Mylonitic mantle peridotites exposed at the Tosa Megamullion in the Shikoku Basin, Philippine Sea, provide direct evidence for amagmatic ductile shear deformation of the upper mantle beneath a back-arc spreading center. Oceanic core complexes (OCCs), or megamullions, are dome-shaped structures formed by detachment faulting and occur locally along slow-spreading mid-ocean ridges and back-arc basins, where they expose fault rocks derived from ductile shear zones in the lower crust and upper mantle. The Shikoku Basin hosts several OCCs, including the Tosa Megamullion, which formed during the early stage of back-arc spreading. In this study, nine ultramafic rocks were collected from the Tosa Megamullion using the submersible Shinkai6500 during cruise YK23-05S. Although all samples were highly serpentinized, several preserved primary peridotitic textures were composed mainly of olivine, orthopyroxene, with subordinate clinopyroxene, plagioclase, and spinel. Seven samples exhibit well-developed foliation and porphyroclastic textures dominated by orthopyroxene porphyroclasts, ranging from rounded to strongly elongated forms, commonly showing microkinks and undulose extinction. Crystallographic preferred orientations (CPOs) of three representative samples, analyzed using SEM-EBSD, reveal E-type-dominant olivine fabrics characterized by the (001)[100] slip system, with a subordinate contribution from C-type (100)[001] slip. These CPOs suggest deformation under non-dry conditions involving moderate hydration and/or elevated differential stress. These results indicate that the ultramafic rocks from the Tosa Megamullion represent mantle-derived mylonitic peridotites formed by ductile shear beneath the spreading axis and subsequently exhumed under strongly magma-poor, amagmatic conditions. The Tosa Megamullion thus represents an amagmatic end-member of the OCC formation in back-arc basins, dominated by tectonic strain localization rather than by magmatic accretion.

Oceanic detachment faulting plays a key role in oceanic crust formation at mid-ocean ridge systems, but the temporal evolution of melt/fluid circulation during the life cycle of an oceanic detachment fault remains poorly constrained. In this study, we present trace element data on amphiboles from deformed gabbroic rocks and amphibolites recovered at both the breakaway (initial stage) and termination (late stage) areas of the Godzilla Megamullion in the Parece Vela Rift, Philippine Sea. We focus on brown and green amphiboles, which are classified into several microstructural forms reflecting their different origins. Our dataset allows us to distinguish previously unrecognized geochemical variations among microstructural types and between different portions of the Godzilla Megamullion. Amphiboles from the breakaway area record multiple episodes of melt infiltration followed by retrograde hydrothermal alteration, suggesting the involvement of distinct magmatic sources such as the Fe-Ti oxide-rich melt that formed the oxide gabbro and felsic melts. In contrast, amphiboles from the termination area display uniform compositions across different microstructural types, pointing to a widespread and homogenized melt/fluid circulation independent of deformation intensity. These results highlight that the final amphibolitization event at Godzilla Megamullion was not solely driven by fault activity but rather by melt stagnation in a thickened lithosphere, consistent with spatially associated peridotites. By comparing the two evolutionary stages of the Godzilla Megamullion, this study provides new information on the magmatic-hydrothermal transition during detachment faulting and offers a framework applicable to melt/fluid processes at oceanic core complexes.

The Shikoku Basin, a Miocene back-arc basin in the Philippine Sea, hosts numerous oceanic core complexes (OCCs) that reflect melt-poor lithospheric extension. Based on bathymetric mapping, gravity and magnetic surveys, dredging, and submersible dives conducted between 2007 and 2023, we identified multiple OCCs in its southern part, including the Mado, Sui-Shin, Tosa, Sanuki, and Awa Megamullions. These OCCs expose mantle and lower crustal rocks and are associated with high mantle Bouguer anomalies, indicating thin crust. Geochemical and geochronological data from the Sui-Shin Megamullion suggest its formation immediately after rifting of the proto-Izu-Bonin arc ( 24 Ma). The Tosa Megamullion, dominated by plagioclase-peridotites with only minor gabbros, exemplifies melt-poor lithospheric conditions and may represent Hess-type oceanic crust. The spatial and temporal distribution of OCCs, including the triplet OCCs (Tosa, Sanuki, and Awa), implies multiple ridge jumps during basin evolution. These findings suggest that a significant portion of the Shikoku Basin formed largely through amagmatic or melt-poor processes, contrasting with typical mid-ocean ridge settings. The OCCs serve as tectonic windows into back-arc lithosphere evolution and provide a framework for future investigations into the role of melt supply and slab-derived fluids in shaping oceanic crustal architecture.

Our understanding of the processes at work in the lower crust/upper mantle transition zone during subduction initiation and early arc development has suffered from a general lack of in situ samples. Here, we present the results of petrographic and geochemical analysis of 34 samples (9 harzburgites, 13 dunites, 2 orthopyroxenites, 3 olivine-gabbros, and 7 wehrlites) collected from the inner trench wall of the Bonin Ridge, Izu-Bonin forearc. The sample suite records three main melt-rock reaction events involving melts with forearc basalt (FAB)-like, boninitic, and transitional compositions. The wehrlitic and gabbroic rocks trend towards more transitional to FAB compositions and the rest towards more boninitic compositions. The crosscutting occurrence of all three events in a single sample (wehrlite D31-106) establishes a relative timing of the events like that reported for the volcanic edifice of the Bonin Ridge, which transitioned from forearc basalt volcanism at subduction initiation (c.a., 51-52 Ma) to boninitic volcanism (c.a., 50-51 Ma) as the subduction system matured. We therefore suggest that the lower crust/upper mantle transition of the Bonin Ridge preserves a record of the transition from FAB melts created by decompression melting at subduction initiation to arc-type flux melting and boninitic volcanism thereafter. Orthopyroxenites and two anomalously fresh harzburgites from the sample suite are suggested to represent the later boninitic melts and possibly the result of hybridization between such melts and residual peridotites, respectively. Diffuse melt-rock reaction between the later boninites and/or subduction-related fluids and the earlier-formed FAB-related crust is recorded by enrichments in fluid mobile elements and depletions in first row transition metals in clinopyroxenes from a metasomatic vein in wehrlite sample D31-106. The chemistry of the wehrlitic and gabbroic clinopyroxenes suggests that they crystallized from hydrous, highly depleted melts which lack a slab fluid signature. We thus suggest that highly depleted melt fractions might be created early on during subduction initiation by the introduction of seawater into the proto-mantle wedge. The overall FAB-like nature of the crustal wehrlites and gabbros would suggest that most of the lower arc crust was created by forearc extension during/following subduction initiation and that later, mature arc volcanism may have contributed little or no material to the lower crust/upper mantle record in the outer forearc.

At mid-ocean ridges, melts formed during adiabatic melting of a heterogeneous mantle migrate upwards and ultimately crystallize the oceanic crust. In this context, the lower crustal gabbros represent the first crystallization products of these melts and the processes involved in the accretion of the lowermost crust drive the chemical evolution of the magmas forming two thirds of Earth’s surface. At fast-spreading ridges, elevated melt supply leads to the formation of a ⁓6 km-thick layered oceanic crust. Here, we provide a detailed petrochemical characterization of the lowermost portion of the fast-spread oceanic crust drilled during IODP Leg 345 at the East Pacific Rise (IODP Site U1415), together with the processes involved in crustal accretion. The recovered gabbroic rocks are primitive in composition and range from olivine-rich troctolites to troctolites, olivine gabbros, olivine gabbronorites and gabbros. Although textural evidence of dissolution-precipitation processes is widespread within this gabbroic section, only the most interstitial phases record chemical compositions driven by melt-mush interaction processes during closure of the magmatic system. Yet, the occurrence of primitive orthopyroxene in most of the olivine-bearing samples indicates that reactive processes allowed for its local saturation within the percolating MORB-type melt. Comparing mineral compositions from this lower crustal section with its slow-spreading counterparts, we propose that the impact of reactive processes on the chemical evolution of the parental melts is dampened in the lowermost gabbros from magmatically productive spreading centres. Oceanic accretion thereby seems driven by in situ crystallization in the lowermost gabbroic layers, followed by upward reactive percolation of melts towards shallower sections. In addition, we here furnish a first estimate of the trace element composition of the parental melts that led to the accretion of the lower crust at Hess Deep, Atlantis Massif and Atlantis Bank; we show that the primary melts of the East Pacific Rise are more depleted in incompatible trace elements compared to those formed at slower spreading rates, as a result of higher melting degrees of the underlying mantle.

Mantle deformation processes leading to seafloor spreading are often difficult to infer due to the highly serpentinized and weathered state of most abyssal peridotites. We investigated the development of high-temperature crystal-plastic deformation and lower temperature mylonitization processes in relatively fresh (<50% modal serpentine) and ultra-fresh (<1% serpentine) mantle peridotites derived from the heterogeneous mantle in the sparsely magmatic zone of ultraslow-spreading Gakkel Ridge system by analyzing 12 peridotites from two dredge sites (<1 km apart). Microstructurally, these 12 peridotites consist of seven high-T deformed samples and five mylonites. Modally, the 12 samples include harzburgites, lherzolites, an olivine websterite, and a plagioclase-bearing lherzolite. Based on their mineral major and trace element compositions, the lherzolites, harzburgites, and olivine websterite are residual peridotites. The lherzolites containing clinopyroxenes with flat REE patterns likely underwent refertilization with a high influx of melt. The plagioclase-bearing lherzolites probably formed by subsolidus reaction after the partial melting process. Microstructural observations support that high-T crystal-plastic deformation (most likely at temperatures exceeding 1000 degrees C) was active in the peridotites of the high-T deformation group, accommodating mantle flow beneath the Gakkel Ridge. The identified melt refertilization process may have contributed to the formation of [0101-fiber olivine fabrics in these peridotites. Mylonitic microstructures, decreasing fabric strength and grain-size reduction of olivine suggest that mylonitization occurred under relatively low-temperature mantle conditions (-800 degrees C) and probably accommodated strain localization. Water did not greatly affect the peridotites during the development of the shear zones, although amphibole with "dusty" zones developed in one mylonitic peridotite after mylonitization, indicating that late-stage metasomatism occurred locally within the shear zone. This low-T mylonitization is likely to have affected mantle peridotites of this region independently of petrogenetic processes. The development of these deformation processes in Gakkel Ridge suggests a shift from flow in the uppermost mantle to shear zone formation in the rift valley walls.

The Mado Megamullion is an oceanic core complex (OCC) in the Shikoku back‐arc basin within the Philippine Sea Plate. Mantle peridotites (serpentinized) recovered by six dredge and submersible cruises exhibit signatures of extensive deformation. Amorphous pseudomorphs after plagioclase in many of the samples, as well as plagioclase‐spinel intergrowths, are clear evidence of melt stagnation and mantle reaction. Spinels show a wide range of compositions in terms of their Cr#, Mg#, and TiO 2 content. The presence of apparently magmatic high‐temperature pargasitic amphibole in veins and as replacement of clinopyroxene suggests that it may be a primary or near‐primary mineral crystallized from a hydrous melt which is unusual for abyssal peridotites. Two trace‐element populations of clinopyroxenes are in equilibrium with depleted and enriched basaltic melts, respectively. Rare‐earth element (REE) in the most depleted clinopyroxenes are modeled by 10% fractional melting except for a ubiquitous La‐Ce “kick.” Multiple models of open system melting combined with subsequent mixing of an enriched melt can explain the REE data. Broadly it appears that the peridotites underwent variable degrees of partial melting with moderate influx of enriched melts, which agrees with the other textural and chemical evidence of melt‐rock reaction and re‐fertilization. The compositions of the accumulated melts simulated by the open system models reproduce the enrichments in fluid mobile elements (Ba, U, and Pb) observed in basalts dredged from the Shikoku basin. Back‐arc basin peridotites at Mado Megamullion appear to have a unique petrographic and geochemical character that is distinct from those of peridotites exposed at the seafloor after formation from mid‐ocean ridges.

This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

We determined the mineralogical and petrological characteristics of ultramafic rocks dredged from two oceanic core complexes: the Mado Megamullion and 23 degrees 30 ' N non-transform offset massif, which are located within the Shikoku back-arc basin in the Philippine Sea. The ultramafic rocks are strongly serpentinized, but can be classified as harzburgite/lherzolite or dunite, based on relict primary minerals and their pseudomorphs. Strongly elongated pyroxene porphyroclasts with undulatory extinction indicate high-temperature (>= 700 degrees C) strain localization on a detachment fault within the upper mantle at depths below the brittle-viscous transition. During exhumation, the peridotites underwent impregnation by magmatic or hydrothermal fluids, lizardite/chrysotile serpentinization at <= 300 degrees C, antigorite crystallization, and silica metasomatism that formed talc. These features indicate that the detachment fault zones formed a fluid pathway and facilitated a range of fluid-peridotite interactions.

Abstract Oceanic core complexes (OCCs) represent tectonic windows into the oceanic lower crust and mantle; they are key structures in understanding the tectono‐magmatic processes shaping the oceanic lithosphere. We present a petrological and geochemical study of gabbros collected at the Mado Megamullion, a recently discovered OCC located in the extinct Shikoku back‐arc basin. Bathymetry of the Mado Megamullion reveals spreading‐parallel corrugations extending 25 km from the breakaway to the termination. Samples from several locations include peridotites, gabbros, dolerite, and rare pillow basalts. Gabbros range from granular to varitextured olivine gabbros and oxide gabbros. The emplacement of these gabbroic rocks within the oceanic lithosphere was followed by a multiphase tectono‐metamorphic evolution including (i) dynamic recrystallization within shear zones, developed under granulite‐ to upper‐amphibolite‐facies conditions, and (ii) intrusion of highly evolved melts forming felsic segregations. This tectono‐metamorphic evolution recalls that of the lower crust from other OCCs worldwide, demonstrating that this OCC exposes deep‐seated intrusions progressively exhumed by detachment faulting. Nonetheless, the Mado Megamullion lower crustal gabbros show an unusual crystal line of descent, different from what is reported from mid‐ocean ridge lower crustal rocks. We infer that the water‐bearing character of the primary melts in this back‐arc basin triggered the early precipitation of clinopyroxene, soon followed by amphibole and Fe‐Ti oxides. Such modifications in phase saturation are likely to be directly related to the back‐arc setting of the Mado Megamullion. If so, the phase assemblages of oceanic gabbros may be a diagnostic for the tectonic setting of lower crustal rocks in ophiolites.

Oceanic core complexes (OCCs) are domal bathymetric highs with axis-normal corrugations, and with exposure of serpentinized peridotite and gabbroic rocks, interpreted as exhumed footwalls of low-angle detachment faults. OCCs provide a valuable opportunity to directly study the architecture of oceanic lithosphere, together with the tectono-magmatic processes associated with its formation and evolution. A significant fraction of the ocean floor is created in backarc basins where water plays a major role in generating backarc basin basalts, strikingly contrasting to magmatic process at mid-ocean ridges. The opportunity to explore the recent formation of backarc basin lower crust and upper mantle is, therefore, an important contribution to understanding the ocean basins. The extensive multi-beam bathymetric mapping by Japan’s continental shelf survey has revealed the presence of potential OCCs in the multiple portions of the Philippine Sea Plate, such as in the Parece Vela Basin, Shikoku Basin, Kita-Daito Basin and West Philippine Basin. Among these, Godzilla Megamullion in the Parece Vela Basin is the largest known OCC on the Earth, and is one of the well-studied OCCs in the world.

Multiple generations of amphibole may form in the lower crust due to magmatism and metamorphism during the development of oceanic core complexes. We investigated the occurrence and chemical compositions of amphibole in gabbro mylonites from the medial area of the Godzilla Megamullion along the Parece Vela Rift in the Philippine Sea. The samples contain brown and green amphiboles with a variety of different textures that may have different origins. The brown amphibole occurs mainly as blebs in clinopyroxene porphyroclasts (Bleb amphibole), the rims around clinopyroxene porphyroclasts (Coronitic amphibole), and as porphyroclasts and fine-grained amphibole within the matrix (Matrix amphibole). The trace element and Cl contents of the bleb and green amphiboles indicate magmatic and metamoprhic origins, respectively. The bleb amphibole is interpreted to have crystallized from a hydrous silicate melt derived from an oxide gabbro-forming melt prior to retrograde metamorphism. In contrast, the compositions of the coronitic amphibole and matrix amphibole vary between those of typical magmatic and metamorphic amphiboles, suggesting that the amphibole-forming reactions were continuously retrogressive. Retrograde metamorphism is generally interpreted to have involved seawater-derived fluids, but the trace element contents of the coronitic and matrix amphiboles do not differ significantly from those of the original minerals (i.e., clinopyroxene and plagioclase). One sample of gabbro mylonite (KH07–02-D18–1) contains amphiboles with high concentrations of light rare earth elements, indicating a large influx of externally derived LREE-enriched fluids. These fluids are interpreted to have formed from an interaction between hydrous silicate melt with LREE-enriched composition and seawater-derived fluid. Our results suggest that multiple phases of melt–fluid infiltration occurred during the development of the detachment fault at the Godzilla Megamullion.

three distinct regimes with different abundance of rock types on the Gakkel Ridge: a western volcanic zone, sparsely magmatic zone on central zone and an eastern volcanic zone. The sparsely magmatic zone is characterized by abrupt morphological changes with no volcanic ridges and large exposures of mantle peridotite. Previous geochemical study proposed that osmium isotopic results of some refractory peridotites have 2 billion years-old, implying the long-term preservation of refractory peridotites in the asthenospheric mantle and the heterogeneous mantle existed beneath the Gakkel Ridge is the consequence of ancient melting, combined with subsequent melt percolation and entrapment. an olivine fabric in such heterogeneous mantle has not studied in detail. Here, we the detailed rock descriptions, analyses of mineral fabrics, and geochemical analyses of the minerals in the deformed peridotite samples recovered from two dredge sites (PS59-235 and PS66-238) on the

The Santa Elena Ophiolite (SEO), located on the northwestern coast of Costa Rica, consists primarily of preserved oceanic mantle and crustal rocks thrust above an accretionary complex. The SEO is predominantly characterized by mantle peridotites (i.e., primarily spinel lherzolite with minor amounts of harzburgite and dunite) cut and intruded by minor pegmatitic gabbros, layered gabbros, plagiogranites, and doleritic and basaltic dykes. Previous studies have concluded that the complex formed in a suprasubduction zone (SSZ) setting based on the geochemical nature of the layered gabbros and plagiogranites (i.e., depleted LREE and HFSE and enriched LILE and Pb), as well, as the peridotites (i.e., low-TiO2, Zr, and V, and high MgO, Cr, and Ni)(Denyer and Gazel, 2009). Eighteen ultramafic samples collected during the winter 2010/2011 field season (SECR11) exhibit abundant evidence for melt-rock reaction (e.g., disseminated plagioclase and plagioclase-spinel, clinopyroxene-spinel, and plagioclase-clinopyroxene symplectites) and provide a unique opportunity to characterize the textural and chemical nature of melt-rock reaction in the SEO. We present the results of a petrologic investigation (i.e., petrography and electron probe microanalysis) of 28 thin sections (19 spinel lherzolites, of which 14 are plagioclase-bearing, 4 pyroxenite veins, and 5 harzburgites) derived from the SECR11 sample set. Major element analyses of spinels (N=788) in the peridotites exhibit chemistries characteristic of abyssal peridotites that overlap with forearc peridotites (e.g., Cr#s 7-57, Mg#s 48-81, TiO2 0.01-0.77). In some cases, we observe extreme variations in spinel chemistry between grains separated by no more than 300 um. The results of this investigation have the potential to better our understanding of the nature of melt generation and migration and melt-rock interaction in the SEO mantle section and shed further light on the complex petrogenetic history of the SEO.