THE PATH FROM A BIG TANK OF MAGMA TO SLOW INCREMENTAL GROWTH OF A PLUTON

Progress can be held up by reticence to flout conventional wisdom, which long held that a pluton forms by ascent and crystallization of a magma body of the same size. Indeed, field observation is the traditional ‘acid test’ for geologic theories and most plutons lack obvious internal contacts. The ‘big tank’ view for decades guided thinking about magma generation, differentiation, ascent, and its emplacement as a pluton. The notion that plutons grow in small increments at the site of emplacement arose from observations and calculations that are incompatible with a big tank, e.g.: 1) pluton-sized bodies of melt-rich magma are not geophysically imaged under active volcanoes; 2) upper-crustal wall-rock displacement rates must greatly exceed geodetically measured rates to accommodate emplacement of a large melt-rich body; 3) geochronology indicates that large plutons grew over millions of years, compatible with space-making at geodetic rates but requiring the active magma body to be a small fraction of the pluton; 4) products of supereruptions commonly yield evidence that multiple discrete magma reservoirs were tapped rather than a single large body. As for the field test, what is observed is guided by what is expected; once geochronology indicated that the Half Dome Granodiorite must be composite, abundant evidence emerged for internal discontinuities. Nevertheless, much of the textural record of incremental growth must have been destroyed in many plutons. Such post-magmatic textural modification conflicts with the conventional wisdom that granite textures record crystallization from a melt. Re-examination of outcrop evidence, rock textures, and mineral chemistry in the light of geochronology and thermodynamic constraints leads us to conclude that post-emplacement textural modification in plutons is widespread and profound. The recrystallization likely results from sustained elevated and fluctuating temperatures after initial magmatic crystallization, facilitated by a grain-boundary flux of water released by crystallization of younger increments added to the pluton. Ideas regarding processes and time scales of magmatic generation, differentiation, ascent, and emplacement that are predicated on the conventional view of plutons thus need to be reconsidered in the light of incremental growth.