Segregation of tonalitic-trondhjemitic melts in the continental crust: The mantle connection

Rates of melt segregation into fractures during fluid-absent partial melting above a mafic heat source are investigated numerically over a range of viscosities appropriate for tonalitic-trondhjemitic melts. Static melt segregation is dependent upon large tensile stresses (> 10 MPa) set up during fluid-absent partial melting reactions that promote fracturing and vein formation in the region above the mafic intrusion. Porous flow of partial melt into veins and fractures (regions of reduced pressure) will occur provided the melt is less dense than the surrounding protolith matrix. Where vein size is much less than compaction length, maximum vein filling rates are of the order of 2.5 m/yr. Melt segregation into veins and fractures is likely to be most effective near the intrusion-protolith interface, where high temperatures result in relatively low melt viscosities (<10(6) Pa s) and high porosities (melt fractions). For relatively low viscosity tonalitic melts (similar to 10(4) Pa s), extraction may occur at values below the critical melt fraction, provided the melt is everywhere interconnected. Larger veins may propagate upward as dikes to feed high level plutons. These conditions for melt extraction are considered likely in tectonic settings where lower (amphibolitic) crust is being partially melted by underplating of mantle derived basalt. The model does not require the existence of large magma chambers in the source region.