Rifting and Arc-Related Early Paleozoic Volcanism along the North Gondwana Margin: Geochemical and Geological Evidence from Sardinia (Italy) Laura Gaggero,1,* Giacomo Oggiano,2 Antonio Funedda,3 and Laura Buzzi1 1. Department for the Study of Territory and Its Resources, University of Genoa, Corso Europa 26, 16132 Genoa, Italy; 2. Department of Botanics, Ecology, and Geology, University of Sassari, Via Piandanna 4, 07100 Sassari, Italy; 3. Department of Earth Sciences, University of Cagliari, Via Trentino 51, 09127 Cagliari, 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 ␧ Ϫ Ϫ external Variscan nappes, overlies the Sardic Unconformity. Negative Ndi values ( 3.03 to 5.75) 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 Ndi values that indicate a depleted mantle source. Trachyandesite lavas have trace element contents characteristic of within-plate basalt differentiates, with evidence of minor crustal con- tamination. Online enhancements: appendix figures. Introduction An exceptional geodiversity in the Earth history accompanied by a complex magmatic evolution re- has been evidenced at the Paleozoic, between the corded by a bimodal intraplate volcanism associ- two major events of continental accretion that pro- ated with terrigenous sedimentation (Etxebarria et duced Gondwana first and then Pangea (Stampfli al. 2006; Chichorro et al. 2008; Linnemann et al. et al. 2002; von Raumer and Stampfli 2008; Nance 2008), mid-ocean ridge basalt (MORB)-type mag- et al. 2010). Thus, the Paleozoic Era begins and ends matism associated with lithospheric rifting and with a similar geographic configuration, in that one oceanization (between Cambrian and Devonian; supercontinent is inferred both in the Neoprote- Murphy et al. 2008, 2011), arc magmatism during rozoic and in the Permian. Between the extremes subduction and continental collision (Middle Or- of this Wilson cycle ranging over an interval of 300 dovician; Sa´nchez-Garcı´a et al. 2003), and post- million years, the supercontinent Rodinia progres- orogenic alkalic magmatism (Upper Ordovician– sively broke up into macro-microcontinents, which Silurian boundary; Lo´ pez-Moro et al. 2007; Keppie later further fragmented, collided, and subse- et al. 2008). quently reassembled in Pangea. This process was In particular, the significance of the Paleozoic Rheic Ocean to the building of Laurussia and Gond- wana continental blocks before the Variscan- Manuscript received February 17, 2011; accepted October 19, 2011. Alleghanian-Ouachita collision has been recently * Author for correspondence; e-mail: [email protected]. emphasized and its Cambrian-Paleozoic evolution [The Journal of Geology, 2012, volume 120, p. 273–292] ᭧ 2012 by The University of Chicago. All rights reserved. 0022-1376/2012/12003-0002$15.00. DOI: 10.1086/664776 273 274 L. GAGGERO ET AL. analyzed and dissected along the western, central, southern European Variscides (Casini et al. 2010), and eastern Gondwana margins (Nance et al. 2010). and it can be divided into the following tectono- Despite the occurrence in Sardinia of an almost metamorphic zones: a foreland zone in the SW, complete cross section of the Variscan orogen, this with either very low-grade or no metamorphism; a segment has barely been included in comprehen- nappe zone in the SE and central parts of the island sive reconstructions of the precollisional history of (subdivided into external and internal nappes, with the Variscan belt, probably due to the scarcity of several stacked tectonic units), affected by low- geochronological and geochemical data on the Sar- grade metamorphism; and an inner zone in the dinian pre-Variscan basement. A set of recent U-Pb north, with medium- to high-grade metamorphism geochronological data (obtained using excimer laser (fig. 1). ablation–inductively coupled plasma mass spec- The foreland and nappe zones are also charac- trometry; Oggiano et al. 2010) provides evidence terized by a Middle Ordovician angular unconfor- that in Sardinia the precollisional volcanic activity mity (Sardic Unconformity; Carmignani et al. 2001, along the North Gondwana margin, or in related and references therein), which is also recognized in terrane assemblages, developed in at least three the Eastern Iberian Plate (Casas et al. 2010; Navidad stages, each stage being characterized by a different et al. 2010). Along the southern boundary of the geodynamic environment: (1) a Late Cambrian– inner zone, an eclogite-bearing belt is exposed, Early Ordovician episode of volcanism (ca. 492–480 which was interpreted as a suture zone (Cappelli Ma) within a stratigraphically well-constrained et al. 1992; Carmignani et al. 1994). The protolith Ma; the 2 ע Cambro-Ordovician clastic sequence, (2) Middle of the eclogite has been dated at457 Ordovician calc-alkalic activity ascribed to the high-pressure event is Devonian (Cortesogno et al. Dapingian-Sandbian on the basis of paleontology 2004; Giacomini et al. 2005; Franceschelli et al. and now dated radiometrically at ca. 465 Ma, and 2007). These eclogites have MORB signatures and (3) an uppermost Ordovician (ca. 440 Ma) volcanic are embedded within a metapelitic-metarenaceous event of alkaline affinity that is widespread in all complex hosting also homogeneous quartzite beds the tectonic units of the Sardinian Variscides. In (metacherts?), orthogneisses, and metabasite with general, however, the various tectonic units are high- to medium-P granulite metamorphic imprint characterized by wide variations in these volcanic (Franceschelli et al. 2007). The deformation, local- rocks in space, time, and volume, and this is typ- ized in low-strength shear zones, and the geometric ically combined with a lack of adequate age control association of rock bodies with different metamor- on the associated clastic sediments. Our interest, phic records point to a me´lange of rocks tectoni- therefore, was raised in obtaining more data, valid cally sampled from diverse crustal levels within a for reconstruction of the paleogeography and geo- channel flow, probably linked to the subduction of dynamic events (rifting, breakup, drifting, accre- a lower Paleozoic ocean (Cappelli et al. 1992; tion/hypercollision) related to the northern Gond- Stampfli et al. 2002; von Raumer et al. 2003). wana margin and its derived “terranes” over a time Throughout the external nappes (Carmignani et period ranging from the Cambro-Ordovician up to al. 1994), the sedimentary record and fossil content the precollisional setting that gave rise to the Var- is generally preserved, and several volcano-sedi- iscan configuration of the Mesoeuropean crust. mentary complexes are stacked in the nappe zone Moreover, the petrology of Paleozoic processes is (Di Pisa et al. 1992; Carmignani et al. 1994; fig. 2). fundamental to understanding the influence on the The Sardinia-Corsica Microplate at the Cambro- lithospheric setting until the Alpine cycle. Diverse Ordovician Boundary. In the foreland, the pre- sectors of Sardinian Variscides were therefore stud- Sardic sedimentary history is dominated by the de- ied and comparison made with adjacent paleogeo- position of epicontinental sediments (Nebida graphic areas. Group), including carbonate shelf deposits (Gon- The aim of this article is to characterize the geo- nesa Group), which are inferred to grade laterally chemical features of the volcanic rocks in order to into deeper siliciclastic sequences (Iglesias Group), constrain the source region and crustal evolution, all of which are topped by the Sardic Unconformity. as well as the nature of the geodynamic setting. The sedimentary rocks of the shelf-slope transition indicate passive continental margin conditions dur- ing the Late Cambrian–Early Ordovician (Cocozza Geological Setting 1979; Galassi and Gandin 1992; Pillola et al. 1995, Present-Day Geological Framework of Sardinia. and references therein). Only the Capo Spartivento The Sardinia-Corsica Microplate exhibits one of the orthogneiss, basement of the foreland, is referred most complete and best-preserved transects of the to this setting, although the error of the available JournalofGeology PALEOZOIC VOLCANISMS IN NORTH GONDWANA 275 Figure 1. Generalized tectonic map of the Variscan basement of Sardinia and tectonic and metamorphic zones of the Variscan basement of Sardinia (a). metamorphism (Arenarie di San Vito). The ages of ע date does not allow a clear age attribution (478 13 Ma; Delaperrie`re and Lancelot 1989). the metasedimentary deposits, based on acritarch The pre-Sardic lithostratigraphic succession of biostratigraphy,