Episodic granite accumulation and extraction from the mid‐crust

Abstract

The processes of long-range granitic magma transfer from mid-and lower crustal anatectic zones to upper crustal pluton emplacement sites remain controversial in the literature. This is partly because feeder networks that could have accommodated this large-scale magma transport remain elusive in the field. Existing granite ascent models are based largely on numerical and theoretical studies that seek to demonstrate the viability of fracture-controlled magma transport through dykes or self-propagating hydrofractures. In most cases, the models present very little supporting field evidence, such as sufficiently voluminous near-or within-source magma accumulations, to support their basic premises. We document large (deca-to hectometre-scale), steeply dipping and largely homogeneous granite lenses in suprasolidus (~5 kbar,~750°C) mid-crustal rocks in the Damara Belt in Namibia. The lenses are surrounded by and connected to shallowly dipping networks of stromatic leucogranites in the well-layered gneisses of the deeply incised Husab Gorge. The outcrops define a four-stage process from (i) the initial formation and growth of large, subvertical magma-filled lenses as extension fractures developed at high angles to the subhorizontal regional extension in relatively competent wallrock layers. This stage is followed by (ii) the simultaneous lateral inflation and (iii) subcritical vertical growth of the lenses to a critical length that (iv) promotes fracture destabilization, buoyancy-driven upward fracture mobilization and, consequently, vertical magma transport. These field observations are compared with existing numerical models and are used to constrain, by referring to the dimensions of the largest preserved inflated leucogranite lens, an estimate of the minimum fracture length (~100 m) and volume (~2.4 9 10 5 m 3 ) required to initiate buoyancy-driven brittle fracture propagation in this particular mid-crustal section. The critical values and field relationships compare favourably with theoretical models of magma ascent along vertical self-propagating hydrofractures which close at their tails during propagation. This process leaves behind subtle wake-like structures and thin leucogranite trails that mark the path of magma ascent. Reutilization of such conduits by repeated inflation and drainage is consistent with the episodic accumulation and removal of magma from the mid-crust and is reflected in the sheeted nature of many upper crustal granitoid plutons.

Metamorphic petrology of a high‐T/low‐P granulite terrane (Damara belt, Namibia) – Constraints from pseudosection modelling and high‐precision Lu–Hf garnet‐whole rock dating

“…Previous studies (Jung & Mezger, ; Ward et al., ) indicated that the high‐grade metapelitic rocks of the Central Damara orogen have undergone partial melting processes. The continuous gradation from small‐scale leucosomes to decametre‐scale leucogranite sheets within the high‐grade metamorphic zone indicates that leucosomes have coalesced into bodies of leucogranite (Hall & Kisters, ; Kruger & Kisters, ; Toé et al., ). Such leucogranites have been dated at c .…”

“…In view of the close association of the studied high‐ T metapelites with hundreds of synmetamorphic high‐ T granites that invaded the terrane as metre‐ to decametre‐wide sills and dykes, we postulate that crystallization of felsic lower crustal magma has supplied at least some of the heat. If large‐scale plutons are absent, pervasive mesoscale felsic magma migration through hot crust (Hall & Kisters, ; Kruger & Kisters, ) may provide some extra heat (Brown, ; Collins & Sawyer, ; Leitch & Weinberg, ; Weinberg, ). However, Clark et al.…”

“…More recently, Longridge, Gibson, Kinnaird, and Armstrong (2017) showed that the metamorphic grade in the southern Central Zone is substantially higher, recording granulite facies conditions of 820°C. Migmatites are important rock types in the highest grade part of the Central Zone (Hall & Kisters, 2016;Toé, Vanderhaeghe, André-Mayer, Feybesse, & Milési, 2013) and are, at the lowermost structural levels, mostly related to the intrusion of hot, leucocratic melts from the underlying basement (intrusion-related migmatites). Here, diatexites are common and are interpreted as to result from incomplete separation of melt and residues (Toé et al, 2013;Vanderhaeghe, 2009).…”

“…In view of the close association of the studied high-T metapelites with hundreds of synmetamorphic high-T granites that invaded the terrane as metre-to decametre-wide sills and dykes, we postulate that crystallization of felsic lower crustal magma has supplied at least some of the heat. If large-scale plutons are absent, pervasive mesoscale felsic magma migration through hot crust (Hall & Kisters, 2016;Kruger & Kisters, 2016) may provide some extra heat (Brown, 2004;Collins & Sawyer, 1996;Leitch & Weinberg, 2002;Weinberg, 1999). However, Clark et al (2011) recently concluded that the movement of felsic magmas through the crust will not add extra heat and is therefore unable to contribute to high-T conditions because the emplacement temperatures of medium-scale granite bodies are likely lower than the peakmetamorphic temperatures.…”

Microstructure and compositional changes across biotite‐rich reaction selvedges around mafic schollen in a semipelitic diatexite migmatite

“…Partial melting has been studied in detail through melting experiments on a variety of rock types (Acosta‐Vigil & London, ; Conrad, Nicholls, & Wall, ; Patino Douce & Beard, ; Skjerlie, Patino Douce, & Johnston, ), thermodynamic modelling in various systems (Johnson & Brown, ; Palin, White, & Green, ) and through field and petrological studies (Jones & Brown, ; Otamendi & Patino Douce, ; Sawyer, Cesare, & Brown, ). Similarly, melt segregation has been investigated through experiments on analogue materials (Barraud, Gardien, Allemand, & Grandjean, ; Rosenberg & Handy, ) and rocks (Kohlstedt, King, & Holtzman, ; Rutter & Neumann, ), by numerical modelling (Hier‐Majumder, Ricard, & Bercovici, ; Simakin & Talbot, ; Stevenson, ), and by field‐based (Hall & Kisters, ; Johnson, Hudson, & Droop, ; Sawyer, ; Weinberg, Hasalová, Ward, & Fanning, ; Weinberg & Mark, ) and petrological (Holness & Watt, ; Sawyer, ) studies. In marked contrast, the changes in microstructure and composition that resulted from chemical exchanges between compositionally different parts of migmatites have not yet been studied in detail.…”