Eclogite Facies Research Articles

Petrological and zircon evidence for anatexis of UHP quartzite during continental collision in the Sulu orogen

AbstractPetrological evidence is provided for anatexis of ultrahigh‐pressure (UHP) metamorphic quartzite in the Sulu orogen. Some feldspar grains exhibit elongated, highly cuspate shapes or occur as interstitial, cuspate phases constituting interconnected networks along grain boundaries. Elongated veinlets composed of plagioclase + quartz ± K‐feldspar also occur in grain boundaries. These features provide compelling evidence for anatexis of the UHP quartzite. Zircon grains from impure quartzite are all metamorphic growth with highly irregular shape. They contain inclusions of coesite, jadeite, rutile and lower pressure minerals, including multiphase solid inclusions that are composed of two or more phases of muscovite, quartz, K‐feldspar and plagioclase. All zircon grains exhibit steep REE patterns, similar U–Pb ages and Hf isotope compositions with a weighted mean of 218 ± 2 Ma. Most grains have similar δ18O values of −0.6 to 0.1‰, but a few fall in the range −5.2 to −4.3‰. Thus, these grains would have grown from anatectic melts at various pressures. Zircon O isotope differences indicate that anatectic melts were derived from different sources with contrasting O isotopes, but similar Hf isotopes, that is, one from the quartzite itself and the other probably from the country‐rock granitic gneiss. Zircon grains from pure quartzite contain relict magmatic cores and significant metamorphic overgrowths. Domains that contain eclogite facies minerals exhibit flat HREE patterns, no Eu anomalies and concordant U–Pb ages of c. 220 Ma. Similar U–Pb ages are also obtained for domains that contain lower pressure minerals and exhibit steep REE patterns and marked negative Eu anomalies. These observations indicate that zircon records subsolidus overgrowth at eclogite facies conditions but suprasolidus growth at lower pressures. Zircon enclosed by garnet gave consistent U–Pb ages of c. 214 Ma. Such garnet is interpreted as a peritectic product of the anatectic reaction that involves felsic minerals and possibly amphibole and titanite. The REE patterns of epidote and titanite also record multistage growth and metasomatism by anatectic melts. Therefore, the anatexis of UHP metamorphic rocks is evident during continental collision in the Triassic.

High-pressure garnet amphibolite from the Funaokayama unit, western Kii Peninsula and the extent of eclogite facies metamorphism in the Sanbagawa belt

Quantitative analyses of garnet-bearing epidote amphibolite (hereafter garnet amphibolite) from the Funaokayama unit, western Kii Peninsula of the Sanbagawa belt reveal a previously unrecognized high-grade part of the Sanbagawa metamorphism. Petrographic observations suggest that the garnet amphibolite underwent three stages of metamorphism (M1, M2 and M3). Records of M1 are preserved as syn-tectonic prograde-zoned garnet and its inclusions. Pseudosection modeling reproduces the observed M1 assemblage garnet + amphibolite + epidote + phengite + quartz and the growth zoning of garnet records the pressure (P)-temperature (T) evolution from 0.8 GPa, 570 °C to 1.3 GPa, 590 °C. Therefore, this garnet growth and associated deformation (D1) took place during subduction up to eclogite facies conditions. The garnet rim compositions and inclusions of epidote, titanite, rutile and quartz (5titanite + 2clinozoisite = 5rutile + 3grossular + 2quartz + H2O) give consistent peak-P estimates of 1.3-1.9 GPa. The garnet contains quartz inclusions retaining residual pressures of up to ∼ 0.7 GPa, which is as high as the values reported in well-characterized eclogite samples in central Shikoku. The inferred peak-P conditions in the eclogite facies is further supported by the occurrence of aragonite in associated pelitic schist. The assemblage hornblende + epidote + titanite + quartz + albite in the matrix and its equilibrium conditions of ∼ 0.8 GPa, 580 °C suggest decompression to the epidote-amphibolite facies (M2). The M2 minerals locally show replacement by the mineral assemblage actinolite + epidote + chlorite + titanite + calcite + quartz + albite, suggesting a partial re-equilibration in the greenschist facies (M3). M2 and M3 are synchronous with the main phase of ductile deformation during exhumation (D2). Despite the absence of the omphacite + quartz assemblage, it is likely that eclogite facies metamorphism in the Sanbagawa belt can be extended to western Kii Peninsula.

Petrology, geochemistry, and origin of metamorphosed mafic rocks of the Trans Vietnam Orogenic Belt, Southeast Asia

The Trans Vietnam Orogenic Belt (TVOB) of Southeast Asia is thought to have formed by a continent-continent collision between the South China and Indochina cratons in Permian-Triassic times. Here we focus on the nature and origin of metamorphosed mafic rocks that are widely distributed throughout the TVOB, including rocks from the Cangshan Mountains (northern TVOB), the Red River Shear Zone and Song Ma Suture Zone (central TVOB), and the Kontum Massif (southern TVOB). Amphibolite facies metamorphosed mafic rocks are widely distributed throughout all of these areas, while granulite facies and eclogite facies metamorphosed mafic rocks occur only in the Kontum Massif and Song Ma Suture Zone, respectively. Major and trace element compositions of these TVOB metamorphosed mafic rocks indicate an overall tholeiitic affinity, but suggest a wide array of tectonic settings for the precursor mafic magmas, including volcanic-arc basalt (VAB), mid-ocean ridge basalt (MORB), and within-plate basalt (WPB). In addition, Fe-rich gabbro-derived mafic metamorphic rocks occur in the southern part of the TVOB (in the Kontum Massif). On the basis of the distributions of each type of mafic metamorphic rocks and their metamorphic grades, there were arc and oceanic crust between the South China Craton and the Indochina Craton.

Evidence of the lawsonite eclogite facies metamorphism from an epidote-glaucophane eclogite in the Kotsu area of the Sanbagawa belt, Japan

Evidence of the lawsonite eclogite facies metamorphism from an epidote-glaucophane eclogite in the Kotsu area of the Sanbagawa belt, Japan

P-TEstimates and Timing of the Sapphirine-Bearing Metamorphic Overprint in Kyanite Eclogites from Central Rhodope, Northern Greece

Sapphirine bearing symplectites that replace kyanite in eclogites from the Greek Rhodope Massif have previously been attributed to a high pressure granulite facies metamorphic event that overprinted the eclogitic peak metamorphic assemblage. The eclogitic mineralogy consisted of garnet, omphacitic pyroxene, rutile and kyanite and is largely replaced by low pressure minerals. Omphacite was initially replaced by sym plectites of diopside and plagioclase that were subsequently replaced by symplectites of amphibole and pla gioclase. Garnet reacted during decompression to form a corona of plagioclase, amphibole and magnetite. Rutile was partly transformed to ilmenite and kyanite decomposed to produce a high variance mineral assemblage of symplectitic spinel, sapphirine, plagioclase and corundum. The presence of quartz and corun dum in the kyanite eclogites is evidence for the absence of bulk equilibrium and obviates a conventional anal ysis of phase equilibria based on the bulk rock composition. To circumvent this difficulty we systematically explored the pressure temperature composition (P–T–X) space of a thermodynamic model for the symplec tites in order to establish the pressure temperature (P T) conditions at which the symplectites were formed after kyanite. This analysis combined with conventional thermometry indicates that the symplectites were formed at amphibolite facies conditions. The resulting upper pressure limit (~0.7 GPa) of the sapphirine producing metamorphic overprint is roughly half the former estimate for the lower pressure limit of the sym plectite forming metamorphic event. Temperature was constrained (T ~ 720°C) using garnet amphibole mineral thermometry. The P T conditions inferred here are consistent with thermobarometry from other lithologies in the Rhodope Massif, which show no evidence of granulite facies metamorphism. Regional geo logical arguments and ion probe (SHRIMP) zircon dating place the post eclogite facies metamorphic evo lution in Eocene times. DOI: 10.1134/S0869591113050032

Elemental responses to subduction-zone metamorphism: Constraints from the North Qilian Mountain, NW China

Subduction zone metamorphism (SZM) and behaviors of chemical elements in response to this process are important for both arc magmatism and mantle compositional heterogeneity. In this paper, we report the results of our petrographic and geochemical studies on blueschist and eclogite facies rocks of sedimentary and basaltic protoliths from two metamorphic sub-belts with different metamorphic histories in the North Qilian Mountain, Northwest China. The protolith of low-grade blueschists is basaltic in composition and is most likely produced in a back-arc setting, while the protoliths of high-grade blueschists/eclogites geochemically resemble the present-day normal and enriched mid-oceanic ridge basalts plus some volcanic arc rocks. The meta-sedimentary rocks, including meta-graywacke, meta-pelite, meta-chert and marble, show geochemical similarity to global oceanic (subducted) sediments. Assuming that high field strength elements (HFSEs) are relatively immobile, the correlated variations of rare earth elements (REEs) and Th with HFSEs suggest that all these elements are probably also immobile, whereas Pb and Sr are mobile in rocks of both basaltic and sedimentary protoliths during SZM. Ba, Cs and Rb are immobile in rocks of sedimentary protoliths and mobile in rocks of basaltic protolith. The apparent mobility of U in rocks of basaltic protolith may be inherited from seafloor alterations rather than caused by SZM.On the basis of in situ mineral compositional analysis (both major and trace elements), the most significant trace element storage minerals in these subduction-zone metamorphic rocks are: lawsonite, pumpellyite, apatite, garnet and epidote group minerals for REEs, white micas (both phengite and paragonite) for large ion lithophile elements, rutile and titanite for HFSEs. The presence and stability of these minerals exert the primary controls on the geochemical behaviors of most of these elements during SZM. The immobility of REEs, Th and U owing to their redistribution into newly formed minerals suggests that subduction-zone dehydration metamorphism will not contribute to the enrichment of these elements in arc magmatism. These observations require the formation and contribution of supercritical fluids or hydrous melts (these can effectively transport the aforementioned incompatible elements) from greater depths to arc magmatism. In addition, the overall sub-chondritic Nb/Ta ratio retained in rutile-bearing eclogites indicates that the subducting/subducted residual ocean crust passing through SZM cannot be responsible for the missing Nb (relative to Ta) in the bulk silicate earth.

Análise estrutural e metamórfica da região de Sucuru (Paraíba): implicações sobre a evolução do Terreno Alto Moxotó, Província Borborema

Inserted on the Alto Moxoto Terrane of the Borborema Province, the Sucuru region (Paraiba state, Brazil) includes two dominantly pre-Ediacaran tectono-stratigraphic domains, bounded by an expressive thrust shear zone (Carmo SZ). The first domain has a metaplutonic nature, being formed by granodioritic to migmatitic orthogneisses (Floresta Complex) cut by several intrusive suites. These suites comprise a unit of mafic-ultramafic nature (Malhada Vermelha), a second one mainly of granitic-granodioritic composition (Pedra d'Agua) and a last suite whose composition ranges from syenitic to syenogranitic (Serra da Barra). On the second domain predominate migmatitic paragneisses from of the Sertânia Complex. All these units were cut by Cambrian-Ediacaran felsic dykes and A-type granites. Three tectonic events were recognized. The first Dn episode represents a thrusting with tectonic transport to NW-NNW, being important the Sucuru and Carmo shear zones. The Dn+1 episode is a transcurrent event of Ediacaran age, being important in regional context the Coxixola and Congo shear zones. The final progressive late Ediacaran-Cambrian Dn+2 episode, culminated with the emplacement of the Sucuru dyke swarm and the Prata and Serra da Engabelada A-type granites. Petrographic evidences show that the Dn episode reached its metamoprhic peak on the granulitic or eclogitic facies, whose paragenesis was after re-equilibrated to the amphibolite during the Dn+1 event. The Dn+1 episode developed mylonitic corridors on the amphibolite facies with associated migmatization, while the Dn+2, event reached a greenschist facies restrict to influence of the late and post-tectonic intrusives. The occurrence of garnet amphibolites with symplectite texture along the metamorphic path Mn-Mn+1 suggests that the thrust event represented a high pressure metamorphic episode, indicating a probable Paleoproterozoic suture.

Metamorphic evolution of garnet amphibolite in the western Dabieshan eclogite belt, Central China: Evidence from petrography and phase equilibria modeling

The western Dabieshan terrane in central China involves numerous blocks of garnet amphibolite and eclogite enclosed in felsic gneiss and schist. Two representative garnet amphibolite samples SHS22 and BSW05 were selected for detailed study. They are composed of garnet, hornblendic amphibole, sodic plagioclase, epidote, quartz, biotite and/or muscovite, and small amounts of rutile, sphene and ilmenite. A few phengite flakes are found as inclusions in garnet in sample SHS22. Garnet varies in grain size and rim composition. The coarse-grained garnet porphyroblasts are mostly anhedral and exhibit compositional zoning with increasing pyrope and decreasing or constant grossular content from core to rim. Pseudosections calculated in the system NCKMnFMASHO suggest that this garnet zoning reflects prograde P–T paths of early-stage eclogite facies from 1.9GPa at 490°C to 2.3GPa at 555°C for sample SHS26 and from 1.8GPa at 450°C to 2.2GPa at 540°C for sample BSW05. The peak P–T conditions are constrained by the garnet rim compositions with the maximum pyrope (Xprp) and minimum grossular (Xgrs) contents. The Si contents in phengite from sample SHS22 were modeled to yield pressure conditions similar to that constrained by garnet. The other garnet rim compositions with decreasing Xprp and increasing Xgrs contents were modeled in P–M (H2O) pseudosections to reflect different degrees of the modification of eclogite facies to amphibolite facies in fluid-unsaturated conditions, indicating decreasing pressures from 0.7 to 0.5GPa and increasing water contents from 3.6 to 5.2mol.% at T=575°C for sample SHS22, and decreasing pressures from 0.8 to 0.55GPa and increasing water contents from 3.5 to 4.7mol.% at T=530°C for sample BSW05, respectively. The garnet rims with minimum Xprp and maximum Xgrs were predicted to be close to an equilibrium state with the matrix minerals involving hornblendic amphibole, plagioclase, epidote, biotite and quartz. The constituting assemblage was closely saturated with water. As it is hard to evaluate if the relevant minerals are in equilibrium or not in the garnet amphibolite with multi-generation assemblages, the conventional thermobarometers are not very useful for estimating the P–T conditions for this type of rock.

Insights on deep, accretionary subduction processes from the Sistan ophiolitic “mélange” (Eastern Iran)

The Sistan ophiolitic belt, formed by the closure of the N–S trending Sistan Ocean during late Cretaceous times, comprises several branches and basins across a 100×700km area along the Iran–Afghanistan border. One of these, the Ratuk complex, exposes disrupted HP ophiolitic blocks from a paleo-subduction complex generally interpreted as a tectonic “mélange”.In order to better understand its overall structure and evaluate the degree of mixing within this mélange, an extensive set of serpentinized peridotites, mafic rocks and metasediments was collected in the Sulabest area (Ratuk complex). A detailed geological and structural map of the Sulabest area is herein provided, in which three main units (the Western, Upper and Eclogitic Units) separated by relatively sharp tectonic contacts were identified. The latter two of these slices exhibit metamorphic evidence for burial along the same HP–LT gradient (up to blueschist and eclogite facies, respectively). Sharp differences in peak metamorphic conditions and retrograde parageneses nevertheless suggest that they followed two distinct P–T trajectories. Geochemical signatures of ultramafic rocks indicate an abyssal origin for the non-metamorphic Western Unit while the presence of mantle wedge serpentinites is inferred for some samples from the high-pressure units. The differences in peak temperatures (between 520 and 650°C) and the geochemical heterogeneity of mafic rocks suggest that tectonic mixing occurred (only) within the high-pressure units, possibly within the hydrated mantle wedge.Our results show that this portion of the Sistan ophiolitic belt did not form, as earlier proposed, by chaotic tectonic “mélange” (i.e. where small tectonic blocks with distinct P–T histories are mixed in a mechanically weak matrix). We instead propose that this segment of the ophiolitic belt formed via accretionary processes deep in the subduction zone, whereby distinct slices with different P–T histories were tectonically juxtaposed at ca. 20km depth during exhumation.

New evidence of blueschist facies rocks and their geotectonic implication for Variscan suture(s) in the Bohemian Massif

AbstractBlueschist facies rocks, exposed within consolidated continental blocks, provide some of the best evidence for the existence of previous suture(s). They usually occur as lenses or layers embedded within greenschist or amphibolite facies rocks and indicate reequilibration at medium‐ to low‐pressure conditions. In the Bohemian Massif, a few occurrences of blueschists have been reported, and here, new evidence of high‐pressure (HP) metamorphism in various lithologies is presented that suggests a larger extent of blueschist facies rocks along the northern border of this Massif. An earlier blueschist facies metamorphism is documented by inclusions of glaucophane in garnet, epidote and titanite from metabasites along with zoned white mica having a phengitic core and a muscovite rim in metapelites and orthogneisses. The estimated P–T conditions, obtained using pseudosections and mineral isopleths, correspond to blueschist and low‐temperature eclogite facies conditions (1.1–2.0 GPa at 350–550 °C). Together with medium‐temperature eclogites from different units in the Bohemian Massif they indicate a geothermal gradient of 8–10 °C km−1, which is typical for cool subduction. Radiometric dating on phengite from metapelites confirms an early Palaeozoic cooling age of c. 360 Ma for this HP metamorphic event. The presence of blueschist facies rocks, their P–T relations and age constraint together with those from eclogite facies rocks allows us to locate the Variscan suture, which straddles the SE margin of Saxothuringian Zone from Erzgebirge to Sudetes, and its possible continuation to the Moldanubian Zone, where eclogite facies and UHPM rocks are abundant.

Geochemistry and trace element behaviors of eclogite during its exhumation in the Xitieshan terrane, North Qaidam UHP belt, NW China

The Xitieshan terrane, located in the central part of the North Qaidam ultrahigh pressure (UHP) metamorphic belt, China, is mainly composed of orthogneiss and paragneiss and a few intercalated eclogite layers and boudins. Based on their bulk-rock TiO2-contents, the eclogites can be subdivided into a high-Ti group (TiO2>2%) and a low-Ti group (TiO2<2%). Whole-rock major and trace element analyses revealed that the protoliths of the low-Ti eclogites are normal-type mid-ocean ridge basalts (N-MORB), whereas those of the high-Ti eclogites are either enriched-type mid-ocean ridge basalts (E-MORB) or near ridge seamount basalts, respectively. The Sr–Nd isotopes of eclogites of both groups are similar to those of MORB. Those of the low-Ti eclogites are characterized by positive εNd(T) and restricted ISr values and therefore provide further evidence for the formation of the protoliths of the eclogites in an oceanic environment. On the other hand, the Sr–Nd isotopes of high-Ti eclogites show mainly positive but also some negative εNd(T) values and relatively broadly distributed ISr values, indicating minor crustal contamination of the ocean floor basalts. Considering available 750–877Ma protolith ages preserved in zircon cores, it is inferred that some of the eclogites derived from Neoproterozoic protoliths were emplaced onto the crust far ahead of the Paleozoic deep subduction, while the other eclogites originate from a different oceanic crust, e.g., the Paleo-Qilian ocean, indicating multiple orogenies in the geological history of the Xitieshan terrane, China.Whole-rock and in-situ LA-ICP-MS mineral trace element analyses of eclogites revealed two stages of fluid behavior during retrogression that correspond to the two exhumation stages uncovered by phase equilibrium calculations. The mineral scale trace element distributions and trace element inheritance of newly formed amphibole from its precursors indicate that, at the peak metamorphic stage (M1) and at the earlier (eclogite facies) overprint (M2), the fluid was internally controlled by the rock itself. Within a mafic lens, the amount of water-soluble elements (e.g., Rb, Sr, Ba, U, Pb and LREE), observed in the whole-rock compositions as well as in amphiboles, increases from the core (phengite-bearing eclogite) to the rim (amphibolite) and implies an external fluid source for the amphibolite facies retrogression (M3) which should be helpful for the final exhumation of UHP eclogite.

Zircon U–Pb and garnet Lu–Hf geochronology of eclogites from the Lhasa Block, Tibet

The Tibetan Plateau is the result of several collision events between a series of Gondwana-derived blocks (e.g., Qiangtang, Lhasa and India) and the Asian continent since the early Paleozoic. The occurrence of high-pressure eclogites has a significant bearing on the Paleo-Tethys subduction and plate suturing processes in this area. A quartzite, together with its host eclogite from the Jilang County in the Lhasa Block of northern Tibet, was investigated in terms of petrology, geochemistry, and combined Lu–Hf and U–Pb geochronology. The composition of the Jilang eclogite is similar to supra-subduction-zone basalts found in back-arc basins (LREE depleted, negative Nb–Ta anomalies, positive Pb anomalies, εNd(t)=~+8). The observations of coesite/coesite pseudomorph in the eclogites and the estimated P–T conditions of 3.4–3.8GPa and 753–790°C for the eclogites imply that the Jilang eclogites have undergone ultra-high-pressure metamorphism. The Lu–Hf age of 265.9±1.1Ma (2σ, 14 points, MSWD=1.8) of the Jilang eclogite most likely reflects garnet growth and is interpreted to reflect eclogite–facies metamorphism due to the occurrence of omphacite inclusions from core to rim in garnets and the spherical geometry effect despite the well-preserved prograde zoning in the garnets. U–Pb analyses of the metamorphic rims of zircons from eclogites indicate that two episodes of zircon growth occurred at ~261Ma and ~238Ma. The identical zircon 206Pb/238U age of 291±4Ma (2σ, MSWD=2.2, n=12) for the quartzite and 290.6±6.2Ma (2σ, MSWD=1.1, n=11) for the magmatic cores of the eclogite suggest that the detrital zircon in quartzite has the same origin as the protolith zircon in the eclogite. The initial opening of the back-arc basin in the Lhasa Block was no later than ca. 290Ma; the final closure of the back-arc basin in the Lhasa Block, led by the arc–continent collision between the Gangdese island arc and the northern Lhasa Block, was no earlier than ca. 266Ma.

Low stress deformation of garnet by incongruent dissolution precipitation creep

Microstructures indicating incongruent dissolution precipitation creep of garnet in eclogite-facies graphitic micaschist (Tauern window, Eastern Alps) are investigated. Garnet dissolution is observed where garnet poikiloblasts grown at eclogite facies metamorphism approached each other as a consequence of progressive deformation during exhumation, with estimated P-T-conditions between 570 °C, 1.7 GPa and 470 °C, 0.9 GPa. The poikiloblasts are separated by a dissolution seam and flanked by strain shadows filled with quartz, white mica, and chlorite; there is no evidence for crystal plastic deformation of garnet. Two cases are investigated: (A) stylolitic contact zone, (B) smooth contact zone. In both cases, internal fabrics of the poikiloblasts and concentric chemical zoning are truncated. Material previously forming inclusions in the garnet poikiloblasts is now passively enriched in a dissolution seam, the original microstructure of fine-grained mica–graphite aggregates remaining preserved. Though microstructures suggest that garnet dissolution was driven by local stress concentration, the level of differential stress remained too low for plastic deformation of the fine-grained white mica-graphite aggregates set free from the stress supporting garnet. Incongruent dissolution precipitation creep appears to be a particularly effective deformation mechanism at low stress in a subduction channel.

Fabrics produced mimetically during static metamorphism in retrogressed eclogites from the Zermatt-Saas zone, Western Italian Alps

Lattice preferred orientations (LPOs) are commonly interpreted to form by dislocation creep. Consequently they are used to infer deformation at the metamorphic grade at which the minerals were stable, especially if those minerals show a shape fabric. Here we show that LPOs can occur through mimicry of a pre-existing LPO, so they formed statically, not during deformation. Omphacite and glaucophane LPOs occur in eclogite facies rocks from the Zermatt-Saas Unit of the Northwest Italian Alps. Barroisite grew during greenschist facies retrogression and has an LPO controlled significantly by the eclogite facies omphacite and glaucophane LPOs, rather than directly by deformation. Using spatially resolved lattice orientation data from the three key minerals, collected using electron backscatter diffraction, we deploy a new technique of interphase misorientation distribution analysis to prove this. Barroisite LPO develops by mimicry of omphacite (via a particular lattice orientation relationship) and by direct topotactic and epitactic replacement of glaucophane. LPO in turn influenced anisotropic grain growth, resulting in a barroisite grain shape fabric. Thus regional retrogression during exhumation of the Zermatt-Saas high-pressure rocks was, in large part, static, rather than dynamic as previously interpreted. In general the possibility of mimetic fabrics forming during metamorphic reactions must be borne in mind when interpreting direct structural observations and seismic anisotropy data in terms of deformation, in both crust and mantle.

Garnet clinopyroxenite layers from the mantle sequences of the Northern Apennine ophiolites (Italy): Evidence for recycling of crustal material

This study aims to define the origin of garnet clinopyroxenite layers from the mantle sequences of the External Ligurian ophiolites. These mantle sequences retain a subcontinental origin and were exposed at a Jurassic ocean–continent transition. The garnet clinopyroxenites are mafic rocks with Mg# values of 66–71. Their chondrite-normalised REE patterns are characterised by severe LREE depletion (CeN/SmN=0.1–0.2) and nearly flat (Type-A pyroxenites) to moderately enriched HREE (Type-B pyroxenites). In addition, Type-A pyroxenites display a small positive Eu anomaly. The whole-rock REE variations are paralleled by the garnet REE compositions. We attribute the major and trace element characteristics of the garnet clinopyroxenites to recycling of gabbroic protoliths that underwent partial melting under eclogite facies conditions. The garnet clinopyroxenites may represent variably evolved garnet+clinopyroxene cumulates formed by eclogite-derived melts. In an alternative hypothesis, Type-A and -B pyroxenites are residual rocks after eclogite melting and cumulates derived from the eclogite melts, respectively. The high pressure fractionation event that gave rise to the garnet clinopyroxenites is considered of Triassic age on the basis of Sm–Nd and Lu–Hf isotope correlations. The Nd–Hf isotopic compositions of the garnet clinopyroxenites in the Triassic (εNd=+4.7 to +7.6, εHf=+4.4 to +12.8) lie below the mantle array, in agreement with recycled ancient MOR-type material. The oxygen isotopic composition of garnet and clinopyroxene from the garnet clinopyroxenites (δ18O=+4.9‰ to +5.2‰) may be reconciled with subduction-related recycling of the lowermost oceanic crust, or delamination and foundering of underplated gabbros from the continental lithosphere. The potential involvement of the garnet clinopyroxenites in the melting processes that gave rise to the MOR-type oceanic crust in the Jurassic would account for the moderate Nd isotope variability and the garnet geochemical signature of the ophiolitic basalts.

Provenance of eclogitic metasediments in the north Qilian HP/LT metamorphic terrane, western China: Geodynamic implications for early Paleozoic subduction-erosion

In this contribution, we present new petrological, geochemical, zircon U–Pb and Hf isotopic data for eclogitic metasediments from the north Qilian orogenic belt, in which early Paleozoic ophiolitic sequences and HP/LT metamorphic rocks have been previously recognized. The studied metasediments contain eclogite facies assemblages reflecting P–T conditions of 450–560°C and 1.9–2.4GPa, consistent with those of adjacent eclogites. REE compositions of the eclogitic metasediments overlap those of average upper continental crust. The SHRIMP and LA-MC-ICPMS U–Pb data of zircons from metasediments demonstrate input from sources with major age components about 500Ma and 1600–1900Ma (peak at ~1800Ma), with a smaller population at 660–800Ma (peak at 750Ma), and minor >1.9Ga zircon ages. The youngest detrital zircons suggest a maximum depositional age of ca. 475Ma, and combined with P–T conditions and previously determined metamorphic age suggest a burial rate of >0.6–0.7cm/yr. Zircon Hf isotopic analyses show that 500Ma zircons have positive εHf(t) (mainly between +8 and +16). By contrast, Meso- to Paleoproterozoic detrital zircons show a broad spectrum of εHf(t) (mainly between −5 and +10) with TDMHf of 1800–2500Ma. These data suggest that eclogitic metasediments are derived from a mixture of Proterozoic continent crust and juvenile early Paleozoic oceanic and/or island arc crust, and their protoliths likely were deposited in a terrigenous-dominated continental margin basin rather than a pelagic oceanic crust environment. The new results are consistent with subduction erosion of the active continental margin during south-dipping subduction, as these sediments, formed in a fore-arc environment close to the Qilian block to the south, were transported in the subduction zone to 60–70km depth prior to their exhumation.

Nb–Ta fractionation induced by fluid‐rock interaction in subduction‐zones: constraints from UHP eclogite‐ and vein‐hosted rutile from the Dabie orogen, Central‐Eastern China

AbstractNiobium and Ta concentrations in ultrahigh‐pressure (UHP) eclogites and rutile from these eclogites and associated high pressure (HP) veins were used to study the behaviour of Nb–Ta during dehydration and fluid‐rock interaction. Samples were collected through a ∼2 km profile at the Bixiling complex in the Dabie orogenic belt, Central‐Eastern China. All but one eclogite away from veins (EAVs) display nearly constant Nb/Ta ratios ranging from 16.1 to 19.2, with an average of 16.9 ± 0.8 (2 SE), similar to that of their gabbroic protolith from the Yangtze Block. Nb/Ta ratios of rutile from the EAVs range from 12.7 to 25.3 among different individual grains, with the average values close to those of the corresponding bulk rocks. These observations show that Nb and Ta were not significantly fractionated by prograde metamorphism up to eclogite facies when no significant fluid‐rock interaction occurs. In contrast, Nb/Ta ratios of rutile from eclogites close to veins (ECVs) are highly variable from 17.8 to 49.8, which are systematically higher (by up to 17) than those of rutile from the veins. These observations demonstrate that Nb and Ta were mobilized and fractionated during localized fluid flow and intensive fluid‐rock interaction. This is strongly supported by Nb/Ta zoning patterns in single rutile grains revealed by in situ LA‐ICP‐MS analysis. Ratios of Nb/Ta in the ECV‐hosted rutile decrease gradually from cores towards rims, whereas those in the EAV‐hosted rutile are nearly invariable. Furthermore, the vein rutile shows Nb/Ta zoning patterns that are complementary to those in rutile from their immediate hosts (ECVs), suggesting an internal origin for the vein‐forming fluids. The Nb/Ta ratios of such fluids evolved from low values at the early stage of subduction to higher values at later supercritical conditions with increased temperature and pressure. Quantitative modelling was conducted to constrain the compositional evolution of metamorphic fluids during dehydration and fluid‐rock interaction focusing on Nb–Ta distribution. The modelling results based on our proposed multistage fluid phase evolution path can essentially reproduce the natural observations reported in the present study.

Petrology and geochemistry of the Běstvina granulite body metamorphosed at eclogite facies conditions, Bohemian Massif

Petrology and geochemistry of the Běstvina granulite body metamorphosed at eclogite facies conditions, Bohemian Massif

Slow subduction and buoyant exhumation of the Sanbagawa eclogite

The Western Iratsu body of the Sanbagawa belt (SW Japan) represents a <2km thick garnet–epidote amphibolite slice with minor intercalated occurrences of eclogite. Equilibrium phase assemblage diagrams (pseudosections) for eclogite and garnet–epidote amphibolite from this body can be successfully used to reconstruct the pressure (P)–temperature (T) path and the evolution of the associated mineral assemblages. Microstructural observations combined with calculated pseudosections reveal two distinct stages of subduction-related metamorphism referred to as M1 and M2. P–T conditions of M1 evolved from 0.9GPa, 570°C to 1.2GPa, 660°C, implying relatively high thermal gradients possibly developed shortly after the initiation of subduction. The available age data and the characteristics of the P–T path suggest that the Western Iratsu body was detached from the slab at 1.2GPa (~40km depth) and the subsequent subduction occurred at a very slow rate due to weak mechanical coupling to the downgoing slab. The prograde metamorphism associated with the second subduction phase (M2) reached the eclogite facies (~1.8GPa, 510–560°C), but mafic rock compositions recrystallized into either eclogite or garnet–epidote amphibolite assemblages, depending on the bulk rock (CaO+Na2O)/(FeO+MgO) ratio. The thermal peak of M2 (~620°C at 1.6GPa) was attained during the early phase of exhumation. Further decompression and re-equilibration (M3) took place in the epidote-amphibolite facies. The M2 to M3 P–T evolution is associated with a progressive increase in thermal gradient (T/P ratio), which can be explained by thermal modeling as the result of the inflow of a very young hot section of the slab. The bulk density of the whole Western Iratsu body at the point of onset of exhumation—the return point—is estimated to be ~3180kgm−3, which is less than the density of mantle, implying that buoyant rise is a viable mechanism for its initial exhumation. Buoyant rise is in accordance with the widespread early-exhumation related deformation that has a dip-parallel stretching lineation. Exhumation of the buoyant discrete mafic slice along the decoupled slab–mantle interface could have been triggered by the heating that lead to fluid liberation and a resultant reduction of the rock strength.

A high-pressure folded klippe at Tehuitzingo on the western margin of an extrusion zone, Acatlán Complex, southern México

High-pressure (HP) rocks at Tehuitzingo, on the western margin of the HP belt within the Paleozoic Acatlán Complex (southern México), occur in a klippe that was thrust over low-grade clastic rocks. The youngest detrital zircon cluster in the low-grade rocks yielded U-Pb ages of 481±16Ma, which provide an older limit for deposition. The HP rocks are composed of metabasites, serpentinite, granite (482±3Ma) and mica schist (youngest concordant detrital zircon: 433±3Ma). The schist and granite are inferred to be high-grade equivalents of lower Paleozoic, low-grade rocks exposed elsewhere in the Acatlán Complex, from which they are inferred to have been removed by subduction erosion. Mineral analyses indicate that the subducted rocks underwent HP metamorphism and polyphase deformation at depths of ~50km (~16kbar and 750°C: eclogite facies). Subsequent retrogression passed through epidote-amphibolite to greenschist facies, which was synchronous with W-vergent thrusting over the low-grade clastic rocks. Deposition of the low-grade rocks and thrusting are bracketed between either 481–329Ma (Ordovician-Mississippian), and was followed by F3 synformal folding. Cooling through ca. 385°C is indicated by 329±1 and 316–317±2Ma, 40Ar/39Ar muscovite plateau ages in HP rocks, which are 5–17 my younger than those of the adjacent Piaxtla eclogites suggesting younger exhumation. The petrology, P-T conditions and ages of the Piaxtla Suite is consistent with an extrusion channel within the Acatlán Complex along the active western margin of Pangea during the Carboniferous. Detrital zircon populations in the low-grade psammite (ca. 481, 520–650, 720, 750, 815, 890, 1050 and 2750Ma) and the HP schist (ca. 457–480, 534, 908, 954–1150, 1265, 1845 and 2035Ma) indicate derivation from the Ordovician Acatlán granitoids, Neoproterozoic Brasiliano orogens, 900–750Ma Goiás arc (Amazonia), 1–1.3Ma Oaxaquia, and more ancient sources in Oaxaquia/Amazonia.