We investigated the tectonic evolution of amphibolite and blueschist tectonic blocks in the serpentinite‐ or pelite‐matrix mélange, 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°C, and 0.8–1.3 GPa and 475–550°C, respectively, (at an apparent thermal gradient ranging between 13 to 17°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°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°C (most probably 0.8–0.85 GPa and 360–480°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° to ~10°C/km over ~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 mélange, including these tectonic blocks, were exhumed together with the coherent accretionary units as the former was emplaced over the latter.
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.
The Bahla 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 WNW-ESE, 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 °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 cross-cut 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 UPb 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 décollements.
Parent melts of the Bahla 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 Bahla 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 Bahla 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.
•The Bahla massif crops out along the thrust front of the Oman ophiolite.•Its crustal section presents distinctive structural and petrological features contrasting with the ones of most other Oman massifs.•It is affected by intense faulting inherited from syn-accretion tectonics and/or from the very early stage of intra-oceanic thrusting.•Some lithological facies from the lower crustal cumulates seem specific to the frontal massifs and are attributable to magma mixing and contamination by hydrous fluids.•Our results highlight the heterogeneity of the Oman ophiolite.•The paradigm considering the Oman ophiolite as a whole the archetype of ocean lithosphere accreted in a fast-spreading setting should be nuanced.
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.
•We report hydrothermal fluid/peridotite-gabbro reaction products in the Oman ophiolite.•The reaction products are of great importance for tracing temporal evolution of the fluids.•Aqueous fluid inclusions were characterized in chemistry and entrapment temperature.•Whole-rock and in-situ chemical analyses restrict formation circumstance of the reaction products.•The results delineated chemical evolution of the fluids at high temperature (530–600°C).
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.
•We examine Sr–Nd isotopic feature of basal peridotites of the Oman ophiolite.•Geochemistry provides evidence of a genetic link between oceanic crust and mantle.•Sr isotopic ratios suggest metasomatism by hydrous fluid from the metamorphic sole.
This paper describes a comprehensive study of the chromian spinels present in mafic–ultramafic dykes cropping out along the mantle section of the Oman ophiolite. We studied about 1100 samples in thin ...section and with the electron microprobe. Chromian spinel is almost ubiquitous in primitive dykes (high-Mg# troctolites and pyroxenites) and less common in more differentiated ones (olivine gabbros and gabbronorites). The Cr#, TiO
2 content, and other compositional parameters are well correlated to the nature and composition of the co-genetic silicate assemblage. Chromian spinel composition contributes to establish that the mantle dykes of Oman are more or less evolved cumulates that crystallised from two main types of primary melts: tholeitic melt similar to Mid-Ocean Ridge basalts (MORB; 0.45
<
Cr#
<
0.63; 0.3
<
Mg#
<
0.6; TiO
2 up to2 wt.%), and more silicic melts issued from a highly depleted mantle source, similar to boninitic–andesitic melts that preferentially form in subduction zone settings (0.35
<
Cr#
<
0.80; 0.1
<
Mg#
<
0.7; TiO
2 up to 0.2 wt.%). The chromian spinel composition presents a higher variability than the associated silicates and allows us to further unravel the petrological evolution and segmentation of the Oman ophiolite. The composition of chromian spinel in mantle dykes and in the spatially related residual harzbugites display well correlated variations at the scale of the Oman ophiolite. This shows that these two lithologies share a common magmatic history, even if, strictly speaking, they cannot be related through direct parent–daughter relationships. The Cr# is on average higher, and the TiO
2 lower in the NW than in the SE, consistent with an increasing influence of “marginal” magmatic processes in the NW, while the southeastern area has petrological characteristics closer to those of an “open” ocean. In this southeastern part, compositional variations of the chromian spinel are correlated to structural characteristics related to the spreading history: the distribution of the Cr# around a former mantle diapir cropping out in the Maqsad area is concentric, with the highest values in the centre of this structure, consistent with higher degrees of partial melting of the mantle. On the other hand, unusually low Cr# – diagnostic of a low melting degree – in a ridge-parallel band of about 20 km to the northeast of the Maqsad area can be related either to temporal variations in the partial melting degree or to off-axis magmatic activity.
We conducted a comprehensive field, petrographic, and microprobe study of the dykes and porous flow channels cropping out in the Oman harzburgites. The 36 rock types we recognized among of about 1000 ...samples can be grouped in two main magma suites contrasted in terms of structural and textural characteristics, modal composition, order of crystallization, and phase chemistry. One suite (troctolites, olivine gabbros, opx‐poor gabbronorites, and rare oxyde gabbros) derives from MORB‐like melts. The other suite (pyroxenites, opx‐rich gabbronorites, diorites, and tonalite‐trondhjemites) derives from melts richer in silica and water than MORBs and ultradepleted in incompatible elements. Dykes and porous flow channels from the MORB suite are restricted to a few areas, covering only 25% of the mantle section. This is an unexpected result as the deep Oman crust is made essentially of cumulates from MORB‐like melts. Their composition, texture, and relations with the host harzburgites point to high mantle temperatures at the time of crystallization (likely above 1100°C, up to 1200°C for part of them), i.e., conditions close to the “asthenosphere/lithosphere” boundary. The largest outcrop of mantle harzburgites enclosing MORB like dykes is a 80 km long and 10 km wide corridor, parallel to the strike of the sheeted dyke complex and centered on an area where a former mantle upwelling has been unambiguously defined (the Maqsad “diapir”). A few other occurrences of mantle cumulates from the MORB suite are smaller than the Maqsad area and have a lesser abundance of troctolites (i.e., of high‐temperature cumulates). We interpret the troctolite zones of Oman as the witnesses of former diapirs frozen at various stages of their development. Dykes belonging to the depleted suite are the most common in Oman harzburgites. Their structural and textural characteristics show that they crystallized in a mantle colder than the melt (likely in the range 600°C to 1100°C). A possible origin for the parent melts of this suite is in situ partial melting of the shallow and partly hydrated lithosphere residual after MORB extraction. Our data support the view that feeding magma chambers with MORBs is a focused (and likely episodic) process involving the rise of hot mantle to the base of the crust through a lithospheric lid accreted during a previous diapiric event. They suggest also that the shallow mantle beneath spreading centers is a place of important petrologic processes, some of them predicted on the basis of MORB composition (e.g., fractionation inside melt conduits) and other ones unexpected (e.g., remelting of the depleted lithosphere).
Whole-rock chemical compositions are significant for igneous petrology, especially volcanic rock studies. ICP-MS is widely used as a conventional method to determine the trace-element compositions ...using solutions prepared by digestion of rocks. In the past decades, LA-ICP-MS technique has been well developed to measure whole-rock compositions by using pressed powdered pellet or fused glass of rock samples. In this study, trace-element concentrations (Li, Sc, Ti, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th, U and REEs) of basaltic and andesitic rocks were determined by using flux-free fused glasses and LA-ICP-MS. The fused glass was prepared by the direct fusion method where the powdered rock sample is melted with an iridium-strip heater. The heating temperature was manually controlled by monitoring the current and the heating lasted less than 30 sec to achieve melting. We evaluated our routine analytical work performed by measurements of 3 random target positions in each fused glass. The data accuracy was evaluated by measurement of 11 fused glasses created from USGS and GSJ international reference materials (BCR-2, BHVO-2, BIR-1a, JB-2, JA-1 and JGb-1). The homogeneity of our fused glasses is represented by less than 5% relative standard deviation (1σ) in most elements. The mean values of most elements are in agreement with the reference values within 10% deviation. However, Pb concentration cannot be reproduced because of strong volatilization during the melting, and Lu, Hf and Ta are occasionally enriched by the elemental contamination/interference from the iridium-strip heater. 9 fused glasses created from selected petrological samples, such as the Ogasawara volcanic rocks and Oman diopsidite, were also measured to examine our routine measurement work and glass preparation method for a wider range of the trace-element abundance, especially the Cr abundance. The analytical results of 20 fused glasses demonstrate that our method is capable of determining the whole-rock trace-element compositions for petrological study.
We studied basal lherzolites that are exposed along the metamorphic sole at the base of the central Oman ophiolite (Wadi Sarami). We recognized two types of lherzolites (Types I and II) based on ...field occurrences, textures, and mineral compositions. Type I lherzolites are massive and transition into harzburgites, whereas Type II lherzolites are strongly foliated with mylonitic to porphyroclastic textures. Type II lherzolites only crop out within a direct contact with the amphibolitic sole to few meters above this sole and are overlain and/or surrounded by Type I. The clinopyroxenes Mg# = Mg/(Mg + Fe) = 0.89-0.94 of Type II lherzolites show higher contents of Al2O3 (4.5-7.3 wt%), Na2O (0.5-1.2 wt%), Cr2O3 (0.6-1.4 wt%), and TiO2 (0.2-0.4 wt%) than those of Type I lherzolites. Positive correlations among the Al2O3, Na2O, and TiO2 contents of clinopyroxenes show a pronounced residual trend from Type I lherzolites to depleted harzburgite, giving rise to chemical heterogeneities at the base of the mantle section. Clinopyroxenes in lherzolites and harzburgites show compositional trends that are similar to those in abyssal peridotites from normal ridge segments. Olivines (Fo89.4-Fo91.5) show a residual character of the Sarami peridotites. Primary spinels show a wide range of Cr# = Cr/(Cr + Al) from 0.04 to 0.53 and low YFe Fe3+/(Cr + Al + Fe3+), <0.046, similar to spinels in abyssal peridotites. The wide range of spinel Cr# is a result of a wide range of partial-melting degrees, which are up to 10% for lherzolites and ∼ 10-25% for harzburgites. The Type II lherzolites, which occur near the paleo-fracture zone located to the east of Wadi Sarami, represent a remnant of asthenospheric materials trapped at the base of oceanic lithosphere mantle (Type I) during detachment and obduction. The Type I lherzolites experienced high-degree partial melting, resulting in the formation of harzburgites at the refractory end. The modal and compositional variations of Sarami pyroxenes and spinels indicate intrinsic mantle heterogeneity of Oman ophiolite formed as residues at an oceanic spreading center.
This paper details a new discovery of anorthite-bearing diopsidites (= anorthite diopsidites) from the lowermost crust along Wadi Fizh in the northern Oman ophiolite. The anorthite diopsidites occur ...as networks within layered gabbros 50 m above the gabbro/peridotite boundary, and are mainly composed of high-Mg diopsidic clinopyroxenes and anorthites with various amounts of uvarovite. They are intermediate in mineral chemical characteristics between the crustal gabbros and the diopsidites, and interpreted as an interaction product between high-temperature Ca-rich hydrothermal fluids and peridotites within the mantle section. The anorthite diopsidites were probably formed by interaction between the fluids involved in diopsidization within the mantle section and the layered gabbro. The fluids responsible for the formation of the anorthite diopsidite were also Ca-rich and carried Cr to form uvarovite from the underlying mantle section. High-temperature hydrothermal circulation plays an important role in the transportation of elements, e.g., Ca and Cr, from the upper mantle to the lower crust across the Moho in the ocean floor.
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