Eclogitic Metagabbro Research Articles

Mobilization of isotopically heavy sulfur during serpentinite subduction.

Primitive arc magmas are more oxidized and enriched in sulfur-34 (34S) compared to mid-ocean ridge basalts. These findings have been linked to the addition of slab-derived volatiles, particularly sulfate, to arc magmas. However, the oxidation state of sulfur in slab fluids and the mechanisms of sulfur transfer in the slab remain inconclusive. Juxtaposed serpentinite and eclogitic metagabbro from the Voltri Massif (Italy) provide evidence for sulfur mobilization and associated redox processes during infiltration of fluids. Using bulk rock and in situ δ34S measurements, combined with thermodynamic calculations, we document the transfer of bisulfide-dominated, 34S-enriched fluids in equilibrium with serpentinite into adjacent metagabbro. We argue that the process documented in this study is pervasive along the subduction interface and infer that subsequent melting of these reacted slab-mantle interface rocks could produce melts that display the characteristic oxygen fugacity and sulfur isotope signatures of arc magmas worldwide.

Ti-Bearing Minerals: from the Ocean Floor to Subduction and Back

Abstract Rutile, titanite, and ilmenite are the most common Ti-bearing minerals in metamorphic rocks. Experimental constraints have shown that titanite is stable at low-grade metamorphic conditions, rutile at high pressure (HP), and ilmenite at high temperature, low pressure (HT-LP) conditions. Yet, petrological evidence suggests that titanite can also be stable at low temperature, HP (LT-HP). This implies that both titanite and rutile can be used to develop proxies to track HP metamorphism, which can have interesting applications. In this study, we have investigated the natural occurrence and chemistry of Ti-bearing minerals in gabbroic rocks and basalts that record different degrees of metamorphism, including LP amphibole-bearing gabbros from the ocean floor (Mid-Atlantic and Indian ridge IODP LEGs) and from an obducted ophiolite (Chenaillet) and HP Alpine metagabbros and metabasalts, including blueschist and eclogite facies rocks from the Western Alps and Corsica. We have performed detailed petrography, Raman spectroscopy and analyzed major and trace elements mineral chemistry using EPMA and LA-ICPMS. We found that rutile is stable at low pressure (< 2 kbar) in ocean-floor amphibole-bearing gabbros, lower than experimental constraints had previously suggested. Rutile is also found in eclogitic metagabbros from the Western Alps and can be chemically distinguished from LP rutile. Blueschist metagabbros from the Western Alps and eclogitic metabasalts from Corsica have titanite stable instead of rutile. While the titanite to rutile transition is pressure- and temperature-dependent, we demonstrate how small variations in bulk-rock Ti/Ca and Ca/Al values within the NCKFMASHTO chemical system may shift their stabilities. High-pressure titanite from these metamafic rocks exhibits La depletion and low La/SmN values in comparison to titanite from amphibolite-facies mafic rocks. La/SmN or Nb together with Yb and V can be used to distinguish HP titanite from titanite formed under other P–T settings. These new systematics can be useful in studies using detrital Ti-bearing minerals to probe the HP metamorphic record through time.

Inheritance versus subduction-related δ11B signatures of eclogites: Insights from the Voltri Massif (Ligurian Western Alps, Italy)

Trace element and isotopic compositions of exhumed high-pressure (P) mafic rocks are an important archive to investigate chemical processes in subduction zones. Here we report the B isotope (δ11B) composition of eclogitic mafic rocks enclosed in high-P serpentinite from the Voltri Massif, Ligurian Alps (Italy). Combined with bulk δ18O values, 87Sr/86Sr ratios and trace element data, the δ11B of eclogitic mafic rocks were investigated to test oceanic inheritance vs. subduction-related processes and to provide inferences on the timing of B uptake in eclogitic metagabbros.Petrographic observations along with major and trace element data indicate that the eclogite facies mafic rocks derived from primitive Mg-Al-bearing gabbros (Erro-Tobbio eclogites) and differentiated Fe-Ti-bearing oxide gabbros (Vara eclogites). Metarodingite from Vara shares similar REE pattern to that of the Erro-Tobbio eclogites, suggesting a plagioclase-rich gabbroic protolith. The studied rocks show variable enrichments in fluids-mobile elements (e.g., Cs, Ba, Li, Sr; high Li/Y, B/Nb). Boron concentrations range between 2.3 and 7.6 ppm, with no significant differences among the different samples. Oxygen isotope compositions (1σ ± 0.1‰) for the Vara and the Erro-Tobbio eclogites range from +5.4 to +6.4‰ and from +3.1 to +5.3‰, respectively, whereas the 87Sr/86Sr ratios range between 0.7036 and 0.7042 for the Vara eclogites and from 0.7030 to 0.7034 for Erro-Tobbio metagabbros. The B isotope compositions for the Vara eclogites range between −3.2 to +0.9‰, which are in striking contrast with the positive δ11B signatures of the Erro-Tobbio eclogitic metagabbros ranging from +4.3 to +8.9‰, the highest positive δ11B signatures observed in eclogitic metagabbros so far. Metarodingite from Vara has a δ18O of +3.7‰ and 87Sr/86Sr of 0.7046 and a δ11B value of +11.5‰.We argue that the δ18O signatures and the 87Sr/86Sr ratios of metamafic rocks from the Voltri Unit mainly reflect inherited signatures from the seafloor, whereas subduction-related processes are mainly traced through δ11B variations. Progressive B isotope fractionation due to dehydration processes during subduction is responsible for the dominantly negative δ11B signatures of eclogites from the Vara area and elsewhere, whereas the positive δ11B of the high-P metarodingite is compatible with an inherited oceanic signature, although interaction with fluids released by the surrounding serpentinites at peak conditions cannot be ruled out. We discuss three scenarios to explain the positive δ11B imprint reported for the Erro-Tobbio eclogites: (i) inheritance of oceanic signatures, (ii) records of B isotope fractionation from an extreme 11B-rich protolith, or (iii) interaction with 11B-rich slab fluids derived from serpentinite hydration at high-P. Based on these scenarios, two potential implications of global relevance are presented: (1) the transfer of largely unmodified oceanic signatures up to eclogite facies conditions, and (2) the formation of newly (subduction-related) 11B-rich reservoirs. We propose that the protolith composition of the mafic crust controls the development of hydrous phases at high-P thus enabling the preservation of high δ11B signatures. This mechanism of chemical transfer may generate a 11B-rich reservoir that, together with high-P serpentinites, may form positive δ11B domains in the mantle.

Fluid‐Mediated Mass Transfer Between Mafic and Ultramafic Rocks in Subduction Zones

AbstractMetasomatic reaction zones between mafic and ultramafic rocks exhumed from subduction zones provide a window into mass‐transfer processes at high pressure. However, accurate interpretation of the rock record requires distinguishing high‐pressure metasomatic processes from inherited oceanic signatures prior to subduction. We integrated constraints from bulk‐rock geochemical compositions and petrophysical properties, mineral chemistry, and thermodynamic modeling to understand the formation of reaction zones between juxtaposed metagabbro and serpentinite as exemplified by the Voltri Massif (Ligurian Alps, Italy). Distinct zones of variably metasomatized metagabbro are dominated by chlorite, amphibole, clinopyroxene, epidote, rutile, ilmenite, and titanite between serpentinite and eclogitic metagabbro. Whereas the precursor serpentinite and oxide gabbro formed and were likely already in contact in an oceanic setting, the reaction zones formed by diffusional Mg‐metasomatism between the two rocks from prograde to peak, to retrograde conditions in a subduction zone. Metasomatism of mafic rocks by Mg‐rich fluids that previously equilibrated with serpentinite could be widespread along the subduction interface, within the subducted slab, and the mantle wedge. Furthermore, the models predict that talc formation by Si‐metasomatism of serpentinite in subduction zones is limited by pressure‐dependent increase in the silica activity buffered by the serpentine‐talc equilibrium. Elevated activities of aqueous Ca and Al species would also favor the formation of chlorite and garnet. Accordingly, unusual conditions or processes would be required to stabilize abundant talc at high P‐T conditions. Alternatively, a different set of mineral assemblages, such as serpentine‐ or chlorite‐rich rocks, may be controlling the coupling‐decoupling transition of the plate interface.

Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps

AbstractA subduction complex of the northwestern Alps consists of serpentinites, eclogitic metagabbros, flysch‐like metasediments, meta‐ophicarbonates, and gneissic slices. Unlike other subduction complexes, it contains unusual hybridized rocks described here for the first time in the northwestern Alps. They are preserved as patches interstitial in the metagabbro and as layers within metagabbros and serpentinites. The hybridized rocks are made of high modal zoisite/clinozoisite + white mica pseudomorphs of lawsonite, garnet, and amphibole associated with an Alpine eclogite‐facies fabric. While these eclogitic metagabbros are chemically comparable to oceanic oxide gabbros from the ultraslow Southwest Indian Ridge, the layers are extremely enriched in Al2O3 and CaO and depleted in TiO2, MgO, and SiO2 relative to metagabbros. Patches have a geochemical signature that is intermediate between that of layers and metagabbros. Trace element compositions of hybridized rocks suggest a contribution from a fluid derived from a mixed source made of sediments and serpentinites. Except for Ba, Rb, and K, layers are comparable to the global subducting sediments, indicating a sedimentary contribution, whereas the enrichment in Cr indicates a serpentinite contribution. Metasediment dehydration and chemical exchange of Ca and Sr have resulted in significant lawsonite crystallization in the subduction zone, as reflected by the ubiquitous presence of lawsonite pseudomorphs. In light of the unique textures and geochemical signature of the lawsonite pseudomorph‐bearing hybridized rocks, an origin by fluid‐rock interaction and Ca‐metasomatism in the subduction environment is inferred and considered in the Western Alps context.

Lawsonite-bearing eclogite from a tectonic mélange in the Ligurian Alps: new constraints for the subduction plate-interface evolution

AbstractLawsonite eclogites are rare rocks and have been described from only a few localities in the world. Lawsonite-bearing assemblages are highly unstable and physico-chemical processes linked to exhumation may destroy them; only aggregates interpreted as pseudomorphs after lawsonite could be often recognized. In this paper, we present a detailed structural and petrological study of an area in the northwestern sector of the metaophiolitic high-pressure Voltri Massif (Ligurian Western Alps, Italy). The study area is characterized by a lawsonite-bearing eclogitic metagabbro associated with carbonated serpentinites and glaucophanic metasediments. The metagabbro body reached eclogitic metamorphic peak conditions atT= 465–477°C andP= 20.9–24.4 kbar, with H2O continuously supplied to the system. H2O under-saturated conditions, with the occurrence of both lawsonite and epidote, characterized the exhumation path. Both the low temperature recorded by the body and the occurrence of variously carbonated serpentinites led us to interpret this area as a portion of the top of the subducted slab, coupled with a ‘cool’ mantle wedge, where both aqueous fluids and carbonate-rich fluids were present. The occurrence of rocks belonging to different paleogeographic domains (e.g. continent versus ocean) and the multiple deformations recorded by the metagabbro suggest that this area was nearby the slab–mantle interface. This sector was thus affected by a shear regime that acted in a low-viscosity serpentinite channel, bringing these high-pressure rocks back to the surface.

Late Neoproterozoic gabbro emplacement followed by early Cambrian eclogite-facies metamorphism in the Menderes Massif (W. Turkey): Implications on the final assembly of Gondwana

Numerous (meta-)gabbroic dikes or stocks occur within the latest Neoproterozoic–early Cambrian series of the Menderes Massif (Anatolide–Tauride Block, western Turkey). These well-preserved rocks were locally converted into eclogitic metagabbros and garnet amphibolites along the contacts or shear zones. Both bulk-rock composition and compositions of igneous clinopyroxenes suggest continental tholeiitic affinity. U–Pb dating of igneous zircons from gabbroic rocks yielded a mean age of 563±1Ma (2σ), indicating emplacement during the latest Neoproterozoic (Ediacaran). On the other hand, rims of zircons from eclogitic metagabbro gave 535±3Ma (2σ) (early Cambrian), in addition to 558±3Ma (2σ) obtained from the igneous core of zircons. These ages are interpreted as the time of high-P metamorphism and crystallization age of gabbroic protolith, respectively. Given the estimated paleogeographic position of the Anatolide–Tauride Block during the late Neoproterozoic and early Cambrian, this orogenic event can be spatially and temporally related to the northward continuity of 600–500Ma orogenic event (Malagasy/Kuunga orogeny) extending from western margin of India, Madagascar, via Arabia up to northern margin of Gondwana beneath thick Phanerozoic cover series in Arabian Peninsula. Therefore, the high-P evolution of the basement of the Menderes Massif and associated basic intrusions can be interpreted to mark the latest stages of consumption of the basin/oceanic branches and final amalgamation of the Gondwana during the late Neoproterozoic–early Cambrian around the Arabian region.

Eclogitic metagabbro from the Lanzada Window, eastern Central Alps: confirmation of subduction beneath the Malenco Unit

Epidote-rich eclogitic metagabbro forms a small body within the Lanzada Window, upper Val Malenco, where it is associated with serpentinites and supracrustal rocks of the Lanzada–Santa Anna Zone (LSZ), which lies structurally beneath the Malenco unit. Conventional garnet–clinopyroxene geothermometry and garnet–clinopyroxene–phengite geobarometry indicate that the peak-metamorphic mineral assemblage (garnet + omphacite + epidote + phengite + titanite + apatite + pyrite) equilibrated at ca. 2.0 GPa and 525 °C. The bulk composition is Ca-rich (wollastonite-normative), suggesting that the rock underwent Ca-metasomatism prior to high-P metamorphism. The presence of eclogite within the LSZ strengthens the correlation of the LSZ with the blueschist-bearing Avers Bundnerschiefer, and confirms the former existence of a southerly-dipping subduction zone beneath the Malenco unit.

High-Mg garnets from pelitic schists adjacent to the Sebadani eclogitic metagabbro mass, Sambagawa metamorphic belt, central Shikoku, Japan

Pelitic schists adjacent to the Sebadani metagabbro mass in the Sambagawa metamorphic belt of central Shikoku, Japan consist mainly of garnet, phengite, albite and quartz, and small or scarce amounts of Na-Ca and Ca-amphiboles, epidote, omphacite, kyanite, rutile and carbonaceous matter. Garnets are zoned from pale reddish brown cores to pale yellowish-green or colorless rims. The cores of the garnets are Mg- and Ca-rich (MgO 3.50-11.30 wt%; CaO 7.27-12.98 wt%), and are overgrown by Fe-rich inner and outer rims. Kyanite occurs as inclusions in the high-Mg cores of the garnets. Inclusion assemblages of the high-Mg core of the garnets indicate relatively high-T conditions such as the kyanite eclogite facies (821 ± 32 °C and 19.4 ± 1.6 kbar). The cores of the garnets are anhedral with irregular embayments, and the outlines of the inner rims also show similar texture. The cores are also intensely fractured and filled by Fe-rich garnet with the same composition as the inner rims. The texture and chemical compositions of the high-Mg cores in the garnets are similar to those of garnets in the eclogites in the Sebadani metagabbro mass. The possible origin of the high-Mg cores are (i) in situ high-T regional metamorphism, (ii) detrital origin, and (iii) mechanical mixing of eclogites in the Sebadani metagabbro and surrounding pelitic schists. It is probable that high-Mg garnets were stripped from the Sebadani metagabbro mass and mechanically introduced into the pelitic schists when the Sebadani metagabbro mass was incorporated into the pelitic schists.

Geochronological Evidence of Pan-African Eclogites from the Central Menderes Massif, Turkey

The Menderes Massif in western Anatolia documents a complex geodynamic history from Precambrian to recent. Eclogitic relics found in metagabbros in the Precambrian basement were dated by the U/Pb method. The zircon age data from granulitic (coronitic) and eclogitic metagabbros is consistent with geological constraints as well as relative and radiometric ages of the host granulitic gneisses and augen gneisses. A Pan-African intrusion age (540 Ma) of the metagabbros and a shortly later eclogite facies event (530 Ma) are inferred. This scenario fits with the geodynamic evolution deduced form the acidic country rocks of the eclogitic metagabbros. Direct links between the Menderes Massif and Mozambique belt are obliterated by Alpine deformation. Nevertheless, the tectonic setting and age of the Menderes eclogites support a terminal collision of East and West Gondwana and the final suturing of the Mozambique Ocean during the Early Cambrian.

Crystal chemistry, structure analyses and phase transition experiment on an omphacite from eclogitic metagabbro from Syros island, Greece

Pyroxene samples, from the Greek island of Syros, taken from a blueschist-eclogite facies Mg-rich metagabbro, were investigated by chemical and XRD analyses and Mossbauer spectroscopy.

Origin of eclogitic metagabbro mass in the Sambagawa belt: Geological and geochemical constraints

Eclogite-bearing metagabbro masses in the high- P/ T Sambagawa metamorphic belt in SW Japan have long been regarded as tectonic blocks derived from the mantle wedge or hanging-wall side of the former Sambagawa subduction zone. The origin of one of the metagabbro masses (the Seba metagabbro) is reexamined by: (i) geological (chemico-structural) study focusing on the relationship between this body and the surrounding metasedimentary schists; and (ii) comparative major- and trace-element bulk-chemistry with other gabbros/metagabbros from the world. The geological studies reveal the presence of a marginal shear zone that: (i) is located between the enveloping pelitic schist and the central metagabbro; (ii) has a bulk chemical composition intermediate between the two; and (iii) shows up to 10 cm-scale prominent mafic–felsic banding. Based on the difficulty of forming such marginal shear zones by a tectonic block hypotheses, and key field observations in neighboring regions indicating formation of the marginal shear zone as sediments, the Seba metagabbro is concluded to have originated on the ocean floor as an olistolith. Comparative bulk-chemical studies show that Sr content of the Seba metagabbro is too high for gabbros derived from fracture zones associated with a spreading ridge. A comparison with other eclogitic metagabbro masses in the Sanbagawa belt reveals numerous similarities with the Seba metagabbro. This suggests all the metagabbro masses can be regarded as members of an ‘olistostrome complex’ that subducted and underwent the eclogite-facies metamorphism. The eclogitic complex includes the Western Iratsu mass that is considered to be an ancient seamount. This suggests that the metagabbro masses represent fragments of the ancient Western Iratsu seamount.

Accretionary Complex Origin of the Mafic-Ultramafic Bodies of the Sanbagawa Belt, Central Shikoku, Japan

In the high-grade Cretaceous Sanbagawa high-pressure (HP) metamorphic belt, our new 1:5000 scale mapping of eclogitic mafic-ultramafic bodies and their surrounding epidote-amphibolite—facies schists has revealed a duplex structure formed by the subduction of the Izanagi-Pacific oceanic plate. Lithologies of the two largest mafic-ultramafic bodies in the Sanbagawa belt, the Iratsu eclogite and the Higashi-Akaishi peridotite, strike WNW-ESE and dip N; the upper boundary with the surrounding schist is a normal fault, whereas the lower boundary is a thrust. The Iratsu body is subdivided into at least two tectonic units; the unit boundary is subparallel to a lithological boundary. Protoliths of the upper unit are gabbro, basalt, minor quartz rock, and pelite, and those of the lower unit are pyroxenite, gabbro, basalt, chert, and marble, in ascending order. The lower unit is characterized by layers of alternating eclogitic metagabbro and pyroxenite. The layers are extensive at the bottom of the Iratsu eclogite, and transient toward the Higashi-Akaishi body. Eclogitefacies metapsammite is intercalated between the Iratsu and Higashi-Akaishi bodies. Our mapping has revealed the following: (1) a duplex structure of the mafic-ultramafic bodies indicating their accretionary complex origin; (2) reconstructed oceanic plate stratigraphy in ascending order of peridotite, gabbro, basalt, limestone, minor chert, and pelite, suggesting that different parts of the protolith were derived from a mid-oceanic topographic high, an oceanic island or plateau, and an overlying trench turbidite; and (3) a change in the convergent motion of the oceanic plate from NW to NE during the accretion of the large oceanic island or plateau.

Thermobaric structure and metamorphic evolution of the Iratsu eclogite body in the Sanbagawa belt, central Shikoku, Japan

The Iratsu body is the largest eclogitic body in the Sanbagawa belt, central Shikoku, composed of schists with pumpellyite–actinolite, through blueschist/greenschist transition and epidote-amphibolite, up to eclogite-facies grades. The Iratsu body and the underlying Higashi-Akaishi peridotite body are situated in the intermediate structural level of the Sanbagawa belt which occurs as a subhorizontal thin slab. In the Iratsu body, eclogites and eclogitic metagabbros (garnet-clinopyroxenites) contain garnet+omphacite+epidote+quartz+rutile±phengite±glaucophane±paragonite±barroisite±kyanite, and garnet+sodic augite+rutile±epidote±zoisite±quartz, respectively, as mineral assemblages at the peak P– T conditions. Textural relations between these minerals and secondary minerals indicate that the eclogites were retrograded by hydration under eclogite through epidote-amphibolite to greenschist-facies conditions. Geothermobarometry for the peak metamorphism of the eclogites yielded conditions of 1.4–2.5 GPa and 500–800 °C in the Iratsu body; the estimated P– T conditions gradually change across the body. Eclogites with higher P– T estimates are predominant in the southern part of the body, and the eclogites near a contact with the Higashi-Akaishi body show the highest P– T conditions. Integration of these results with previously published P– T estimates for various rock types suggests a sandwiched thermobaric structure. The highest P– T area is situated in an intermediate structural level, i.e., in the upper part of the Higashi-Akaishi body near the boundary with the Iratsu body, and the metamorphic conditions systematically decrease towards the upper and lower levels not only within the Iratsu body but also in the surrounding lower grade schists. Our thermobaric structure implies that the Iratsu and Higashi-Akaishi composite mass is not a tectonic block but is the integral part of the Sanbagawa belt, which has been formed together with the surrounding schists under a specific thermal gradient at depths in the subduction zone. The Sanbagawa metamorphic facies series, newly proposed, draws a convex curve towards the lower P and higher T side in a P– T diagram, and is similar to a steady-state P– T path in a subduction zone, calculated by numerical models.

O–H isotope ratios of high pressure ultramafic rocks: implications for fluid sources and mobility in the subducted hydrous mantle

We examine the O–H isotope signatures of Alpine ultramafic rocks and eclogitic metagabbros of the Erro–Tobbio peridotite Unit (western Italian Alps), which record a subduction and exhumation cycle. Localization of subduction-related deformation along serpentinite mylonite shear zones favoured preservation of pre-subduction mantle and low temperature (oceanic) alteration assemblages within undeformed (meta)peridotite that underwent partial static recrystallization to high-pressure metamorphic parageneses. Bulk rock and mineral separate (clinopyroxene and serpentine) oxygen isotope ratios of the serpentinized mantle peridotites (5–8‰) are slightly enriched in 18O compared with those of the high-pressure metaperidotites and the serpentinite mylonites (4.4–7.6‰). The lowest values occur in high-pressure veins (3.5–5.7‰) and eclogitic metagabbros (3.1–5.3‰). These variations are comparable to variations observed in modern oceanic rocks and in non-subducted ophiolites. Preservation of pre-eclogitic δ18O signatures of the Erro–Tobbio rocks and a lack of oxygen isotope re-equilibration between different shear zones imply local-scale fluid flow at low water/rock ratios and closed system behaviour during high-pressure metamorphism. Different serpentine generations show a bimodal distribution in δD values: pre-eclogitic lizardite and chrysotile range from –102 to –77‰; high-pressure antigorite in the mylonites and in low strain metaperidotites range from –71 to –57‰ and –83 to –60‰, respectively. Comparable ranges occur in antigorite in the associated high-pressure veins, suggesting that the hydrogen signatures were acquired prior to veining. We propose that the isotopic variations reflect multiple events of fluid uptake in different geodynamic environments. The H- and O-isotope ratios in the eclogitic mylonites suggest that initial hydration occurred over a range of temperatures during local interaction with altered seawater along oceanic shear zones. The 18O-enriched and H-depleted compositions of chrysotile and lizardite in the mantle peridotites suggest that a second hydration event may have occurred as a result of interaction with metamorphic fluids at the early stages of burial in a forearc setting, where slabs undergo large-scale, low-temperature fluid fluxing. The oceanic mantle is thus a candidate for continuous hydration during its oceanic and early subduction history. The Erro–Tobbio unit thus represents an example of cycling of internally-derived fluids, whereby the different structural and textural domains behaved as relatively closed systems to fluid circulation during high-pressure metamorphism.

Pan-African high-pressure metamorphism in the Precambrian basement of the Menderes Massif, western Anatolia, Turkey

The Menderes Massif is made up of Pan-African basement and a Paleozoic to Early Tertiary cover sequence imbricated by Late Alpine deformation. The Precambrian basement comprises primarily medium- to high-grade schists, paragneisses, migmatites, orthogneisses, metagranites, charnockites, and metagabbros. High-pressure relics in the Pan-African basement are divided into two groups: eclogites and eclogitic metagabbros. The mineral assemblage in the eclogites is omphacite (Jd 44)–garnet–clinozoisite–rutile. The eclogites occur as pods and boudinaged layers in the basement schists and paragneisses. Inclusions found in the cores of the garnets indicate a medium-pressure protolith. The eclogitic metagabbros are closely related to Precambrian gabbroic stocks. The igneous texture and relic magmatic phases are preserved in these high-pressure rocks, which are characterized by the mineral assemblage omphacite (Jd 25)–garnet–rutile±kyanite. The P–T conditions of the Pan-African high-pressure metamorphism in the eclogites are estimated to be 644°C with a minimum pressure of approximately 15 kbar. The eclogites are partly to completely retrograded to garnet amphibolites by a Barrovian-type overprint which developed under isothermal decompression conditions. For this post-eclogitic event, the P–T estimates are 7 kbar and 623°C. The eclogite relics provide strong support for a correlation of the Menderes Massif with the Bitlis Massif in terms of common Pan-African high-pressure evolution.

STRUCTURES AND MICROSTRUCTURES IN A THRUST-RELATED, GREENSCHIST FACIES TECTONIC MÉLANGE, VOLTRI GROUP (NW ITALY)

The ophiolitic Voltri Group in the eastern part of the Ligurian Alps (NW Italy) is made up of a number of thrust sheets emplaced during Alpine collision. These thrust sheets include (1) the Voltri-Rossiglione calcschist unit of Mesozoic high-pressure calcareous micaschists, metavolcanics and slices of serpentinite, overlain by (2) the Beigua serpentinite unit, of mainly antigorite serpentinite and eclogitic metagabbro, in turn overlain by (3) the Erro-Tobbio peridotites. In the northern part of the Voltri Massif, a conspicuous melange-type lithology occurs along the contact of the Beigua serpentinite unit and the Voltri-Rossiglione calcschist unit. In the vicinity of the contact, the structure in the hanging-wall serpentinites is dominated by kink-type crenulations. Towards the base of the serpentinite nappe these crenulations become intense, and veins and patches of talc + chlorite + tremolite + carbonate replace the original antigorite-dominated assemblage. The thrust itself is marked by a layer, at least several tens of metres thick, of intensely deformed and foliated antigorite-bearing talc-chlorite-tremolite schist, enclosing rounded and lense-shaped (phacoid) blocks, up to 25 metres across, of retrogressed eclogitic metagabbro, antigorite serpentinite, metabasic rock, calcschist and schistose micaceous marble. The main features of this chaotic lithology meet the descriptive criteria of a tectonic melange. The structures and assemblages in the wall rock units and those in the melange indicate that the melange lithology developed at a relatively late stage of greenschist facies ductile thrusting, emplacing the Beigua unit onto the rocks of the Voltri-Rossiglione unit. The structures indicate that the blocks and lenses were formed during localized deformation in a relatively narrow zone along the thrust plane via intense stretching and boudinage of the various lithologies in the foot- and hanging wall. The development of talc-chlorite-tremolite-carbonate assemblages at the expense of the overriding antigorite serpentinites require significant calciummetasomatism, hence extensive fluid activity, whilst the microstructures in the melange matrix suggest that the talc-chlorite-tremolite-carbonate assemblage was mechanically weak. It is suggested that both fluid activity and the associated metamorphic reactions strongly facilitated ductile to semi-brittle deformation along the thrust, leading to progressive fragmentation and mixing of the different lithologies and development of a tectonic melange.

Chlorine cycling during subduction of altered oceanic crust

Eclogitic rocks that have experienced devolatilization, with little or no interaction with external fluid sources, can be viewed as analogues of crustal material which may be transferred back into the mantle during subduction. Thus they can be used to evaluate the extent of the recycling of volatile elements, such as chlorine. We report new oxygen isotope ratios of omphacite, and fluid inclusion data determined from eclogitic metagabbros, of the Rocciavré massif (Italian Alps). The data are compared with those obtained for the Monviso, Cyclades and the Franciscan Complex high-pressure rocks. In all localities, relics of early dehydration fluids are preserved as primary fluid inclusions in the cores of omphacite megacrysts (Rocciavré, Monviso and Franciscan Complex) or garnet (Cyclades). Salinity estimates of the inclusion fluids range from 32 to 45 wt% NaCl in Rocciavré, 17 to 21 wt% NaCl in Monviso, and are similar to seawater in other areas. Omphacite and bulk-rock δ 18O values of Rocciavré (5.1–6.8‰) and Monviso (3.0–5.3‰) metagabbros are markedly lower than those of Cyclades (6.8–14.3‰) and the Franciscan (6.7–13.1‰) metabasites. The fluid salinity–δ 18O systematics of eclogitic rocks is similar to that documented along a typical section of the altered oceanic crust and unmetamorphosed ophiolites. This suggests that high-pressure metamorphism, and associated processes, did not modify significantly the variability in chlorine concentrations and oxygen isotope ratios, inherited from a stage of sea-floor hydrothermal alteration under low- (basaltic layer) and high-temperature (gabbroic layer) conditions, respectively. Extrapolating the estimated H 2O and Cl contents of eclogitic rocks to a representative section of the subducted oceanic crust indicates that a minimum of 100 to 200 ppm Cl could be recycled into the mantle during subduction. This yields a Cl/H 2O ratio of 3.6 to 7.5×10 −2 for the subducted oceanic crust, which is similar to E-MORB. On the basis of available Cl isotopic data, we infer that a large proportion (70%) of the Cl stored in the altered crust should be recycled to the mantle to generate an isotopic composition of the subducted crust equivalent to the source of unaltered mid-ocean ridge basalt (δ 37Cl = 4.7‰).

Age and nature of protoliths in the Caledonian blueschist-eclogite complex of western Spitsbergen: a combined approach using UPb, SmNd and REE whole-rock systems

High-pressure metamorphic rocks (≈20 kb) (eclogites, eclogitic metagabbros and glaucophane-schists) from western Spitsbergen were investigated for major and trace element (including REE) geochemistry, as well as Nd isotopes and UPb zircon characteristics. The distinct nature of protoliths of the various high-pressure rocks has been preserved to some extent. Eclogites and eclogitic metagabbros represent Lower Proterozoic (2.1 Ga) alkali basalts and transitional MORB, respectively. Glaucophane-schists probably represent Upper Proterozoic spilitized FeTi gabbros or volcaniclastic mixtures, or both. The dating results indicate that the Caledonian high-pressure metamorphism occurred at ca. 475 Ma, whereas a Lower Proterozoic magmatic activity — derived from a depleted mantle source — is also recognized. This event is the oldest magmatism detected so far in the Svalbard Caledonides. The age of the protolith of the glaucophane-schists is not strictly constrained; however, it formed between 1.0 Ga and 475 Ma.

Retrograde fluid inclusions in eclogitic metagabbros from the Ligurian Western Alps

Quartz veins and apatites of the eclogitic Fe-Ti metagabbros from the Ligurian Western Alps contain a variety of aqueous fluid inclusions. The quartz veins are related to fluid flow occurred at different stages of the metamorphic history and formed on a pressure - temperature span ranging from high pressure to greenschist facies conditions. The microstructures and microthermometry allow to relate all the analyzed inclusions to the greenschist retrogression of the eclogitic mineral assemblages. Two main types of inclusions can be distinguished: CO 2 -free aqueous inclusions and CO 2 -H 2 O-rich inclusions with a few moles percent N 2 mixed with the CO 2 . No clear relationships have been observed between the above two groups, but the textural and microthermometric features allow to relate the aqueous fluid inclusions in apatite to the earliest episodes of syn-greenschist fluid infiltration. Fluid/rock interactions during water consuming reactions may have the major effect to concentrate salts and CO 2 (when present) in the metamorphic brine. The interaction between the experimental curves defining the stability field of the greenschist assemblages and the calculated isochores of inclusions in apatite, give an approximate trapping estimate of about 3 kbars and 500 o C. Such evaluation consents to derive anomalously high geothermal gradients, related to high uplift rates occurring at the brittle-ductile transition, and preventing the thermal reequilibration of rocks at depth