Melt Segregation in Deep Crustal Hot Zones: a Mechanism for Chemical Differentiation, Crustal Assimilation and the Formation of Evolved Magmas

Mantle-derived basaltic sills emplaced in the lower crust provide a mechanism for the generation of evolved magmas in deep crustal hot zones (DCHZ). This study uses numerical modelling to characterize the time required for evolved magma formation, the depth and temperature at which magma formation occurs, and the composition of the magma. The lower crust is assumed to comprise amphibolite. In an extension of previous DCHZ models, the new model couples heat transfer during the repetitive emplacement of sills with mass transfer via buoyancy-driven melt segregation along grain boundaries. The results shed light on the dynamics of DCHZ development and evolution. The DCHZ comprises a mush of crystals plus interstitial melt, except when a new influx of basaltic magma yields a short-lived (20-200 years) reservoir of melt plus suspended crystals (magma). Melt segregation and accumulation within the mush yields two contrasting modes of evolved magma formation, which operate over timescales of c. 10 kyr-1 Myr, depending upon emplacement rate and style. In one, favoured by emplacement via over-accretion, or emplacement at high rates, evolved magma forms in the crust overlying the intruded basalt sills, and is composed of crustal partial melt, and residual melt that has migrated upwards out of the crystallizing basalt. In the other, favoured by emplacement via under- or intra-accretion, or by emplacement at lower rates, evolved magma forms in the intruded basalt, and the resulting magma is composed primarily of residual melt. In all cases, the upward migration of buoyant melt yields cooler and more evolved magmas, which are broadly granitic in composition. Chemical differentiation is therefore driven by melt migration, because the melt migrates through, and chemically equilibrates with, partially molten rock at progressively lower temperatures. Crustal assimilation occurs during partial melting, and mixing of crustal and residual melt occurs when residual melt migrates into the partially molten crust, yielding chemical signatures indicative of a mixed crustal and mantle origin. However, residual melt is volumetrically more significant than crustal melt, except at the highest emplacement rates. Contamination of crustal melt by residual melt from basalt crystallization appears to be an inevitable consequence of melt segregation in DCHZ, and can explain the mixed crust-mantle origin of many granites.