P30 Field Trip
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Volume n° 4 - from P14 to P36 32nd INTERNATIONAL GEOLOGICAL CONGRESS METALLOGENY IN SARDINIA (ITALY): FROM THE CAMBRIAN TO THE TERTIARY Leaders: A. Marcello, S. Pretti, P. Valera Associate Leaders: M. Agus, M. Boni, M. Fiori Field Trip Guide Book - P30 Field Trip Florence - Italy August 20-28, 2004 Post-Congress P30 P30_copertina_R_OK C 21-06-2004, 9:34:57 The scientific content of this guide is under the total responsibility of the Authors Published by: APAT – Italian Agency for the Environmental Protection and Technical Services - Via Vitaliano Brancati, 48 - 00144 Roma - Italy Series Editors: Luca Guerrieri, Irene Rischia and Leonello Serva (APAT, Roma) English Desk-copy Editors: Paul Mazza (Università di Firenze), Jessica Ann Thonn (Università di Firenze), Nathalie Marléne Adams (Università di Firenze), Miriam Friedman (Università di Firenze), Kate Eadie (Freelance indipendent professional) Field Trip Committee: Leonello Serva (APAT, Roma), Alessandro Michetti (Università dell’Insubria, Como), Giulio Pavia (Università di Torino), Raffaele Pignone (Servizio Geologico Regione Emilia-Romagna, Bologna) and Riccardo Polino (CNR, Torino) Acknowledgments: The 32nd IGC Organizing Committee is grateful to Roberto Pompili and Elisa Brustia (APAT, Roma) for their collaboration in editing. Graphic project: Full snc - Firenze Layout and press: Lito Terrazzi srl - Firenze P30_copertina_R_OK D 25-05-2004, 16:37:54 Volume n° 4 - from P14 to P36 32nd INTERNATIONAL GEOLOGICAL CONGRESS METALLOGENY IN SARDINIA (ITALY): FROM THE CAMBRIAN TO THE TERTIARY AUTHORS: A. Marcello¹, S. Pretti¹, P. Valera¹, M. Agus³, M. Boni², M. Fiori³ 1 Dipartimento di Geoingegneria e Tecnologie Ambientali, Università di Cagliari - Italy 2 Dipartimento di Geofi sica e Vulcanologia, Università “Federico II” di Napoli - Italy 3 Istituto di Geologia e Geoingegneria Ambientale del C.N.R., Sezione di Cagliari - Italy Florence - Italy August 20-28, 2004 Post-Congress P30 P30_R_OK A 21-06-2004, 9:37:00 Front Cover: the southwestern coast of Sardinia. P30_R_OK B 25-05-2004, 16:47:35 METALLOGENY IN SARDINIA (ITALY): FROM THE CAMBRIAN TO THE TERTIARY P30 Leader: A. Marcello, S. Pretti, P. Valera Associate Leaders: M. Agus, M. Boni, M. Fiori Introduction continental arc-related volcanic suite (Nappe Zone); In spite of its relatively small surface (24,090 km2), III) an inner zone that includes a high-grade the island of Sardinia was one of the most important metamorphic complex juxtaposed to a medium-grade mining regions in Europe. For this reason it has one along a mylonitic belt (Internal Zone), located at always called the attention of miners and geologists. the north-eastern side. The general metallogenic outline of Sardinia, The stratigraphically lowermost sequence (Carmignani its consequent distribution of ores and industrial et al., 1994), which occurs in SW Sardinia (Iglesiente- minerals, and their controlling factors will be given Sulcis region), is probably of Pre-Cambrian age a brief overview in this initial section, in order and consists mainly of terrigenous metasediments to provide the readers with the main elements of represented by feldspathic metasandstones, quartzites, Sardinian metallogeny. metaconglomerates, and thin dolomitic intercalations that grade upwards into shales, metasiltites, and Regional geologic setting metasandstones (Bithia Formation). The overlying The Sardinia-Corsica Massif is a microplate that has Nebida Formation, the oldest fossiliferous terrain, had a very long geological history (Oggiano et al., mostly consists of terrigenous metasediments with 2001). In Sardinia the sedimentary, magmatic, and minor oolitic limestones bearing Early Cambrian metamorphic records, over a time span that includes Archaeocyathas, Trilobites and algal stromatolites, the entire Phanerozoic, are well represented within and is believed to represent a continental shelf three main lithologic complexes: environment with eastward prograding deltaic • a mainly Palaeozoic basement that underwent systems (Matoppa Member) that evolved into an repeated phases of deformation and metamorphism oolitic lagoonal environment. This terrigenous during the Caledonian and Hercynian orogenic formation grades upwards into a thick carbonate cycles, and was eventually intruded extensively by sequence consisting of dolostones and limestones (the calc-alkaline granitoids; Gonnesa Formation), which represent an arid tidal fl at • a Late Palaeozoic epicontinental sequence system. The “drowning” of this carbonate platform and a Mesozoic carbonate platform sequence, marks the beginning of the Cabitza Formation, and representative of stable shelves, that bounded the is recorded by the occurrence of nodular limestones passive margin of Southern Europe; (“Calcescisti” Auctt.) rich in Middle Cambrian • a Cainozoic to Quaternary volcanic and trilobites, echinoderms, and brachiopods. The sedimentary cover consisting of shallow-water overlying deeper-environment member consists of a marine carbonates, siliciclastic sediments, 400-m thick neritic terrigenous succession in which continental conglomerates, as well as volcanic rocks the youngest levels contain achritarcs and graptolites represented by a calc-alkaline suite, and by within- (Dictyonerna fl abelliforme) of Tremadocian age plate basalts. (“Argilloscisti di Cabitza”, Auctt). These lagoonal and epicontinental carbonate and terrigenous deposits The basement of Sardinia is a segment of the South correspond to thick siliciclastic sequences in the Variscan chain which, after the Cainozoic drifting of Nappe Zone (the San Vito and Solanas Formations). the island, shows a NW-SE trend and crops out with All over Central and SE Sardinia, Middle Cambrian- good continuity. From south to north its structural Early Ordovician metasedimentary sequences are framework includes the following three different overlain by an Arenigian-Caradocian metavolcanic zones: complex, which includes several effusive episodes, P14 to P36 n° 4 - from Volume I) a thrust-and-fold belt foreland consisting of a with abundant pyroclastic fl ows, and intrusions into sedimentary succession ranging in age from Upper the basement. The magmatic products include a Vendian to Lower Carboniferous, which crops out in complete sub-alkaline suite ranging in composition the SW of the island (External Zone); from basaltic-andesitic to rhyolitic, with acid II) a SW-verging nappe building that equilibrated terms being more abundant than the intermediate under greenschist facies conditions and occupies and basic ones. These characters are typical of an the centre of the island. It consists of a Palaeozoic orogenic suite involving continental crust. Extensive metasedimentary succession hosting a thick evidence supports the hypothesis of a magmatic arc 3 - P30 P30_R_OK 3 25-05-2004, 16:47:47 Leaders: A. Marcello, S. Pretti, P. Valera P30 Volume n° 4 - from P14 to P36 n° 4 - from Volume Figure 1 - Geological sketch map of Sardinia P30 - 4 P30_R_OK 4 27-05-2004, 14:22:56 METALLOGENY IN SARDINIA (ITALY): FROM THE CAMBRIAN TO THE TERTIARY P30 connected to a subduction of the oceanic lithosphere margin, and the gravitational collapse of the under the northern Gondwanian margin. The arc- collisional orogenic wedge. trench gap was incorporated in the Internal Nappes The Hercynian collisional event is well preserved (Mount Gennargentu, Baronie region). The back-arc in the Sardinian basement. The overthrusting basin in the Iglesiente-Sulcis region is devoid of continental margin is represented by the “High Grade calc-alkaline magmatism and underwent an Early Metamorphic Complex” (Internal Zone) of northern Hercynian compressional event. This post-Tremadoc Sardinia and Corsica; the underthrust continental and pre-Caradoc phase of deformation, which is margin is represented by the Internal (Nappe Zone) found in many parts of Europe, is very evident and External (External Zone) Nappe Complexes of here, especially in the Iglesiente region, where the central and southern Sardinia. The two domains are Cambrian-Lower Ordovician sequences were folded separated by the “Posada-Asinara Line” suture zone. before the Caradocian (“Sardinian Unconformity”). The External Zone Complex, cropping out in the The products of the subsequent erosion reaches up to Iglesiente-Sulcis region, is a classic fold-thrust belt several hundreds of metres in thickness (“Puddinga” characterised by medium- to steeply-dipping thrusts, Auctt.). This angular unconformity has also been fold axial planes and cleavages, and very low-grade reported in south-eastern (Sarrabus-Gerrei region) Hercynian metamorphism. For the late stages of the and central Sardinia, where the Cambrian-Lower Hercynian collision or early uplift, an age of 344 Ma Ordovician successions are often separated from has been proposed. the Middle- to Upper-Ordovician volcanics and An important extensional event also developed in the sediments by conglomerates mainly derived from the Sardinian Variscides as a response to gravitational volcanic arc. re-equilibration within the collisional structure. Both the “Puddinga” continental clastics and the The extensional evolution is confi ned to a time Middle-Ordovician metavolcanics of central and interval extending from the end of the collision to south-eastern Sardinia are covered by terrigenous the emplacement of the widespread calc-alkaline continental to littoral sediments that show a large plutonism of the Sardinian-Corsican batholith, and variability in thickness and facies “Caradocian to the development of the largely coeval