Ofioliti https://www.ofioliti.it/index.php/ofioliti <p>Since 1976, <em>Ofioliti</em> provides an international forum for original contributions and reviews in the field of the geodynamics, petrology, geochemistry, biostratigraphy, stratigraphy, tectonics and paleogeography applied to ophiolitic terrains and modern oceanic lithosphere, including their sedimentary cover. Studies of topics such as geodynamics of the mantle, the evolution of orogens including ophiolites and paleoceanography are also welcome.</p> <p>Ofioliti is a FREE ACCESS journal. </p> <p><em>2019 Impact Factor: 0.941</em></p> <p><em>5-year Impact Factor: 1.41</em></p> <p> </p> en-US editor@ofioliti.it (Alessandra Montanini) journals@edizioniets.com (Giovanni Campolo) Sat, 24 Oct 2020 18:23:19 +0200 OJS http://blogs.law.harvard.edu/tech/rss 60 SUBDUCTED BASALTS AND SEDIMENTS AS SOURCES FOR FELSIC DYKES IN THE MAWAT OPHIOLITE, NE IRAQ https://www.ofioliti.it/index.php/ofioliti/article/view/610 <p>The Mawat ophiolite is a fragment from the Neo-Tethyan oceanic lithosphere and is regarded as the largest and best-exposed ophiolite within the Zagros Suture Zone, northeast Iraq. Field work, petrology, and geochemical data of felsic intrusive rocks in Mawat ophiolite are investigated. The felsic dykes are classified into two types: oceanic plagiogranites in western (Mirza and Ismail, 2007), and leucogranites in central (Mohammad et al. 2014) Mawat ophiolite, here called tonalites and granites, respectively. Both types, but variously mixed, occur in the eastern felsic dykes. The tonalites are weakly peraluminous to metaluminous and are low in K<sub>2</sub>O and TiO<sub>2</sub> and high in Na<sub>2</sub>O. The granites are strongly peraluminous, moderate to high in K<sub>2</sub>O and Na<sub>2</sub>O, and very low in TiO<sub>2</sub>. The western tonalites are similar to adakites and are interpreted to have derived from melts from subducted oceanic slab interacted with the mantle. The eastern tonalites have gone through hornblende and plagioclase fractionation in a shallow-level magma chamber modifying their original compositions. The granites are interpreted to have derived from the melted psammitic sediments on top of the subducted slab. The tonalitic and granitic melts were partially mixed and fractionated in shallow level in eastern Mawat.&nbsp;</p> Heider Sabri Al Humadi, Markku Väisänen, Sabah A. Ismail, Marja Lehtonen, Bo Johanson Copyright (c) 2020 Ofioliti https://www.ofioliti.it/index.php/ofioliti/article/view/610 Sat, 24 Oct 2020 00:00:00 +0200 METASOMATIC HORIZON SEALING SERPENTINITE-METASEDIMENTS PAIR IN THE ZERMATT-SAAS METAOPHIOLITE (NORTHWESTERN ALPS): RECORD OF A CHANNEL FOR FOCUSSED FLUID FLOW DURING SUBDUCTION https://www.ofioliti.it/index.php/ofioliti/article/view/583 <p>A metasomatic horizon (MH) occurs between the metaophiolite (serpentinite and metaophicarbonates) basement and metasedimentary sequence (chaotic rocks and calcschists) of the Lake Miserin Ophiolite, in the high pressure Zermatt-Saas Zone of the Northwestern Alps. Macro- and microstructural analyses combined with petrological and geochemical investigations of the MH and surrounding lithologies unravelled a polyphase blastesis-deformation history, which led to the formation of a complex fabric and minero-chemical alteration of the serpentinite basement-metasediments interface. Dehydration, decarbonation and carbonation interplayed from early Alpine subduction up to HP-LT metamorphic peak (T=550-630 °C, P=1.8-2.5 GPa), to produce a distinctive, pervasive amphibole (tremolite/actinolite) replacement both in carbonate-rich and serpentinite-rich domains pertaining to the MH protoliths, i.e. serpentinite and carbonate-bearing metabreccia of the chaotic rock unit. This characteristic amphibole metasomatism is more pronounced toward the contact with the metaophicarbonates, and the average δ<sup>18</sup>O<sub>VSMOW</sub> and δ<sup>13</sup>C<sub>VPDB</sub> values of dolomite within the MH (+14.4‰ and +0.7‰ respectively) lie between those of the metaophicarbonates and of calcschist. These results suggest that Mg- H<sub>2</sub>O-rich fluids from the dehydrating slab, CO<sub>2</sub> released by decarbonation and SiO<sub>2</sub>-rich fluids evolved in calcschists mixed together and circulated mostly along the metaophiolite basement/metasediments interface, where the MH developed and recorded a preferential channel for mixed metamorphic fluid flow. These findings highlight and confirm that the study of metasomatic rocks in convergent systems is crucial to comprehend the behaviour of different fluids circulating, mixing and interacting with lithologies along slab-parallel discontinuities, which act as major fluid conduits for deep volatile recycling.</p> Francesca Claudia Rotondo, Paola Tartarotti, Sara Guerini, Giovanna Della Porta, Nicola Campomenosi Copyright (c) 2020 Ofioliti https://www.ofioliti.it/index.php/ofioliti/article/view/583 Sat, 24 Oct 2020 00:00:00 +0200 GEOCHEMICAL EVIDENCE FOR LOCAL VARIABILITY IN REDOX CONDITIONS DURING OCEANIC ANOXIC EVENT 2 IN A DEEP-WATER BONARELLI EQUIVALENT SECTION FROM SOUTHERN THETYS (FONTANA VALLONETO SECTION, SOUTHERN ITALY) https://www.ofioliti.it/index.php/ofioliti/article/view/588 <p>Identifying the depositional redox conditions is useful to evaluate the interplay between climate changes, biological feedbacks and de-oxygenation processes in the oceans during the Oceanic Anoxic Events (OAEs). Here, we focus on the about 56m-thick Albian-Turonian Fontana Valloneto stratigraphic section outcropping in of Southern Italy (Potenza, Basilicata), belonging to the «Flysch Rosso» Formation, and containing an equivalent of the Bonarelli Horizon (globally called OAE2 occurred ~94 Ma). Inorganic geochemical compositions and Total Organic Carbon contents obtained from this section are here used to assess depositional environment and redox conditions. The paucity of carbonates during the entire sequence and a gradual decreases in Y, Zr and Al contents along the section suggest a deep depositional environment (below the Calcite Compensation Depth) and an overall decrease in the terrigenous supply. Samples within the Bonarelli Horizon (BH) show highly variable TOC contents (~0 to ~30 wt%) that, mirrored by variations in redox sensitive and nutrient-related elements (e.g. V, Mo and U) and Mn, suggest variation of the seawater primary productivity associated to changes of the local redox conditions between suboxic to strongly euxinic. We infer that during OAE2 the accumulation of the black shales was associated to high OM productivity, low detrital input, fine-grained sedimentations (mainly aeolian dust and illite) in a period of “sluggish” oceanic circulation and stagnant conditions. These periods were alternated by moments of more active oceanic circulation and enhanced runoff, leading to the local deposition of radiolarites with very low TOC contents. Finally, a comparison with other section from the proto Atlantic Ocean and the Mesozoic Tethys sustains the idea that the drawdown of redox sensitive elements (V, Mo and U) was a global process during the deposition of OAE 2, providing a link between the environmental changes detected in our section with the global perturbations developed during this oceanic anoxic event.</p> Greta Bonacina, Alessio Sanfilippo, Simone Zana, Alberto Bosino, Elisabetta Previde Massara, Paolo Viaggi, Luisa Sabato, Salvatore Gallicchio, Paolo Scotti Copyright (c) 2020 Greta Bonacina, Alessio Sanfilippo, Simone Zana, Alberto Bosino, Elisabetta Previde Massara, Paolo Viaggi, Luisa Sabato, Salvatore Gallicchio https://www.ofioliti.it/index.php/ofioliti/article/view/588 Wed, 09 Dec 2020 00:00:00 +0100