Melt segregation and magma flow in migmatites: implications for the generation of granite magmas

Abstract

ABSTRACT:To form a granite pluton, the felsic melt produced by partial melting of the middle and lower continental crust must separate from its source and residuum. This can happen in three ways: (1) simple melt segregation, where only the melt fraction moves; (2) magma mobility, in which all the melt and residuum move together; and (3) magma mobility with melt segregation, in which the melt and residuum move together as a magma, but become separated during flow. The first mechanism applies to metatexite migmatites and the other two to diatexite migmatites, but the primary driving forces for each are deviatoric stresses related to regional-scale deformation. Neither of the first two mechanisms generates parental granite magmas. In the first mechanism segregation is so effective that the resulting magmas are too depleted in FeOT, MgO, Rb, Zr, Th and the REEs, and in the second no segregation occurs. Only the third mechanism produces magmas with compositions comparable with parental granites, and occurs at a large enough scale in the highest grade parts of migmatite terranes, to be considered representative of the segregation processes occurring in the source regions of granites.

Role of strain localization and melt flow on exhumation of deeply subducted continental crust

“…3D, 3E, 4B, and 4D). Nevertheless, destruction of the deformation layering may occur at lower melt fractions if the deformation mechanism is dominated by grain boundary sliding (granular flow) (Sawyer, 1994(Sawyer, , 1996White et al, 2005). Therefore, complete fabric disintegration and granofels formation can be also explained by continuous reactive porous grain-scale melt flow through dynamic porosity during the granular flow of the grains below the melt connectivity threshold marked by 7-10 vol% (Rosenberg and Handy, 2005).…”

Microstructures of melt inclusions in anatectic metasedimentary rocks

“…Stevens et al, 2007). More recently the incorporation in the melt of variable amounts of peritectic phases (Sawyer, 1996;Lavaure & Sawyer, 2011), e.g. garnet and ilmenite (Stevens et al, 2007;Villaros et al, 2009), has been proposed as one of the leading processes in shaping the S-type granite geochemistry.…”

“…However, several processes, such as cumulus phenomena, fractional crystallization, entrainment of xenocrysts and residual minerals, may lead to a composition that does not represent a liquid. Thus any estimation based only on the chemical composition of leucosomes is questionable (Ellis & Obata, 1992;Brown et al, 1995;Sawyer, 1996Sawyer, , 2008Marchildon & Brown, 2001). To overcome these problems, during the last few decades an experimental approach has become a well-established approach in partial melting investigation of metapelites and metagraywackes, the most fertile (melt-producing) lithologies in the middle crust (e.g.…”

Syn‐deformational migmatites and magmatic‐arc metamorphism in the Xolapa Complex, southern Mexico

“…The large amount of leucosome in the metasedimentary migmatites, the absence of muscovite, the abundance of sillimanite, the occurrence of hercynite, as well as the relics of orthopyroxene, along with the evidence of partial melting in mafic protoliths, are all indicators that temperatures exceeding biotite-breakdown melting were reached in the evolution of the XC. Multivariant reactions responsible for biotite-breakdown melting in complex chemical systems have been extensively studied experimentally by Vielzeuf & Holloway (1988), Patin˜o-Douce & Johnston (1991), Vielzeuf & Montel (1994), Patin˜o-Douce & Beard (1995, 1996 and Montel & Vielzeuf (1997). It has been shown that melt productivity and melt evolution (Patin˜o-Douce & Johnston, 1991;Vielzeuf & Schmidt, 2001) are closely related to the relative abundance of micas and plagioclase in the protolith (Fig.…”

“…Field relations commonly show a close relationship between foliated plutonic rocks, high-grade metamorphosed rocks and migmatites, with variable percentages of leucosome distributed as pods, layered leucosomes and/or anatectic granitoids (Sawyer & Barnes, 1988;Lucassen & Franz, 1996;Brown, 1998Brown, , 2001Whitney et al, 1999;Gibbons & Moreno, 2002;Valley et al, 2003). Complex morphologies of migmatitic structures are a result of the progressive change of temperature (Mehnert, 1968), of degree of partial melting (Clemens & Vielzeuf, 1987;Rushmer, 2001) and of the relationships with differential stress (Brown et al, 1995;Sawyer, 1996Sawyer, , 2001Brown & Rushmer, 1997;Rosenberg & Handy, 2001). Therefore, relative synchronicity (Sawyer, 1994;Vigneresse et al, 1996) and/or feedback relations (Brown & Solar, 1998) between heat, deformation, partial melting and regional metamorphism have been shown to control the lithological differentiation in deep magmatic-arc environments.…”