EXTREMES OF RESPONSES OF GRANITIC MAGMA TO ENGULFING BASALTIC MAGMA, COASTAL MAINE MAGMATIC PROVINCE

The coastal Maine magmatic province hosts many bimodal plutons in which magma mingling and hybridization textures attest to the intrusion of granitic magma by basaltic magma (e.g., Cadillac Mountain, Isle au Haut, Vinalhaven) and the likely presence of basaltic magma as a heat source for crustal melting. This area also hosts granitic plutons that are surrounded by gabbro, but were only minimally invaded by basaltic magma, despite the fact that textural evidence indicates that the mafic and felsic magmas were at least partially liquid contemporaneously. The Tunk Lake pluton is an example of such a granitic pluton; it is strongly mineralogically zoned, in a way that suggests that the hot surrounding basalt established a gradient in temperature and water activity across the granitic magma chamber.

Granitic magmas may be influenced by engulfing basaltic magmas in a variety of ways in tectonic settings in which large volumes of basaltic magma invade the upper crust. One end-member of response is the development of a mafic and silicic layered intrusion (MASLI) as described by Wiebe (1993, 1994, 1996), in which large volumes of basaltic magma intrude the granitic magma chamber and produce a layered complex characterized by varying degrees of hybridization of mafic and felsic magmas. The other end-member is a granitic magma that becomes zoned in terms of temperature and water concentration, producing a granitic pluton in which higher liquidus temperature, anhydrous mineral assemblages occur nearest the surrounding mafic magma, and lower liquidus temperature, more hydrous mineral assemblages occur toward the center of the granitic pluton.

The main control on the types of plutons that result from engulfment of a granitic magma chamber by gabbroic magma may be the timing of engulfment relative to the progress toward crystallization of the granitic magma. If the basalt arrives on the scene early in the crystallization history of the granite, large-scale contamination of the granitic magma by the basalt is likely. If the basalt arrives late in the crystallization history of the granitic magma, intrusion of the basalt into the granitic magma chamber is less likely to occur, but the magma chamber still responds to the thermal and water concentration contrast of the surrounding basaltic magma.