The Tarr albitite: a metasomatic plagiogranite from mainly non- intrusive protoliths.

The Tarr albitite complex of SE Sinai is structurally controlled by two large shear zones and is spatially associated with a tectonically emplaced ophiolitic suite consisting of hornblendite, gabbro, talc-serpentinite and carbonate. Based on un- or partially metasomatized remnants of protoliths throughout albitite bodies, gradational contacts, ghost structures and other features, the albitite is shown to have originated by Na-metasomatism. The metasomatic albitites are typified by pseudo-igneous textures, which developed from a variety of meta-sedimentary and meta-pyroclastic country rocks and tonalite. Chemical changes are extreme with bulk additions of >5% Na₂O and depletions of >5% K₂O, >4% MgO and >8% total Fe. The primary metasomatic flux may have originated as sea-water. Albitites occur in a wide variety of geological environments and apparently formed through igneous and metasomatic processes. Those in ophiolites (leucocratic plagiogranites), generally associated with gabbros, are considered to have formed originally either by liquid immiscibility or as late residual fractions, and are commonly hydrothermally or metasomatically altered. It is suggested that their high Na₂O/K₂O ratios are better explained by metasomatic processes, and that certain leucocratic plagiogranites may be metasomatic throughout. (Authors' abstract)-A.W.H.