Ofioliti. An international journal on ophiolites and oceanic litosphere https://journal.edizioniets.com/index.php/ofioliti <p>Ofioliti was established in 1976 and provides an international forum for original contributions and reviews in the field of the geodynamics, petrology, geochemistry, stratigraphy, tectonics, and paleogeography of both ophiolites and the crust of present oceans, with their sedimentary cover. Studies of related topics such as the geodynamics of the mantle and the evolution of orogens including ophiolites are also welcome.</p> Edizioni ETS en-US Ofioliti. An international journal on ophiolites and oceanic litosphere 0391-2612 PLAGIOCLASE-FACIES THERMOBAROMETRIC EVOLUTION OF THE EXTERNAL LIGURIDE PYROXENITE-BEARING MANTLE (SUVERO, ITALY) https://journal.edizioniets.com/index.php/ofioliti/article/view/175 <p>Plagioclase peridotites are an important marker of the shallow geodynamic evolution of the lithospheric mantle at extensional settings. Based on lowpressure experiments, a recent study by Fumagalli et al. (2017) defined and calibrated a geobarometer for peridotitic bulk compositions, based on the Forsterite-Anorthite-Ca-Tschermak-Enstatite (FACE) pressure-sensitive equilibrium. The Suvero plagioclase-bearing peridotites, on which the FACE geobarometer was calibrated, are primarily associated to plagioclase pyroxenites. Assuming that the pyroxenites record the same Pressure-Temperature evolution than the plagioclase peridotites, they represent ideal candidates to test the applicability of the FACE geobarometer on pyroxenitic compositions. As documented in the plagioclase peridotites, the pyroxenites are characterized by the development of fine-grained neoblastic assemblages, indicative of partial recrystallization under plagioclase-facies conditions. Chemical zonations in these neoblastic mineral aggregates suggest equilibration stages at variable pressure and temperature and allowed to document two re-equilibration stages corresponding to the onset of plagioclase-facies recrystallization (830-850°C, 6.9-8.1±0.5 kbar) and a shallower colder re-equilibration (770-790°C, 5.8-5.9±0.5 kbar), respectively. The decompressional evolution reported for pyroxenitic bulk compositions is consistent with the exhumation history documented in the associated Suvero peridotite, although at slightly higher equilibrium pressures (~ 1 kbar). Remarkably, the much lower XCr in pyroxenites reflects in lower Cr incorporation in pyroxenes and, consequently, in significantly higher Ca-Tschermak activity in clinopyroxene that might introduce the systematic pressure overestimation by FACE geobarometer.</p> Valentin Basch Giulio Borghini Patrizia Fumagalli Elisabetta Rampone Andrea Gandolfo Carlotta Ferrando ##submission.copyrightStatement## 2020-01-30 2020-01-30 45 1 1 11 MELT-ROCK INTERACTION BETWEEN GRANITIC PEGMATITES AND HOSTING AMPHIBOLITES FROM THE CHIAVENNA OPHIOLITIC UNIT (TANNO PEGMATITIC FIELD, CENTRAL ALPS, NORTH ITALY) https://journal.edizioniets.com/index.php/ofioliti/article/view/176 <p>The Tanno pegmatitic field, placed southward of Chiavenna (Central Alps, Sondrio, Italy), develops a large number of subplanar dykes that crosscut the Chiavenna Unit, an ophiolitic complex mainly composed, in the study area, of amphibolite rocks. This study focuses on the contact between a pegmatitic dyke and the amphibolitic country rock. We distinguished four zones across the contact: I) inner amphibolite, II) contact amphibolite, III) contact pegmatite, IV) inner pegmatite. The inner amphibolite, not affected by melt-rock interaction, is composed of amphibole, phlogopite, ilmenite, titanite and rutile. Two amphibole generations occur, both of them showing a patchy compositional zoning. Amphibole I are Mg-hornblende, whereas Amphibole II have a pargasitic composition. The contact amphibolite shows an enrichment of mica belonging to the phlogopite-biotite series, titanite and the presence of fluorapatite and plagioclase (Ab45-60), that is absent in the inner amphibolite. Close to the contact, amphiboles display no zoning and gain an Mg-horneblenditic composition. The contact pegmatite has quartz, albitic plagioclase, garnet (almandine-spessartine series), muscovite, K-feldspar and fluorapatite. It shows a comb texture, with elongation of plagioclase crystals normal to the contact itself. Far from the contact, the inner pegmatite has an increasing grain-size and a less organized texture. In this zone several accessory phases occur, including gahnite, columbite-(Fe), monazite-(Ce), xenotime-(Y), uraninite and betafite. Whole rock analyses suggest that a chemical exchange, concerning both major elements and trace elements, occurred between the pegmatitic melt and the hosting amphibolite. A considerable increase of SiO2, Na2O and, to a lesser extent, of Al2O3 is observed from the amphibolite towards the pegmatite; K2O and CaO show a decrease at the same extent. The REE pattern in the pegmatite highlights an enrichment in HREE at the contact. Mineral chemistry confirms this trend with variations observable in plagioclase, gradually more albitic from the amphibolite to the pegmatite. Mineralogical characters and geochemical features allow to classify the Tanno pegmatite in the LCT (Lithium, Cesium, Tantalium) family. Based on the metamorphic peak conditions reported from the Lepontine Dome the ambient conditions during pegmatite intrusion were ca. 550°C and 5 kbar. The reduced thermal difference between pegmatite and wall rock explains the diffuse contact observed by X-ray micro-computed tomography. The collected data suggest a chemical interaction between melt and wall rock, according to the following reaction taking place in the amphibolite</p> <p>Amphibole I + Amphibole II + Ilmenite + Pegmatitic melt → Amphibole III + Plagioclase + Phlogopite + Titanite + Fluorapatite</p> Francesco Arrigoni Patrizia Fumagalli Stefano Zanchetta Alessandro Guastoni ##submission.copyrightStatement## 2020-01-30 2020-01-30 45 1 13 23 LARGE-SCALE STRUCTURE OF THE DOLDRUMS MULTI-FAULT TRANSFORM SYSTEM (7-8ºN EQUATORIAL ATLANTIC): PRELIMINARY RESULTS FROM THE 45TH EXPEDITION OF THE R/V A.N. STRAKHOV https://journal.edizioniets.com/index.php/ofioliti/article/view/177 <p>The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transform<br>domains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs).<br>Among these megatransforms, the Doldrums system (7-8ºN) is arguably the less studied, although it may be considered the most magmatically active. New<br>geophysical data and rock samples were recently collected during the 45th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allow<br>to reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled with<br>extensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km2. The new data clearly indicate that<br>the Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward the<br>central part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminary<br>interpretation is that a region of intense mantle melting is located in the central part of the Doldrums system as consequence of either a general transtensive regime<br>or the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections.<br>We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphere<br>by tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, and<br>the exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys</p> Sergey G. G. Skolotnev Alessio Sanfilippo Alexander Peyve A. Filippo Muccini Sergey Yu. Sokolov Camilla Sani Kseniia O. Dobroliubova Carlotta Ferrando Nikolai P. Chamov Camilla Palmiotto Alexey N. Pertsev Enrico Bonatti Marco Cuffaro Anastasiya C. Gryaznova Konstantin N. Sholukhov Artem S. Bich Marco Ligi ##submission.copyrightStatement## 2020-01-30 2020-01-30 45 1 25 41 PETROLOGY, GEOCHEMISTRY AND ORIGIN OF THE SIERRA DE BAZA OPHIOLITES (BETIC CORDILLERA, SPAIN) https://journal.edizioniets.com/index.php/ofioliti/article/view/178 <p>In this work we present for the first time a petrological-geochemical and genetic study of the Sierra de Baza ophiolites, which represent one of the<br>ophiolitic occurrences of the Betic Cordillera (Southern Spain). They are composed of ultramafic, mafic and sedimentary rocks, largely affected both<br>by ocean floor and polyphasic metamorphism during the Alpine orogeny. Ultramafic rocks are serpentinized lherzolites and harzburgites, whereas the<br>metabasites are meta-gabbros and meta-basalts. On the whole, Sierra de Baza ophiolites show striking geochemical similarities with those from other<br>Betic occurrences, as well as with other Tethyan ophiolites of the Western Mediterranean (Calabria, Internal and External Ligurides, Platta, Corsica and<br>Western Alps). In particular, metabasites show petrological and geochemical features similar to the E-MORB magmatism of the Atlantic Ridge between<br>45 and 63ºN generated under ultra-slow spreading ridge conditions. This process originated a strip of few hundreds km of ocean floor at the western end<br>of the Tethys, located SE of the Iberian-European margin during the Mesozoic. The inversion of the stress regime in the European-Iberian and African<br>geodynamics, starting from the Late-Middle Cretaceous, caused subduction and metamorphism in the eclogite facies of oceanic slices that were partially<br>exhumed on the continental margin, forming the Betic Ophiolites. These ophiolites were disarticulated and dismembered as a result of the shift towards<br>SW of the Alboran continental block, progressively separated from the AlKaPeCa (Alboran, Kabilias, Peloritani, Calabria) microplate, finally occupying<br>their current position in the Betic Internal Zones.</p> J.C. Lozano Rodriguez J.C. Lozano Rodriguez Encarnacion Puga Claudio Natali Gianluca Bianchini Luigi Beccaluva ##submission.copyrightStatement## 2020-01-30 2020-01-30 45 1 43 69