Abstract
New data and interpretations are presented for the igneous albitites of the Wadi Tarr area, southern Sinai, Egypt. The albitite masses are isolated in outcrop from any granitic intrusions and have intrusive contacts against the country rocks without any structural control. They have marginal zones of breccias with jigsaw-fit angular clasts suggesting explosive, in-situ formation. The albitites are of two types: the western, medium-grained, hypabyssal albitite and the eastern, fine-grained porphyritic albitite. The field relations suggest emplacement at different levels in a magmatic cupola: the hypabyssal texture and steeply dipping slope of the upper contact of the western albitite imply deeper emplacement whereas the gently dipping contacts and porphyritic texture of the eastern albitite masses indicate that they define the probable location of the cupola apex. Both types of albitites consist of albite (92–97%) with minor amounts of quartz, K-feldspar and biotite. The accessory minerals include Fe-oxides, augite, sulphides, zircon, rutile, xenotime, titanite, allanite and monazite. The whole-rock compositions of the hypabyssal and porphyritic albitites are closely related, but the porphyritic type has lower abundances of Sr, Ba, Y, Nb, Th and Zr. We show that the hypabyssal and porphyritic albitites have a common petrogenetic origin, most likely as late-stage cumulates from a fractionating, strongly alkaline A-type magma, consistent with the compositions of the mafic minerals. The source magma was probably a tephritic liquid; we use MELTS models to show that only a sufficiently alkaline magma follows a differentiation path that both avoids quartz saturation and encounters the alkali feldspar solvus, reaching a residual liquid in equilibrium with highly sodic feldspar. Although the MELTS results show a chemically consistent means of forming igneous albitite, they are incomplete in that physical segregation mechanisms are still required to isolate the albite from mafic minerals and or a low-temperature aqueous alteration stage is needed to leach K from the feldspar. Alteration surrounding the Wadi Tarr albitites is extensive and dominated by alkali metasomatism similar to fenitization. Alteration in the marginal breccia zone of the albitite is dominated by precipitation of amphibole and carbonate in veins and in the breccia matrix, whereas the volcanic country rocks show replacement of feldspars by sericite, carbonate and epidote as well as vein carbonate. The altered volcanic country rocks show lower concentrations of Fe2O3, Sr, Cu, Pb, Ba and Ce, accompanied by higher concentrations of Na2O and MgO compared to unaltered equivalent samples.
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•The hypabyssal and porphyritic Tarr albitites have a common petrogenetic origin.•Tarr albitites formed in a magmatic cupola accompanied by fenitization.•Dips of the contacts are consistent with the cupola apex above porphyritic phase.•Tarr albitites are late cumulates from fractionation of alkaline, tephritic magma.