Very Low-grade Metamorphic Evolution of Pelitic Rocks under High-pressure/Low-temperature Conditions, NW New Caledonia (SW Pacific)

The P–T gradient in a Late Eocene low-T high-P metamorphicbelt in northern New Caledonia increases from SW to NE. Metapelitesin the pumpellyite–prehnite and blueschist zones containlawsonite, Mg-carpholite, Fe-stilpnomelane and Fe-glaucophane.Thermodynamic calculations indicate a progression of metamorphicconditions from less than 0·3 GPa and 250°C in akaolinite-bearing rock in the SW, up to 1·5 GPa and 410°Cin a lawsonite–glaucophane-bearing sample in the NE ofthe Diahot terrane. Through a multi-method investigation ofphyllosilicates, organic matter and fluid inclusions, we demonstratethat the evolution of organic matter and illite crystallinitydepends strongly on the evolution of the P–T path withtime. In addition, we show that the illite–muscovite bcell dimension provides a robust estimate of maximum pressurereached in low-temperature domains with polyphase metamorphichistories, despite subsequent high-temperature–low-pressureevents. Fluid inclusion study reveals an isothermal decompressionin the Diahot terrane. KEY WORDS: low-temperature/high-pressure metapelites; illite crystallinity; coal rank; illite–muscovite b cell dimension; New Caledonia     

An Experimentally Constrained Petrogenetic Grid in the Silica-Saturated Portion of the System KFMASH at High Temperatures and Pressures

Experiments in the quartz-saturated part of the system KFMASHunder fO₂ conditions of the haematite–magnetite bufferand using bulk compositions with X_Mg of 0·81, 0·72,0·53 define the stability limits of several mineral assemblageswithin the P–T field 9–12 kbar, 850–1100°C.The stability limits of the mineral assemblages orthopyroxene+ spinel + cordierite ± sapphirine, orthopyroxene + garnet+ sapphirine, sapphirine + cordierite + orthopyroxene and garnet+ orthopyroxene + spinel have been delineated on the basis ofP–T and T–X pseudosections. Sapphirine did not appearin the bulk composition of X_Mg = 0·53. A partial petrogeneticgrid applicable to high Mg–Al granulites metamorphosedat high fO₂, developed in our earlier work, was extended tohigher pressures. The experimental results were successfullyapplied to several high-grade terranes to estimate P–Tconditions and retrograde P–T trajectories. KEY WORDS: KFMASH equilibria; experimental petrogenetic grid at high fO₂     

Variation in Metamorphic Style along the Northern Margin of the Damara Orogen, Namibia

The northern margin of the Inland Branch of the Pan-AfricanDamara Orogen in Namibia shows dramatic along-strike variationin metamorphic character during convergence between the Congoand Kalahari Cratons (M₃ metamorphic cycle). Low-P contact metamorphismwith anticlockwise P–T paths dominates in the westerndomains (Ugab Zone and western Northern Zone), and high-P Barrovianmetamorphism with a clockwise P–T path is documented fromthe easternmost domain (eastern Northern Zone). The sequenceof M₃ mineral growth in contact aureoles shows early growthof cordierite porphyroblasts that were pseudomorphed to biotite–chlorite–muscoviteat the same time as an andalusite–biotite–muscovitetransposed foliation was developed in the matrix. The peak-Tmetamorphic assemblages and fabrics were overprinted by crenulationsand retrograde chlorite–muscovite. The KFMASH P–Tpseudosection for metapelites in the Ugab Zone and western NorthernZone contact aureoles indicates tight anticlockwise P–Tloops through peak metamorphic conditions of 540–570°Cand 2·5–3·2 kbar. These semi-quantitativeP–T loops are consistent with average P–T calculationsusing THERMOCALC, which give a pooled mean of 556 ± 26°Cand 3·2 ± 0·6 kbar, indicating a high averagethermal gradient of 50°C/km. In contrast, the eastern NorthernZone experienced deep burial, high-P/moderate-T Barrovian M₃metamorphism with an average thermal gradient of 21°C/kmand peak metamorphic conditions of c. 635°C and 8·7kbar. The calculated P–T pseudosection and garnet compositionalisopleths in KFMASH, appropriate for the metapelite sample fromthis region, document a clockwise P–T path. Early plagioclase–kyanite–biotiteparageneses evolved by plagioclase consumption and the growthof garnet to increasing X_Fe, X_Mg and X_Ca and decreasing X_Mncompositions, indicating steep burial with heating. The developedkyanite–garnet–biotite peak metamorphic parageneseswere followed by the resorption of garnet and formation of plagioclasemoats, indicating decompression, which was followed by retrogressivecooling and chlorite–muscovite growth. The clockwise P–Tloop is consistent with the foreland vergent fold–thrustbelt geometry in this part of the northern margin. Earlier formed(580–570 Ma) pervasive matrix foliations (M₂) were overprintedby contact metamorphic parageneses (M₃) in the aureoles of 530± 3 Ma granites in the Ugab Zone and 553–514 Magranites in the western Northern Zone. Available geochronologicaldata suggest that convergence between the Congo and KalahariCratons was essentially coeval in all parts of the northernmargin, with similar ages of 535–530 Ma for the main phaseof deformation in the eastern Northern Zone and Northern Platformand 538–505 Ma high-grade metamorphism of the CentralZone immediately to the south. Consequently, NNE–SSW-directedconvergent deformation and associated M₃ metamorphism of contrastingstyles are interpreted to be broadly contemporaneous along thelength of the northern margin of the Inland Branch. In the westheat transfer was dominated by conduction and externally drivenby granites, whereas in the east heat transfer was dominatedby advection and internally driven radiogenic heat production.The ultimate cause was along-orogen variation in crustal architecture,including thickness of the passive margin lithosphere and thicknessof the overlying sedimentary succession. KEY WORDS: Pan-African Orogeny; P–T paths; pseudosections; low-P metamorphism; contact metamorphism; Barrovian metamorphism     

Talc-, Magnesite- and Ti-Clinohumite-Bearing Ultrahigh-Pressure Meta-Mafic and Ultramafic Complex in the Dabie Mountains, China

The Bixiling mafic-ultramafic metamorphic complex is a 1•5km² tectonic block within biotite gneiss in the southern Dabieultrahigh-pressure terrane, central China. The complex consistsof banded eclogites that contain thin layers of garnet-bearingcumulate ultramafic rock. Except for common eclogitic phases(garnet, omphacite, kyanite, phengite, zoisite and rutilc),banded eclogites contain additional talc and abundant coesiteinclusions in omphacite, zoisite, kyanite and garnet. Some metaultramaficrocks contain magnesite and Ti-clinohumite. Both eclogites andmeta-ultramafic rocks have undergone multi-stage metamorphism.Eclogite facies metamorphisrn occurred at 610–700C andP>27 kbar, whereas amphibolite facies retrograde metamorphismis characterized by symplectites of plagioclase and hornblendeafter omphacite and replacement of tremolite after talc at P<6–15kbar and T <600C. The meta-ultramafic assemblages such asolivine + enstatite + diopside + garnet and Ti-clinohumite +diopside + enstatite + garnet + magnesite olivine formed at700–800C and 47–67 kbar. Investigation of the phaserelations for the system CaO-MgO-SiO₂-H₂O-CO₂ and the experimentallydetermined stabilities of talc, magnesite and Ti-clinohumiteindicate that (1) UHP talc assemblages are restricted to Mg-Algabbro composition and cannot be an important water-bearingphase in the ultramafic mantle, and (2) Ti-clinohumite and magnesiteare stable H₂O-bearing and CO₂-bearing phases at depths >100km. The mafic-ultramafic cumulates were initially emplaced atcrustal levels, then subducted to great depths during the Triassiccollision of the Sine-Korean and Yangtze cratons. KEY WORDS: eclogite; magnesite; meta-ultramafics; talc; ultrahigh-P metamorphism ^*Corresponding author     

Metamorphic Conditions and Fluid Compositions of Scapolite-Bearing Rocks from the Lapis Lazuli Deposit at Sare Sang, Afghanistan

Scapolite and other halogen-rich minerals (phlogopite, amphibole,apatite, titanite and clinohumite) occur in some high-pressureamphibolite facies calc-silicates and orthopyroxene-bearingrocks at Sare Sang (Sar e Sang or Sar-e-Sang), NE Afghanistan.The calc-silicates are subdivided into two groups: garnet-bearingand garnet-free, phlogopite-bearing. Besides garnet and/or phlogopite,the amphibolite facies mineral assemblages in the calc-silicatesinclude clinopyroxene, calcite, quartz and one or more of theminerals scapolite, plagioclase, K-feldspar, titanite, apatiteand rarely olivine. Orthopyroxene-bearing rocks consist of clinopyroxene,garnet, plagioclase, scapolite, amphibole, quartz, calcite andaccessory dolomite and alumosilicate (kyanite?). Retrogradephases in the rocks are plagioclase, scapolite, calcite, amphibole,sodalite, haüyne, lazurite, biotite, apatite and dolomite.The clinopyroxene is mostly diopside and rarely also hedenbergite.Aegirine and omphacite with a maximum jadeite content of 29mol % were also found. Garnet from the calc-silicates is Grs_45–95Py_0–2and from the orthopyroxene-bearing rocks is Grs_10–15Py_36–43.Peak P–T metamorphic conditions, calculated using availableexchange thermobarometers and the TWQ program, are 750°Cand 1·3–1·4 GPa. Depending on the rock type,the scapolite exhibits a wide range of composition (from Eq_An= 0·07, X_Cl =0·99 to Eq_An = 0·61, X_Cl =0·07).Equilibria calculated for scapolite and coexisting phases atpeak metamorphic conditions yield X_CO2 = 0·03–0·15.X_NaCl (fluid), obtained for scapolite, ranges between 0·04and 0·99. Partitioning of F and Cl between coexistingphases was calculated for apatite–biotite and amphibole–biotite.Fluorapatite is present in calc-silicates, but orthopyroxene-bearingrocks contain chlorapatite. Cl preferentially partitions intoamphibole with respect to biotite. All these rocks have sufferedvarious degrees of retrogression, which resulted in removalof halogens, CO₂ and S. Halogen- and S-bearing minerals formedduring retrogression and metasomatism are fluorapatite, sodalite,amphibole, scapolite, clinohumite, haüyne, pyrite, andlazurite, which either form veins or replace earlier formedphases. KEY WORDS: scapolite; fluid composition; high-pressure; amphibolite facies; Western Hindukush; Afghanistan     

Synthesis of Staurolite in Melting Experiments of a Natural Metapelite: Consequences for the Phase Relations in Low-Temperature Pelitic Migmatites

We document experiments on a natural metapelite in the range650–775°C, 6–14 kbar, 10 wt % of added water,and 700–850°C, 4–10 kbar, no added water. Staurolitesystematically formed in the fluid-present melting experimentsabove 675°C, but formed only sporadically in the fluid-absentmelting experiments. The analysis of textures, phase assemblages,and variation of phase composition and Fe–Mg partitioningwith P and T suggests that supersolidus staurolite formed at(near-) equilibrium during fluid-present melting reactions.The experimental results are used to work out the phase relationsin the system K₂O–Na₂O–FeO–MgO–Al₂O₃–SiO₂–H₂Oappropriate for initial melting of metapelites at the upperamphibolite facies. The P–T grid developed predicts theexistence of a stable P–T field for supersolidus staurolitethat should be encountered by aluminous Fe-rich metapelitesduring fluid-present melting at relatively low temperature andintermediate pressures (675–700°C, 6–10 kbarfor X_H2O = 1, in the KNFMASH system), but not during fluid-absentmelting. The implications of these findings for the scarcityof staurolite in migmatites are discussed. KEY WORDS: metapelites; migmatites; partial melting; P–T grid; staurolite     

Complex Growth Textures in a Polymetamorphic Metabasite from the Kraubath Massif (Eastern Alps)

Zoned garnet and amphibole occur in metabasites of the KraubathMassif, Eastern Alps, that contain relic magmatic clinopyroxene.The amphibole composition gradually changes from core (X_Mg =0·83) to rim (X_Mg = 0·6–0·7). A numberof compositional varieties of garnet occur in the metabasite.An older porphyroblastic garnet (Py_23–27, Alm_41–43,Grs_29–33) has two different compositional domains, onerelatively rich in Mg (Py_27–30) and the other rich inCa (Grs_35–38) with a low Mg (Py_20–25) content. Theyoungest variety, which forms rims on, or microveins in, theporphyroblastic garnet, has high Ca and low Mg (Grs_40–57,Py_2–7, Alm_46–51). The amphibole cores and garnetporphyroblasts are interpreted to represent minerals formedduring Variscan regional metamorphism under amphibolite-faciesconditions. Alpine metamorphism is represented by the most recentCa-rich and Mg-poor variety of garnet that coexists with theamphibole rims, epidote and chlorite. Fracturing in the porphyroblasticgarnet probably originated during retrogression of the Variscanamphibolite-facies assemblages. Textural relations suggest thatthe garnet in the microveins formed by dehydration of hydrousphases during an Alpine metamorphic overprint that reached P–Tconditions of 550–583°C at 1·0 GPa. KEY WORDS: microveins; garnet; metabasites; Kraubath Massif; Eastern Alps     

Phase Relations and Stability of Magnetoplumbite- and Crichtonite-Series Phases under Upper-Mantle P-T Conditions: an Experimental Study to 15 GPa with Implications for LILE Metasomatism in the Lithospheric Mantle

High-pressure–high-temperature experiments were performedin the range 7–15 GPa and 1300–1600°C to investigatethe stability and phase relations of the K- and Ba-dominantmembers of the crichtonite and magnetoplumbite series of phasesin simplified bulk compositions in the systems TiO₂–ZrO₂–Cr₂O₃–Fe₂O₃–BaO–K₂Oand TiO₂–Cr₂O₃–Fe₂O₃–BaO–K₂O. Both seriesof phases occur as inclusions in diamond and/or as constituentsof metasomatized peridotite mantle xenoliths sampled by kimberlitesor alkaline lamprophyres. They can accommodate large ion lithophileelements (LILE) and high field strength elements (HFSE) on awt % level and, hence, can critically influence the LILE andHFSE budget of a metasomatized peridotite even if present onlyin trace amounts. The Ba and K end-members of the crichtoniteseries, lindsleyite and mathiasite, are stable to 11 GPa and1500–1600°C. Between 11 and 12 GPa, lindsleyite breaksdown to form two Ba–Cr-titanates of unknown structurethat persist to at least 13 GPa. The high-pressure breakdownproduct of mathiasite is a K–Cr-titanate with an idealizedformula KM₇O₁₂, where M = Ti, Cr, Mg, Fe. This phase possessesspace group P6₃/m with a = 9·175(2) Å, c = 2·879(1)Å, V = 209·9(1) ų. Towards high temperatures,lindsleyite persists to 1600°C, whereas mathiasite breaksdown between 1500 and 1600°C to form a number of complexTi–Cr-oxides. Ba and K end-members of the magnetoplumbiteseries, hawthorneite and yimengite, are stable in runs at 7,10 and 15 GPa between 1300 and 1400°C coexisting with anumber of Ti–Cr-oxides. Molar mixtures (1:1) of lindsleyite–mathiasiteand hawthorneite–yimengite were studied at 7–10GPa and 1300–1400°C, and 9–15 GPa and 1150–1400°C,respectively. In the system lindsleyite–mathiasite, onehomogeneous Ba–K phase is stable, which shows a systematicincrease in the K/(K + Ba) ratio with increasing pressure. Inthe system hawthorneite–yimengite, two coexisting Ba–Kphases appear, which are Ba rich and Ba poor, respectively.The data obtained from this study suggest that Ba- and K-dominantmembers of the crichtonite and magnetoplumbite series of phasesare potentially stable not only throughout the entire subcontinentallithosphere but also under conditions of an average present-daymantle adiabat in the underlying asthenosphere to a depth ofup to 450 km. At still higher pressures, both K and Ba may remainstored in alkali titanates that would also be eminently suitablefor the transport of other ions with large ionic radii. KEY WORDS: crichtonite; magnetoplumbite; high-P–T experiments; phase relations; upper mantle     

Kimberlite-like Metasomatism and 'Garnet Signature' in Spinel-peridotite Xenoliths from Sal, Cape Verde Archipelago: Relics of a Subcontinental Mantle Domain within the Atlantic Oceanic Lithosphere?

A mantle xenolith suite from two Late Tertiary necks on SalIsland (Cape Verde Archipelago) consists of nearly equivalentamounts of anhydrous spinel-bearing lherzolites and harzburgites,in which secondary metasomatic textural domains are superimposedon the original protogranular textures. Detailed petrographicstudies, coupled with in situ major and trace element analysesof the constituent minerals and interstitial glasses, revealthe complex evolutionary history of the Cape Verde lithosphericmantle, from depletion in the garnet facies to re-equilibrationand re-enrichment in the spinel stability field. Low CaO (16·4–18·0wt %) and heavy rare earth element (HREE; Yb_n = 2·4–4·8),and high Cr₂O₃ (1·06–1·84 wt %) contentsin the clinopyroxenes of the lherzolites can be quantitativelyaccounted for by (1) low-degree (4%) partial melting of a PrimitiveMantle-like garnet lherzolite followed by (2) partial re-equilibrationof the melting residuum from the garnet to the spinel stabilityfield. This model is further supported by thermobarometric estimates(T = 975–1210°C; P = 1·3–2·1 GPa),which cluster around the spinel–garnet boundary in theperidotite system. Secondary parageneses, regardless of theprimary lithologies, are characterized by (1) two clinopyroxenes,cpx2-O and cpx2-C, respectively related to orthopyroxene andclinopyroxene destabilization after reaction with metasomaticfluids, and (2) glasses with anomalously high, even for continentalsettings, K₂O contents (up to 8·78 wt %), together withK-feldspar. Major and trace element mass balance calculationsbetween the primary and secondary parageneses suggest infiltrationof a kimberlite-like metasomatizing agent (on volatile-freebasis, MgO 17–27 wt %; K₂O/Na₂O 1·6–3·2molar; (K₂O + Na₂O)/Al₂O₃ 1·1–3·0 molar;Rb 91–165 ppm; Zr 194–238 ppm). The kimberlite-likemetasomatism in the Cape Verde lithospheric mantle, togetherwith the presence of lherzolitic domains, partially re-equilibratedfrom the garnet to the spinel stability field, may suggest thepresence of subcontinental mantle lithosphere relicts left behindby drifting of the African Plate during the opening of the CentralAtlantic Ocean. KEY WORDS: Cape Verde; mantle metasomatism; garnet signatures; clinopyroxenes; kimberlites     

The Substitution of Al and F in Titanite at High Pressure and Temperature: Experimental Constraints on Phase Relations and Solid Solution Properties

Experimental studies were carried out to evaluate phase relationsinvolving titanite–F–Al-titanite solid solutionin the system CaSiO₃–Al₂SiO₅–TiO₂–CaF₂. Theexperiments were conducted at 900–1000°C and 1·1–4·0GPa. The average F/Al ratio in titanite solid solution in theexperimental run products is 1·01 ± 0·06,and X_Al ranges from 0·33 ± 0·02 to 0·91± 0·05, consistent with the substitution [TiO²⁺]_–1[AlF²⁺]₁.Analysis of the phase relations indicates that titanite solidsolutions coexisting with rutile are always low in X_Al, whereasthe maximum X_Al of titanite solid solution occurs with fluoriteand either anorthite or Al₂SiO₅. Reaction displacement experimentswere performed by adding fluorite to the assemblage anorthite+ rutile = titanite + kyanite. The reaction shifts from 1·60GPa to 1·15 ± 0·05 GPa at 900°C, from1·79 GPa to 1·375 ± 0·025 GPa at1000°C, and from 1·98 GPa to 1·575 ±0·025 GPa at 1100°C. The data show that the activityof CaTiSiO₄O is very close to the ideal molecular activity model(X_Ti) at 1100°C, but shows a negative deviation at 1000°Cand 900°C. The results constrain     

Mesoproterozoic Reworking of Palaeoproterozoic Ultrahigh-temperature Granulites in the Central Indian Tectonic Zone and its Implications

In the southern periphery of the Sausar Mobile Belt (SMB), thesouthern component of the Central Indian Tectonic Zone (CITZ),a suite of felsic and aluminous granulites, intruded by gabbro,noritic gabbro, norite and orthopyroxenite, records the polymetamorphicevolution of the CITZ. Using sequences of prograde, peak andretrograde reaction textures, mineral chemistry, geothermobarometricresults and petrogenetic grid considerations from the felsicand the aluminous granulites and applying metamorphosed maficdyke markers and geochronological constraints, two temporallyunrelated granulite-facies tectonothermal events of Pre-Grenvillianage have been established. The first event caused ultrahigh-temperature(UHT) metamorphism (M₁) (T 950°C) at relatively deepercrustal levels (P 9 kbar) and a subsequent post-peak near-isobariccooling P–T history (M₂). M₁ caused pervasive biotite-dehydrationmelting, producing garnet–orthopyroxene and garnet–rutileand sapphirine–spinel-bearing incongruent solid assemblagesin felsic and aluminous granulites, respectively. During M₂,garnet–corundum and later spinel–sillimanite–biotiteassemblages were produced by reacting sapphirine–spinel–sillimaniteand rehydration of garnet–corundum assemblages, respectively.Applying electron microprobe (EMP) dating techniques to monazitesincluded in M₁ garnet or occurring in low-strain domains inthe felsic granulites, the UHT metamorphism is dated at 2040–2090Ma. Based on the deep crustal heating–cooling P–Ttrajectory, the authors infer an overall counterclockwise P–Tpath for this UHT event. During the second granulite event,the Palaeoproterozoic granulites experienced crustal attenuationto 6·4 kbar at T 675°C during M₃ and subsequentnear-isothermal loading to 8 kbar during M₄. In the felsic granulites,the former is marked by decomposition of M₁ garnet to orthopyroxene–plagioclasesymplectites. During M₄, there was renewed growth of garnet–quartzsymplectites in the felsic granulites, replacing the M₃ mineralassemblage and also the appearance of coronal garnet–quartz–clinopyroxeneassemblages in metamorphosed mafic dykes. Using monazites frommetamorphic overgrowths and metamorphic recrystallization domainsfrom the felsic granulite, the M₄ metamorphism is dated at 1525–1450Ma. Using geochronological and metamorphic constraints, theauthors interpret the M₃–M₄ stages to be part of the sameMesoproterozoic tectonothermal event. The result provides thefirst documentation of UHT metamorphism and Palaeo- and Mesoproterozoicmetamorphic processes in the CITZ. On a broader scale, the findingsare also consistent with the current prediction that isobaricallycooled granulites require a separate orogeny for their exhumation. KEY WORDS: Central Indian Tectonic Zone; UHT metamorphism; counterclockwise P–T path; monazite chemical dating     

Processes and Conditions During Contact Anatexis, Melt Escape and Restite Formation: the Huntly Gabbro Complex, NE Scotland

The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic,mid-crustal, layered, mafic intrusions, emplaced into Proterozoicmetaclastic rocks of NE Scotland soon after the thermal peakof static, high-T, low-P regional metamorphism. This gabbroand its associated contact metamorphic rocks illustrate a varietyof processes operating during contact anatexis and subsequentmelt segregation and extraction. These processes may closelymirror those occurring at much larger scales in the deep crustduring high-grade regional metamorphism and the generation ofgranitic magmas. The emplacement of the Huntly mafic magma resultedin high-grade contact metamorphism and, locally, anatexis ofmetapelites, leading to the formation of migmatites. The migmatitesand country-rock schists were studied to establish the physicalconditions of metamorphism and anatexis, the nature of the meltingreactions, the compositions of the melts produced, and the extentto which melting was a closed- or open-system process. The country-rockschists immediately to the south of the Huntly Complex containmineral assemblages characteristic of the regional andalusitezone. Thermobarometry of an andalusite schist yields regionalmetamorphic conditions of 537 ± 42°C and 0·27± 0·12 GPa, consistent with previously publishedP–T estimates. The contact metamorphic rocks include sillimanitehornfelses, metatexites and diatexites. The metatexites consistof cordierite–K-feldspar hornfels melanosomes and K-feldspar-richgarnetiferous leucosomes. The diatexites consist of schollenof fine-grained granoblastic hornfels and metatexite suspendedin igneous-textured matrix rocks composed of abundant sub/euhedralgarnet, cordierite, plagioclase and, locally, orthopyroxene,with minor interstitial biotite, K-feldspar and quartz. Thehornfels melanosomes and schollen retained their structuralintegrity during partial melting, but the matrix rocks did not.In the highest-grade diatexites, the assemblage Grt + Opx +Crd + Hc + Pl characterizes both the hornfels schollen and thesub/euhedral minerals of the matrix rocks. Application of phaseequilibria to Opx-bearing rocks yields estimated peak-metamorphicconditions of 900 ± 50°C, 0·45 ± 0·1GPa and a_H2O < 0·3. The pressure estimate impliesan emplacement depth of     

Calculated Phase Relations in High-Pressure Metapelites in the System NKFMASH (Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O)

Using an internally consistent thermodynamic dataset and updatedmodels of activity–composition relation for solid solutions,petrogenetic grids in the system NKFMASH (Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O)and the subsystems NKMASH and NKFASH have been calculated withthe software THERMOCALC 3.1 in the P–T range 5–36kbar and 400–810°C, involving garnet, chloritoid,biotite, carpholite, talc, chlorite, kyanite/sillimanite, staurolite,phengite, paragonite, albite, glaucophane, jadeite, with quartz/coesiteand H₂O in excess. These grids, together with calculated AFMcompatibility diagrams and P–T pseudosections, are shownto be powerful tools for delineating the phase equilibria andP–T conditions of Na-bearing pelitic assemblages for avariety of bulk compositions from high-P terranes around theworld. These calculated equilibria are in good agreement withpetrological studies. Moreover, contours of the calculated phengiteSi isopleths in P–T pseudosections for different bulkcompositions confirm that phengite barometry is highly dependenton mineral assemblage. KEY WORDS: phase relations; HP metapelite; NKFMASH; THERMOCALC; phengite geobarometry     

Calculated Phase Relations in the System NCKFMASH (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O) for High-Pressure Metapelites

Petrogenetic grids in the system NCKFMASH (Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O)and the subsystems NCKMASH and NCKFASH calculated with the softwareTHERMOCALC 3.1 are presented for the P–T range 7–30kbar and 450–680°C, for assemblages involving garnet,chloritoid, biotite, carpholite, talc, chlorite, kyanite, staurolite,paragonite, glaucophane, jadeite, omphacite, diopsidic pyroxene,plagioclase, zoisite and lawsonite, with phengite, quartz/coesiteand H₂O in excess. These grids, together with calculated compatibilitydiagrams and P–T and T–X_Ca and P–X_Ca pseudosectionsfor different bulk-rock compositions, show that incorporationof Ca into the NKFMASH system leads to many of the NKFMASH invariantequilibria moving to lower pressure and/or lower temperature,which results, in most cases, in the stability of jadeite andgarnet being enlarged, but in the reduction of stability ofglaucophane, plagioclase and AFM phases. The effect of Ca onthe stability of paragonite is dependent on mineral assemblageat different P–T conditions. The calculated NCKFMASH diagramsare powerful in delineating the phase equilibria and P–Tconditions of natural pelitic assemblages. Moreover, contoursof the calculated phengite Si isopleths in P–T and P–X_Capseudosections confirm that phengite barometry in NCKFMASH isstrongly dependent on mineral assemblage. KEY WORDS: phase relations; metapelites; NCKFMASH; THERMOCALC; phengite geobarometry     

Sediment Melts at Sub-arc Depths: an Experimental Study

The phase and melting relations in subducted pelites have beeninvestigated experimentally at conditions relevant for slabsat sub-arc depths (T = 600–1050°C, P = 2·5–4·5GPa). The fluid-present experiments produced a dominant paragenesisconsisting of garnet–phengite–clinopyroxene–coesite–kyanitethat coexists with a fluid phase at run conditions. Garnet containsdetectable amounts of Na₂O (up to 0·5 wt%), P₂O₅ (upto 0·56 wt%), and TiO₂ (up to 0·9 wt%) in allexperiments. Phengite is stable up to 1000°C at 4·5GPa and is characterized by high TiO₂ contents of up to 2 wt%.The solidus has been determined at 700°C, 2·5 GPaand is situated between 700 and 750°C at 3·5 GPa.At 800°C, 4·5 GPa glass was present in the experiments,indicating that at such conditions a hydrous melt is stable.In contrast, at 700°C, 3·5 and 4·5 GPa, asolute-rich, non-quenchable aqueous fluid was present. Thisindicates that the solidus is steeply sloping in P–T space.Fluid-present (vapour undersaturated) partial melting of thepelites occurs according to a generalized reaction phengite+ omphacite + coesite + fluid = melt + garnet. The H₂O contentof the produced melt decreases with increasing temperature.The K₂O content of the melt is buffered by phengite and increaseswith increasing temperature from 2·5 to 10 wt%, whereasNa₂O decreases from 7 to 2·3 wt%. Hence, the melt compositionschange from trondhjemitic to granitic with increasing temperature.The K₂O/H₂O increases strongly as a function of temperatureand nature of the fluid phase. It is 0·0004–0·002in the aqueous fluid, and then increases gradually from about0·1 at 750–800°C to about 1 at 1000°C inthe hydrous melt. This provides evidence that hydrous meltsare needed for efficient extraction of K and other large ionlithophile elements from subducted sediments. Primitive subduction-relatedmagmas typically have K₂O/H₂O of 0·1–0·4,indicating that hydrous melts rather than aqueous fluids areresponsible for large ion lithophile element transfer in subductionzones and that top-slab temperatures at sub-arc depths are likelyto be 700–900°C. KEY WORDS: experimental petrology; pelite; subduction; UHP metamorphism; fluid; LILE     

The origin and P–T evolution of peridotites and serpentinites of NE New Caledonia: prograde interaction between continental margin and the mantle wedge

The Pouébo and Diahot terranes of NE New Caledonia mostly comprise eclogite to blueschist facies metabasite and metasedimentary rocks that experienced c. 40 Ma metamorphism. This Eocene high‐P event has been linked with the SW‐directed obduction of the New Caledonian Ophiolite, an extensive ultramafic nappe that dominates outcrop in the south of the island. In the north, ultramafic lithologies are found only as sheets or discrete lenticular masses interleaved with, but separated from, the eclogites and blueschists by foliated talc–chlorite–serpentine–carbonate‐bearing rocks. The base of the largest and best‐preserved ultramafic body at Yambé is marked by a distinctive (2 m thick) layer of high‐P mylonite that preserves evidence for early blueschist facies conditions (S1) as inclusions in eclogite facies minerals. Textural evidence preserved in olivine‐bearing serpentinites and their bounding mafic mylonites suggest that the ultramafic bodies were emplaced within the structurally highest levels of the high‐P terrane as serpentinite tectonites sourced from hydrated mantle, formerly in the hangingwall of the Eocene subduction zone. Serpentinite emplacement accompanied burial of the NE New Caledonian margin at T<500 °C and P<16 kbar. The ultramafic fragments were buried to depths of 50–60 km in the subduction zone, where olivine was stable and coarse‐grained garnet–omphacite‐rich assemblages developed in low strain domains within enclosing mylonites. Host metabasic and metasedimentary rocks from the structurally highest portions of the high‐P belt have a prograde record identical to that of the ultramafic tectonites. The early emplacement and similar P–T history of host rocks and ultramafic masses suggest that NE New Caledonia preserves a fossil slab/mantle–wedge boundary reactivated during exhumation.     

Mineralogy, Textures and P-T Relationships of a Suite of Xenoliths from the Monaro Volcanic Province, New South Wales, Australia

Intraplate basalts of the Eocene–Oligocene Monaro VolcanicProvince (MVP), in southeastern New South Wales, include lower-crustaland refractory to weakly metasomatized upper-mantle xenoliths.Lower-crustal-derived xenoliths appear to be all two-pyroxeneplagioclase granulites (Cpx_{Fe:Mg:Ca 0·17–0·56:0·63–0·77:0·28–0·89}Opx_{Fe:Mg:Ca 0·39–0·52:1·37–1·47:0·02}An_72–86 and An_48–50) but may also include garnetpyroxenites at depth. Mantle-derived xenoliths are principallyspinel-bearing lherzolites (Fo_{89·8–90·6}Cpx_{Fe:Mg:Ca 0·07–0·45:0·70–1·70:0·01–0·94}Opx_{Fe:Mg:Ca 0·16–0·19:1·62–1·75:0·01–0·10})but also include amphibole ± spinel-bearing lherzolite(Fo_{88·7–89·1} Cpx_{Fe:Mg:Ca 0·09–0·21:0·61–0·91:0·73–0·93}Opx_{Fe:Mg:Ca 0·09–0·31:0·70–1·54:0·03–0·91}),spinel-bearing harzburgite (Fo_{90·5–90·7}Cpx_{Fe:Mg:Ca 0·08:0·91–0·93:0·74–0·84}Opx_{Fe:Mg:Ca 0·16–0·18:1·73–1·79:0·00–0·02}),wehrlite, pyroxenite (Cpx_{Fe:Mg:Ca 0·08–0·10:0·84–0·90:0·80–0·85}Opx_{Fe:Mg:Ca 0·16–0·33:1·51–1·73:0·02–0·03})and rare garnet pyroxenite (Gt_{Fe:Mg:Ca 0·83–0·95:1·60–1·70:0·45–0·48}Cpx_{Fe:Mg:Ca 0·14–0·21:0·69–0·77:0·78–0·86}Opx _{Fe:Mg:Ca 0·31–0·42:1·43–1·56:0·02–0·03})and amphibole–apatite composites. Xenolith textures aregenerally weakly to moderately foliated, a few are mosaic-porphyroblasticand rare samples are veined or highly strained. MVP xenolithsappear to have equilibrated under similar pressure–temperature(P–T) conditions to other southeastern Australian xenolithsequivalent to the South Eastern Australia (SEA) palaeogeotherm.P–T estimates for the MVP suite of xenoliths reveal aheterogeneous lower crust and upper mantle that is thickly underplatedto c. 1·8 GPa or c. 50 km depth. MVP xenolith P–Tdata are compared with those used to derive the SEA palaeogeotherm,which is shown to be in need of revision using more modern geothermometersand geobarometers and new xenolith coexisting mineral data. KEY WORDS: xenolith; petrography; texture; geotherm; Monaro; eastern Australia     

Continuous Gradations among Primary Carbonatitic, Kimberlitic, Melilititic, Basaltic, Picritic, and Komatiitic Melts in Equilibrium with Garnet Lherzolite at 3-8 GPa

Multianvil melting experiments in the system CaO–MgO–Al₂O₃–SiO₂–CO₂(CMAS–CO₂) at 3–8 GPa, 1340–1800°C, involvingthe garnet lherzolite phase assemblage in equilibrium with CO₂-bearingmelts, yield continuous gradations in melt composition betweencarbonatite, kimberlite, melilitite, komatiite, picrite, andbasalt melts. The phase relations encompass a divariant surfacein P–T space. Comparison of the carbonatitic melts producedat the low-temperature side of this surface with naturally occurringcarbonatites indicates that natural magnesiocarbonatites couldbe generated over a wide range of pressures >2·5 GPa.Melts analogous to kimberlites form at higher temperatures alongthe divariant surface, which suggests that kimberlite genesisrequires more elevated geotherms. However, the amount of waterfound in some kimberlites has the potential to lower temperaturesfor the generation of kimberlitic melts by up to 150°C,provided no hydrous phases are present. Compositions resemblinggroup IB and IA kimberlites are produced at pressures around5–6 GPa and 10 GPa, respectively, whereas the compositionsof some other kimberlites suggest generation at higher pressuresstill. At pressures <4 GPa, an elevated geotherm producesmelilitite-like melt in the CMAS–CO₂ system rather thankimberlite. Even when a relatively CO₂-rich mantle compositioncontaining 0·15 wt % CO₂ is assumed, kimberlites andmelilitites are produced by <1% melting and carbonatitesare generated by even smaller degrees of melting of <0·5%. KEY WORDS: carbonatite; CO₂; kimberlite; melilitite; melt generation     

Garnetiferous Metabasites from the Sausar Mobile Belt: Petrology, P-T Path and Implications for the Tectonothermal Evolution of the Central Indian Tectonic Zone

A suite of garnetiferous amphibolites and mafic granulites occuras small boudins within layered felsic migmatite gneiss in thenorthern part of the Sausar Mobile Belt (SMB), the latter constitutingthe southern component of the Proterozoic Central Indian TectonicZone (CITZ). Although the two types of metabasites are in variousstages of retrogression, textural, compositional and phase equilibriastudies attest to four distinct metamorphic episodes. The earlyprograde stage (M_o) is represented by an inclusion assemblageof hornblende₁ + ilmenite₁ + plagioclase₁ ± quartz andgrowth zoning preserved in garnet. The peak assemblage (M₁)consists of porphyroblastic garnet + clinopyroxene ±quartz ± rutile ± hornblende in mafic granulitesand garnet + quartz + hornblende in amphibolites and stabilizedat pressure–temperature conditions of 9–10 kbarand 750–800°C and 8 kbar and 675°C, respectively.This was followed by near-isothermal decompression (M₂), andpost-decompression cooling (M₃) events. In mafic granulites,the former resulted in the development of early clinopyroxene_2A–hornblende_2A–plagioclase_2Asymplectites at 8 kbar and 775°C (M_2A stage), synchronouswith D₂ and later anhydrous clinopyroxene_2B–plagioclase_2B–ilmenite_2Bsymplectites and coronal assemblages at 7 kbar, 750°C (M_2Bstage) and post-dating D₂. In amphibolites, ilmenite + plagioclase+ quartz ± hornblende symplectites appeared during M₂at 6·4 kbar and 700°C. During M₃, coronal garnet+ clinopyroxene + quartz ± hornblende-bearing symplectitesin metabasic dykes and hornblende₃–plagioclase₃ symplectitesembaying garnet in mafic granulites were formed. P–T estimatesshow near-isobaric cooling from 7 kbar and 750°C to 6 kbarand 650°C during M₃. It is argued that the decompressionin the mafic granulites is not continuous, being punctuatedby a distinct heating (prograde?) event. The latter is alsocoincident with a period of extension, marked by mafic dykeemplacement. The combined P–T path of evolution has aclockwise sense and provides evidence for a major phase of earlycontinental subduction in parts of the CITZ. This was followedby a later continent–continent collision event duringwhich granulites of the first phase became tectonically interleavedwith younger lithological units. This tectonothermal event,of possibly Grenvillian age, marks the final amalgamation ofthe North and the South Indian Blocks along the CITZ to producethe Indian subcontinent. KEY WORDS: Central Indian Tectonic Zone; clockwise P–T path; continental collision; metabasite     

The Effect of Reaction Overstep on Garnet Microtextures in Metapelitic Rocks of the Ilesha Schist Belt, SW Nigeria

Garnet-bearing assemblages of K-rich and K-poor metapelitesfrom the Ilesha Schist belt, SW Nigeria, are investigated. K-richsamples contain the assemblages (A) garnet–staurolite–muscovite–chlorite–magnetite,(B) andalusite–garnet–staurolite–muscovite–chlorite–magnetiteand (C) sillimanite–andalusite–garnet–muscovite–chlorite–magnetite.K-poor samples contain the assemblages (D) garnet–staurolite–cordierite–chloriteand (E) garnet–cordierite–chlorite ± staurolite.All assemblages contain quartz, plagioclase, biotite and ilmenite.P–T pseudosections calculated in the system CaO–Na₂O–K₂O–TiO₂–MnO–FeO–MgO–Al₂O₃–SiO₂ –H₂O ± O₂ suggest peak metamorphismat 590 ± 20°C at 5 ± 0·5 kbar, followedby retrogression to 550°C at 3·0 kbar, in agreementwith field evidence, domain assemblages, mineral compositions,modes and geothermobarometry. The absence of compositional zonationshows that garnet in all investigated rocks nucleated and grewat constant P–T–X in equilibrium with associatedminerals on the thin-section scale. However, the garnet-in reactiondid not begin until the establishment of a significant temperatureoverstep of