Mineral chemistry of the Beacon Heights Phonolite, an agpaitic ignimbrite in the Mesoproterozoic Pilanesberg alkaline complex, South Africa

Abstract The Beacon Heights Phonolite (BHP) in the Mesoproterozoic, alkaline Pilanesberg Complex (North West province, South Africa) is a welded ash-flow tuff that was deposited in an explosive eruption towards the end of the igneous evolution history of the complex, probably related to caldera collapse. The rock has a highly peralkaline composition, and carries characteristic magmatic mineral assemblages indicating an agpaitic crystallisation regime in which the High Field Strength elements (HFSE) Zr and Ti are hosted in alkali- and volatile-bearing minerals, e.g. members of the eudialyte group and the seidozerite and astrophyllite supergroups, rather than the more common zircon and ilmenite. An electron microprobe study of petrologically important minerals in the BHP shows that the composition of clinopyroxene, which is the dominant non-HFSE mafic silicate mineral ranges from Hd50Aeg40Di10 (cores) to Aeg100 rims, with (Ti+Zr)M1 below 0.05 cation per formula unit. In addition, a small number of clinopyroxene grains carry resorbed, xenocrystic diopside cores. Eudialyte-group minerals form a eudialyte-kentbrooksite-ferrokentbrooksite trend, but have suffered post-magmatic loss of sodium. Ti-bearing minerals are early-crystallised titanite, followed by rinkite (including cation-deficit rinkite approaching mosandrite in composition), kupletskite and members of the lamprophyllite group, trending from lamprophyllite towards barytolamprophyllite. Alkali feldspar (microcline) and feldspathoids (nepheline, sodalite) have endmember compositions, probably reflecting late- or post-magmatic reequilibration; secondary analcime and natrolite are also close to theoretical endmember compositions. The igneous mineral assemblage and composition of the major minerals closely resemble those of intrusive members of the Green Foyaite Suite in the Pilanesberg Complex, and a genetic relationship to Green Foyaite magma is indicated, but the rock is more strongly affected by late- to post-magmatic, fluid-driven alteration than its intrusive counterparts.