Lawsonite Eclogite Research Articles

Metamorphic evolution of ultrahigh-pressure rocks from Chinese southwestern Tianshan and a possible indicator of UHP metamorphism using garnet composition in low-T eclogites

How to identify ultrahigh-pressure (UHP) metamorphism in the absence of coesite is a key problem to gain the correct P–T history for an orogenic belt. In this study, garnet composition combined with the pseudosection approach was used to identify ultrahigh-pressure metamorphism and to determine P–T paths for eclogites and metapelites from Chinese southwestern Tianshan. Porphyroblastic garnets from both eclogites and metapelites develop pronounced chemical core-rim textures: the relatively homogeneous core with low pyrope [Prp; Mg/(Ca+Mn+Mg+Fe2+)×100] and grossular [Grs; Ca/(Ca+Mn+Mg+Fe2+)×100] content is overgrown by a thin rim with sharply increased Prp and Grs. Phase equilibria modeling indicates that the ultrahigh-pressure rocks have undergone a clockwise P–T path characterized by heating during early exhumation with peak P–T at 31–33kbar and 490–520°C. The P–T pseudosections for eclogites show that isopleths of Prp and Grs strongly depend on temperature and pressure, respectively, especially in the stability fields of glaucophane–lawsonite-bearing eclogite facies assemblages. This indicates that garnet composition provides robust thermobarometric constraints. Consequently, we propose a Prp–Grs diagram which is subdivided into a high-pressure region and an ultrahigh-pressure region by the quartz–coesite–transition curve. Those garnet compositions which fall into the ultrahigh-pressure region are regarded to have experienced ultrahigh-pressure metamorphism. This approach is expected to be a useful tool to qualitatively identify ultrahigh-pressure metamorphism for glaucophane–lawsonite-bearing eclogites and its particular strength is the quick examination of large datasets comprising samples with similar bulk composition. Using this method, garnet compositions of eclogites and mafic blueschists from Chinese southwestern Tianshan and lawsonite eclogites worldwide are plotted in the Prp–Grs diagram and several possible ultrahigh-pressure eclogite occurrences are newly identified.

Lawsonite blueschists and lawsonite eclogites as proxies for palaeo‐subduction zone processes: a review

AbstractLawsonite‐bearing metamorphic/metasomatic rocks form at high‐pressure–ultrahigh pressure (HP–UHP) and low‐temperature (LT) conditions, commonly in Pacific‐type subduction zones. TheP–Tstability fields of lawsonite blueschists and lawsonite eclogites represent subfacies of the blueschist and eclogite facies, respectively. Although the lawsonite–epidote transition boundary has a positive Clapeyron (dP/dT) slope, the blueschist‐to‐eclogite transformation within the lawsonite stability field in metabasaltic rocks is gradual and cannot be defined by a specific discontinuous reaction inP–Tspace. The oldest occurrences of lawsonite‐bearing blueschists are latest Neoproterozoic, suggesting that subduction‐zone thermal structures evolved towards the necessary LTconditions for lawsonite formation only by the late Neoproterozoic. A clear difference in frequency between Phanerozoic lawsonite and epidote blueschists does not exist, but our new compilation found a global lawsonite hiatus in the Permian that is a robust indication of relatively warm subduction‐zone thermal regimes. Lawsonite eclogites have been confirmed from at least 19 localities; they are classified as L‐ (lawsonite only), E‐ (lawsonite + epidote), and U‐type (lawsonite + coesite). Complete preservation of L‐type lawsonite eclogites attending their return to the surface is uncommon. Rare evidence of progressive eclogitization within the lawsonite stability field is preserved in some zoned garnets, as growth isolates a significant volume of precursor phases and textures during incipient eclogitization. Brittle fracturing and fluid infiltration are common during prograde eclogite facies metamorphism. Certain lawsonite‐bearing metasomatic rocks record multiple fluid‐infiltration events. Significant cooling and continuous H2O supply from the dehydrating oceanic plate to exhuming HPserpentinite mélange may cause lawsonite blueschist facies overprinting and prevent breakdown of lawsonite during decompression. The subduction records of lawsonite blueschists and eclogites agree with numerical modelling of subduction zones.

Fluid-mediated metal transport in subduction zones and its link to arc-related giant ore deposits: Constraints from a sulfide-bearing HP vein in lawsonite eclogite (Tianshan, China)

High-pressure (HP) veins in eclogites provide insight into element mobility during fluid–rock interaction in subduction zones. Here, we present a petrological–geochemical study of a sulfide-bearing HP vein and its massive lawsonite eclogite host rock from the Chinese Tianshan (ultra-)high-pressure/low-temperature metamorphic belt. The omphacite-dominated vein is enveloped by a garnet-poor, sulfide-bearing eclogite-facies reaction selvage. Lawsonite, garnet, omphacite, glaucophane and other HP minerals occur as inclusions in pyrite porphyroblasts of the selvage rock, indicating that the selvage formed prograde under eclogite-facies conditions. Relicts of wall-rock garnet (Grt_I) cores in recrystallized selvage garnet (Grt_II) close to the wall rock and the Ca distribution in Grt_II, which images the overgrown selvage matrix, indicate that the selvage formed due to dissolution–precipitation processes as a consequence of fluid–rock interaction of the wall rock eclogite with the vein-forming fluid. The peak metamorphic P–T conditions of the wall-rock eclogite are estimated at ca. 590°C and 23kbar. Mass-balance calculations indicate that the reaction selvage experienced: (1) a depletion of the large-ion lithophile elements (K–Rb–Ba) of up to 100% relative to their concentrations in the wall-rock eclogite; (2) a moderate depletion of the HREE and some transition metal elements including Fe, Cu, Ni, Zn, Co, Cr, and Mn (10–40%); (3) a significant enrichment of CaO (up to 50–80%), Sr (up to ∼200%), Pb (up to ∼170%) and S (up to ∼210%); (4) a slight to moderate enrichment of the LREE (10–20%) and MREE (0–40%); whereas (5) the HFSE show no significant variations.The chemical changes in the selvage suggest that the fluid, which caused the dissolution of the wall-rock and the precipitation of the selvage assemblage while the vein formed, was probably a mixture of an “internal” fluid derived from the prograde dehydration (e.g., lawsonite breakdown) of the wall rock and an “external” fluid most likely derived from dehydrating altered oceanic crust located in stratigraphically lower units of the subducting slab. The external fluid introduced Ca, S, Sr, Pb and at least parts of the LREE and the MREE into the selvage, whereas some elements, such as the remaining LREE and MREE, may have been derived from the wall rock eclogite via diffusional transport into the selvage. The enrichment of Ca and L-MREE is displayed by the abundant growth of selvage epidote. In contrast, the dissolution of garnet and phengite released significant amounts of HREE and LILE (K–Rb–Ba) into the passing fluid, because the chemical changes within the selvage prevented the formation of a mineral assemblage with sufficiently high bulk-fluid partition coefficients for these elements. Significant amounts of transitional metal elements were released into the fluid during the dissolution of white mica and the dissolution–precipitation behavior of garnet, omphacite, dolomite and sulfides. Thus the LILE and HREE along with some transition metal elements (e.g., Fe, Cu, Ni and Zn) were mobilized during the dissolution–precipitation processes that led to the selvage formation. Accordingly the slab fluids are not only strongly enriched in LILE and depleted in HFSE, but also carry significant amounts of transition metals. It is most likely that slab fluids strongly contribute to the metal flux into the arc magma systems finally resulting in giant arc-related ore deposits.

Jadeitite from Guatemala: new observations and distinctions among multiple occurrences

In Guatemala, jadeitite occurs as blocks in serpentinite melange in distinct settings on opposite sides of the Motagua fault. Jadeitites north of the Motagua fault are associated with eclogites, blueschists, and garnet amphibolites and distributed over a 200km E-W area. Omphacitite, omphacite - taramite metabasite, albitite, and phengite rock are found with jadeitite. The assemblages indicate formation at 6-12kbar and 300-400°C, however jadeite - omphacite pairs yield T from ~200 to gt;500oC for jadeite crystallization. Jadeitites south of the Motagua fault are sourced from three separate fault slices of serpentinite in Jalapa and Zacapa departments and are distinctive: 1) Jadeitite near Carrizal Grande is found in serpentinite with lawsonite eclogites, variably altered to blueschist, and rarely in schists. A large jadeite – omphacite gap and lawsonite suggests T=300-400°C, but at high P as indicated by the presence of quartz: Pgt;12-20kbar. Lawsonite eclogites (P=20-25kbar, T=350-450°C) occur with these jadeitites. 2) At La Ceiba, jadeitites coexist with omphacite blueschists and contain late-stage veins of quartz, diopside, cymrite, actinolite, titanite and vesuvianite. A large jadeite – omphacite gap suggests 300-400°C, but at lower P as indicated by quartz + albite: P=10-14kbar. 3) At La Ensenada jade i tites occur with lawsonite-glaucophane blueschists and chloritite. It is a fine-grained jadeite-pumpellyite rock, intensely deformed and veined with grossular, omphacite, albite and titanite, but no quartz. A large jadeite–omphacite gap and pumpellyite suggest ~200-~300°C at lower P consistent with primary albite: P=6-9kbar. The silicates contain little iron

Revised activity–composition models for clinopyroxene and amphibole

AbstractRecent activity–composition models for clinopyroxene and amphibole are revised to provide better consistency with observed phase relations in natural rocks. For clinopyroxene, the calibration in NCFMAS is retained, but the incorporation of acmite is revised to improve the partitioning of ferric iron between coexisting clinopyroxenes. For amphibole, the NCFMASH calibration is retained, but the addition of ferric iron is changed to provide consistency with the clinopyroxenes. The thermodynamics of orthoamphibole (gedrite) is also adjusted to resolve an unrelated inconsistency. The effects of these improvements are illustrated through comparison of calculated pseudosections produced with the existing and new models with natural data from lawsonite eclogites.

Metamorphic evolution of lawsonite eclogites from the southern Motagua fault zone, Guatemala: insights from phase equilibria and Raman spectroscopy

AbstractLawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a progradeP–Tpath that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐Tof ∼520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ∼10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived progradeP–Tpath associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐Tsubduction zone.

Calculated phase equilibria for MORB compositions: a reappraisal of the metamorphic evolution of lawsonite eclogite

Pseudosections calculated with thermocalc predict that lawsonite-bearing assemblages, including lawsonite eclogite, will be common for subducted oceanic crust that experiences cool, fluid-saturated conditions. For glaucophane–lawsonite eclogite facies conditions (500–600 °C and 18–28 kbar), MORB compositions are predicted in the NCKMnFMASHO system to contain glaucophane, garnet, omphacite, lawsonite, phengite and quartz, with chlorite at lower temperature and talc at higher temperature. In these assemblages, the pyrope content in garnet is mostly controlled by variations in temperature, and grossular content is strongly controlled by pressure. The silica content in phengite increases linearly with pressure. As the P–T conditions for these given isopleths are only subtly affected by common variations in bulk-rock compositions, the P–T pseudosections potentially present a robust geothermobarometric method for natural glaucophane-bearing eclogites. Thermobarometric results recovered both by isopleth and conventional approaches indicate that most natural glaucophane–lawsonite eclogites (Type-L) and glaucophane–epidote eclogites (Type-E) record similar peak P–T conditions within the lawsonite stability field. Decompression from conditions appropriate for lawsonite stability should result in epidote-bearing assemblages through dehydration reactions controlled by lawsonite + omphacite = glaucophane + epidote + H2O. Lawsonite and omphacite breakdown will be accompanied by the release of a large amount of bound fluid, such that eclogite assemblages are variably recrystallized to glaucophane-rich blueschist. Calculated pseudosections indicate that eclogite assemblages form most readily in Ca-rich rocks and blueschist assemblages most readily in Ca-poor rocks. This distinction in bulk-rock composition can account for the co-existence of low-T eclogite and blueschist in high-pressure terranes.

Mass flow in serpentinite-hosted subduction channels: P– T– t path patterns of metamorphic blocks in the Rio San Juan mélange (Dominican Republic)

Serpentinite mélanges formed in the subduction zone channel of an intra-oceanic arc system are exposed in the Rio San Juan complex of northern Hispaniola. This study presents an array of pressure–temperature paths for 14 metamorphic blocks. The P– T paths were constructed from isochemical P– T phase diagrams (“pseudosections”) as well as multi-equilibrium calculations on mineral assemblages, providing for the first time the means to compare P– T paths in both time and space. The study involves metabasic, meta-andesitic and metagranitic lithologies. Subduction was slow, so that the uninterrupted thermal development of the subduction zone can be followed over a time span of more than 60 Myr. Early eclogites mirror “warm” subduction zone gradients (∼18°/km); both clockwise and anticlockwise paths are found. The maximum P– T conditions reached are ∼800 °C and 26 kbar, thus indicating sufficient buoyancy for return flow from depths exceeding the stability of serpentine. As the subduction zone matures, P/ T gradients gradually steepen to ∼6°/km, and the maximum depth from which blocks are exhumed decreases in a regular way from 26 to 17 kbar. The exhumed metamorphic products are initially omphacite-bearing blueschists and evolve to jadeite- and lawsonite-bearing blueschists with veins of jadeitite jade. Lawsonite eclogites are not found, because in the latest history of the subduction zone the predominantly basic geochemistry of the blocks evolved to more intermediate compositions inappropriate for this rock type. There are no indications of systematic differences or episodic variations in subduction/exhumation velocities, so that a continuous cycling of material in the subduction zone channel is indicated. In two blocks of eclogite and garnet blueschist an earlier amphibolite-facies history at 575–700 °C and 7–8 kbar can be documented, suggesting erosion of the base of the overlying intra-oceanic island arc. The fossil subduction zone studied in detail here is part of a much larger feature exposed at intervals around the perimeter of the Caribbean plate where the intra-oceanic arc has collided with North and South America. The continuous history documented here from before 120 Ma to after 60 Ma constrains regional plate tectonic models to incorporate a similarly uninterrupted continuous history of eastward movement of the Caribbean plate with respect to North and South America.

Petrotectonics of lawsonite eclogite exhumation: Insights from the Sivrihisar massif, Turkey

The Sivrihisar massif of the Tavşanlı Zone of Turkey is 1 of less than 10 known lawsonite eclogite localities worldwide. Rocks of the Sivrihisar massif consist of eclogite and blueschist in contact with metasedimentary host rocks and record decreasing maximum pressure conditions across three WNW‐ESE striking belts from 16–24 kbar in the northern Halilbağı belt to 14–16 kbar in the Karacaören belt and 8–10 kbar in the Kertek belt. Where present, sodic‐amphibole, phengite, chlorite, and quartz define a pervasive S0/S1 foliation; garnet, omphacite, and lawsonite define stretching lineations and kinematic indicators. D1 and D2 structures are similar in the Halilbağı and Karacaören belts but differ to those in the Kertek belt. D3 structures are uniform across the massif including fibrous calcite that occurs parallel to F3 fold axes. Shear sense indicators from field observations and asymmetric type‐I cross girdle of quartz c axes obtained from electron backscatter diffraction (EBSD) show top to the south thrusting throughout much of the massif. D1 and D2 structures are interpreted to have formed during exhumation by extrusion along a ∼5°C/km gradient. The Halilbağı and Karacaören belts were juxtaposed possibly as deep as 45 km within the subduction channel and exhumed by the arrival of the Anatolide microcontinent at approximately 70 Ma. Homogeneity of F3 axes and calcite fibers across the massif suggests that assembly occurred at blueschist conditions before exhumation through the aragonite‐calcite transition (∼350°C) above 8 kbar.

Lawsonite vorticity and subduction kinematics

The exceptional preservation of lawsonite in eclogite and blueschist in a subduction complex (Sivrihisar, Turkey) allows evaluation of high-pressure (H P ) deformation and exhumation using vorticity analysis of a H P index mineral. The Sivrihisar H P rocks were deformed during H P metamorphism and initial decompression by oblique (normal and/or dextral) shearing beneath serpentinized peridotite. We evaluated kinematic vorticity (W m pure shear = 0, W m simple shear = 1) using the shape fabric of lawsonite crystals in blueschist layers and meter-scale eclogite pods. Pods record dominant simple shear (W m > 0.8) in their eclogitic core and at pod margins partially transformed to blueschist; map-scale blueschist layers record a large component of pure shear (W m = 0.4–0.6), consistent with oblique extrusion at depth in the subduction zone, and local simple shear (W m = 0.8) at the fault contact with serpentinite. A combination of oblique unroofing from beneath a weak serpentinized hanging wall and pure shear extrusion in the subduction channel accounts for rapid exhumation along a low geothermal gradient and the rare preservation of lawsonite eclogite during exhumation to the Earth9s surface.

Prograde P–T Evolution of a Lawsonite Eclogite from the Monviso Meta-ophiolite (Western Alps): Dehydration and Redox Reactions during Subduction of Oceanic FeTi-oxide Gabbro

Lawsonite eclogites represent fossil records of processes occurring during subduction of cold- and fast-subducting slabs, and provide an opportunity to investigate the thermal and metamorphic evolution of palaeosubduction zones. Occurrences of lawsonite eclogites are rare because lawsonite is often replaced during exhumation. We report here, for the first time, the occurrence of a lawsonite eclogite from the Monviso meta-ophiolite (Western Alps), representing the product of the Alpine metamorphism of a FeTi-oxide gabbro. The prograde metamorphic evolution of this lawsonite eclogite has been investigated using the petrological approach of pseudosections. The reconstruction of its prograde P-T evolution allowed us to monitor changes in chemical and physical properties (mineral assemblages and compositions, density, H2O content, oxygen fugacity) during subduction. The pseudosection modelling suggests peak metamorphic conditions of T epsilon 550 degrees C, P = 25-26 kbar, and a decrease in the thermal gradient during subduction from about 9 degrees C km(-1) to less than 7 degrees C km(-1), which may be interpreted as related to an increase in the subduction rate. During its prograde evolution, at a depth of about 65-70 km, the eclogite-facies metagabbro experienced a significant dehydration passing from the Grt(1) + Omp(1) + Lws + Chl + Qtz + Rt assemblage (stage I) to the Grt(2) + Omp(2) + Tlc + Qtz + Rt assemblage (stage II). The breakdown of lawsonite and chlorite caused the release of up to 3 wt % of H2O and also of oxygen, as modelled by the redox equilibrium Lws + Qtz + Chl + Omp(1) = Grt + Omp(2) + H2O + O-2. This redox-equilibrium represents the boundary between an earlier, more oxidized assemblage (stage I), stable at lower T, and a later, more reduced assemblage (stage II), stable at higher T. These results have possible implications for the understanding of the complex interactions between crust and mantle in subduction zones, especially in clarifying the processes locally involved in the oxidation of the mantle wedge overlying the subducting slab.

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.

Calculated phase equilibria for a morb composition in a P–T range, 450–650 °C and 18–28 kbar: the stability of eclogite

AbstractLower temperature eclogite (with T = 600 °C) represents a significant part of the occurrences of eclogite in orogenic belts. ‘True’ eclogite, with, for example, garnet + omphacite >70%, is well represented in such an occurrence. Calculated phase equilibria in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO), for just one rock composition – that of a representative mid‐ocean ridge basalt, morb– are used to see under what circumstances ‘true’ eclogite is predicted to occur. The variables considered are not only pressure (P) and temperature (T) but also water content and oxidation state. The latter two variables are known to exert a significant control on mineral assemblage but are difficult to establish retrospectively from the observed rocks themselves. It is found that whereas oxidation state does have a strong effect on mineral assemblage, the key control on developing ‘true’ eclogite is shown to be temperature and water content. If temperature is established to be <600 °C, water content has to be low (less or much less than that for H2O saturation) in order for ‘true’ eclogite to form. Moreover, unless pressure is at the high end in the range considered, lawsonite eclogite and ‘true’ eclogite will tend to be mutually exclusive, with the former requiring high water content at the lower temperature where it occurs, but the latter requiring low water content.

Antigorite Peridotite, Metaserpentinite, and other Inclusions within Diatremes on the Colorado Plateau, SW USA: Implications for the Mantle Wedge during Low-angle Subduction

Antigorite peridotite, Cr-magnetite dunite, chlorite harzburgite, and other ultramafic rock fragments included in the Green Knobs diatreme in NW New Mexico are used to investigate the evolution of the mantle wedge during low-angle subduction. The diatreme, part of the Navajo Volcanic Field, has a matrix of serpentinized ultramafic microbreccia (SUM). The meta-peridotite inclusions have been characterized by petrographic, electron microprobe, and bulk-rock analysis. The assemblage antigorite–diopside–olivine formed and was stable in the mantle. In a Cr-magnetite dunite, olivine compositions span the range Fa9–3, and chlorite-rich pockets contain extraordinarily pure pyrope (Py94·4Alm5·5Gr0·1). The Cr-magnetite dunite is a product of prograde metamorphism of a brucite-bearing serpentinite. Second generations of olivine in that dunite and other samples formed during a sharp temperature increase caused by intrusion of the magma that triggered the eruption. Histories of lawsonite eclogite, garnetite, and other inclusions in SUM diatremes have been interpreted to provide context: some are from the lithospheric part of the mantle wedge, hydrated near the trench, tectonically eroded and transported some 700 km in a mélange, and emplaced below the Colorado Plateau during low-angle subduction of the Farallon plate. The Cr-magnetite dunite was also derived from that mélange, whereas some other inclusions represent mantle hydrated in place above the slab. Tectonic erosion of the Colorado Plateau mantle lithosphere, or serpentinite diapirism into that lithosphere, or both, accompanied the low-angle subduction. Serpentinization far from the trench and low-angle transport of parts of the lithospheric mantle wedge are consistent with models of the Laramide orogeny and may be common during low-angle subduction.

Cold subduction and the formation of lawsonite eclogite – constraints from prograde evolution of eclogitized pillow lava from Corsica

AbstractA new discovery of lawsonite eclogite is presented from the Lancône glaucophanites within the Schistes Lustrés nappe at Défilé du Lancône in Alpine Corsica. The fine‐grained eclogitized pillow lava and inter‐pillow matrix are extremely fresh, showing very little evidence of retrograde alteration. Peak assemblages in both the massive pillows and weakly foliated inter‐pillow matrix consist of zoned idiomorphic Mg‐poor (<0.8 wt% MgO) garnet + omphacite + lawsonite + chlorite + titanite. A local overprint by the lower grade assemblage glaucophane + albite with partial resorption of omphacite and garnet is locally observed. Garnet porphyroblasts in the massive pillows are Mn rich, and show a regular prograde growth‐type zoning with a Mn‐rich core. In the inter‐pillow matrix garnet is less manganiferous, and shows a mutual variation in Ca and Fe with Fe enrichment toward the rim. Some garnet from this rock type shows complex zoning patterns indicating a coalescence of several smaller crystallites. Matrix omphacite in both rock types is zoned with a rimward increase in XJd, locally with cores of relict augite. Numerous inclusions of clinopyroxene, lawsonite, chlorite and titanite are encapsulated within garnet in both rock types, and albite, quartz and hornblende are also found included in garnet from the inter‐pillow matrix. Inclusions of clinopyroxene commonly have augitic cores and omphacitic rims. The inter‐pillow matrix contains cross‐cutting omphacite‐rich veinlets with zoned omphacite, Si‐rich phengite (Si = 3.54 apfu), ferroglaucophane, actinolite and hematite. These veinlets are seen fracturing idiomorphic garnet, apparently without any secondary effects. Pseudosections of matrix compositions for the massive pillows, the inter‐pillow matrix and the cross‐cutting veinlets indicate similar P–T conditions with maximum pressures of 1.9–2.6 GPa at temperatures of 335–420 °C. The inclusion suite found in garnet from the inter‐pillow matrix apparently formed at pressures below 0.6–0.7 GPa. Retrogression during initial decompression of the studied rocks is only very local. Late veinlets of albite + glaucophane, without breakdown of lawsonite, indicate that the rocks remained in a cold environment during exhumation, resulting in a hairpin‐shaped P–T path.

Metamorphic reworking of a high pressure–low temperature mélange along the Motagua fault, Guatemala: A record of Neocomian and Maastrichtian transpressional tectonics

The Guatemala suture zone is a major east–west left-lateral strike slip boundary that separates the North American and Caribbean plates in Guatemala. The Motagua fault, the central active strand of the suture zone, underwent two major collisional events within a system otherwise dominated by strike–slip motion. The first event is recorded by high-pressure/low temperature (HP/LT) eclogites and related rocks that occur within serpentinites both north and south of the Motagua fault. Lawsonite eclogites south of the fault are not significantly retrograded and give 40Ar/ 39Ar ages of 125–116 Ma and Sm–Nd mineral isochrons of 144–132 Ma. Eclogites north of the fault give similar Sm–Nd isochron ages (131–126 Ma) but otherwise differ in that they are strongly overprinted by a lower pressure assemblage and, along with associated HP/LT rocks, give much younger 40Ar/ 39Ar ages of 88–55 Ma indicating a later amphibolite facies metamorphic event. We propose therefore that all serpentinite hosted eclogites along the Motagua fault formed at essentially the same time in different parts of a laterally extensive Lower Cretaceous forearc subduction system, but subsequently underwent different histories. The southern assemblages were thrust southwards (present coordinates) immediately after HP metamorphism whereas the northern association was retrograded during a later collision that thrust it northward at ca. 70 Ma. They were subsequently juxtaposed opposite each other by major strike slip motion. This model implies that the HP rocks on opposing sides of the Motagua fault evolved along a plate boundary that underwent both dip slip and strike slip motion throughout the Late Cretaceous as a result of oblique convergence. The juxtaposition of a convergent and strike slip system means that HP/LT rocks within serpentinites can be found at depth along much of the modern Guatemala suture zone and its eastward extension into the northern Caribbean. Both sets of assemblages were exhumed relatively recently by the uplift of mountain ranges on both sides of the fault caused by movement along a restraining bend. Recent exhumation explains the apparently lack of offset of surface outcrops along a major strike slip fault.

Tectonics and HP–UHP metamorphism of northern Tibet – Preface

This special issue has been prepared for the 8th International Eclogite Conference (IEC) to be held in the Qilian–Qaidam regions, western China during August, 2009. Most papers in this issue summarize results of many current petrological–geochemical–geochronological and tectonic studies of both Qilian Pacific-type and Qaidam Alpine-type lithotectonic assemblages. Important new occurrences include the Qilian lawsonite eclogites which required an extremely cold subduction of the Early Paleozoic oceanic lithosphere, and the mixing of oceanic and supracrustal protoliths in the Qaidam continental subduction channel. These two Early Paleozoic sutures in northern Tibet provide additional constraints on the regional tectonic evolution of Paleo- and Neo-Tethys in south-central Asia.

Lawsonite eclogites from the Pinchi Lake area, British Columbia—new P– T estimates and interpretation

Lawsonite eclogites from Pinchi Lake, British Columbia, occur as tectonic blocks associated with lawsonite, aragonite, and jadeite-bearing blueschists. One tectonic block from Tezzeron Lake shows extensive lower-temperature retrogression. The block from Beaver Lake Road (BLR) shows less retrogression and the garnet crystals contain abundant inclusions of lawsonite and omphacite suggesting these minerals were stable throughout most of the crystallization history. Compositional profiles and element maps of garnets show: (1) Mn enrichment in cores with decrease toward rims; (2) little change in Ca from core to rim; (3) general decrease in Fe/Mg from core to rim. Phase diagram sections for sample BLR-3 and garnet-core compositions suggest garnet-core growth at 505 °C and 24.3 kbar (1 kbar = 0.1 GPa). Temperature estimates based upon garnet–clinopyroxene Fe–Mg exchange geothermometry are, for inclusions in garnet at 25 kbar, 450 ± 30 °C (4 spots); 435 ± 15 (10); and 445 ± 25 (6). For garnet rims and matrix clinopyroxene the results are: 515 ± 50 °C (2); 440 ± 25 (5); 475 ± 20 (2). These results suggest very little if any statistically significant changes in temperature during crystal growth. Estimates of pressure are based upon several different calibrations of the garnet–clinopyroxene–phengite equilibria. Using the calibration of Krogh-Ravna we obtain pressures of: 24.2–26.8 kbar, compatible with that estimated for garnet-core growth in BLR-3. For the glaucophane = talc + jadeite geobarometer, the activity relations for glaucophane are not well constrained. At 450 °C a pressure of 26 kbar is calculated (one sample). A pressure of 25 kbar at 450 °C suggests depths of ~ 90 km, leading to a P– T gradient of ~ 5 °C km − 1 . This gradient passes through the inferred stability field of lawsonite eclogites and the calculated stability fields of lawsonite for Pinchi Lake samples. This is very near the P– T stability field of coesite, which is characteristic of Ultra-High-Pressure metamorphism.

Petrogenesis and structural petrology of high-pressure metabasalt pods, Sivrihisar, Turkey

Lawsonite eclogite pods ranging in size from 3 cm to 6 m occur in lawsonite blueschist and eclogite facies metasedimentary and metabasaltic rocks in the Sivrihisar Massif, Turkey. Some pods have a core of lawsonite eclogite surrounded by alternating, centimeter-scale layers of lawsonite blueschist, eclogite, and transitional eclogite–blueschist, all with similar basaltic bulk composition. These pods also contain texturally late lawsonite-rich veins and layers. Most eclogites and blueschists within the pods lack reaction textures, but some blueschists near pod margins contain texturally complex garnet as well as glaucophane rims on omphacite, suggesting retrogression of eclogite to blueschist. Phase diagrams (pseudosections) calculated for the lawsonite eclogite core of a meter-scale pod indicate that the eclogite equilibrated at ∼22–24 kbar, ∼520°C. Lawsonite eclogite and blueschist at the tectonized margin of the same pod equilibrated at similar temperatures and slightly lower pressures. The composite eclogite–blueschist pod is foliated, lineated, and folded. An earlier generation of lineated omphacite in the pod core has a different spatial orientation than the lineation at the pod margin, although electron backscattered diffraction data show that core and rim omphacite have similar lattice preferred orientation patterns. Petrologic and structural data are consistent with mechanical formation of pods by folding and dissection of eclogite layers at high-P, and localized retrogression at pod margins during initial stages of exhumation at P–T conditions >425°C, 16 kbar.

Pressure–temperature evolution of lawsonite eclogite in Sivrihisar; Tavşanlı Zone–Turkey

The Cretaceous blueschist belt, Tavşanlı Zone, representing the subducted and exhumed northern continental margin of the Anatolide–Tauride platform is exposed in Western Anatolia. The Sivrihisar area east of Tavşanlı is made up of tectonic units consisting of i) metaclastics and conformably overlying massive marbles (coherent blueschist unit), ii) blueschist-eclogite unit, iii) marble–calcschist intercalation and iv) metaperidotite slab. The metaclastics are composed of jadeite–lawsonite–glaucophane and jadeite–glaucophane–chloritoid schists, phengite phyllites, and calcschists with glaucophane–lawsonite metabasite layers. The blueschist-eclogite unit representing strongly sheared, deeply buried and imbricated tectonic slices of accreted uppermost levels of the oceanic crust with minor metamorphosed serpentinite bodies consists of lawsonite-bearing eclogitic metabasites (approximately 90% of the field), lawsonite eclogites, metagabbros, serpentinites, pelagic marbles, omphacite–glaucophane–lawsonite metapelites and metacherts. The mineral assemblage of the lawsonite eclogite (garnet + omphacite > 70%) is omphacite, garnet, lawsonite, glaucophane, phengite and rutile. Lawsonite eclogite lenses are enclosed by garnet–lawsonite blueschist envelopes. Textural evidence from lawsonite eclogites and country rocks reveals that they did not leave the stability field of lawsonite during subduction and exhumation. The widespread preservation of lawsonite in eclogitic metabasites and eclogites can be attributed to rapid subduction and subsequent exhumation in a low geothermal gradient of the oceanic crust material without experiencing a thermal relaxation. Peak P– T conditions of lawsonite eclogites are estimated at 24 ± 1 kbar and 460 ± 25 °C. These P– T conditions indicate a remarkably low geotherm of 6.2 °C/km corresponding to a burial depth of 74 km.