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

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.     

Petrology of Mt Etinde Nephelinite Series

Mt Etinde is a volcano situated on the southwestern flank ofthe large Mt Cameroon. Its eruptions are dated at 065 Ma andtook place during the Mt Cameroon eruptive cycle (6 Ma withrecorded recent activity). The lava types, unrelated to theMt Cameroon alkali basalts, are melanephelinites, nephelinitessensu stricto, and numerous and varied nephelinites that containone or more of the following species: nosean, melilite, perovskite,garnet, aenigmatite, leucite, feldspar and haynophyres. Clinopyroxeneis the dominant mafic phase, with a variable composition betweenAl–Ti augite and aegirine. Zoning is also present in garnets,with conspicuous Ti enrichment in the border. Aenigmatite includesa fair proportion of Fe³⁺ Tschermak's component. Melilite issystematically Sr rich; its SrO contents increase continuouslywith MgO decrease, reaching 16 wt % in some facies. The chemicalcomposition of the lavas is extreme, with unusual concentrationsof volatiles (CO₂, H₂O, SO₃), most incompatible elements, suchas Ba, Sr and Zr, and the light rare earth elements (LREE).The Mt Etinde lavas define two lineages (MgO poor and MgO rich)that partly overlap. The chemical evolution of these two lineagescan be reproduced for major elements using a simple model ofcrystal fractionation. The major fractionating phase is an aluminousclinopyroxene, in accord with the petrographical observations.The scheme proposed can only be validated if the alkalis arenot taken into account, a hypothesis warranted by observationsof other nephelinite provinces or ijolite massifs and theirfenite aureoles. Nephelinite magmas were obviously generatedat great mantle depth, but their exotic composition can onlybe produced by partial melting of a metasomatic mantle. Comparisonwith other provinces would point to a source that has undergonecarbonatitic mtasomatism. KEY WORDS: nephelinites; Mt Etinde; Cameroon; petrogenesis; differentiation ^*Present address: Dpartement des Sciences de la Terre, Facult des Sciences, Universit de Yaound, BP 812, Yaound, Cameroon.Corresponding author.