Mafic Eclogites Research Articles

The signature of devolatisation: Extraneous 40Ar systematics in high-pressure metamorphic rocks

The validity of using the 40Ar/39Ar system for thermochronology relies on the assumption that the source mineral is surrounded by a grain boundary reservoir defined by an effective 40Ar concentration of zero. However, the presence of extraneous 40Ar (Are) in metamorphic rocks shows that this assumption is invalid for a significant number of cases. Are is common in micas that have equilibrated under (ultra-)high pressure ((U)HP) conditions: metasediments from six Phanerozoic (U)HP terranes yield apparent 40Ar/39Ar phengite ages ≲50% in excess of the age of peak (U)HP conditions, whereas cogenetic mafic eclogites yield ages up to ∼700% older despite lower K2O concentrations. A model is developed that calculates Are age fractions as a function of variable mica–fluid KD, bulk K2O and porosity under closed system conditions. Measured Are concentrations in mafic eclogites are reproduced only when porosities are ≲10-4 volume fraction, showing that mafic protoliths operate as closed systems to advective solute transport during subduction. Porosities in eclogite-facies metapelites are ≲10-2, reflecting loss of significant volumes of lattice-bound H2O relative to mafic rocks during subduction. Retention of locally-generated 40Ar in mafic eclogites shows that the oceanic crust is an efficient vehicle for volatile transport to the mantle.

Integrated pressure–temperature–time constraints for the Tso Morari dome (Northwest India): implications for the burial and exhumation path of UHP units in the western Himalaya

AbstractNorthward subduction of the leading edge of the Indian continental margin to depths greater than 100 km during the early Eocene resulted in high‐pressure (HP) quartz‐eclogite to ultrahigh‐pressure (UHP) coesite–eclogite metamorphism at Tso Morari, Ladakh Himalaya, India. Integrated pressure–temperature–time determinations within petrographically well‐constrained settings for zircon‐ and/or monazite‐bearing assemblages in mafic eclogite boudins and host aluminous gneisses at Tso Morari uniquely document segments of both the prograde burial and retrograde exhumation path for HP/UHP units in this portion of the western Himalaya. Poikiloblastic cores and inclusion‐poor rims of compositionally zoned garnet in mafic eclogite were utilized with entrapped inclusions and matrix minerals for thermobarometric calculations and isochemical phase diagram construction, the latter thermodynamic modelling performed with and without the consideration of cation fractionation into garnet during prograde metamorphism. Analysis of the garnet cores document (M1) conditions of 21.5 ± 1.5 kbar and 535 ± 15 °C during early garnet growth and re‐equilibration. Sensitive high resolution ion microprobe (SHRIMP) U–Pb analysis of zircon inclusions in garnet cores yields a maximum age determination of 58.0 ± 2.2 Ma for M1. Peak HP/UHP (M2) conditions are constrained at 25.5–27.5 kbar and 630–645 °C using the assemblage garnet rim–omphacite–rutile–phengite–lawsonite–talc–quartz (coesite), with mineral compositional data and regional considerations consistent with the upper P–T bracket. A SHRIMP U–Pb age determination of 50.8 ± 1.4 Ma for HP/UHP metamorphism is given by M2 zircons analysed in the eclogitic matrix and that are encased in the garnet rim. Two garnet‐bearing assemblages from the Puga gneiss (host to the mafic eclogites) were utilized to constrain the subsequent decompression path. A non‐fractionated isochemical phase diagram for the assemblage phengite–garnet–biotite–plagioclase–quartz–melt documents a restricted (M3) P–T stability field centred on 12.5 ± 0.5 kbar and 690 ± 25 °C. A second non‐fractionated isochemical phase diagram calculated for the lower pressure assemblage garnet–cordierite–sillimanite–biotite–plagioclase–quartz–melt (M4) documents a narrow P–T stability field ranging between 7–8.4 kbar and 705–755 °C, which is consistent with independent multiequilibria P–T determinations. Th–Pb SHRIMP dating of monazite cores surrounded by allanite rims is interpreted to constrain the timing of the M4 equilibration to 45.3 ± 1.1 Ma. Coherently linking metamorphic conditions with petrographically constrained ages at Tso Morari provides an integrated context within which previously published petrological or geochronological results can be evaluated. The new composite path is similar to those published for the Kaghan UHP locality in northern Pakistan, although the calculated 12‐mm a−1 rate of post‐pressure peak decompression at Tso Morari would appear less extreme.

The temperature and compositional dependence of disordering in Fe-bearing dolomites

Dolomite occurs in a wide range of rock compositions, from peridotites to mafic eclogites and metasediments, up to mantle depths of more than 200 km. At low-temperatures dolomite is ordered ( R 3), but transforms with increasing temperature into a disordered higher symmetry structure ( R 3 c ). To understand the thermodynamics of dolomite, we have investigated temperature, pressure, kinetics, and compositional dependence of the disordering process in Fe-bearing dolomites. To avoid quench effects, in situ X-ray powder diffraction experiments were performed at 300–1350 K and 2.6–4.2 GPa. The long-range order parameter s , quantifying the degree of ordering, has been determined using structural parameters from Rietveld refinement and the normalized peak area variation of superstructure Bragg peaks characterizing structural ordering/disordering. Time-series experiments show that disordering occurs in 20–30 min at 858 K and in a few minutes at temperatures ≥999 K. The order parameter decreases with increasing temperature and X Fe. Complete disorder is attained in dolomite at ~1240 K, 100–220 K lower than previously thought, and in an ankeritic-dolomites.s. with an X Fe of 0.43 at temperatures as low as ~900 K. The temperature-composition dependence of the disorder process was fitted with a phenomenological approach intermediate between the Landau theory and the Bragg-Williams model and predicts complete disorder in pure ankerite to occur already at ~470 K. The relatively low-temperature experiments of this study also constrain the breakdown of dolomite to aragonite+Fe-bearing magnesite at 4.2 GPa to temperature lower than ~800 K favoring an almost straight Clapeyron-slope for this disputed reaction.

Metamorphic rocks seek meaningful cooling rate: Interpreting 40Ar/39Ar ages in an exhumed ultra-high pressure terrane

40Ar/39Ar dating of metamorphic white mica is commonly used to elucidate the timing of cooling and/or exhumation of metamorphic terranes. In theory, so long as the dated mineral originally crystallised above its closure temperature, the yielded 40Ar/39Ar ages should not vary significantly within or between samples, and should vary only due to grain size (~3Ma variation depending on the estimated cooling rate). Infra-red laser single grain fusion and ultra-violet laser spot muscovite and/or phengite data from mafic eclogite bodies and their host gneisses in the Nordfjord region of the Western Gneiss Complex (WGC), Norway, show that the ages yielded both within and between white micas from the same mafic eclogite sample vary by up to 284Ma. Ages yielded within the same pelitic gneiss sample vary by up to 28Ma. Overall the 40Ar/39Ar phengite dates from mafic eclogites range from ~750 to 440Ma and are significantly older than the previously reported 405–400Ma U–Pb zircon age constraining peak metamorphism in the Nordfjord WGC. The felsic host gneiss samples, which recrystalised during a post-peak, exhumation-related, amphibolite-facies overprint, yield single grain fusion and laser spot 40Ar/39Ar white mica dates spanning ~420 to 385Ma, encompassing previous estimates of the timing of the entire metamorphic cycle. These data suggest that Ar is heterogeneously distributed both within grains and between grains on the cm to sub-mm scale in all samples, and imply restricted Ar transport and inefficient removal during the metamorphic cycle. Open system (zero grain boundary Ar concentration) behaviour is an inherent assumption in the commonly applied closure temperature formulation, but unless this can be demonstrated, linking 40Ar/39Ar ages to temperature is no longer trivial. The high spatial and chronological precision afforded by UV laser ablation mean that intra-grain Ar concentration distributions may be used to assess open vs. closed system behaviour. The data presented here suggest that open system behaviour cannot be assumed in metamorphic systems, even those in which the pressure, temperature and time conditions in theory allow for efficient diffusion during the metamorphic cycle. High spatial resolution 40Ar/39Ar data and robust comparison with numerical models of open system behaviour provide the greatest future potential for unravelling the complexities of metamorphic exhumation timescales.

Crustal stacking and expulsion tectonics during continental subduction: P-T deformation constraints from Oman

[1] The northeastern continental margin of Oman in the Saih Hatat region is characterized by high-pressure (HP) chloritoid- or carpholite-bearing metasediments and highly deformed mafic eclogites and blueschists in a series of tectonic units bounded by high-strain ductile shear zones. New data on the upper cover units of this HP nappe stack indicate that all of them underwent similar P conditions to the underlying Hulw structural unit (with a cooler exhumation pressure-temperature path). Early SSW directed crustal thickening during ophiolite emplacement created recumbent folds and strong schistose fabrics in these Permian-Mesozoic shelf carbonates and was followed by later NNE dipping normal sense shear zones and normal faults. The Mayh unit shows high strain in a 15–25 km long sheath fold that likely formed at carpholite grade pressures of 8–10 kbar. We show that there are no significant P differences across the Hulw shear zone (upper plate–lower plate discontinuity) or between the overlying Mayh, Yenkit-Yiti, and Ruwi units. Postpeak metamorphic exhumation of the HP rocks was therefore accomplished by bottom-to-SSW (rather than top-to-NNE) active footwall extrusion beneath a fixed, static, passive hanging wall. Footwall uplift beneath these passive roof faults resulted in progressive expulsion of the HP rocks from depths of ∼80–90 km (eclogites) and mainly 30–35 km (blueschists and chloritoid-/carpholite-bearing units) during the Campanian–Early Maastrichtian. Oman thus provides a detailed record of how continental material (thick platform shelf carbonates) progressively jammed a subduction zone and emphasizes the contrasting behavior between cover units and their underlying basement.

Initial water budget: The key to detaching large volumes of eclogitized oceanic crust along the subduction channel?

We herein investigate the extent to which extensive hydration of the oceanic lithosphere influences the preservation and exhumation of large-scale ophiolite bodies from subduction zones. The Zermatt–Saas ophiolite (ZS, W. Alps), which was subducted during the late stages of oceanic subduction, preserves a complete section of Mesozoic Tethys oceanic lithosphere and particularly fresh eclogites, and represents, so far, the largest and deepest known portion of exhumed oceanic lithosphere. Pervasive hydrothermal processes and seafloor alteration led to the incorporation of large amounts of fluid bound in the hydrated upper layers of the oceanic crust (now as lawsonite eclogites, glaucophanites, and chloritoschists) and in associated ultramafic rocks. Internally, the ZS ophiolite is made up of a series of tectonic slices of oceanic crust (150–300 m thick) which are systematically separated by a 5 to 100 m thick layer of serpentinite. This stack of slices is separated from the underlying eclogitized continental crust (e.g., Monte Rosa) by a thick (~ 500 m) serpentinite sole. Field observations, textural relationships and pseudosection modelling reveal that lawsonite was abundant and widespread in mafic eclogites when the ophiolite detached from the slab at around 550 °C and 24 kbar. Comparison between fresh eclogitic samples and pseudosection modelling shows that (i) water remained in excess from burial to eclogitic peak conditions, (ii) the lightest eclogitized metabasalts correspond to the portions of oceanic crust where metasomatism was the strongest, (iii) crystallization of widespread hydrated parageneses (such as lawsonite, glaucophane and phengite) instead of garnet and omphacite decreased by 5 to 10% the rock density and subsequently enhanced its buoyancy. We propose that this density decrease acted as a ‘float’ which prevented the slices from an irreversible sinking in the mantle. These slices were subsequently detached from the downgoing slab and stacked in the serpentinized subduction channel at pressures between 15 and 20 kbar, in the epidote blueschist facies. Exhumation of the underlying, positively buoyant continental crust dragged this “frozen” nappe-stack from the subduction channel towards the surface.

Garnet–chloritoid–kyanite assemblages: eclogite facies indicators of subduction constraints in orogenic belts

AbstractThe assemblage garnet–chloritoid–kyanite is shown to be quite common in high‐pressure eclogite facies metapelites from orogenic belts around the world, and occurs over a narrowly restricted range of temperature ∼550–600 °C, between 20 and 25 kbar. This assemblage is favoured particularly by large Al2O3:K2O ratios allowing the development of kyanite in addition to garnet and chloritoid. Additionally, ferric iron and manganese also help stabilize chloritoid in this assemblage. Pseudosections for several bulk compositions illustrate these high‐pressure assemblages, and a new thermodynamic model for white mica to include calcium and ferric iron was required to complete the calculations. It is extraordinary that so many orogenic eclogite facies rocks, both mafic eclogites sensu stricto as well as metapelites with the above assemblage, all yield temperatures within the range of 520–600 °C and peak pressures ∼23±3 kbar. Subduction of oceanic crust and its entrained associated sedimentary material must involve the top of the slab, where mafic and pelitic rocks may easily coexist, passing through these P–T conditions, such that rocks, if they proceed to further depths, are generally not returned to the surface. This, together with the tightly constrained range in peak temperatures which such eclogites experience, suggests thermal weakening being a major control on the depths at which crustal material is decoupled from the downgoing slab.

Pressure‐temperature history of titanite‐bearing eclogite from the Western Iratsu body, Sanbagawa Metamorphic Belt, Japan

AbstractEvidence for eclogite‐facies metamorphism is widespread in the Western Iratsu body of the oceanic subduction type Sanbagawa Belt, Southwest Japan. Previous studies in this region focused on typical mafic eclogites and have revealed the presence of an early epidote‐amphibolite facies metamorphism overprinted by a phase of eclogite facies metamorphism. Ca‐rich and titanite‐bearing eclogite, which probably originated from a mixture of basaltic and calc‐siliceous sediments, is also relatively common in the Western Iratsu body, but there has been no detailed petrological study of this lithology. Detailed petrographic observations reveal the presence of a relic early epidote‐amphibolite facies metamorphism preserved in the cores of garnet and titanite in good agreement with studies of mafic eclogite in the area. Thermobarometric calculations for the eclogitic assemblage garnet + omphacite + epidote + quartz + titanite ± rutile ± phengite give peak‐P of 18.5–20.5 kbar at 525–565°C and subsequent peak‐T conditions of about 635°C at 14–16 kbar. This eclogite metamorphism initiated at about 445°C/11–15 kbar, implying a significantly lower thermal gradient than the earlier epidote‐amphibolite facies metamorphism (∼650°C/12 kbar). These results define a P–T path with early counter‐clockwise and later clockwise trajectories. The overall P–T path may be related to two distinct phases in the tectono‐thermal evolution in the Sanbagawa subduction zone. The early counter‐clockwise path may record the inception of subduction. The later clockwise path is compatible with previously reported P–T paths from the other eclogitic bodies in the Sanbagawa Belt and supports the tectonic model that these eclogitic bodies were exhumed as a large‐scale coherent unit shortly before ridge subduction.

Nitrogen recycling in subducted oceanic lithosphere: The record in high- and ultrahigh-pressure metabasaltic rocks

This paper provides the first measurements of the nitrogen (N) concentrations and isotopic compositions of high- and ultrahigh-pressure mafic eclogites, aimed at characterizing the subduction input flux of N in deeply subducting altered oceanic crust (AOC). The samples that were studied are from the Raspas Complex (Ecuador), Lago di Cignana (Italy), the Zambezi Belt (Zambia) and Cabo Ortegal (Spain), together representing subduction to 50–90 km depths. The eclogites contain 2–20 ppm N with δ 15N air values ranging from −1 to +8‰. These values overlap those of altered oceanic crust, but are distinct from values for fresh MORB (for the latter, ∼1.1 ppm N and δ 15N air ∼ −4‰). Based on N data in combination with other trace element data, the eclogite suites can be subdivided into those that are indistinguishable from their likely protolith, AOC, with or without superimposed effects of devolatilization (Lago di Cignana, Cabo Ortegal), and those that have experienced metasomatic additions during subduction-zone metamorphism (Zambezi Belt, Raspas). For the former group, the lack of a detectable loss of N in the eclogites, compared to various altered MORB compositions, suggests the retention of N in deeply subducted oceanic crust. The metasomatic effects affecting the latter group can be best explained by mixing with a (meta)sedimentary component, resulting in correlated enrichments of N and other trace elements (in particular, Ba and Pb) thought to be mobilized during HP/UHP metamorphism. Serpentinized and high-pressure metamorphosed peridotites, associated with the eclogites at Raspas and Cabo Ortegal, contain 3–15 ppm N with δ 15N air values ranging from +3 to +6‰, significantly higher than the generally accepted values for the MORB mantle (δ 15N air ∼ −5‰). Based on their relatively high N contents and their homogeneous and positive δ 15N values, admixing of sedimentary N is also indicated for the serpentinized peridotites. One possible pathway for the addition of sediment-derived N into eclogites and peridotites involves mixing with fluids along the slab–mantle wedge interface. Alternatively, sedimentary N could be incorporated into peridotites during serpentinization at bending-related faults at the outer rise and, during later deserpentinization, released into fluids that then infiltrate overlying rocks. Deep retention of N in subducting oceanic crust should be considered in any attempt to balance subduction inputs with outputs in the form of arc volcanic gases. If materials such as these eclogites and serpentinized peridotites are eventually subducted to beyond sub-arc depths into the deeper mantle, containing some fraction of their forearc–subarc N inventory (documented here), they could deliver isotopically heavy N into the mantle to potentially be sampled by plume-related magmas.

Metamorphic P–T conditions and thermal structure of Chinese Continental Scientific Drilling main hole eclogites: Fe–Mg partitioning thermometer vs. Zr‐in‐rutile thermometer

AbstractCore rocks recovered from the main hole (5158 m deep) of the Chinese Continental Scientific Drilling (CCSD‐MH) project, southern Sulu UHP terrane, east‐central China, consist of eclogites, various gneisses and minor metaperidotite cumulates; this lithological section underwent subduction‐zone UHP metamorphism. Coesite‐bearing eclogites are mainly present between the depths of 100–2000 m, but below 2000 m, mafic eclogites are rare. Selected elements (Zr, Nb, Cr, Fe, Si, Mg, Al & Ti) in rutile from 39 eclogite cores from 100 to 2774 m, and major elements of minerals from representative eclogites were analysed by electron microprobe. Zirconium and Nb concentrations of rutile cluster ∼100–400 and 200–700 ppm respectively. However, Zr and Nb contents in rutile from strongly retrograded eclogites show larger variations than those of fresh or less retrograded eclogites, implying that somehow fluid infiltration affected rutile chemistry during retrograde metamorphism. Zr contents in rutile inclusions in garnet and omphacite are slightly lower than those of the matrix rutile, suggesting that the rutile inclusions formed before or close to the peak temperature. The P–T conditions of the CCSD‐MH eclogites were estimated by both Fe–Mg exchange and Zr‐in‐rutile thermometers, as well as by the Grt–Cpx–Phn–Ky geothermobarometer. The maximum temperature range of 700–811 °C calculated at 40 kbar using the Zr‐in‐rutile thermometer is comparable with temperature estimates by the Fe–Mg exchange thermometer. The temperature estimates of eclogites in a ∼3000 m thick section define a continuous gradient, and do not show a distinct temperature gap, suggesting that the rocks from 100 to 3000 m depth might belong to a single, large‐scale UHP slab. These data combined with P–T calculations for CCSD‐MH peridotites yield a low geotherm (∼5 °C km−1) for the Triassic subduction zone between the Sino‐Korean and Yangtze cratons; it lies ∼30–35 mW m−2 conductive model geotherm.

Partial melting of deeply subducted continental crust and the formation of quartzofeldspathic polyphase inclusions in the Sulu UHP eclogites

Two types of quartzofeldspathic inclusions hosted by omphacite and garnet were identified in the Sulu UHP eclogites. The first consists of albite, quartz, and various amounts of K-feldspar. In contrast, the second consists predominantly of K-feldspar and quartz without any albite. The presence of quartzofeldspathic inclusions within the UHP mafic eclogites indicates that partial melting occurred in deeply subducted continental crust via mica dehydration melting reactions at an early stage of rapid exhumation. Such a melting event generated hydrous Na-K-Al-Si melts. These melts infiltrated into the mafic eclogite and were captured by recrystallizing garnet or omphacite, which together followed by dehydration and crystallization to form feldspar-bearing polyphase inclusions. Formation of silicate melts within the deeply subducted continental slab not only provides an excellent medium to transport both mobile (LILE) and immobile (HFSE) elements over a large distance, but also induces effective changes in the physical properties of the UHP slab. This process could be a major factor that enhances rapid exhumation of a deeply subducted continental slab.

Geochronology of granulitized eclogite from the Ama Drime Massif: Implications for the tectonic evolution of the South Tibetan Himalaya

The Ama Drime Massif (ADM) is an elongate north‐south trending antiformal feature that extends ∼70 km north across the crest of the South Tibetan Himalaya and offsets the position of the South Tibetan Detachment system. A detailed U(‐Th)‐Pb geochronologic study of granulitized mafic eclogites and associated rocks from the footwall of the ADM yields important insights into the middle to late Miocene tectonic evolution of the Himalayan orogen. The mafic igneous precursor to the granulitized eclogites is 986.6 ± 1.8 Ma and was intruded into the paleoproterozoic (1799 ± 9 Ma) Ama Drime orthogneiss, the latter being similar in age to rocks previously assigned to the Lesser Himalayan Series in the Himalayan foreland. The original eclogite‐facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite facies metamorphism at 750°C and 0.7–0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with synkinematic sillimanite‐grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at <13.2 ± 1.4 Ma. High‐grade metamorphism was followed by postkinematic leucogranite dyke emplacement at 11.6 ± 0.4 Ma. This integrated data set indicates that high‐temperature metamorphism, decompression, and exhumation of the ADM postdates mid‐Miocene south directed midcrustal extrusion and is kinematically linked to orogen‐parallel extension.

The Léon Domain (French Massif Armoricain): a westward extension of the Mid-German Crystalline Rise? Structural and geochronological insights

The Léon Domain in the NW part of the French Massif Armoricain is a stack of synmetamorphic nappes displaced from south to north in ductile conditions. From bottom to top, an orthogneissic basement is overthrusted successively by (1) a Lower Nappe of gneiss including mafic eclogites, (2) an Intermediate Nappe of biotite–garnet–staurolite micaschists with mafic blocks, and (3) an upper nappe made up of Neoproterozoic phyllites covered by unmetamorphosed Paleozoic sedimentary series. This microstructural study documents a polyphase evolution with firstly a top-to-the-N shearing, secondly followed by upright folding of the stack of nappes coeval with migmatization, and lastly, a dextral wrenching along the North Armorican Shear Zone associated with emplacement of synkinematic plutons. New U–Th/Pb chemical dating of monazites from biotite–garnet–staurolite micaschists, migmatites, and granitoids argue for 340–335 Ma, 335–327 Ma, and about 320 Ma ages for synthrusting metamorphism, anatexis, and wrenching, respectively. A metagabbro from Le Conquet yields a zircon LA-ICP-MS age of 478 ± 4 Ma, which corresponds to magma emplacement time. The Léon Domain is interpreted as a microcontinent separated from Armorica by the Le Conquet-Penzé suture to the south and east, and from Laurussia by the Rheic suture to the north. A possible correlation with the Mid-German Crystalline Rise of Central Europe is discussed.

Ultrahigh pressure macro diamonds from Copeton (New South Wales, Australia), based on Raman spectroscopy of inclusions

Mining of Cenozoic alluvial deposits at Copeton and Bingara (Eastern Australia) has produced two million macrodiamonds (0.25 ct median size). Raman spectroscopy is used to identify included minerals within uncut Copeton diamonds, with sealed chamber remnant pressures of 31.7 to 35.6 kbar for coesite, 13.6 and 22.7 kbar for clinopyroxene, and 7.6 kbar for grossular garnet. Assuming elastic behaviour, these values generate inclusion entrapment PT loci which intersect, restricting diamond formation conditions: from 250 °C, 43 kbar to 800 °C, 52 kbar. Larger than error (± 100 °C and ± 4 kbar), this range shows a systematic variation in inclusion composition with diamond zoning and N properties. Published research shows 1) Copeton and Bingara diamonds are unique, and 2) modern alluvium in the Bingara district carries mantle-formed garnet, captured by post-tectonic alkali basalt from an extensive diamondiferous ultrahigh pressure (UHP) terrane that stalled at depth because it is dominated by mafic eclogite. The combined Raman and geological results indicate two sets of subduction UHP diamond formation conditions/protolith are required, firstly cooler oceanic slab and secondly including higher temperature continental crust. The Copeton and Bingara stones are UHP macrodiamonds, and Carboniferous 40Ar/ 39Ar age dates on clinopyroxene inclusions should be interpreted as ages of crystallisation, representing the termination of subduction. The characteristic features of ruptured inclusions and etched percussion marks on Copeton and Bingara diamond indicate volcanic delivery to the earth's surface. Alluvial deposits elsewhere in Eastern Australia may carry similar diamond along with diamond of different origin.

High-Pressure Metamorphism in the Texas Grenville Orogen: Mesoproterozoic Subduction of the Southern Laurentian Continental Margin

Collisional orogenesis in the Llano Uplift of central Texas during the late Mesoproterozoic drove metamorphism that comprised both an initial high-pressure (HP) phase (610-775°C at 1.4-2.4 GPa) and a subsequent moderate-pressure (MP) phase (~700°C at ~0.7 GPa). A later, low-pressure overprint (525-625°C at 0.3 GPa) took place under largely static conditions. Evidence for HP metamorphism is geographically widespread, but confined principally to boudins of mafic eclogite encased in felsic gneisses. The geographic distribution of P-T conditions for HP metamorphism inferred from the eclogites, combined with evidence from the relative degrees of homogenization of growth zoning in garnet from both eclogites and felsic gneisses, suggests a general increase from northeast to southwest in depths of burial during HP metamorphism. Exhumation of HP rocks to shallower depths prior to MP metamorphism appears to have been rapid: ages for the two phases of metamorphism overlap, on the basis of the very limited data now available. The region's early metamorphic history is best explained by southwestward subduction of the Laurentian continental margin during collision with a still-unidentified continental mass, followed by buoyancy-driven uplift to lower crustal levels while collisional contraction continued.

P-T Path for Ultrahigh-Pressure Garnet Ultramafic Rocks of the Cuaba Gneiss, Rio San Juan Complex, Dominican Republic

Ultrahigh-pressure (UHP) rocks in the Cuaba Gneiss include Grt ultramafic rocks, mafic eclogite, and partially retrograded equivalents. The Grt ultramafic rocks (Spl-bearing Grt peridotite, Splbearing Grt clinopyroxenite, Crn-Spl-bearing Grt clinopyroxenite) are of igneous origin, with magmatic conditions of P > 3.5 GPa, T > 1550°C. The magmatic history took place in the asthenosphere. New chemical analyses of minerals give the following subsolidus conditions: Grt peridotite, 3.0-4.2 GPa, 838-867°C; Grt clinopyroxenite (Grt + Cpx + Spl + Crn), 2.75 GPa, 807°C. The ultramafic rocks are associated with partially retrograded eclogite, interpreted as deep-subducted oceanic crust. New chemical analyses of minerals in the eclogite give conditions that relate to the retrograde decomposition of Grt + Omp + Qtz to Grt + Pl + Di + Qtz, 1.8 GPa, 730°C. The P-T path for the Grt ultramafic rocks is modeled in three parts: (1) slow, isobaric (> 4 GPa) cooling in the mantle, >1550°C down to ∼850°C; (2) relatively rapid, nearly adiabatic (?) decompression, ∼4 GPa (∼850°C) down to ∼1 GPa (∼700°C); and (3) relatively rapid, non-adiabatic decompression and cooling. The first part of the path (1) took place in the mantle above the subduction zone and relates to delivery of the Grt ultramafic rocks to the subduction zone. Incorporation of Grt ultramafic rocks in the deep-subducted oceanic crust (eclogite) marked the end of this part of the path. The second part of the path (2) was associated with transport up the subduction zone. Retrograde P-T conditions for the eclogite fall on this part of the path, supporting the idea that the Grt ultramafic rocks were transported as blocks in the eclogite. The third part of the path (3) relates to final uplift to the surface.

The geodynamic evolution of the Southern European Variscides: constraints from the U/Pb geochronology and geochemistry of the lower Palaeozoic magmatic-sedimentary sequences of Sardinia (Italy)

The high-grade metamorphic complex of northern Sardinia consists of a strongly deformed sequence of migmatitic ortho- and paragneisses interlayered with minor amphibolites preserving relic eclogite parageneses. The protolith ages and geochemical characteristics of selected gneiss samples were determined, providing new constraints for reconstructing the Palaeozoic geodynamic evolution of this sector of the Variscan chain. The orthogneisses are metaluminous to peraluminous calcalkaline granitoids with crustal Sr and Nd isotopic signatures. One orthogneiss from the high-grade zone and one metavolcanite from the volcanic belt in southern Sardinia were dated by LAM-ICPMS (and SHRIMP) zircon geochronology. The inferred emplacement ages of the two samples are 469 ± 3.7 and 464 ± 1 Ma, respectively. The analysed paragneisses are mainly metawackes with subordinate metapelites and rare metamarls. Three paragneiss samples were dated: zircon ages scatter between 3 Ga and about 320 Ma, with a first main cluster from 480 to 450 Ma, and a second one from about 650 to 550. Variscan zircon ages are rare and mostly limited to thin rims and overgrowths on older grains. These data indicate that the high-grade complex principally consists of middle Ordovician orthogneisses associated with a thick metasedimentary sequence characterised by a maximum age of deposition between 480 and 450 Ma. The association of nearly coeval felsic-mafic magmatic rocks with immature siliciclastic sedimentary sequences points to a back-arc setting in the north Gondwana margin during the Early Palaeozoic. The Variscan metamorphic evolution recorded by the high-grade gneisses (Ky-bearing felsic gneisses and mafic eclogites) testifies to the transformation of the Late Ordovician–Devonian passive continental margin into an active margin in the Devonian–Early Carboniferous.

Lithological interpretation of crustal composition in the Fennoscandian Shield with seismic velocity data

In this study, we report the results of an investigation of lithological interpretation of the crust in the central Fennoscandian Shield (in Finland) using seismic wide-angle velocity models and laboratory measurements on P- and S-wave velocities of different rock types. The velocities adopted from wide-angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal PT conditions in the study area. The wide-angle velocity models indicate that the P-wave velocity does not only increase step-wise at boundaries of major crustal layers, but there is also gradual increase of velocity within the layers. On the other hand, the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends. Thus, there must be gradual vertical changes in rock composition. The downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth. We have calculated vertical velocity profiles for a range of possible crustal lithological compositions. The Finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest. For instance, on the SVEKA'81 transect, the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic–granodioritic rocks with a minor contribution of amphibolite and diabase. In the middle crust, the amphibolite proportion increases. The lower crust consists of tonalitic gneiss, mafic garnet granulite, hornblendite, pyroxenite and minor mafic eclogite. Assuming that these rock types are present in sufficiently extensive and thick layers, they would also have sufficiently high acoustic reflection coefficients for generating the generally well-developed reflectivity in the crust in the central part of the shield. Density profiles calculated from the lithological models suggest that there is practically no density contrast at Moho in areas of the high-velocity lower crust. Comparison of reflectors from FIRE-1 and FIRE-3 transects and the velocity model from SVEKA'81 wide-angle transect indicated that the reflectors correlate with velocity layering, but the three-dimensional structures of the crust complicate such comparisons.

Early Miocene continental subduction and rapid exhumation in the western Mediterranean

New Lu-Hf garnet growth ages from high-pressure rocks from the structurally lowest terrane (Nevado-Filabride Complex) in the Betic Cordillera of southern Spain indicate early Miocene subduction of continental crust, requiring significant revision of the timing and processes of orogeny in the western part of the Alpine orogenic system. Mafic eclogite and pelitic schist give Lu-Hf ages in the range 18–14 Ma, and fission-track data show that the terrane was exhumed by 12–8 Ma. The overlying terrane (Alpujarride Complex) shows early Tertiary high pressure-temperature metamorphism; this was followed by a thermal event that accompanied rapid crustal extension at 22–18 Ma, leading to the formation of the Alboran Sea backarc basin. The evidence therefore suggests two phases of continental subduction separated by a phase of large-scale intraorogenic extension. The two terranes are now separated by a mid-late Miocene low-angle extensional fault. The timing and paleogeographic setting suggest that the Nevado-Filabride terrane originally was close to the rifted South Iberian margin, and was subducted during westward motion of the extending upper terrane between the converging African and Iberian plates.

Adakites from collision‐modified lithosphere

Adakitic melts from Papua New Guinea (PNG) show adakitic geochemical characteristics, yet their geodynamic context is unclear. Modern adakites are associated with hot‐slab melting and/or remelting of orogenic mafic underplate at convergent margins. Rift‐propagation over collision‐modified lithosphere may explain the PNG adakite enigma, as PNG was influenced by rapid creation and subduction of oceanic microplates since Mesozoic times. In a new (rift) tectonic regime, decompressional rift melts encountered and melted remnant mafic eclogite and/or garnet‐amphibolite slab fragments in arc collisional‐modified mantle, and partially equilibrated with metasomatized mantle. Alternatively, hot‐slab melting in a proposed newborn subduction zone along the Trobriand Trough could generate adakitic melts, but recent seismic P‐wave tomographic models lack evidence for subducting oceanic lithosphere in the adakite melt region; however they do show deep subduction zone remnants as a number of high P‐wave anomalies at lithospheric depths, which supports our proposed scenario.