Influence of Sr on an Established Petrological Incompatibility: The Association Melilite + K-Feldspar in a Nephelinite from Djebel Targou, Central Morocco |
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| Authors: | VELDE DANIELLE; EN-NOUFOUS RACHDI HAYAT |
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| Institution: | Laboratoire de Petrologie Min?ralogique, U.A. 0736 du C.N.R.S. Tour 26, E3, 4, Place Jussieu, 75252 Paris Cedex 05 |
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| Abstract: | A peralkaline olivine-free nephelinite from Morocco containsan unusual minerological assemblage of Ti-rich garnet, nosean,clinopyroxene, nepheline, leucite, K-feldspar, and melilite.This occurrence appears to be the first report of coexistingK-feldspar and melilite in a lava. The rock bulk compositionshows unusually high SrO content (0.90%) but the calculatedCIPW norm indicates only moderate silica-undersaturation. Amongthe minerals present nosean contains minor amounts of Sr butmost of the available Sr is concentrated in melilite and largezoned crystals of xenocrystic apatite. Apatite contains up to25% SrO. Experimentally determined partition coefficients ofSr between apatite and melt indicate that it could not haveprecipitated from a nephelinite magma. Isotopic compositionof both Nd and Sr dispel any contamination by either sediments,metamorphic basement, or carbonatites. It is deduced that partialor complete dissolution of Sr-rich xenocrysts (apatite and possiblycarbonates) from an ultrabasic alkaline complex in a peralkalinenephelinite composition induces the precipitation of Sr-bearingmelilite in the presence of the normal nepheline +leucite +K-feldspar assemblage. The Moroccan nephelinite thus providesan interesting example where a minor element influences therelations and commonly observed petrological incompatibilitiesbetween phases. |
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