bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐pressure granulite facies metamorphism in the Larsemann Hills area, East Antarctica; petrology and tectonic implications for the evolution of the Prydz Bay area
Authors: STÜWE K. , POWELL R.
Published: May 2007

ABSTRACT Thermobarometric studies on various granulite facies areas along the Prydz Bay coast, East Antarctica (73°‐79°E, 68°‐70°S), show that, at around 1100 Ma, during a late Proterozoic orogeny, the rocks of the Larsemann Hills suffered a lower pressure metamorphic peak than the surrounding areas. Along the Prydz Bay coast, the rocks affected by this event include parts of the Vestfold Hills block plus all of the Rauer Group, the Larsemann Hills and the Munro Kerr Mountains. The dykes in the south‐west corner of the Vestfold Hills were recrystallized during this event with little deformation at temperatures not quite as high as in the areas further south‐west (650°C, 6.5 kbar) (Collerson et al., 1983), the Rauer Group was metamorphosed at 800°C and 7.5 kbar (Harley, 1987a), the Larsemann Hills at 750°C and 4.5 kbar, and the Munro Kerr Mountains probably at around 850°C and 5 kbar. Retrograde equilibration in the different areas occurred during decompression to about 10 km depth in all areas, followed by isobaric cooling at this depth.

This paper shows that the peak metamorphism in the Larsemann Hills occurred at a pressure which is too low to have been the consequence of thermal relaxation of overthickened crust with normal mantle heat flow. Although other areas in Prydz Bay were metamorphosed at sufficiently high pressures so that their decompression paths are not inconsistent with a continental collision model, the inferred pre‐metamorphic peak histories and the requirement of consistency with the Larsemann Hills, make it unlikely that collision followed by erosion‐driven decompression is an appropriate model. We suggest that the thermal regime of the crust in the Larsemann Hills region was controlled by a perturbation in the asthenosphere, with magma invasion of the crust. We suggest that the 500 Ma event, represented in Prydz Bay by granitic outcrops at Landing Bluff and by several K/Ar ages from the Larsemann Hills area, was responsible for the final excavation of the terrane.
Metamorphism of an ophiolitic tectonic mélange, northern California Klamath Mountains, USA
Authors: DONATO M. M.
Published: May 2007

ABSTRACT Mineral assemblages in pelitic, mafic, calcareous and ultramafic rocks within a metamorphosed tectonic mélange indicate that the Marble Mountain terrane and adjacent Western Hayfork subterrane (northern California) underwent regional low‐ to medium‐pressure amphibolite facies metamorphism. Metamorphic conditions estimated by comparison of observed assemblages with experimentally‐determined reaction boundaries and by geothermometry constrain metamorphic temperatures between about 500° and 570°C. The occurrence of andalusite in regionally metamorphosed pelites indicates pressures below about 370 MPa. Metabasite amphibole compositions also suggest low to intermediate metamorphic pressures.

Metaserpentinites containing the upper amphibolite facies assemblage (olivine + enstatite + anthophyllite) are found locally within the study area and have been reported previously by other workers elsewhere in the Marble Mountain terrane. These assemblages may reflect higher temperatures of recrystallization than assemblages in surrounding rocks and may represent vestiges of an earlier high‐temperature metamorphic event undergone by the ultramafic rocks prior to incorporation in the mélange.

Although the age of the low‐ to intermediate‐pressure metamorphism is poorly constrained, cross‐cutting plutons indicate that metamorphism must be older than about 162 Ma. Therefore this regional metamorphic event, which probably marks the accretion of these terranes to the North American continental margin, is older than the currently accepted 151–147 Ma age of the Nevadan event in the Klamath Mountains. The inferred low to intermediate pressures of metamorphism and the lithologies of the protoliths suggest a near‐arc tectonic setting and refute a subduction zone model for this event.
Amphibolites from Panama: anticlockwise P‐T paths from a Pre‐upper Cretaceous metamorphic basement in Isthmian Central America
Authors: TOURNON J. , TRIBOULET C. , AZEMA J.
Published: May 2007

Rocks from the metamorphic basement of the Azuero and Sona peninsulas, Panama, consist of schistose amphibolites and minor amounts of metasediment. In the Sona peninsula, strongly zoned amphiboles indicate that the amphibolites followed a progressive anticlockwsie P‐T path prograde from low T/low P to medium T/high P, and are retrograded into the greenschist facies. In contrast, the amphibolites of the Azuero peninsula are affected by a low to medium T/low P metamorphism.

The metamorphic events of the Sona amphibolites occurred prior to the intra‐Senonian tectonic phase which affects the Mesozoic formations along the Pacific coast of Costa Rica and Panama. The regional significance of such a basement in Isthmian Central America is discussed.
Controls on the first appearance of jadeitic pyroxene, northern Diablo Range, California
Authors: PATRICK B. E. , DAY H. W.
Published: May 2007

A reaction producing jadeitic pyroxene in metagreywackes of the northern Diablo Range has been identified on the basis of mineral distribution, isograd patterns and composition of coexisting minerals. The appearance of jadeitic pyroxene (∼Jd80) is closely followed by the disappearance of pumpellyite, which indicates that pumpellyite plays a major role in the pyroxene‐producing reaction. A new projection from hematite, lawsonite, chlorite, quartz and H2O on to the NaAlO2‐FeO‐MgO ternary confirms the role of pumpellyite in pyroxene production and suggests a reaction of the form: 1.00 pumpellyite + 0.31 chlorite + 8.71 albite + 0.70 hematite + 2.00 H2O = 8.54 jadeite + 0.57 glaucophane + 3.09 lawsonite + 5.26 quartz. Metagreywackes of the northern Diablo Range were metamorphosed under conditions of PH2O=Ptotal at 200‐300 °C and 7.5‐10.0 kbar. Despite the low temperatures attained during metamorphism, the assumption of equilibrium yields results consistent with field observations and phase relations.
Calibration of the anchizone: a critical comparison of illite ‘crystallinity’scales used for definition
Authors: KISCH H. J.
Published: May 2007

Instrumental settings for determination of illite/muscovite ‘crystallinity’(half‐height width of the 10‐Å X‐ray diffraction peak) and the limits of the anchimetamorphic zone adopted by various authors fall into several groups.

The variation in the limiting peak widths between the authors that have adopted Kubler's Neuchâtel boundary values of 0.42° and 0.25°Δ2θ can be interpreted in terms of variation in the instrumental settings. The choice of time constants higher than those given by the formula causes peak broadening; this effect is particularly marked at high scan rates. The peak broadening is by constant increments that are virtually independent of the absolute peak width.

The differences between the Kubler (Neuchâtel)‐derived limiting values and some other scales are appreciably greater than can be accounted for by these differences in instrumental settings: many of these scales are not equivalent. In particular, the limits adopted by Dunoyer de Segonzac (1969) and subsequent workers at Strasbourg are too broad; their anchizone represents a range of grades of metamorphism lower than that of Kubler, widely overlapping the latter's ‘diagenetic’zone. Those adopted by some other, mainly French, authors are too narrow.

The limits of the anchizone should be calibrated by inter‐laboratory standards, and the instrumental settings should be specified in full.
An orthopyroxene–biotite geothermometer and its application in crustal granulites and mantle‐derived rocks
Authors: SENGUPTA P. , DASGUPTA S. , BHATTACHARYA P. K. , MUKHERJEE M.
Published: May 2007

Using the experimental data on Fe–Mg exchange between orthopyroxene and biotite of Fonarev & Konilov (1986), an orthopyroxene–biotite geothermometer is developed. The thermometer is corrected for mixing of Ti and Al in octahedral sites in biotite and also for non‐ideal mixing of Fe and Mg in orthopyroxene. The thermometer is applied to several amphibolite–granulite transition facies and granulite facies rocks and also to mantle xenoliths. It yields consistent results in rocks of widely varying bulk composition, and highly magnesian mantle xenoliths. This thermometer removes the difficulty of estimating temperature in garnet‐free rocks in high‐grade terrains and also provides independent estimates of temperature in garnet‐bearing assemblages.
Sequential porphyroblast growth during deformation in a low‐pressure metamorphic terrain, Orrs Island–Harpswell Neck, Maine
Authors: LANG H. M. , DUNN G. R.
Published: May 2007

Poikiloblastic index minerals in pelitic rocks from the Orrs Island–Harpswell Neck area of coastal Maine contain inclusion textures that indicate sequential growth of progressively higher grade metamorphic minerals during development of a near‐vertical crenulation foliation. The sequence of zones in the field is garnet, staurolite, staurolite–andalusite, staurolite–sillimanite and sillimanite. Inclusion fabrics characteristic of different stages in crenulation cleavage development indicate that index minerals nucleated and grew sequentially: biotite began to grow before deformation, garnet began to grow during early stages of crenulation cleavage development, staurolite grew during intermediate stages, and andalusite grew relatively late, when transposition of the foliation was nearly complete. Muscovite pseudomorphs and sillimanite were mainly post‐kinematic. The fact that metamorphic index minerals grew sequentially in individual rocks in the same order in which they appear across the field area indicates that the high temperature part of the pressure–temperature path was similar to the metamorphic field gradient.

Metamorphism in the Orrs Island–Harpswell Neck area is consistent with the magmatic heating model that has been proposed for western Maine. Sequential development of index minerals in pelitic rocks in the Orrs Island–Harpswell Neck area apparently resulted from sequential nucleation after substantial overstepping of mineral‐forming reactions. Once nucleation of an index mineral had taken place, initial growth was rapid and poikiloblasts preserved inclusion trails characteristic of the prevailing stage of crenulation cleavage development. Because nucleation of sillimanite may have required more overstepping of the andalusite–sillimanite reaction than nucleation at dehydration reactions, determination of metamorphic conditions for rapidly heated rocks such as these by comparison with a petrogenetic grid is problematic. Garnet zoning patterns in these rocks should reflect the fact that growth of garnet interiors occurred early during metamorphism in equilibrium with a low‐grade assemblage. Only garnet rims would be expected to record the subsequent pressure–temperature path.
The effect of heterogeneous stress and strain on metamorphic fluid flow, Mary Kathleen, Australia, and a model for large‐scale fluid circulation *
Authors: OLIVER N. H. S. , VALENTA R. K. , WALL V. J.
Published: May 2007

In the Proterozoic Mary Kathleen Fold Belt, northern Australia, infiltration of large volumes of externally derived fluid occurred synchronously with regional amphibolite‐facies metamorphism and deformation. This paper develops a model of structurally controlled fluid migration by comparing the distribution of fossil fluid pathways with the inferred stress and strain patterns during the deformation. Intense fluid flow was localized within strong, relatively brittle meta‐intrusive bodies, and in discrete, veined, brecciated and altered zones around their margins. In metasediments folded in a ductile manner outside these areas, fluid infiltration was negligible. The direct correlation between structural styles and the magnitude of veining and metasomatism suggests control of permeability enhancement, and hence fluid flow, by deformation. Finite difference modelling of a strong body in a weaker matrix has been used to evaluate the variation of stresses during the deformation, from which it is clear that stress and strain heterogeneities have systematically influenced the development and maintenance of metamorphic fluid pathways. Particular regions in which mean stress may be significantly lower than the average lithostatic pressures include the ‘strain shadow’zones adjacent to the strong bodies, other dilatant zones around the bodies, and the bodies themselves. This geometry is favourable not only for localized brittle deformation under amphilobite facies conditions, but also for focused fluid flow in the low mean stress regions, as evidenced by the abundance of veins. Fluid access through these metamorphic aquifers occurred during tensile failure episodes, with particularly large dilations and decimetre‐scale veining in areas of strain incompatibility. It appears likely that fluid circulated many times through the Fold Belt, with flow concentrated in the metamorphic aquifers. A model is developed that explains both the structurally focused fluid flow and the postulated multi‐pass recirculation by dilatancy pumping, the ‘pump engines’comprising the low mean stress zones.
Glaucophane‐bearing assemblage overprinted by greenschist‐facies metamorphism in the Variscan Kaczawa complex, Sudetes, Poland
Authors: KRYZA R. , MUSZYNSKI A. , VIELZEUF D.
Published: May 2007

An Early Palaeozoic (Ordovician ?) metamudstone sequence near Wojcieszow, Kaczawa Mts, Western Sudetes, Poland, contains numerous metabasite sills, up to 50 m thick. These subvolcanic rocks are of within‐plate alkali basalt type. Primary igneous phases in the metabasites, clinopyroxene (salite) and kaersutite, are veined and partly replaced by complex metamorphic mineral assemblages. Particularly, the kaersutite is corroded and rimmed by zoned sodic, sodic–calcic and calcic amphiboles. The matrix is composed of actinolite, pycnochlorite, albite (An ≤ 0.5%), epidote (Ps 27–33), titanite, calcite, opaques and, occasionally, biotite, phengite and stilpnomelane.

The sodic amphiboles are glaucophane to crossite in composition with NaBfrom 1.9 to 1.6. They are rimmed successively by sodic–calcic and calcic amphiboles with compositions ranging from magnesioferri‐winchite to actinolite. No compositions between NaB= 0.92 and NaB= 1.56 have been ascertained.

The textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist (or blueschist–greenschist transitional) assemblage. The presence of glaucophane and no traces of a jadeitic pyroxene + quartz association indicate pressures between 6 and 12 kbar during the high‐pressure episode. Temperature is difficult to assess in this metamorphic event. The replacement of glaucophane by actinolite + chlorite + albite, with associated epidote, allows restriction of the upper pressure limit of the greenschist recrystallization to <8 kbar, between 350 and 450°C. The mineral assemblage representing the greenschist episode suggests theP–Tconditions of the high‐pressure part of the chlorite or lower biotite zone. The latest metamorphic recrystallization, under the greenschist facies, may have taken place in the Viséan.
Biotite and garnet compositional variation and mineral equilibria in Grenville gneisses of the Otter Lake area, Quebec
Authors: KRETZ RALPH
Published: May 2007

Garnet‐biotite gneisses, some of which contain sillimanite or hornblende, are widespread within the Otter Lake terrain, a portion of the Grenville Province of the Canadian Shield. The metamorphic grade is upper amphibolite to, locally, lower granulite facies.

The atomic ratio Fe2+/(Fe2++ Fe3+) in biotite ranges from 0.79 to 0.89 (ferrous iron determinations in 10 highly pure separates), with a mean of 0.86. Mg and Fe2+ atoms occupy 67–78% of the octahedral sites, the remainder are occupied by Fe3+, Ti, and Al, and some are vacant. Mg/(Mg + Fe2+), denoted X, in the analysed samples ranges from 0.32 to 0.65. Garnet contains 1–24% grossular, 1–12% spessartine and X ranges from 0.07 to 0.34.

Compositional variation in biotite and garnet is examined in relation to three mineral equilibria:

(I) biotite + sillimanite + quartz = garnet + K‐feldspar + H2O;

(II) pyrope + annite = almandine + phlogopite;

(III) anorthite = grossular + sillimanite + quartz.

Measurements of X (biotite) and X (garnet) are used to construct an illustrative model for equilibrium (I) which relates the observed variation in X to a temperature range of 70°C or a range in H2O activity of 0.6; the latter interpretation is preferred.

In sillimanite‐free gneisses, the distribution of Mg and Fe2+ between garnet (low in Ca and Mn) and biotite is adequately described by a distribution coefficient (KD) of 4.1 (equilibrium II). The observed increase in the distribution coefficient with increasing Ca in garnet is ln KD= 1.3 + 2.5 × 10−2 [Ca]

where [Ca] = 100 Ca/(Mg + Fe2++ Mn + Ca). The distribution coefficient is apparently unaffected by the presence of up to 12% spessartine in garnet.

In several specimens of garnet‐sillimanite‐plagioclase gneiss, the Ca contents of garnet and of plagioclase increase in unison, as required by equilibrium (III). The mean pressure calculated from these data (n= 17) is 5.9 kbar, and the 95% confidence limits are ±0.5 kbar.
Pseudotachylite from the Main Zone of the Hidaka metamorphic belt, Hokkaido, northern Japan
Authors: TOYOSHIMA T.
Published: May 2007

Pseudotachylite veins have been found in the mylonite zone of the Hidaka metamorphic belt, Hokkaido, northern Japan. They are associated with faults with WNW‐ESE to ENE‐WSW or NE‐SW trends which make a conjugate set, cutting foliations of the host mylonitic rocks with high obliquity. The mylonitic rocks comprise greenschist facies to prehnite‐pumpellyite facies mineral assemblages. The mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near‐surface environments. An analysis of the deformational and metamorphic history of the Hidaka Main Zone suggests that the ambient rock temperature was 200–300° C immediately before the formation of the Hidaka pseudotachylite.

Three textural types of veins are distinguished: cryptocrystalline, microcrystalline and glassy. The cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline‐type veins. The microcrystalline type is largely made up of quench microlites of orthopyroxene, clinopyroxene, biotite, plagioclase and opaque minerals with small amounts of amphibole microlites. The interstices of these microlites are occupied by glassy and/or cryptocrystalline materials. The presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km.

The bulk chemical composition of the pseudotachylite veins is characterized by low SiO2 and a high water content and is very close to that of the host mylonitic rocks. This indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases. Local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts. Pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high Al contents. The presence of pigeonite and high‐Al pyroxene microlites, of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins. The melt temperatures were estimated to be up to 1100° C using a two‐pyroxene geothermometer. Using published data relating water solubilities in high‐temperature andesitic magmas to pressure, a depth estimate of about 4 km is inferred for the Hidaka pseudotachylites.

Evidence derived from pseudotachylites in the Hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals.
Prograde–retrograde P–T–t–deformation path of Austroalpine micaschists during Variscan continental collision (Eastern Alps)
Authors: SCHULZ B.
Published: May 2007

A complete prograde P–T path, defined by 10 calculated P–T fields in succession, is recognized from metapelites by using geothermobarometry on garnet‐bearing assemblages with microstructural control. Overstacking of several tectonic units during an early Variscan continental collision explains the complex prograde P–T history. Isostatic uplift and deformation controlled the retrograde P–T path.

Deformation with changing character acted continuously during all stages of the evolution of the Austroalpine basement complex. After the intrusion of Caledonian granitoids, metapelites and magmatic rocks suffered a shearing deformation D1–D2, which produced sheath folds as well as the main foliation S2. Spessartine‐rich first‐generation garnets, situated in microlithons enclosed by S2, record the onset of shearing under increasing high‐pressure–low‐temperature conditions (7 kbar/380°C). Geothermobarometry on second‐generation garnets which have been rotated during growth indicates isothermal decompression from 9 kbar to 5 kbar/500°C and subsequent recompression/heating during continuing shearing. This is explained by overthrusting of a tectonic unit (unit 2) from NE to SW upon the micaschist unit (unit 1), followed by isostatic uplift and further overstocking of a third unit (unit 3). The resulting Pmax of 12 kbar at 650°C and further increasing temperatures up to 680°C accompanied by decompression have been calculated using a third generation of garnets. These high‐pressure–high‐temperature conditions may explain the occurrence of eclogitic metabasites in adjacent regions.

Staurolite and kyanite first appeared under decreasing pressures at the last stage of prograde P–T evolution. Shortening deformation D3 and simultaneous growth of typical amphibolite facies minerals (staurolite 2, kyanite 2, sillimanite, andalusite) occurred during the retrograde path. A final step of Variscan evolution was marked by an oppositely directed shearing D4 (at T > 300°C and P > 3 kbar), possibly indicating backthrusting or extension.

Apart from acid intrusions, no signs of a previous Caledonian thermotectonic history were found in the area to the south of the Defereggen–Antholz–Vals Line.
Thermal granulite‐facies metamorphism with diffuse retrogression in Archaean orthogneisses, Fiskefjord, southern West Greenland
Authors: GARDE A. A.
Published: May 2007

Around Fiskefjord, southern West Greenland, Archaean amphibolite‐facies, granulite‐facies and retrograde orthogneisses occur in lithological and structural continuity with each other. The granulite‐facies rocks here—and elsewhere in West Greenland—are surrounded by extensive areas of retrograde gneisses. Both the prograde and retrograde metamorphism took place in a major event of continental crust formationc. 3000 Ma ago, which gave rise to granulite‐facies conditions in part of the rock complex exposed today. In the Fiskefjord area distributions of major and trace elements, as well as strontium and lead isotopes, show that the fades transformations were accompanied by pronounced metasomatism, and mineral chemistry indicates that the hydrous retrograde metamorphism took place under amphibolite‐facies conditions and was gradual and incomplete. The metamorphic and metasomatic processes in the Fiskefjord area are believed to have been controlled by heat from continuous intracrustal injection of large masses of tonalitic magma, which caused gradual dehydration and partial melting, followed by liberation of aqueous fluids during crystallization of anatectic melts. These fluids partially retrograded previously dehydrated gneisses. In contrast, South Indian high‐grade gneisses have mainly prograde amphibolite–granulite‐facies transitions which are distinct and well preserved, later than penetrative deformation, and are likely to have been controlled by CO2streaming. These amphibolite–granulite‐facies transitions are reported to be near‐isochemical. It is suggested that there are (at least) two different kinds of granulite‐facies metamorphism: a near‐isochemical prograde type in stabilized tectonic environments, perhaps controlled by influx of CO2(e.g. in South India) and significantly post‐dating original crust formation; and a fluid‐deficient type with widespread anatexis, hydrous retrogression and metasomatism, which takes place during accretion of continental crust, and in which heat is the governing factor (e.g. in southern West Greenland).
From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean
Authors: LARDEAUX J. M. , SPALLA M. I.
Published: May 2007

The Sesia zone (Italian Western Alps) offers one of the best preserved examples of pre‐Alpine basement reactivated, under eclogite facies conditions, during the Alpine orogenesis. A detailed mineralogical study of eclogitized acid and basic granulites, and related amphibolites, is presented. In these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished.

(a) A medium‐ to low‐Pgranulite stage giving rise to the development of orthopyroxene + garnet + plagioclase + brown amphibole + ilmenite ± biotite in basic granulites and garnet + K‐feldspar + plagioclase + cordierite + sillimanite + biotite + ilmenite in acid granulites.

(b) A post‐granulite re‐equilibration, associated with the development of shear zones, producing discrete amphibolitization of the basic granulites and widespread development of biotite + sillimanite + cordierite + spinel in the acid rocks.

(c) An eo‐Alpine eclogite stage giving rise to the crystallization of high‐Pand low‐Tassemblages.

In an effort to quantify this evolution, independent well‐calibrated thermobarometers were applied to basic and acid rocks. For the granulite event,P‐Testimates are 7–9 kbar and 700–800° C, and for subsequent retrograde evolution,P‐Twas 4–5 kbar and 600° C. For the eo‐Alpine eclogite metamorphism, pressure and temperature conditions were 14–16 kbar and 550° C.

The inferredP‐Tpath is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks. In the light of the available geochronological constraints we propose to relate the pre‐Alpine granulite and post‐granulite retrograde evolution to the Permo‐Jurassic extensional regime. The complex granulite‐eclogite transition is thus regarded as a record of the opening and of the closure of the Piedmont ocean.
Structural and metamorphic controls on the distribution of zircon in an evolving quartzofeldspathic migmatite: an example from the Reynolds Range, central Australia
Authors: DIRKS P. , HAND M.
Published: May 2007

Three types of zircon occur in a complexly deformed and variably migmatized quartzofeldspathic gneiss from the Reynolds Range, central Australia. The oldest type is inherited from the granitic precursor of the gneiss, and is overgrown by a second group of zircon grains that formed during prograde, granulite facies metamorphism. Partial melting of the gneiss resulted in solution of both the inherited and metamorphic zircon. No new zircon growth accompanied crystallization of the partial melt, suggesting loss of zirconium–rich residual fluids. Hydrous, amphibolite facies retrogression of the gneiss and its migmatized variants during late shearing produced new, idiomorphic zircon in both the shear zone and its wall rocks.

Important implications of this study are that (i) zircon has a tendency to dissolve if it comes into direct contact with a melt produced from anhydrous biotite breakdown in a quartzofeldspathic granulite, (ii) melt crystallization is not necessarily accompanied by zircon growth, and (iii) euhedral zircon can grow from a hydrous fluid phase under subsolidus, amphibolite facies conditions, e.g. within shear zones.
Pressure–temperature evolution of metapelitic granulites in a polymetamorphic terrane: the Rauer Group, East Antarctica
Authors: HARLEY S. L. , FITZSIMONS I. C. W.
Published: May 2007

Distinctive lithological associations and geological relationships, and initial geochronological results indicate the presence of an areally extensive region of reworked Archaean basement containing polymetamorphic granulites in the Rauer Group, East Antarctica.

Structurally early metapelites from within this reworked region preserve complex and varied metamorphic histories which largely pre‐date and bear no relation to a Late Proterozoic metamorphism generally recognized in this part of East Antarctica. In particular, magnesian metapelite rafts from Long Point record extreme peak P–T conditions of 10–12 kbar and 100–1050°C, and an initial decompression to 8 kbar at temperatures of greater than 900°C. Initial garnet–orthopyroxene–sillimanite assemblages contain the most magnesian (and pyrope‐rich) garnets (XMg= 0.71) yet found in granulite facies rocks. A high‐temperature decompressional P–T history is consistent with reaction textures in which the phase assemblages produced through garnet breakdown vary systematically with the initial garnet XMg composition, reflecting the intersection of different divariant reactions in rocks of varied composition as pressures decreased. This history is thought to relate to Archaean events, whereas a lower‐temperature (c. 750–800°C) decompression to 5 kbar reflects Late Proterozoic reworking of these relict assemblages.

The major Late Proterozoic (c. 1000 Ma) granulite facies metamorphism is recorded in a suite of younger Fe‐rich metapelites and associated paragneisses in which syn‐ to post‐deformational decompression, through 2–4 kbar from maximum recorded P–T conditions of 7–9 kbar and 800–850°C, is constrained by geothermobarometry and reaction textures. This P–T evolution is thought to reflect rapid tectonic collapse of crust previously thickened through collision.
A decompressional P–T path, Reinbolt Hills, East Antarctica
Authors: NICHOLS G. T. , BERRY R. F.
Published: May 2007

Granulites exposed in the Reinbolt Hills, East Antarctica, are part of the extensive Late Proterozoic granulite complex of East Antarctica, which includes the Rauer Group to the east and the northern Prince Charles Mountains to the west. The deformation history includes three pervasive deformation phases. No chemical or mineralogical distinction between these phases has been detected and this is interpreted to be the result of complete re‐equilibration at the end of the third deformation phase. Two late deformation phases post‐date the metamorphism and record a medium‐temperature cooling path. A short segment of the P–T path of these rocks was inferred from mineral reactions that occurred during these late deformation phases. The path passes from 800°C, 7 kbar to 690°C, 5 kbar, indicating strong decompression, which is typical of a thrust‐dominated crustal thickening followed by rapid erosion or extensional collapse.
Reaction textures in calc‐silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica: implications for the retrograde P–T path
Authors: MOTOYOSHI Y. , THOST D. E. , HENSEN B. J.
Published: May 2007

Calc‐silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica, exhibit a sequence of reaction textures that have been used to elucidate their retrograde P–T path. The highest temperature recorded in the calc‐silicates is represented by the wollastonite‐ and scapolite‐bearing assemblages which yield at least 760°C at 6 kbar based on experimental results. The calc‐silicates have partially re‐equilibrated at lower temperatures (down to 450°C) as evidenced by the successive reactions: (1) wollastonite + scapolite + calcite = garnet + CO2, (2) wollastonite + CO2= calcite + quartz, (3) wollastonite + plagioclase = garnet + quartz, (4) scapolite = plagioclase + calcite + quartz, (5) garnet + CO2+ H2O = epidote + calcite + quartz, and (6) clinopyroxene + CO2+ H2O = tremolite + calcite + quartz.

The reaction sequence observed indicates that aCO2 was relatively low in the wollastonite‐bearing rocks during peak metamorphic conditions, and may have been further lowered by local infiltration of H2O from the surrounding migmatitic gneisses on cooling. Fluid activities in the Bolingen calc‐silicates were probably locally variable during the granulite facies metamorphism, and large‐scale CO2 advection did not occur.

A retrograde P–T path, from the sillimanite stability field (c. 760°C at 6 kbar) into the andalusite stability field (c. 450°C at <3 kbar), is suggested by the occurrence of secondary andalusite in an adjacent cordierite–sillimanite gneiss in which sillimanite occurs as inclusions in cordierite.
Discontinuous grain growth of quartz in metacherts: the influence of mica on a microstructural transition
Authors: MASUDA T. , KOIKE T. , YUKO T. , MORIKAWA T.
Published: May 2007

Abstract
The microstructure of quartz in metacherts of the Ryoke metamorphic belt in central Japan develops from polygonal, through duplex to irregular with increasing metamorphic grade. The polygonal microstructure is composed of small (mostly 90–160 μm), equant, equigranular, polygonal quartz grains, whereas the irregular microstructure is characterized by large (>300 μm) grains with irregular grain boundaries. The duplex microstructure is a mixture of small polygonal and large irregular grains. The development of these microstructures is interpreted as being due to secondary recrystallization. The size of polygonal grains is greatly influenced by the presence of second‐phase minerals, such as mica, whereas that of large irregular grains is unaffected by second‐phase minerals. There seems to be a critical grain size for quartz to occur as polygonal aggregates: no polygonal aggregates occur in rocks with larger than the critical grain size. The size (about 140 μm) decreases slightly with increasing volume fraction of mica. The mean grain sizes of polygonal quartz (D) and coexisting mica (d) in the duplex microstructure are systematically related to the volume fraction of mica (f) by D= 0.728d(1/f)0.629.
Metamorphic evidence for an inverted crustal section, with constraints on the Main Karakorum Thrust, Baltistan, northern Pakistan
Authors: ALLEN T. , CHAMBERLAIN C. P.
Published: May 2007

Abstract
The Shyok Suture Zone separates rocks in the Asian plate from rocks in the Kohistan‐Ladakh island arc. In Baltistan, this suture has been reactivated by the late ‘break‐back’Main Karakorum Thrust (MKT). The P‐T histories of metamorphic rocks both north and south of the MKT have been determined in an effort to place constraints on the tectonic history of this zone. The terranes north and south of the MKT have different, unrelated metamorphic histories. Rocks from the Kohistan‐Ladakh island arc south of the MKT have undergone a static low‐P (2–4 kbar, c. 500° C) thermal metamorphism. The P‐T paths and metamorphic textures of these rocks are consistent with metamorphism due to emplacement of plutonic rocks into the island arc. This metamorphism pre‐dates folding and deformation of these rocks. Rocks in the Karakorum Metamorphic Complex, north of the MKT, have experienced a complex deformational and metamorphic history. Prograde metamorphic isograds have been deformed by subsequent south‐verging folding and by gneiss dome emplacement. However, decompression metamorphic reactions occurred during nappe emplacement. Higher pressure rocks are associated with higher level nappes, creating an inverted pressure metamorphic sequence (8–9‐kbar rocks over 5–6‐kbar rocks). There is little variation in temperature with structural level (550–625° C). These two different terranes have been juxtaposed after metamorphism by the late south‐directed MKT.
Fluid channelling during ductile shearing: transformation of granodiorite into aluminous schist in the Tauern Window, Eastern Alps
Authors: SELVERSTONE J. , MORTEANI G. , STAUDE J.‐M.
Published: May 2007

Abstract
Ductile shearing in the core of the Tauern Window, Austria, transformed metagranodiorite into Si‐undersaturated garnet‐chlorite‐staurolite schist at a depth of c. 35–40 km during the Alpine orogeny. Four distinct zones have been recognized extending from the wallrock into the centre of the shear zone: Zone I—unaltered metagranodiorite with subordinate amphibolite; Zone II—biotite‐white mica‐garnet schist; Zone III—biotite‐phengite schist; Zone IV—quartz‐absent, garnet‐chlorite‐staurolite schist with garnets up to 10 cm across. Whole‐rock analyses show a dramatic decrease in SiO2 from >65 wt% in Zone I to <35 wt% in Zone IV; Ca, Na, and Sr also decrease across the shear zone, whereas Al, Ti, Fe, Mg, P, Cr, Ni, Zn, and Rb all increase towards Zone IV. Mass‐balance calculations indicate that shearing was accompanied by up to 60% volume loss near the centre of the shear zone. Comparison of the Tauern Window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in Mg, Fe, and Ti is typical for volume‐loss shear zones, but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour. In the samples studied here, volume loss appears to have resulted from channellized fluid flow during shearing, producing time‐integrated fluid fluxes of ± 108 cm3 cm−2 in Zone IV. This large volume of fluid may have originated, in part, from dehydration of flysch carried beneath the metagranodiorites during Eocene movement on the North Penninic subduction zone. Development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating.
Himalayan structure and metamorphism south of the Main Mantle Thrust, Lower Swat, Pakistan
Authors: DIPIETRO J. A. , LAWRENCE R. D.
Published: May 2007

Abstract
During the Eocene‐Oligocene, the Indian plate collided with the Kohistan arc along the Main Mantle Thrust (MMT) zone. The structure of the Lower Swat rock sequence, on the Indian plate directly south of the MMT, is a dome with a basement of granitic gneiss and quartz‐rich schist unconformably overlain by amphibolitic and calcareous schist. The earliest superposed small‐scale folds (F1 & F2) represent a progressive F1/F2 deformation that is associated with a single set of WSW‐vergent large‐scale folds (termed F2). These folds are inferred to have developed during oblique, WSW‐directed overthrusting of the MMT suture complex onto the Lower Swat rock sequence. Metamorphism began during F1/F2 as indicated by an S1 foliation that developed during biotite‐grade metamorphism. S1 is preserved as a relict texture in porphyroblasts that grew during a subsequent interkinematic phase during garnet‐ and higher grade metamorphism. The dominant, regional foliation (S2) developed following the interkinematic phase. S2 is associated with transposition of S1 and rotation or dismemberment of porphyroblasts. Annealing recrystallization followed S2 and continued during F3 thereby destroying or masking possible pre‐existing stretching fabrics. Superposed F3 folds are upright and open with N‐S axial trends. They may correlate with early doming of the Lower Swat rock sequence and with strike‐slip displacement in the northern part of the MMT zone, north of the Lower Swat area. F3 was followed by retrograde metamorphism and development of E‐W‐trending, S‐vergent F4 folds. F4 may be associated with a final phase of southward directed thrusting and inactivity in the MMT zone. Correlation of published 40Ar/39Ar ages with the metamorphic fabrics suggests that F1/F2 and F3 occurred in the Eocene, and that F4 developed in the Oligocene. F4 is the earliest indication of southward verging structures on this part of the Indian plate.
Reviews
Authors: Waters D.
Published: May 2007

Petrology of the Metamorphic Rocks, 2nd Edn. By R. Mason. Unwin Hyman

Atlas of Metamorphic Rocks and their Textures. By B. W. D. Yardley, W. S. Mackenzie and C. Guilford.
The Alpine geodynamic evolution of Penninic nappes in the eastern Central Alps
Authors: RING U.
Published: May 2007

The eastern Central Alps consist of several Pennine nappes with different tectonometamorphic histories. The tectonically uppermost units (oceanic Avers Bündnerschiefer, continental Suretta and Tambo nappes, oceanic Vals Bündnerschiefer) show Cretaceous/early Tertiary W‐directed thrusting with associated blueschist facies metamorphism related to subduction of the Pennine units beneath the Austroalpine continental crust. This event caused eclogite facies metamorphism in the underlying continental Adula nappe. The gross effect was crustal thickening. The tectonically lower, continental Simano nappe is devoid of any imprint from this event. In the course of continent‐continent collision, high‐T metamorphism and N‐directed movements occurred. Both affected the whole nappe pile more or less continuously from amphibolite to greenschist facies conditions. Crustal thinning commenced during the regional temperature peak. A final phase is related to differential uplift under retrograde P–T conditions. Further thinning of the crust was accommodated by E‐ to NE‐directed extensional deformation.
Metamorphism, geochemistry and origin of magnesian volcanic rocks, Klamath Mountains, California
Authors: HACKER B. R. , ERNST W. G. , BARTON M. D.
Published: May 2007

Metabasaltic rocks in the Klamath Mountains of California with ‘komatiitic’ major element concentrations were investigated in order to elucidate the origin of the magnesian signature. Trace‐element concentrations preserve relict igneous trends and suggest that the rocks are not komatitic basalts, but immature arc rocks and within‐plate alkalic lavas. Correlation of ‘excess’ MgO with the volume per cent hornblende (±clinopyroxene) suggests that the presence of cumulus phases contributes to the MgO‐rich compositions. Early submarine alteration produced regional δ18O values of +10±1.5%° and shifts in Al2O3, Na2O, and K2O concentrations. Regional metamorphic grade in the study area varies from biotite‐zone greenschist facies (350–550°C, c. 3 kbar) southward to prehnite–actinolite facies (200–400°C, ≤3 kbar), but little isotopic or elemental change occurred during the regional recrystallization. The greenschist facies assemblage is actinolitic hornblende + phengite + epidote + sodic plagioclase + microcline + chlorite + titanite + hematite + quartz in Ti‐poor metabasaltic rocks; in addition to these phases biotite is present in Ti‐rich analogues. Lower grade greenstones contain prehnite and more nearly stoichiometric actinolite. The moderate to low pressures of regional metamorphism are compatible with P–T conditions in a magmatic arc. Later contact metamorphism at 2–2.9±0.5 kbar and at peak temperatures approaching 600° C around the English Peak and Russian Peak granodiorites produced 3–4–km‐wide aureoles typified by gradual, systematic increases in the pargasite content of amphibole, muscovite content of potassic white mica, and anorthite content of plagioclase compositions. Metasomatism during contact metamorphism produced further increases in bulk‐rock δ18OSMOW of as much as +6%°. Thus, the unusually MgO‐rich nature of the Sawyers Bar rocks may be attributed at least partly to metasomatism and the presence of magnesian cumulus phases.
High‐pressure metamorphism in the Western Cordillera of North America
Authors: WHITNEY D. L.
Published: May 2007

The Skagit Gneiss, a major component of the crystalline core of the North Cascades, was metamorphosed during a mid‐Cretaceous(?) to early Tertiary high‐P event driven by the collision of the Insular and Intermontane superterranes. Maximum pressures recorded by metapelitic rocks are 8–10 kbar at 650–725° C. High pressures are also indicated by coexisting staurolite and hornblende in amphibolites in the Skagit Gneiss and adjacent Cascade River Schist.

Mineral reactions continued during nearly isothermal decompression from 8–10 kbar to c. 3–5 kbar. Early high‐P minerals (e.g. kyanite) are present as armoured relics in garnet in gneisses that contain sillimanite and cordierite in the groundmass. Skeletal relics of kyanite are also present in the groundmass of lower‐grade, staurolite‐bearing schists that contain texturally later cordierite. This matrix kyanite may have been preserved as a result of rapid uplift following initial decompression at high temperature.

These results represent a revision of the metamorphic history of the Skagit Gneiss, which was formerly thought to have experienced only relatively low‐P Barrovian metamorphism. Qualitative estimates of metamorphic conditions based on stable matrix mineral assemblages result in an underestimation of maximum pressures because mineral reactions continued during decompression.

Geobarometric results for the Skagit Gneiss are interpreted as evidence for major crustal thickening in the North Cascades. Recognition that pressures of c. 9 kbar were attained supports a contractional model for North Cascades orogenesis and requires that tectonic syntheses account for the burial of the Skagit Gneiss protoliths to a depth of c. 25–30 km.
Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes
Authors: BOUNDY T. M. , FOUNTAIN D. M. , AUSTRHEIM H.
Published: May 2007

Caledonian eclogite facies shear zones developed from Grenvillian garnet granulite facies anorthosites and gabbros in the Bergen Arcs of western Norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation (CPO) in lower continental crust. Field relations on the island of Holsnøy show that the eclogites formed locally from granulite facies rocks by progressive development of: (1) eclogite adjacent to fractures; (2) eclogite in discrete shear zones (> 2 m thick); (3) eclogite breccia consisting of >80% well‐foliated eclogite that wraps around rotated granulite blocks; and (4) anastomosing, subparallel, eclogite facies shear zones 30–100 m thick continuous over distances > 1 km within the granulite terrane. These shear zones deformed under eclogite facies conditions at an estimated temperature of 670 ± 50°C and a minimum pressure of 1460 MPa, which corresponds to depths of >55 km in the continental crust. Detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite/garnet‐rich and kyanite/zoisite‐rich layers. A consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite. The CPO of omphacite, determined from five‐axis universal stage measurements, shows a strong b‐axis maximum normal to foliation, and a c‐axis girdle within the foliation plane with weak maxima parallel to the lineation direction. These patterns are consistent with deformation of omphacite by slip parallel to [001] and suggest glide along (010). The lineation and CPO data reveal a consistent sense of shear zone movement, although the displacement was small. Localized faulting of high‐grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust. Field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth. Deformation of granulite and partial conversion to eclogite, such as is exposed on Holsnøy Island, may be an orogenic‐scale process in the lowermost crust of collisional orogens.