Skeletal quartz and dendritic biotite: Witnesses of primary disequilibrium growth textures in an alkali-feldspar granite

Abstract : We describe a case of S-type alkali-feldspar granite (quartz/K-feldspar/plagioclase/ biotite/garnet ∼ 30/35/20/13/2 vol%) showing skeletal and dendritic textures. It consists of three nested units distinguishable by the size of biotite crystals: a first-emplaced unit characterized by small biotite flakes (0.4–9 mm); a second unit with cm-sized biotite platelets (0.3–28 mm) outlining a syn-magmatic foliation; and a third unit formed of large biotite plates (0.7–850 mm) showing dendritic and radial growth patterns. Quartz occurs mostly as rounded to euhedral grains, but also shows complex shapes (serrated outline, re-entrants, etc.). Cathodoluminescence imaging reveals that grains consist of both rounded and skeletal bluish cores (quartz-1) rimmed by reddish overgrowths (quartz-2) that lead locally to the coalescence of neighbouring grains and equilibrated texture. Biotite is poikilitic enclosing dominantly quartz grains but also in lesser amount plagioclase and garnet. In the third unit, biotite shows textures typical of dendritic growth (bifurcating, radiating or branching structures, skeletal core). Plagioclase (An≤25) occurs as small euhedral to subhedral grains scattered in the matrix or forming glomerophyric aggregates. K-feldspar (Or74–100) mostly forms an almost equilibrated mosaic of anhedral perthitic crystals; it also appears in association with albite as a secondary phase replacing plagioclase. The crystallization sequence started with garnet, then proceeded with quartz, plagioclase, biotite and K-feldspar, ended with cordierite, and was followed by subsolidus reequilibration. The variability in size and morphology of quartz and biotite is interpreted in terms of variable undercooling depending on temperature of the host-rocks and on more or less intense degassing of magma batches with variable melt water content. Here, rapid crystallization prevented significant textural reequilibration and allowed the initial crystallization textures of quartz to be preserved. Overall, this study shows that behind the common “coarse-grained” texture of quartz in granite, subsist primary disequilibrium crystallization textures.
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Submitted on : Wednesday, October 23, 2019 - 10:43:55 AM
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Pierre Barbey, François Faure, Jean-Louis Paquette, Karine Pistre, Cyrille Delangle, et al.. Skeletal quartz and dendritic biotite: Witnesses of primary disequilibrium growth textures in an alkali-feldspar granite. Lithos, Elsevier, 2019, 348-349, pp.105202. ⟨10.1016/j.lithos.2019.105202⟩. ⟨hal-02328268⟩



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