THE GRANITE AQUEDUCT AND AUTOMETAMORPHISM OF PLUTONS

Ian Carmichael (2002) wrote of an “andesite aqueduct” that conveys vast amounts of water from the magma source region of a subduction zone to the Earth’s surface. Diverse observations indicate that intermediate subduction zone magmas contain 5 wt% or more H₂O. A time-averaged plutonic/volcanic ratio of 5 means that more than 80% of the water in the andesite aqueduct ends up in intrusions; upon crystallization, >90% of this water is released to play a central role in contact metamorphism and the genesis of ore deposits. This water also is important to thermal histories and petrologic characteristics of plutonic rocks.

Field and geochronological data indicate that many plutons were constructed incrementally from the top down, commonly over 1 m.y. or more. Early-formed increments are wall rocks to later increments; heat and water released by each crystallizing increment must pass through older increments before they exit the growing pluton. The water released from a 5 km thick granodiorite pluton formed from dacite magma that is 5 wt% H₂O amounts to a condensed-water equivalent depth of ~500 m per unit of horizontal area. Field and petrographic observations indicate that the water ascends via multiple pathways. Hydrothermal veins record ascent via fracture conduits. Myriad pipe-like hydrothermal conduits in the Tuolumne Intrusive Suite are located in or near aplite-pegmatite dikes, which themselves are crystalline products of hydrous late-stage magmatic liquids. Pervasive grain-boundary infiltration is recorded by albite selvages that formed by solution-precipitation reactions along feldspar grain boundaries and, more generally, by fluid-mediated sub-solidus modification of mineral compositions and textures. The flood of magmatic water that ascends through a growing pluton transmits thermal energy more rapidly than conduction. Faster heat transfer enhances fluctuating post-emplacement thermal histories that result from incremental pluton growth and which contribute to late-stage textural coarsening of plutonic rocks. The effects of water conveyed into the crust by ascending subduction zone magma thus are central not only to metamorphism and mineralization of surrounding rocks, but also to the petrology and thermal history of the plutons themselves.