Showing papers in "Journal of Metamorphic Geology in 2014"

New mineral activity-composition relations for thermodynamic calculations in metapelitic systems

Abstract: New activity–composition (a–x) relations for minerals commonly occurring in metapelites are presented for use with the internally consistent thermodynamic dataset of Holland & Powell (2011, Journal of Metamorphic Geology, 29, 333–383). The a–x relations include a broader consideration of Fe2O3 in minerals, changes to the formalism of several phases and order–disorder in all ferromagnesian minerals where Fe–Mg mixing occurs on multiple sites. The a–x relations for chlorite, biotite, garnet, chloritoid, staurolite, cordierite, orthopyroxene, muscovite, paragonite and margarite have been substantially reparameterized using the approach outlined in the companion paper in this issue. For the first time, the entire set of a–x relations for the common ferromagnesian minerals in metapelitic rocks is parameterized simultaneously, with attention paid to ensuring that they can be used together to calculate phase diagrams of geologically appropriate topology. The a–x relations developed are for use in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system for both subsolidus and suprasolidus conditions. Petrogenetic grids in KFMASH and KFMASHTO are similar in topology to those produced with earlier end-member datasets and a–x relations, but with some notable differences. In particular, in subsolidus equilibria, the FeO/(FeO + MgO) of garnet is now greater than in coexisting staurolite, bringing a number of key staurolite-bearing equilibria into better agreement with inferences from field and petrographic observations. Furthermore, the addition of Fe3+ and Ti to a number of silicate phases allows more plausible equilibria to be calculated in relevant systems. Pseudosections calculated with the new a–x relations are also topologically similar to equivalent diagrams using earlier a–x relations, although with many low variance fields shifting in P–T space to somewhat lower pressure conditions.

The effect of Mn on mineral stability in metapelites revisited: new a–x relations for manganese-bearing minerals

Abstract: The a-x relations recently presented in White et al. (, Journal of Metamorphic Geology, 32, 261-286) are extended to include MnO. This provides a set of internally consistent a-x relations for metapelitic rocks in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (MnNCKFMASHTO) system. The mixing parameters for the Mn-bearing minerals were estimated using the micro-ϕ{symbol} approach of Powell et al. (, Journal of Metamorphic Geology, 32, 245-260). Then the Mn-end-member thermodynamic properties were calibrated using a database of co-existing minerals involving literature data from rocks and from experiments on natural materials. Mn-end-members were calibrated for orthopyroxene, cordierite, staurolite, chloritoid, chlorite, biotite, ilmenite and hematite, assuming known properties for the garnet end-member spessartine. The addition of MnO to phase diagram calculations results in a marked expansion of the stability of garnet-bearing assemblages. At greenschist facies conditions garnet stability is extended down temperature. At amphibolite facies conditions, the garnet-in boundary shifts to lower pressure. While the addition of MnO greatly influences the stability of garnet, it has relatively little effect on the stability of other common metapelitic minerals, with the resultant diagrams being topologically very similar to those calculated without MnO. Furthermore, the addition of MnO in the amounts measured in most metapelites has only a small effect on the mode of garnet, with calculated garnet modes remaining smaller than 1% in the P-T range outside its predicted Mn-free P-T range.

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

Abstract: Lawsonite-bearing metamorphic/metasomatic rocks form at high-pressure–ultrahigh pressure (HP–UHP) and low-temperature (LT) conditions, commonly in Pacific-type subduction zones. The P–T stability 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 in P–T space. The oldest occurrences of lawsonite-bearing blueschists are latest Neoproterozoic, suggesting that subduction-zone thermal structures evolved towards the necessary LT conditions 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 HP serpentinite melange 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.

Low-pressure - high-temperature metamorphism during extension in a Jurassic magmatic arc, Central Pontides, Turkey

Abstract: Magmatic arcs are zones of high heat flow; however, examples of metamorphic belts formed under magmatic arcs are rare. In the Pontides in northern Turkey, along the southern active margin of Eurasia, high temperature–low pressure metamorphic rocks and associated magmatic rocks are interpreted to have formed under a Jurassic continental magmatic arc, which extends for 2800 km through the Crimea and Caucasus to Iran. The metamorphism and magmatism occurred in an extensional tectonic environment as shown by the absence of a regional Jurassic contractional deformation, and the presence of Jurassic extensional volcaniclastic marine basin in the Pontides, over 2 km in thickness, where deposition was coeval with the high‐T metamorphism at depth. The heat flow was focused during the metamorphism, and unmetamorphosed Triassic sequences crop out within a few kilometres of the Jurassic metamorphic rocks. The heat for the high‐T metamorphism was brought up to crustal levels by mantle melts, relicts of which are found as ultramafic, gabbroic and dioritic enclaves in the Jurassic granitoids. The metamorphic rocks are predominantly gneiss and migmatite with the characteristic mineral assemblage quartz + K‐feldspar + plagioclase + biotite + cordierite ± sillimanite ± garnet. Mineral equilibria give peak metamorphic conditions of 4 ± 1 kbar and 720 ± 40 °C. Zircon U–Pb and biotite Ar–Ar ages show that the peak metamorphism took place during the Middle Jurassic at c. 172 Ma, and the rocks cooled to 300 °C at c. 162 Ma, when they were intruded by shallow‐level dacitic and andesitic porphyries and granitoids. The geochemistry of the Jurassic porphyries and volcanic rocks has a distinct arc signature with a crustal melt component. A crustal melt component is also suggested by cordierite and garnet in the magmatic assemblage and the abundance of inherited zircons in the porphyries.

Grain-scale pressure variations and chemical equilibrium in high-grade metamorphic rocks

Abstract: In the classical view of metamorphic microstructures, fast viscous relaxation (and so constant pres- sure) is assumed, with diffusion being the limiting factor in equilibration. This contribution is focused on the only other possible scenario - fast diffusion and slow viscous relaxation - and brings an alter- native interpretation of microstructures typical of high-grade metamorphic rocks. In contrast to the pressure vessel mechanical model applied to pressure variation associated with coesite inclusions in various host minerals, a multi-anvil mechanical model is proposed in which strong single crystals and weak grain boundaries can maintain pressure variation at geological time-scales in a polycrystalline material. In such a mechanical context, exsolution lamellae in feldspar are used to show that feldspar can sustain large differential stresses (>10 kbar) at geological time-scales. Furthermore, it is argued that the existence of grain-scale pressure gradients combined with diffusional equilibrium may explain chemical zoning preserved in reaction rims. Assuming zero net flux across the microstructure, an equilibrium thermodynamic method is introduced for inferring pressure variation corresponding to the chemical zoning. This new barometric method is applied to plagioclase rims around kyanite in fel- sic granulite (Bohemian Massif, Czech Republic), yielding a grain-scale pressure variation of 8 kbar. In this approach, kinetic factors are not invoked to account for mineral composition zoning preserved in rocks metamorphosed at high grade.

Lawsonite geochemistry and stability – implication for trace element and water cycles in subduction zones

Abstract: This contribution reviews the existing data on lawsonite stability and trace element geochemistry, and provides new data for metabasaltic and metasedimentary (quartzite) rocks from New Caledonia, Turkey and California. Lawsonite is a major host of REE, Sr, U, Th and Pb in basaltic compositions. Trace element-rich lawsonite also occurs in metasedimentary rocks, in which comparatively fewer phases compete for trace elements than in metabasaltic rocks. Trace element patterns in lawsonite are influenced by the coexistence or breakdown of allanite, titanite, apatite and garnet that compete for these elements in high-P metamorphic rocks. Lawsonite is restricted to cool geotherms and therefore is an indicator mineral for subduction-zone metamorphism. The lawsonite stability field shows a strong dependence on temperature and composition and it is largest in rocks with a high normative anorthite content and, in basaltic systems, carbon content. Along cold geotherms, lawsonite can transport water and trace elements to great depths, providing a source for these elements in the deep mantle. Along warmer geotherms, lawsonite disappears on a continuous reaction, gradually releasing water over a temperature interval of several tens of degrees. During lawsonite breakdown in complex systems, Th and LREE remain trapped in newly formed accessory allanite. However, owing to extreme LREE content, allanite has lower Pb/Ce and Sr/Nd than lawsonite, resulting in a relative enrichment of Sr and Pb compared with Ce and Nd in the fluids produced during lawsonite breakdown. Existing experimental data on the solidus of altered oceanic crust suggest that the lawsonite-breakdown reaction is within 50 °C of the solidus at sub-arc pressures of 3–4 GPa.

Lawsonite metasomatism and trace element recycling in subduction zones

Abstract: Although lawsonite-bearing rocks are rare in exhumed high-pressure (HP) terranes, they are considered to exert a primary role in subduction dynamics. Recent observations in natural settings have shown that fluid–rock interaction at HP conditions, including metasomatism, may lead to unusually high lawsonite amounts even in rocks that originally contained little or no lawsonite. This process may therefore bear important implications for element recycling in subduction zones. A detailed characterization of the geochemical fingerprints associated with lawsonite metasomatism is presented in this contribution. The studied rocks belong to the HP terranes of Alpine Corsica (France), which is the largest documented exposure for lawsonite metasomatism. Metasomatic lawsonite displays complex compositional zoning, including high trace element, Cr and Ti content. The trace element content is much higher compared with the average of non-metasomatic lawsonite, and is in line with the re-incorporation of large amounts of trace elements (e.g. REE, Sr, Pb, Th) in the rock during metasomatism, as shown by mass transfer calculations. Our data suggest that serpentinites represented the main fluid source for the metasomatism, with concurrent contribution of other, possibly Ca-rich lithologies, such as mafic or meta-sedimentary rocks. We propose that the breakdown of metasomatic lawsonite may contribute to the genesis of magmas and their characteristic geochemical signatures.

The processes that control leucosome compositions in metasedimentary granulites: perspectives from the Southern Marginal Zone migmatites, Limpopo Belt, South Africa

Abstract: Anatexis of metapelitic rocks at the Bandelierkop Quarry (BQ) locality in the Southern Marginal Zone of the Limpopo Belt occurred via muscovite and biotite breakdown reactions which, in order of increasing temperature, can be modelled as: (1) Muscovite + quartz + plagioclase = sillimanite + melt; (2) Biotite + sillimanite + quartz + plagioclase = garnet + melt; (3) Biotite + quartz + plagioclase = orthopyroxene ± cordierite ± garnet + melt. Reactions 1 and 2 produced stromatic leucosomes, which underwent solid-state deformation before the formation of undeformed nebulitic leucosomes by reaction 3. The zircon U–Pb ages for both leucosomes are within error identical. Thus, the melt or magma formed by the first two reactions segregated and formed mechanically solid stromatic veins whilst temperature was increasing. As might be predicted from the deformational history and sequence of melting reactions, the compositions of the stromatic leucosomes depart markedly from those of melts from metapelitic sources. Despite having similar Si contents to melts, the leucosomes are strongly K-depleted, have Ca:Na ratios similar to the residua from which their magmas segregated and are characterized by a strong positive Eu anomaly, whilst the associated residua has no pronounced Eu anomaly. In addition, within the leucosomes and their wall rocks, peritectic garnet and orthopyroxene are very well preserved. This collective evidence suggests that melt loss from the stromatic leucosome structures whilst the rocks were still undergoing heating is the dominant process that shaped the chemistry of these leucosomes and produced solid leucosomes. Two alternative scenarios are evaluated as generalized petrogenetic models for producing Si-rich, yet markedly K-depleted and Ca-enriched leucosomes from metapelitic sources. The first process involves the mechanical concentration of entrained peritectic plagioclase and garnet in the leucosomes. In this scenario, the volume of quartz in the leucosome must reflect the remaining melt fraction with resultant positive correlation between Si and K in the leucosomes. No such correlation exists in the BQ leucosomes and in similar leucosomes from elsewhere. Consequently, we suggest disequilibrium congruent melting of plagioclase in the source and consequential crystallization of peritectic plagioclase in the melt transfer and accumulation structures rather than at the sites of biotite melting. This induces co-precipitation of quartz in the structures by increasing SiO2 content of the melt. This process is characterized by an absence of plagioclase-induced fractionation of Eu on melting, and the formation of Eu-enriched, quartz + plagioclase + garnet leucosomes. From these findings, we argue that melt leaves the source rapidly and that the leucosomes form incrementally as melt or magma leaving the source dumps its disequilibrium Ca load, as well as quartz and entrained ferromagnesian peritectic minerals, in sites of magma accumulation and escape. This is consistent with evidence from S-type granites suggesting rapid magma transfer from source to high level plutons. These findings also suggest that leucosomes of this type should be regarded as constituting part of the residuum from partial melting.

Long‐lived high‐T, low‐P granulite facies metamorphism in the Arunta Region, central Australia

Abstract: In situ LA–ICP–MS U–Pb monazite geochronology from the Boothby Hills in the Aileron Province, central Australia, indicates that the region records more than 80 Ma of high-T, low-P (HTLP) anatectic conditions during the Early Mesoproterozoic. Monazite ages from granulite facies rocks and leucosomes span the interval 1576–1542 Ma. Pegmatites that overprint the regional gneissic fabric and are interpreted to record the last vestiges of melt crystallization give ages between 1523 and 1513 Ma. Calculated P–T pseudosections suggest peak metamorphic conditions in excess of 850 °C at 0.65–0.75 GPa. The retrograde evolution was characterized by a P–T path that involved minor decompression and then cooling, culminating with the development of andalusite. Integration of the geochronological data set with the inferred P–T path trajectory suggests that suprasolidus cooling must have been slow, in the order of 2.5–4 °C Ma−1. In addition, the retrograde P–T path trajectory suggests that HTLP conditions were generated within crust of relatively normal thickness. Despite the long duration over which anatectic conditions occurred, there is no evidence for external magmatic inputs or evidence that HTLP conditions were associated with long-lived extension. Instead, it seems probable that the long-lived HTLP metamorphism was driven to a significant extent by long-lived conductive heating provided by high crustal heat production in voluminous pre-metamorphic granitic rocks.

On parameterizing thermodynamic descriptions of minerals for petrological calculations

Abstract: A new regularization approach, termed micro-ϕ, is outlined for parameterizing activity–composition (a–x) relations and other aspects of the thermodynamic descriptions of minerals for petrological calculations. In the context of the symmetric formalism, a formulation of a–x relations that is easily generalizable to multi-component minerals, parameterization with micro-ϕ extends from where there are good data available to constrain, for example, interaction energies, to where there are little or no data. This involves decomposing the interaction energies, which are macroscopic between end-members, into their microscopic components involving interactions between elements on sites. Micro-ϕ involves heuristics to simplify and approximate these microscopic interaction energies prior to their reassembly as macroscopic interaction energies. Micro-ϕ also allows parameterization of Fe2+–Mg order–disorder between the sites of minerals. Application of micro-ϕ is illustrated by making a thermodynamic description of chlorite in FeO–MgO–Al2O3–SiO2–H2O–O, FMASHO. Micro-ϕ has been developed for use with the significant upgrade of the Holland & Powell internally consistent thermodynamic data set which required the re-evaluation of a–x relations of several common rock-forming minerals.

Phase equilibria modelling of retrograde amphibole and clinozoisite in mafic eclogite from the Tso Morari massif, northwest India: constraining the P–T–M(H2O) conditions of exhumation

Abstract: Phase equilibria modelling of post-peak metamorphic mineral assemblages in (ultra)high-P mafic eclogite from the Tso Morari massif, Ladakh Himalaya, northwest India, has provided new insights into the potential behaviour and source of metamorphic fluid during exhumation, and constrained the P–T conditions of hydration. A series of P–M(H2O) pseudosections constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) system show that a number of petrographically distinct hydration episodes occurred during exhumation from peak P–T conditions (~640 °C, 27–28 kbar), resulting in the formation of abundant compositionally zoned amphibole and minor clinozoisite poikiloblasts at the expense of a peak assemblage dominated by garnet and omphacite. Initial hydration is interpreted to have occurred as a result of the destabilization of talc following isothermal decompression to ~23 kbar, which led to the formation of barroisite–winchite amphibole core domains. An episode of fluid infiltration from an external source at ~19 kbar, with or without syn-decompressional cooling to ~560 °C, resulted in further barroisitic–winchitic amphibole growth, followed by the formation of clinozoisite poikiloblasts. Continued buoyancy-driven exhumation to the base of the lower crust is constrained to have taken place with no additional fluid input. A final hydration event is characterized by the formation of magnesiohornblende rims on the barroisite–winchite cores, with the former interpreted to have formed during later prograde overprinting in the middle crust associated with the final stages of exhumation. Notably, the vast majority of externally sourced H2O, comprising just over half of the current bulk rock fluid content, was added during this later hydration event. In a middle crustal setting, this is interpreted as the result of devolatilization reactions occurring in migmatitic host orthogneiss and/or metasedimentary units, or following the crystallization of partial melt.

Cold subduction of the Neotethys: the metamorphic record from finely banded lawsonite and epidote blueschists and associated metabasalts of the Nagaland Ophiolite Complex, India

Abstract: In this study, we have deduced the thermal history of the subducting Neotethys from its eastern margin, using a suite of partially hydrated metabasalts from a segment of the Nagaland Ophiolite Complex (NOC), India. Located along the eastern extension of the Indus-Tsangpo suture zone (ITSZ), the N–S-trending NOC lies between the Indian and Burmese plates. The metabasalts, encased within a serpentinitic melange, preserve a tectonically disturbed metamorphic sequence, which from west to east is Greenschist (GS), Pumpellyite–diopside (PD) and Blueschist (BS) facies. Metabasalts in all the three metamorphic facies record prograde metamorphic overprints directly on primary igneous textures and igneous augite. In the BS facies unit, the metabasalts interbedded with marble show centimetre- to metre-scale interlayering of Lawsonite Blueschist (LBS) and Epidote Blueschist (EBS). Prograde HP/LT metamorphism stabilized lawsonite + omphacite (X Jd = 0.50–0.56 to 0.26–0.37) + jadeite (X Jd = 0.67–0.79) + augite + ferroglaucophane + high-Si phengite (Si = 3.6–3.65 atoms per formula unit, a.p.f.u.) + chlorite + titanite + quartz in LBS and lawsonite + glaucophane/ferroglaucophane ± epidote ± omphacite (X Jd = 0.34) + chlorite + phengite (Si = 3.5 a.p.f.u.) + titanite + quartz in EBS at the metamorphic peak. Retrograde alteration, which was pervasive in the EBS, produced a sequence of mineral assemblages from omphacite and lawsonite-absent, epidote + glaucophane/ferroglaucophane + chlorite + phengite + titanite + quartz through albite + chlorite + glaucophane to lawsonite + albite + high-Si phengite (Si = 3.6–3.7 a.p.f.u.) + glaucophane + epidote + quartz. In the PD facies metabasalts, the peak mineral assemblage, pumpellyite + chlorite + titanite + phengitic white mica (Si = 3.4–3.5 a.p.f.u.) + diopside appeared in the basaltic groundmass from reacting titaniferous augite and low-Si phengite, with prehnite additionally producing pumpellyite in early vein domains. In the GS facies metabasalts, incomplete hydration of augite produced albite + epidote + actinolite + chlorite + titanite + phengite + augite mineral assemblage. Based on calculated T–M(H 2 O), T–M(O 2 ) (where M represents oxide mol.%) and P–T pseudosections, peak P–T conditions of LBS are estimated at ∼11.5 kbar and ∼340°C, EBS at ∼10 kbar, 325°C and PD facies at ∼6 kbar, 335°C. Reconstructed metamorphic reaction pathways integrated with the results of P–T pseudosection modelling define a near-complete, hairpin, clockwise P–T loop for the BS and a prograde P–T path with a steep dP/dT for the PD facies rocks. Apparent low thermal gradient of 8°C km −1 corresponding to a maximum burial depth of 40 km and the hairpin P–T trajectory together suggest a cold and mature stage of an intra-oceanic subduction zone setting for the Nagaland blueschists. The metamorphic constraints established above when combined with petrological findings from the ophiolitic massifs along the whole ITSZ suggest that intra-oceanic subduction systems within the Neotethys between India and the Lhasa terrane/the Karakoram microcontinent were also active towards east between Indian and Burmese plates.

The inception and growth of leucosomes: microstructure at the start of melt segregation in migmatites

Abstract: The beginning stages of melt segregation and the formation of leucosomes are rarely preserved in migmatites. Most arrays of leucosomes record a more advanced stage where flow dominates over segregation. However, the early stages in the formation of leucosomes and the segregation of melt are preserved in a partially melted meta-argillite from the metatexite zone (>800 °C) of the contact aureole around the Duluth Complex, Minnesota. The rock contains 2.4 modal% leucosome in a matrix consisting of 40.5% in situ neosome and 57.1% cordierite + plagioclase framework. The domainal microstructure in the matrix is a pre-anatectic feature resulting from the bulk composition. Terminal chlorite reactions produced a large volume of cordierite which, with plagioclase, formed a framework that enclosed patches of biotite + quartz + plagioclase ± K-feldspar. Upon melting, these fertile domains became patches of in situ neosome. Plagioclase in the neosome is less sodic than in the leucosome, hence segregation of melt occurred during crystallization, not melting. Segregation was delayed because the cordierite + plagioclase framework was strong enough to resist dilatation and compaction until after crystallization started. The leucosomes are small (i.e. they are microleucosomes) and display a systematic progression in morphology as length and aspect ratio increase from ~1 to 19 mm and from ~2.5 to >30 respectively. Small equant micropores form first, and in places these coalesce into small (~1 mm, aspect ratio ~2.5), isolated, blunt-ended, elliptical microleucosomes. In the next stage, micropores develop ahead of, and at ~45° to the left and right of the blunt tip of a microleucosome; one of these develops into an elliptical leucosome and an en echelon array of either a left- or right-stepping elliptical microleucosome forms. Each elliptical microleucosome in the en echelon arrays is separated by a bridge of matrix. Next, microleucosomes of greater length (>4 mm) and aspect ratio (>5) form when the bridges of cordierite + plagioclase matrix rupture and the elliptical microleucosomes link together to form a zigzag-shaped microleucosome. Finally, still longer microleucosomes with greater aspect ratios (~30) are formed by the joining of zigzag arrays. Such a progression is characteristic of the way ductile fractures grow. The segregation of melt was driven by the pressure gradient between the dilatant fracture and an adjacent in situ neosome, which drew melt to the growing fracture, thereby creating a microleucosome. The microleucosomes are filled arrays of ductile fractures. Melt was contiguous only between microleucosomes and adjacent patches of in situ neosome. The length-scale of segregation was ~5 mm, the size of a typical patch of in situ neosome, and restricted by the surrounding impermeable cordierite + plagioclase framework. The melt in the microleucosome was the most fractionated and the last to crystallize. All microleucosomes contain entrained minerals as a consequence of their mechanism of growth. Rupture of the bridges resulted in the entrainment of pre-anatectic phases. However, microleucosomes that cross patches of in situ neosome are also contaminated with peritectic phases that were transported with the melt.

Metamorphic evolution of relict lawsonite‐bearing eclogites from the (U) HP metamorphic belt in the Chinese southwestern Tianshan

Abstract: In the Chinese southwestern Tianshan (U)HP belt, former lawsonite presence has been predicted for many (U)HP metamorphic eclogites, but only a very few lawsonite grains have been found so far. We discovered armoured lawsonite relicts included in quartz, which, on its part, is enclosed in porphyroblastic garnet in an epidote eclogite H711-14 and a paragonite eclogite H711-29. H711-14 is mainly composed of garnet, omphacite, epidote and titanite, with minor quartz, paragonite and secondary barroisite and glaucophane. Coarse-grained titanite occasionally occurs in millimetre-wide veins in equilibrium with epidote and omphacite, and relict rutile is only preserved as inclusions in matrix titanite and garnet. H711-29 shows the mineral assemblage of garnet, omphacite, glaucophane, paragonite, quartz, dolomite, rutile and minor epidote. Dolomite and rutile are commonly rimed by secondary calcite and titanite respectively. Porphyroblastic garnet in both eclogites is compositionally zoned and exhibits an inclusion-rich core overgrown by an inclusion-poor rim. Phase equilibria modelling predicts that garnet cores formed at the P-peak (490–505 °C and 23–25.5 kbar) and coexisted with the lawsonite eclogite facies assemblage of omphacite + glaucophane + lawsonite + quartz. Garnet rims (550–570 °C and ~20 kbar) grew subsequently during a post-peak epidote eclogite facies metamorphism and coexisted with omphacite + quartz ± glaucophane ± epidote ± paragonite. The results confirm the former presence of a cold subduction zone environment in the Chinese southwestern Tianshan. The P–T evolution of the eclogites is characterized by a clockwise P–T path with a heating stage during early exhumation (thermal relaxation). The preservation of lawsonite in these eclogites is attributed to isolation from the matrix by quartz and rigid garnet, which should be considered as a new type of lawsonite preservation in eclogites. The complete rutile–titanite transition in H711-14 took place in the epidote eclogite facies stage in the presence of an extremely CO2-poor fluid with X(CO2) [CO2/(CO2 + H2O) in the fluid] 0.01) in the coexisting fluid at the epidote eclogite facies stage.

The duration of near-peak metamorphism from diffusion modelling of garnet zoning

Abstract: Garnet in a staurolite–kyanite zone sample from central Vermont displays a bell-shaped Mn growth zoning with diffusional modification over the outer 100 μm. The diffusion is driven by the prograde net transfer reaction garnet + chlorite = kyanite + biotite as is evidenced by a well-defined resorption zone on the rim. Analysis of the reaction history and resorbed garnet composition suggests that the peak temperature attained was 620–660 °C. Diffusional modelling of the rim diffusion provides an estimate of the duration of the metamorphic episode over which significant garnet diffusion occurs. The duration is a function of the assumed peak temperature and garnet diffusivities and range from a few hundred thousand years to a few million years. Such short durations require rapid tectonic burial and exhumation of relatively thin tectonic slices.

Cl/Br of scapolite as a fluid tracer in the earth's crust: insights into fluid sources in the Mary Kathleen Fold Belt, Mt. Isa Inlier, Australia

Abstract: A combination of analytical methods, including trace element analysis of Br in scapolite by LA-ICP- MS, was employed to unravel the fluid–rock interaction history of the Mary Kathleen Fold Belt of northern Australia. Halogen ratios in the metamorphic and hydrothermally derived scapolite from a range of rock-types record interaction between the host rocks and magmatic-hydrothermal fluids derived from granite plutons and regional metamorphism. The results show that halite-dissolution supplied at best only minor chlorine to fluids in the Fold Belt. Chlorine/bromine ratios in metamorphic scapolite indicate that fluids were dominantly derived from basinal brines formed from sub-aerial evaporation of seawater beyond the point of halite saturation. This bittern fluid infiltrated the under- lying sedimentary sequences prior to regional metamorphism. Zoned scapolite in a major late meta- morphic mineralized shear-zone records three discrete pulses of magmatic and metamorphic fluid, and it is suggested that fluid mixing may have assisted mineralization along and around this shear zone. As a crucial prerequisite for halogen fluid tracer studies using scapolite, we find in our samples that Cl and Br do not fractionate when incorporated in scapolite. Furthermore, unaltered rims of heavily retrogressed scapolite show indistinguishable Cl/Br signatures compared with fresh grains from the same sample indicating retrograde metamorphism did not significantly affect Cl and Br signatures in scapolite group minerals.

Zircon U–Pb ages and Hf isotope compositions of migmatites from the North Qinling terrane and their geological implications

Abstract: Migmatites are predominant in the North Qinling (NQ) orogen, but their formation ages are poorly constrained. This paper presents a combined study of cathodoluminescence imaging, U–Pb age, trace element and Hf isotopes of zircon in migmatites from the NQ unit. In the migmatites, most zircon grains occur as new, homogeneous crystals, while some are present as overgrowth rims around inherited cores. Morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting. The inherited cores have oscillatory zoning and yield U–Pb ages of c. 900 Ma, representing their protolith ages. The early Neoproterozoic protoliths probably formed in an active continental margin, being a response to the assembly of the supercontinent Rodinia. The migmatite zircon yields Hf model ages of 1911 ± 20 to 990 ± 22 Ma, indicating that the protoliths were derived from reworking of Palaeoproterozoic to Neoproterozoic crustal materials. The anatexis zircon yields formation ages ranging from 455 ± 5 to 420 ± 4 Ma, with a peak at c. 435 Ma. Combined with previous results, we suggest that the migmatization of the NQ terrane occurred at c. 455–400 Ma. The migmatization was c. 50 Ma later than the c. 490 Ma ultra-high-P (UHP) metamorphism, indicating that they occurred in two independent tectonic events. By contrast, the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the NQ unit, which collectively argue for their formation due to the northward subduction of the Shangdan Ocean. UHP rocks were distributed mainly along the northern margin and occasionally in the inner part of the NQ unit, indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process.

Dating subduction‐zone metamorphism with combined garnet and lawsonite Lu–Hf geochronology

Abstract: Studies of ancient and active subduction zones are critically important to understanding processes of interplate coupling, crust-mantle recycling and arc magmatism. Dating subduction metamorphism along prograde and retrograde paths in order to constrain such processes, however, has proved to be extremely difficult. The complex thermal history of subduction systems poses significant challenges to low-T geochronometers and subduction-zone assemblages commonly lack suitable minerals for higher temperature geochronology. Garnet and lawsonite, however, are critical index minerals of high- and low-T subduction-zone metamorphism and are well suited to Lu–Hf geochronology. In addition, the closure temperature for Lu–Hf in garnet and lawsonite should be sufficiently high that an age will date mineral crystallization and therefore the timing of subduction zone metamorphism. The relative stability and timing of garnet and lawsonite formation will be controlled by the bulk composition, peak metamorphic conditions and shape of the P–T path experienced by a particular sample. To test the influence of metamorphic conditions and P–T path on Lu–Hf geochronology, garnet and lawsonite-bearing samples were dated from rocks where lawsonite formed after garnet along a retrograde path, stabilized prior to garnet along a prograde path and formed contemporaneously with garnet. In the Franciscan Complex, the ages of garnet–epidote amphibolite, garnet–epidote blueschist, garnet–lawsonite blueschist and lawsonite blueschist range from c. 166–130 Ma and generally decrease with decreasing metamorphic grade, consistent with previous studies. Garnet–lawsonite blueschist/eclogite formed along an apparent prograde path at Ward Creek records an apparent age of c. 152 Ma. Lower temperature lawsonite blueschist at Ward Creek, however, failed to provide a geologically significant date and likely reflects isotopic disequilibrium at low temperatures. The apparent temperature–time history from Franciscan Complex Lu–Hf ages most likely reflects samples derived from various portions of the subduction zone or that were subducted and metamorphosed at different times in the thermal evolution of the subduction zone. In the Sivrihisar Massif, lawsonite eclogite and garnet–lawsonite blueschist record distinctly different ages of 91.1 ± 1.3 Ma and 83.3 ± 1.8 Ma. The different ages date the timing of high-P metamorphism within each protolith and suggest that garnet–lawsonite eclogite metamorphism pre-dated garnet–lawsonite blueschist metamorphism in these samples by c. 8 Ma. The age of lawsonite eclogite metamorphism extends the timing of high-P metamorphism and requires subduction initiation prior to 91 Ma. The results indicate that the Lu–Hf system provides a reliable tool for dating the wide range in P–T conditions of subduction-zone metamorphism. Lawsonite dating, in particular, provides a reliable method by which to date low-T retrograde and prograde metamorphism in the absence of garnet. Lawsonite may not be ideal for geochronology if sufficient garnet coexists in the mineral assemblage, the lawsonite has very low-T stability, or if extremely fine-grained Hf phases such as zircon are present in lawsonite. In poly-metamorphic assemblages where the pressure and temperature can be estimated for separate garnet and lawsonite sub-assemblages, the age discrepancy between garnet and lawsonite may provide the ability to quantify the rates of heating or cooling and subduction or exhumation.

Temporal links between pluton emplacement, garnet granulite metamorphism, partial melting and extensional collapse in the lower crust of a Cretaceous magmatic arc, Fiordland, New Zealand

Abstract: Garnet granulite facies mid-to lower crust in Fiordland, New Zealand, provides evidence for pulsed intrusion and deformation occurring in the mid-to lower crust of magmatic arcs. 238U-206Pb zircon ages constrain emplacement of the ∼595 km2 Malaspina Pluton to 116–114 Ma. Nine Sm-Nd garnet ages (multi-point garnet-rock isochrons) ranging from 115.6 ± 2.6 to 110.6 ± 2.0 Ma indicate that garnet granulite facies metamorphism was synchronous or near synchronous throughout the pluton. Hence, partial melting and garnet granulite facies metamorphism lasted 900 °C, garnet contained ∼4000 ppm Ti; subsequently, rutile inclusions grew during declining temperature with limited pressure change. Garnet granulite metamorphism of the Malaspina Pluton is c. 10 Ma younger than similar metamorphism of the Pembroke Granulite in northern Fiordland; therefore, high-P metamorphism and partial melting must have been diachronous for this >3000 km² area of mid-to-lower crust. Thus, two or more pulses of intrusion shortly followed by garnet granulite metamorphism and extensional strain occurred from north to south along the axis of the lower crustal root of the Cretaceous Gondwana arc.

Production of metaluminous melt during fluid‐present anatexis: an example from the Maghrebian basement, La Galite Archipelago, central Mediterranean

Abstract: Garnet brought to the surface by late Miocene granitoids at La Galite Archipelago (Central Mediterranean, Tunisia) contains abundant primary melt and fluid inclusions. Microstructural observations and mineral chemistry define the host garnet as a peritectic phase produced by biotite incongruent melting at ~800 °C and 0.5 GPa, under fluid-present conditions. The trapped melt is leucogranitic with an unexpected metaluminous and almost peralkaline character. Fluid inclusions are one phase at room temperature, and contain a CO2-dominated fluid, with minor H2O, N2 and CH4. Siderite and an OH-bearing phase were identified by Raman and IR spectroscopy within every analysed inclusion, and are interpreted as products of a post-entrapment carbonation/hydration reaction between the fluid and the host during cooling. The fluid present during anatexis is therefore inferred to have been originally richer in both H2O and CO2. The production of anatectic melt with a metaluminous signature can be explained as the result of partial melting of relatively Al-poor protoliths assisted by CO2-rich fluids.

Structural, petrological and chemical analysis of syn-kinematic migmatites: insights from the Western Gneiss Region, Norway

Abstract: Evidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnets that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (1) as zoned segregations found in high-pressure (HP) (ultra) mafic bodies, (2) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss, and (3) as undeformed pegmatites cutting the main Caledonian structures. Segregations post-date the eclogite facies foliation and predate the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation and undeformed pegmatites are post-kinematic and were formed at the end of the deformation history. Geochemistry of the segregations, leucosomes and pegmatites in the WGR defines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca-poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K-poor compositions corresponds to segregation hosted in (ultra) mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. The REE patterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in the WGR before or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward-buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800°C implies the presence of an H2O-rich fluid phase at great depth to facilitate H2O-present partial melting.

Thermal evolution of the Tseel terrane, SW Mongolia and its relation to granitoid intrusions in the Central Asian Orogenic Belt

Abstract: The timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are important for understanding the growth process of continental crust. In this study, the petrology and geochronology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to understand the relationship between igneous activity and metamorphism during crustal evolution in the Central Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressive changes in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimanite and cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones) occur in the central part of the Tseel area, where granitoids are abundant. To the north and south of these granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in the pelitic gneisses varies systematically across the mineral zones, from grossular-rich garnet in the garnet zone to zoned garnet with grossular-rich cores and pyrope-rich rims in the staurolite zone, and pyrope-rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individual garnet crystals reveal two main stages of metamorphism: (i) a high-P and low-T stage (as recorded by garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar in the kyanite stability field and (ii) a low-P and high-T stage (garnet rims in the staurolite zone and garnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stability field. The earlier high-P metamorphism resulted in the growth of kyanite in quartz veins within the staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids reveal that (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low-P and high-T metamorphism occurred at 377 ± 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intrusion at 385 ± 7 Ma. The age of the earlier low-T and high-P metamorphism is not clearly recorded in the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low-P and high-T metamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoid body. These findings indicate that an elevation of geotherm and a transition from high-P and low-T to low-P and high-T metamorphism occurred, associated with continuous emplacement of several granitoids, during the crustal evolution in the Devonian CAOB.

Coexistence of garnet blueschist and eclogite in South Tianshan, NW China: dependence of P–T evolution and bulk-rock composition

Abstract: Coexisting garnet blueschist and eclogite from the Chinese South Tianshan high-pressure (HP)–ultrahigh-pressure (UHP) belt consist of similar mineral assemblages involving garnet, omphacite, glaucophane, epidote, phengite, rutile/sphene, quartz and hornblendic amphibole with or without paragonite. Eclogite assemblages generally contain omphacite >50 vol.% and a small amount of glaucophane ( 0.75) or fluid composition with higher X(CO2) (>0.15), but eclogite assemblages preferentially occur in rocks with higher X(CaO) and lower A/CNK or fluid composition with lower X(CO2). Moreover, phase modelling suggests that the coexistence of blueschist and eclogite depends substantially on P–T conditions, which would commonly occur in medium temperatures of 500–590 °C under pressures of ~17–22 kbar. The modelling results are in good accordance with the measured bulk-rock compositions and modelled temperature results of the coexisting garnet blueschist and eclogite from the South Tianshan HP–UHP belt.

Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage-Colton Mylonite Zone, Adirondack Mountains, NY, USA

Abstract: Oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen- bearing minerals and the dependence of mineral 18 O/ 16 O ratios on temperature and fluid composi- tion. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10-lm scale by secondary ion mass spectrometry. The patterns of d 18 O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxy- gen exchange, partial recrystallization, grain-size reduction, and grain growth. Using the chrono- logical framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Car- thage-Colton Mylonite Zone (CCMZ), a long-lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ott- awan phase (1090-1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW-dipping protomylonitic fabric (S1) is crosscut by near-vertical ultramylonitic shear zones (S2), which are locally reoriented by a NNW-dipping mylonitic fabric (S3). Texturally early titanite (pre-S2) shows diffusion-domi- nated d 18 O zoning that records cooling from peak Ottawan, granulite-facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed d 18 O profiles for cooling rates of 50 � 20 ° CM a � 1 , which are considerably faster than the 1-5 ° CM a � 1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cool- ing rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn-S2 )h as higher overall d 18 O and shows a transition from diffusion-dominated to recrystallization-dominated d 18 O zoning, indicating infiltration of elevated-d 18 O fluids along S2 shear zones and continued shear- ing below the blocking temperature for oxygen (~≤500 °C for grain sizes at the study site). The tex- turally latest titanite (post-S3 )h as growth-dominatedd 18 O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.

Anatectic record and contrasting P–T paths of aluminous gneisses from the central Grenville Province

Abstract: Anatectic aluminous gneisses, some derived from sedimentary rocks of broadly pelitic composition and others from hydrothermally altered felsic volcanic rocks, are exposed in the mid-P and high-P segments of the hinterland in the central Grenville Province. These gneisses consist dominantly of garnet, biotite, K-feldspar, plagioclase and quartz, with sillimanite or kyanite, and display microstructural evidence of anatexis by fluid-absent reactions consuming muscovite and/or biotite. Melt-related microstructures, such as inter-granular films and/or interstitial quartz or feldspar enclosing relict phases, are most abundant in the metasedimentary samples. Despite anatexis at granulite facies conditions, the hydrothermally altered rocks preserve earlier features attributed to the circulation of hydrothermal fluids, such as sillimanite seams, dismembered quartz veins and garnet-rich aluminous nodules in a K-feldspar-dominated matrix. Microstructural and mineral chemical data, integrated with P–T pseudosections calculated with thermocalc for the metasedimentary rocks, permit qualitative constraints on the P–T paths. Data from a high-P kyanite-bearing sample are consistent with a steep prograde P–T path up to ~14.5 kbar and 860–900 °C, followed by decompression with minor cooling to the solidus at ~11 kbar and 870 °C. This pressure-dominated P–T path is similar to those inferred in other parts of the high-P segment in the central Grenville Province. In contrast, the P–T path predicted from a mid-P sillimanite-bearing paragneiss has a strong temperature gradient with P–T of ~9.5 kbar and 850 °C at the thermal peak, and a retrograde portion down to ~8 kbar and 820 °C. In a broad sense, these two contrasting P–T patterns are consistent with predictions of thermo-mechanical modelling of large hot orogens in which P–T paths with strong pressure gradients exhume deeper rocks in the orogenic flanks, whereas P–T paths with strong temperature gradients in the orogenic core reflect protracted lateral transport of ductile crust beneath a plateau.

Influence of ferric iron on phase equilibria in greenschist facies assemblages: the hematite-rich metasedimentary rocks from the Monti Pisani (Northern Apennines)

Abstract: We have investigated the effects of different Fe2O3 bulk contents on the calculated phase equilibria of low-T/intermediate-P metasedimentary rocks. Thermodynamic modelling within the MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNKFMASHTO) chemical system of chloritoid-bearing hematite-rich metasedimentary rocks from the Variscan basement of the Pisani Mountains (Northern Apennines, Italy) fails to reproduce the observed mineral compositions when the bulk Fe2O3 is determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P–XFe2O3 diagrams, obtaining equilibration conditions of 475 °C and 9–10 kbar related to a post-compressional phase of the Alpine collision. The introduction of ferric iron affects the stability of the main rock-forming silicates that often yield important thermobaric information. In Fe2O3-rich compositions, garnet- and carpholite-in curves shift towards higher temperatures with respect to the Fe2O3-free systems. The presence of a ferric-iron oxide (hematite) prevents the formation of biotite in the mineral assemblage even at temperatures approaching 550 °C. The use of P–T–XFe2O3 phase diagrams may also provide P–T information in common greenschist facies metasedimentary rocks.

Subduction and exhumation mechanisms of ultra‐high and high‐pressure oceanic and continental crust at Makbal (Tianshan, Kazakhstan and Kyrgyzstan)

Abstract: The Makbal Complex in the northern Tianshan of Kazakhstan and Kyrgyzstan consists of metasedimentary rocks, which host high-P (HP) mafic blocks and ultra-HP Grt-Cld-Tlc schists (UHP as indicated by coesite relicts in garnet). Whole rock major and trace element signatures of the Grt-Cld-Tlc schist suggest a metasomatized protolith from either hydrothermally altered oceanic crust in a back-arc basin or arc-related volcaniclastics. Peak metamorphic conditions of the Grt-Cld-Tlc schist reached ~580 °C and 2.85 GPa corresponding to a maximum burial depth of ~95 km. A Sm-Nd garnet age of 475 ± 4 Ma is interpreted as an average growth age of garnet during prograde-to-peak metamorphism; the low initial eΝd value of −11 indicates a protolith with an ancient crustal component. The petrological evidence for deep subduction of oceanic crust poses questions with respect to an effective exhumation mechanism. Field relationships and the metamorphic evolution of other HP mafic oceanic rocks embedded in continentally derived metasedimentary rocks at the central Makbal Complex suggest that fragments of oceanic crust and clastic sedimentary rocks were exhumed from different depths in a subduction channel during ongoing subduction and are now exposed as a tectonic melange. Furthermore, channel flow cannot only explain a tectonic melange consisting of various rock types with different subduction histories as present at the central Makbal Complex, but also the presence of a structural ‘dome’ with UHP rocks in the core (central Makbal) surrounded by lower pressure nappes (including mafic dykes in continental crust) and voluminous metasedimentary rocks, mainly derived from the accretionary wedge.

Dehydration and anatexis of UHP metagranite during continental collision in the Sulu orogen

Abstract: Dehydration and anatexis of ultrahigh-pressure (UHP) metamorphic rocks during continental collision are two key processes that have great bearing on the physicochemical properties of deeply subducted continental crust at mantle depths. Determining the time and P–T conditions at which such events take place is needed to understand subduction-zone tectonism. A combined petrological and zirconological study of UHP metagranite from the Sulu orogen reveals differential behaviours of dehydration and anatexis between two samples from the same UHP slice. The zircon mantle domains in one sample record eclogite facies dehydration metamorphism at 236 ± 5 Ma during subduction, exhibiting low REE contents, steep MREE–HREE patterns without negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 651–750 °C and inclusions of quartz, apatite and jadeite. A second mantle domain records high-T anatexis at 223 ± 3 Ma during exhumation, showing high REE contents, steeper MREE–HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, high temperatures of 698–879 °C and multiphase solid inclusions of albite + muscovite + quartz. In contrast, in a second sample, one zircon mantle domain records limited hydration anatexis at 237 ± 3 Ma during subduction, exhibiting high REE contents, steep MREE–HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, medium temperatures of 601–717 °C and multiphase solid inclusions of albite + muscovite + hydrohalite. A second mantle domain in this sample records a low-T dehydration metamorphism throughout the whole continental collision in the Triassic, showing low REE contents, steep MREE–HREE patterns with weakly negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 524–669 °C and anhydrite + gas inclusions. Garnet, phengite and allanite/epidote in these two samples also exhibit different variations in texture and major-trace element compositions, in accordance with the zircon records. The distinct P–T–t paths for these two samples suggest separate processes of dehydration and anatexis, which are ascribed to the different geothermal gradients at different positions inside the same crustal slice during continental subduction-zone metamorphism. Therefore, the subducting continental crust underwent variable extents of dehydration and anatexis in response to the change in subduction-zone P–T conditions.

Deformation-driven, regional-scale metasomatism in the Hamersley Basin, Western Australia

Abstract: Mafic volcanic rocks of the Fortescue Group form the lowermost stratigraphic unit of the 100,000 km2 Hamersley Basin on the southern margin of the Archean Pilbara Craton, Western Australia. A regional burial metamorphic gradient extends across the basin from prehnite–pumpellyite facies in the north to greenschist facies in the south. Phase equilibria modelling of mafic rocks with the computer program thermocalc, in subsets of the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–Fe2O3, successfully reproduces observed metamorphic mineral assemblages, giving conditions of ~210 °C, 2 kbar in the north and 335 °C, 3.2 kbar in the south. Superimposed on this metamorphic gradient, regional-scale metasomatism in the Fortescue Group progressively produces a suite of prehnite-bearing and pumpellyite–quartz/epidote–quartz-dominated assemblages. Further modelling of variably metasomatized samples consistently estimates conditions of 260–280 °C, 2.5–3 kbar across the basin. All modelled samples were likely metasomatized at approximately the same structural level, following regional deformation during the Ophthalmian orogeny. Folding during the Ophthalmian orogeny produced topographic and/or tectonic driving forces for regional-scale fluid flow, pushing metasomatic fluid northwards across the Hamersley Basin. These new phase equilibria calculations support previous interpretations linking the Ophthalmian orogeny, fluid flow and upgrading of Hamersley iron ore deposits. We propose an extension of this fluid flow to the Fortescue Group, the metasomatism of which may have contributed a source of Fe to the Hamersley iron ore deposits.