The granitoids of the Corsica Batholith exhibit a magmatic flow pattern, the trend of which ... Corsica and Sardinia islands are located in the northern sector of the ...
International Conference in Honor of Ron H. Vernon “Sheared Magmas in Nature and Experiment: Bridging the Brittle and Ductile Fields”
October 1 - 4, 2005 Technische Universität München, Germany Ludwig-Maximilians-Universität München, Germany
Hercynian Magmatism in Corsica EXCURSION October 5 – 11, 2005
Ivan Zibra(1), Maria Rosaria Renna(2), Jörn H. Kruhl(3), Riccardo Tribuzio(2) (1) Universitá di Pisa, Italy, (2) Universitá di Pavia, Italy, (3) TU München, Germany
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Introduction
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Granitoids and syntectonic magmatism within the Variscan Belt of Europe (Kruhl)
The Variscan orogen, exposed in central and southern Europe, (Fig. 2A) predominantly comprises medium- to high-grade metamorphic rocks and a large number and variety of magmatic rocks, mainly granitoids. Studies during the last 2 decades revealed that many of these granitoids were injected into a deforming continental crust, i.e., crystallized under stress, particularly in contractional regimes. A decisive role for the large-scale reorganization of the continental crust as well as for the ascent of granitic magma during the Variscan orogeny is played by deep-rooted, melt-bearing strike-slip shear zones or low-angle thrusts (e.g., Vernon et al. 1989, D’Lemos et al. 1992, Krohe 1992, Dörr et al. 1998, Büttner 1999, Liotta et al. 2004, Kruhl & Vernon 2005) which may change from early, low-angle dip-slip to later, steep strike-slip (Büttner et al. 1997, Büttner 1999). In general, the Variscan Belt represents an assembly of (micro)plates which derived from Gondwana and, during early-Variscan times, collided with Laurentia and, finally, again with Gondwana (Franke 1989, von Raumer et al. 2003). The continent collision led to nappe thrusting and piling and intensive granitoid magmatism. The origin of the large amount of late- to post-Variscan melts is still debated but, in general, is seen as a result of crustal thickening during continent collision and/or heat transfer to the lower/middle crust by underplating of basaltic magma. Most plutons were emplaced during the final stages of plate collision, which occurred at different times in different parts of the Variscides. Emplacement varies between ~ 350 and ~ 270 Ma. The chemical evolution of the plutonism is explained in terms of mantle-crust interactions at decreasing depths during post-orogenic extension (Ferré & Leake 2001). Specifically, for Corsica an evolution from high-K calc-alkaline to alkaline composition has been proposed (Fig. 4). In most parts of the Variscides syntectonic granitoids are present. During the early stages of continent collision they are related to low-angle thrusts, for example in the southern Bohemian Massif (Mühlviertel/ Büttner 1999) or in Sardinia (Baronie/ Kruhl & Vernon 2005). During subsequent stages of crustal shortening and thickening, low-angle thrusting switches to strike-slip movements. The strike-slip faults may extend over several hundred kilometres and are frequently accompanied by granitoid intrusions, for example in the Bretagne (Gapais & Balé 1990, D’Lemos et al. 1992), in the Bohemian Massif (Pfahl and Danube Fault/ Büttner 1999, Central Bohemian Shear Zone/ Scheuvens & Zulauf 2000), in Calabria (Liotta et al. 2004), or in the Odenwald (Krohe 1992). Moreover, the formation, ascent and emplacement of granitic melts may be related to the uplift of sections of the lower continental crust to midcrustal levels, e.g., in Calabria or in the Ivrea Zone. All these syntectonic granitoids show more or less intensive magmatic foliations and lineations, mainly represented by the alignment of feldspar phenocrysts in a more fine-grained magmatic groundmass. The granitoids of the Corsica Batholith exhibit a magmatic flow pattern, the trend of which is visible on the large-scale geological map (Fig. 3). In the early K-calc-alcaline granitoids (U1 and U2, according to Cocherie et al. 2005; Fig. 3), the overall N-S trend of this generally steep foliation swings towards an E-W orientation in the central part of the Batholith and back again to a ~ N-S orientation at the southern part. This pattern is intensified by a large-scale magmatic layering, mainly kilometre long lenses and layers of mafic intrusions into the granitoids. In addition, on the meso-scale, magma mingling and mixing are present and reflect the complex intrusion history and the compositional variability of the Corsica Batholith on different scales. The younger of the U2 intrusives have a less developed fabric whereas some of the subsequent alkaline granitoids (U3, Fig. 3) show different magmatic flow patterns. In general, magmatic flow patterns may be highly variable on smaller scales.
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Fig. 2 (left): (A) Variscan massifs of central and southern Europe. Tectonometamorphic domains after Franke (1989). (B) Simplified geological map of Corsica with the three main Variscan plutonic suites and the Alpine domain. From Ferré & Leake (2001). Fig. 3 (2 pages above): Geology of the Corsica Batholith. The dashed signature indicates the general trend of the magmatic foliation of the late-Variscan granitoids. From Cocherie et al. (2005). Fig. 4 (right): Evolution of the chemistry of late- to post-orogenic plutonism in the European Variscides, as a result of crustal thinning and post-collisional extension following crustal thickening. From Ferré & Leake (2001).
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Geology of Corsica (Tribuzio)
Corsica and Sardinia islands are located in the northern sector of the western Mediterranean. They are considered as a microblock welded to southern France-northern Iberia until Middle Oligocene to Early Miocene, when rifting and drifting from the European mainland took place (e.g. Carminati et al., 1998; Speranza et al., 2002). Corsica is subdivided into two main geological domains. The northeastern area (allochthonous “Alpine Corsica”) is formed by a complex stack of nappes, derived from
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Middle Jurassic-Early Cretaceous oceanic lithosphere (ophiolitic “Schistes Lustrés”) and associated continental margins (e.g. Mattauer et al., 1981; Durand Delga, 1984; Jolivet et al., 1990; Malavieille et al., 1998; Molli and Tribuzio, 2004). These tectonic units were deformed and metamorphosed in response to the Alpine orogeny. In particular, most nappes record Late Cretaceous to Eocene peak metamorphic conditions in the blueschist to lowtemperature eclogite facies (Caron et al., 1981; Caron and Pequignot, 1986; Lahondère, 1988; Lahondère and Guerrot, 1997; Brunet et al., 2000; Tribuzio and Giacomini, 2002). The Alpine nappes were thrusted on a pre-Mesozoic crystalline basement that is mostly unaffected by the Alpine tectono-metamorphic event, the autochthonous “Hercynian Corsica” of the western area. This basement consists mainly of granitoid plutons emplaced at middle to upper crustal levels (Bonin et al., 1978; Egeberg et al., 1993; Rossi and Cocherie, 1991; Cocherie et al., 1994; Poitrasson et al., 1994a; 1995; Tommasini et al., 1995; Ferré and Leake, 2001) during the late to post-collisional phases of the Variscan orogeny, in Carboniferous to Early Permian (Paquette et al., 2003; Cocherie et al., 2005). The granitoid plutons are locally associated with coeval gabbroic rocks (see also Ohnenstetter and Rossi, 1985; Bonin, 2004; Renna et al., 2005) and scarce remnants of a low- to high-grade metamorphic basement (Lardeaux et al., 1994, and references therein). The “Hercynian Corsica” comprises also some volcanic sequences, which are mostly represented by andesitic to dacitic lavas and ignimbritic rhyolites (Cabanis et al., 1990). The “Alpine Corsica” is commonly interpreted as the southern prolongation of the Penninic domain of the western Alps, and the “Hercynian Corsica” is correlated with the MauresEsterel basement in southern France (Durand Delga, 1984; Edel, 1981).
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The western Corsica batholith
The plutonic bodies from the “Hercynian Corsica” are as a whole known as the western Corsica batholith, which is traditionally subdivided into three geochronologically distinct suites (e.g. Rossi and Cocherie, 1991; Cocherie et al., 1994; 2005). The oldest intrusions are exposed in western and northwestern Corsica and consist mainly of K- and Mg-rich granitoids, commonly biotite-hornblende-titanite-bearing, defining a shoshonitic affinity (Rossi and Cocherie, 1991; Cocherie et al., 1994; Ferré and Leake, 2001). Structural data at a regional scale and K-feldspar megacryst fabric analysis provide evidence for a syntectonic character of these intrusions (Laporte et al., 1986). Most geochronological determinations on zircons have provided 336-340 Ma for the emplacement of K- and Mg-rich granitoids (Paquette et al., 2003; see also Cocherie et al., 2005). The Kand Mg-rich granitoids are intimately associated with minor mafic bodies, mainly represented by intrusive stocks and enclaves made of biotite-hornblende-titanite-rich diorites, which were related to lamprophyric magmas derived from enriched mantle sources (Cocherie et al., 1994; Ferré and Leake, 2001). The hybrid nature of K- and Mg-rich granitoids, which most likely involved both metasedimentary and mantle source materials, is confirmed by their trace element and Nd isotope compositions (Cocherie et al., 1994; Paquette et al., 2003). The second plutonic episode is characterised by hornblende-biotite-bearing granitoids, commonly containing microgranular mafic enclaves and small diorite septa, which define typical calc-alkaline differentiation trends (Cocherie et al., 1984; Zorpi et al., 1989; Rossi and Cocherie, 1991). These plutonic bodies are volumetrically dominant in the central and southern sectors of the batholith and according to the U-Pb geochronological determinations on zircons by Paquette et al. (2003) formed at about 305 Ma, during a short time interval (
