REEXAMINING PLUTON EMPLACEMENT PROCESSES

Previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement. In fact, the only means of 'making space' during emplacement of mantle-derived magmas in the crust arc (1) lowering the Moho or (2) outward displacement of the Earth's surface. Other 'pluton emplacement mechanisms' are material transfer processes (MTPs) that do not increase the volume of the crust. One frequently cited MTP for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall-rocks around the plutons. The expected structures and total strains near such plutons arc dependent on the three-dimensional pluton shape, the width of the deforming aureole, whether the pluton is a piercing or non-piercing diapir, and, in the latter case, the number of body radii the pluton travels. We have re-examined a few such plutons and calculated the amount of material transfer caused by ductile flow. In no case is more than 40%, and in most cases only 15-35%, of the required material transfer accommodated by wall-rock flow. The contact aureoles around these plutons are too narrow and/or strains too low, indicating that the plutons are predominantly discordant bodies with narrow (0.1-0.4 pluton radii) concordant aureoles. A comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains. We suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models arc too simplistic. Instead, we argue that the ascent and emplacement of magmas require multiple near-field and far-field MTPs. These MTPs will show gradients with depth, distance from the magma, and with time. Rates of near-field MTPs must be rapid, whereas far-field rates are probably nearer to long-term average rates of orogenic processes.