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 with low </p> <p><a href="https://www.ofioliti.it/index.php/ofioliti/about"><strong>Read more..</strong></a></p> Edizioni ETS - Pisa en-US Ofioliti 0391-2612 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 carbonatebearing<br />metabreccia of the chaotic rock unit. This characteristic amphibole metasomatism is more pronounced toward the contact with the metaophicarbonates, and the average δ18OVSMOW and δ13CVPDB 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-H2O-rich fluids from the dehydrating slab, CO2 released by decarbonation and SiO2-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</p> Francesca Claudia Rotondo Paola Tartarotti Sara Guerini Giovanna Della Porta Nicola Campomenosi Copyright (c) 2020 Ofioliti 2021-01-29 2021-01-29 46 1 1 25 10.4454/ofioliti.v46i1.535 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 considered as the largest and best-exposed ophiolite within the Zagros Suture Zone, northeast Iraq. Felsic dykes are encountered in three locations in the Mawat ophiolite. Here, we use field work, petrology and geochemical data to investigate the magma sources of the felsic dykes. The felsic dykes are classified into two types: oceanic plagiogranites in the western (Mirza and Ismail, 2007) and leucogranites in the central (Mohammad et al., 2014) Mawat ophiolite, here called tonalites and granites, respectively. Both types, mixed in various proportions occur in the eastern felsic dykes. The eastern tonalites are weakly peraluminous to metaluminous and are low in K2O (0.13-0.24 wt%) and TiO2 (0.01-0.07 wt%) and high in Na2O (6.63-11.02 wt%). The eastern granites are strongly peraluminous, moderate to high in K2O (1.16-6.57 wt%) and Na2O (2.83-6.47 wt%), and very low in TiO2 (0.03-0.07 wt%). The western tonalites are similar to adakites and are interpreted to have crystallized from melts of subducted oceanic crust interacting with the mantle. The eastern tonalites underwent hornblende and plagioclase fractionation in shallow-level magma chambers modifying their original compositions. The granites are interpreted to derive from melting of psammitic sediments on top of the subducted slab. Parental melts of the tonalites and granites were partially mixed in shallow magma chambers in eastern Mawat. </p> Heider Sabri Al Humadi Markku Väisänen Sabah Ismail A. Marja Lehtonen Bo Johanson Copyright (c) 2021 Ofioliti 2021-01-29 2021-01-29 46 1 27 41 10.4454/ofioliti.v46i1.536 GEOCHEMICAL EVIDENCE FOR LOCAL VARIABILITY IN REDOX AND DEPOSITIONAL CONDITIONS IN A DEEP-WATER BONARELLI EQUIVALENT SECTION FROM SOUTHERN TETHYS (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 56 m-thick Albian-Turonian Fontana Valloneto stratigraphic section cropping out in Southern Italy (Potenza, Basilicata), belonging to the “Flysch Rosso” Formation, and containing an equivalent of the Bonarelli Horizon (globally called OAE2 which occurred at~ 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 within the entire sequence and a gradual decrease 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-e) 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 Organic Matter (OM) productivity, high biogenic silica production and fine-grained sedimentation (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 OAE2, 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 2021-01-29 2021-01-29 46 1 43 62 10.4454/ofioliti.v46i1.537 PETROLOGICAL CONSTRAINTS ON THE ORIGIN OF PYROXENITE DYKES IN THE LITHOSPHERIC MANTLE OF THE CHESHMEH-BID OPHIOLITIC MASSIF, SOUTHERN IRAN https://www.ofioliti.it/index.php/ofioliti/article/view/620 <p>The Cheshmeh-Bid ophiolitic massif in the Khajeh-Jamali district (Southern Iran) is dominated by harzburgite-dunite tectonites locally intruded by orthopyroxenite dikes. These latter are composed of dominant coarse orthopyroxene with minor olivine, Cr-spinel, clinopyroxene and amphibole. Estimated equilibrium temperatures for Mg-hornblende and edenitic amphibole reveal a late stage magmatic origin. The Cheshmeh-Bid orthopyroxenites are characterized by very low Al2O3, CaO, Na2O and TiO2 abundances coupled to relatively high MgO and SiO2 contents. They display U-shaped REE patterns, selective LILE enrichment and positive Pb and Sr anomalies. The host harzburgites are highly refractory mantle residues resulting from fluid-assisted melting. Field observations and mineral assemblages suggest that the pyroxenites formed by melt injection along fractures within rather cold ambient harzburgites and chromitites at moderate pressure (P &gt; 1 GPa). Based on bulk-rock compositions and mineral chemistry, we infer that the Cheshmeh-Bid orthopyroxenites originated from the intrusion and crystallization of hydrous Si-rich, low-Ca melts with a boninite signature in a suprasubduction environment. Fine-grained neoblastic domains developed in the pyroxenites in response to subsolidus ductile deformation and recrystallization, which were most likely related to the exhumation of the Cheshmeh-Bid ophiolite massif.</p> Alireza Eslami Giulio Borghini Alessandra Montanini Giovanni Grieco Claudio Marchesi Copyright (c) 2021 Alireza Eslami, Giulio Borghini, Alessandra Montanini, Giovanni Grieco, Claudio Marchesi 2021-01-29 2021-01-29 46 1 10.4454/ofioliti.v46i1.538 SEAFLOOR SPREADING AND TECTONICS AT THE CHARLIE GIBBS TRANSFORM SYSTEM (52-53ºN, MID ATLANTIC RIDGE): PRELIMINARY RESULTS FROM R/V A. N. STRAKHOV EXPEDITION S50 https://www.ofioliti.it/index.php/ofioliti/article/view/618 <p>The Charlie Gibbs offsetting by ~ 340 km the Mid Atlantic Ridge (MAR) axis at 52°-53° N is one of the main transform systems of the North Atlantic. Located between long mid-ocean ridge segments influenced from the south by the Azores and from the north by the Iceland mantle plumes, this transform system has been active since the early phases of North Atlantic rifting. Object of several surveys in the ’70 and ’80, Charlie Gibbs received great attention for its unique structure characterized by two long-lived right-lateral transform faults linked by a short ~ 40 km-long intra-transform spreading centre (ITR) with parallel fracture zone valleys extending continuously towards the continental margins. In October 2020 expedition S50 of the R/V A.N. Strakhov surveyed an area of 54,552 km2 covering the entire Charlie Gibbs transform system and the adjacent MAR spreading segments. We collected new bathymetric, magnetic and high-resolution single channel seismic data, along with basaltic, gabbroic and mantle rocks from 21 dredges. This work contains preliminary data from cruise S50 and discusses the large-scale architecture of this unique, long-lived transform system.</p> Sergey Skolotnev Alessio Sanfilippo Alexander Peyve Yago Nestola Sergey Sokolov Lorenzo Petracchini Kseniya O. Dobrolyubova Valentin Basch Alexei N. Pertsev Carlotta Ferrando Alexander N. Ivanenko Camilla Sani Anatoly A. Razumovskiy Filippo Muccini Artem S. Bich Camilla Palmiotto Yuri V. Brusilovsky Enrico Bonatti Konstantin N. Sholukhov Marco Cuffaro Ilya A. Veklich Vitaly N. Dobrolyubov Marco Ligi Copyright (c) 2021 Sergey Skolotnev, Alessio Sanfilippo, Alexander Peyve, Yago Nestola, Sergey Sokolov, Lorenzo Petracchini, Kseniya O. Dobrolyubova, Valentin Basch, Alexei N. Pertsev, Carlotta Ferrando, Alexander N. Ivanenko, Camilla Sani, Anatoly A. Razumovskiy, Filippo Muccini, Artem S. Bich, Camilla Palmiotto, Yuri V. Brusilovsky, Enrico Bonatti, Konstantin N. Sholukhov, Marco Cuffaro, Ilya A. Veklich, Vitaly N. Dobrolyubov, Marco Ligi 2021-01-29 2021-01-29 46 1 10.4454/ofioliti.v46i1.539