Le complexe d’Ile-Rousse, Balagne, Corse du Nord-Ouest : pétrologie et cadre de mise en place des granitoïdes magnésiopotassiques

The northern part of the carboniferous Corsican-Sardinian batholith is made up of K₂O- and MgO-rich, calcalkaline granitoids, thereafter referred to as magnesiopotassic granitoids [Orsini, 1980; Rossi, 1986]. These granitoids are well exposed in Balagne, northwestern Corsica, where they are divided in two main units: Calvi intrusion on the West and Ile-Rousse complex on the East. A detailled petrological and structural analysis of the latter was performed [Laporte, 1987] with the purposes of: (1) contributing to a better knowledge of magnesiopotassic plutonism in Corsica; and (2) assessing the relative importance of emplacement mechanisms and regional deformation on the development of magmatic fabrics in Balagne granitoids.

Granitoid types. Ile-Rousse complex comprises two main petrogenetic types of granitoids:

(1) the volumetrically-dominant magnesiopotassic granitoids have a mixed origin; they contain abundant enclaves and decametric bodies of K₂O- and MgO-rich basic rocks (the so-called « vaugnerites » [Sabatier, 1980; Michon, 1987]) which indicate that a mantle-derived component was involved in their genesis.

(2) aluminous granitoids from Corbara unit (and presumably Pioggiola unit; fig. 1) compare to australian S-type granites and are presumed to have a purely crustal origin.

Magnesiopotassic granitoids are usually porphyroid, with up to 46 volume % potash feldspar megacrysts. Mafic-rich end-members have modal compositions of granodiorites and quartzmonzonites (fig. 2 and tabl. I) and contain magnesian biotite (fig. 5), magnesiohornblende (fig. 4) and a CaO- and MgO-rich clinopyroxene (fig. 3); leucocratic end-members are monzogranites with biotite alone or biotite + hornblende ± clinopyroxene.              Ile-Rousse magnesiopotassic granitoids comprise four subtypes: (1) Monticello granodiorites and monzogranites; (2) Ginebaru monzogranites; (3) Santa Reparata quartzmonzonites; and (4) Percepina monzogranites. Subtypes (1) and (2) define a regular trend presumed to result from fractional crystallization (figs. 2, 6). Subtypes (3) and (4) do not belong to this trend: Santa Reparata quartzmonzonites have K₂O contents much higher than Monticello granodiorites (6% versus 4%, at SiO₂=62%; tabl. I) while Percepina monzogranites show some distinctive features -  no hornblende and no clinopyroxene, ferrous biotite (fig. 5), higher contents of Fe₂O_3T and lower contents of Na₂O (tabl. I) - which discriminate them from Monticello and Ginebaru monzogranites. At present, we do not know if Santa Reparata quartzmonzonites and Percepina monzogranites belong to a single, discontinuous sequence (as suggested by their close spatial association; fig. 1) or if they correspond to independent batches of magnesiopotassic magmas.

Corbara granodiorites are medium-grained, biotite-rich granitoids which may contain garnet or cordierite. They differ from magnesiopotassic granitoids by: (1) higher modal contents of plagioclase (fig. 2 and tabl. I); (2) higher AI₂O₃ contents in biotite (fig. 5); (3) the absence of hornblende and clinopyroxene; and (4) peraluminous chemical compositions (fig. 7). The abun- ance of xenoliths (mostly gneisses) and the absence of vaugnerites suggest that Corbara granodiorites have a crustal origin, in agreement with their S-type affinity.

Diversity of magnesiopotassic plutonism in Balagne. Large variations in K₂O contents of magnesiopotassic granitoids from Balagne - specially, the mafic-rich end-members are displayed in figure 6 (data from Pézeril [1977] on the quartzmonzonites-monzogranite sequence from Calvi intrusion have been plotted in addition to our data). These variations do not correlate with prominent mineralogical changes (e.g., with changes in the nature and composition of mafic minerals): they mostly result from increasing modal abundances of potash feldspar megacrysts from Monticello granodiorites (~20%) to Santa Reparata quartzmonzonites (40% to 50%); Aregno quartzmonzonites from Calvi intrusion contain ~30% megacrysts. A tentative model to explain these large variations in K₂O contents is summarized in figure 8.

A model for the construction of Ile-Rousse complex: Structural analysis of Ile-Rousse complex was performed to explain the close association - as steeply dipping sheets of hectometric to kilometric thickness (fig. 1) - of petrogenetically-contrasted granitoids. Our main conclusions [Laporte et al., 1986,1989] are that Balagne granitoids are synkinematic and that they under- went considerable shortening during and/or after their emplacement (with a ~EW trending, horizontal direction of shortening). This regional deformation was responsible for the development of NS-trending, steeply dipping magmatic foliations. Statistical analysis of preferred dimensional orientation of potash feldspar megacrysts indicates that shortening percentages were high, locally in excess of 75% [Laporte, 1987; see also Fernandez and Laporte, 1991]. From these observations, it is suggested that granitic sheets from Ile-Rousse complex are in fact « pancakes » and that their large aspect ratios (> 10/1; fig. 1) result from the shortening of initially sub-equant magmatic bodies. Our model for the construction of Ile-Rousse complex involves the two following events (fig. 9): (1) the nesting of genetically-independent granitic pulses to build a large, composite body; and (2) the flattening of this body, before consolidation.