Rifting and arc-related early Paleozoic volcanism along the North Gondwana margin: geochemical and geological evidence from Sardinia (Italy)

Three series of volcanic rocks accumulated during the Cambrian to Silurian in the metasediment-dominated Variscan basement of Sardinia. They provide a record of the changing geodynamic setting of the North Gondwana margin between Upper Cambrian and earliest Silurian. A continuous Upper Cambrian-Lower Ordovician succession of felsic submarine and subaerial rocks, dominantly transitional alkaline in character (c. 492-480 Ma) is present throughout the Variscan nappes. Trace element data, together with Nd isotope data that point to a depleted mantle source, indicate an ensialic environment. A Middle Ordovician (c. 465 Ma) calc-alkaline bimodal suite, restricted to the external Variscan nappes, overlies the Sardic Unconformity. Negative εNdi values (-3.03 to - 5.75) indicate the suite is a product of arc volcanism, from a variably enriched mantle. A Late Ordovician-Early Silurian (c. 440 Ma) volcano-sedimentary cycle consists of an alkalic mafic suite in a post-Caradocian transgressive sequence. Feeder dykes cut the pre-Sardic sequence. The alkali basalts are enriched in Nb-Ta, have Zr/Nb ratios in the range 4.20-30.90 (typical of a rift environment), and positive εNdi values that indicate a depleted mantle source. Trachy-andesite lavas have trace element contents characteristic of within-plate basalt differentiates, with evidence of minor crustal contamination.