NO BIG TANK: SLOW INCREMENTAL GROWTH OF PLUTONS BY MAGMATIC CRACK-SEAL

Granitic plutons are central to unraveling processes and rates of tectonism and crustal growth, yet mechanisms by which plutons grow and are emplaced remain uncertain, and estimated rates of magma ascent and volumetric addition vary by orders of magnitude. A growth model that emerges from studies of pluton growth histories, wall-rock interactions, pluton shapes and internal structures, and petrology and petrochemistry, promises to reconcile diverse observations into an integrated framework. A central aspect of the model is that a large body of igneous rock does not grow from a large body of magma. Magma composes a small fraction of a pluton at any one time, and a pluton may grow over millions of years such that parts of the same intrusion differ substantially in age. Growth occurs by episodic incremental addition at short-term rates that may greatly exceed the long-term growth rate. The volume of a pluton that contains partial melt at any time differs in shape and size from growth increments because increments solidify diachronously and because parts of earlier increments may be partially remelted. An incrementally growing pluton thus contains two types of internal contacts—intrusive contacts between increments, and solidification fronts between solid and partially molten rock—that tend to be discordant even where intrusive contacts are mutually parallel. The volume of a magma body relative to the final volume of a pluton is governed by the rate at which magma, and thus heat, is added to the system. The composite nature of plutons is commonly cryptic in the field 1) owing to recrystallization at migrating and oscillating solidification fronts, and 2) because incremental growth promotes subsolidus recrystallization as much of the pluton remains at elevated temperature for long times because early increments lie in the metamorphic aureoles of later additions. At least in the upper crust, plutons are emplaced mainly by opening fractures to form dikes, sills, and laccoliths. Pluton shapes commonly reflect this process, but plutons of arbitrarily complex shape can by assembled from sheet-like intrusions in varying orientations. A pluton formed by incremental fracture opening—magmatic crack-seal— does not grow at depth and ascend en masse, but rather grows in situ as magma is incrementally added via conduits such as dikes.