Quantitative Modeling of Granitic Melt Generation and Segregation in the Continental Crust
journal contributionposted on 09.07.2003, 00:00 by M. D. Jackson, Michael Cheadle, M. P. Atherton
We present a new quantitative model of granitic (in a broad sense) melt generation and segregation within the continental crust. We assume that melt generation is caused by the intrusion of hot, mantle-derived basalt, and that segregation occurs by buoyancy-driven flow along grain edges coupled with compaction of the partially molten source rock. We solve numerically the coupled equations governing heating, melting, and melt migration in the source rock, and cooling and crystallization in the underlying heat source. Our results demonstrate that the spatial distribution and composition of the melt depends upon the relative upward transport rates of heat and melt. If melt transport occurs more quickly than heat transport, then melt accumulates near the top of the source region, until the rock matrix disaggregates and a mobile magma forms. As the melt migrates upward, its composition changes to resemble a smaller degree of melting of the source rock, because it thermodynamically equilibrates with rock at progressively lower temperatures. We demonstrate that this process of buoyancy-driven compaction coupled with local thermodynamic equilibration can yield large volumes of mobile granitic magma from basaltic and meta-basaltic (amphibolitic) protoliths over timescales ranging from similar to 4000 years to similar to 10 Myr. The thickness of basaltic magma required as a heat source ranges from similar to 40 m to similar to 3 km, which requires that the magma is emplaced over time as a series of sills, concurrent with melt segregation. These findings differ from those of previous studies, which have suggested that compaction operates too slowly to yield large volumes of segregated granitic melt.