Protoliths of the Variscan Terranes in Central Southern Alps and Austroalpine nappes in Valtellina

Serie dei Laghi, Orobic basement and Austroalpine Units of lower Valtellina consist of metasediments and metaplutonites with Variscan metamorphism (331-340Ma, Boriani and Villa, 1997), mostly in amphibolite facies. The Serie dei Laghi (low T amphibolite facies) includes: Scisti dei Laghi metapelites, Strona Ceneri meta-arenites (Cenerigneiss and Gneiss Minuti), and Strona Ceneri Border Zone metabasites (equivalent to the "Leptynite-Amphibolite Group") with ultramafite, metagabbro and eclogite relics. All these rocks contain granitic-granodioritic orthogneisses. Cenerigneiss and Gneiss Minuti are geochemically similar to turbidite sands from continental island arcs, mainly derived from calcalkaline magmatic rocks with variable contributions of recycled crustal materials. Their Sr isotopic ratios correspond to that of the Ordovician ocean water; ?Nd values are typical of a crustal component. The Pb isotopic ratios show a range of variation broader than that of the Sr ratios. The metasediments show higher Pb isotopic ratios than the associated metaplutonites; both lie on the same linear trend with positive slope. U-Pb SHRIMP studies on Cenerigneiss reveal the presence of older (either 600-700 and 1000-1100 Ma) detrital zircons as well as younger (465 Ma) igneous zircons, which suggest some magmatic infiltration. In the SCBZ, the amphibolites have irregular LILE patterns and relatively flat HFSE patterns, recalling some back-arc tholeiites. The leptynites display a moderate enrichment in LREE with (La/Sm)N = 2.0-3.0, a slight negative Eu anomaly and a flat HREE pattern (ca. 20-40 times the chondrite level) with (Tb/Lu)N = 1.0 - 1.2, similar to those of the Brevenne metavolcanics. Both amphibolites and leptynites plot in the depleted mantle field, with ?Nd466 varying from +3.21 to +7.73. They represent arc/back arc bimodal volcanics from a common mantle source. The zircons of the leptynites are weakly luminescent, with evidence of magmatic growth (555 Ma). Those of the amphibolites show highly-luminescent domains overprinting the original growth. Their U-Pb age is about 349 Ma. The Orobic basement (low-T amphibolite facies) consists of two metasedimentary units with gradual transition: Scisti di Edolo, mostly metapelites with quartzdioritic to granitic orthogneisses (M.te Fioraro Complex), and Gneiss di Morbegno, mostly meta-arenites with metaleucogranites ("Gneiss Chiari"). The REE patterns of Gneiss di Morbegno are uniform and show LREE enrichment. Their chemical composition is close to the NASC. They show geochemical affinity with sediments from recent trailing-edge margin; a TDM (Nd) between 1.7-2.0 Ga suggests Proterozoic source rocks. The "Southern Steep Belt" of the Austroalpine units include: the Tonale Series, "kinzigitic" metapelites (high-T amphibolite facies), without metaplutonites; the Punta di Pietra Rossa Unit (Languard-Campo Nappe), mostly meta-arenites with granitic orthogneisses (low T amphibolite facies); the Mt Canale Unit (Bernina Nappe), mostly granitic orthogneisses with metapelites and metacarbonate rocks (greenschist facies). Most metaplutonic rocks result from mixing of a subcrustal and a crustal component, followed by fractionation. They display well pronounced Eu anomalies. Compared to the Serie dei Laghi and Austroalpine orthogneisses, the Mt Fioraro complex of the Orobic basement is enriched in REE, Nb and Y, recalling an OIB-type mafic component. Chemical and isotope characteristics of the leucogranitic protolith of the Gneiss Chiari suggest a derivation from the melting of a sedimentary source of mixed nature and age, dominated by Neoproterozoic components. The emplacement ages (U-Pb SHRIMP on zircon) of the magmatic protoliths are 462+11 Ma for the Mt Fioraro complex and 475+8 Ma for the Gneiss Chiari. Chemical signatures of the Ordovician magmatism suggest a geodynamic environment from continental arc to continental rifting.