The Arburese igneous complex (SW Sardinia, Italy) — an example of dominant igneous fractionation leading to peraluminous cordierite-bearing leucogranites as residual melts

Abstract

The Arburese massif (SW Sardinia, Italy) is a zoned shallow pluton emplaced in the frontal part of a nappe pile at the end of the Hercynian orogeny. Its main lithologies are (a) pyroxene-bearing biotite granodiorites, and (b) cordierite-bearing peraluminous leucogranites. They form a sequence of sharply bounded multiple intrusions. The leucogranites (LG) form the core of the intrusive complex and are younger than the granodiorites. Three types of granodiorites (GD 1, GD 2, GD 3) have been distinguished on the basis of their field relations and their pyroxene content. Dark-coloured enclaves are common in the granodiorites. Their compositions range from biotite-bearing norite to pyroxene-biotite quartzdiorite/quartz-monzodiorite. A small body of olivine-bearing quartz monzogabbro-norite (MG) forms an independent intrusion in the border zone of the igneous massif. It is crosscut by the granodiorite. The whole-rock sequence from MG to GD 3 is characterized by the same mafic assemblage Opx (±Cpx) + Bt + Ilm. It formed by dominant crystal/liquid fractionation processes from a mantle-derived olivine-bearing magma. From GD 3 to three types of leucogranites (LG 1, LG 2 and LG 3), the rocks are progressively enriched in Al reaching a peraluminous composition. In the leucogranites, cordierite makes its appearance among the primary mineral phases, though confined to the later products of crystallization. Remarkably, cordierite was also observed among the latest crystallization products in some granodiorites and even in some dark monzodioritic enclaves. Independent whole-rock age determinations were made on the MG to GD 3 series and on the leucogranites. These gave very similar values of 309 ± 19 and 304 ± 21 Ma, respectively. Some textural and mineralogical features common to both GD 3 granodiorite and the leucogranites (e.g., both contain Ti-biotites characterized by a common trend of Al- and Fe-enrichment) suggest a genetic linkage between these two rock-types. The extreme iron-rich compositions displayed by the mafic minerals (biotite, cordierite) of the leucogranites suggest that they crystallized from highly evolved melts. Petrochemical, trace element and REE distribution patterns strongly support a common origin for the whole igneous sequence, including the leucogranites. Strontium isotope compositions do not conflict with the above conclusion. The oxygen isotopic composition changes progressively with time of emplacement: δ 18O increases from + 7.6 in the granodiorites up to + 11.9 in the leucogranites. This probably reflects some degree of magma contamination due to assimilation of crustal material. The Sr and O isotope systematics reveal that the MGGD series is characterized by very small variations of the Sr ratio and by pronounced changes in δ 18O. On account of the rather high Sr initial ratios, the observed trend is interpreted as due to AFC (assimilation-fractional crystallization) processes of a source-contaminated parental magma. Modelling the evolution path from MG to LG in terms of AFC processes shows the following: (a) the assumed fractionation steps give realistic mineral assemblages compatible with the observed lithologies; (b) they are able to produce high proportions of both leucogranitic and granodioritic rocks (as observed in the field); and (c) they can produce a progressive increase of alumina up to the peraluminous compositions observed in the more evolved members. Good results were also obtained by modelling the behaviour of trace elements such as Rb, Sr and Ba, whose distribution is mainly governed by the major mineral phases. The geochemical fingerprint of the whole igneous sequence is typical of a K-rich calc-alkaline series. Only some rare “grey enclaves” in the GD 3 granodiorite and in the leucogranites display a typical shoshonitic character. Their presence probably reflects the open-system condition of the deep-seated magma reservoir.