The pressure quench formation of rapakivi texture

Chemical and textural data for rapakivi granites are combined with experimentally determined phase equilibria to provide constraints on the growth of mantled feldspars by a pressure quench mechanism. In water saturated melts of granitic composition the mantled texture develops in response to a decrease in pressure of as little as 1–0.5 kbar. In water undersaturated melts a similar mechanism is operable over a wider range of pressures and a restricted range of water contents. Water undersaturated granites containing 6–10 wt % water undergoing a change in pressure from 5–2 kbar will develop mantling as will a more calcic granodiorite containing 1–5wt% water undergoing a pressure change from 10-1 kbar. The mantling process is interpreted as a reflection of the reduction of pressure accompanying the emplacement of a magma.