The high-pressure (HP) continental Etirol-Levaz slice (ELS) is exposed on the right side of the Valtournenche (Aosta) at the contact between the overlying blueschist-facies Combin Zone and the underlying eclogite-facies Zermatt-Saas Zone. The ELS contains a layered metagabbro showing a complex evolution, which includes the pre-Alpine high-temperature (HT) recrystallization of the igneous mineralogies, and the polyphase Alpine metamorphism, initially at high-pressure (HP) quartz eclogite-facies, and greenschist-facies (GS) retrogression.In this paper, an unusual corundum-bearing ultramafic rocks associated with metagabbros are described. The studied sample is a Mg-chloritite with relict green spinel partly replaced by corundum, Mg-beltrandoite-2N3S (a new mineral of the hogbomite supergroup), Mg-chlorite and dolomite. The rock is crossed by mm- to cm-thick veins composed of coarse-grained corundum + Mg-chlorite + dolomite. P-T phase-diagram projections indicate that the corundum-bearing assemblages formed in the presence of a water-rich fluid (X(CO2) ≤ 0.04), during retrograde decompressional evolution. This stage follows the prograde HP-peak of the associated eclogites. The newly inferred prograde-to-retrograde P-T path suggests that the ELS and the underlying Zermatt-Saas Zone shared a common Alpine metamorphic evolution.The detailed study of the relict minerals preserved in the chloritite indicates its pre-Alpine protolith, a green spinel websterite, and its evolution, characterized by a high-T recrystallization of the original igneous asemblages. During the Alpine orogeny, the spinel websterite experienced metasomatic hydration that converted the original igneous rock into a chloritite.

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.

The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transformdomains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs).Among these megatransforms, the Doldrums system (7-8oN) is arguably the less studied, although it may be considered the most magmatically active. Newgeophysical data and rock samples were recently collected during the 45th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allowto reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled withextensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km2. The new data clearly indicate thatthe Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward thecentral part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminaryinterpretation 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 regimeor the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections.We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphereby tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, andthe exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys

The lower part of chert sequence of Kiselevka-Manoma accretionary complex, Sikhote-Alin system (Eastern Russia) is characterized by the following radiolarian assemblages (in ascending order of age): 1. Early Pliensbachian to early Toarcian with Parahsuum izeenze , Bagotum maudense and Katroma clara. 2. Early Toarcian with Bistarkum rigidium and Citriduma hexaptera. 3. Middle-late Toarcian to early-middle Aalenian with Paronaella grahamensis, Praeparvicingula gigantocornis and Hsuum exiguum. 4. Early-middle Aalenian with Parahsuum ? grande, Parahsuum cruciferum and Hsuum altile. 5. Late Aalenian with Parahsuum ? hiconocosta, Transhsuum hisuikyoense and Nassellaria gen. et sp. indet. 1. 6. Early-middle Bajocian with Sella beniderkoulensis and Emiluvia splendida. New species Olanda inflata Bragin, n. sp. is described from this assemblage. Studied radiolarian assemblages are Tethyan in affinity, which allows to utilize the low-latitude zonal radiolarian scales for the Jurassic (Baumgartner et al., 1995b; Carter et al., 2010). The total stratigraphic range of chert sequence of the Kiselevka-Manoma accretionary complex can be estimated as Early Jurassic (Pliensbachian-early Toarcian)-Early Cretaceous (Barremian).

Late Mesozoic neritic-continental strata of East Asia show intercalations of conglomerates including microfossil-bearing clasts. Some clasts are presumed to have originated in mid-Mesozoic accretionary complexes (ACs) of East Asia. Microfossils ( e.g ., radiolaria, conodonts) in these clasts can provide age constraints. Relationships between rock facies and age in sequences of the mid-Mesozoic ACs have been clarified in previous studies; therefore, the rock facies and the age of the clasts offer a clue about denudation history and provenance of the mid-Mesozoic ACs. This study compiles previous studies of microfossil-bearing clasts within the Late Mesozoic in the Outer and Inner Zone of Southwest Japan and in the Gyeongsang Supergroup in the southeastern Korean Peninsula. The compilation, at present, recognizes three denudation stages (Stages A, B, and C) of the mid-Mesozoic accretionary complexes. Stage A (Oxfordian-Hauterivian) is characterized by an initial and narrow denudation. The denudation of the mid-Mesozoic ACs started at some point in this stage, but it was of limited extent. Stage B (Barremian-early Albian) is characterized by wide denudation: the mid-Mesozoic ACs became largely exposed. Afterwards, some younger geologic bodies of the mid-Mesozoic ACs, which were structurally underlying, were exposed and denuded. Stage C (late Albian–) is characterized by denudation of almost all the mid-Mesozoic accretionary complexes. This denudational evolution might be related to some tectonic and igneous events in East Asia, such as the uplift of the Tetori Group in late Hauterivian-early Barremian and the initiation of the granitic magmatic activity in middle Albian.

Within the ophiolitic melange of the Central Dinaridic Ophiolitic Belt (CDOB) that stretches throughout the Balkans region in SE Europe, a latest Bajocian-early Bathonian radiolarian assemblage was obtained from chert-rich shaly to silty matrix. The sampling locality in northern Bosnia and Herzegovina is characterized by a highly-diversified ophiolitic suite, consisting of basic and ultrabasic rocks of different geotectonic provenances. This makes the radiolarian dating a convenient complementary tool for studying the geodynamic history of CDOB within a broader regional context. The host sediments and the nature of their associated crystalline rocks suggest that radiolarian deposition occurred relatively close to the Adria shelf margins, predating or being contemporaneous to the rapid transitions in the Dinaridic Neotethys geotectonic setting, changing from active ridge magmatism to an intraoceanic subduction environment and island-arc volcanism. The minimum age of ophiolite melange formation is defined by the mineral equilibration ages in metamorphic sole (161±4 Ma), with the obduction tectonics that must have lasted at least until the Oxfordian time (i.e. termination of MOR activity in the Dinarides). This age correlates well with the ages of sediments reported elsewhere in the melange of the Dinaride-Hellenide orogenic system.

In this paper, we present the preliminary data on the age of the radiolarian cherts deposited on top of basalts belonging to the Eastern Ankara Mélange (part of the Izmir-Ankara Mélange). Petrological studies on the basalts were carried out in order to constrain the tectonic setting of the studied basalt-chert\nsequences. Nine sections were sampled East and Northeast of Ankara and twenty seven samples were collected for biostratigraphic and geochemical analyses.\nThe oldest radiolarian cherts dated in the present paper are referable to the Late Triassic (Section 6: late Norian) and are associated with basaltic rocks of OIB character. OIB type volcanic rocks are also found in other sections, associated with cherts of Late Jurassic (Section 3: middle-late Oxfordian to late Kimmeridgian-early Tithonian) and Early Cretaceous (Section 1: late Valanginian to late Hauterivian) ages.\nE-MORB type rocks are associated with radiolarian cherts of Cretaceous age (Section 4: middle late Barremian-early early Aptian and Section 7: Valanginian to middle Aptian-early Albian), whereas the oldest N-MORBs were found in a section of Late Jurassic age (Section 5: early-early late Tithonian).\nOther N-MORBs are associated with radiolarian cherts of Early Cretaceous age (Section 8: late Valanginian-early Barremian). P-MORBs type rocks were found only in a section of Middle Jurassic age (Section 2: early-middle Bajocian to late Bathonian-early Callovian age).\nIn this work, we document the occurrence of Late Triassic OIB-type rocks and of rocks showing different geochemical affinities (N-, E-, P-MORBs and OIB) generated within the same time span (Middle-Late Jurassic - Early Cretaceous). N-MORBs are compatible with composition of melts generated by partial\nmelting of a depleted MORB mantle source. In contrast, OIBs are compatible with partial melting of an enriched-type mantle source. E-MORBs may have derived from mantle source slightly enriched with respect to a DMM source, whereas P-MORBs are compatible with melts generated from a mantle source\nsignificantly enriched, compared to DMM.

The Ayli Dag ophiolites occur as an independent tectonic unit within the Intra-Pontide suture zone, central-northern Turkey. They crop out, together withthe other units of the Intra-Pontide suture zone, at the top of the Late Cretaceous-Middle Paleocene foredeep sediments of the Sakarya Terrane. The Ayli Dagophiolites sequence include in its lower part a mantle sequence consisting of not less than 2-3 km-thick peridotites, topped by 500-600 m-thick layered gabbroswith alternating, dm- to m-thick layers of spinel-bearing dunites, melatroctolites, troctolites, ol-gabbros and leucogabbros. The gabbro sequence is overlainby a sheeted dyke complex, that shows a transition to 100-200 m-thick massive basaltic lava flows followed by 600-800 m-thick massive and pillow lavasand breccias alternating with ophiolite-bearing arenites and cherts.Geochemical evaluation of the mafic lavas and dykes reveals three distinct chemical groups that reflect melt generation within an intra-oceanic subductionsystem. Among these, the first group shows island-arc tholeiite (IAT)-like features, showing very-depleted characteristics (very high Zr/Nb, low Zr/Y and Nb/Y)ratios coupled with light rare-earth (LREE)-depleted chondrite-normalized patterns. The second group is more akin to back-arc basin basalts (BABB); it displaysnormal mid-ocean ridge basalt (N-MORB)-like high-field strength element (HFSE) distribution except for depletion in Nb, and flat REE patterns. The third groupis somewhat similar to the second one, displaying BABB-like characteristics, but it is more enriched in terms of absolute trace element abundances.The radiolarian cherts sampled from the top of the pillow lavas yielded less-diverse but characteristic radiolarian assemblages indicating the middleBathonian to early Callovian ages.The Ayli Dag Ophiolite is the first finding of back-arc type oceanic lithosphere in the Intra-Pontide suture zone. Together with previous data obtainedfrom basalts in the melange from the Intra-Pontide suture zone, this finding represents a proof that an intra-oceanic subduction within the Intra-Pontide oceanicbasin occurred in the Middle Jurassic time.

Vein and stock-work cryptocrystalline magnesite deposits, known also as Kraubath type, Gelmagnesite, and Khalilovo type, are widespread in the Tethyan ophiolites from the Alps to the Zagros mountains, including the Dinarides, but also in other ophiolite suites like California, etc. Thirteen samples from the magnesite deposits of the Dinaric and Vardar zone ophiolite belts were analyzed for C- and O-isotopes, major and trace elements, and REE. Genesis of cryptocrystalline magnesite has received two controversial interpretations, involving circulation of Mg-bearing ore fluids, “ per ascendum ” or “ per descendum ” mechanisms. Stable isotope data on C- and O-isotopes in magnesite deposits show significant positive correlation along the light-heavy isotope ratio from vein to stock-work and sedimentary type ore bodies. The thirteen new samples confirmed the same regularity, as a rule for cryptocrystalline magnesite. It should be stressed that there are some exceptions, marked by different isotope patterns, represented by deposits located close to the products of the Tertiary volcanic activity. This paper brings arguments both in favor and against the two genetic models, supported by the new geochemical data. The authors favour, however, the per descendum model, stating generation beneath the lateritic weathering crusts, which covered a wide area of the obducted ophiolites, subjected to warm, humid tropical climate in the Early Cretaceous. The laterite crusts acted as a chromatographic column separating immobile elements from mobilized magnesia in solution, which percolating down through a fracture system, deposited magnesite as a result of increased pH. The self refining of ore forming fluids produced almost monomineralic magnesite veins, with or without selvages. The fractionation of light C- and O-isotopes in magnesite needs isotopic light CO 2 , commonly interpreted as a derivate of deep seated decarboxylation of organic rich sediments. However, this behaviour of light C- and O-isotopes on a regional scale can be convincingly explained also by closed/semiclosed fractionation between atmospheric CO 2 and magnesite in the descending fluid, controlled by the Rayleigh equation.

Age data of Armenian ophiolites are of great importance for understanding the palaeogeographic and geodynamic evolution of Tethys in the Caucasus area and for lateral correlations of Tethyan ophiolitic belts in the Middle East area. Moderately well-preserved radiolarian assemblages obtained during this study from the Sarinar section of eastern Armenia allow to investigate the sedimentary and volcanic history recorded in a ca. 30 m-thick radiolarite sequence associated with spilitic lavas of the Sevan ophiolitic suture zone. The latest Bajocian-early Bathonian assemblage yielded from a sample situated close to the base of the studied sequence suggests that radiolarian ooze accumulation took place on oceanic crust of late Middle Jurassic age (probably Bajocian). Several tuffite levels are intercalated within the lower to middle part of the radiolarite sequence; a late Bathonian-early Oxfordian assemblage from a sample taken just below the uppermost tuffitic bed, suggests that during this time the oceanic domain of Sevan ophiolites was close to subaerial volcanic activity.