Volume 38: January - December 1997

Issue 1: January 1997

Abstract

Continental lithospheric contribution to alkaline magmatism: isotopic (Nd, Sr, Pb) and geochemical (REE) evidence for Serra de Monchique and Mount Ormonde complexes

J. Bernard-Griffiths, G. Gruau¹, G. Cornen², B. Azambre³ and J. Mace¹ ¹Geosciences Rennes, UPR 4661 CNRS, Campus de Beaulieu, Avenue du General Leclerc, 35042 Rennes Cedex, France, ²Laboratoire de Petrologie Structurale, UER des Sciences de la Nature, Faculte des Sciences de Nantes, 2 Rue de la Houssiniere, 44072 Nantes cedex 03, France, ³Department de Petrologie, Universite P et M Curie, 4 Place Jussieu, 75252 Paris cedex 05, France and Corresponding author
ABSTRACT
Isotopic results (Sr, Nd, Pb), as well as concentrations of major and trace elements (REE) are reported for whole-rock samples and mineral separates for the onland alkaline complex of Serra de Monchique (South Portugal) and the offshore alkali basalt volcanic suite of Mount Ormonde (Gorringe Bank). These two genetically related alkaline complexes were emplaced at the east Atlantic continent-ocean boundary during the Upper Cretaceous, i.e. 66-72 m.y. ago. Taken together, Serra de Monchique and Mount Ormonde may be seen as one of the few examples of within plate magmatism that straddles the continent-ocean boundary. Major and trace element compositions fail to reveal any significant differences between onland and offshore complexes. This is particularly true regarding less differentiated samples (mg-number <= 0.40) which show the same progressive and continuous enrichment of their trace element patterns, with no specific anomaly (e.g. negative Nb anomaly) being present in samples from the onland complex. Initial Pb and Sr isotopic compositions also do not allow any distinction to be made between Serra de Monchique and Mount Ormonde samples. Initial Pb isotope ratios are moderately high (19.1<²⁰⁶Pb/²⁰⁴Pb<19.8; ²⁰⁷Pb/²⁰⁴Pb=15.6) in both cases. Moreover, once the effects of Sr contamination by seawater are taken into account and the most contaminated samples discarded using data from fresh clinopyroxene separates and results of leaching experiments, the initial Sr isotopic compositions of Mount Ormonde samples are found to be unradiogenic (⁸⁷Sr/⁸⁶Sr=0.7031+-1) and identical to those obtained at Serra de Monchique (⁸⁷Sr/⁸⁶Sr=0.7032+-1). In contrast, a systematic mean difference of 2 [epsilon]Nd units is observed between Serra de Monchique [[epsilon]Nd(T)=+4.8] and Mount Ormonde [[epsilon]Nd(T)=+6.6] whole-rock samples. Surprisingly, a variation is also observed at Mount Ormonde between the whole-rock samples and one of the two analysed clinopyroxene separates. Whereas Mount Ormonde whole-rock samples invariably yielded [epsilon]Nd(T)=+6.6 (mean value), a value of +0.5 is obtained for one clinopyroxene separate, whereas another gives +6.0. The above geochemical and isotopic results make it possible to assign respective roles to the asthenosphere, lithosphere and crust in the petrogenesis of Serra de Monchique and Mount Ormonde complexes. We propose that both complexes share a common mantle source whose isotopic characteristics are very similar to the source of oceanic island basalts. Continental mantle lithosphere, already characterized isotopically by studies of periodite massifs within the Iberian peninsula, acts as a contaminant which is evident onland on the whole-rock scale, and also present offshore as discrete clinopyroxine xenocrysts. The continental crust appears to play no role in the petrogenesis of the Serra de Monchique alkalinr rocks.

Keywords: alkaline complexes; continental lithosphere; isotope geochemistry, passive continental margin; within-plate volcanics.

Pages: 115 - 132

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