Melt segregation and far-field melt transfer in the mid-crust

Abstract

Migmatites in the Damara Belt in Namibia preserve relationships between anatectic leucosomes and intrusive leucogranite sheets illustrating the consecutive stages of partial melting, local melt segregation and far-field mobilization of melts over hundreds of metres and out of the anatectic region. Initial melting and localized segregation of melts are controlled by gradients in fluid pressure created by pervasively developed, shallowly dipping dilatant fractures. Subsequent melt transfer out of these initial sites of melting and melt storage occurs along subvertical, disc-shaped leucogranite sheets that intersect the leucosomes. The leucogranite sheets propagate as isolated, melt-filled hydrofractures, driven by the pressure differential along the subvertical fractures. The collapse of wall rocks into former melt-bearing fractures, and the presence of residual peritectic phases of the melting reaction trapped in wall rocks testify to the efficient extraction of melt by the mobile hydrofractures during their ascent. This process of melt drainage into and transport by hydrofractures leaves almost no trace of the ascent conduits. This study also shows that melt transport and the stability of melt pathways in strongly layered mid-crustal levels characterized by low deviatoric stresses are determined by the presence of pre-existing anisotropies and the progressively evolving structure in deforming orogens.

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

“…7D, 7E). Contacts between these two discordant sets of leucosomes are gradational, which suggests that they were interconnected and contained melt at the same time (Vanderhaeghe 1999(Vanderhaeghe , 2001Barraud et al, 2004;Vernon and Paterson, 2001;Sawyer, 2008;Kisters et al, 2009;Hall and Kisters, 2012).…”

“…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.…”

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

“…For the most part, these leucogranites form sheet-like bodies and geometrically complex, commonly interconnected networks (e.g. Kinnaird and Nex, 2007;Miller, 2008;Kisters et al, 2009Kisters et al, , 2012Longridge et al, 2011;Kisters, 2012, 2016;Corvino and Pretorius, 2013).…”

“…In contrast, more recent geochronological (Jung, 2000;Jung and Mezger, 2003;Longridge, 2012) and structural work (Kisters et al, 2009;Longridge et al, 2011;Kisters, 2012, 2016) highlight the structurally controlled emplacement of the leucogranites during regional northwest-southeast directed subhorizontal shortening related to the main phase of crustal convergence and collisional tectonics (D2, after Jacob, 1974;D3 after Miller, 1983D3 after Miller, , 2008. Importantly, the wellexposed sections along the Khan and Swakop rivers illustrate systematic changes in the geometry and orientation of granite injection complexes depending on (1) their structural position with respect to regional structures (folds and thrusts) and (2) wall-rock lithologies.…”

“…Clemens and Mawer, 1992;Petford et al, 1994Petford et al, , 2000, self-propagating hydrofractures (e.g. 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).…”