The inception and growth of leucosomes: microstructure at the start of melt segregation in migmatites

Abstract

The beginning stages of melt segregation and the formation of leucosomes are rarely preserved in migmatites. Most arrays of leucosomes record a more advanced stage where flow dominates over segregation. However, the early stages in the formation of leucosomes and the segregation of melt are preserved in a partially melted meta‐argillite from the metatexite zone (>800 °C) of the contact aureole around the Duluth Complex, Minnesota. The rock contains 2.4 modal% leucosome in a matrix consisting of 40.5% in situ neosome and 57.1% cordierite + plagioclase framework. The domainal microstructure in the matrix is a pre‐anatectic feature resulting from the bulk composition. Terminal chlorite reactions produced a large volume of cordierite which, with plagioclase, formed a framework that enclosed patches of biotite + quartz + plagioclase ± K‐feldspar. Upon melting, these fertile domains became patches of in situ neosome. Plagioclase in the neosome is less sodic than in the leucosome, hence segregation of melt occurred during crystallization, not melting. Segregation was delayed because the cordierite + plagioclase framework was strong enough to resist dilatation and compaction until after crystallization started. The leucosomes are small (i.e. they are microleucosomes) and display a systematic progression in morphology as length and aspect ratio increase from ~1 to 19 mm and from ~2.5 to >30 respectively. Small equant micropores form first, and in places these coalesce into small (~1 mm, aspect ratio ~2.5), isolated, blunt‐ended, elliptical microleucosomes. In the next stage, micropores develop ahead of, and at ~45° to the left and right of the blunt tip of a microleucosome; one of these develops into an elliptical leucosome and an en echelon array of either a left‐ or right‐stepping elliptical microleucosome forms. Each elliptical microleucosome in the en echelon arrays is separated by a bridge of matrix. Next, microleucosomes of greater length (>4 mm) and aspect ratio (>5) form when the bridges of cordierite + plagioclase matrix rupture and the elliptical microleucosomes link together to form a zigzag‐shaped microleucosome. Finally, still longer microleucosomes with greater aspect ratios (~30) are formed by the joining of zigzag arrays. Such a progression is characteristic of the way ductile fractures grow. The segregation of melt was driven by the pressure gradient between the dilatant fracture and an adjacent in situ neosome, which drew melt to the growing fracture, thereby creating a microleucosome. The microleucosomes are filled arrays of ductile fractures. Melt was contiguous only between microleucosomes and adjacent patches of in situ neosome. The length‐scale of segregation was ~5 mm, the size of a typical patch of in situ neosome, and restricted by the surrounding impermeable cordierite + plagioclase framework. The melt in the microleucosome was the most fractionated and the last to crystallize. All microleucosomes contain entrained minerals as a consequence of their mechanism of growth. Rupture of the b...

Magma accumulation and segregation during regional-scale folding: The Holland’s dome granite injection complex, Damara belt, Namibia

“…Weertmann, 1971;Sleep, 1988;Bons et al, 2004;Kisters et al, 2009) or ductile fractures (e.g. Weinberg and Regenauer-Lieb, 2010;Sawyer, 2014; see comprehensive reviews by Brown, 2007Brown, , 2013 for a compilation of different processes). The relatively fast strain rates associated with most different modes of fracturecontrolled magma migration prevent the granitic magmas from freezing, particularly in subsolidus crust (e.g.…”

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

“…Grain-scale to mesoscale melt topology studies mostly focus on anatectic high-temperature-medium-pressure regions that commonly show protracted episodes of melting (e.g., Sawyer, 2001;Brown and Solar, 1999;Hasalová et al, 2008aHasalová et al, , 2008bKisters et al, 2009;Bonamici and Duebendorfer, 2010;Sawyer, 2014;Kisters, 2012, 2016). In contrast, petrostructural studies of migmatitic UHP terranes have aimed to identify the incipient melting reactions close to the UHP conditions (e.g., Gordon et al, 2013;Chen et al, 2013Ganzhorn et al, 2014;Zhang et al, 2015;Stepanov et al, 2016), and rather neglected the grain-scale melt topology and deformation mechanisms with respect to the kinematic framework of their host.…”

Crystallization of Heterogeneous Pelitic Migmatites: Insights from Thermodynamic Modelling

“…During melt segregation, residuum material and peritectic phases may be entrained into the meltdominated domains (Stevens et al, 2007;Villaros et al, 2009;Taylor et al, 2010;Sawyer, 2014), making the leucosome composition not simply the solids that crystallized from a melt, as modelled here. Several experimental studies have found that equilibrium with Pl is not attained during melting experiments at T < 800 C; instead, Pl dissolves stoichiometrically with little or no change in Pl composition (Johannes, 1978(Johannes, , 1980(Johannes, , 1984Acosta-Vigil et al, 2006).…”

“…However, forming a migmatite also requires consideration of the crystallization reactions and processes (Brown, 2002;White et al, 2004;White & Powell, 2010), which take place after the melt has segregated from its source and accumulated in melt-dominated sites (e.g. Stevenson, 1989;Brown, 1994;Sawyer, 1994Sawyer, , 2014Simakin & Talbot, 2001;Rabinowicz & Vigneresse, 2004). By understanding the crystallization history, the composition of the melt lost from partially molten terrains can be better understood (Johannes et al, 2003;Johnson et al, 2003), and the retrograde solidification Fig.…”