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

Published online by Cambridge University Press:  06 July 2012

N. M. S. Rock
N. M. S. Rock
Affiliation:
Institute of Geological Sciences, West Mains Road, Edinburgh
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Synopsis

The term ‘monchiquite’ has suffered such conflicting usage that a redefinition is here proposed: monchiquites are 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.

Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 70 , Issue 5-9 , 1979 , pp. 149 - 170
Copyright
Copyright © Royal Society of Edinburgh 1979

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