Rifting and Arc-Related Early Paleozoic Volcanism along the North Gondwana Margin: Geochemical and Geological Evidence from Sardinia (Italy)

Abstract

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 (ca. 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 (ca. 465 Ma) calc-alkaline bimodal suite, restricted to the external Variscan nappes, overlies the Sardic Unconformity. Negative Nd i values (Ϫ3.03 to Ϫ5.75) indicate that the suite is a product of arc volcanism from a variably enriched mantle. A Late Ordovician-Early Silurian (ca. 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 and have Zr/Nb ratios in the range 4.20-30.90 (typical of a rift environment) and positive Nd i values that indicate a depleted mantle source. Trachyandesite lavas have trace element contents characteristic of within-plate basalt differentiates, with evidence of minor crustal contamination.

Magmatic evolution in the N-Gondwana margin related to the opening of the Rheic Ocean—evidence from the Upper Parautochthon of the Galicia-Trás-os-Montes Zone and from the Central Iberian Zone (NW Iberian Massif)

“…Although the onset of continental rifting may differ in age from one part of the margin to the other, the opening and subsequent widening of the Rheic Ocean is thought to have occurred before the Upper Ordovician (Cocks and Torsvik 2002;Fortey and Cocks 2003), with the Armorican Quartzite facies (c. 477 Ma; Gutiérrez- Alonso et al 2007) representing the establishment of an stable platform in the early stages of the riftdrift process (Gutiérrez-Marco et al 2002). Yet, the U-Pb ages presented in this paper confirm that the igneous activity in the autochthonous and parautochthonous domains of the margin exposed today in NW Iberia lasted at least until the Upper Ordovician, in similarity to what is described in NE Iberia (e.g., Castiñeiras et al 2010;Navidad et al 2010;Casas et al 2011;Liesa et al 2011;Martínez et al 2011), in the Armorican Massif (e.g., Ballèvre et al 2002), in the French Massif Central (Roger et al 2004;Faure et al 2010;Melleton et al 2010) and in the Mediterranean realm (e.g., Trombetta et al 2004;Helbing and Tiepolo 2005;Oggiano et al 2010;Gaggero et al 2012). Interestingly, neither of these events represents the last magmatic pulses along the continental platform, which remained magmatically active at least up to the latest Ordovician in NE Iberia (446 Ma; Casas 2010) and up to the Silurian in NW Iberia (Ancochea et al 1988;Valverde-Vaquero et al 2007).…”

“…These tectono-sedimentarymagmatic relationships can be traced not only in the Iberian Massif but also throughout a significant part of the Variscan orogen, from the Bohemian Massif to Corsica and Sardinia (Helbing and Tiepolo, 2005;Linnemann et al 2007Linnemann et al , 2008Nance et al 2008;Oggiano et al 2010;Gaggero et al 2012). The interaction between (at least) two tectonic plates is considered the most plausible rifting trigger (Nance et al 2010), but plume activity can also be accounted for (e.g., Díez Fernández et al 2012a).…”

Metavolcanics from Capo Malfatano, SW Sardinia, Italy: New insight on the age and nature of Ordovician volcanism in the Variscan foreland zone

“…Three main Cambro–Ordovician magmatic cycles have been distinguished in the Sardinian Variscan chain (Gaggero, Oggiano, Funedda, & Buzzi, ; Oggiano et al, ). The first cycle is characterized by intermediate and felsic transitional volcanic rocks interlayered in the Cambro–Ordovician terrigenous succession.…”

Rare earth elements (REE)—Minerals in the Silius fluorite vein system (Sardinia, Italy)

“…The Ordovician magmatic and volcano-sedimentary complex is unconformably overlain by 111 siliciclastic continental to neritic sediments, which were deposited during a period of active intraplate 112 magmatism, resulting in alkali basalt flows, sills and dykes (late Ordovician-Silurian) (Oggiano et al,113 2010; Gaggero et al, 2012) (Fig. 1B).…”

“…In particular, the presence of Eu anomalies is strongly temperature-dependent, because at and also show positive Eu anomalies in the REE chondrite-normalized pattern (Gaggero et al 2012). …”