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     

Disequilibrium in the Ross of Mull Contact Metamorphic Aureole, Scotland: a Consequence of Polymetamorphism

The Ross of Mull pluton consists of granites and granodioritesand intrudes sediments previously metamorphosed at amphibolitefacies. The high grade and coarse grain size of the protolithis responsible for a high degree of disequilibrium in many partsof the aureole and for some unusual textures. A band of metapelitecontained coarse garnet, biotite and kyanite prior to intrusion,and developed a sequence of textures towards the pluton. InZone I, garnet is rimmed by cordierite and new biotite. In ZoneII, coarse kyanite grains are partly replaced by andalusite,indicating incomplete reaction. Coronas of cordierite + muscovitearound kyanite are due to reaction with biotite. In the higher-gradeparts of this zone there is complete replacement of kyaniteand/or andalusite by muscovite and cordierite. Cordierite chemistryindicates that in Zone II the stable AFM assemblage (not attained)would have been cordierite + biotite + muscovite, without andalusite.The observed andalusite is therefore metastable. Garnet is unstablein Zone II, with regional garnets breaking down to cordierite,new biotite and plagioclase. In Zone III this breakdown is welladvanced, and this zone marks the appearance of fibrolite andK-feldspar in the groundmass as a result of muscovite breakdown.Zone IV shows garnet with cordierite, biotite, sillimanite,K-feldspar and quartz. Some garnets are armoured by cordieriteand are inferred to be relics. Others are euhedral with Mn-richcores. For these, the reaction biotite + sillimanite + quartz garnet + cordierite + K-feldspar + melt is inferred. Usinga petrogenetic grid based on the work of Pattison and Harte,pressure is estimated at 3·2 kbar, and temperature atthe Zone II–III boundary at 650°C and in Zone IV asat least 750°C. KEY WORDS: contact metamorphism; disequilibrium     

Metamorphic Evolution of Iron-rich Mafic Cumulates from the Otztal-Stubai Crystalline Complex, Eastern Alps, Austria

Olivine-rich rocks containing olivine + orthopyroxene + spinel+ Ca-amphibole ± clinopyroxene ± garnet are presentin the central Ötztal–Stubai crystalline basementassociated with eclogites of tholeiitic affinity. These rockscontain centimetre-sized garnet layers and lenses with garnet+ clinopyroxene ± corundum. Protoliths of the olivine-richrocks are thought to be olivine + orthopyroxene + spinel dominatedcumulates generated from an already differentiated Fe-rich () tholeiitic magma that was emplaced into shallowcontinental crust. Protoliths of the garnet-rich rocks are interpretedas layers enriched in plagioclase and spinel intercalated ina cumulate rock sequence that is devoid of, or poor in, plagioclase.U–Pb sensitive high-resolution ion microprobe dating ofzircons from a garnet layer indicates that emplacement of thecumulates took place no later than 517 ± 7 Myr ago. Aftertheir emplacement, the cumulates were subjected to progressivemetamorphism, reaching eclogite-facies conditions around 800°Cand >2 GPa during a Variscan metamorphic event between 350and 360 Ma. Progressive high-P metamorphism induced breakdownof spinel to form garnet in the olivine-rich rocks and of plagioclase+ spinel to form garnet + clinopyroxene ± corundum inthe garnet layers. Retrogressive metamorphism at T 650–680°Cled to the formation of Ca-amphibole, chlorite and talc in theolivine-rich rocks. In the garnet layers, högbomite formedfrom corundum + spinel along with Al-rich spinel, Ca-amphibole,chlorite, aspidolite–preiswerkite, magnetite, ilmeniteand apatite at the interface between olivine-rich rocks andgarnet layers at P < 0·8 GPa. Progressive desiccationof retrogade fluids through crystallization of hydrous phasesled to a local formation of saline brines in the garnet layers.The presence of these brines resulted in a late-stage formationof Fe- and K-rich Ca-amphibole and Sr-rich apatite, both characterizedby extremely high Cl contents of up to 3·5 and 6·5wt % Cl, respectively. KEY WORDS: cumulates; Variscan metamorphism; SHRIMP dating; högbomite; saline brines     

Metamorphic and post-metamorphic fluid flow in the low-grade rocks of the Harlech Dome, north Wales

A combination of fluid inclusion, stable isotope and geochemical techniques has been used to study the nature of fluids present and their behaviour during Caledonian low-grade metamorphism of the Harlech Dome, north Wales. Fluid inclusion studies show that in most of the metasedimentary sequence the peak metamorphic fluid was an aqueous Na–K–Cl brine but in the graphitic Clogau Formation and in parts of the overlying Maentwrog Formation immiscible H₂O-rich and CH₄-rich fluids coexisted. Late-stage inclusions are of calcium-rich brine and a dilute aqueous fluid. The chemical composition of chlorite in metamorphic veins and rocks varies between different formations and quartz-oxygen isotopic compositions show considerable variation between different units. Both of these features are taken to indicate that there was little or no pervasive movement of fluid between different units at the peak of metamorphism. After the metamorphic peak there was focused flow of fluid upward through the sequence along fractures, in response to end-Caledonian uplift and unloading. Where the migrating fluid crossed the graphitic shales, interaction between the fluid and the shales gave rise to the formation of the auriferous veins of the Dolgellau Gold Belt. Subsequent to this mineralizing event there was widespread development of ¹⁸O-enriched calcites and micas. In the case of vein minerals it is possible that these crystallized directly from late-stage fluids at lower temperature than the quartz in the same veins. Alternatively, the original vein minerals may have re-equilibrated with later ¹⁸O-enriched or cooler fluid. In the case of muscovites in the rock matrix it is proposed that the isotopically heavy compositions are the result of re-equilibration of initially light grains with an introduced fluid, requiring considerable influx of fluid. This event may relate to either of two late-stage fluids observed as inclusions.     

Metamorphic and Thermal History of a Fore-Arc Basin: the Fossil Bluff Group, Alexander Island, Antarctica

The Himalia Ridge Formation (Fossil Bluff Group), AlexanderIsland is a 2·2-km-thick sequence of Upper Jurassic–LowerCretaceous conglomerates, sandstones and mudstones, derivedfrom an andesitic volcanic arc and deposited in a fore-arc basin.The metamorphic and thermal history of the formation has beendetermined using authigenic mineral assemblages and vitrinitereflectance measurements. Metamorphic effects include compaction,pore-space reduction, cementation and dissolution and replacementof detrital grains by clay minerals (smectite, illite/smectite,corrensite and kaolinite), calcite, chlorite, laumontite, prehnite,pumpellyite, albite and mica, with less common quartz, haematite,pyrite and epidote. The authigenic mineral assemblages exhibita depth-dependence, and laumontite and calcite exhibit a strongantipathetic relationship. Detrital organic matter in the argillaceouslayers has vitrinite reflectance values (R_o) ranging from 2·3to 3·7%. This indicates considerable thermal maturation,with a systematic increase in reflectivity with increasing depth.There is good correlation of metamorphic mineral assemblageswith chlorite crystallinity and vitrinite reflectance values—allindicating temperatures in the range of 140 ± 20°Cat the top of the sequence to 250 ± 10°C at the baseof the sequence. The temperatures suggest a geothermal gradientof 36–64°C/km and a most likely gradient of 50°C/km.It is suggested that this higher-than-average gradient for afore-arc basin resulted either from rifting during basin formationor from a late-stage arc migration event. KEY WORDS: Antarctica; diagenesis; fore-arc basin; low-temperature metamorphism; vitrinite reflectance     

Petrogenesis of Ultramafic Rocks from the Ultrahigh-pressure Metamorphic Kimi Complex in Eastern Rhodope (NE Greece)

Widespread bodies of garnet–spinel metaperidotites withpyroxenitic layers occur in the ultrahigh-pressure metamorphicKimi Complex. In this study we address the origin of such peridotite–pyroxeniteassociations in the context of polybaric melting regimes. Weconduct a detailed geochemical investigation of major and traceelement relations and compare them with a range of major elementmodelling scenarios. With increasing bulk-rock MgO content,the garnet–spinel metaperidotites exhibit decreasing CaO,Al₂O₃, TiO₂, and Na₂O along with increasing Ni and a graduallyincreasing Zr/Zr* anomaly, consistent with an origin as residuesafter variable degrees of melt extraction. The major elementmodelling further suggests a polybaric adiabatic decompressionmelting regime beginning at high to ultrahigh pressure, withan intermediate character between pure batch and fractionalmelting and a mean extent of melting of 9–11%. The pyroxenitesexhibit major element compositions that cannot be reproducedby experimental or calculated melts of peridotite. Moreover,the Kimi pyroxenites have highly variable Ni and Sc contentsand a wide range of Mg-number (0· 76–0·89), inconsistent with an origin as frozen melts or the productsof melt–peridotite interaction. However, both the majorelement systematics and the observed rare earth element patterns,with both convex and concave shapes, can be explained by anorigin as clinopyroxene-rich, high-pressure cumulates involvinggarnet and/or Cr-spinel. KEY WORDS: peridotite; pyroxenite; partial melting; UHP metamorphism; cumulate     

Fluid Transfer in High-grade Metamorphic Terrains Intruded by Anorogenic Granites: The Thor Range, Antarctica

A composite intrusive igneous complex in the central mountainrange of Queen Maud Land (Thor Range), Antarctica, displayscharacteristic features of anorogenic granites. A suite of massiveintrusives and various sets of dykes and satellite intrusionsare ferroan, alkalic to alkali–calcic, and weakly peraluminous.An early set of plutons consists of charnockitic alkali-granites;a later group of plutons comprises fayalite Qtz-syenites. Coarsemesoperthite is the dominant mineral in all rocks, quartz isabundant and plagioclase is a minor mineral. Olivine (fayalite)is the characteristic mafic mineral, but subcalcic augite andoccasionally pigeonite or orthopyroxene are present. In mostsamples, amphibole is the dominant mafic mineral and its compositionis close to end-member hastingsite. It contains high concentrationsof F and Cl. Some samples contain igneous fluorite. Thermobarometrysuggests a temperature of 900 ± 25°C and a pressureof 0·4 ± 0·1 GPa for the crystallizationconditions of the pyroxene–olivine assemblages. The solidustemperature of 800–850°C for both suites of plutonicrocks is typical of water-deficient granitic melts. The estimatedlow water activity of 0·3–0·5 at solidusconditions is consistent with the high halogen content of thebulk-rocks and their constituent minerals. In the absence ofan aqueous fluid, the halogens remained in the minerals at thesolidus. Oxygen fugacity stayed below QFM in all igneous rocksabove solidus. This is typical of melts derived from partialmelting of mafic source rocks. The igneous rocks were locallyaffected by at least three distinct episodes of hydration. Asthe melt approached solidus conditions, fayalite and pyroxenewere locally transformed into hastingsite as a result of increasingfugacity of volatile components. Fayalite-free and fayalite-bearingigneous rocks are arranged in banded structures. Subsolidushydration locally modified the igneous rocks and transformedpyroxene- and fayalite-bearing granites into biotite-granitesand hornblende-granites in which all evidence of former high-Thistory was erased. This local hydration of igneous rocks occurredin response to uptake of H₂O that had been given off by gneissicxenoliths as a result of progressing, continuous, dehydrationreactions. The reactions in the gneiss xenoliths were drivenby contact metamorphism. This exchange of H₂O between igneousand metamorphic rocks occurred in a fluid-absent regime at temperaturesof about 750°C. Late reaction veins formed by hydraulicfracturing of the plutonic rocks and indicate the presence ofa low-density fluid phase at amphibolite facies conditions. KEY WORDS: anorogenic granite; fayalite; hastingsite; fluid recycling; Antarctica     

Correlations between U, Th Content and Metamorphic Grade in the Western Namaqualand Belt, South Africa, with Implications for Radioactive Heating of the Crust

The digital image of airborne radiometric data across SouthAfrica reveals that the largest anomaly, 100 nGy/h, is causedby the granulite-facies rocks of the Namaquan metamorphic complex,whereas most of the country is <60 nGy/h. This observationis consistent with geochemical data that show that the 1900± 100 Ma greenschist-facies Richtersveld Terrane nearNamibia (max. U = 3·4 ppm; Th = 20·1 ppm) andthe adjacent, 1100 ± 100 Ma, amphibolite-facies Aggeneys/SteinkopfTerranes (max. U 10 ppm; Th 52 ppm) are the least enrichedin U, Th and K. In contrast, the lower-T granulite-facies OkiepTerrane near Springbok hosts more enriched granites (max. U 17 ppm; Th 66 ppm) and noritic intrusions (max. U = 14 ppm;Th = 83 ppm). The most enriched rocks are found in the 1030Ma higher-T granulite-facies core of the Namaquan belt and includequartzo-feldspathic gneisses (max. U = 46 ppm; Th = 90 ppm)and charnockites (max. U = 52 ppm; Th = 400 ppm). Our findingscontradict the notion that granulite-facies terrains are characteristicallydepleted in U and Th. In this study we modeled the heat productionin the core of the Namaquan complex, where the granulites havehad a very unusual metamorphic history, and show that ultra-high-T(1000°C, P 10 kbar) metamorphic conditions could have beenachieved by radiogenic heating without invoking external heatsources. However, monazite-rich veins of charnockite and patchesof granulites mark the passage of CO₂-dominated melts and fluidsderived from fractionated noritic intrusions. KEY WORDS: charnockite; granulite; Namaqualand; thorium; uranium; radioactive heating; metamorphism     

Calculation of Garnet Fractionation in Metamorphic Rocks, with Application to a Flat-Top, Y-rich Garnet Population from the Ruhla Crystalline Complex, Central Germany

A mathematical approach is presented for the calculation ofthe major and trace element fractionation that is caused bygrowth of zoned garnet in metamorphic rocks. This approach isbased on textural and compositional parameters directly obtainedfrom natural examples. It takes into account the mode and compositionof all unzoned minerals, as well as the mode, crystal size distributionand zonation patterns of garnet grains of different sizes withina certain rock volume. These parameters can be used to fit functionsfrom which the amount of garnet fractionation at each step ofa garnet growth history can be calculated. The approach is testedfor two compositionally distinct domains within a single garnet–biotitegneiss sample from the Ruhla Crystalline Complex. This samplecontains unusual flat-top garnet grains with Y₂O₃-rich cores.It is shown that MnO, FeO and Y₂O₃ are extremely fractionatedduring garnet growth, but in different ways, and that MnO fractionationdoes not obey a Rayleigh function. To demonstrate the influenceof garnet fractionation on P–T path estimates, quantitativephase diagrams in the model system Na₂O–K₂O–CaO–MnO–FeO–MgO–Al₂O₃–TiO₂–SiO₂–H₂Oare constructed by means of the computer software THERMOCALC.The good agreement between calculated and observed mineral assemblagesand garnet compositions for all fractionation steps indicatesthat the new approach can be used to infer detailed P–Tpaths, even for rocks that contain complexly zoned garnet grains.The results indicate that garnet growth in the metapelite underinvestigation occurred along a linear P–T path from 470°Cand 2·7 kbar to 580°C and 8·5 kbar. The resultsalso show that garnet cores with high Y₂O₃ contents of about1 wt % nucleated over a temperature interval of c. 90°C,indicating that Y in garnet is relatively insensitive to temperaturechanges. KEY WORDS: garnet; fractionation; pseudosection; yttrium; THERMOCALC     

Phase Relations and Chemical Composition of Phengite and Paragonite in Pelitic Schists During Decompression: a Case Study from the Monte Rosa Nappe and Camughera-Moncucco Unit, Western Alps

The metamorphic evolution of metapelites from the eastern partof the Monte Rosa nappe and the Camughera–Moncucco unit,both situated in the upper Penninic units SW of the Simplonline, were investigated using microstructural relationshipsand equilibrium phase diagrams. The units under considerationexperienced pre-Alpine amphibolite-facies conditions and underwenta complex metamorphic evolution during the Alpine orogeny. Peakpressures during an early Alpine high-pressure stage of 12·5–16kbar were similar in the Monte Rosa nappe and Camughera–Moncuccounit. A pronounced thermal gradient is indicated during decompressionleading to an amphibolite-facies overprint, as the decompressionpaths went through the chlorite, biotite and plagioclase stabilityfields in most of the Monte Rosa nappe, through the staurolitefield in the easternmost Monte Rosa nappe and in the Camughera–Moncuccounit, and through the sillimanite field in the easternmost Camughera–Moncuccounit. In high-Al metapelites the initial formation of stauroliteis related to continuous paragonite breakdown and associatedformation of biotite. In the course of this reaction phengitebecomes successively sodium enriched. In low-Al metapelites,in contrast, the initial staurolite formation occurs via thecontinuous breakdown of sodium-rich phengite. In both low- andhigh-Al metapelites the largest volume of staurolite is formedduring the continuous breakdown of sodium-rich phengite belowP–T conditions of about 9·5 kbar at 600–650°C.During this reaction phengite becomes successively potassiumenriched as sodium from phengite is used to form the albitecomponent in plagioclase. For ‘normal’ pelitic chemistries,phengite becomes Na enriched during decompression through thebreakdown of paragonite along a near-isothermal decompressionpath. The Na content in phengite reaches its maximum when paragoniteis entirely consumed. During further decompression the paragonitecomponent in phengite decreases again because Na is preferentiallyincorporated into the albite component of plagioclase. KEY WORDS: metapelites; white mica; high pressure; equilibrium diagrams; Western Alps     

Petrographic, Chemical and B-Isotopic Insights into the Origin of Tourmaline-Rich Rocks and Boron Recycling in the Martinamor Antiform (Central Iberian Zone, Salamanca, Spain)

Tourmaline in the Martinamor antiform occurs in tourmalinites(rocks with >15–20% tourmaline by volume), clasticmetasedimentary rocks of the Upper Proterozoic Monterrubio formation,quartz veins, pre-Variscan orthogneisses and Variscan graniticrocks. Petrographic observations, back-scattered electron (BSE)images, and microprobe data document a multistaged developmentof tourmaline. Overall, variations in the Mg/(Mg + Fe) ratiosdecrease from tourmalinites (0·36–0·75),through veins (0·38–0·66) to granitic rocks(0·23–0·46), whereas Al increases in thesame order from 5·84–6·65 to 6·22–6·88apfu. The incorporation of Al into tourmaline is consistentwith combinations of ^xAl(NaR)_–1 and AlO(R(OH))_–1exchange vectors, where ^x represents X-site vacancy and R is(Mg + Fe²⁺ + Mn). Variations in ^x/(^x + Na) ratios are similarin all the types of tourmaline occurrences, from 0·10to 0·53, with low Ca-contents (mostly <0·10apfu). Based on field and textural criteria, two groups of tourmaline-richrocks are distinguished: (1) pre-Variscan tourmalinites (probablyCadomian), affected by both deformation and regional metamorphismduring the Variscan orogeny; (2) tourmalinites related to thesynkinematic granitic complex of Martinamor. Textural and geochemicaldata are consistent with a psammopelitic parentage for the protolithof the tourmalinites. Boron isotope analyses of tourmaline havea total range of ¹¹B values from –15·6 to 6·8;the lowest corresponding to granitic tourmalines (–15·6to –11·7) and the highest to veins (1·9to 6·8). Tourmalines from tourmalinites have intermediate¹¹B values of –8·0 to +2·0. The observedvariations in ¹¹B support an important crustal recycling ofboron in the Martinamor area, in which pre-Variscan tourmaliniteswere remobilized by a combination of mechanical and chemicalprocesses during Variscan deformation, metamorphism and anatexis,leading to the formation of multiple tourmaline-bearing veinsand a new stage of boron metasomatism. KEY WORDS: tourmalinites; metamorphic and granitic rocks; mineral chemistry; whole-rock chemistry; boron isotopes     

Ultrahigh-temperature Metamorphism (1150{degrees}C, 12 kbar) and Multistage Evolution of Mg-, Al-rich Granulites from the Central Highland Complex, Sri Lanka

Mg- and Al-rich granulites of the central Highland Complex,Sri Lanka preserve a range of reaction textures indicative ofa multistage P–T history following an ultrahigh-temperaturemetamorphic peak. The granulites contain a near-peak assemblageof sapphirine–garnet–orthopyroxene–sillimanite–quartz–K-feldspar,which was later overprinted by intergrowth, symplectite andcorona textures involving orthopyroxene, sapphirine, cordieriteand spinel. Biotite-rims, kornerupine and orthopyroxene-rimson biotite are considered to be late assemblages. Thermobarometriccalculations yield an estimated P–T of at least 1100°Cand 12 kbar for the near-peak metamorphism. Isopleths of Al₂O₃in orthopyroxene are consistent with a peak temperature above1150°C. The P–T path consists of four segments. Initialisobaric cooling after peak metamorphism (Segment A), whichproduced the garnet–sapphirine–quartz assemblage,was followed by near-isothermal decompression at ultrahigh temperature(Segment B), which produced the multiphase symplectites. Furtherisobaric cooling (Segment C) resulted in the formation of biotiteand kornerupine, and late isothermal decompression (SegmentD) formed orthopyroxene rims on biotite. This evolution canbe correlated with similar P–T paths elsewhere, but thereare not yet sufficient geochronological and structural dataavailable from the Highland Complex to allow the tectonic implicationsto be fully assessed. KEY WORDS: central Highland Complex; granulites; multistage evolution; Sri Lanka; UHT metamorphism     

A Theoretical Study on the Formation of Growth Zoning in Garnet Consuming Chlorite

Chemical zoning of garnet is often used to deduce P–Tpaths of rocks by inverse calculation. To validate the derivedP–T paths, it is desired to establish a method to predictthe chemical compositions of garnet theoretically. This studyproposes a new forward calculation of the formation of Mg–Fe–Mngarnet from chlorite, which solves the non-linear simultaneousequations using nested iterative calculations. Growth of garnetconsuming chlorite and quartz was modelled in a MnO–FeO–MgO–Al₂O₃–SiO₂–H₂Osystem, using the most recent thermodynamic data for the minerals.The prograde P–T history of the Sambagawa metamorphicbelt, SW Japan, was modelled. To reproduce growth zoning, crystallizedgarnet was removed step by step from the system; perfect diffusionwas assumed for chlorite. The proposed model derived the evolutionof molar amounts and chemical compositions of Mg–Fe–Mnchlorite and garnet. It successfully reproduced the shape ofthe observed chemical profile of garnet, although the temperaturecondition was higher than general observations. The Mn contentof the garnet core was generally high, and Mg/Fe ratio alwaysstarted rising rapidly after Mn was depleted. Thermodynamicproperties of minerals, initial chlorite composition, P–Tpath, H₂O partial pressure, and Ca content in garnet were variedto test the behaviour of the system. The properties of Mn phasesinfluenced only the chemical composition of the garnet core.The temperature range in which garnet grew depended on the H₂Opartial pressure or the Ca content in garnet. KEY WORDS: chemical equilibrium; chemical zoning; garnet; forward modelling; Sambagawa metamorphic belt     

Assigning Dates to Thin Gneissic Veins in High-Grade Metamorphic Terranes: A Cautionary Tale from Akilia, Southwest Greenland

A granodiorite from Akilia, southwest Greenland, previouslysuggested to date putative life-bearing rocks to 3·84Ga, is re-investigated using whole-rock major and trace-elementgeochemistry, and detailed cathodoluminescence image-guidedsecondary ion mass spectrometer analyses of zircon U–Th–Pband rare earth elements. Complex zircon internal structure revealsthree episodes of zircon growth and/or recrystallization datedto c. 3·84 Ga, 3·62 Ga and 2·71 Ga. Rareearth element abundances imply a significant role for garnetin zircon generation at 3·62 Ga and 2·71 Ga. The3·62 Ga event is interpreted as partial melting of ac. 3·84 Ga grey gneiss precursor at granulite facieswith residual garnet. Migration of this 3·62 Ga magma(or melt–crystal mush) away from the melt source placesa maximum age limit on any intrusive relationship. These earlyArchaean relationships have been complicated further by isotopicreworking in the 2·71 Ga event, which could have includeda further episode of partial melting. This study highlightsa general problem associated with dating thin gneissic veinsin polyphase metamorphic terranes, where field relationshipsmay be ambiguous and zircon inheritance can be expected. KEY WORDS: Archaean; geochronology; Greenland; secondary ion mass spectrometry; zircon     

Monazite-Xenotime Thermochronometry and Al2SiO5 Reaction Textures in the Picuris Range, Northern New Mexico, USA: New Evidence for a 1450-1400 Ma Orogenic Event

Al₂SiO₅ reaction textures in aluminous schist and quartziteof the northern Picuris range, north-central New Mexico, recorda paragenetic sequence of kyanite to sillimanite to andalusite,consistent with a clockwise P–T loop, with minor decompressionnear the Al₂SiO₅ triple-point. Peak metamorphic temperaturesare estimated at 510–525°C, at 4·0–4·2kbar. Kyanite and fibrolite are strongly deformed; some prismaticsillimanite, and all andalusite are relatively undeformed. Monaziteoccurs as inclusions within kyanite, mats of sillimanite andcentimetre-scale porphyroblasts of andalusite, and is typicallyaligned subparallel to the dominant regional foliation (S₀/S₁or S₂) and extension lineation (L₁). Back-scatter electron imagesand X-ray maps of monazite reveal distinct core, intermediateand rim compositional domains. Monazite–xenotime thermometryfrom the intermediate and rim domains yields temperatures of405–470°C (±50°C) and 500–520°C(±50°C), respectively, consistent with the progradeto peak metamorphic growth of monazite. In situ, ion microprobeanalyses from five monazites yield an upper intercept age of1417 ± 9 Ma. Near-concordant to concordant analyses yield²⁰⁷Pb–²⁰⁶Pb ages from 1434 ± 12 Ma (core) to 1390± 20 Ma (rim). We find no evidence of older regionalmetamorphism related to the 1650 Ma Mazatzal Orogeny. KEY WORDS: Al₂SiO₅; metamorphism; monazite; thermochronometry; triple-point     

A Metamorphosed Early Cambrian Crust-Mantle Transition in the Eastern Alps, Austria

In the Speik Complex (Eastern Alps, Austria), highly melt-depleted,metamorphosed harzburgites with abundant pods and layers ofchromitite are interlayered with a suite of metamorphosed orthopyroxenites,clinopyroxenites and gabbros. Coarse-grained orthopyroxenitesoccur as centimetre- to metre-wide veinlets and pods, but alsoas intrusive plugs several tens of metres wide. Intimately associatedmetaclinopyroxenite and metagabbro are present as bodies upto several metres thick at a distinct stratigraphic level withinthe complex. In the ultramafic rocks, relict magmatic olivine,orthopyroxene, clinopyroxene and spinel have been overprintedby a metamorphic assemblage of forsterite, diopside, tremolite,anthophyllite, chlorite, serpentine, talc and Cr–Fe-richspinel. Hornblende, epidote, zoisite and chlorite dominate themetamorphic paragenesis in metagabbros, in addition to rarerelicts of clinopyroxene and two phases of Ca-rich garnet. Thepolymetamorphic evolution of the Speik Complex includes rarelypreserved pre-Variscan (400 Ma) eclogite-facies conditions,Variscan (330 Ma) amphibolite-facies conditions (600–700°C,>5 kbar) and Eoalpine (100 Ma) greenschist- to amphibolite-faciesconditions reaching 550°C and 7–10 kbar. Orthopyroxenitesare characterized by high concentrations of SiO₂, MgO and Cr,and by U-shaped chondrite-normalized rare earth element (REE)patterns similar to those of their harzburgite hosts. The REEpatterns of the clinopyroxenites are flat to slightly enrichedin light REE. Metagabbro compositions are variable, but generallycharacterized by low SiO₂ and high mg-numbers (61–78).Their REE patterns all have Gd_N/Yb_N > 1; some samples havelarge positive Eu anomalies implying the original presence ofcumulus plagioclase. In the orthopyroxenites, clinopyroxenitesand some peridotites, Pt, Pd and Re are distinctly enrichedcompared with Os, Ir and Ru, whereas most harzburgites haveunfractionated to slightly fractionated platinum-group element(PGE) patterns with respect to average upper mantle. The Re–Osisotope compositions of the pyroxenites define an errorchronat 550 ± 17 Ma and a supra-chondritic ¹⁸⁷Os/¹⁸⁸Os of0·179 ± 0·003. An isochron age of 554 ±37 Ma with _Nd(i) +0·7 is indicated by the Sm–Ndisotope compositions of whole-rock pyroxenite and gabbro samples,whereas the harzburgites plot on an errorchron of 745 ±45 Ma and _Nd(i) +6. The pyroxenites and gabbros probably representa cogenetic suite of magmatic dykes intruded into uppermost,highly depleted, suboceanic mantle below the crust–mantletransition zone in an oceanic basin close to the northwesternmargin of Gondwana. KEY WORDS: pyroxenite; metagabbro; geochemistry; Re–Os isotopes; Sm–Nd isotopes     

Time Markers for the Evolution and Exhumation History of a Late Palaeozoic Paired Metamorphic Belt in North-Central Chile (34{degrees}-35{degrees}30'S)

A multi-method geochronological approach is applied to unravelthe dynamics of a paired metamorphic belt in the Coastal Cordilleraof central Chile. This is represented by high-pressure–low-temperaturerocks of an accretionary prism (Western Series), and a low-pressure–high-temperatureoverprint in the retro-wedge with less deformed metagreywackes(Eastern Series) intruded by magmas of the coeval arc. A pervasivetransposition foliation formed in metagreywackes and interlayeredoceanic crust of the Western Series during basal accretion nearmetamorphic peak conditions (350–400°C, 7–11kbar) at 292–319 Ma (⁴⁰Ar/³⁹Ar phengite plateau ages).⁴⁰Ar/³⁹Ar UV laser ablation ages of phengite record strain-freegrain growth and recrystallization with a duration of 31–41Myr during a pressure release of 3–4 kbar. During earlyaccretion the main intrusion in the arc occurred at 305 Ma (Pb–Pbevaporation; zircon) and the Eastern Series was overprintedby a short high-temperature metamorphism at 3 kbar, 296–301Ma (⁴⁰Ar/³⁹Ar muscovite plateau ages). Fission-track ages ofzircon (206–232 Ma) and of apatite (80–113 Ma) aresimilar in both series, indicating synchronous cooling duringdistinct periods of exhumation. Early exhumation (period I)during continuing basal accretion proceeded with mean ratesof 0·19–0·56 mm/yr, suggesting that erosionin a tectonically active area was an important unroofing mechanism.At the same time mean rates were 0·03–0·05mm/yr in the Eastern Series, where crustal thickening was minor.A shallow granite intruded into the Western Series at 224 Ma,at the end of basal accretion activity, when exhumation ratesdecreased to 0·04–0·06 mm/yr in both seriesduring period II (100–225 Ma). Major extension, basinformation and local bimodal dyke intrusion at 138 Ma were accompaniedby mean cooling rates of 1–2°C/Myr. Accelerated coolingof 3–5°C/Myr at 80–113 Ma suggests a mid-Cretaceousconvergence event (period III). After 80 Ma cooling rates decreasedto 1–2°C/Myr (period IV). The pressure–temperature–deformation–timeinformation for subduction, basal accretion and exhumation inthe accretionary wedge of central Chile illustrates that theseprocesses reflect a continuous cyclic mass flow that lastednearly 100 Myr, while the retro-wedge remained stable. Afterthe cessation of accretion activity a similarly long periodof retreat of the subducting slab occurred; this ended withrenewed convergence and shortening of the continental margin. KEY WORDS: exhumation rates; Ar/Ar geochronology; fission-track geochronology; Chile; paired metamorphic belt     

The Role of Water Retention in the Anatexis of Metapelites in the Bushveld Complex Aureole, South Africa: an Experimental Study

Highly restitic metapelites occur at the contact of the RustenburgLayered Suite (Bushveld Complex). On the basis of previous experimentalstudies, the high (     

Evidence of Magma Mixing in the 'Daly Gap' of Alkaline Suites: a Case Study from the Enclaves of Pantelleria (Italy)

The island of Pantelleria consists of trachytes, pantelleritesand minor mildly alkaline basalts. Rocks of intermediate composition(falling in the so-called ‘Daly Gap’) such as mugearites,benmoreites and mafic trachytes occur only in the form of enclavesin trachytes and pantellerites inside the main caldera of theisland (Caldera ‘Cinque Denti’), which collapsedduring the ‘Green Tuff’ ignimbrite eruption at 50ka. The enclaves include volcanic, subvolcanic and intrusiverock types. The enclaves in host trachyte contain traces ofglass; devitrified glass occurs within enclaves in host pantellerites.Minerals in the enclaves show regular compositional variationswith whole-rock silica content. Glass present in the medium-grainedsamples is interpreted to be the result of incipient melting.The major and trace element compositions of the enclaves showregular and linear variations between an evolved mafic magma(hawaiite) and a felsic end-member similar to the ‘GreenTuff’ trachyte. Fractional crystallization modelling ofcompatible and incompatible trace elements (V, Ni, Zr, La, Sm,Lu, Nb, Y, Th) does not reproduce the observed trends. Rocksof intermediate composition within the ‘Daly Gap’can be explained only by magma mixing between an already differentiatedmafic magma (hawaiite) and an anorthoclase-rich trachytic meltin the lower and higher parts, respectively, of a stratifiedmagmatic chamber. Medium-grained enclaves are interpreted asthe result of fragmentation of solidified mixing layers in theroof of the magma chamber during the eruption of the ‘GreenTuff’, when the collapse of the caldera took place. Diffusioncalculations suggest a residence time of <5 days for theenclaves in their host magmas. KEY WORDS: Daly Gap; enclaves; magma mixing; Pantelleria     

Metamorphic Evolution of the Ribeira Belt: Evidence from Outcrops in the Rio de Janeiro Area, Brazil

Migmatitic granulites and arc-related felsic intrusives of Pan-Africanage form the bedrock in the Rio de Janeiro area, SE Brazil.These rocks preserve a partial record of three parageneses.The earliest assemblage (M₁) grew during fabric formation inthe rocks (D₁) and is characterized by the mineral assemblagePl + Bt + Sil + Kfs + Qtz. Peak metamorphic conditions (M₂)are characterized by the assemblage Bt + Crd + Kfs + Pl + Grt+ liq + Qtz and are inferred to have developed during D₂ foldingof the rocks at T = 750–800°C and P = 7 kbar. M₃ reactiontextures overprint the M₂ assemblage and comprise symplectiticintergrowth of cordierite(II) and quartz that formed after garnet,whereas secondary biotite formed as a result of reactions betweengarnet and K-feldspar. By comparing the observed modal abundanceswith modal contours of garnet, cordierite and quartz on therelevant pseudosection a post M₂ P–T vector indicatingcontemporaneous cooling and decompression can be deduced. Theinferred equilibrium assemblage and reaction textures are interpretedto reflect a clockwise P–T path involving heating followedby post-peak decompression and associated cooling. We inferthat metamorphism occurred in response to advective heatingby the abundant syn-collisional (arc-related) I-type granitoidsin the region, consistent with the unusually high peak T/P ratio. KEY WORDS: advective heating; Ribeira belt; granulite; partial melting; P–T pseudosection