The Role of Sediment Subduction and Crustal Growth in Hercynian Plutonism: Isotopic and Trace Element Evidence from the Sardinia-Corsica Batholith

The calc-alkaline association of the Hercynian Sardinia-CorsicaBatholith consists of multiple coalescent granitoid plutonsand minor gabbroic complexes. Isotopic and trace element dataare presented for selected gabbros and I-type granitoids representativeof the parental mantle- and crust-derived magmas, respectively.The gabbros belong to normal calc-alkaline suites and have markedrelative enrichments in Rb, Ba, K and Pb in primitive mantle-normalizedtrace element diagrams. The granitoids belong to high-K calc-alkalinesuites and have fairly uniform trace element compositions resemblingvolcanic arc granitoids (VAG). A significant overlap in Sr andNd isotope compositions is observed between gabbros and granitoids. Geochemical and isotopic data provide evidence for the originof the gabbros from mantle sources enriched in incompatibletrace elements through recycling of sediments via subductionzones, whereas the granitoids were derived from crustal sourcescomposed mainly of igneous protoliths with relatively homogeneouscomposition. Sr and Nd isotope compositions of gabbros and granitoidsare consistent with both the mantle enrichment process and theformation of the igneous crustal sources occurring at 450 Ma,during the earlier calc-alkaline igneous activity. The connection between Hercynian and Ordovician igneous activityhas important and new implications for the Palaeozoic evolutionof the Sardinia and Corsica lithosphere, and permits the Hercynianorogeny to be placed in a wider geodynamic setting, consistingof three main phases. The Ordovician precollisional phase wascharacterized by a N-NE-dipping subduction of an oceanic plateunder a continental plate with emplacement of acid and subordinatebasic-intermediate volcanic and intrusive rocks. The subcontinentalmantle underneath Sardinia and Corsica experienced enrichmentin incompatible trace elements through recycling of sediments.Major crustal accretion also occurred with underplating of basalticmagmas. The Devonian collisional phase was characterized bythe collision of two continental plates after the total consumptionof the oceanic plate. Crustal thickening processes took placetogether with regional metamorphic events that recorded a clockwiseP-T-t path. The Carboniferous post-collisional phase was characterizedby isostatic and thermal readjustments following crustal thickeningthat caused extensive partial melting. Large quantities of I-typegranitoids and subordinate gabbroic complexes were emplacedin the middle-upper crust and formed the mainframe of the Sardinia-CorsicaBatholith. This geodynamic model is consistent with the Palaeozoic evolutionof other sectors of Western Europe suggested on the basis ofgeological, geochronological and palaeomagnetic data. The palaeomagneticrestoration of the Late Palaeozoic position of Sardinia andCorsica close to Southern France suggests that Sardinia andCorsica could have been portions of the southern edge of theArmorican plate that, during Siluro-Devonian, collided withthe Ibero-Aquitanian plate after the total consumption of theLate Cambro-Ordovician South Armorican and/or Massif CentralOcean. KEY WORDS: crustal growth; Hercynian orogeny; mantle enrichment; radiogenic isotopes; Sardinia-Corsica Batholith ^*Corresponding author. Present address Dipartimento di Scienza del Suolo e Nutrizione della Pianta, Piazzale Delle Cascine i6, 50144 Firenze, Italy