A popular model for the emplacement of roughly spherical plutons is that of “ballooning” or in situ inflation of a magma chamber. In a common version of this model magma ascends until loss of heat or buoyancy causes the outermost magma to crystallize and cease ascent, while the hotter “tail” of magma continues to rise and expand the already crystallized outer margin. This expansion forms a concentric, gneissic to mylonitic foliation and flattening-type strain in the outer margin of the pluton by means of subsolidus deformation and pushes aside the surrounding country rock to form a dynamothermal aureole that postdates regional structures. Our reexamination of three supposedly ballooned plutons (Ardara, Ireland; Cannibal Creek, Australia; Papoose Flat, California) and evaluation of published descriptions of many others indicate that this model is largely incorrect. Deflections of country-rock structures, strains, and porphyroblast-matrix relationships indicate that only minor to moderate expansion (usually 30% or less) occurred during emplacement, that other emplacement mechanisms must have occurred, and that regional deformation continued during and after emplacement. Internal structures indicate that when magma chamber expansion did occur, it did so by flow of magma, that magmatic foliations and lineations formed late in the magma chamber evolution, that enclave shapes are neither good strain markers nor indicators of the magnitude of expansion, and that only minor internal subsolidus deformation results from emplacement. This study indicates that many plutons previously interpreted as post-tectonic ballooning plutons are better viewed as syntectonic, nested diapirs emplaced by a variety of country-rock material-transfer processes. This nested diapir model implies that magma ascent may occur by rise of large magma batches (instead of transport in dikes followed by ballooning), that magma chamber dynamics differ from that in the ballooning model, and that normally zoned plutons may form by intrusion of several pulses of magma rather than by in situ crystal fractionation from a single parent melt.

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