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Layering and schlieren in granitoids: A record of interactions between magma emplacement, crystallization and deformation in growing plutons (The André Dumont medallist lecture)
ABSTRACT. A review of the literature shows that layering in granitoids is the expression of three processes occurring concurrently during the growth of plutons (injection, hydrodynamic processes coupled with fractional crystallization, and deformation). Layering may result from aggregation (± mingling/hybridization) of magma pulses of contrasting compositions or with variable crystal contents, leading to stratified plutons characterized by macrorhythmic units, or exceptionally to cyclic units in the case of low-viscosity magmas. At smaller scale, layering formation is dependent on the injection dynamics and the rheological state of magmas. Rhythmic layering and depositional features related to gravity- or flow-driven crystal-melt segregation do not necessarily imply pluton-scale convective overturn. They occur preferentially close to rheological boundaries (intramagmatic or country-rock walls), close to eruption vents, in relation with local magma plumes, or in association with mafic injections in mafic-silicic layered intrusions. Rhythmically layered series in the periphery of some plutons may result from sidewall crystallization in relation with pulsed injections, whereas aplite-pegmatite layering results from segregation of undercooled residual melts. However, in situ fractional crystallization of single magma batch is unlikely to produce, in most cases, the large rock units constituting plutons. Ductile deformation and more widely the regional tectonic context appear to exert major controls on the formation of pluton-scale compositional layering related to emplacement of heterogeneous magmas. Deformation-assisted melt segregation also common in syntectonic granites may lead to sheet-like melt segregations or structures analogous to migmatitic layering.
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About: Pierre BARBEY
CRPG, Nancy-Université, CNRS, B.P. 20, F-54501 Vandœuvre-lès-Nancy cedex. E-mail: email@example.com