Igneous banding, schlieren and mafic enclaves in calc-alkaline granites: The Budduso pluton (Sardinia)

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 Sr-86/Sr-86 initial ratios (0.7090 +/- 4) and epsilon(Nd(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 X-Fe values from the core of enclave (similar to 0.65) to the host monzogranite (similar to 0.72): schlieren in the monzogranite show biotite X-Fe values similar to that of the host rock, whereas schlieren close to mafic enclaves show lower X-Fe values (similar to 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. (C) 2007 Elsevier B.V. All rights reserved.