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. © 1978 Springer-Verlag.