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.     

Metamorphic, Thermal, and Tectonic Evolution of Central New England

A new, detailed tectonic model is presented for the Acadianorogenic belt of central New England (Vermont and New Hampshire)that accounts for a wide range of petrological and structuralobservations. Three belts are considered: the Eastern Vermont,Merrimack, and intervening Bronson Hill belts. Specific observationsin eastern Vermont that are accounted for in the model includethe following. P–T paths are clockwise with maximum pressuresnear the Athens, Chester, and Strafford domes of 8–11kbar, but with maximum pressures decreasing to 3–5 kbarat the boundary with the Bronson Hill belt. Differential exhumationof the Vermont domes relative to the rocks in easternmost Vermontis required by the recorded differences in maximum pressure(5–6 kbar; 15–20 km) and the present-day geographicalseparation (7–10 km). Specific observations in New Hampshirethat are explained include the following. P–T paths inthe Merrimack belt are counter-clockwise with maximum pressuresof 4–5 kbar and are related to high regional heat flowand heat transfer by early Acadian plutons. P–T pathsin the Bronson Hill belt are intimately associated with structuralposition. An early contact metamorphism is evidenced in theSkitchewaug and Fall Mountain nappes near contacts with theearly Acadian Bethlehem gneiss (     

Structural effects on the Proterozoic Ullensvang Group (West Norway) relatable to forceful emplacement of expanding plutons

An isolated synclinorium, comprising the up to 5 km thick Ullensvang Group of metavolcanic and metasedimentary rocks, is surrounded by granitoid plutons and has no recognizable older basement. From an earlier interpretation based on regional data this area is inferred to be part of a Proterozoic, cordilleran-type volcanic-plutonic belt. Major structures in the synclinorium are interpreted to have evolved as follows: 1) Early folds were produced by regional compression; 2) within a part of the synclinorium, such folds were tightened and reoriented during a subsequent deformation phase caused by forceful emplacement of the Kvinnherad batholith, flanking the synclinorium to the southwest; 3) this deformation was accompanied by thrust faulting towards the northeast, away from the batholith; 4) forceful emplacement of plutons belonging to the Eidfjord-Kinsarvik batholith, on the northeastern flank of the synclinorium, produced southwest-vergent overfolding of the earlier structures in the synclinorium. In addition, local deformation around a pluton intruding the northern part of the synclinorium near Utne produced areally restricted deformation structures, approximately corresponding in time with the second deformation event (2) above. Structures in plutonics rocks of the batholiths are interpreted in terms of an emplacement model involving distension diapirism. The deformation zones associated with diapiric plutons in this area appear to be more similar to some of those reported from Archaean greenstone belts than to those found in volcanic-plutonic belts of younger cordilleran-type orogens.     

Transformation of Spinel Lherzolite to Garnet Lherzolite in Ultramafic Lenses of the Austridic Crystalline Complex, Northern Italy

Numerous lenticular bodies of ultramafic rocks occur withinthe upper amphibolite- to granulitefacies metamorphic terraneof the Austrides between the Non and Ultimo valleys (Nonsbergregion), northern Italy. The ultramafic rocks are divided intotwo textural types: (a) coarse-type; and (b) finetype. The coarse-typerocks have the protogranular texture and are predominantly spinellherzolite. Some coarse-type spinel lherzolites have partlytransformed to garnet lherzolite. The fine-types are consideredto be metamorphic derivatives of the former, and the observedmineral assemblages are: (1) olivine + orthopyroxene + clinopyroxene+ garnet + amphibole ? spinel, (2) olivine + orthopyroxene +garnet + amphibole + spinel; (3) olivine + orthopyroxene + amphibole+ spinel; and (4) olivine+ orthopyroxene + amphibole + chlorite.Based on the microprobe analyses of constituent minerals fromten representative peridotite samples, physical conditions ofthe metamorphism, particularly that of the spinel to garnetlherzolite transformation, are estimated. Applications of pyroxenegeothermometry yield temperature estimates of 1100–1300?Cfor the formation of the primary spinel lherzolite, and 700–800?Cfor that of the fine-type peridotites. A pressure range of 16–28kb is obtained for the garnet lherzolite crystallization dependingon the choice of geobarometers. Two alternative P-T paths, i.e.(1) isobaric cooling or (2) pressure-increase and temperaturedecrease are considered and their geodynamic implications discussed.     

A tertiary fold and thrust belt in the Valle del Cauca Basin, Colombian Andes

Surface geology and heophysical data, supplemented by regional structural interpretations, indicate that the Valle del Cauca basin and adjacent areas in west-central Colombia form a west-vergent, basement-involved fold and thrust belt. This belt is part of a Cenozoic orogen developed along the west side of the Romeral fault system. Structural analysis and geometrical constraints show that the Mesozoic ophiolitic basement and its Cenozoic sedimentary cover are involved in a “thick-skinned” west-vergent foreland style deformation. The rocks are transported and shortened by deeply rooted thrust faults and stacked in imbricate fashion. The faults have a NE---SW regional trend, are listric in shape, developed as splay faults which are interpreted as joining a common detachment at over 10 km depth. The faults carry Paleogene sedimentary strata and Cretaceous basement rocks westward over Miocene strata of the Valle del Cauca Basin. Fold axes trend parallel or sub parallel to the thrust faults. The folds are westwardly asymmetrical with parallel to kink geometry, and are interpreted to be fault-propagation folds stacked in an imbricate thrust system. Stratigraphic evidence suggests that the Valle del Cauca basin was deformed between Oligocene and upper Miocene time. The kinematic history outlined above is consistent with an oblique convergence between the Panama and South American plates during the Cenozoic.A negative residual Bouguer anomaly of 20–70 mgls in the central part of the Valle del Cauca basin indicates that a substantial volume of low density sedimentary rocks is concealed beneath the thrust sheets exposed at the land surface. The hydrocarbon potential of the Valle del Cauca should be reevaluated in light of the structural interpretations presented in this paper.     

中条山早元古代变质岩石的PTt轨迹和构造演化

中条山早元古代变质岩石呈北东向分布，经历了中低级变质作用。本文在变质作用研究的基础上，结合地质构造和显微构造，利用近来发展的石榴石环带定量测量Ｐ－Ｔ轨迹的技术，建立了绛县群和中条群泥质岩石经历的ＰＴｔ轨迹。ＰＴｔ轨迹表明绛县群经历拉张作用，地壳发生抬升，产生绿片岩相变质；在中条第Ⅰ期运动中，地壳略有增厚或不变升温和抬升，产生低角闪岩相变质；而中条群岩石经历低绿片岩相埋深变质后，于中条第Ⅰ期运动中，     

P-T Conditions of Metamorphism in the Neoproterozoic Rocks of the Negele Area, Southern Ethiopia

Neoproterozoic rocks constitute the Kenticha, Alghe and Bulbul litho-tectonic domains in the Negele area of southern Ethiopia. Structural features and fabrics in these rocks were developed during north-south folding (D1), thrusting (D2) and shearing (D3) deformation. From micro-structural inferences and fabric relationships in semi-pelitic schists/gneisses of the Kenticha and Alghe domains, three episodes of metamorphic mineral growths (M1, M2 and M3) are inferred to have accompanied the deformational events. Pressure-Temperature estimates on equilibrium garnet-plagioclase-biotite and garnet-biotite assemblages from semi-pelitic schists/gneisses of the two domains indicate metamorphic recrystallization at temperatures of 520–580°C and 590–640°C, and pressures of 4–5 kb and 6–7 kb in the Kenticha and Alghe domains, respectively. These results correspond to regional metamorphism at a depth of 16–20 km for the Kenticha and 22–25 km for the Alghe domains. The P-T results suggest that the protoliths to the rocks of the Kenticha and Alghe domains were subjected to metamorphism at different crustal depths. This implies exhumation of the Alghe gneissic rocks from intermediate crustal level (ca. 25 km) before juxtaposition with the Kenticha sequence along a north-south trending thrust at the present crustal level during the Neoproterozoic. The combined deformation, fabric and mineral growth data suggest that rocks in the Kenticha and Alghe domains evolved under similar tectono-metamorphic conditions, which resulted from crustal thickening and uplift followed by extension and orogenic collapse, exhumation and cooling before litho-tectonic domains coalesced and cratonized in the Neoproterozoic southern Ethiopian segment of the East African Orogen.     

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.     

Exhumation Paths of High-Pressure-Low-Temperature Metamorphic Rocks from the Lycian Nappes and the Menderes Massif (SW Turkey): a Multi-Equilibrium Approach

The Menderes Massif and the overlying Lycian Nappes occupy anextensive area of SW Turkey where high-pressure–low-temperaturemetamorphic rocks occur. Precise retrograde P–T pathsreflecting the tectonic mechanisms responsible for the exhumationof these high-pressure–low-temperature rocks can be constrainedwith multi-equilibrium P–T estimates relying on localequilibria. Whereas a simple isothermal decompression is documentedfor the exhumation of high-pressure parageneses from the southernMenderes Massif, various P–T paths are observed in theoverlying Karaova Formation of the Lycian Nappes. In the uppermostlevels of this unit, far from the contact with the MenderesMassif, all P–T estimates depict cooling decompressionpaths. These high-pressure cooling paths are associated withtop-to-the-NNE movements related to the Akçakaya shearzone, located at the top of the Karaova Formation. This zoneof strain localization is a local intra-nappe contact that wasactive in the early stages of exhumation of the high-pressurerocks. In contrast, at the base of the Karaova Formation, alongthe contact with the Menderes Massif, P–T calculationsshow decompressional heating exhumation paths. These paths areassociated with severe deformation characterized by top-to-the-eastshearing related to a major shear zone (the Gerit shear zone)that reflects late exhumation of high-pressure parageneses underwarmer conditions. KEY WORDS: exhumation; high-pressure–low-temperature metamorphism; multi-equilibrium P–T estimates; Lycian Nappes; Menderes Massif     

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.     

Metamorphism during Ophiolite Emplacement the Petrology of the St. Anthony Complex

The St. Anthony Complex consists of the White Hills Peridotite(the ultramafic remnant of an ophiolite), and an underlyingmetamorphic aureole which grades from metagabbro and hornblendegranulite through amphibolite, epidote amphibolite, and greenschistinto undeformed volcanic rocks at the base of the sequence.The petrology and mineralogy of the complex show that the basalperidotite mylonites recrystallized at 900–950 °C,the coronitic metagabbros at 850–900 °C, two-pyroxeneamphibolites at 860 °C, marbles at 680 °C, epidote amphibolitesat 550–650 °C, and greenschists at 350–550 °C.The metamorphic pressures range from 7–10 kb at the peridotitecontact to 3–5 kb in the greenschists and amphibolites,and less than 2 kb in the volcanics. The geology suggests thatthe metamorphic rocks originally formed from a strip of turbidites,alkali basalts, tholeiites, and gabbros adjacent to a continentalmargin. This sequence was subjected to dynamothermal metamorphismresulting from a combination of conduction of heat from theoverlying ultramafic rocks and shear heating. The rocks weregradually accreted onto the base of the overriding ophiolitesheet, forming the composite metamorphic sequence now observed.     

Thrust kinematics and transposition fabrics from a basal detachment zone, eastern Australia

Emplacement of an upper crustal, leading imbricate-fan thrust belt in the Lachlan Fold Belt of eastern Australia was accomplished along a 0.5–1 km thick zone of heterogeneously deformed, low grade phyllonite in pelitic rock. Continuous recrystallization and neocrystallization of mica in a zone of transposition layering has provided a weak zone at the base of a 100 km wide × 150 km exposed length × 10 km thick thrust system. The basal deformation zone is characterized by a low-moderately dipping, strong-intense transposition foliation enclosing elongate fault-bounded slices (up to 20 km long × 5 km wide in map view) of disrupted Cambrian metavolcanics and Upper Ordovician black shales and slates. These are derived from a structurally lower zone of duplexing or from the overturned limbs of anticlinorial structures. The detachment zone is a 10–15 km wide zone of intense deformation showing a transition from open, upright folds with weak cleavage to inclined, tightisoclinal folds with strong axial surface cleavage. The intensity of minor faults also increases into the zone. Leading imbricate fan thrust belts show maximum deformation effects along the basal detachment which forms the frontal or leading fault. The leading imbricate geometry is due to emplacement of the basal detachment zone up the lowest and last formed imbricate thrust. Movement is along a relatively ductile, low viscosity ‘layer’ at the base where strain softening occurs with development of transposition layering. This enables confined ‘flow’ along the basal zone with transport and emplacement of the fold system and duplex zone to higher structural levels. Reaction-enhanced ductility and grain boundary sliding may be important deformation mechanisms responsible for this flow. Localized polydeformation, marked by mesofolds and crenulation cleavage, reflects the interaction between thrust sheets and the movement on faults.     

Perspectives of phosphorite mineralization in Lower Paleozoic rocks of the Zilair synclinorium,Southern Urals

Data on the phosphorite mineralization, lithology, and variability of Lower Paleozoic rocks in the Zilair synclinorium and Yuryuzan basin are given. Comparison of Lower Paleozoic rocks with type sections of phosphate-bearing rocks in the world suggested their compliance with the phosphorite-enclosing rocks of the fine-grained phosphorite association. Analysis of the facies variability of rocks indicate the possibility of discovery of large phosphorite deposits in the Yuryuzan and Syuren subzones of the Western structural-facies zone. Phosphorite deposits are unknown in the synclinorium because of their instability during hypergenesis and burial under allochthonous loose sediments of the Cenozoic.     

Geochemical and High Pressure Experimental Studies of Garnet Pyroxenite and Pyroxene Granulite Xenoliths from the Delegate Basaltic Pipes, Australia

Subsolidus phase relationships have been determined to pressuresof 15–27 kb for a garnet clinopyroxenite, a garnet-plagioclaseclinopyroxenite, a spinel-garnet websterite, and a two-pyroxenegranulite occurring as xenoliths in the Delegate basaltic brecciapipes. Assuming all the garnet pyroxenite suite xenoliths formedtogether or last equilibrated together, the experimental dataconstrain the P-T conditions of their formation to 13–17kb and 1050–1100 °C; for the pyroxene granulites,pressures of formation of 6–10 kb at temperatures around1100 °C are indicated. In the case of the spinel-garnetwebsterite, the texturally implied exsolution of garnet andorthopyroxene from clinopyroxene, and reaction of spinel withclinopyroxene to yield garnet, are shown to be explicable interms of approximately isobaric cooling of a pre-existing aluminousclinopyroxene+spinel aggregate. The garnet of the garnet andgarnet—plagioclase clinopyroxenites cannot, however, havebeen derived wholly by exsolution processes. New chemical data are presented for the xenoliths studied experimentallyand for several similar examples from Delegate and other easternAustralian localities. Consideration of available major andtrace element and isotopic data for garnet pyroxenite suitexenoliths from Delegate and elsewhere in the world stronglysuggests genetic relationships with their host basaltic rocks.The Delegate examples are interpreted as a series of accumulatesfrom local pockets of alkaline basaltic magma within the Earth'supper mantle, and which have subsequently undergone exsolutionand/or recrystallization in response to subsolidus cooling.A similar origin is suggested for the analogous garnet pyroxenitesfound as layers within western Mediterranean peridotite massifs.The Delegate two-pyroxene granulite xenoliths are consideredto be accidental fragments of metamorphic rocks from the deepcrust beneath eastern Australia.     

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.     

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.     

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     

Prograde amphiboles in hematite-bearing basic and quartz schists in the Sanbagawa belt, central Shikoku: relationship between metamorphic field gradient and P-T paths of individual rocks

The prograde amphibole that coexists with chlorite, epidote, muscovite, albite, quartz and hematite in Sanbagawa schists was examined to investigate the relationship between the prograde P-T paths of individual rocks and the metamorphic field gradient in the Sanbagawa metamorphic belt, central Shikoku. The amphibole changes from actinolite, through ferri-winchite and crossite, to barroisite and hornblende with increasing grade along the metamorphic field gradient. However, the sequence of prograde amphibole compositions in each sample varies in different mineral zones. The general scheme can be summarized as: magnesioriebeckite-riebeckite crossite in the upper chlorite zone of lower-grade rocks; crossite or glaucophane barroisite in the garnet zone of medium-grade rocks; and actinolite or winchite barroisite hornblende in the albite-biotite zone of higher-grade rocks. Changes of amphibole composition indicate that the prograde P-T path recorded in the higher-grade rocks was situated on the higher-temperature side of that of the lower-grade rocks and on the lower-pressure side of the metamorphic field gradient. The systematic change of P-T paths implies an increasing d P /d T during continuous subduction. These features can be interpreted as documenting prograde metamorphism within a young subduction zone that has a non-steady-state geotherm.     

Deep-crustal granulites with migmatitic and mylonitic fabrics from the Zambezi Belt, northeastern Zimbabwe

Abstract High- P granulites contained in two allochthonous tectonic units were thrust southwards onto the northern margin of the Zimbabwe craton during the Pan-African Zambezi orogeny. In the lower sheet, the Masoso Metamorphic Suite contains mafic garnet granulite assemblages formed during a high- P-T metamorphic event, although the suite as a whole is predominantly granitic. The garnet granulites occur as relicts within narrow mafic layers characterized by migmatitic and mylonitic fabrics. The annealed mylonites represent surfaces of deep-crustal tectonic imbrication that formed immediately before the Pan-African orogeny. Gabbros which intruded the granulites after the main phase of migmatization have formed corona textures that document a low- P-T metamorphic event at mid-crustal levels. The style of deformation then changed and the Masoso Suite with its mylonitic layers was folded and thrusted southwards onto the Archaean Zimbabwe craton.     

Garnet-Orthopyroxene Bearing Granulites from Enderby Land, Antarctica: Metamorphic Pressure Temperature-Time Evolution of the Archaean Napier Complex

Garnet-orthopyroxene bearing granulite assemblages from theArchaean Napier Complex, Enderby Land, Antarctica, display avariety of exsolution, recrystallization and corona textureswhich result both from near-isobaric cooling from the peak ofmetamorphism and from later overprinting. Compositional dataon distinct generations of phases and on zoning patterns incoexisting minerals, have been used to estimate (a) peak metamorphicconditions attained between the first and second major deformationphases (Dl and D2); (b) cooling paths from this peak, and (c)ambient metamorphic conditions at the time of a later deformation(D3). Experimentally calibrated geothermobarometers indicateinitial metamorphism at 900–950?C and 7–10 kb duringand subsequent to Dl and D2, at 3100–3000 Ma. The presentlyexposed granulites indicate a regional increase in the pressuresof this metamorphism south-west to the Scott Mountains-CaseyBay region, where minimum crustal thicknesses of 10 kb wereattained at c. 3000 Ma. Subsequently, the Napier Complex granulitesevolved through a prolonged period of near-isobaric coolingprior to further metamorphism at 600–750 and 4–8kb during D3 at c. 2500 Ma. The near-isobaric pressure-temperature-timepath (P-T-t) suggests that the Napier Complex acted as an essentiallystable craton as early as 3000 Ma, and that the major magmaticand tectonic crustal thickening events associated with Dl precededthe thermal peak represented by the earliest recognized metamorphism.