DIAPIRIC BASAL ENTRAINMENT OF MAFIC INTO FELSIC MAGMA

Abstract

One consequence of partial melting of the lower crust by heat transfer from a mantle-derived underplate is that the resultant buoyant, felsic magma layer (density ρ2, viscosity μ2, thickness h2) will overlie a denser mafic layer (density ρ3, viscosity μ3, thickness h3) which can be fully liquid to completely solid, depending on its thermal history. Laboratory experiments and finite-difference numerical models have been used to determine the conditions that favour the entrainment of the mafic layer into the overlying felsic magma as it ascends diapirically. Large amounts of entrainment occur when R = (ρ - ρ1)(ρ1 - ρ2) ~ 0 (where ρ1 is the density of the crust), m = μ3μ2 = 1-0.001, and tr = h3h2 > 1. When these conditions occur, the buoyancy and viscous effects acting to maintain the stability of the felsic-mafic layer interface are minimized. The role of m is much more important in the diapiric entrainment phenomenon than in the comparable problem of axial withdrawal from a density- and viscosity-stratified magma chamber with rigid walls.Favourable conditions for entrainment are likely to occur during the evolution of many lower crustal felsic magma source regions with a mafic underplate. Low amounts of entrainment result in minimal interaction (i.e., mixing) between the felsic host and entrained mafic material. If a large amount of entrainment occurs, our models combined with other studies show that mafic magma can remain in the centre of the conduit (low to high Reynolds number (Re), m ~ 1), become fully mixed with the felsic host (high Re, low m), or become encapsulated by the felsic magma (low Re, m < 0.6). Such mechanical processes may account for the textural and compositional complexity shown by some plutons.