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The ophiolitic Villa Clara serpentinite-matrix mélange, central Cuba, forms part of the large ophiolitic belt of the Greater Antilles. The composition of ultramafic and mafic rocks allows classifying them into two main groups, revealing a complex multi-stage formation of oceanic lithosphere in varied tectonic settings: i) group 1 matches fertile MORB-like mantle typical of abyssal/transform fault peridotites, while ii) group 2 shows characteristics of refractory mantle-wedge forearc peridotites. These features are consistent with moderate extent of partial melting (4–8%) of depleted mantle at a mid-ocean ridge setting that created residual abyssal peridotite (group 1) and intense remelting (14–18%) of the latter in a forearc setting that created highly refractory peridotite (group 2). The elemental and isotopic characteristics of groups 1 and 2 serpentinites denote the influx of fluids evolved from the subducting slab. Mafic rocks appear as variably sized tectonic blocks within serpentinite that underwent variable alteration at low pressure. Immobile element concentrations allow identifying two main groups: i) group 1 is consistent with forearc basalts formed during the second stage of melting of (abyssal) peridotite, while ii) group 2 has a compositional pattern like island-arc tholeiitic basalts. A geodynamic evolution from abyssal to forearc settings is presented to explain the nature of the oceanic lithosphere of the Villa Clara ophiolitic bodies and ensuing serpentinization and mélange formation. This model may explain other ophiolitic bodies of the Caribbean arc in Cuba, Dominican Republic and Puerto Rico and elsewhere formed in abyssal/transform fault or forearc settings.

Exotic blocks of massive antigorite-serpentinite (antigoritite) document a deep-seated subduction channel in the Villa Clara serpentinite-matrix mélange, central Cuba. The petrological and geochemical characteristics of antigoritite allow distinguishing two types of rock: i) antigoritite and ii) dolomite-bearing antigoritite. Both types are intimately related in field exposures and represent deep peridotite infiltrated by HO-CO fluid mixtures that triggered antigoritization and local carbonation. Fluid infiltration continued after antigoritization forming a vein network as a potential response to hydrofracturing that precipitated tremolitite in the veins and triggered fluid-antigoritite reaction forming blackwalls. The mineralogical and chemical zoning in the blackwalls (Atg + Chl + Tr adjacent to antigoritite and Chl + Tr adjacent to the tremolitite vein) attest for multi-step metasomatic processes during fluid-rock interaction characterized by advection of infiltrating fluid towards the blackwall and, possibly, by diffusion out of the blackwall towards the fluid-filled vein. Tentative thermodynamic modeling of the blackwall domain Atg + Chl + Tr points vein network formation at 400–500 °C and 5–10 kbar during exhumation in the subduction channel, suggesting the infiltration of deep-seated pressurized fluid that triggered hydrofracturing. The chemical compositions of antigoritites, veins and blackwalls indicate a LILE- and LREE-enriched fluid evolved from the subducting plate, while Sr[sbnd]Nd isotope systematics are compatible with an external fluid composed of a mixture of fluids evolved from sediments and, probably to a lesser extent, altered oceanic crust.

Exotic blocks of massive antigorite-serpentinite (antigoritite) document a deep-seated subduction channel in the Villa Clara serpentinite-matrix mélange, central Cuba. The petrological and geochemical characteristics of antigoritite allow distinguishing two types of rock: i) antigoritite and ii) dolomite-bearing antigoritite. Both types are intimately related in field exposures and represent deep peridotite infiltrated by H2O-CO2 fluid mixtures that triggered antigoritization and local carbonation. Fluid infiltration continued after antigoritization forming a vein network as a potential response to hydrofracturing that precipitated tremolitite in the veins and triggered fluid-antigoritite reaction forming blackwalls. The mineralogical and chemical zoning in the blackwalls (Atg + Chl + Tr adjacent to antigoritite and Chl + Tr adjacent to the tremolitite vein) attest for multi-step metasomatic processes during fluid-rock interaction characterized by advection of infiltrating fluid towards the blackwall and, possibly, by diffusion out of the blackwall towards the fluid-filled vein. Tentative thermodynamic modeling of the blackwall domain Atg + Chl + Tr points vein network formation at 400–500 °C and 5–10 kbar during exhumation in the subduction channel, suggesting the infiltration of deep-seated pressurized fluid that triggered hydrofracturing. The chemical compositions of antigoritites, veins and blackwalls indicate a LILE- and LREE-enriched fluid evolved from the subducting plate, while Srsingle bondNd isotope systematics are compatible with an external fluid composed of a mixture of fluids evolved from sediments and, probably to a lesser extent, altered oceanic crust.

Volcanic rocks of the Zamoranos Formation exposed in the Betic Cordillera of southern Spain reveal Late Triassic CAMP volcanism extending from southwest to southeast of the Iberian massif. Comparison with volcanic rocks from the Upper Triassic of the Algarve region in Portugal enables correlation between these different magmatic provinces based on the striking geochemical similarity. The studied mafic magmatic rocks in the Betic Cordillera reveal three volcanic episodes during the deposition of the Zamoranos Formation with two types of geochemical affinity: alkaline (Group 1) and sub-alkaline (Group 2 and 3). The sub-alkaline rocks resemble the studied Triassic rocks of Portugal and the magmatism described as CAMP on a global scale. In the present study, the identification of alkaline rocks preceding the main CAMP sub-alkaline phase by ca. 15 Ma, is the first evidence of alkaline precursors prior to CAMP magmatism and indicates a change from a prior deeper enriched mantle source to a subsequent, shallower one. The temporal evolution of alkaline magmatism followed by a CAMP-type sub-alkaline magmatism is best explained in a context of initial rifting during Norian-Rhaetian times. It is inferred that rifting gradually developed from west to east in the southern area of Iberia.

An increasing number of seismological studies report transient seismicity clusters in the mantle wedge several kilometers above the subduction interface. Their physical significance with respect to subduction zone seismo-tectonics remains poorly understood. Jadeitites are known to form and/or be associated with mantle wedge serpentinites in the c. 30–70 km depth range, and thus may yield information on deformation mechanisms in this region of deep subduction environments. We herein document and compare brittle-viscous features recorded in jadeitites from Polar Urals (Russia), Kashin state (Myanmar) and Motagua fault region (Guatemala) – some of the most important jadeitite occurrences worldwide. In the Polar Urals we identified ultramafic-hosted pristine jadeitite-bearing veins c. 1 km above a Devonian paleo-subduction interface, interpreted as metasomatized former felsic dyke networks crosscutting the mantle wedge peridotites. Here, both jadeitites and associated amphibole-rich dark granofels display widespread brittle-ductile deformation fabrics such as shear bands, foliated cataclasites and breccias, cemented through dissolution-precipitation processes by omphacite and sodic amphiboles, a mineral assemblage typical of high-pressure–low-temperature subduction zone conditions. Electron probe and laser ablation ICP-MS mapping indicate that these brittle-viscous networks display a substantial metasomatic imprint highlighted in the dark granofels by variations in major and trace elements. Switches between viscous and brittle deformation patterns are attested by crystallographic-preferred orientations of jadeite in some of the shear zones that crosscut the host jadeitites. Strikingly similar mineral assemblages and deformation patterns were observed in the Kashin and Motagua samples. Observed deformation features in these localities can be classified into three categories (tectonic breccias, foliated cataclasites and hydraulic breccias), which may occasionally form in

Volcanic rocks of the Zamoranos Formation exposed in the Betic Cordillera of southern Spain reveal Late Triassic CAMP volcanism extending from southwest to southeast of the Iberian massif. Comparison with volcanic rocks from the Upper Triassic of the Algarve region in Portugal enables correlation between these different magmatic provinces based on the striking geochemical similarity. The studied mafic magmatic rocks in the Betic Cordillera reveal three volcanic episodes during the deposition of the Zamoranos Formation with two types of geochemical affinity: alkaline (Group 1) and sub-alkaline (Group 2 and 3). The sub-alkaline rocks resemble the studied Triassic rocks of Portugal and the magmatism described as CAMP on a global scale. In the present study, the identification of alkaline rocks preceding the main CAMP sub-alkaline phase by ca. 15 Ma, is the first evidence of alkaline precursors prior to CAMP magmatism and indicates a change from a prior deeper enriched mantle source to a subsequent, shallower one. The temporal evolution of alkaline magmatism followed by a CAMP-type sub-alkaline magmatism is best explained in a context of initial rifting during Norian-Rhaetian times. It is inferred that rifting gradually developed from west to east in the southern area of Iberia.

The physical and mechanical processes rooted in the hydrated, serpentinized mantle above subduction zones remain insufficiently explored despite fundamental implications for our understanding of rheology and fluid recycling along subduction interfaces. Through a field-based investigation, serpentinized peridotites and jadeitite samples from a fossil forearc mantle in the Polar Urals (Russia) are studied here to document fluid–rock interaction processes in the high-P field, as well as the long-term evolution of the base of the mantle wedge. Petrographic, geochemical and microstructural observations reveal a complex, protracted evolution of the jadeitite-forming fluid pathway throughout the gradual cooling of the forearc mantle and increasing serpentinization of the host. It is shown that the jadeitite lenses in the studied locality (a) derive for a large part from a trondhjemitic dyke earlier emplaced in a warm subduction environment, and (b) record the cooling of the subduction hangingwall under high-P conditions associated with increasing host serpentinization. In the studied locality, the majority of the jadeitites formed at relatively high temperatures (>600°C) by the influx of Na–Al-rich, slab-derived metamorphic fluids that were drained along the base of the mantle wedge, parallel to the subduction interface. Changes in bulk-rock geochemical signatures and in paragenetic sequences also constrain the compositional evolution of the fluid channelized along this drainage, with an increasing sedimentary component. The phlogopite-bearing walls of the dyke exhibit Rb–Sr and Ar–Ar ages ranging between c. 405 and c. 390 Ma, a range partly overlapping within uncertainty with the previously dated zircons from the jadeitite core (410–⁠400 Ma; U–Pb). This study opens a unique window on the pristine structures formed above the plate interface by melting and fluid–rock interaction in the early subduction stages, as well as their evolution during secular cooling of the base of

High pressure–low temperature metamorphic rocks from the late Paleozoic accretionary wedge exposed in central Chile (Pichilemu region) are characterized by a greenschist-blueschist litholog-ical association with interbedded metasediments that reached peak burial conditions of ~400 °C and 0.8 GPa during late Carboniferous times. We herein combine new extensive field observations, structural measurements, and geochemical and petrological data on vein and matrix material from Pichilemu transitional greenschist-blueschist facies rocks. The studied veins were first filled by albite, fol-lowed by quartz and calcite as well as glaucophane and winchite. Field, structural, and microscopic zoning patterns show that these rocks underwent a protracted sequence of prograde vein-opening events, which have been largely transposed to the main foliation before and during underplating in the basal accretion site near 25–30 km depth. While some of the earliest albite-filled vein sets may have formed after prograde breakdown of sub–green-schist facies minerals (<250 °C), our thermodynamic modeling shows that relatively minor amounts of fluid are produced in the subducted pile by dehy-dration reactions between 250 and 400 °C along the estimated geothermal gradient. It also confirms that the formation of interlayered blueschist and greenschist layers in Pichilemu metavolcanics is a consequence of local bulk composition variations, and that greenschists are generally not formed due to selective exhumation-related retrogression of blueschists. The early vein sets are a consequence of prograde internal fluid production followed by sets of hydrofractures formed at near-peak burial that are interpreted as a record of external fluid influx. We postulate that such a fractured sequence represents a close analogue to the high-Vp/Vs regions documented by seismological studies within the base of the seismogenic zone in active subduction settings.