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Journal of Petrology | Volume 29 | Number 3 | Pages 585-597 | 1988
© Oxford University Press 1988

research-article

Influence of Sr on an Established Petrological Incompatibility: The Association Melilite + K-Feldspar in a Nephelinite from Djebel Targou, Central Morocco

DANIELLE VELDE and HAYAT EN-NOUFOUS RACHDI

Laboratoire de Petrologie Minéralogique, U.A. 0736 du C.N.R.S. Tour 26, E3, 4, Place Jussieu, 75252 Paris Cedex 05

Received December 2, 1985; Revised typescript accepted November 10, 1987

ABSTRACT

A peralkaline olivine-free nephelinite from Morocco contains an unusual minerological assemblage of Ti-rich garnet, nosean, clinopyroxene, nepheline, leucite, K-feldspar, and melilite. This occurrence appears to be the first report of coexisting K-feldspar and melilite in a lava. The rock bulk composition shows unusually high SrO content (0.90%) but the calculated CIPW norm indicates only moderate silica-undersaturation. Among the minerals present nosean contains minor amounts of Sr but most of the available Sr is concentrated in melilite and large zoned crystals of xenocrystic apatite. Apatite contains up to 25% SrO. Experimentally determined partition coefficients of Sr between apatite and melt indicate that it could not have precipitated from a nephelinite magma. Isotopic composition of both Nd and Sr dispel any contamination by either sediments, metamorphic basement, or carbonatites. It is deduced that partial or complete dissolution of Sr-rich xenocrysts (apatite and possibly carbonates) from an ultrabasic alkaline complex in a peralkaline nephelinite composition induces the precipitation of Sr-bearing melilite in the presence of the normal nepheline +leucite + K-feldspar assemblage. The Moroccan nephelinite thus provides an interesting example where a minor element influences the relations and commonly observed petrological incompatibilities between phases.


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