Melt Segregation Mechanism Controls on the Geochemistry of Crustal Melts

Abstract

Crustally-derived granites may be generated in a variety of tectonic settings, but whatever the ultimate causse of heating crustal thickening, crustal extension, enhanced heat flux from the mantle, or magmatic advection by mantle-derived melts crustal melting likely occurs in a dynamic environment. Under these conditions deviatoric stress acting on an anisotropic crustal source region will lead to heterogeneous deformation at all scales. Deformation-enhanced melt segregation and melt ascent through the crust along major shear zones and fracture systems are to be expected. This dynamic environment will be reflected in the metamorphic P-T-t path for the source, which will be clockwise in P-T space for most orogenic belts regardless of whether the crust is responding to overthickening or extension. In both cases upper crust is replenished from below so that former middle and lower crust is displaced towards the surface relative to some material marker, and decompression melting and melt segregation leading to magma ascent and granite emplacement may occur. However, our knowledge of what happens between the beginning of anatexis in the middle and lower crust and the emplacement of granite plutons at structurally shallower levels remains poor. Understanding the physical mechanisms of melt segregation will allow evaluation of controls on initial magma chemistry. For example, why does melt segregation occur in some circumqtanees whereas bulk magma mobility with restite entrainment occurs in other eirenmstances, and why do some melts segregate before equilibrium with the residues has been achieved? This paper evaluates what we can learn from migmatites and residual granulites concerning the physical mechanisms of melt segregation and the control of these on magma chemistry, and indicates how we can relate this knowledge to information derived from upper crustal granites deafly separated from their sources.