DIAPIRIC BASAL ENTRAINMENT OF MAFIC INTO FELSIC MAGMA

One consequence of partial melting of the lower crust by heat transfer from a mantle-derived underplate is that the resultant buoyant, felsi c magma layer (density rho(2), viscosity mu(2), thickness h(2)) will o verlie a denser mafic layer (density rho(3), viscosity mu(3), thicknes s h(3)) which can be fully liquid to completely solid, depending on it s thermal history. Laboratory experiments and finite-difference numeri cal models have been used to determine the conditions that favour the entrainment of the mafic layer into the overlying felsic magma as it a scends diapirically. Large amounts of entrainment occur when R = (rho( 3) - rho(1))/(rho(1) - rho(2)) approximate to 0 (where rho(1) is the d ensity of the crust), m = mu(3)/mu(2) = 1-0.001, and t(r) = h(3)/h(2) > 1. When these conditions occur, the buoyancy and viscous effects act ing to maintain the stability of the felsic-mafic layer interface are minimized. The role of m is much more important in the diapiric entrai nment phenomenon than in the comparable problem of axial withdrawal fr om 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 maf ic underplate. Low amounts of entrainment result in minimal interactio n (i.e., mixing) between the felsic host and entrained mafic material. If a large amount of entrainment occurs, our models combined with oth er studies show that mafic magma can remain in the centre of the condu it (low to high Reynolds number (Re), m approximate to 1), become full y 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 a ccount for the textural and compositional complexity shown by some plu tons.