petrological, geothermometric and geochemical evidence is presented to suggest that these dykes resulted from two mixing processes: (i) a version of Bowen's classic reaction between mafic crystals and alkaline liquids—here between cumulus divines and foyaitic magma; (ii) hybridisation of hydrous theralitic and nepheline syenite magmas, restricted by liquid immiscibility. The secondary origin of certain dykes is evinced by extreme mineralogical disequilibrium. The monchiquites could represent a more advanced stage of such processes than the camptonites and sannaites.