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Exceptionally well-preserved pillowed andmassive phenocryst-free metabasaltic lava flows in theuppermost part of the Palaeoarchaean Hooggenoeg Complexof the Barberton Greenstone Belt exhibit both flowbanding and large leucocratic varioles. The flow bandingis defined by blebs and bands of pale and dark greenmetabasalt and was the result of mingling of two types ofbasalt (Robins et al. in Bull Volcanol 72:579–592, 2010a).Varioles occur exclusively in the dark chlorite-, MgO- andFeO-rich metabasalt. Varioles are absent in the outermostrinds of pillows and increase in both abundance and sizetowards the centres of pillows. In the central parts of somepillows, they impinge to form homogeneous pale patches,bands or almost homogenous cores. Individual variolesconsist essentially of radially orientated or outwardlybranching dendritic crystals of albite. Many variolesexhibit concentric zones and finer-grained rims. Somevarioles seem to have grown around tiny vesicles andvesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding theocelli contains acicular pseudomorphs of actinolite andchlorite after chain-like, skeletal Ca-rich pyroxenes that arepartly overgrown by the margins of varioles. Varioles areenriched in the chemical constituents of feldspar but containconcentrations of immobile TiO2, Cr, Zr and REE thatare similar to the host metabasalts. The shape, distribution,texture and composition of the varioles exclude liquidimmiscibility and support an origin by spherulitic crystallisationof plagioclase from severely undercooled basaltmelt and glass. Nucleation of plagioclase was stronglyinhibited and took place on vesicles, on the bases ofdrainage cavities and along early fractures. Eruption indeep water and retention of relatively high concentrationsof volatiles in the melt may be the principal cause ofspherulitic crystallisation in the interiors of pillows ratherthan only in their margins as in younger submarine flows

The origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex, Barberton Greenstone Belt, South Africa