Granite: From genesis to emplacement

Abstract

At low temperatures (<750 °C at moderate to high crustal pressures), the production of suffi cient melt to reach the melt connectivity transition (~7 vol%), enabling melt drainage, requires an infl ux of aqueous fl uid along structurally controlled pathways or recycling of fl uid via migration of melt and exsolution during crystallization. At higher temperatures, melting occurs by fl uid-absent reactions, particularly hydrate-breakdown reactions involving micas and/or amphibole in the presence of quartz and feldspar. These reactions produce 20-70 vol%, melt according to protolith composition, at temperatures up to 1000 °C. Calculated phase diagrams for pelite are used to illustrate the mineralogical controls on melt production and the consequences of different clockwise pressure-temperature (P-T) paths on melt composition. Preservation of peritectic minerals in residual granulites requires that most of the melt produced was extracted, implying a fl ux of melt through the suprasolidus crust, although some may be trapped during transport, as recorded by composite migmatitegranite complexes. Peritectic minerals may be entrained during melt drainage, consistent with observations from leucosomes in migmatites, and dissolution of these minerals during ascent may be important in the evolution of some crustal magmas. Since siliceous melt wets grains, suprasolidus crust may become porous at only a few volume % melt, as evidenced by microstructures in residual migmatites in which quartz or feldspar pseudomorphs form after melt fi lms and pockets. With increasing melt volume and decreasing effective pressure, assuming the residue is able to deform and compact, the source becomes permeable at the melt connectivity transition. At this threshold, a change from distributed shear-enhanced compaction to localized dilatant shear failure enables melt segregation. The result is a highly permeable vein network that allows transfer of melt to ascent conduits at the initiation of a melt-extraction event. Melt is drained from the anatectic zone via several extraction events, consistent with evidence for incremental construction of plutons from multiple batches of magma. Buoyancy-driven magma ascent occurs via dikes in fractures or via high-permeability zones controlled by tectonic fabrics; the way in which these features relate to compaction and the generation of porosity waves is discussed. Emplacement of laccoliths (horizontal tabular intrusions) and wedge-shaped plutons occurs around the ductile-to-brittle transition zone, whereas steep tabular sheeted and blobby plutons represent back freezing of melt in the ascent conduit or lateral expansion localized by instabilities in the magma-wallrock system, respectively.

The composition of nanogranitoids in migmatites overlying the Ronda peridotites (Betic Cordillera, S Spain): the anatectic history of a polymetamorphic basement

“…Since both environments are important regarding the investigation of crustal genesis and differentiation processes (e.g. Vielzeuf et al 1990;Schmidt et al 2004;Hermann and Spandler 2008;Hacker et al 2011;Sawyer et al 2011;Brown 2013), high P granulites are particularly relevant rocks for the study of MI.…”

LP-HT anatectic processes and lithological heterogeneity in the Mindelo Migmatite Complex (NW Portugal)

“…The leucogranite and leucogranite veins composition, namely the high HSFE depletion relatively to metatexites and the distinct REE patterns can be produced by removal of leucosome before complete equilibration due to the inhibited dissolution of accessory phases, during an early stage of segregation and extraction (Barbero & Villaseca, 1995;Watt et al, 1996;Zeng et al, 2005a and2005b;Brown, 2008Brown, , 2013. Evidence for channelized flow and deformation-enhanced melt segregation on an outcrop scale is present all over the MMC where melt migration occurs over several distances.…”

“…It is also significant that the leucosomes and leucogranites are cordierite-free, as opposed to what occurs in diatexites. The first result from fluid-present melting and the second are the product of fluid-absent hydrate-breakdown melting that carry anhydrous (peritectic) minerals, such as cordierite or garnet or pyroxene (Milord et al, 2001;Brown, 2013).…”

Geochemical characteristics and conditions of formation of the Chah-Bazargan peraluminous granitic patches, ShahrBabak, Iran

“…Low concentrations of Ni, Cr, V, Sr, Y, Nb, and REE indicate that they probably remained in the residuum (metapelitic xenoliths) during partial melting and segregation. Based on Brown (1994), Brown (2007), García-Arias et al (2012), Brown (2013), Sawyer (2014), and García-Arias & Stevens Fig. 4.…”