Anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity, features that have not been explained satisfactorily but which are related to physical processes of melt segregation. We evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith. Compaction by gravity-driven two-phase flow results in asymmetric separation, yields calculated segregation times that are slow or geologically unreasonable, and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites. Although calculated segregation times for "very wet" granite might allow segregation by compaction, some driving force in addition to gravity is needed to cause widespread melt segregation in the crust. For segregation at low volume fraction of melt, we develop a two-dimensional two-phase (matrix and fluid) viscous flow cell model (length much greater than thickness) with phase changes which predicts either expansion or contraction convection (depending on whether volume change, /xV•, is positive or negative). Convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale. At moderate volume fraction of melt, segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress. Melt migration is by porous medium flow driven by differences in mean normal stress between the layers, as a consequence of differences in rheology, and shear-enhanced matrix collapse. Calculated segregation rates are fast, and the model yields adequate volume of leucosome. The positive/xV• for volatile phaseabsent melting reactions under crustal pressures promotes melt escape, unless extensional deformation facilitates melt accumulation. If the rate of melt production exceeds the rate of melt escape, then the increase in melt pressure may lead to melt-enhanced embrittlement and fracture, and melt may migrate out of the system. In contrast, water-rich volatile phase-present melt-producing reactions have a negative/xV•, which promotes melt retention after segregation, unless deformation generates melt escape pathways. Evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy, and magma may become mobile and entrain residual material ("restite"). 15,655 15,656 BROWN ET AL.: MELT SEGREGATION IN MIGMATITES anatectic migmatites, at least some of the segregated melt has frozen in situ in the protolith during the transfer process.Petrological, geochemical, and microstructural studies have demonstrated that many migmatites were formed by some type of local mesoscopic differentiation [e.g., Mehnert, 1968;Brown, 1973;Ashworth...

Abstract. A method for the quantitative analysis of the spatial relations of minerals is described. Dispersed distributions are formed by annealing and destroyed in post-tectonic migmatuation. Aggregate distributions characterize solid-state differentiation, whereas leucosomes formed in systems of high fluid:rock ratio (in the examples studied, anatectic melts) show random distributions.Quantitative textural analysis can be used to indicate whether migmatization was post-tectonic or earlier, though caution is necessary if postmigmatite cooling is slow or if there is some minor deformation. More importantly, it can be used to discriminate melt-present from melt-absent leucosomes; this is exemplified by a suite of metamorphic and anatectic migmatites from the Scottish Caledonides.The textural evolution of anatexites with increasing melt percentage is traced. Initial feldspar porphyroblastesis occurs by Ostwald ripening via grain boundary melts; subsequently ophthalmites develop with fabrics and chemistry inherited from the palaeosome. At greater than 30% melt these inherited fabrics are wholly destroyed. Deformation prompts segregation into melanosome and leucosome; resultant leucosomes contain no inherited crystals. The scale of anatectic systems is fixed at the point at which segregation begins; ophthalmites provide evidence for melt and crystal transfer beyond original palaeosome boundaries. In contrast, metamorphic migmatites are necessarily small-scale systems because of diffusive constraints, and melanosomes are invariably produced.

Abstract. Migmatite structures in the Coast Plutonic-Metamorphic Complex are well exposed in the inlet of Boca de Quadra, southeast Alaska. Two types of anatectic migmatites are present. Patch migmatites formed by in situ melting and subsequent crystallization of melt. Diktyonitic migmatites comprise a discontinuous veined network of leucocratic material, in which leucosomes enclose boudins of host rock. The margins of these boudins show the development of both melanosomes and shear band fabrics.Strain analysis of diktyonitic melanosomes indicates that these regions have undergone volume decreases of 2&27%. This volume decrease is attributed to melt extraction into the adjacent fracture-filling leucosomes. Thus, diktyonitic migmatites formed by shear-induced segregation of partial melt, whereas in patch migmatites the lack of shear stresses inhibited melt segregation. The variable structural style of anatectic migmatites in Boca de Quadra is not related to host-rock composition, but may be due to differences in the amount of differential stress during migmatization. These in turn may be controlled by host-rock strength andlor diachroneity of migmatization and deformation.Determination of volume changes during migmatization using strain analysis is potentially capable of discriminating intrusive and anatectic migmatites and consequently of documenting melt segregation and subsequent migration across crustal levels.

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