Probing crustal and mantle lithosphere origin through Ordovician volcanic rocks along the Iberian passive margin of Gondwana

Abstract

Northwestern Iberia preserves one of the most complete Paleozoic sequences that document the origin and development of a passive margin along the southern (Gondwanan) flank of the Rheic Ocean. In addition to a well preserved sedimentary record, there is widespread Ordovician volcanic activity that can be used to probe the nature of the lower crust and mantle lithosphere that sourced the volcanic rocks during the Rheic ocean opening. The Ordovician rift-related volcanic sequences provide first-order constraints on the early evolution of the Rheic Ocean. In addition to published and new lithogeochemical data, we provide Sm/Nd isotopic data which together constrain the mantle or crustal source and allow an assessment on the role of the basement in Rheic Ocean magmatism. The data imply that the mafic rocks are derived from a variety of sources, including juvenile mantle that was contaminated by subduction coeval with Early Ordovician magmatism, suggesting the importance of arc activity in northwest Iberia during the opening of the Rheic Ocean. Other basalts were derived from a subcontinental lithospheric mantle that was enriched at about 1.0 Ga. Basalts derived from a mantle enriched at ca. 1.0 Ga occur along other parts of the Gondwanan margin (Avalonia, Oaxaquia) and so the Iberian basalts may be a local representation of a regionally significant enriched mantle. The Sm–Nd isotopic characteristics permit a genetic connection between this mantle source and the basement rocks recently identified in northwest Iberia. Felsic magmas are predominantly intracrustal magmas derived from melting a Mesoproterozoic crust, lending support to other lines of data that the Gondwanan margin of northwest Iberia was predominantly underlain by a South American (Rio Negro) source.