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-Corsica Bathol ith consists of multiple coalescent granitoid plutons and minor gabbro ic complexes. Isotopic and trace element data are presented for select ed gabbros and I-type granitoids representative of the parental mantle - and crust-derived magmas, respectively. The gabbros belong to normal calc-alkaline suites and have marked relative enrichments in Rb, Ba, K and Pb in primitive mantle-normalized trace element diagrams. The gr anitoids belong to high-X calc-alkaline suites and have fairly uniform trace element compositions resembling volcanic are granitoids (VAG). A significant overlap in Sr and Nd isotope compositions is observed be tween gabbros and granitoids. Geochemical and isotopic data provide ev idence for the origin of the gabbros from mantle sources enriched in i ncompatible trace elements through recycling of sediments via subducti on zones, whereas the granitoids were derived from crustal sources com posed mainly of igneous protoliths with relatively homogeneous composi tion. Sr and Nd isotope compositions of gabbros and granitoids are con sistent with both the mantle enrichment process and the formation of t he igneous crustal sources occurring at similar to 450 Ma, during the earlier calc-alkaline igneous activity. The connection between Hercyni an and Ordovician igneous activity has important and new implications for the Palaeozoic evolution of the Sardinia and Corsica lithosphere, and permits the Hercynian orogeny to be placed in a wider geodynamic s etting, consisting of three main phases. The Ordovician pre-collisiona l phase was characterized by a N-NE-dipping subduction of an oceanic p late under a continental plate with emplacement of acid and subordinat e basic-intermediate volcanic and intrusive rocks. The subcontinental mantle underneath Sardinia and Corsica experienced enrichment in incom patible trace elements through recycling of sediments. Major crustal a ccretion also occurred with underplating of basaltic magmas. The Devon ian collisional phase was characterized by the collision of two contin ental plates after the total consumption of the oceanic plate. Crustal thickening processes took place together with regional metamorphic ev ents that recorded a clockwise P-T-t path. The Carboniferous post-coll isional phase was characterized by isostatic and thermal readjustments following crustal thickening that caused extensive partial melting. L arge quantities of I-type granitoids and subordinate gabbroic complexe s were emplaced in the middle-upper crust and formed the mainframe of the Sardinia-Corsica Batholith. This geodynamic model is consistent wi th the Palaeozoic evolution of other sectors of Western Europe suggest ed on the basis of geological, geochronological and palaeomagnetic dat a. The palaeomagnetic restoration of the Late Palaeozoic position of S ardinia and Corsica close to Southern France suggests that Sardinia an d Corsica could have been portions of the southern edge of the Armoric an place that, during Siluro-Devonian, collided with the Ibero-Aquitan ian plate after the total consumption of the Late Cambro-Ordovician So uth Armorican and/or Massif Central Ocean.