This study reassesses the development of compositional layering during the growth of granitic plutons, with emphasis on fractional crystallization and its interaction with both injection and inflation-related deformation. The Dolbel batholith (SW Niger) consists of 14, kilometre-sized plutons emplaced by pulsed magma inputs. Each pluton has a coarse-grained core and a peripheral layered series. Rocks consist of albite (An≤11), K-feldspar (Or96–99, Ab1–4), quartz, edenite (XMg = 0·37–0·55), augite (XMg = 0·65–0·72) and accessories (apatite, titanite and Fe–Ti-oxides). Whole-rock compositions are metaluminous, sodic (K2O/Na2O = 0·49–0·62) and iron-rich [FeOtot/(FeOtot + MgO) = 0·65–0·82]. The layering is present as size-graded and modally graded, sub-vertical, rhythmic units. Each unit is composed of three layers, which are, towards the interior: edenite ± plagioclase (Ca/p), edenite + plagioclase + augite + quartz (Cq), and edenite + plagioclase + augite + quartz + K-feldspar (Ck). All phases except quartz show zoned microstructures consisting of external intercumulus overgrowths, a central section showing oscillatory zoning and, in the case of amphibole and titanite, complexly zoned cores. Ba and Sr contents of feldspars decrease towards the rims. Plagioclase crystal size distributions are similar in all units, suggesting that each unit experienced a similar thermal history. Edenite, characteristic of the basal Ca/p layer, is the earliest phase to crystallize. Microtextures and phase diagrams suggest that edenite cores may have been brought up with magma batches at the site of emplacement and mechanically segregated along the crystallized wall, whereas outer zones of the same crystals formed in situ. The subsequent Cq layers correspond to cotectic compositions in the Qz–Ab–Or phase diagram at PH2O = 5 kbar. Each rhythmic unit may therefore correspond to a magma batch and their repetition to crystallization of recurrent magma recharges. Microtextures and chemical variations in major phases allow four main crystallization stages to be distinguished: (1) open-system crystallization in a stirred magma during magma emplacement, involving dissolution and overgrowth (core of edenite and titanite crystals); (2) in situ fractional crystallization in boundary layers (Ca/p and Cq layers); (3) equilibrium ‘en masse’ eutectic crystallization (Ck layers); (4) compaction and crystallization of the interstitial liquid in a highly crystallized mush (e.g. feldspar intercumulus overgrowths). It is concluded that the formation of the layered series in the Dolbel plutons corresponds principally to in situ differentiation of successive magma batches. The variable thickness of the Ck layers and the microtextures show that crystallization of a rhythmic unit stops and it is compacted when a new magma batch is injected into the chamber. Therefore, assembly of pulsed magma injections and fractional crystallization are independent, but complementary, processes during pluton construction.
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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