We investigated the tectonic evolution of amphibolite and blueschist tectonic blocks in the serpentinite- or pelite-matrix melange, which are distributed at the highest structural level of the high-P/T type Kamuikotan metamorphic rocks in northern Japan. The tectonic blocks in this study area are divided into six rock types: garnet-epidote amphibolite, epidote amphibolite, amphibolite, plagioclase-poor amphibolite, epidote blueschist and glaucophane-bearing quartz schist. Based on phase equilibrium modelling, garnet-epidote amphibolite and epidote amphibolite experienced peak metamorphism at pressure and temperature conditions of 1.1-1.25 GPa and 550-590 degrees C, and 0.8-1.3 GPa and 475-550 degrees C, respectively, (at an apparent thermal gradient ranging between 13 to 17 degrees C/km). By contrast, although the peak-metamorphic conditions for each one of amphibolite, plagioclase-poor amphibolite, and glaucophane-bearing quartz schist are not well constrained, they may have been originally metamorphosed at amphibolite to epidote-amphibolite facies at thermal gradients exceeding 20 degrees C/km, inferred from the core composition of amphibole (edenite/magnesiohornblende/barroisite). The epidote blueschist experienced peak metamorphism at pressure and temperature conditions of 0.8-1.6 GPa and 360-520 degrees C (most probably 0.8-0.85 GPa and 360-480 degrees C). Although different types of tectonic blocks experienced a variety of peak metamorphism under different P/T conditions, all of them underwent epidote blueschist facies metamorphism at the peak or retrograde stage (as shown by the glaucophane rims of amphibole with different core compositions). The overall P-T paths appear counter-clockwise, which could be interpreted to reflect the cooling history of the subduction channel from the beginning to the steady state of subduction. The geothermal gradient could have changed from 15-17 degrees to similar to 10 degrees C/km over similar to 20-25 Myr, as estimated by previously reported radiometric ages. The protoliths to the tectonic blocks could have begun to subduct into the subduction channel at different times (where the thermal structure evolved with time), acquiring different prograde P-T paths. Subsequently, these tectonic blocks were juxtaposed at a certain depth and incorporated into the overlying serpentinite during the subduction stage. Finally, the serpentinite- or pelite-matrix melange, including these tectonic blocks, were exhumed together with the coherent accretionary units as the former was emplaced over the latter.

The Bahia massif exposes the lower crustal section of the Oman ophiolite located close to the thrust front of the Semail nappe. It is affected by intense faulting previously attributed to tectonic events that dismembered a classical ophiolitic sequence during or after the obduction. Here we show that most of this complexity is primary, inherited from syn-accretion tectonics. The crustal section is exposed in a 15 by 8 km tectonic enclave surrounded by mantle peridotite. Its northern boundary corresponds to a major, steeply dipping normal fault striking WNWESE, at low angle to the paleo-ridge axis. Movement along this fault was accommodated by intense plastic deformation of the crustal cumulates and adjacent mantle peridotites at temperature conditions >= 900 degrees C. The thickness of the deformed zone reaches several hundred meters. The flattening of the cumulate layering away from the fault is correlated to a decrease in the deformation intensity. Undeformed olivine-gabbro dykes crosscut this "tectonic Moho" indicating that the tilting occurred before the end of the igneous activity. To the southwest, the crustal enclave is bounded by a NW-SE trending transtentional shear zone that was active in the amphibolite to greenschist facies and was intensely injected by syn- to post-kinematic gabbronorite and tonalite/ trondhjemite dykes and plugs. The age of one felsic sample (95.214 +/- 0.032 Ma, high-precision U-Pb zircon dating) is within error of the age of intrusive felsic intrusions into the mantle and lowermost axial crust from the length of the Oman ophiolite, which slightly post-dates the mean crystallization age of the Semail crust (V1 magmatism; 96.1-95.6 Ma). Other contacts are low temperature features including cataclastic faults, serpentine-carbonate breccias and flat-lying decollements. Parent melts of the Bahia crustal cumulates were more siliceous and hydrous, i.e. more andesitic, than typical mid-ocean ridge basalt (MORB) as deduced from the frequent occurrence of early crystallizing orthopyroxene (opx) and late crystallizing amphibole. Some facies such as cumulate harzburgite and opx-troctolite have not been documented elsewhere in the Oman ophiolite and may be specific to the tectonic context in which the frontal massifs accreted. The chemical composition of the lower crustal cumulates can be accounted for by the hybridization in various proportions between MORB and a primitive andesite from a depleted source whose origin can be looked for in melts from a nascent subduction zone or from high temperature hydrothermal processes. The structure of the Bahia lower crustal section is reminiscent of the plutonic growth faults documented along present-day slow-spreading centres in both mid-ocean ridge and back arc settings. The distinctive characteristics of the Moho and lower crustal section in the Bahia massif are tentatively related to their position at the leading edge of the ophiolite, i.e. closer to the Arabian continental margin at the time of accretion than the massifs from the internal part of the ophiolite that have a more continuous and less deformed lower crust. It indicates that the style of crustal accretion may have changed during the opening of the oceanic basin from which the Oman ophiolite issued.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

A small peridotite massif (similar to 5 kmx similar to 0.6 km) crops out on the Northern flank of the Jabal Nakhl close to the Taww oasis in the Northern Oman mountains. It rests directly on the autochthonous and is over-thrusted by the Hawasina nappes, which contrasts with the structural position of the Semail ophiolite. The lack of pervasive high-temperature mylonitization and the absence of a metamorphic sole at its base is a distinctive feature of the Taww massif which appears more structurally linked to the autochthonous than to the allochthonous units. Its exhumation was synchronous with the one of the autochthonous with no relative displacement during or after this event. The Taww peridotite is characterized by the abundance of clinopyroxene (Cpx); its modal composition is transitional between Cpx-harzburgite and lherzolite (> 5% Cpx). Cpx is associated in clusters with orthopyroxene porphyroclasts. This texture and mineral composition of the Taww peridotite are consistent with residues left after a moderate degree of partial melting of a fertile peridotite. The Taww peridotite has a mineralogical and chemical composition contrasting with that of the Semail peridotites made essentially of harzburgite generally devoid of Cpx and with trace element patterns pointing to melt-rock reaction. When present in the Semail harzburgite, Cpx is mostly interstitial and attributable to melt (+/-hydrous fluids) percolation and crystallization, an origin that can be excluded in the case of Taww Cpx. The Semail ophiolite evolved in a highly productive magmatic environment, likely a fast spreading centre. Conversely, the Taww peridotite is closer to peridotites from highly stretched continental margins or post-orogenic subcontinental mantle. The Taww massif clearly experienced a different emplacement, partial melting, meltmigration and melt-rock interaction history than the Semail ophiolite. Whatever its origin, which remains puzzling, the Taww peridotite does not seem to belong to the same obducted units as the Semail and Hawasina nappes. (C) 2020 Published by Elsevier B.V.

IODP Expedition 345 aimed to drill lower crust gabbros at Hess deep rift (East Pacific Rise, 2°14’N-101° 30’W), which is located at the junction between EPR and the Cocos, Nazca and Ridge. Lower oceanic gabbros were sampled on a about ~200 m wide bench located on the intrarift's southern slope between 4675 and 4850 m below sea level, and total of 11 holes (A to P) were drilled among which two reached a depth over 100 m below seafloor (Holes 1415J and 1415P) (IODP Expedition 345 Scientific Report, 2013). Primitive troctolites and olivine-rich gabbros were the main lithologies recovered from these two holes. Shipboard data showed a high Mg# whole rock chemistry in concordance with their primitive nature.

The Maqsad or Sumail massif in the Oman Ophiolite is known for its well defined mantle structural diapir well matching other petrographical and chemical data, allowing us to locate the axis of a former spreading centre. However, most of the studies conducted on this famous massif concerned the mantle section and published data include mantle harzburgites and mantle dykes and intrusions petrology and chemistry. The crustal section in this massif is only locally known in its lowest part located near the mantle/crust boundary. In this study, we conducted a sequential study of the 2 km thick Wadi Mahram crustal section (from mantle-crust boundary to sheeted dyke transition) located directly above the mantle diapir, i.e. directly above the Oman spreading axis. The crustal column above the Maqsad diapir is exceptionally rich in troctolites as this lithology represent more than the three quaters of the observed facies and is present at any level from the basis to top of the section. Troctolites are layered in the lower levels, close to the mantle-crust transition zone, intercaled with minor olivine gabbros layers, and the layering is crosscut by centimeter scale olivine gabbro dykes. From the middle to the top of the section, vary textured troctolites progressively become become isotropic at the boundary with the sheeted dyke complex. Meter scale olivine gabbros and olivine-free layers are more abundant at the top. Dolerite dyke cutting troctolite and gabbro structures are abundant near the sheeted dyke complex and probably represent its root system. Down section mineral chemistry evolution show a considerable contrast between the troctolites and the olivine gabbros. The troctolites chemical characteristics seem to be evolving on a large scale with 2 superposed main bodies at the top and the bottom of the section, about 800 m thick, showing a regular evolution probably in relation to magmatic differentiation during fractional crystallisation. A minor block between these 2 main troctolite bodies show a strong chemical scatter in association with variabke textures and minéralogies suggesting that melt mixing and melt/rock reaction were the dominant processes during its formation. Between the layers of troctolites, olivine gabbros layers show petrographical and chemical properties that could have been aquired by local magmatic processes like differentiation in small scaled trapped melt pocket or local melt/rock reaction during melt migration to the surface. As it was already shown by studies on magmatic dykes in the mantle, the presence of differentiated dykes cutting the layered structure at the section bottom show that significant differentiation degree may be reached within the mantle and differentiated melt can be injected from the mantle into the crust. These differentiated melts melt s are however injected during during the late magmatic history of the section as they cut already cooled lower troctolite. Traces of mixing with these differentiated melts are found only at 2 levels kn the crust: at the intermediate vary textured level or at the topmost level, below below the sheeted dyke complex. SMP41-11 JpGU-AGU Joint Meeting 2017

International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

Reaction products between hydrothermal fluids and uppermost mantle harzburgite-lowermost crustal gabbro have been reported along Wadi Fizh, northern Oman ophiolite. They are named mantle diopsidite or crustal diopsidite. They construct network-like dike crosscutting structures of surrounding harzburgite or gabbro. The mantle diopsidite is mainly composed of diopsidic clinopyroxene, whereas the crustal diopsidite is of diopsidic clinopyroxene and anorthitic plagioclase. Here, we report new reaction product, crustal anorthosite, collected in the lowermost crustal section. It is always placed in the center of the crustal diopsidite network. It mainly consists of anorthitic plagioclase with minor titanite and chromian minerals as chromite and uvarovitic garnet. Aqueous fluid inclusions trapped in negative crystal are evenly distributed in the crustal anorthosite. Some of them include angular-shaped or rounded daughter minerals as calcite or calcite-anhydrite composite, which were identified by Raman spectroscopic analyses. We estimated their captured temperature at 530oC at least by conducting microthermometric analyses of the fluid inclusions by Heating-cooling stage. Furthermore, we examined their chemical characteristics by direct laser-shot sampling method operated by laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS). The results indicate that the trapped aqueous fluids contain an appreciable amount of Na, but no K. Hydrothermal fluids involved in the crustal anorthosite formation transported hydrothermally immobile Cr, which was probably provided from chromite seam in the uppermost mantle section to precipitate chromites and uvarovitic garnet in the lowermost crustal section. Cr got soluble by forming complexes with anions as SO4 , CO3 2and Cl. In addition, these hydrothermal fluids transported Fe, Mg, Ti and rare-earth elements. Our temperature estimation for the crustal anorthosite formation requires rather lower temperatures (530–600oC) with considering microthermometric results and mineral equilibria, thus later circumstance than the mantle diopsidite and crustal diopsidite formation. Therefore, a series of high-temperature hydrothermal events had been significantly contributing to the chemical flux occurring around the boundary between the mantle and crustal sections.

Reaction products between hydrothermal fluids and uppermost mantle harzburgite-lowermost crustal gabbro have been reported along Wadi Fizh, northern Oman ophiolite. They are named mantle diopsidite (MD) or crustal diopsidite (CD) depending on the stratigraphic level. They construct network-like dikes crosscutting structures of the surrounding harzburgite or gabbro. The MD is mainly composed of diopsidic clinopyroxene, whereas the CD is of diopsidic clinopyroxene and anorthitic plagioclase. Here, we report a new reaction product, crustal anorthosite (CA), from the lowermost crustal section. The CA is always placed in the center of the CD network, and mainly consists of anorthitic plagioclase with minor titanite and chromian minerals such as chromite and uvarovite.Aqueous fluid inclusions forming negative crystals are evenly distributed in minerals of the CA. The fluid inclusions contain angular-shaped or rounded daughter minerals as calcite or calcite-anhydrite composite, which were identified by Raman spectroscopic analysis. We estimated their captured temperature at 530°C at least by conducting microthermometric analysis of the fluid inclusions. Furthermore, we examined their chemical characteristics by direct laser-shot sampling conducted by laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS). The results indicate that the trapped aqueous fluids contain an appreciable amount of Na, but no K and Cr.Hydrothermal fluids involved in the CA formation transported Cr, which was probably taken up from chromite seams in the uppermost mantle section. Cr got soluble by forming complexes with anions as SO42–, CO32– and Cl–. In addition, these hydrothermal fluids transported Fe, Mg and trace elements (Ti, Sr, Y, Zr and rare-earth elements) governing whole-rock chemical compositions of the MDs, CDs and CAs. Our estimation for the condition of CA formation yielded rather low temperatures (530–600°C), which indicates a later stage production of the CA than the MD and CD (~800°C). A series of high-temperature hydrothermal events had been significantly contributed to the chemical flux occurring around the Moho, boundary between the mantle and crustal sections.

The Oman ophiolite is one of the best preserved sections of oceanic crust and upper mantle worldwide, and consists of multiple massifs that lie along more than 400km of the Arabian coast. In the northernmost massifs, the oceanic crust preserves a record of polygenetic magmatism from mid-ocean ridge to subduction-related stages. The lherzolites and clinopyroxene (Cpx)-rich harzburgites of the Fizh block are located a few tens to a hundred meters above the metamorphic sole of the ophiolite and the geochemistry of these Cpx-rich peridotites provides evidence of a genetic link between oceanic crust and mantle. These Cpx-rich peridotites contain olivine with a restricted range of forsterite contents (90–91), but variable Cr-spinel Cr# (Cr/(Cr+Al) atomic ratio) values (0.12–0.33), suggesting that these Cpx-rich peridotites have undergone variable degrees of melt extraction. Cpxs within the Cpx-rich peridotites have chondrite-normalised trace element variation patterns that slope either gently or steeply between the heavy rare earth elements (REEs) and the middle REEs ((Sm/Yb)N=0.08–0.55, where N chondrite-normalised) and are enriched in highly incompatible elements such as Rb, Ba and Nb. This Cpx chemistry can be explained by a polygenetic evolution whereby an initial 4–12% of melt was extracted from the depleted mantle source before this mantle was metasomatised by interaction with fluids derived from dehydration of the metamorphic sole during subduction initiation and obduction. A comparison between 143Nd/144Nd versus 147Sm/144Nd for Cpx in the Fizh basal Cpx-rich peridotites and a mineral–whole rock Sm–Nd isochron for a gabbro from the same massif suggests a genetic link between crustal and mantle rocks in this area. In addition, Cpxs within the basal Cpx-rich peridotites have highly variable Sr isotopic compositions that are indicative of a significant contribution of seawater from the metamorphic sole, originally derived from subducted oceanic crustal material.