DIAPIR-DRIVEN CRUSTAL CONVECTION - DECOMPRESSION MELTING, RENEWAL OF THE MAGMA SOURCE AND THE ORIGIN OF NESTED PLUTONS

This paper studies numerically the rise of diapirs and the consequence s of diapir-driven crustal flow. The ascent of diapirs imposes a conve ctive flow pattern concentrated mostly in the low-viscosity lower crus t in which the rocks immediately surrounding the diapir are dragged up wards and rocks a few radii to the sides of the diapir are pushed down wards to fill the gap left by the rising mass. There are two main cons equences of this flow. One, warm and water undersaturated rocks may un dergo decompression melting when dragged upwards. This is shown to enh ance the ability of a diapir to intrude the crust, because the partial ly molten wall rocks gain buoyancy and add to the total buoyancy of th e diapir, which regains part of the energy it spent on heating and dra gging the surroundings. Two, the downward flow of rocks renews the mag ma source with potentially fertile rocks, which may undergo melting an d give rise to a new diapir which will follow the same path as the fir st one and repeat the process, giving rise to a sequence of diapirs. W hereas source renewal is a direct result of diapirism, magma transport through dykes leads to the accumulation of refractory restite in the source which will eventually prevent further melting. Diapirism and de compression melting may lead to strong recycling of the lower crust an d give rise to large volumes of melt. Sequential diapirism may explain nested plutons and multipulse mid- to upper-crustal batholiths.