Thermobarometry and Geotectonic Significance of High-Pressure Granulites: Examples from the Moldanubian Zone of the Bohemian Massif in Lower Austria

Petrographic details together with mineral and whole-rock compositiondata are provided for acid-intermediate garnet granulites fromexposed granulite complexes in Lower Austria. Thermobarometricevaluation integrated with available isotopic age data indicatesthe initial equilibration of early Variscan ({small tilde}370Ma) high-pressure granulite assemblages at {small tilde}16 kbarand 1000C and their partial overprinting by retrograde assemblageswhich reflect the blocking of mineral exchange reaction equilibriaat {small tilde}6•5 kbar and 725C during subsequent Variscanuplift and cooling. These calculated P-T estimates, togetherwith general phase equilibria constraints and evidence frompreserved prograde coronitic reaction textures and garnet compositionalzoning profiles, indicate a clockwise P-T-t evolutionary pathof the type expected during crustal thickening in a major platecollision orogen and characterized by near-isothermal decompressionduring initial uplift. Geochemical characterization of the rockprotoliths as calc-alkaline igneous rocks and the high metamorphictemperatures suggest that garnet granulite formation involvedthe subduction of a magmatic arc at a continental plate margin. Reviewed evidence from granulites in the Central European Variscidesruns counter to suggestions by Bohlen (1987, 1991) that high-pressuregranulites are of little regional geotectonic significance incomparison with low- to medium-pressure granulites. The differentevolutionary P-T paths for these two important groups of granulitespoint to formation in contrasting plate settings. However, questionsare raised regarding petrogenetic models for low- to medium-pressuregranulites which have emphasized the importance of magmatic,rather than tectonic, crustal thickening and the recognitionof stabilization along deduced anti-clockwise P-T-t paths characterizedby post-peak near-isobaric cooling. It is suggested here that the reality of stabilization of atleast some low- to medium-pressure granulites in a collisionaltectonic regime may have been concealed either because lower-pressureassemblages have overprinted mineralogical evidence for an earlierhigh-pressure history at deeper crustal levels or through invaliddeduction of near-isobaric cooling trajectories as a resultof the different closure temperatures for the mineral reactionsused to monitor the equilibration temperatures and pressuresin granulites. However, the sequential underthrusting modelfavoured for the tectonometamorphic evolution of the Variscannappe pile in the Bohemian Massif renders it unlikely that alllate Variscan low- to medium-pressure granulites have experiencedthe early Variscan high-pressure metamorphism.     

Regional progressive high-pressure metamorphism, Seward Peninsula, Alaska

Abstract Blueschist-facies rocks on the Seward Peninsula constitute a structurally coherent terrane measuring at least 100 × 150 km. Radiometric age data indicate that high-pressure metamorphism probably occurred in Jurassic rather than in Palaeozoic or Precambrian time, as previously suggested. Protolith sediments (Nome Group) are of intracontinental basin or continental margin type, and of lower Palaeozoic and possibly late Precambrian age, thus predating the high pressure metamorphism by more than 200 m.y. Blueschist-facies mineral assemblages were developed in almost all lithologies of the Nome Group, and are best preserved in FeTi-rich metabasites (glaucophane + almandine + epidote) and pelites (glaucophane + chloritoid + phengite). A lawsonite–crossite subfacies was developed in possible Nome Group rocks on the east flank of the Darby Mountains. Albite–epidote–amphibolite facies assemblages characterize Nome Group rocks in the southwestern part of the Peninsula. Metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote–almandine–glaucophane schist subfacies with the local development (east of the Nome River) of eclogitic assemblages. The high pressure metamorphic minerals were synkinematic with the development of mesoscopic-scale intrafolial isoclinal folds and a flattening foliation of consistent orientation. Initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles, and partial post-kinematic greenschist facies replacements record later stages of decompression. Ophiolites and melange are not associated with the Seward Peninsula blueschists. The high-pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin. The PT conditions of high pressure metamorphism were approximately 9–11 kbar, 400–450°C, thus falling between the PT paths of the Shuksan and Franciscan terranes.     

High-Grade Fluid Metasomatism on both a Local and a Regional Scale: the Seward Peninsula, Alaska, and the Val Strona di Omegna, Ivrea-Verbano Zone, Northern Italy. Part II: Phosphate Mineral Chemistry

This study explores the origin and geochemical evolution ofapatite, monazite, and xenotime along two metamorphic traverses.The first, from the Kigluaik Mountains, Seward Peninsula, Alaska,consists of a localized (85 cm) orthopyroxene–clinopyroxene-bearingdehydration zone. The second consists of orthopyroxene ±clinopyroxene-bearing granulite facies metabasite layers interlayeredwith metapelites over a 3–4 km traverse, along the ValStrona, Ivrea–Verbano Zone, Northern Italy (IVZ). In bothdehydration zones small Th- and U-poor inclusions of monaziteand/or xenotime occur in the apatite. These inclusions are metasomaticallyinduced and nucleated within the apatite via the coupled substitutionsNa⁺ + (Y + REE)³⁺ = 2 Ca²⁺ and Si⁴⁺ + (Y + REE)³⁺ = P⁵⁺ + Ca²⁺.These are not present in apatite from the original amphibolitefacies gneiss. Apatite, in both dehydration zones, also showsa relative increase in both F and Cl compared with apatite fromthe amphibolite facies zone. Granulite facies metabasites inthe IVZ also contain isolated monazite grains, which range fromuniform to complexly zoned in Th the (13–30·1 mol% ThSiO₄). These are the product of breakdown and subsequentmobilization of the lanthanides and actinides from monazite-(Ce)in the metapelite layers into the metabasite layers at the startof granulite facies metamorphism. KEY WORDS: apatite; monazite; xenotime; KCl–NaCl brines; metasomatism; phosphate minerals; charnockite–enderbite; granulite facies metamorphism     

Experimental Determination of the Upper Thermal Stability of Fe-Staurolite+Quartz at Medium Pressures

The thermal equilibrium for the reaction Fe-staurolite+quartz=almandine+sillimanite+H₂Ohas been reversed at 3?25 and 5?00 kb pressure using a well-characterizednatural Fe-rich staurolite. Long run times of 100 days at 3?25kb and 60 days at 5 kb, in addition to pretreatments of thealmandine+sillimanite+quartz by annealing at the experimentalP and T for 30 days prior to the experimental run, increasedthe probability that equilibrium was attained and that sufficientamounts of reaction had occurred to allow its detection. Reaction direction was determined by directly observing surfacemorphologies of the staurolites with the Scanning Electron Microscope(SEM), a technique that permits evaluation of stability andinstability even when the extent of reaction is minor. Growthand dissolution appear to be crystallographically controlledbut produce distinct morphologies that allow mterpretation ofthe reaction direction. Growth of staurolite develops by a face-selectiveprocess, such that small step-like features overgrow the originalseed staurolite surface. Dissolution produces simpler, blockierforms locally transected by etch pits. Based on textural criteria for staurolite stability and instability,the equilibrium boundary is located between 643' and 658?C at3?25 kb and between 673 and 688?C at 5 kb. This phase boundaryhas a shallower dP/dT slope and lies {small tilde}25?C lowerthan the previous experimental investigation at low pressure(Richardson, 1968). However, this study has not solved the apparentdiscrepancy between the experimentally determined thermal stabilityof staurolite and natural occurrences of staurolite (the stauroliteproblem). For the experimentally determined staurolite curveto agree with natural staurolite occurrences, the experimentalequilibrium boundary would have to be {small tilde}50? lowerthan that indicated by the results of this study. Additionalthermochemical discrepancies are most likely related to thecomplex crystal chemistry of staurolite. ^*Present address: Department of Geology & Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803     

Metamorphic P-T Paths from Cordillera Darwin, a Core Complex in Tierra del Fuego, Chile

P–T conditions and prograde P–T paths have beencalculated for amphibolite-grade pelites and amphibolites fromCordillera Darwin, Tierra del Fuego, Chile. Peak P–T conditionsare nearly all within the kyanite stability field; temperaturesgenerally show an increase with increasing grade, but pressureshave a less consistent trend, possibly increasing slightly fromgarnet to kyanite grade. P–T paths from pelites show heatingby 80–100C during loading of 0•2–3 kbar. Texturalanalysis and previous structural work indicate that this segmentof the path correlates with back-folding deformation. P–Tpaths from two Mg-rich garnet amphibolites suggest a decreasein pressure of as much as 3 kbar with 25–50C of heatingfrom the kyanite stability field to the sillimanite, and areconsistent with pervasive, minor development of fibrolitic sillimanitealong plagioclase grain boundaries. Together, the P–Tpath segments from pelites and amphibolites constitute a clockwiseP–T trajectory. The proposed clockwise P–T paths are consistent with theinterpretation that Cordillera Darwin represents an extensionallyexhumed metamorphic core complex, in which loading during garnetgrowth in the pelitic rocks was succeeded by differential upliftduring garnet growth in magnesian amphibolites. ^* Present address: Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706     

Decompression Reactions and P--T Conditions in High-grade Rocks, Northern Labrador: P--T--t Paths from Individual Samples and Implications for Early Proterozoic Tectonic Evolution

The boundary between the Archaean Nain and Rae Provinces, knownas the Early Proterozoic Torngat orogen, has been examined inthe Saglek Fiord area of northern Labrador. Torngat orogen, up to 40 km wide, is principally composed ofgranulite and amphibolite facies gneisses that are characterizedby transcurrent sinistral shear zone fabrics that were subsequentlypartially reworked in an event involving east-directed thrusting.This paper documents the Early Proterozoic metamorphic historyof some gneisses from the Saglek Fiord area of Torngat orogen.Petrographic and thermobarometric studies show that evidenceof both peak thermal conditions and subsequent decompressionreactions are preserved and that portions of this part of theP–T path (here called the P–T vector to emphasizethe direction of P–T changes), are preserved in individualsamples. Highest P and T ({small tilde} 10 kb and 800?C) arerecorded by cores of coexisting minerals in equigranular aggregates,whereas rims record variable degrees of post-peak re-equilibration.Substantial decompression accompanied by cooling (down to {smalltilde}5 kb and 650?C) is recorded by symplectites developedat garnet rims adjacent to clinopyroxenes. These symplectitesare interpreted to have developed during erosion and upliftimmediately following the peak metamorphic event. A tectonic model incorporating the structural and metamorphicobservations is presented. It is suggested that the Proterozoic-Archaeanboundary in the Saglek area developed through crustal thickeningand north-south oriented transcurrent shearing followed by east-directedthrusting, possibly through a continuous process of obliquecollision.     

Low-grade Metamorphism of the Karmutsen Volcanics, Vancouver Island, British Columbia

Low-variance assemblages occurring within amygdules of Karmutsenlavas from the Elk Creek and Upper Campbell Lake areas, VancouverIsland, British Columbia, provide important constraints on thepressure and temperature of metamorphism as well as on the compositionof the attendant fluid. The P-T stability of the assemblagesepidote-muscovite-K-feldspar-prehnite and epidote-prehnite-quartz-wairakitecoupled with mean isochores derived from homogenization temperaturesof H₂O inclusions within amygdaloidal quartz indicate that theUpper Campbell Lake area was subjected to metamorphism at 1?5kb (?0?5 kb), 260 ?C (? 15?C) and the Elk Creek area at somewhatlower P or higher T. Isobaric T-a(CO₂) diagrams show that the occurrence of epidote-oligoclase,prehnite-orthoclase-albite, and prehnite-andesine assemblagescollected from the Elk Creek area is consistent with the P-Tconstraints and that these assemblages formed in water-richfluids containing very low concentrations of CO₂. The presenceof Ca-zeolite-epidote assemblages in the Upper Campbell Lakearea is also compatible with P-T estimates. The consistencyof epidote and prehnite rim compositions in low-variance assemblagesand the lack of incompatible phases in these assemblages demonstratethat equilibrium was obtained on limited domains within amygdules. Because epidote and prehnite compositions in low-variance assemblagesare very sensitive to changes in concentration of CO2, low-varianceassemblages involving these phases can serve as monitors offluid composition. It is postulated that low-variance assemblagesin Karmutsen flows originated by reaction of previously formed,high-variance assemblages with infiltrating CO₂-bearing aqueousfluids during a subsequent hydrothermal event. These fluidspreferentially exploited more permeable amygdaloidal portionsof the Karmutsen flows. The low-variance assemblages not onlyrecord the extremely H₂O-rich composition of the permeatingfluid, but also outline the paths the fluid took. ^* Offprint requests to B. R. Frost     

Garnet Peridotite Xenoliths from the Vitim Volcanic Field, Baikal Region: the Nature of the Garnet--Spinel Peridotite Transition Zone in the Continental Mantle

A suite of large and fresh peridotite xenoliths from a picritetuff deposit in the Cenozoic Vitim volcanic field, {small tilde}200km east of Lake Baikal, shows a continuous gradation from protogranularspinel through garnet–spinel to very abundant garnet peridotites.This includes composite nodules in which all these lithologiescoexist on the scale of a few centimeters. Garnet and many spinellherzolites are remarkably fertile in terms of their ‘basaltic’major element contents (CaO 30–37%, MgO 37–40%,Ca/Al=11, Cr/Al<013), whereas some garnet–spineland spinel peridotites are moderately depleted (Cr/Al 014–045).T estimates are 850–880C for the fertile spinel lherzolitesapparently brought up from shallow depths of 40–50 km.This contrasts with 980–1030C for depleted spinel peridotitesand 1000–1150C for the garnet-bearing peridotites forwhich equilibration pressures between 16 and 23 kbar are inferred.The data suggest that garnet and spinel peridotites coexistin the sub-Vitim mantle at a pressure of {small tilde}18 kbarover an interval of {small tilde}2 kbar, with the appearanceof garnet, and with the garnetto-spinel ratio in this transitionalzone primarily being controlled by bulk rock contents of Ca,Al, Cr, and Cr/Al ratios, in addition to P–T conditions. The Vitim peridotites show little evidence for metasomatic enrichment:they commonly show depletion of LREE compared with intermediateREE; this includes also rare amphibole-bearing veins. The fertilespinel and garnet lherzolites have very similar bulk rock majoroxide contents and REE distribution patterns; these featuresindicate a lack of significant chemical vertical mantle stratificationin that region. Garnet peridotites from Vitim show large differencesin modal and chemical composition from garnet peridotite xenolithsfrom Yakutian and South African kimberlites, suggesting distinctlithospheric mantle structure and composition in Archean cratonsand post-Archean mobile belts. ^* Present address: School of Earth Sciences, Macquarie University, N.S.W. 2109, Australia     

Reconsideration of the age of blueschist facies metamorphism on the Seward Peninsula, Alaska, based on phengite 40Ar/39Ar results

Abstract ⁴⁰Ar/³⁹Ar step-heating and single-grain laser fusion ages from phengites from the polydeformed and polymetamorphosed blueschist-greenschist facies Nome Group fall into two groups. Samples from the upper part of the structural section that have experienced a relatively weak metamorphic and deformational post-blueschist facies overprint and one sample from the Cape Nome orthogneiss yield plateau ages of 116-125 Ma. More intensely overprinted samples yield hump-shaped spectra with minimum ages of 123 Ma and maximum ages of 334 Ma. Samples with hump-shaped spectra are derived from a greater structural depth than most samples with plateau ages. Unreasonably old maximum ages from some of the disturbed spectra suggest that the hump-shaped spectra result from the incorporation of excess ⁴⁰Ar. This interpretation conflicts with previous interpretations of similarly disturbed spectra from the Brooks Range, which have been argued to provide minimum ages for blueschist facies metamorphism. Since the maximum temperatures achieved by all samples were probably above the blocking temperature of Ar in phengite, the 116-125 Ma plateau ages are a minimum age for blueschist facies metamorphism on the Seward Peninsula, Alaska.     

Middle Pleistocene age of the Nome River glaciation, northwestern Alaska

During the middle Pleistocene Nome River glaciation of northwestern Alaska, glaciers covered an area an order of magnitude more extensive than during any subsequent glacial intervals. The age of the Nome River glaciation is constrained by laser-fusion ⁴⁰Ar/³⁹Ar analyses of basaltic lava that overlies Nome River drift at Minnie Creek, central Seward Peninsula, that average 470,000 ± 190,000 yr (±1σ). Milligram-size subsamples of the lava were dated to identify and eliminate extraneous ⁴⁰Ar enrichments that rendered the mean of conventional K---Ar dates on larger bulk samples of the same flow too old (700,000 ± 570,000 yr). While the ⁴⁰Ar/³⁹Ar analyses provide a minimum limiting age for the Nome River glaciation, maximum ages are provided by a provisional K---Ar date on a basaltic lava flow that underlies the Nome River drift at nearby Lave Creek, by paleomagnetic determinations of the drift itself at and near the type locality, and by amino acid epimerization analysis of molluscan fossils from nearshore sediments of the Anvilian marine transgression that underlie Nome River drift on the coastal plain at Nome. Taken together, the new age data indicate that the glaciation took place between 580,000 and 280,000 yr ago. The altitude of the Anvilian deposits suggests that eustatic sea level during the Anvilian transgression rose at least as high as and probably higher than during the last interglacial transgression; by correlation with the marine oxygen-isotope record, the transgression probably dates to stage 11 at 410,000 yr, and the Nome River glaciation is younger still. Analyses of floor altitudes of presumed Nome River cirques indicate that the Nome River regional snowline depression was at least twice that of the maximum late Wisconsin. The cause of the enhanced snowline lowering appears to be related to greater availability of moisture in northwestern Alaska during the middle Pleistocene.     

Distribution of Ferric Iron in some Upper-Mantle Assemblages

The distribution of ferric iron among the phases of upper-mantlerocks, as a function of pressure (P), temperature (T) and bulkcomposition, has been studied using ⁵⁷Fe Mssbauer spectroscopyto determine the Fe³⁺/Fe ratios of mineral separates from 35peridotite and pyroxenite samples. The whole-rock Fe³⁺ complementof a peridotite is typically shared approximately evenly amongthe major anhydrous phases (spinel and/or garnet, orthopyroxeneand clinopyroxene), with the important exception of olivine,which contains negligible Fe³⁺. Whole-rock Fe³⁺ contents areindependent of the T and P of equilibration of the rock, butshow a well-defined simple inverse correlation with the degreeof depletion in a basaltic component. Fe³⁺ in spinel and inboth pyroxenes from the spinel Iherzolite facies shows a positivecorrelation with temperature, presumably owing to the decreasein the modal abundance of spinel. In garnet peridotites, theFe³⁺ in garnet increases markedly with increasing T and P, whereasthat in clinopyroxene remains approximately constant. The complexnature of the partitioning of Fe³⁺ between mantle phases resultsin complicated patterns of the activities of the Fe³⁺ -bearingcomponents, and thus in calculated equilibrium f_O2, which showlittle correlation with whole-rock Fe³⁺ or degree of depletion.Whether Fe³⁺ is taken into account or ignored in calculatingmineral formulae for geothermobarometry can have major effectson the resulting calculated T and P. For Fe-Mg exchange geothermometers,large errors must occur when applied to samples more oxidizedor reduced than the experimental calibrations, whose f_O2 conditionsare largely unknown. Two-pyroxene thermometry is more immuneto this problem, and probably provides the most reliable P—Testimates. Accordingly, the convergence of P—T valuesderived for a given garnet peridotite assemblage may not necessarilybe indicative of mineral equilibrium. The prospects for thecalculation of accurate Fe³⁺ contents from electron microprobeanalyses by assuming stoichiometry are good for spinel, uncertainfor garnet, and distinctly poor for pyroxenes. KEY WORDS: mantle; oxidation; partitioning; peridotite; thermobarometry ^*Corresponding author. Present address: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700, Victoria, B.C., V8W 2Y2, Canada     

Metamorphism of Calcic Pelitic Schists, Strafford Dome, Vermont: Compositional Zoning and Reaction History

Four assemblages from calcic pelitic schists from South Strafford,Vermont, have been studied in detail to determine the relationshipbetween reaction history and compositional zoning of minerals.The lowest-grade assemblage is garnet + biotite + chlorite +plagioclase + epidote + quartz + muscovite + graphite + fluid.Along a path of isobaric heating, the net reaction is Chl +Ms + Ep + Gr = Grt + Bt + Pl + fluid. Garnet grows with decreasingFe/(Fe + Mg) and X_Spa, (from 0•2 to 0•05), X_Gra staysnearly constant between 0•20 and 0•25, and plagioclasegrows with X_An increasing from peristerite to 0•2–0•5. The subsequent evolution depends on whether chlorite or epidotereacts out first. If chlorite is removed from the assemblagefirst, the net reaction along an isobaric heating path becomesGrt + Ms + Ep + Qtz + Gr = Bt + Pl + fluid. X_An of plagioclaseincreases to 0•20–0•70, depending on the bulk-rockcomposition and changes in pressure and temperature. If epidoteis removed first, the assemblage becomes a simple pelite andthe net reaction becomes Chl + Pl + Ms + Qtz = Grt + Bt + H₂O.Plagioclase is consumed to provide Ca for growing garnet, andX_An, Fe/(Fe + Mg) of garnet, X_Gra, and X_Spa all decrease. Afterboth chlorite and epidote are removed, continued heating upto the metamorphic peak of {small tilde}600C produces littleprogress of the reaction Grt + Ms = Bt + Pl; and X_An increases. The four assemblages have been numerically modeled using theGibbs method starting with measured compositions. The modelssuccessfully predict the observed compositional zoning and trendsof mineral growth and consumption along the computed P–Tpaths. The models also predict the compositional mineral zoningthat would have resulted from other P–T paths. ^* Present address: Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487     

High-Pressure Metamorphism in the SW Tauern Window, Austria: P-T Paths from Hornblende-Kyanite-Staurolite Schists

The hornblende garbenschist horizon of the Lower Schieferhulleseries (LSH) in the SW Tauern Window, Austria, contains theassemblage hornblende + kyanite + staurolite + garnet + biotite+ epidote + plagioclase + ankerite + quartz + rutile + ilmenite,with either chlorite or paragonite present in all samples. Theseassemblages are divariant in the system SiO₂-Al₂O₃-TiO₂-Fe₂O₃-MgO-FeO-MnO-CaO-Na₂O-K₂O-H₂O-CO₂.Garnet-biotite geothermometry yields temperatures of final equilibrationof {small tilde}550 °C, and garnet-plagioclase-kyanite-quartzgeobarometry indicates pressures of 6–8 kb for the matrixassemblage and 9–10 kb for plagioclase inclusions in garnet.Quantitative modelling of zoned garnet, hornblende, and plagioclaseindicates growth and equilibration along a decompression pathfrom {small tilde}530 °C, 10 kb to {small tilde}550 °C,7 kb. Fluid inclusion data constrain the uplift path to havepassed through a point at {small tilde} 375 °C, 1.5 kb. These data permit the construction of a relatively completeP-T loop for metamorphism associated with the Alpine orogeniccycle in the LSH of the SW Tauern Window. The maximum pressureconditions ({small tilde}10 kb at 530 °C) recorded alongthis loop are considerably higher than previous estimates of5–7 kb for the region. Simple overthrust models developedfor the Tauern Window cannot account for pressures of this magnitude;a more likely scenario involves partial subduction of the rocksto a depth of {small tilde}35 km, followed by prolonged heatingin response to decay of the subduction isotherms. Initial upliftappears to have been rapid and occurred along a nearly isothermalpath. Significant cooling did not occur until the rocks werewithin {small tilde}5 km of the surface. Detailed tectonic modelsfor the evolution of the Tauern Window must be able to accountfor the quantitative features of the P-T loop.     

Metapelites of the Kanskiy Granulite Complex (Eastern Siberia): Kinked P-T Paths and Geodynamic Model

The Southern Yenisey Range (Eastern Siberia) consists of thegranulite-facies Kanskiy complex bordered by the lower-gradeYeniseyskiy and Yukseevskiy complexes. Samples of metapeliteof the Kanskiy complex typically show characteristic garnet-formingreaction textures and near-isobaric cooling P–T paths.An important new result of this study concerns the differencein shape of the P–T paths from different parts of theKanskiy granulite complex: metapelites collected 8 km from theboundary with the Yeniseyskiy complex followed a linear pathwith dP/dT 0·006 kbar/°C; metapelites collected3 km from this boundary reveal a kinked P–T path withan interval of burial cooling (dP/dT –0·006 kbar/°C).The difference in the shape of the P–T paths is supportedby the chemical zoning of garnet studied in the second groupof samples. A mechanism of buoyant exhumation of granulite issuggested by comparison with the results of numerical modelling,which indicate that such a diversity of P–T paths mayresult from a transient disturbance of the thermal structureby rapid differential movement of material from different crustallevels. To arrive at a correct tectonic interpretation, thewhole assemblage of interrelated P–T paths of metamorphicrocks collected from different localities within the same complexmust be studied. KEY WORDS: crustal diapirism; exhumation; granulites; numerical modelling; P–T path     

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     

Phase Equilibria in the Pyroxene Quadrilateral

Experimental phase equilibrium data on compositions of coexistingpyroxenes in the quadrilateral enstatite-diopside-ferrosilite-hedenbergitehave been used to model pyroxene solid solutions and to formulatepyroxene geothermometers. Each pyroxene is treated as a solidsolution of four quad-components using the Kohler formulation where G_ij^* is the excess free energy of mixing in a binary solutioncalculated with binary mole fractions (e.g. X_i^o = X_i/(X_i+X_j))and X_i is the mole fraction in a multicomponent solution. Thefit to the experimental data is achieved by minimizing the totalGibbs free energy of the assemblage. The following set of thermochemicaldata and simple mixture parameters (W_ij) are found to be bestsuited. Standard (T = 298?15 K) enthalpy and entropy of formationfrom elements for fictive orthohedenbergite are –1416?8kJ and 84?88 J K^–1 mol ^–1 respectively. The heatcapacity is given by 114?67+17?09E-3T–31?40E5T^–2.The W_ij data are: Opx: W₁₂ = W₂₁ = 25 W₁₃ = (13?1–0-015T),W₃₁ = (3?37–0?005T), W₂₃ = 20, W₃₂ = 16, W₂₄ = 5, W₄₂= 7, W₃₄ = 15, W₄₃ = 15; Cpx: W₁₂ = (25?484+0?0812P), W₂₁ =(31?216–0?0061P),W₃₁ = W₁₃ = 0W₁₄ = (93?3–0?045T), W₄₁ = (–20?0+0?028T),W₂₃ = 24, W₃₂ = 15, W₂₄ = 12, W₄₂ = 12, W₃₄ = (16?941+0?00592P),W₄₃ = (20?697–0?00235P). Coexisting pyroxene compositionshave been computed in the temperature range of 700 to 1400?C. Two geothermometers have been constructed, one based on atomicfraction of iron (Fe/(Fe + Mg)) in orthopyroxene and the Fe-Mgdistribution coefficient and the other, based on wollastonitecontent of clinopyroxene. The two scales yield different temperatureswhen applied to the same rock. In igneous pyroxenes, the Catransfer ceased at 150 to 200?C above the closure temperatureof the Fe-Mg ion-exchange. In metamorphic rocks an oppositeeffect seems to have prevailed.     

Orthopyroxene-Corundum in Mg-Al-rich Granulites from the Oygarden Islands, East Antarctica

High-Mg–Al, silica-undersaturated metapelites from theOygarden Group of islands, East Antarctica, preserve clear evidencefor the stable coexistence of the assemblage orthopyroxene +corundum in natural rocks. The quartz-absent metapelite occursas pods and isolated layers within a high-strain zone relatedto deformation during the c. 0·93 Ga Rayner StructuralEpisode. Assemblages that include orthopyroxene, corundum, sapphirine,sillimanite, cordierite, garnet and kornerupine are developedacross a pre-existing compositional zoning, leading to contrastingmineral Fe–Mg ratios. The assemblage orthopyroxene–corundumis shown to exist in only a very restricted range of bulk compositionsand P–T histories. Simplified qualitative FMAS grids havebeen constructed for kornerupine-absent and -present systems,illustrating MAS terminations and divariant equilibria thathelp to describe the mineral assemblage and reaction history.Reaction textures that include coronas of sapphirine and sillimaniteseparating orthopyroxene and corundum, and symplectites of orthopyroxene+ sapphirine ± cordierite/plagioclase, orthopyroxene+ sillimanite ± cordierite/plagioclase and orthopyroxene+ sapphirine + sillimanite embaying garnet, imply a clockwiseP–T–t evolution. Conditions of P > 9–10kbar and T     

Magmatism in the Gregory Rift, East Africa: Evidence for Melt Generation by a Plume

The volume and composition of volcanic rocks associated withthe Gregory rift are interpreted in the light of inversionsperformed on the REE concentrations of the most magnesian basalts.When the estimated volume of salic rock ({small tilde}88 000km³) is converted into basalt ({small tilde}792 000 km³) thetotal volume of basaltic melt generated over the last 30 Myis at least 924 000 km³, corresponding to an average rate ofmelt production of {small tilde}0•03 km³/yr and an averagemelt thickness of between 7 and 26 km everywhere beneath therift. The mean compositions of the basaltic magmas erupted withinthe rift and on the rift flanks during the Upper Oligocene andMiocene, the Pliocene, and the Quaternary are taken to be representativeof the average compositions of melts produced by fractionalmelting in the asthenospheric mantle. When the REE concentrationsof the observed average compositions are inverted they suggestthat much of the melt was produced in the depth and temperaturerange of the transition from garnet to spinel peridotite. Fora mantle potential temperature of {small tilde}1500C the topof the melting region predicted from the inversions is at {smalltilde}70 km beneath the rift axis and {small tilde}80 km beneaththe rift flanks. Within the rift zone the predicted thicknessof melt increases from the Upper Oligocene and Miocene to thePliocene and is always greater than that predicted for the riftflanks, and the timeaveraged thickness of melt predicted is0/5 km. To generate the observed volume of melt the asthenosphericmantle must continually upwell through the melting region (extendingfrom 70 to 150 km) with a vertical velocity of between 40 and140 mm/yr. The results suggest that, volumetrically and compositionally,magmatic activity associated with the Gregory rift is quantitativelyconsistent with a model of a mantle plume upwelling beneaththinned continental lithosphere. Predictions made by such amodel are in broad agreement with geophysical observations. ^* Present address/reprint requests to: B.P. Exploration, 4/5 Long Walk, Stockley Park, Uxbridge UB11 1BP, UK     

Subsolidus Emplacement of Mantle Peridotites during Incipient Oceanic Rifting and Opening of the Mesozoic Tethys (Voltri Massif, NW Italy)

The Erro–Tobbio (ET) peridotite (Voltri Massif, NW Italy)represents a fragment of subcontinental mantle, emplaced athigh crustal levels during rifting and opening of the Piemonte–Ligurianocean, the Alps–Apennine part of the Mesozoic Tethys.The ET peridotite is dominated by spinel-bearing lherzolites,with minor dunites, spinel websterites and plagioclase-bearinglherzolites. Granular spinel lherzolites in the ET peridotiteare transected by five generations of shear zone structures:porphyroclastic spinel-bearing tectonites, plagioclase-, hornblende-and chlorite-bearing peridotite mylonites, and serpentinitemylonites. There is a systematic correlation between the microstructuresand the composition of the constituent mineral phases. Thesecompositional trends are related to changing conditions of Pand T during the pertinent stages of syntectonic recrystallizationin the shear zones. Geothermobarometry shows that the shearzone structures developed at progressively lower P and T conditions.The P–T path obtained for the ET peridotite indicatessubsolidus uplift, from deep levels in the subcontinental mantletowards the ocean floor. Uplift is associated with limited ‘wet’partial melting. This subsolidus trajectory is particularlyconsistent with the thermal history expected for the footwallof a lithosphere-scale, dipping extensional shear zone, anddiffers from those of more oceanic peridotites showing an adiabaticuplift history at much higher temperatures presumably relatedto convective upwelling. The shear zone structures in the ETperidotites are therefore interpreted as fragments of an extensionaldetachment system. This interpretation is consistent with theoverall asymmetric architecture of the coeval passive marginsbordering the Piemonte–Ligurian ocean. It is suggestedthat the uplift of the ET peridotites occurred by tectonic denudation,in a slightly to strongly asymmetric oceanic rift. ^* Present address: Shell Int. Petroleum Maatschappij, P.O. Box 162, 2501 AN The Hague, Netherlands     

Pressure, Temperature, and Structural Evolution of the Orfordville Belt, West-Central New Hampshire

Metamorphic P-T paths have been derived for staurolite-kyanitegrade and garnet grade rocks from the Orfordville Belt, west-centralNew Hampshire. P-T paths calculated from garnet zoning are consistentwith parageneses observed in amphibolites as determined froma petrogenetic grid derived for amphibolites. The P-T pathsfrom the staurolite-kyanite zone show a pressure maximum at6.5 to 7.5 kb and {small tilde} 500?C followed by heating anddecompression to approximately 5 kb, 580?C, and a final phaseof near isobaric cooling. The path from the garnet zone is similar,but does not show the final phase of isobaric cooling. Both nappe-stage and dome-stage folds are observed in the OrfordvilleBelt. Comparison of mesoscale structures with mineral growthindicates that the nappe stage deformation occurred near orbefore the pressure maximum and dome stage deformation tookplace along the decompression-heating path. The last phase ofnear isobaric cooling may have resulted from rapid verticalreadjustment of the Orfordville Belt.