Spherulitic textures formed during crystallization of partially melted arkose, Rum, Scotland

Abstract

Melting of Torridonian arkoses, due to intrusion of shallow bodies of mafic melts on the Isle of Rum, was locally extensive, reaching up to 95 vol.%. Crystallization (to form granophyre) of these highly melted rocks was generally static in the vicinity of minor intrusions, although disruption of bedding is apparent in the aureole of the main mafic pluton. A distinctive millimetre- to centimetre-scale spherulitic texture developed during solidification of the partially melted arkose, in both the aureole of the main intrusion, and also in those of associated minor gabbro plugs. The spherulites are formed of radiating, fan-shaped intergrowths of plagioclase and quartz set in a matrix of quartz, plagioclase and K-feldspar. The importance of nucleation rates in determining texture development is demonstrated by a dominance of Na-rich compositions in the first feldspar to form, even in non-spherulitic rocks. It is suggested that the spherulites formed by the early growth of metastable plagioclase due to difficulties in nucleating feldspar.