Eclogite Formation Research Articles

Eclogite at the Antarctic palaeo-Pacific active margin of Gondwana (Lanterman Range, northern Victoria Land, Antarctica)

Well-preserved eclogites were found for the first time in Antarctica, at the Lanterman Range, northern Victoria Land. They are part of a mafic–ultramafic belt that lies between the Wilson Terrane, representing part of the palaeo-Pacific margin of Gondwana, and the Bowers Terrane, a Cambro-Ordovician volcanic are and related sediments, accreted to the margin during the Ross Orogeny. The eclogites formed at temperatures in the range 750–850°C and pressures above 15 kbar and subsequently experienced a decompressional path to low pressure amphibolite facies conditions. The formation and exhumation of eclogites and the attainment of the metamorphic peak in adjacent rock units is consistent with a plate convergent setting model at the palaeo-Pacific margin of Gondwana.

Evidence for a 2 Ga subduction zone: Eclogites in the Usagaran belt of Tanzania

U-Pb geochronology on metamorphic minerals from a 35-km-long belt of eclogite-facies rocks in central Tanzania yields a Paleoproterozoic age of 2 Ga for the time of metamorphism. Peak metamorphic conditions found in eclogites (± kyanite) and metapelites reached about 750 °C and 18 kbar. A clockwise pressure-temperature path is deduced from mineral zonations, inclusion relations, and retrograde reaction textures. Near-isothermal decompression can be explained by erosion or tectonically controlled exhumation that followed tectonic thickening of the crust during subduction. Trace and rare earth element geochemistry indicates a mid-ocean ridge basaltlike mantle source for the precursors of the mafic members of the eclogite-facies rock suite. All the observations combined indicate that these high-pressure rocks are the oldest-known large-scale outcrops of eclogites formed during subduction of oceanic lithosphere. Linking eclogite formation to a Paleoproterozoic subduction event adds credibility to models of crust dynamics that advocate the operation of plate-tectonic processes early in Earth9s history. The paucity of Precambrian eclogites may then be addressed as a problem of preservation rather than lack of formation.

Isotopic constraints on fluid infiltration from an eclogite facies shear zone, Holsenøy, Norway

ABSTRACTGranulite facies anorthosites on Holsenøy Island in the Bergen Arcs region of western Norway are transected by shear zones 0.1–100 m wide characterized by eclogite facies assemblages. Eclogite formation is related to influx of fluid along the shears at temperatures of c. 700d̀C and pressures in excess of 1.7 GPa. Combined carbon and nitrogen stable isotope, 40Ar/36Ar, trace‐element and petrological data have been used to determine the nature and distribution of fluids across the anorthosite‐eclogite transition.A metre‐wide drilled section traverses the eclogitic centre of the shear into undeformed granulite facies garnet‐clinopyroxene anorthosite. Clinozoisite occurs along grain boundaries and microcracks in undeformed anorthosite up to 1 m from the centre of the shear and clinozoisite increases in abundance as the edge of the shear zone is approached. The eclogite‐granulite transition, marked by the appearance of sodic pyroxene and loss of albite, occurs within the most highly sheared section of the traverse. The jadeite‐in reaction coincides with increased paragonite activity in mica. The separation between paragonite and clinozoisite reaction fronts can be semiquantitatively modelled assuming advective fluid flow perpendicular to the shear zone. The inner section of the traverse (0.25 m wide) is marked by retrogressive replacement of omphacite by plagioclase + paragonite accompanied by veins of quartz‐phengite‐plagioclase.C‐N‐Ar characteristics of fluid inclusions in garnet show that fluids associated with precursor granulite, eclogite and retrogressed eclogite are isotopically distinct. The granulite‐eclogite transition coincides with a marked change in CO2 abundance and δ13C (<36ppm, δ13C=‐2% in the granulite; <180 ppm, δ13C=‐10% in the eclogite). The distribution of Ar indicates mixing between influxed fluid (40Ar/36Ar > 25 times 103) and pre‐existing Ar in the granulite (40Ar/36Ar < 8 times 103). δ15N values decrease from +6% in the anorthosite to +3% within the eclogite shear. The central zone of retrogressed eclogite post‐dates shearing and is characterised by substantial enrichment of Si, K, Ba and Rb. Fluids are CO2‐rich (δ13C ∼ ‐5%) with variable N2 and Ar abundances and isotopic compositions.Both Ar and H2O have penetrated the underformed granulite fabric more than 0.5m beyond the granulite/eclogite transition during eclogite formation. Argon isotopes show a mixing profile consistent with diffusion through an interconnecting H2O‐rich fluid network. In contrast, a carbon‐isotope front coincides with the deformation boundary layer, indicating that the underformed anorthosite was impervious to CO2‐rich fluids. This is consistent with the high dihedral angle of carbonic fluids, and may be interpreted in terms of evolving fluid compositions within the shear zone.

SmNd and UPb dating of eclogites and granulites from the Oberpfalz, NE Bavaria, Germany

The formation of eclogites and felsic granulites of the Moldanubian basement of the Oberpfalz (area of Winklarn, NE Bavaria) has been dated at 424 Ma using the SmNd method on minerals (garnet, zircon, rutile) and whole rock. Only estimates on the age of the tholeiitic protoliths of the eclogite boudins are possible from the scattered U-Pb zircon data and from SmNd systematics. Both methods suggest protolith ages around 1 Ga and indicate, together with the major-, trace- and rare-earth element data, extraction of the tholeiitic melts from a suboceanic mantle with an initial ϵ Nd of + 7.5. Based on the geochronological data of three typical Moldanubian cordierite-bearing paragneisses which surround the Winklarn area in distances of 30–60 km, tectonic contacts to the low-pressure country rocks of the eclogites are probable as these paragneisses yielded Ordovician ages for their amphibolite-facies metamorphism and post-Pan-African ages for the deposition of their sedimentary precursors. Low-pressure-high-temperature metamorphism at 323 Ma fully reset monazites in the granulites. This widespread and synchronous Carboniferous overprinting of the Moldanubian basement is probably also responsible for the variable opening of UPb zircon, RbSr whole-rock and SmNd mineral systems. A terrane or microcontinent model still appears most suitable to explain the Ordovician, Silurian, Devonian and Carboniferous high-pressure (subduction) events detected all over the European Variscan belt. In this model, oceanic basins between Gondwana-derived microcontinents were successively closed since the Ordovician, causing differently old subduction zone-related high-pressure metamorphisms.

Collision of the North China and Yangtse Blocks and formation of coesite-bearing eclogites: Timing and processes

Various types of eclogite, including coesite-bearing varieties formed under high- P-high- T conditions (up to 27 kbar, 700–800°C), and glaucophane schist occur in the Dabie Mountains and the SuLu terrane, eastern China. Dating of these high-pressure rocks by the SmNd mineral isochron and 40 Ar 39 Ar method suggests that the occurrence of the high-pressure metamorphism was during the early Triassic. LREE-enriched chondrite-normalized patterns for type-II eclogites and low initial ϵ Nd of − 14 to − 3 and the 87 Sr 86 Sr-values (0.706–0.710) for various types of eclogites suggest that their protoliths were mainly derived from Precambrian island arc or intraplate basalts in the basement of the Yangtse Block and the enriched pyroxenite layer in alpine peridotite. The P-T-t path of eclogite from the southern Dabie Mountains suggests that the uplift history of eclogite in the Dabie Mountains can be subdivided into two stages: (1) fast uplift driven by thrust during continental-continental collision and deep subduction (at 221 Ma) of the continental crust; (2) later gentle uplift with rise of the Dabie Mountains in the late Jurassic and Cretaceous (at 134 Ma). It is proposed that the collision between the North China Block and Yangtse Block began in the late Permian or early Triassic with a north-dipping subduction zone. This was followed by subduction of the continental crust of the Yangtse Block under the North China Block during the Triassic. These two continental blocks were welded into a single tectonic unit in the late Jurassic or early Cretaceous. Different cooling histories for a coexisting gneiss and eclogite pair from Shima area suggest that the eclogite and their country rocks are not always coherent to each other. Some of them may have been juxtaposed through tectonic processes from different levels.

Different P-T-t paths and leucogranite occurrences along the High Himalayan Crystallines : Implications for subduction and collision along the northern Indian margin

Different P-T-t paths and a heterogeneous distribution of Tertiary leucogranites are recorded along the High Himalayan Crystallines (HHC), in the collided Indian plate. Rapid subduction with eclogite formation during the early-Eocene is apparently restricted to part of the HHC of northern Pakistan, that is the zone of the first collision between the narrow NW Indian spur and the Kohistan island arc. In northern Pakistan rapid exhumation occurred along a cooling path and the crustally derived leucogranites of Tertiary age are rare or absent. East of the spur the delayed India-Eurasia collision occurred at lower rates of convergence and determined less rapid and deep continental subduction. Here exhumation of the HHC occurred at lower rates and at increasing T, giving a relatively warm thickened crust which underwent extensive anatexis during uplift in Oligocene-Miocene times. In eastern Himalayas, anatexis at medium- and low-P produced numerous cordierite-andalusite-bearing leucogranites which emplaced along extensional shear zones at high structural levels during the Miocene.

Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts

The sizes of continental blocks, basaltic oceanic plateaus, and island arcs that would cause collisional orogenesis when they enter a subduction zone are calculated in an analysis based upon the assumption of local isostasy and the assumption that plate subduction is primarily driven by the negative buoyancy of the lithosphere. Buoyancy analysis indicates that the bulk density contrast between 80-m.y.-old oceanic lithosphere capped by a 7-km-thick basaltic crust and the less dense underlying asthenosphere is on the order of 0.04 gm/cm 3 . Oceanic lithosphere that is ∼10 m.y. old is the youngest that is more dense than the asthenosphere and hence inherently susceptible to subduction. Subduction zone metamorphism causes the crustal layer of basalt/gabbro to transform into more dense amphibolite and eclogite. Where eclogite formation is extensive, the descending oceanic lithosphere increases in bulk density by as much as 0.04 gm/cm 3 . Lithosphere that is 100 km thick with a 30-km-thick granitic continental crust resists Subduction because it is ∼0.09 gm/cm 3 less dense than the asthenosphere. Contrasts in lithospheric bulk density (crust + mantle) of 3 are the difference between whether subduction is nearly inevitable (as for normal ocean crust) or greatly resisted (as for thick, ancient continents). Collisional orogenesis is defined as a plate interaction of the sort that causes a rearrangement of plate motions, generally with the initiation of a new subduction zone and the creation of mountains. Buoyancy analysis indicates that only bodies of continental and oceanic island are crust that are > ∼15 km thick make the lithosphere buoyant enough to jam a subduction zone. Oceanic island arc complexes built upon ocean crust typically must be active for more than ∼20 m.y. to attain crustal thicknesses so that their attempted subduction causes collisional orogenesis. Oceanic plateaus where basaltic crust as much as ∼17 km thick caps 100-km-thick lithosphere are inherenty subductable and actually less buoyant than normal oceanic lithosphere following subduction metamorphism. Basaltic plateaus must have crustal thicknesses >∼30 km to typically cause collisional orogenesis during subduction. Short subducting seamounts (

Precambrian tectonics and the physicochemical evolution of the continental crust. I. The gabbro-eclogite transition revisited

We report the first occurrence of retrograde eclogite shear zones cutting Proterozoic granulites from Australia. The Musgrave Range granulites were metamorphosed at about 12 kbar pressure and 850–900°C. The granulites then cooled isobarically and developed a variety of lower-temperature mineral reaction coronas typical of garnet granulites. These rocks are cut by a major shear zone, the Davenport Shear, in which lower-temperature, fine-grained eclogite developed. This demonstrates that mafic rocks in the deep crust can cool down into the thermodynamic stability field of eclogite without undergoing transformation to eclogite. We agree with Harte et al. (1981) that rocks lacking a fluid phase in stable tectonic regions are unlikely to maintain local equilibrium at ambient geotherm conditions until depths well within the upper mantle are reached. It is proposed that many isobaric, high-pressure granulites could have cooled down into the eclogite field before their later excavation to the surface, and did not transform to an eclogite mineralogy. The inability of the lower crust to transform into eclogite by retrograde reaction has important implications for the seismic, petrologic and geochemical character of the lower crust. The continental Moho cannot be caused by the gabbro-eclogite transition. Preservation of relatively low density, high-temperature granulite and gabbroic mineral assemblages in the lower continental crust prohibits major recycling of lower crust back into the mantle once it is removed from active plate boundaries. However, in island arcs with magmatically thickened crust, mafic rocks at a depth of 60 to 70 km will be in the eclogite stability field at high enough temperatures (800–900°C) for eclogite to rapidly form and sink back into the mantle. This could result in a net accreted crust of broadly andesitic composition. In contrast, later growth of continental crust by basalt underplating in stabilised shield regions will not result in subsequent eclogite formation, and lower crustal recycling back into the mantle will not occur. Thus, estimates of the average composition of the continental crust which are based on a mass balance of plate margin basaltic input from the mantle will not be adequate.

Eclogite formation and dynamics of crustal roots under continental collision zones

ABSTRACTField observations on deep parts of the root zone of the Caledonian orogen are combined with the geophysical data available for recent continental collision zones such as the Alps, to provide a model for dynamics of crustal root zones.Rocks descending into the root zone are locally infiltrated by fluids which cause eclogitization along shear zones and fluid pathways. The eclogite facies rocks vary in density from 3.1 to 3.6 g cm‐3, reflecting the compositional variation in their protoliths. Where the amount of highly ductile eclogitic material reaches about 40% the descending crustal segment loses its coherence and disintegrates into a breccia where blocks of granulite facies rocks float in a matrix of eclogite material. A density‐stratified root zone develops, where light unreacted or partly eclogitized material floats on an ‘ocean’ dominated by eclogite facies rocks, the top of which may correspond to the Moho. Catastrophic decent of this eclogitic layer may (1) transport blocks of light material to depths of > 90 km, which upon further disintegration due to eclogitization are released and return by buoyancy forces to the crust; (2) result in extension and uplift of the partly‐eclogitized lighter material.

Comparison of PTt paths in allochthonous high pressure metamorphic terrains from the Scandinavian Caledonides and the French Massif Central: Contrasted thermal structures during uplift

A regional comparison of retrograde PTt paths of high pressure metamorphic terrains in the Scandinavian Caledonides (Seve Nappe Complex (S.N.C.)) and the French Massif Central (F.M.C.) reveals the following similarities: physical conditions of eclogite formation are roughly identical (600–780°C / 13.5–16.5 Kb), dominantly isothermal decompression. However in detail two major differences have been recognized: 1- In the S.N.C., PT estimates of eclogite formation are constant within individual tectonic lenses but vary when different tectonic lenses are compared. During uplift these temperature differences are maintained and diminish only at the final stages of the uplift history; 2- In the F.M.C. the physical conditions of eclogite formation are relatively constant (700–750°C/15Kb) but discrepancies in temperature (> 100°C) appear during the first stages of the uplift history if one compares eastern with western domains.

Evolution of the Devonian Hornelen Basin, west Norway: new constraints from petrological studies of metamorphic clasts

Abstract The Middle Devonian Hornelen Basin of western Norway is a continental fault-basin which formed during the late stages of the Caledonian orogeny, closely following the Late Silurian crustal thickening and high pressure metamophism responsible for the formation of eclogites in the adjacent Western Gneiss Region. Clasts from alluvial fan conglomerates which fringe the basin provide important constraints on the type of basin tectonism and the uplift history of this part of the orogen. The clasts lack a petrographic linkage with the Western Gneiss Region (from which they are separated by a fault), but show a strong petrographic linkage with units of the allochthon (which lie unconformably below the basin fill). The clast sourceland had a similar metamorphic evolution to the allochthon, and both lack evidence for the high pressures recorded in the Western Gneiss Region. Both clasts and allochthon cooled through muscovite argon-retention temperatures about 40 Ma earlier than the Western Gneiss Region. Depth-time curves derived from isotopic and petrological results show that prior to basin initiation the Western Gneiss Region was uplifting faster than the clast source, but still lay at about 10 km depth during basin formation. A tectonic scenario is suggested in which much of the uplift of the Western Gneiss Region took place before the deposition of the Hornelen Basin sediments, aided by crustal thinning along a major extensional mylonitic shear-zone. However, final exposure of this high-pressure metamorphic terrain only took place after the Hornelen Basin sediments were deposited, aided by uplift in the footwall of the basin sole fault.

Petrology of eclogites and clinopyroxene-garnet metabasites from the Cabo Ortegal Complex (northwestern Spain)

High-pressure, subduction-related metamorphism is recorded in two structural units of the Cabo Ortegal complex of Spain: (i) an upper thrust unit (Concepenido-La Capelada) composed of granulites, eclogites and metaperidotites and (ii) a structurally underlying unit (Chimparra-Banded) formed of gneisses with minor metabasic intercalations. In the upper unit, a metamorphic episode of Lower Ordovician age caused the formation of eclogites from basic rocks with N-MORB compositions, while high-pressure, garnet-clinopyroxene granulites were formed from basic to intermediate protoliths with calc-alkaline affinities. Garnet replaces spinel in ultramafics of the same unit and may have originated simultaneously. The PT-conditions were c. 800°C at 13.5 kbar in the granulites, whereas similar temperatures and higher pressures ( > 17 kbar) prevailed in the eclogites. The migmatitic Chimparra and Banded gneisses contain boudins of garnet and omphacite-bearing rocks, with or without plagioclase. The basic protoliths of these rocks are similar to LREE-enriched basalts with alkaline affinities. The Silurian metamorphism in this gneissic unit took place at c. 700°C or lower and at pressures around 15 kbar. This metamorphism could have occurred when a volcano-sedimentary complex, formed in a thinned-crust environment, was subducted beneath an overlying active margin/volcanic arc complex. An ophiolitic unit, made up of the N- and T-type MORB metabasites of the Candelaria and Purrido-Peña Escrita Formations, underlies the gneisses. Metamorphism in this unit took place during the Early-Middle Devonian, mainly giving rise to amphibolite-facies associations. Sporadic coronitic metagabbros/dolerite and garnet-clinopyroxene metabasites indicate that amphibolite-granulite transitional conditions ( > 700°C, 8 kbar) were attained locally. Retrograde amphibolitic assemblages in the upper units are more or less coeval with this episode, which suggests that it may have been related to the cessation of obduction and the stacking of the overlying high-grade units.

Behaviour of rare-earth elements during high-pressure metamorphism

The behaviour of rare-earth elements (REE) during high-pressure metamorphism has been studied to estimate their possible use in determining the geodynamic settings of eclogite procoliths. Eclogites exhibiting various degrees of amphibolitization have been studied in a number lf metamorphic complexes in the Urals-Mongolian fold belt (the Kokchetav massif in northern Kazakhstan and the Atbashi range in the Central Tien-Shan, all in the U.S.S.R.). In addition, an eclogitized metagabbro from the Koralpe, Austrian Alps and gabbroids transformed into garnet amphibolites from the Kokhchetav massif have been investigated. The analytical data suggest that about 30–60% of the light rare-earth elements (LREE) in eclogites is present in the intergranular space. The results of the investigation demonstrate that the REE were virtually immobile during eclogite formation. Their low mobility can be explained either by absence of complex formers in the fluid phase or by low fluid/rock ratio.

Diffusion gradients in an eclogite xenolith from the Roberts Victor kimberlite pipe: (2) kinetics and implications for petrogenesis

In a bimineralic eclogite xenolith (sample JJG41) from the Roberts Victor kimberlite, compositional gradients in clinopyroxene are related to garnet exsolution. Two principal reactions involving clinopyroxene and garnet occur: (i) The net-transfer Al2Si-1Mg-1 which is responsible for garnet growth according to the equation 2Di+Al2Si-1Mg-1=Grossular+MgCa-1 (reaction 1). This has created substantial compositional gradients in Al, Si and Mg within clinopyroxene. (ii) The exchange of Fe−Mg between garnet and clinopyroxene (reaction 2). During the stage of garnet growth (reaction 1) the lamellae crystallized sequentially as a result of a temperature decrease from around 1400 to 1200° C. This exsolution growth-stage was under the control of Al diffusion in clinopyroxene and at around 1200° C further growth of garnet lamellae became impeded by the sluggishness of Al diffusion in the clinopyroxene host. However, reaction 2 continued during further cooling down to about 1000° C; this temperature being inferred from the constant Fe−Mg partitioning at clinopyroxene-garnet interfaces for the whole set of lamellae. The initial clinopyroxene in JJG41 was probably formed by crystallization from a melt in Archaean time. The cessation of Fe−Mg exchange between garnet and clinopyroxene at about 1000° C may well predate the eruption of the eclogite in kimberlite at around 100 Ma. Kinetic models of reaction are examined for both reactions. Modelling of reaction 1, involving both diffusion and interface migration, allows several means of estimating the diffusion coefficient of Al in clinopyroxene; the estimates are in the range 10-16-10-20 cm2/s at 1200° C. These estimates bracket the experimentally determined data for Al diffusion in clinopyroxene, and from these experimental data a preferred cooling rate of about 300° C/Ma is obtained for the period of growth of garnet exsolution lamellae. A ‘geospeedometry’ approach (Lasaga 1983) suitable for a pure-exchange process (reaction 2) is used to estimate the cooling rate in the later stages of the thermal history (after garnet growth); values 4–40° C/Ma are consistent with the shape of the Fe-diffusion gradients in the clinopyroxene. The extensive thermal history recorded by JJG41, including probable melt involvement at ca. 1400° C, demonstrates the complex evolution of rocks within the mantle. Whilst the notion of formation of mantle eclogites from subducted oceanic crust has become fashionable, it is clear that tracing eclogite geochemical and P-T characteristics backwards from their nature at the time of xenolith eruption, through high-temperature mantle events to the characteristics of the original subducted oceanic crust, will be very complex.

Fluid controlled eclogitization of granulites in deep crustal shear zones, Bergen arcs, Western Norway

During the Caledonian orogeny large parts of the western margin of the Baltic shield were disrupted, sliced and stacked. Caledonian deformation resulted in a massif thickening of the continental crust. Mafic granulites and granulite facies meta-anorthosites build up a large portion of the Bergen Arcs terrane in southwestern Norway. The rocks represent typical Precambrian continental lower crust. These rocks experienced extensive eclogitization in response to stacking and crustal thickening during the Caledonian orogenic cycle. Eclogite formation resulted from shear deformation and associated infiltration of H2O-rich fluids (XH2O≥0.75). During an early stage, eclogite facies mineralogy formed in extension fractures (veins). The veins are probably related to hydraulic fracture systems which transported the inferred fluid phase. During the main stage, eclogitization occurred along shear zones ranging from centimeters to tens of meters in thickness. Eclogite forming reactions are shown to consume H2O, alkalies and to release SiO2. Much of the SiO2 released by the eclogitization process can be found in late quartz vein systems. The eclogitization took place at a temperature of about 700°C and a pressure between 18 and 21 kbar. Fluid infiltration was supported by a decrease in rock volume during reaction (ΔVsolids<0). The negative volume change of reaction occurs despite that the process of eclogitization involves hydration reactions. The formation of eclogite from granulite produces approximately 15 KJ heat per 100 cm3 original granulite. Numerical modeling of the regional temperature effects associated with partial hydration of the lower crust suggests that these processes may not cause large perturbations on the geotherm. Both, transport of heat and matter by advection of the fluid phase is negligible on a regional scale.

P-T evolution and fluid inclusion characteristics of retrograded eclogites, M�nchberg Gneiss Complex, Germany

Kyanite eclogites occur as part of the Munchberger nappe pile in NE-Bavaria, West Germany. Eclogites are overprinted by subsequent amphibolite facies metamorphism. The preservation of primary eclogitic textures as well as symplectitic textures are indicative of rapid decompression. Eclogite formation is estimated to have occurred under conditions of high H2O-activities at pressures between 20 and 26 kbar and temperatures ranging between 590 and 660° C, as is shown by the coexistence of omphacite (Jd 50), kyanite, zoisite and quartz. Minimum pressure estimates, independent of the water activity, range between 9 and 16 kbar at the relevant temperatures. Detailed studies of fluid inclusion reveal two predominant groups of aqueous-brine inclusions: high salinity (14–17 wt% NaCl equiv.) and low salinity (0–8 wt% NaCl equiv.) inclusions. Fluid compositions of both groups of inclusions yield isochores passing close to the estimated amphibolite facies PT-field. The compositions of these fluids are in good agreement with fluid compositions considered from mineral equilibria. None of the fluid inclusions has densities appropriate for eclogite facies metamorphism, but probably reflect later amphibolite facies metamorphism.

Lawsonite, blueschist and eclogite formation in the southern Sesia zone (western Alps, Italy)

Lawsonite, blueschist and eclogite formation in the southern Sesia zone (western Alps, Italy)

Metamorphic evolution of the palaeozoic series of the Betic Cordilleras (Nevado-Filabride complex, SE Spain) and its relationship with the alpine orogeny

The Palaeozoic rocks of the Veleta nappe (Nevado-Filabride complex, Betic Cordilleras, Southern Spain) consist of graphite-mica schists with oxychlorite — chloritoid — albite — garnet porphyroblasts. Crystal chemistry of the minerals, their phase relations and microfabrics are consistent with a normal prograde metamorphic evolution and a slight retrograde overprint. The basis of the overlying Mulhacen nappe consists also of Palaeozoic graphite-mica schists, but with clear signs of polymetamorphic development. An early low-pressure metamorphism with andalusite and chloritoid is overprinted by high-pressure metamorphism with kyanite and Mg-rich chloritoid. This later development is of Alpine age, which is demonstrated by the metamorphic conditions of Permo-Triassic rocks in the upper part of the Mulhacen nappe, included formation of eclogites, followed by an amphibolite facies overprint. The metamorphism of the Veleta nappe is regarded as pre-Alpine: a possible Alpine overprint did not cause an increase in temperatures. No inverse metamorphic gradient exists between Veleta and Mulhacen nappes, because of the different ages of metamorphism.

‘On the eclogites of Norway’—65 years later

AbstractThe Western Gneiss Region (WGR) of Norway consists largely of Proterozoic orthogneisses, but also contains paragneisses, peridotites, anorthosites, gabbros and coarse-grained intermediate-acid ‘rapakivi granites’. All of these lithologies enclose eclogites. Structural and isotopic data suggest that many of these rocks were juxtaposed by early Caledonian thrusting prior to eclogite formation at ca. 425 Ma.Low-P protoliths can be demonstrated for many eclogites. Prograde metamorphism to eclogite facies is demonstrated by inclusion suites within garnet grains and zoning of eclogite minerals. The regional distribution of KD (gnt/cpx) and Xcpxjd shows a decrease in Tmax, and in the corresponding P, away from the present coastline. The lowest values (500 °C, 10 kbar) are found in the Sunnfjord area and the highest (∼ 800 °C) along the coast of Sunnmøre and Nordmøre.Maximum pressures were reached at temperatures 100–200° &lt; Tmax. This P-T-t path is consistent with the preservation of jadeite-rich cpx (and possible coesite) in the coastal regions. Earlier overestimates of Pmax, based on partitioning of Al between opx and gnt, resulted from combination of early low-T (low-Al) opx and T values derived from cpx/gnt equilibrated at Tmax. Despite pervasive later amphibolitization, high-P assemblages (phengite + kyanite, omphacite + quartz) are locally preserved within gneisses near the coast. The high-P metamorphism can be explained by westward subduction of the Baltic continental plate beneath the Greenland plate, during the Caledonian orogeny.At least some of the Mg-Cr garnet peridotites of the WGR were derived from low-P protoliths (spinel ± chlorite peridotites, enclosing high-Al pyroxenites). While Sm-Nd mineral ages of most eclogites cluster around 425 Ma, garnet peridotites and their enclosed garnet pyroxenites give Proterozoic Sm-Nd mineral ages (1700–1000 Ma). The tectonic position of the Mg-Cr garnet peridotites, relative to the Caledonian high-pressure metamorphism, remains to be resolved.

Metagabbro and associated eclogites in the Lubrin area, Nevado‐Filabride Complex, Spain

Abstract Several small bodies of metabasite (maximum dimensions of 1000 m x 500 m) are included in the metamorphic rocks of the Nevado‐Filabride Complex in the Betic Cordilleras (Almeria Region). The body of 400 m x 100 m, located 200 m due west of the Lubrin village, contains troctolitic gabbro with well‐preserved igneous textures and mineral compositions, wholly amphibolitized gabbro, garnet‐bearing metagabbro eclogite. Along with the textural and mineral changes, sensible and regular geochemical variations can be observed, where the content of MgO decreases from 24% to 11%, while that of CaO and Na2O increases from 7% to 11% and from 2% to 3%, respectively. In addition, the content of some minor elements such as Sr, Y, Nb, Zr and Sc increases while that of Ni and Cr decreases from troctolitic gabbro to the eclogite. The amphibolitized gabbro shows values scattered around those of the troctolitic gabbro. These geochemical variations are ascribed to inherited differences in the pre‐metamorphic protolith, i.e. a fractionated gabbro which varies from olivine‐rich to clinopyroxene‐rich gabbro. Nevertheless, some metasomatism affected the Lubrin body without changing the main chemical trends, as documented by the significantly different 87Sr/86Sr ratios of each rock‐type. This points to a metasomatism which involved the introduction of crustal radiogenic strontium. The petrographical and mineral chemical features are interpreted to be the result of syn‐metamorphic fluid circulation possibly combined with deformation by shearing. The igneous texture and mineral chemistry have been retained wherever both fluid circulation and shearing were ineffective. On the contrary, where both events were effective, the formation of eclogite occurred. Later, the entire body underwent a retrogressive amphi‐bolitic stage under greenschist facies conditions, which was probably responsible for the formation of the amphibolitized gabbro portion and for the retrogression of the eclogite.