Igneous banding, schlieren and mafic enclaves in calc-alkaline granites: The Budduso pluton (Sardinia) |
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| Authors: | P Barbey D Gasquet C Pin AL Bourgeix |
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| Institution: | aCRPG, Nancy Université, CNRS, 15 rue Notre-Dame-des-Pauvres BP 20, 54501 Vandoeuvre-lès-Nancy Cedex, France;bUMR5204 “EDYTEM”, Université de Savoie, 73376 Le Bourget du Lac Cedex, France;cLaboratoire de Géologie, CNRS and Université B. Pascal, 5 rue Kessler, 63038 Clermont-Ferrand Cedex, France;dInstitute of Mineralogy and Geochemistry, BFSH-2, University of Lausanne, CH-1015 Lausanne, Switzerland |
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| Abstract: | This study deals with the origin of igneous layering in plutons, and, especially, the extent layering is related to mafic–silicic magma interactions. The Budduso pluton (Sardinia) shows three main scales of organization.(i) Large scale lithological variations correspond to three main magmatic units, with differentiation increasing from the Outer (hornblende-bearing biotite granodiorite/monzogranite) to the Middle (biotite monzogranite) and the Inner (leucomonzogranite) units. The striking homogeneity of 87Sr/86Sr initial ratios (0.7090 ± 4) and εNd(t) values (− 5.6 ± 0.1) strongly suggests that magma isotopic equilibration was achieved prior to emplacement, whereas mixing/mingling structures observed within the pluton reflect second-stage processes involving broadly cogenetic components.(ii) Metre to decametre-scale igneous layering may be isomodal or modally-graded, locally with cross-layering. Biotite and plagioclase compositions are similar in both biotite-rich and quartzofeldspathic layers, as are the trace-element patterns which differ only by relative abundances. This precludes an origin by fractional crystallization. A penetrative submagmatic fabric superimposed on the layering and corresponding mainly to flattening can be ascribed to interference between pluton growth and regional deformation.(iii) Composite layering and schlieren are commonly associated to mafic microgranular enclaves, locally within synmagmatic shear zones or disrupted synplutonic dykes. In that case, there is a progressive shift in biotite XFe values from the core of enclave ( 0.65) to the host monzogranite ( 0.72): schlieren in the monzogranite show biotite XFe values similar to that of the host rock, whereas schlieren close to mafic enclaves show lower XFe values ( 0.69) towards those of enclave rims.These features can be ascribed to three main processes: (i) assembly of differentiated (± mixed/mingled) magmatic pulses; (ii) local hydrodynamic sorting related to density currents in a mush, and segregation of residual melt; (iii) mechanical disruption and chemical hybridization of mafic magmas during ascent or within the pluton related to magma dynamics. None of these processes affect the whole pluton but they are limited to specific magmatic units. Therefore, pluton growth by incremental assembly of magma batches is not incompatible with magma chamber processes. |
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| Keywords: | Igneous layering Schlieren Mafic enclaves Calc-alkaline pluton Budduso Sardinia |
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