Ascent of felsic magmas and formation of rapakivi

The wide compositional range of rapakivi-bearing rocks strongly suggests that process, not composition, governs the formation of rapakivi texture. A computational investigation was undertaken to determine whether decompression can induce the formation of the common features of rapakivi, i.e., the presence of mantled ovoid megacrysts of alkali feldspar (occasionally with inclusions of plagioclase and quartz), embayed early-formed quartz, and the presence of two generations of the phenocrystic phases. Crystallization path calculations were conducted for seven rapakivi-bearing magma compositions at pressures in the range of 1–8 kbar to determine the effect of decompression on the phase stabilities ofplagioclase, quartz, and alkali feldspar. These calculations indicate that decompression can produce parageneses that can lead to all ofthe characteristic features of rapakivi if (1) H₂O saturation is not attained prior to or during ascent, (2) the cooling rate during ascent lies in the range of ~5–10 °C/kbar, (3) ascent occurs after the magma has become saturated with both alkali feldspar and plagioclase, and (4) the temperature at the onset of decompression is low enough to prevent complete resorption of alkali feldspar during ascent. These constraints readily explain why rapakivi-trearing rocks are relatively uncommon in nature, although the compositions that could produce rapakivi are common and decompression is not an unusual process in the evolution of many felsic magmas.