Di Bella_periodico 03/09/10 12.59 Pagina 1 Period. Mineral. (2010), 79, 2, 1-20 doi: 10.2451/2010pM0007 http://go.to/permin An International Journal of PEriOdiCO di MinEralOgia MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment , Archeometry and Cultural Heritage Evidence of Early Oligocene submarine volcanism in the Caltanissetta Basin (Central-Southern Sicily) Marcella Di Bella 1, S elMa ruSSo 1, *, G iuSeppe SaBatino 1,paolo pino 1 and anGela BalDanza 2 1 Dipartimento di Scienze della terra - università di Messina, Salita Sperone, 31 - 98122 Messina, italy 2 Dipartimento di Scienze della terra - università di perugia, piazza università, 1 - 06100 perugia, italy Submitted, February 2010 - Accepted, June 2010 aBStract - petrological and geochemical data of Basin results as different magmatic pulses intruded volcanic rocks from several sites of the caltanissetta along lithospheric fractures originated in response to Basin (central-southern Sicily) are discussed to flexure and uplift of the african paleomargin, in the provide information about volcanism in the northern pre-subduction stage and then incorporated in the portion of the african plate. Volcanics occur as large future Sicilian accretionary prism. isolated blocks, mainly as pillows and subordinate lava flows, enclosing sedimentary levels of marly riaSSunto - Dati petrologici e geochimici delle limestones. they are packed in several paleogene and rocce vulcaniche provenienti da vari siti del bacino di neogene clayey lithologies, which are correlated to caltanissetta (Sicilia centro-meridionale) sono the sedimentation into the Miocenic Foredeep. the discussi allo scopo di fornire informazioni sul age of volcanism is referable to early oligocene vulcanismo della porzione settentrionale della placca (rupelian), as revealed relative dating of the africana. le vulcaniti si rinvengono in grandi blocchi interpillow calcareous sediments. the studied samples isolati, in subordinate colate e prevalenti pillows nei are transitional and poorly evolved alkali basalts. quali sono intercalati livelli sedimentari di calcari petrographic study highlights a discrete uniformity for marnosi. Dal punto di vista stratigrafico i corpi the most samples, with porphyritic texture vulcanici sono inclusi in litologie argillose del characterized by olivine, clinopyroxene and paleogene e neogene, che sono correlate alla plagioclase phenocrysts in a microcrystalline sedimentazione nella zona di avanfossa Miocenica. groundmass, composed of the same phases plus l’età del vulcanismo è riferibile all’oligocene opaque minerals. Major and trace element data are (rupeliano), come è stato rivelato dalle datazione poorly variable and the trends show low degree of relative effettuate sui sedimenti calcarei interpillow. i fractional crystallization with some contribution of prodotti vulcanici analizzati hanno mostrato mineral accumulation. abundance and ratios of un’affinità alcalina transizionale, risultando poco incompatible elements resemble oiB-type volcanics evoluti con prevalenti composizioni basaltiche. from intraplate environment. a slightly different l’analisi petrografica ha messo in evidenza una garnet lherzolite sources that underwent a low partial discreta omogeneità tessiturale e composizionale per melting degree can be hypothesized for the studied la maggior parte dei campioni, che sono caratterizzati rocks. the oligocene volcanism of the caltanissetta da una tessitura porfirica con fenocristalli di olivina, * Corresponding author, E-mail: [email protected] Di Bella_periodico 03/09/10 12.59 Pagina 2 2 M. D i Bella , S. r uSSo , p. p ino , a. B alDanza and G. S aBatino clinopirosseno e minore plagioclasio in una massa di been understood about its petrological features fondo microcristallina composta dalle stesse fasi più and magma sources (Beccaluva et al ., 1998; trua variabili quantità di minerali opachi. i trend osservati et al. , 1998). in contrast, few information is sui diagrammi di variazione degli elementi maggiori available on the more ancient Sicilian ed in tracce hanno indicato uno scarso frazionamento magmatism (lucido et al. , 1978; Di Bella, 2007; con il contributo di fenomeni di cumulo. le abbondanze e i rapporti degli elementi incompatibili Di Bella et al , in progress). as regards the sono comparabili con quelle di magmi tipo oiB e western-central Sicily volcanism, only preliminary riconducibili dal punto di vista geodinamico a petrological studies on the caltanissetta Basin vulcaniti originatesi in un ambiente di tipo intraplacca. lavas have been carried out (Di Bella et al. , 2006; i magmi coinvolti deriverebbero da una sorgente 2008a; 2009). all the above volcanic sites, in mantellica lherzolitica a granato interessata da particular the plio-pleistocenic volcanics from differenti gradi di fusione parziale. i risultati ottenuti linosa, pantelleria, some Sicily channel ci consentono di dire che il vulcanismo oligocenico Seamounts, etna, Hyblean Hills and the del Bacino di caltanissetta si sarebbe originato in cretaceous volcanics of capo passero, augusta seguito alla risalita di pulsi magmatici provenenti da fratture litosferiche sviluppatesi come conseguenza and Siracusa, are situated in a similar della flessura e dell’uplift del paleomargine africano, geodynamic setting along the northern sector of nella fase di pre-subduzione. Gli edifici o corpi the african plate. they show oiB-type affinity, vulcanici sarebbero stati poi incorporati nel prisma di but are different in age and for some accrezione siciliano. geochemical-isotopic signatures (Beccaluva et al. , 1998; trua et al. , 1998; Di Bella et al. , 2008b Key Words : Caltanissetta basin; Oligocenic intraplate and references therein). volcanism; biostratigraphic characterization; trace in this work we focus on the undated and element geochemistry; garnet-lherzolite source. poorly studied isolated volcanic outcrops in the caltanissetta basin (central-southern Sicily) (Fig. 1), introDuction reported in several geological maps and papers (Behrmann, 1938; Beneo, 1955; 1956; Decima, Sicily represents a significant area on the 1972; Schmidt Di Friedberg, 1967a; 1967b). the volcanological point of view, because of the volcanic bodies are mainly pillows and occurrence of various episodes of magmatic subordinate lava flows packed in several clayey activity from Mesozoic to present (lucido et al. , deposits correlated to the sedimentation into the 1978; tanguy, 1978; Bianchini et al ., 1998; trua Foredeep Basins. the aim is to constrain their et al ., 1998; Beccaluva et al ., 1998; civetta et origin and geodynamic significance, through al ., 1998; armienti et al ., 2004; Di Bella, 2007; petrological study of four well-preserved Di Bella et al. , 2008b; 2009). During triassic representative volcanic outcrops (cattolica and mainly in the Jurassic times, the Sicilian area eraclea, Siculiana Marina, pietranera, and was interested by distensive tectonics leading to Xirbi), and to discuss their genesis and evolution. opening of the tethys ocean, with formation of the relative age of these volcanic events is various rift basins, along which volcanism took indirectly provided using the calcareous place. Discontinuous magmatic activity also nannofossils and foraminifers present into the occurred during closure of tethys basin, from eocene to present (peccerillo and lustrino, sedimentary levels inserted in a few pillow lavas. 2005; peccerillo, 2005). the plio-pleistocene magmatism of Sicily has GeoloGical SettinG been so far intensively studied and much has central Sicily is part of the Maghrebian Sicilian Di Bella_periodico 03/09/10 12.59 Pagina 3 Evidence of Early Oligocene submarine volcanism in the Caltanissetta Basin (Central-Southern Sicily) 3 Fig. 1 - Geological sketch map of Sicily with localization of the studied volcanic sites (Grey stars): ce = cattolica eraclea; Sic = Siculiana Marina; pn = pietranera; Xir = Xirbi. Front thrusts Belt, a segment of the alpine Moreover, several large bodies of chaotic lenses collisional belt, described as a result of both post- of mud breccia, including large heterogeneous collisional convergence between africa and exotic Meso-cenozoic sedimentary and volcanic europe and roll-back of the subduction hinge of blocks, extensively crop out interlayered with the ionian lithosphere (Grasso et al. , 1991). the different stratigraphic levels. they overlie pre- volcanic bodies here examined are situated in the Burdigalian beds in the north, and central-southern Sicily, where a neogene- tortonian-pliocene beds in the south (roure at al. , Quaternary accretionary wedge, the so-called 1990). these chaotic clayey assemblages, caltanissetta Basin or Gela nappe, widely crops consisting of brecciated clays (ogniben, 1957) out (ogniben, 1957; 1960; 1969; argnani, 1987; or olistostromes related to gravitative trasport on Grasso et al. , 1991; Butler et al. , 1993). the the instable clayey basin slopes (rigo De righi, wedge, up to 6 km thick, incorporates faulted and 1957; Schmidt Di Friedberg, 1967), have been folded materials including cretaceous-eocene to reinterpreted as tectonic melanges (roure et al. , lower Miocene Variegated clays, late oligocene 1988) and more recently as the result of different marls and marly limestones, lower Miocene manifestations of mud diapirism (Monaco and numidian Flysch, covered by neogene to tortorici, 1996). Such structural setting is the pleistocene glauconitic and quartz-rich result of continental