The Sardic Phase: field evidence of Ordovician tectonics in SE Sardinia, Italy

Abstract

Detailed geological mapping, field observations and structural analyses demonstrate that Early Ordovician ('Sardic') deformation occurred in the early Palaeozoic successions that are now incorporated in the Variscan Nappe zone of SE Sardinia. This deformation is represented by folds that formed at a shallow depth, lack a significant syn-folding axial planar foliation, and do not affect the overlying Late Ordovician - Devonian sedimentary sequence. These deformation features can be related to the development of the Sardic Unconformity and to calc-alkaline volcanism in several now-scattered terranes of Ordovician northern Gondwana. This reflects a convergent geodynamic setting that in the study sector appears to have failed to reach a continental collisional end-stage. Associating the structural data from this study with those of several published research studies, a preliminary evaluation about which tectonic setting could better fit is proposed. These conditions affected the eastern side of the northern Gondwana margin more or less contemporaneously with the opening of the Rheic Ocean and the closure of the Qaidam Ocean, before the amalgamation of the Hunia terranes

The pre-orogenic detrital zircon record of the Peri-Gondwanan crust

“…A local tectonic feature of the Peri-Gondwanan crust is the so called ‘Sardic phase’ that is recorded as a 457–443 Ma tectono-metamorphic event in Middle Ordovician rocks of the Agordo basement and the Strona–Ceneri Zone of the Southern Alps and in parts of the Carpathians (Poli & Zanferrari, 1992; Franz & Romer, 2007; Zurbriggen, 2015) and as a pronounced unconformity in the shelf sediments of the Montagne Noire, the Mouthoumet Massif, the Eastern Pyrenees, the Catalonian Coastal Ranges, the Central Iberian Zone, Sardinia, Corsica, Sicily and in southern Turkey (Lotze, 1956; Carmignani et al 1982; Gil Ibarguchi, Navidad & Ortega, 1990; Gutiérrez-Marco, De San José & Pieren, 1990; Hammann, 1992; Robardet et al 1994; Valverde-Vaquero & Dunning, 2000; Trombetta et al 2004; Rossi, Oggiano & Cocherie, 2009; Javier Álvaro et al 2016; Cocco & Funedda, 2017). According to Gutiérrez-Marco et al (2002 and references therein), echoes of the Sardic event may be represented by oolitic iron beds in Iberia.…”

Structural Controls of Ore Mineralization in a Polydeformed Basement: Field Examples from the Variscan Baccu Locci Shear Zone (SE Sardinia, Italy)

“…In this study, ore mineralogy and microtextural studies were performed on polished sections by optical microscopy in reflected light. Several transmitted light studies on thin sections of ores and their host rocks have been performed in past works [3,5,7,19].…”

“…The oldest tectonic features that are recognizable in the Baccu Locci mine district involve the lower Carboniferous rocks and are related to the Variscan orogeny. Indeed, there is no evidence of the pre-Middle Ordovician tectonic structures recognized in adjacent areas [7], as they were probably obliterated by Variscan deformation. The overlap of several early to late Carboniferous deformation events is evidence that a D1 collisional phase with crustal thickening and subhorizontal shortening occurred under ductile conditions and a D2 postcollisional extension with the reactivation of some of D1 structures occurred in the ductile-brittle transition (Figure 4) [6,8,15].…”

Insights into the structural evolution of the pre-Variscan rocks of the Eastern Pyrenees from La Molina quartz veins; constraints on chlorite and fluid inclusion thermometry

“…The existence of this intra-Ordovician unconformity was first established in other NW Gondwana domains such as in Sardinia (Teichmuller, 1931), where it separates the Cambrian-Ordovician from the Upper Ordovician successions (Pillola et al, 2008), although its origin and meaning are still under debate (i.e. Cocco and Funedda, 2019;Pasci et al, 2008). As a result, the large-scale geodynamic processes that led to its formation have been subject to several interpretations (see a review in Álvaro et al, 2018).…”