Fe Ti Gabbros Research Articles

The subduction and exhumation history of the Voltri Ophiolite, Italy: Evaluating exhumation mechanisms for high-pressure metamorphic massifs

The problem of how dense high- (HP) and ultrahigh-pressure (UHP) metamorphosed oceanic mafic and ultramafic rocks are exhumed from deep within subduction zones is crucial to understanding processes occurring at the interface between the subducting slab and the overlying plate and mantle wedge. In this study, we use the P-T-t evolution of metamorphosed FeTi gabbros hosted in meta-serpentinite from the Voltri Ophiolite (Ligurian Alps, Italy) to evaluate potential exhumation mechanisms and assess whether HP mafic rocks were exhumed as blocks within a complex serpentinite-hosted mélange or as coherent sections of oceanic lithosphere. A number of observations provide evidence for subduction and exhumation of the central portion of Voltri Ophiolite as a coherent package of ultramafic and mafic material: (1) a similar evolution in mineral assemblages, textures and compositions suggesting peak metamorphism in the eclogite-facies, followed by a retrograde evolution through blueschist-, amphibolite- and greenschist-facies conditions; (2) uniform P-T conditions of both initial garnet growth (~475–480 °C and 2.0–2.2 GPa) and peak metamorphism (~500–525 °C and 2.3–2.5 GPa); and (3) bulk SmNd garnet ages that reveal a very tight grouping of ages for four well-equilibrated foliated FeTi meta-gabbros clustering at 39.98 ± 0.84 Ma. A massive undeformed gabbro also from the central Voltri area gave a slightly younger age of 38.15 ± 0.89 Ma, which is interpreted to reflect kinetic overstepping and delayed growth of garnet. By contrast, an SmNd garnet age from the NW portion of the Voltri Ophiolite indicates peak eclogite-facies metamorphism occurring ~10–12 Ma earlier at 49.68 ± 0.35 Ma. A compilation of the current geochronological data suggests that the NW portion of the Voltri Ophiolite underwent peak eclogite-facies metamorphism first at ~50 Ma, the sedimentary portion of the Voltri Unit (the Voltri-Rossiglione meta-sediments) experienced peak metamorphism around ~47–44 Ma, whilst subduction and peak metamorphism of the central part of the ophiolite occurred later at 41–38 Ma. Alongside recent studies of other Alpine ophiolites (Zermatt Saas, Monviso, Alpine Corsica), this study suggests that detachment and buoyant exhumation of large kilometric-scale coherent sections of slab material, rather than exhumation within a chaotic serpentinite mélange, is the dominant mechanism of preserving and returning eclogite-facies metamorphic rocks to the surface in the Western Alps. In the case of the Voltri Ophiolite, which lacks any associated continental massif, this exhumation was likely aided by the buoyancy of the serpentinite-dominated oceanic lithosphere, rather than by coupling to continental crust.

Subduction of the Mesoarchaean spreading ridge and related metamorphism, magmatism and deformation by the example of the Gridino eclogitized mafic dyke swarm, the Belomorian Eclogite Province, eastern Fennoscandian Shield

A model of Mesoarchaean spreading ridge subduction and related metamorphism, magmatism and deformation in an active continental margin is presented here. The eclogites exposed along northeastern boundary of the Belomorian orogen in the eastern Fennoscandian Shield were formed as a result of Mesoarchaean–Neoarchaean subduction and collision. The Belomorian eclogite province contains two eclogite associations have been recognized within TTG gneisses: (1) the subduction-type Salma association and (2) the Gridino mafic dykes. Protoliths of the Salma eclogites represent a sequence comprising oceanic layered gabbro, formed at ∼2.9 Ga in a slow-spreading ridge. The subduction began in a time span 2.9–2.87 Ga, and eclogite-facies events occurred between ca.2.87 and ca.2.78 Ga. Injection of a mafic magma into an active continental margin, recorded by the Gridino dyke swarm, is attributed to subduction of a mid-ocean ridge, commencing at 2.87 Ga. The dykes represent typical fissure intrusions and underwent metamorphism to eclogite, granulite and amphibolite facies. The dykes divide into two compositions (Fe-Ti gabbro and Mg-Cr gabbronorite) which represent alternating episodes of arc and slab window magmatism. The Fe-Ti gabbros have high Ti and low abundances of fluid-mobile and other LIL elements compared to HFSE and plot in the MORB and within-plate oceanic basalt fields. The Fe-Ti gabbros formed from low-degree of partial melting from a depleted mantle source. This petrogenesis is consistent with slab window model in which magmas were derived by decompression melting of suboceanic mantle that upwelled into the opening beneath continental margin following spreading ridge subduction. The Mg-Cr gabbronorites have low Ti along with high abundances of fluid-mobile elements and other LILE compared to HFSE and plot in arc basalt fields that imply melting of subduction-modified hydrous subcontinental mantle.As the mid-ocean spreading ridge entered a trench, continental rocks above subduction zone underwent deformations with pseudotachylyte-bearing strike-slip faults development that is an evidence of a fossile earthquake. Paleoseismic event was limited to crustal conditions above the brittle-ductile transition, corresponding to temperatures below 450 °C and depths above 15 km. The slab window opening induced upwelling of anhydrous mantle and formation of the Fe-Ti gabbro dykes. The Mg-Cr gabbronorite was formed at slab window edges from metasomatized mantle.Summarizing evidences, such as ophiolites, eclogite, arc magmatism, we suggest that the modern style of subduction and plate tectonics began already in Mezoarchaean time 2.9-2.8 Ga ago. Special aspect of the paper is an argumentation of Meso-Neoarchaean age of eclogite facies metamorphism in the Belomorian eclogite province.

The Composition and Age of the Mesoarchean Gabbro in the South Vygozersky and Kamennoozersky Greenstone Structures, Karelia

The geochemical and zircon geochronological (U-Pb, SHRIMP-II) study of Mesoarchean gabbros of the South Vygozersky and Kamennoozersky greenstone structures of Central Karelia made it possible to distinguish four gabbro types: (1) Fe–Ti gabbro, 2869 ± 12 Ma, (2) gabbro compositionally close to tholeiitic basalts, 2857 ± 7 Ma, (3) leucogbabbro, 2840 ± 5 Ma; and (4) melanogabbro, 2818 ± 14 Ma. From the early to late gabbros, the rocks are depleted in Ti, Fe, V, Y, Zr, Nb, Hf, REE and enriched in Mg, Ca, Cr, Ni. According to the systematics (Condie, 2005), the Nb/Y, Zr/Y, Zr/Nb ratios in the studied Late Archean gabbros are close to those of primitive mantle, while the gabbros in composition are similar to those of plumederived ocean-plateau basalts. Their magma sources were derived from different mantle reservoirs. The leucogabbro and melanogabbro with similar eNd = +4 were derived from a depleted mantle source (DM). The gabbro close in composition to tholeiitic basalts and having the elevated positive eNd (+4.9) was derived from a strongly depleted mantle source. Insignificant admixture of crustal material or lithospheric mantle is inferred in a source of the Fe–Ti gabbro (with lowest eNd = +2.1).

Geochemistry of metamorphic processes in mafic rocks of the Krasnaya Guba area, Belomorian Mobile Belt

New data are obtained on the major-component and minor- and trace-element composition of metamorphosed gabbronorites and Fe-Ti gabbro in the Krasnaya Guba area, Belomorian Mobile Belt. These two magmatic complexes, which are spatially combined but are of different age and were produced in different geodynamic environments, were simultaneously metamorphosed in the Paleoproterozoic to parameters transitional between the amphibolite and eclogite facies. Eclogite metamorphism was associated with perceivable changes in the concentrations of alkalis and alkali earth elements, and the metamorphic transformation of the plagioclase eclogite (which can be metamorphosed gabbronorite and/or gabbro) into amphibolite and biotite amphibolite was associated with changes in the concentrations of practically all trace elements. This implies that the metamorphic processes were not isochemical and were accompanied by significant removal and introduction of incompatible elements. However, some trace-element ratios (such as Ti/Y, Ti/Zr, Zr/Y, La/Sm, and Nb/Th) remained unmodified in the course of metamorphism and remained the same as in the pristine gabbronorite or gabbro and likely can be employed as indicators of the composition of the probable protolith and the conditions under which it was produced.

Mesoproterozoic high Fe–Ti mafic magmatism in western Shandong, North China Craton: Petrogenesis and implications for the final breakup of the Columbia supercontinent

High Fe–Ti basic rocks (FeOt>12wt% and TiO2>2wt%) are rare in the geological record and display a different evolutionary trend (the ‘Fenner’ trend) from the normal ‘Bowen’ trend of SiO2 enrichment. In this paper, we present geochronological, geochemical and Sr–Nd isotopic data for the Fe–Ti-rich gabbros from western Shandong within the Eastern Block of the North China Craton. Zircon U–Pb dating indicates that they were emplaced at ∼1.21Ga. All high Fe–Ti gabbros are characterized by low MgO (<7wt%) and Mg numbers (<50), and high FeOt (>12wt%), TiO2 (>2wt%) and P2O5 (mostly >0.4wt%). Geochemically, they can be divided into two different series (the alkaline series for the Yishui gabbros and the sub-alkaline series for the Feixian gabbros). Both series of Fe–Ti-rich gabbros show similar depletion in Th, U, Nb and Ta, and have positive Eu anomalies. The main difference is that the alkaline series displays slightly more fractionated REE patterns and more depleted Sr–Nd isotopic compositions than the sub-alkaline series; it also lacks positive P anomalies. Elemental geochemistry indicates that both series were likely generated by the partial melting of lherzolitic mantle, with different proportions of garnet in their sources. The mantle sources were re-enriched through varying degrees of hybridization of lower crustal materials prior to partial melting. The results of modeling based on the Sr–Nd isotopic compositions show that their mantle sources could have formed from the hybridization of N-MORB-like depleted mantle through involvement of around 1% lower crustal components for the alkaline series and 2% for the sub-alkaline series: this accounts for their geochemical signatures, especially for the marked depletion in Th and U, and weak negative Nb and Ta anomalies. A combination of Fe-rich mantle source and subsequent fractional crystallization, coupled with low fO2 during magmatic evolution, likely resulted in the Fe–Ti enrichment trend in both series of gabbros. In combination with regional and global studies, their generation could be related to a prolonged superplume that led to the final breakup of the Columbia supercontinent during Mesoproterozoic time.

The Meso-Neoarchaean Belomorian eclogite province: Tectonic position and geodynamic evolution

The aim of this paper is to review the main features of the Meso-Neoarchaean Belomorian eclogite province (BEP) in the northeastern Fennoscandian Shield, including regional and local geology, geochemistry, petrology and geochronology and to compare the Belomorian eclogites with Precambrian eclogites elsewhere. Two eclogite associations have been recognized within Belomorian TTG gneisses: (1) the subduction-type Salma association and (2) Gridino eclogitized mafic dykes. Protoliths of the Salma eclogites represent a sequence comprising gabbro, Fe–Ti gabbro and troctolites, formed at ~2.9Ga in a slow-spreading ridge setting (like the Southwest Indian Ridge). The main subduction and eclogite-facies events occurred between ~2.87 and ~2.82Ga. Injection of mafic magma into an active continental margin setting, recorded by the Gridino dyke swarm, is attributed to subduction of a mid-ocean ridge, commencing at 2.87Ga. Crustal delamination of the active margin and subsequent involvement of the lower crust in subduction between 2.87 and 2.82Ga ago caused high-pressure metamorphism of the Gridino dykes, culminating in eclogite-facies conditions between 2.82 and 2.78Ga and accompanying amalgamation of the Karelia, Kola and Khetolamba blocks and formation of the Mesoarchaean Belomorian accretionary–collisional orogen. The clockwise P–T paths of the Salma and Gridino associations cross the granulite-facies P–T field. Detailed metamorphic studies indicate a complicated post-eclogite history with thermal events and fluid infiltration, related to plume activity at 2.72–2.70, ~2.4 and ~1.9Ga. The eclogite assemblages were exhumed to mid-to-lower crustal depths at ~1.7Ga, while erosion or younger tectonic events were responsible for final exhumation to the surface. Comparison of P–T–t paths and data for peak metamorphic parameters demonstrates the general similarity of the Archaean and Palaeoproterozoic eclogites worldwide and their association with anomalously “hot” environments. The occurrence of high-T conditions during eclogite-facies metamorphism can be attributed to either subduction of a mid-ocean ridge (Archaean, BEP) or to interaction with mantle plumes (Proterozoic).

Origin and tectonic significance of corundum–kyanite–sapphirine amphibolites from the Variscan French Massif Central

AbstractThe contact zone between two major allochthonous lithotectonic units in the French Massif Central (FMC) is characterized by the presence of corundum‐bearing amphibolites associated with serpentinites, flaser‐gabbros, eclogites and granulites. These unusual amphibolites are best preserved in the Western FMC, where they are found within the lower oceanic crust of the Limousin ophiolite. Mineralogical observations and thermodynamic modelling of the spinel–corundum–sapphirine–kyanite amphibolites in the CMASH system show that they were formed at peak P–T conditions around 800 °C/10 kbar in response to near isothermal burial followed by a retrogressive anticlockwise path. Metamorphic reactions are controlled both by modification of P–T conditions and by local chemical changes linked to fluid infiltration. Pargasite growth has been enhanced by infiltration of Ca‐ and Al‐rich fluids whereas kyanite‐ and sapphirine‐forming reactions are partly controlled by local inputs of MgO–SiO2 components, most probably during infiltration metasomatism. By analogy with worldwide ophiolites (Oman, Tethyan, Appalachian) and published numerical models, subduction of a still‐hot oceanic ridge is proposed to form these Al‐rich amphibolites from plagioclase‐rich troctolites. The trace‐element composition of high‐Ti, fine‐grained amphibolites (former fine‐grained Fe–Ti gabbros) adjacent to the corundum‐bearing ones, further indicates that the oceanic crust was initially created at a mid‐ocean ridge (rather than within a back‐arc basin), followed by the emplacement of supra‐subduction zone‐type magmas, probably due to intraoceanic subduction close to the ridge.

The Eastern Carpathians “ophiolites” (Romania): Remnants of a Triassic ocean

Mesozoic ophiolitic and related rocks in the Eastern Carpathians occur in three areas, from north to south: Rarău, Hăghimaş and Perşani Mts. They are found as blocks ranging from few metres to a few kilometers in size and as centimetre-sized in breccias, most likely embedded in the Late Barremian–Early Albian Wildflysch formation. Compositionally, they range from lherzolites and harzburgites to mafics such as FeTi gabbros, dolerites, basalts, and to andesites. The volcanics comprise highly-depleted basalts/andesites to enriched-type mid-ocean ridge basalts; additionally they include ocean island basalts and calc-alkaline basalts/andesites and trachytes. Based on paleontological evidence, their age is Middle to ?Late Triassic. They can be clearly compared with remnants of the Meliata–Hallstatt Ocean in the Western Carpathians, but do not match the Jurassic ophiolites and island arc volcanics in the Mureş Zone of the Southern Apuseni Mts. We propose a Triassic ocean connected with the Meliata–Hallstatt Ocean, between (a) the Bucovinian/Sub-Bucovinian continental crust, (b) the Infrabucovinian and finally (c) the Northern Apuseni microcontinents. This ocean closed in the Late Triassic to Early Jurassic causing close juxtaposition of all three microcontinents. An ophiolite complex together with ocean island basalts and calc-alkaline basalts/andesites remained from this ocean and was subsequently eroded and transported as blocks of different size into the Lower Cretaceous Wildflysch basin, together with blocks and clasts of limestones similar to the Mesozoic sedimentary sequences in the Northern Apuseni realm. The Wildflysch formation was thrust as an independent unit during the Albian over the Bucovinian Nappe in the Eastern Carpathians and the Northern Apuseni continental crust, respectively.

Jurassic dikes of Vestfjella, western Dronning Maud Land, Antarctica: Geochemical tracing of ferropicrite sources

Ferropicrites and their differentiates make up a geochemically distinctive group of dikes that crosscut Jurassic continental flood basalts of the Karoo large igneous province at Vestfjella, western Dronning Maud Land, Antarctica. The Vestfjella ferropicrites can be divided into two geochemical types: The depleted ferropicrites have (La/Sm) N of 1.2–1.3, (Sm/Yb) N of 4.5, initial ε Nd from + 7 to + 8, initial ε Sr from − 18 to − 19, and show relative depletion of highly incompatible elements, but pronounced enrichment of V; The enriched ferropicrites have (La/Sm) N of 1.7, (Sm/Yb) N of 5.1–5.4, initial ε Nd from + 3 to + 4, initial ε Sr from 0 to + 1, and show general enrichment of incompatible trace elements. The immobile incompatible element signatures of the ferropicrites have not been significantly affected by alteration, fractional crystallization, or contamination. Based on primitive olivine phenocrysts (Fo 79–88) and high ε Nd values, the depleted ferropicrites represent near-primary melts derived from anomalous hot mantle sources. Overall, geochemical compositions favor a pyroxenite source for the ferropicrites. Unusually high (V/Lu) N values of the depleted ferropicrites indicate an affinity to oceanic Fe–Ti gabbros and geochemical modeling favors such a recycled mantle source component in them. The enriched ferropicrites probably represent near-primary melts, but this cannot be confirmed. They may also record an exceptionally Fe-rich source component, but their high Fe contents stem at least partly from relatively low-degree melting at high pressures, as indicated by the high (La/Sm) N and (Sm/Yb) N ratios. Examination of a global ferropicrite dataset reveals that the recycled Fe–Ti gabbro component is detectable in many ferropicrites.

Trace element chemistry and U–Pb dating of zircons from oceanic gabbros and their relationship with whole rock composition (Lanzo, Italian Alps)

The U–Pb ages and the trace element content of zircon U–Pb along with major and trace element whole rock data on gabbroic dikes from the Lanzo lherzolitic massif, N-Italy, have been determined to constrain crustal accretion in ocean–continent transition zones. Three Fe–Ti gabbros were dated from the central and the southern part of the massif providing middle Jurassic ages of 161 ± 2, 158 ± 2 and 163 ± 1 Ma, which argue for magmatic activity over few millions of years. Zircon crystals are characterized by high but variable Th/U ratios, rare earth element patterns enriched in heavy rare earths, pronounced positive Ce and negative Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition coupled with whole rock chemistry was used to reconstruct the crystallization history of the gabbros. A number of gabbros crystallized in situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model in which the emplacement mechanism of gabbroic rocks in ocean–continent transition zones evolves from in situ crystallization to stratified crystallization with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the crystallization history is probably related to the thermal evolution of the underlying mantle lithosphere.

Geochemistry of eclogites from the Dabie–Sulu terrane, eastern China: New insights into protoliths and trace element behaviour during UHP metamorphism

The major and trace element compositions of nine eclogites from the Dabie–Sulu ultrahigh pressure (UHP) metamorphic terrane in eastern China were determined for both whole rock and the main constituent minerals, garnet and clinopyroxene. The results indicate that the eclogite protoliths originated from a basaltic magma, which formed in a continental setting as shown by isotopic and immobile element data. Based on the garnet REE characteristics, the eclogites can be roughly divided into two groups. Group 1 has LREE enrichment with no Eu anomaly for whole rock, and smooth LREE depletion but HREE enrichment pattern for garnet, whereas group 2 shows a depletion of LREE with a pronounced positive Eu anomaly and flat HREE pattern for both whole rock and garnet. From these features, we suggest that the protoliths for group 2 are Fe–Ti–gabbros with relatively high cumulus plagioclase and Fe–Ti oxide, whereas the group 1 eclogites are probably from basalts. Therefore, the unusual garnet REE pattern observed in group 2 can be considered as an important signature for identifying gabbro protoliths for eclogites. The identification of gabbro protoliths from the eclogites in the Dabie–Sulu terrane provides evidence for Neoproterozoic rift magmatism in the northern margin of the Yangtze craton. During ultrahigh pressure metamorphism in the Dabie–Sulu terrane, LILEs (including Ba, Rb, Th, U, K) had high mobility, but REEs and HFSEs were immobile, and trace element distribution equilibrium was approached between garnet and clinopyroxene. An estimate of mass balance indicates that garnet and clinopyroxene host the majority of HREEs and Y, and clinopyroxene is a significant host for Sr, but minor and accessory minerals predominantly account for LREEs, Th, U, and Zr.

Ca-rich Garnet–Clinopyroxene Rocks at Hujialin in the Su–Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates?

Layers of Ca-rich garnet–clinopyroxene rocks enclosed in a serpentinite body at Hujialin, in the Su–Lu terrane of eastern China, preserve igneous textures, relict spinel in garnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite, Fe-rich spinel, and also amphibole in clinopyroxene. In terms of their major and trace element compositions, the studied samples form a trend from arc cumulates towards Fe–Ti gabbros. Reconstructed augite compositions plot on the trend for clinopyroxene in arc cumulates. These data suggest that the rocks crystallized from mantle-derived magmas differentiated to various extents beneath an arc. The Ca-rich garnet þ diopside assemblage is inferred to have formed by compressing Ca-rich augite, whereas the relatively Mg-rich cores of garnet porphyroblasts may have formed at the expense of spinel. The protolith cumulates were subducted from near the crust–mantle boundary (c. 1 GPa) deep into the upper mantle (4 8 0 6 GPa and 750 50 C). Negatively sloped P–T paths for the garnet– clinopyroxene rocks and the corollary of corner flow induced subduction of mantle wedge peridotite are not supported by the available data. Cooling with, or without, decompression of the cumulates after the igneous stage probably occurred prior to deep subduction.

Titanian clinohumite–garnet–pyroxene rock from the Su-Lu UHP metamorphic terrane, China: chemical evolution and tectonic implications

A garnet–pyroxene rock containing abundant Ti-clinohumite (ca. 40 vol.%) occurs along with eclogites as small blocks in quartzo-feldsparthic gneiss in the southern end of the Chinese Su-Lu ultrahigh-pressure (UHP) metamorphic terrane. It consists of three aggregates: (1) Ti-clinohumite-dominated aggregate with interstitial garnet and pyroxene, (2) garnet+pyroxene aggregate with Ti-clinohumite inclusions, and (3) Ti-clinohumite-free aggregate dominated by garnet. Apatite, phlogopite, zircon, hematite, pentlandite, and an unknown Ni-Fe-volatile-Si (NFVS) mineral, which is replaced by Ni-greenalite, occur as accessories. Serpentine is the major secondary mineral. Garnet (Prp 63.9–64.6Alm 25.8–26.9Grs 1.4–7.9Uva 0.5–7.6Sps 1.0) in all three aggregates is pyrope-rich with very low grossular component, with that in the aggregate (2) most enriched in Cr (Cr 2O 3=2.55 wt.%). Orthopyroxene is depleted in Al (Al 2O 3=0.16 wt.% in the cores) and Ca (CaO=0.06–0.09 wt.% in the cores), with X Mg (Mg/(Mg+Fe)) values at ca. 0.900. Clinopyroxene is chromian diopside with Fe 3+≥Fe 2+. Matrix clinopyroxene has a lower X Mg (0.862) than that (0.887) included in Ti-clinohumite. The rock contains modest amount of heavy rare earth elements (HREE) (10 to 12×C1 chondrite), with significant enrichment in Cr, Co, Ni, V, Sr, and light rare earth elements (LREE) (22 to 33×C1 chondrite). The clinopyroxene is very enriched in Cr (Cr 2O 3 is up to 2.09 wt.% in the cores) and Sr (ca. 350 ppm) and LREE (Ce N/Yb N=157.7). Ti-clinohumite is enriched in Ni (1981 ppm), Co (123 ppm), and Nb (85 ppm). While it is possible to enrich ultramafites in incompatible elements in a subducted slab, the high Al, Fe, Ti, and low Si, Ca, and Na contents in the Ti-clinohumite rock are difficult to account for by crustal metasomatism of an ultramafite. On the other hand, the similarity in major and trace element compositions and their systematic variations between the Ti-clinohumite-garnet-pyroxene rock of this study and those of Mg-metasomatised Fe–Ti gabbros reported in the literature suggest that crustal metasomatism occurred in a gabbroic protolith, which resulted in addition of Cr, Co, Ni, and Mg and removal of Si, Ca, Na, Al, and Fe. This implies that the rock was in contact with an ultramafite at low pressure. During subsequent subduction, the metagabbro was thrust into the country gneiss, where gneiss-derived hydrous fluids caused enrichment of Sr and LREE in recrystallised clinopyroxene. P–T estimates for the high-pressure assemblage are ca. 4.2 GPa and ca. 760 °C, compatible with those for the eclogites and gneisses in this terrane. It is possible that the Ti-clinohumite-garnet-pyroxene rock and associated eclogites represent remnants of former oceanic crust that was subducted to a great depth.

Melt migration in ophiolitic peridotites: the message from Alpine-Apennine peridotites and implications for embryonic ocean basins

Abstract Results of a field study as well as petrological and geochemical data demonstrate that substantial portions of the lithospheric mantle, exhumed during opening of the Jurassic Piedmont Ligurian ocean, were infiltrated by and reacted with migrating melts. Intergranular flow of ascending liquids produced by the underlying hot asthenosphere dissolved clinopyroxene ± spinel and precipitated orthopyroxene + plagioclase ± olivine, forming orthopyroxene + plagioclase-rich perioditite. Migrating liquids became progressively saturated in clinopyroxene, and then precipitated microgranular aggregates of clinopyroxene-bearing gabbronorite. Later, diffuse porous melt flow was replaced by focused porous flow, producing a system of discordant dunite bodies. Upon cooling, liquids migrating in dunite channels became progressively saturated in clinopyroxene and plagioclase, forming interstitial clinopyroxene at olivine triple points followed by clinopyroxene ± plagioclase megacrysts and gabbro veinlets within the dunite, and gabbro dykelets within plagioclase peridotites. Subsequent cooling during continued exhumation was accompanied by intrusion of kilometre-scale gabbroic dykes evolving from troctolite to Mg-Al and Fe-Ti gabbros. Migrating liquids, which infiltrated peridotite and formed gabbroic rocks, span a wide range of compositions from silica-rich single melt fractions to T- and N-MORB (mid-ocean ridge basalt), characteristic of the melting column beneath midocean ridges. Explanations for the progressive evolution of an igneous system from diffuse to focused porous flow and finally dyking include the competing effects of heating of the lithospheric mantle by ascending magmas from the underlying hot asthenosphere and conductive cooling by exhumation. Whether or not rift-related melt infiltration and heating is recorded by exhumed subcontinental lithospheric mantle along ocean-continent transitions and/or oceanic lithospheric mantle along slow-spreading ridges depends on the relative position to the underlying upwelling asthenosphere.

INFLUENCE OF THE WHOLE-ROCK COMPOSITION ON THE CRYSTALLIZATION OF SODIC AMPHIBOLES (PIEMONTE ZONE, WESTERN ALPS)

The recent discovery of glaucophane and crossite amphiboles in metaophiolites of the Combin Unit (Piemonte zone, north-western Alps, Italy) confirms their Alpine re-equilibration under epidote-blueschist facies conditions. Sodic amphiboles were found in two rock types: i) coarse titanite-rich metabasites derived from original Fe-Ti gabbros, and ii) epidote-rich metabasites derived from MOR basalts. Crystallisation of Fe-glaucophane and crossite in the gabbroderived titanite-rich metabasites was favoured by their high total Fe contents, whereas crystallisation of glaucophane and crossite in the basalt-derived epidote- rich metabasites was favoured by Mg depletion due to sea-floor metasomatism. The comparison of all the analysed sodic-amphibole bearing metabasites s.l. from the Combin Unit shows that the FeOtot/MgO bulk-rock ratios, independently on their intrusive or extrusive origin, ranges between 2 and 3. Therefore, it can be concluded that the growth of sodic amphiboles at peak-conditions around 0.6-0.7 GPa and 350-400 °C typical of the subduction metamorphism is favoured in metabasites by a FeOtot/MgO ratio around 2 to 3.

The Steinmann Trinity revisited: mantle exhumation and magmatism along an ocean-continent transition: the Platta nappe, eastern Switzerland

Abstract The close association of serpentinites, basalts and radiolarites, later known as the Steinmann Trinity, was clearly described by Steinmann from the south Pennine Arosa zone and its southern prolongation, the Platta nappe of the eastern Swiss Alps. This classical ‘ophiolite’ is distinctly different from typical fast-spreading ridge associations and can be compared with the transitional crust occurring along non-volcanic passive continental margins in present-day oceans. It includes serpentinized peridotites that we interpret as subcontinental mantle rocks, which were exhumed along low-angle detachment faults and locally overlain by extensional allochthons of continental crust, minor gabbroic intrusions, tholeiitic pillow lavas and flows and a succession of oceanic sediments. The serpentinized peridotites record deformation at falling temperatures during extension leading to final exposure of the mantle rocks at the sea floor and their inclusion in tectono-sedimentary breccias (ophicalcites). Field relationships, and mineral-chemical and radiometric data show that the gabbros intruded already serpentinized mantle rocks at shallow depth 161 Ma ago. They are Mg gabbros, Fe gabbros and Fe-Ti gabbros, cut by dioritic pegmatoid veins and albitite dykes, which originated by differentiation from the same parental magma. All gabbros show the same metamorphic evolution, i.e. intrusion at relatively low pressure, oceanic hydration at elevated temperature and a subsequent static oceanic alteration. The pillow lavas stratigraphically overlie the exhumed mantle rocks and the tectono-sedimentary breccias related to the exhumation of both mantle rocks and gabbros. However, both gabbros and pillow basalts are characterized by εNd values typical for an asthenospheric mid-ocean ridge-type source of the melts. They may be the products of a steady process that combined extensional deformation with magma generation and emplacement. They appear to document the onset of sea-floor spreading across an exhumed subcontinental mantle during the earliest phases of a slow-spreading ridge.

Transformation of Fe–Ti gabbro to coronite, eclogite and amphibolite in the Baie du Nord segment, Manicouagan Imbricate Zone, eastern Grenville Province

Fe–Ti gabbros from the Baie du Nord Segment of the Manicouagan Imbricate Zone, metamorphosed under high P–T conditions during the Grenvillian orogeny, have been the focus of a detailed micropetrological study. Textures and mineral chemistry suggest that the mineral assemblages represent progressive stages of metamorphic transformation resulting in the formation of coronas, pseudomorphs after igneous phases (transitional) and true, granoblastic eclogites. The transitional and eclogitic samples also have coronas which are developed locally around igneous xenocrysts of plagioclase and olivine. The deformed margins of coronitic Fe–Ti gabbros are transformed to amphibolite and contain clinopyroxene‐bearing leucosomes with garnet poikiloblasts that are indicative of high‐P–T dehydration melting. Interpretation of garnet zoning and thermobarometry suggest that the highest P–T conditions are recorded by coronas around xenocrysts (c. 720–800 °C at 14–17 kbar) and garnet–clinopyroxene cores in granoblastic assemblages (c. 740–820 °C at 13–17 kbar) in the eclogitic samples. Re‐equilibration during the early stages of exhumation at high‐T conditions (&gt;700 °C) affected all samples, and is evidenced by the widespread development of pargasite‐bearing plagioclase collars in the coronitic and transitional metagabbros and by widespread re‐equilibration of the eclogites giving lower P–T estimates at grain boundaries. However, the difference in calculated pressure conditions between coronite and eclogite samples is consistent with increasing pressure (depth) from the coronites (11–13 kbar) to the eclogites (13–17 kbar). The P–T conditions recorded by these rocks define a metamorphic field gradient which suggests high heat flow through the lower crust during the Grenvillian orogeny.

Petrology and U–Pb geochronology of mafic, high-pressure, metamorphic coronites from the Tshenukutish domain, eastern Grenville Province

A suite of mafic and ultramafic rocks from the Tshenukutish domain, eastern Grenville Province, have been the subject of a detailed petrographic and geochemical study combined with U–Pb geochronology. The Tshenukutish domain is the highest tectonic level in the recently discovered, eclogite-bearing, Manicouagan Imbricate Zone. The mafic and ultramafic rocks are characteristic of two of the lithotectonic segments in the domain, the Lac Espadon segment and the Baie du Nord segment. In the Lac Espadon segment, cumulate textures and geochemical characteristics suggest that the units represent a layered, mafic and ultramafic complex and an unrelated apatite-rich, Fe–Ti mafic pegmatite (nelsonite) body. In contrast, the samples from the Baie du Nord segment represent fairly homogeneous Fe–Ti gabbros and anorthosites. The samples studied show abundant petrographic evidence for high-pressure metamorphism, for instance the widespread development of garnet, clinopyroxene and kyanite bearing metamorphic assemblages. Estimates of the conditions of high-pressure metamorphism in previous studies and melting textures in deformed and hydrated gabbro margins are in the range 750–900°C and 1600–1800 MPa. The gabbroic end-members from both segments suggest within-plate tholeiite petrotectonic signatures. REE-patterns of gabbros and cumulates in both segments suggest fractionation from a tholeiitic source. Cumulate textures, fractionation and inherited zircons suggest high-crustal level emplacement of the magmatic protoliths in the Tshenukutish domain. In the Lac Espadon segment U–Pb ages of igneous zircon and baddeleyite give an age range of 1643–1629 Ma for the emplacement of the layered complex. The nelsonite in the LES gives a poorly constrained igneous zircon age of 1596+126/−99 Ma. In the Baie du Nord segment baddeleyite gives an age of 1170±5 Ma for emplacement of the Fe–Ti gabbros. This is the youngest intrusion in the TD and thus, gives a maximum limit for the onset of Grenvillian metamorphism. Ages of high-P metamorphism are further constrained by zircon formed during partial melting of deformed and hydrated Fe–Ti gabbro margins (1046±3 Ma), overgrowth of igneous zircon (1042+22/−28 Ma) in the nelsonite, saccharoidal zircon replacing baddeleyite (1030+10/−7 Ma and 1030±12 Ma) and new growth of equant zircon (1012±12 Ma) from the undeformed interior of the gabbros. These ages indicate that the high-P metamorphism of the Tshenukutish domain was late-Grenvillian (Ottawan). Retrogression to amphibolite facies is constrained by the ages of titanite (1006–997 Ma) in deformed gabbro margins. Rutile ages range between 960 and 929 Ma and are interpreted as cooling ages. The difference between the ages of titanite and rutile suggest slow cooling of the Tshenukutish domain after initial high-pressure and temperature metamorphism and exhumation.

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.

Blueschist and Eclogite transformations in Fe-Ti Gabbros: A Case from the Western Alps Ophiolites

Blueschist and Eclogite transformations in Fe-Ti Gabbros: A Case from the Western Alps Ophiolites