Metamorphism in the Adirondacks. I. Petrology, Pressure and Temperature

Grenville Supergroup sediments and suites of pre- and syn-tectonicigneous rocks have been metamorphosed to the upper amphiboliteand granulite facies in the Adirondacks of northern New Yorkduring the Grenville orogeny about one billion years ago. Magnetite-ilmenite, alkali feldspar-plagioclase, calcite-dolomiteand garnet-clinopyroxene thermometry indicate that metamorphictemperatures (T) increase from about 650 ?C in the area westand northwest of Gouverneur to 700–750 ?C near Coltonand along the Lowlands-Highlands boundary to 750–800 ?Cin areas within and around the Marcy anorthosite massif. Thepresence of grossular-rich garnet + quartz without wollastonite+ plagioclase in calc-silicate rocks and the apparent absenceof metamorphic ferropigeonite in charnockites restrict maximummetamorphic T to less than 800–850 ?C. Metamorphic pressures (P), determined from coexisting pyrite-pyrrhotite-sphalerite,garnet-rutile-sillimanite-ilmenite-quartz, fayalite-quartz-ferrosilite,fayalite-anorthite-garnet, ferrosilite-anorthite-garnet-quartz,kyanite-sillimanite, anorthite-garnet-sillimanite-quartz andthe stability of akermanite, are 6?5–7?0 kb near Gouverneurand increase to 7?5–8?0 kb in the central Adirondack Highlands. The above P-T data deduced from diverse mineralogical/chemicalsystems are interpreted as peak or near-peak conditions forAdirondack metamorphism. The compositions of thin retrograderims on garnets indicate a post-peak-metamorphic P-T path forthe Adirondacks with appreciable cooling (200–300?) beforedecompression. Peak and retrograde P-T conditions inferred forthe Adirondacks are similar to numerous other granulite terranessuggesting that similar tectonothermal events are necessaryfor the formation of many granulite belts.     

Blueschist and Eclogite transformations in Fe-Ti Gabbros: A Case from the Western Alps Ophiolites

The study of the alpine metamorphism of three suites of Fe-Timetagabbros occurring in the western Alps ophiolites has showna set of reactions governed by T, P, X_H2O, and diffusion. T-Pestimates point to 350?50?C at a minimum of 9–10 kb forthe Queyras blueschist rocks and to 450–500 ?C at a minimumof 12–13 kb for the Lanzo and Rocciavr` eclogitized rocks.These variations are the result of different T-P-time trajectoriesduring subduction/obduction events of alpine age. In the Fe-Timetagabbros, little-deformed volumes showing a crystallizationhistory controlled by local equilibrium are bounded by mylonites.Water-poor and water-rich volumes alternate during eclogitizationof the Rocciavr? suite. The persistence in the little-deformedrocks of prealpine metastable relics, of corona structures andof chemical gradients, demonstrates that a complete high pressureequilibration is inhibited by slow reaction kinetics and slowdiffusion. Only in the mylonites has the catalytic effect ofdeformation favoured an approach to bulk-rock metamorphic equilibration.In the eclogitized coronitic rocks the apparent O₂, releasefrom the alteration of the magmatic opaques plays an importantrole in reaction rates; increasing extent of eclogitizationmay be enhanced either by the release of free O₂ from the rocks,or by a process in which new H₂O formed by the combination ofoxygen with hydrogen introduced into the system.     

Phase Equilibria for Calcic Scapolite, and Implications of Variable Al-Si Disorder for P-T, T-XCO2, and a-X Relations

Carbonate scapolite is a potentially powerful mineral for calculatingCO₂ activities in non-calcareous rocks, but an analysis of thethermodynamics and phase equilibria of carbonate scapolite isfirst necessary. This includes an evaluation of Al-Si disorderin meionite, as this has the greatest effect on derived phaserelations. Available experimental data on meionite stability,X-ray diffraction refinements and nuclear magnetic resonancespectra for calcic scapolite do not uniquely constrain the Al-Siordering state of synthetic meionite. However, the data aremost consistent with a high degree of Al-Si disorder and inconsistentwith long-range Al-Si order. An internally consistent thermodynamicdata set was derived and used to calculate P-T and T-X_CO2 equilibriainvolving meionite in the CaO-Al₂O₃-SiO₂-CO₂-H₂O (CASCH) system.The effect of Al-Si disorder is illustrated by calculating thephase equilibria using an ordered, an arbitrary intermediatedisordered, and a completely Al-Si disordered standard statefor meionite. The Gibbs free energy of meionite was calculatedfrom reversals (at 790–815?C, 2–15 kb) on the reaction 3 Anorthite +Calcite =Meionite The _fG?_{m, 298} for each of the standard states is –13 146?6,–13128?8, and –130930kJ/mol, respectively. Becauseof the steep slope of reaction (1) and limited temperature rangeover which it breaks down, meionite used in the experimentsto constrain reaction (1) must possess a limited range of Al-Sidisorder. The P-T slope of reaction (1) increases, and the slopeof meionite decarbonation equilibria changes from positive tonegative in T-X_CO2 and P-T space, as a function of increasingAl-Si disorder. Meionite has a wide stability field at highT in T-X space at 5 and 10 kb (P_Total=P_Fluid), being stableto X_CO2=0?06. Meionite alone breaks down to undersaturated gehleniteand/or corundum-bearing assemblages at 5 kb, and to clinozoisiteat 10 kb. The effect of solid solutions on the T-X stabilityof meionite is similar to that of increasing pressure, stabilizingmeionite to lower temperature. Variable Al-Si disorder doesnot significantly affect the upper limit of meionite stabilityin T-X_CO2 space. Activity-composition relations for meionitein carbonate scapolite were calculated relative to reaction(1) from data on natural scapolite-plagioclase-calcite assemblages.The extent of departure from ideality varies as a function ofAl-Si disorder. Negative deviations from ideality are indicatedfor natural scapolite solid solutions at T<750?C, based ona disordered Al-Si standard state for meionite. This is likelyto reflect a more ordered Al-Si distribution in natural scapolitescompared with the synthetic endmember standard state. Present address: Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794-2100     

Eclogites and Blueschists from Northeastern Oman: Petrology and P-T Evolution

Eclogites and blueschists occur in the basement and continentalshelf deposits in the Saih Hatat tectonic window of northeasternOman, where they structurally underlie the Semail ophiolite.These eclogites and blueschists constitute part of a coherenthigh-P/T metamorphic terrain comprised of interlayered metabasites,metapelites, and quartzofeldspathic and calcareous mica schists.The metabasites record a continuous regional trend of increasingP and T from west to east, where crossite-epidote schists incentral and western Saih Hatat grade into blueschists and eclogitesat As-Sifah, allowing for the subdivision of this terrain intothree metamorphic zones. P-T conditions range from 4?5 to 5?5kb atT <340?C for zone A to >10?2 kb at 500–580?Cfor zone C. Zoning patterns of garnets, pyroxenes, and phengite, and thecompositions of pyroxene and amphibole inclusions in garnet,indicate that the eclogites and enclosing high-grade blueschistsof zone C followed a ‘clockwise’ P-T rpath of increasingP and T followed by increasing T and decreasing P and a finalstage of retrogression. Garnets and pyroxenes crystallized atdifferent stages of P-T evolution of the terrain in the variouseclogite pods and boudins. This conclusion and the contrastingmineralogies of the eclogites and the enclosing blueschistsare attributed to differences in bulk-rock chemistry, fluidavailability, or fluid compositions within the terrain. The blueschists and eclogites of this terrain formed as a resultof A-type subduction and crustal thickening of the Oman continentalmargin along an east-northeast-dipping thrust or ‘subductionzone’ which was initiated by the change in plate motionbetween Africa and Eurasia at 131 Ma. High P/T metamorphismcontinued until the final stages of ophiolite emplacement inthe late Cretaceous, as indicated by the metamorphism of thethrust sheets overlying the coherent terrain.     

Petrology of Mantle Xenoliths from Harrat al Kishb: The Mantle beneath Western Saudi Arabia

Tertiary to Recent continental rifting and sea floor spreadingformed the Red Sea. Mantle xenoliths from the Saudi ArabianRed Sea margin provide an opportunity to study the mantle beneaththe flanks of this young ocean basin. The Harrat al Kishb mantlexenolith suite consists of Cr-diopside group spinel harzburgiteand lherzolite mantle wall rock, and a variety of pyroxenitesproduced by crystallization from mafic magmas within the mantle.The pyroxenites include two texturally distinct varieties ofCr-diopside group spinel websterites, and Al-augite group spinelpyroxenite, garnet-spinel websterite, and garnet-bearing spinelclinopyroxenite. All Harrat al Kishb xenoliths are deformedto some degree and many are recrystallized. Mineral exsolutionand zoning textures indicate reequilibration to decreasing temperatureconditions. Several xenoliths provide evidence for metasomaticprocesses in the mantle beneath western Saudi Arabia. Estimates of peridotite temperatures are 900–980?C withpressure bracketed between 13 and 19 kb. Al-augite spinel pyroxenitesyield temperatures of 1050–1070?C. Garnet-spinel websteritesyield temperatures and pressures in the range 1000–1030?C,13.8–16.5 kb. These P-T estimates show that mantle temperatures are elevatedwell above those predicted by low surface heat flow measurements.Mantle heating associated with rifting is young enough thatsurface heat flow has not yet equilibrated. The xenolith dataare consistent with a model of asthenosphere upwelling beneaththe Red Sea rift. Comparison of xenolith data with existingseismic refraction data reveals a coherent picture of the compositionof the western Saudi Arabian lithosphere.     

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.     

Proterozoic Granulites from the Rauer Group, East Antarctica. I. Decompressional Pressure-Temperature Paths Deduced from Mafic and Felsic Gneisses

Granulites from the Rauer Group, East Antarctica, were metamorphosedat 860?40?C during a high-grade tectonothermal episode youngerthan 1400 Ma and probably close to 1000 Ma in age. A spatialvariation of pressures of metamorphism at the thermal peak iscalculated for felsic and mafic granulites preserving garnet-orthopyroxene-plagioclaseassemblages with or without additional clinopyroxene and quartz.Pressures of 6 to 7.5 kb are derived for the northern partsof the Rauer Group, whereas 7–8?5 kb pressures are calculatedfor similar granulites some 10–20 km further south. Post-deformational reaction textures including orthopyroxene-plagioclasesymplectites after garnet in basic granulites and plagioclasemoats or rims on garnet and orthopyroxene in felsic granulitesindicate a decompressional pressure-temperature-time evolution(P-T-t) which is confirmed by garnet-orthopyroxene-plagioclase-quartzand garnet-orthopyroxene barometry of zoned and regrown minerals.A pervasive decompression through c. 2 to 3–5 kb in thenorthern Rauer Group and to 5–6 kb in the southern partof the region occurred at temperatures above 700?C and probablyin excess of 750?C. This P-T evolution, which indicates a uniformunroofing of some 6–9 km while quite high mid- to lower-crustaltemperatures only decreased by c. 100?C, is consistent withthe later stages of a prolonged collision-related thermal evolution.Comparisons of the P-T-t paths of the late Proterozoic granulitesfrom the Rauer Group and elsewhere in East Antarctica with calculatedP-T paths for simple collisional models where erosion terminatesthe heating phase show that externally- derived magmatic additionsand an enhanced total heat budget are necessary to produce theobserved high-temperature evolution.     

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     

Empirical Garnet-Biotite-Plagioclase-Quartz (GBPQ) Geobarometry in Medium- to High-Grade Metapelites

On the basis of the net transfer reactions among garnet, biotite,plagioclase and quartz (for both Mg and Fe end-member models),the garnet–biotite–plagioclase–quartz (GBPQ)geobarometer was empirically calibrated under physical conditionsof P = 1·0–11·4 kbar and T = 515–878°C,based on the input garnet–biotite temperatures and garnet–aluminosilicate–plagioclase–quartz(GASP) pressures of 224 natural aluminosilicate-bearing metapeliticsamples collated from the literature. The calibrations are internallyconsistent with the asymmetric quaternary solid solution modelof garnet, the symmetric quaternary solid solution model ofbiotite, and the Al-avoidance ternary solid solution model ofplagioclase in calibrating the garnet–biotite geothermometerand the GASP geobarometer. The resulting two GBPQ barometerformulae reproduce the input GASP pressures well within ±1·0kbar (mostly within ±0·5 kbar). For both aluminosilicate-bearingand aluminosilicate-absent metapelites, the two GBPQ barometryformulae yielded identical pressures, whether the sample wasincluded or not included in calibrating the GBPQ barometry.The random error of the GBPQ barometry may be expected as ±1·2kbar. The dP/dT slopes of these two GBPQ formulae are closeto that of the GASP barometer in P–T space. Applicationsof the GBPQ barometry of aluminosilicate-absent metapelitesto the rocks within a thermal contact aureole, or rocks withina limited geographical area without post-metamorphic structuraldiscontinuity, show no obvious pressure change. It may be concludedthat the two GBPQ barometry formulae derived in this study maybe used as practical tools for metamorphic pelites under theconditions of 515–878°C and 1·0–11·4kbar, in the composition range of X_gros >3% in garnet, X_an>17% in plagioclase, and     

Stability and Phase Relations of Trioctahedral Calcium Brittle Micas (Clintonite Group)

On the pseudobinary join CaO:3MgO:Al₂O₃:2SiO₂:_xH2O–CaO:1.25MgO:2.75 Al₂O₃: 0.25SiO₂:_xH₂O clintonite mixed crystals Ca(Mg_{1+ x}Al_{2 – x}) (Al_{4 – x}Si_xO₁₀)(OH)₂ with x rangingfrom 0.6 to 1.4 occur in the temperature range 600–830?C, 2 kb fluid pressure. On the MgSirich side clintonites coexistwith chlorite, forsterite, diopside, and calcite (due to smallamounts of CO₂ in the gas phase) and, at lower temperatures,also with idocrase, hydrogrossularite, and aluminous serpentine.Decomposition of clintonite over a divariant temperature rangeoccurs above 830 ?C, 2 kb; clintonite-free subsolidus assemblagescomprising three or four solid phases are formed in the temperatureranges 890 ?–1120 ?C. The subsolidus assemblages can berepresented in a polyhedron defined by the corners forsterite,diopside, melilite, spinel, anorthite, corundum, and calciumdialuminate. Above 1120 ?C partial melting occurs. The upper thermal stability limits of three selected compositionshave been reversed in the P-T range 0.5–20 kb and 730–1050 ?C, respectively. Below some 4 kb breakdown is dueto the divariant reactions: (1)Ca(Mg_2.25Al_0.75)(Al_2.75)(Si_1.25O₁₀)(OH)₂ spinel+diopside_ss+forsterite+clintonite_ss+vapor, (2)Ca(Mg₂Al)(Al₃SiO₁₀)(OH)₂ spinelx002B;melilite_ss+anorthite+clintonite_ss+vapor, (3)Ca(Mg_1.75Al_1.25)(Al_3.25)(Si_0.75O₁₀)(OH)₂ spinel+melilite_ss+corundum+clintonite_ss+vapor, At the terminations of the divariant temperature ranges (1)melilite_ss, (2) diopside_ss, and (3) anorthite enter those assemblagesand clintonitess disappears completely. The reactions can berepresented by the following equations (1)log,_H2O = 10.2879–8113/T+0.0856(P–1)/T, (2)log = 9.5852–7325/T+0.0794(P–1)/T, (3)log = 7.8358–5250/T+0.077(P–1)/T, with P expressed in bars and Tin ?K. Above 4 kb the upper thermalstability limit of clintonite is defined by incongruent melting,with grossularite participating at pressures above 9 kb. Thesecurves exhibit a very steep, probably even negative slope inthe P-T diagram. There is a close correspondence between natural clintonite-bearingassemblages and thosefound experimentally. The rarity of clintonitein nature is not due to special conditions of pressure and temperaturebut rather due to special bulk compositions of the rocks.     

Petro-tectonic Imprints in the Sapphirine Granulites from Anantagiri, Eastern Ghats Mobile Belt, India

Sapphirine granulite occurring as lenses in charnockite at Anantagiri,Eastern Ghat, India, displays an array of minerals which developedunder different P-T-X conditions. Reaction textures in conjunctionwith mineral chemical data attest to several Fe-Mg continuousreactions, such as

1. spinel+rutile+quartz+MgFe_–1=sapphirine+ilmenite
2. cordierite=sapphirine+quartz+MgFe_–1
3. sapphirine+quartz=orthopyroxene+sillimanite+MgFe_–1
4. orthopyroxene+sapphirine+quartz=garnet+MgFe_–1
5. orthopyroxene+sillimanite=garnet+quartz+MgFe_–1
6. orthopyroxene+sillimanite+quartz+MgFe_–1=cordierite.

Calculated positions of the reaction curves in P-T space, togetherwith discrete P-T points obtained through geothermobarometryin sapphirine granulite and the closely associated charnockiteand mafic granulite, define an anticlockwise P-T trajectory.This comprises a high-T/P prograde metamorphic path which culminatedin a pressure regime of 8?3 kb above 950?C, a nearly isobariccooling (IBC) path (from 950?C, 8?3 kb, to 675?C, 7?5kb) anda terminal decompressive path (from 7?5 to 4?5 kb). Spinel,quartz, high-Mg cordierite, and sapphirine were stabilized duringthe prograde high-T/P metamorphism, followed by the developmentof orthopyroxene, sillimanite, and garnet during the IBC. Retrogradelow-Mg cordierite appeared as a consequence of decompressionin the sapphirine granulite. Deformational structures, reportedfrom the Eastern Ghat granulites, and the available geochronologicaldata indicate that prograde metamorphism could have occurredat 30001?00 and 2500?100 Ma during a compressive orogeny thatwas associated with high heat influx through mafic magmatism. IBC ensued from P_max and was thus a direct consequence of progrademetamorphism. However, in the absence of sufficient study onthe spatial variation in P-T paths and the strain historiesin relation to time, the linkage between IBC and isothermaldecompression (ITD) has remained obscure. A prolonged IBC followedby ITD could be the consequence of one extensional mechanismwhich had an insufficient acceleration at the early stage, orITD separately could be caused by an unrelated extensional tectonism.The complex cooled nearly isobarically from 2500 Ma. It sufferedrapid decompression accompanied by anorthosite and alkalinemagmatism at 1400–1000 Ma.     

Petrology and Geochronology of Granulites from the McKaskle Hills, Eastern Amery Ice Shelf, Antarctica, and Implications for the Evolution of the Prydz Belt

A combined petrological and geochronological study was carriedout on mafic granulites and associated felsic gneisses fromthe McKaskle Hills, eastern Amery Ice Shelf, East Antarctica.Garnet-bearing mafic granulites exhibit reaction textures andexsolution textures that indicate two-stage metamorphic evolution.Thermobarometric estimates from matrix and symplectite assemblagesyield peak and retrograde P–T conditions of 9·0–9·5kbar and 880–950°C and 6·6–7·2kbar and 700–750°C, respectively. Similar but slightlyscattered peak P–T estimates of 7·9–10·1kbar and 820–980°C are obtained from the core compositionsof minerals from felsic para- and orthogneisses. Evidence forthe prograde history is provided by muscovite inclusions ingarnet from a paragneiss. Sensitive high-resolution ion microprobeU–Pb zircon dating reveals an evolutionary history forthe granulites, including a mafic and felsic igneous intrusionat 1174–1019 Ma, sedimentation after 932–916 Ma,and a high-grade metamorphism at 533–529 Ma. In contrast,Sm–Nd mineral–whole-rock dating mainly yields asingle age population at 500 Ma. This suggests that the McKaskleHills form part of the Prydz Belt, and that the relatively highpeak P–T conditions and a decompression-dominated P–Tpath for the rocks resulted from a single Cambrian metamorphiccycle, rather than two distinct metamorphic events as formerlyinferred for the granulites from Prydz Bay. The age data alsoindicate that the Precambrian history of the McKaskle Hillsis not only distinct from that of the early Neoproterozoic terranein the northern Prince Charles Mountains, but also differentfrom that of other parts of the Prydz Belt. The existence ofmultiple basement terranes, together with considerable crustalthickening followed by tectonic uplift and unroofing indicatedby the clockwise P–T–t evolution, suggests thatthe Prydz Belt may represent a collisional orogen that resultedin the assembly of Gondwana during the Cambrian period. KEY WORDS: Mesoproterozoic basement; Cambrian metamorphism; P–T path; Prydz Belt; East Antarctica     

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     

Eclogites and Associated Albite-Epidote-Garnet Paragneisses between Yambe and Cape Colnett, New Caledonia

High-pressure metamorphic rocks form a coastal belt, 175 kmby 35 km, in northeastern New Caledonia. Metamorphic grade rangesfrom lawsonite-albite schists through glaucophane-epidote schiststo omphacite-garnet-quartz gneisses. In the eclogitic terrane,metabasites, locally containing relict pillow structure andigneous textures, with well-preserved eclogitic mineral assemblages,are intercalated with metasedimentary gneisses containing albite-epidote-garnet? glaucophane and barroisite. Omphacite is partly retrogressedto albite and ferromagnesian minerals in almost every paragneiss.The paragneisses show strong evidence of penetrative foldingand microfracturing and were more permeable to metamorphic fluidsthan were the metabasites. The metabasites are inferred to havebeen relatively ‘dry’ and free of penetrative deformationduring the latter stages of metamorphism and thus were preservedmetastably during uplift, erosion, and cooling. Fe-Mg exchange thermometry between omphacite and garnet suggeststemperatures between 520 and 600 ?C. Omphacite + quartz (molper cent jadeite = 37–43) does not coexist stably withalbite suggesting minimum pressures near 12 kb at 550 ?C. Remnantsof more jadeite-rich pyroxenes in paragneisses (jd_50–60)suggest even higher pressure. The stable coexistence of chloritoidalmandine-quartz in paragneisses suggests relatively H₂O-rich fluids werein equilibrium with this assemblage. The widespread stable occurrenceof sphene suggests relatively low f_co2 during metamorphism.Late stage healed fractures in quartz contain H₂O-rich fluidinclusions with relatively low density isochores. Limited geochronologicdata combined with these petrologic data suggest a fairly rapidinitial rate of uplift followed by a much slower rate of uplift     

Zoned Poikiloblastic Garnets: P-T Paths and Syn-Metamorphic Uplift through 30 km of Structural Depth, Wopmay Orogen, Canada

In the early Proterozoic Wopmay Orogen (Northwest Territories,Canada), the occurrence of garnet-biotite-sillimanite/kyanite-plagioclase-quartzassemblages in pelitic schists at a variety of obliquely exposedstructural levels enables the use of calibrated geothermometersand geobarometers through 30 km of composite structural relief.Direct derivation of multipoint P-T paths from single garnetsis attained from core-to-rim microprobe analyses of zoned poikiloblasticgarnets, which contain biotite, plagioclase, quartz, and lesscommonly Al₂SiO₅ inclusions. The documented garnet zoning andthe entrapment of the mineral inclusions is compatible withpartial-equilibrium growth models. The lack of significant diffusionre-equilibration in the garnet interiors is favored by samplerestriction to medium-grade schists and is attested by the preservationof normal-zoning profiles, the lack of garnet diffusion babesaround biotite inclusions, the matching composition trends ofgarnet-core to -rim plagioclase inclusions with those of zonedmatrix plagioclase grains, and the systematic variation of thederived P-T data. Only the garnet rims, which are characterizedby a reversal of compositional trends and by textural resorption,are interpreted to indicate local post-thermal-peak re-equilibration. The calculated P-T paths quantify segments of uplift trajectoriescorresponding to pressure drops of 2?5–1?5 kb from maximaof 9?3–5?0 kb depending on structural level. This is concurrentwith initial increases of 25–75?C to peak-temperatureconditions and is followed by variable drops in temperatureduring continued decompression. Individual paths are consistentwith modelled variations of metamorphic conditions as a functionof loading, uplift, and erosion in overthrust terrains. Consideredwith U-Pb zircon geochronological data the P-T paths, studiedas a set, indicate an average uplift rate that varies spatiallyfrom 1?5–2?7 mm y^–1. This variation can be relatedto late cross folding of the orogenic internal zone, suggestingthat the syn-metamorphic uplift was structurally controlled.The distribution of peak-temperature conditions attained duringdecompression is independent of structural depth. This, andthe inverted metamorphism documented in the Wopmay Orogen, requirethat final variations in temperature result from thermal relaxationof isotherms in, and away from, a hot crustal allochthon composedin part of high-T rift-fill units and a syntectonic graniticbatholith.     

Provenance and Magmatic-Metamorphic Evolution of a Variscan Island-Arc Complex: Constraints from U-Pb Dating, Petrology, and Geospeedometry of the Kyffhauser Crystalline Complex, Central Germany

The Kyffhäuser Crystalline Complex, Central Germany, formspart of the Mid-German Crystalline Rise, which is assumed torepresent the Variscan collision zone between the East Avalonianterrane and the Armorican terrane assemblage. High-precisionU–Pb zircon and monazite dating indicates that sedimentaryrocks of the Kyffhäuser Crystalline Complex are youngerthan c. 470 Ma and were intruded by gabbros and diorites between345 ± 4 and 340 ± 1 Ma. These intrusions had magmatictemperatures between 850 and 900°C, and caused a contactmetamorphic overprint of the sediments at P–T conditionsof 690–750°C and 5–7 kbar, corresponding toan intrusion depth of 19–25 km. At 337 ± 1 Ma themagmatic–metamorphic suite was intruded by granites, syenitesand diorites at a shallow crustal level of some 7–11 km.This is inferred from a diorite, and conforms to P–T pathsobtained from the metasediments, indicating a nearly isothermaldecompression from 5–7 to 2–4 kbar at 690–750°C.Subsequently, the metamorphic–magmatic sequence underwentaccelerated cooling to below 400°C, as constrained by garnetgeospeedometry and a previously published K–Ar muscoviteage of 333 ± 7 Ma. With respect to P–T–D–tdata from surrounding units, rapid exhumation of the KCC canbe interpreted to result from NW-directed crustal shorteningduring the Viséan. KEY WORDS: contact metamorphism; U–Pb dating; hornblende; garnet; Mid-German Crystalline Rise; P–T pseudosection     

The Genesis of Zoned Skarns in the Sierra Nevada, California

Zoned skarns occur at plutonic-metamorphic contacts, in veinscutting marble, and at contacts between marble and interlayeredamphibolite and biotite-rich rocks. For P = 2 kb, fluid inclusionsand P-T-X_CO2 stability relations of calc-silicate assemblagessuggest T< 650 °C and a H₂O-rich fluid (X_CO2 < 0.1).Small-scale, Ca-rich endoskarns are common near exoskarns. Massbalance calculations suggest that: (a) the formation of exoskarnrequires the influx of solute in an aqueous solution from uncontaminatedmagma in addition to material derived from the endoskarn, (b)some ‘limestone assimilation’ is required to formendoskarns, and (c) skarn formation was essentially a constant-volumeprocess. Applying chromatographic theory, compositional profilesof garnet and pyroxene across zoned skarns suggest that infiltrationmetasomatism was an important process, although diffusion metasomatismappears to have produced local compositional gradients at theinfiltration ‘fronts’. Fluid flow calculations showthat thick exoskarns could readily form by intergranular infiltration of aqueous solutions. Reciprocal diffusional exchangeis suggested as a dominant mechanism in the formation of zonedskarns formed at contacts between interlayered metamorphic lithologies.     

Eclogite Facies Regional Metamorphism of Hydrous Mafic Rocks in the Central Alpine Adula Nappe

The Adula Nappe is a slice of Pre-Mesozoic continental basementaffected by Early Alpine (Mesozoic or Lower Tertiary) high-pressuremetamorphism. Mineral compositions in mafic rocks containingomphacite + garnet + quartz record a continuous regional trendof increasing recrystalliza tion temperatures and pressuresthat can be ascribed to this regional high-pressure metamorphicevent. P-T estimates derived from mineral compositions gradefrom about 12 kb and 500 ?C or less in the north of the nappeto more than 20 kb/800 ?C in the south. The regional P-T trend is associated with a mineralogical transitionfrom assemblages containing additional albite and abundant amphiboles,epidote minerals, and white micas in the north (omphacite-garnetamphibolites) to kyanite eclogites containing smaller amountsof hornblende and zoi.site in the south. Textures and mineralcompositional data show that these hydrous and anhydrous silicatesassociated with omphacite + garnet + quartz arc primary partsof the high-pressure assem blages. Observed phase relationsbetween these primary silicates, theoretical Schreinemakersanalysis, and the thermobarometric results, together indicatethat the regional transition from omphacite amphibolites tokyanite eclogites can be described by two simplified reactions: alb+epi+hbl=omp+kya+qtz+par (H₂O-conserving) (15) par+epi+hbl+qtz=omp+kya+H₂O (dehydration) (12) which have the character of isograd reactions. Local variations of water activity (a_H2O) as indicated by isofacialmineral assemblages, and the H₂O character of the reaction (15),are interpreted to reflect largely H and predominantly fluid-absenthigh-pressure metamorphism within the northern part of the nappe.The omphacite amphibolites and paragonite eclogites in thisarea are thought to have formed by H₂O reactions from Pre-Mesozoichigh-grade amphibolites, i.e. from protoliths of similar bulkH₂O-countent. The second ‘isograd’ (12) is interpreted to markthe regional transition from largely fluid-absent metamorphismin the north to fluid-present metamorphism in the south, wheremetamorphic pressures and temperatures in excess of 12-15kband 500-600?C were sufficient for prograde in-situ dehydrationof similar hydrous protoliths to kyanite eclogites. The observationof abundant veins, filled with quartz+kyanite+omphacite, suggeststhat a free fluid coexisted locally with the kyanite eclogitesof the southern Adula Nappe at some time during progressivedehydration.     

The Influence of Retrograde Cation Exchange on Granulite P-T Estimates and a Convergence Technique for the Recovery of Peak Metamorphic Conditions

Chemical relationships in garnet-orthopyroxene-plagioclase-quartzrocks are governed principally by three equilibria: the Fe-Mgexchange reaction between garnet and orthopyroxene, the solubilityof alumina in orthopyroxene coexisting with garnet, and thereaction of garnet and quartz to form orthopyroxene and plagioclase.Various thermobarometric calibrations of these equilibria havebeen applied to granulite-facies gneisses from two areas ofthe Proterozoic Complex of East Antarctica, and a wide rangeof P-T estimates is obtained for each area. Some of this P-Tvariation reflects the different thermodynamic data and mineralmixing models used by each calibration, but other differencesare attributed to the effects of retrograde Fe-Mg exchange.An inter-specimen spread of temperatures in each area, obtainedfor mineral core compositions with a single calibration of thegarnet-orthopyroxene exchange reaction, is attributed to a variableextent of Fe-Mg exchange on cooling from peak metamorphic conditions.A similar spread of pressures from the garnet-orthopyroxenealumina solubility barometer indicates that this calibrationis also reset by retrograde Fe-Mg exchange. In contrast, pressuresfrom the garnet-orthopyroxene-plagioclase-quartz barometer formineral cores show little variation between specimens from thesame area, indicating that this equilibrium is relatively insensitiveto changes in the Fe-Mg distribution coefficient and that derivedpressures are more likely to reflect peak metamorphic conditionsthan those from the alumina solubility barometer. Temperaturescan be corrected for Fe-Mg exchange using the Fe-Mg distributioncoefficient required to bring pressures from the exchange-sensitivealumina solubility barometer into agreement with reference pressurescalculated from the exchange-insensitive garnet-orthopyroxene-plagioclase-quartzbarometer. These corrected temperatures are closure temperaturesfor Al diffusion, which in many cases are likely to be goodestimates for the peak metamorphic temperature. The extent oftemperature correction in these specimens is 0–140C,and can be qualitatively related to textural features such asgrain size and mutual proximity of garnet and orthopyroxenegrains. Retrograde Fe-Mg exchange has clearly been significantin these rocks, with major consequences for thermobarometry.It is likely that Fe-Mg exchange during cooling is more widespreadthan currently recognized, and that the suggested convergencemethod for retrieving peak metamorphic conditions is applicableto other granulite terrains.     

Petrology of Felsic Granulites, Metapelites, Metabasics, Ultramafics, and Metacarbonates from Southern Calabria (Italy): Prograde Metamorphism, Uplift and Cooling of a Former Lower Crust

A high-grade metamorphic terrane in the southern part of theCalabrian massif (South Italy) has been petrographically mappedand the dominant rock types petrologically investigated. Bothmethods of investigation have led to the recognition of a continuoussection through a former lower crust which is 7 km thick. Itslower part consists predominantly of metabasic rocks togetherwith minor felsic granulites, its upper part of metapeliteswith minor metabasic and metacarbonate rocks. The rocks experienced a common two-stage prograde metamorphicevolution in which the second stage occurred after the lastpenetrative deformation. The prograde metamorphism which, accordingto radiometric dates, ended in late Hercynian time, was of themedium-pressure type of Miyashiro (1961), and equilibrationoccurred in the ‘medium-pressure granulite field’(characterized by the instability of olivine-plagioclase aswell as garnet-clinopyroxene-quartz). Estimates of the highest_P—_T conditions of prograde metamorphism give 7–8kb and approximately 800°C at the base, but 5–6 kband 650–700°C at the top of the section, at whichthe paragenesis staurolite-quartz indicates the transition tothe amphibolite facies. The existence of a metamorphic gradientin the lower crust section is demonstrated by the systematicchange in the compositions of ferro-magnesian minerals in divariantmetapelitic assemblages. The metamorphic evolution during the excavation history of theformer lower crust has been reconstructed using the numerousdisequilibrium reaction textures preserved in most rock types.The highest metamorphic conditions ended with a pressure decreaseof approximately 1.5 to 2 kb, which was followed by a periodof quasi-isobaric cooling in the middle crust. During this cooling,the stability field of the ‘high-pressure granulites’(garnet-clinopyroxene-quartz) was reached. The pressure decrease, which induced the end of the high-temperaturehistory of the lower crust, is interpreted as reflecting theerosion of the uppermost crustal levels as a response to overlappingof large crustal segments during the Hercynian orogeny. Consequently,the deduced P—T path of the upper, i.e. overthrust crustalsegment is thought to have been tectonically controlled.