bluuebunny/crossref_metadata_2025 huggingface dataset:
Isotopic constraints on fluid infiltration from an eclogite facies shear zone, Holsenøy, Norway
Authors: MATTEY D. , JACKSON D. H. , HARRIS N. B. W. , KELLEY S.
Published: May 2007

Granulite facies anorthosites on Holsenøy Island in the Bergen Arcs region of western Norway are transected by shear zones 0.1–100 m wide characterized by eclogite facies assemblages. Eclogite formation is related to influx of fluid along the shears at temperatures of c. 700d̀C and pressures in excess of 1.7 GPa. Combined carbon and nitrogen stable isotope, 40Ar/36Ar, trace‐element and petrological data have been used to determine the nature and distribution of fluids across the anorthosite‐eclogite transition.

A metre‐wide drilled section traverses the eclogitic centre of the shear into undeformed granulite facies garnet‐clinopyroxene anorthosite. Clinozoisite occurs along grain boundaries and microcracks in undeformed anorthosite up to 1 m from the centre of the shear and clinozoisite increases in abundance as the edge of the shear zone is approached. The eclogite‐granulite transition, marked by the appearance of sodic pyroxene and loss of albite, occurs within the most highly sheared section of the traverse. The jadeite‐in reaction coincides with increased paragonite activity in mica. The separation between paragonite and clinozoisite reaction fronts can be semiquantitatively modelled assuming advective fluid flow perpendicular to the shear zone. The inner section of the traverse (0.25 m wide) is marked by retrogressive replacement of omphacite by plagioclase + paragonite accompanied by veins of quartz‐phengite‐plagioclase.

C‐N‐Ar characteristics of fluid inclusions in garnet show that fluids associated with precursor granulite, eclogite and retrogressed eclogite are isotopically distinct. The granulite‐eclogite transition coincides with a marked change in CO2 abundance and δ13C (<36ppm, δ13C=‐2% in the granulite; <180 ppm, δ13C=‐10% in the eclogite). The distribution of Ar indicates mixing between influxed fluid (40Ar/36Ar > 25 times 103) and pre‐existing Ar in the granulite (40Ar/36Ar < 8 times 103). δ15N values decrease from +6% in the anorthosite to +3% within the eclogite shear. The central zone of retrogressed eclogite post‐dates shearing and is characterised by substantial enrichment of Si, K, Ba and Rb. Fluids are CO2‐rich (δ13C ∼ ‐5%) with variable N2 and Ar abundances and isotopic compositions.

Both Ar and H2O have penetrated the underformed granulite fabric more than 0.5m beyond the granulite/eclogite transition during eclogite formation. Argon isotopes show a mixing profile consistent with diffusion through an interconnecting H2O‐rich fluid network. In contrast, a carbon‐isotope front coincides with the deformation boundary layer, indicating that the underformed anorthosite was impervious to CO2‐rich fluids. This is consistent with the high dihedral angle of carbonic fluids, and may be interpreted in terms of evolving fluid compositions within the shear zone.
Metamorphic evolution of granulites in the Minto Block, northern Québec: extraction of peak P‐T conditions taking account of late Fe‐Mg exchange
Authors: BéGIN N. J. , PATTISON D. R. M.
Published: May 2007

In the central Minto Block of northern Québec, the Lake Minto and Goudalie domains are dominated, respectively, by orthopyroxene‐bearing plutonic suites (granite‐granodiorite and diatexite) and a tonalitic gneiss complex, both of which contain scattered remnant paragneisses. Two main granulite‐grade mineral assemblages are observed in the paragneiss: garnet (Grt)‐orthopyroxene (Opx)‐plagioclase‐quartz (GOPQ) and garnet (Grt)‐cordierite (Crd)‐sillimanite‐plagioclase‐quartz (GCSPQ). These show distinct lithological associations, with the GCSPQ assemblages occurring exclusively within the diatexite in the Lake Minto domain. Petrogenetic grid considerations demonstrate that the GOPQ rocks are higher grade than the GCSPQ rocks. Maximum temperatures for GOPQ rocks, obtained from equilibria based on Al solubility in orthopyroxene in equilibrium with garnet, range from 950 to 1000d̀ C, significantly higher than garnet‐orthopyroxene Fe‐Mg exchange temperatures of 700 ± 50d̀ C, the latter probably representing a closure temperature below peak conditions. The Al temperatures were corrected for late cation exchange by adjusting the Fe/(Fe + Mg) ratios in garnet and orthopyroxene, to achieve internal consistency between the GOPQ thermometers and barometers. Grt‐Crd thermometry records temperatures of 750±50d̀ C. Peak P‐T conditions range from 5‐6 kbar and 750‐800d̀ C in the Goudalie and eastern Lake Minto domains, to 7‐10 kbar and 950‐1000d̀ C in the western and central Lake Minto domain. This variability contrasts with the uniform crustal pressures of 5 ± 1 kbar recorded by the GCSPQ assemblages in the diatexites and the hornblende granodiorites (c. 4‐5 kbar) across the same area. The GOPQ rocks are inferred to record earlier P‐T conditions that prevailed before the formation of GCSPQ assemblages and the intrusion of the granodiorites. Partial P‐T paths in GOPQ rocks from both domains, based on net transfer equilibria corrected for Fe‐Mg resetting, document cooling of 100‐250d̀ C from thermal‐peak conditions, concomitant with a modest pressure decrease of 2‐3 kbar. Although textures diagnostic of isobaric cooling are not developed, the paths are consistent with a tectonic model in which granulite metamorphism and crustal thickening in the Minto Block were consequences of magmatic underplating. The progression from higher P‐T conditions recorded by GOPQ assemblages to lower P‐T conditions recorded by GCSPQ assemblages is attributed to variable amounts of synmagmatic uplift and cooling in a single, continuous thermal event in the Minto crust, associated with protracted crustal magmatism. In the Goudalie and eastern Lake Minto domains, where GOPQ and GCSPQ rocks and Hbl granodiorites have similar P‐T conditions of equilibration, the crust may not have been thickened as much as further west, where GOPQ P‐T conditions are significantly higher than those of the hornblende granodiorites and the GCSPQ rocks.
Stable isotopic and fluid inclusion evidence for meteoric fluid penetration into an active mountain belt; Alpine Schist, New Zealand
Authors: JENKIN G. R. T. , CRAW D. , FALLICK A. E.
Published: May 2007

Calcite and quartz veins have formed, and are forming, in steeply dipping fissures in the actively rising Alpine Schist metamorphic belt of New Zealand. The fluids that deposited these minerals were mostly under hydrostatic pressure almost down to the brittle‐ductile transition, which has been raised to 5‐6 km depth by rapid uplift. Some fluids were trapped under lithostatic pressures. Fluids in the fissure veins were immiscible H2O + NaCl‐CO2 mixtures at 200‐350d̀ C. Bulk fluid composition is 15‐20 mol% CO2 and <4.3 total mol CH4+ N2+ Ar/100mol H2O. Water hydrogen isotopic ratio δDH2O in the fissure veins spans ‐29 to ‐68‰, δ18OH2O ‐0.7 to 8.5‰, and bulk carbon isotopic ratio δ13C ranges from ‐3.7 to ‐11.7‰. The oxygen and hydrogen isotopic data suggest that the water has a predominantly meteoric source, and has undergone an oxygen isotope shift as a result of interaction with the host metamorphic rock. Similar fluids were present during cooling and uplift. Dissolved carbon is not wholly derived from residual metamorphic fluids; part may be generated by oxidation of graphite.
Geothermobarometry of mafic granulites and metapelite from the Palghat Gap, South India: petrological evidence for isothermal uplift and rapid cooling
Authors: KUMAR C. R. RAVINDRA , CHACKO T.
Published: May 2007

The Palghat Gap region is located near the centre of the large southern Indian granulite terrane. at the northern edge of the Kodaikanal charnockite massif. The dominant rock types in the region are hornblende‐biotite ± orthopyroxene gneisses and charnockites along with minor amounts of intercalated mafic granulite, metapelite and calc‐silicate. TheP‐Testimates from garnetiferous mafic granulites and metapelite samples are generally in the range 9‐10 kbar and 800‐900d̀ C using both conventional thermobarometric methods and the TWEEQU thermobarometry program. TheseP‐Testimates, which should be taken as minimum values, are among the highest yet reported for South Indian and Sri Lankan granulites. The occurrence of orthopyroxene + plagioclase symplectites around embayed garnet grains in the mafic granulites and cordierite rims around garnet grains in metapelite suggest an isothermal decompression‐type path. Similarly, a core‐rimP‐Ttrajectory indicatesc.3 and 7 kbar decompression at high temperature in the mafic granulites and metapelite, respectively. In both rock types, the key to the determination of the retrogradeP‐Tpath was the recognition of small amounts of second generation plagioclase with a more anorthitic composition than the matrix plagioclase. The preservation of high garnet‐pyroxene temperatures in the mafic granulites (despite small garnet grain size) suggests rapid cooling of the terrane. Calculated minimum cooling rates range from 8 to 80d̀ C Ma‐1. Such cooling rates are more rapid than those associated with normal isostatic processes and suggest that the terrane was tectonically exhumed at high temperature.
The association of poorly ordered graphite, coke and bitumens in greenschist facies rocks of the Poniklá Group, Lugicum, Czech Republic: the result of graphitization of various types of carbonaceous matter
Authors: KRÍBEK B. , HRABAL J. , LANDAIS P. , HLADÍKOVÁ J.
Published: May 2007

Greenschist facies rocks of the Poniklá Group (Ordovician‐Silurian), Czech Republic, contain several types of carbonaceous matter that differ in their morphology, texture, reflectance and Raman characteristics.

The first type consists of large (up to 3 mm) irregularly bound particles of low reflectance (Romin = 0.9%; Romax = 5.6%). The area ratio of the 1585 cm‐1 to 1350 cm‐1 Raman peaks (1.08–1.17) indicates an intermediate degree of graphitization. The formation of this type of highly porous particle, displaying a texture reminiscent of regular or needle coke, is attributed to the thermal alteration of the amorphous (structureless) kerogen of the precursor sediments.

The second type consists of lamellar particles up to 30 μm thick, which can be associated with the latter or can occur independently in white mica‐rich laminae. This type is characterized by high bireflectance (Romin = 0.6%; Romax = 11.9%) and by lower ratios (0.70–0.82) of the Raman peak areas. These particles are interpreted as the product of solid‐state, diffusion‐controlled graphitization of a chemically homogeneous organic material, e.g. of graptolite periderms.

The third type consists of isometric, up to 2 mm large, commonly fractured grains and fragments which mainly occur in quartz‐rich laminae. In reflected light, the texture is either homogeneous or consists of various types of anisotropic mozaics. The Raman peak area ratios (0.75–1.14) indicate a highly variable degree of structural ordering. These particles are considered as the remains of metamorphosed bitumens, accumulated in the sandy laminae of the original sediments.

The fourth type consists of small particles of carbonaceous matter (maximum length 25 μm, thickness 1‐2 μm), which occur adjacent to crystal faces of white micas. This type is probably the product of epitaxial growth of graphite from the gaseous phase.

The results of this work indicate that the differences in the degree of graphitization of the carbonaceous matter in low‐grade metamorphosed rocks can be mainly related to the initial nature of the sedimentary organic matter and to its premetamorphic history.
The three‐dimensional geometry of simulated porphyroblast inclusion trails: inert‐marker, viscous‐flow models
Authors: GRAY N. H. , BUSA M. D.
Published: May 2007

A fluid dynamic model for a rigid spherical porphyroblast growing in a viscous fluid matrix undergoing simple shear deformation predicts an arrangement of captured inert inclusions that is remarkably similar to the spiral pattern observed in synkinematic ‘rolled’ garnets. The model assumes only creeping (Stokes) flow of the matrix and a kinetic model for the growth of the porphyroblast of the form rm=kmΩ, where r is the crystal radius, Ω is the amount of shear‐induced rotation, and m and k are kinetic parameters. For rotation less than 18d̀, J. B. Thompson and J. L. Rosenfeld's simple ring‐model provides a surprisingly accurate description of the three‐dimensional geometry of the central inclusion surface in the simulated porphyroblasts. Although marker planes, originally parallel to the shear plane, are deflected around the porphyroblast, their intersections with its surface remain approximately circular for much larger amounts of rotation than anticipated by Rosenfeld. The polar coordinate equation, T(r, θ), of the double spiral formed by the trace of the central inclusion surface in a section through the centre of simulated porphyroblasts, is also surprisingly simple, as follows: rm∼ (6/5k)mθ. This relationship was observed over the complete range of rotation angles investigated, 0‐720d̀, and could form a reasonable basis for estimating the growth‐rotation history of rolled garnets from the shape of their spiral inclusion trails. Two‐dimensional sections through the simulated porphyroblasts, parallel to the rotation axis, yield ‘clamshell’ (Rosenfeld's term) inclusion geometries similar to the controversial ‘millipede’ patterns observed in many natural porphyroblasts.
Fluid flow and mass transport at the Valentine wollastonite deposit, Adirondack Mountains, New York State
Authors: GERDES M. L. , VALLEY J. W.
Published: May 2007

The Valentine wollastonite skarn in the north‐west Adirondack Mountains, New York, is a seven million ton deposit which resulted from channellized infiltration of H2O‐rich, silica‐bearing fluids. The wollastonite formed by reaction of these fluids with non‐siliceous calcite marble. The skarn formed at the contact of the syenitic Diana Complex and was subsequently overprinted by Grenville‐age granulite facies metamorphism and retrograde hydrothermal alteration during uplift. Calcite marbles adjacent to the deposit have generally high δ18O values (c. 21‰), typical of Grenville marbles which have not exchanged extensively with externally derived fluids. Carbon isotopic fractiona‐tions between coexisting calcite and graphite in the marbles indicate equilibration at 675d̀ C, consistent with the conditions of regional metamorphism. Oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14‰ variation (‐1‰ to 13‰). Some isotopic heterogeneity is preserved from skarn formation, and some represents localized exchange with low‐δ18O retrograde fluids. Detailed millimetre‐ to centimetre‐scale isotopic profiles taken across skarn/marble contacts reveal steep δ18O gradients in the skarn, with values increasing towards the marble. The gradients reflect isotopic evolution of the fluid as it reacted with high δ18O calcite to form wollastonite. Calcite in the marble preserves high δ18O values to within <5 mm of the skarn contact. The preservation of high δ18O values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid. Thus, the marbles were relatively impermeable during both the skarn formation and retrograde alteration. Skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts. Although these steep isotope gradients resemble fluid infiltration fronts, they actually represent the sides of the major flow system. Because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock, such models are not applicable to this hydrothermal system and, by extension, to many other metamorphic systems where low‐permeability rocks restrict fluid migration pathways. Minimum time‐integrated fluid fluxes have been calculated at the Valentine deposit using oxygen isotopic mass balance, reaction progress of fluid buffering reactions, and silica mass balance. All three approaches show that large volumes of fluid were necessary to produce the skarn, but silica mass balance calculations yield the largest minimum flux and are hence the most realistic.
Contact metamorphism around the Stawell granite, Victoria, Australia
Authors: XU G. , POWELL R. , WILSON C. J. L. , WILL T. M.
Published: May 2007

The contact metamorphosed metapelitic and metapsammitic rocks surrounding the Stawell granite, western Victoria, Australia, are divided into three zones: the low‐grade zone, the medium‐grade zone and the high‐grade zone. Detailed petrological study shows consistency of element distributions, implying that equilibrium was widely attained in the rocks, although equilibrium volumes are generally small (millimetre scale) and considerable mineral chemical variations exist between adjacent domains. The metamorphic mineral assemblages are generally of high variance (KFMASH variance ≤ 2). Consequently, the chemical evolution of assemblages is controlled largely by bulk composition and metamorphic temperature, the former factor being more important in most rocks. The chemographic relations of mineral assemblages in low‐ and medium‐ to high‐grade zones are presented in compatibility diagrams projected from biotite, quartz and H2O, and biotite, K‐feldspar and H2O, respectively. These compatibility diagrams have the advantage of showing both quartz‐bearing and quartz‐absent assemblages. The metamorphic reactions are modelled successfully by a calculated petrogenetic grid that combines both KFASH and KMASH equilibria. Based on petrographic observations and with constraints from the calculated petrogenetic grid, the following KFMASH reactions, in the order of increasing metamorphic grade, are responsible for producing the various mineral assemblages in the Stawell rocks:

chl + mu + q = bi + cd + V,

chl + q + cd = g + V,

mu + bi + q = ksp + cd + V,

mu + q = ksp + and + cd + V (or KASH mu + q = ksp + and + V),

mu + cd = ksp + and + bi + V,

mu + bi + and = ksp + sp + V,

and + bi = ksp + sp + cd + V,

mu + bi = ksp + cor + sp + V,

mu = ksp + cor + and + sp + V (or KASH mu = ksp + cor + V),

bi + cd + q = g + ksp + V.

The combined KFASH and KMASH grid provides constraints on reaction coefficients in the above sequence of reactions and on temperature and pressure of metamorphism.
Timing of metamorphism in the Tauern Window, Eastern Alps: Rb‐Sr ages and fabric formation
Authors: INGER S. , CLIFF R. A.
Published: May 2007

The Peripheral Schieferhülle of the Tauern Window of the Eastern Alps represents post‐Hercynian Penninic cover sequences and preserves a record of metamorphism in the Alpine orogeny, without the inherited remnants of Hercynian events that are retained in basement rocks. The temperature‐time‐deformation history of rocks at the lower levels of these cover sequences have been investigated by geochronological and petrographic study of units whose P‐T evolution and structural setting are already well understood.

The Eclogite Zone of the central Tauern formed from protoliths with Penninic cover affinities, and suffered early Alpine eclogite facies metamorphism before tectonic interposition between basement and cover. It then shared a common metamorphic history with these units, experiencing blueschist facies and subsequent greenschist facies conditions in the Alpine orogeny. The greenschist facies phase, associated with penetrative deformation in the cover and the influx of aqueous fluids, reset Sr isotopes in metasediments throughout the eclogite zone and cover schists, recording deformation and peak metamorphism at 28‐30 Ma.

The Peripheral Schieferhülle of the south‐east Tauern Window yields Rb‐Sr white mica ages which can be tied to the structural evolution of the metamorphic pile. Early prograde fabrics pre‐date 31 Ma, and were reworked by the formation of the large north‐east vergent Sonnblick fold structure at 28 Ma. Peak metamorphism post‐dated this deformation, but by contrast to the equivalent levels in the central Tauern, peak metamorphic conditions did not lead to widespread homogenization of the Sr isotopes. Localized deformation continued into the cooling path until at least 23 Ma, partially or wholly resetting Sr white mica ages in some samples.

These isotopic ages may be integrated with structural data in regional tectonic models, and may constrain changes in the style of crustal deformation and plate interaction. However, such interpretations must accommodate the demonstrable variation in thermal histories over small distances.
Reaction textures and fluid behaviour in very high pressure calc‐silicate rocks of the Münchberg gneiss complex, Bavaria, Germany
Authors: KLEMD R. , MATTHES S. , SCHÜSSLER U.
Published: May 2007

Calc‐silicate rocks occur as elliptical bands and boudins intimately interlayered with eclogites and high‐pressure gneisses in the Münchberg gneiss complex of NE Bavaria. Core assemblages of the boudins consist of grossular‐rich garnet, diopside, quartz, zoisite, clinozoisite, calcite, rutile and titanite. The polygonal granoblastic texture commonly displays mineral relics and reaction textures such as post kinematic grossular‐rich garnet coronas. Reactions between these mineral phases have been modelled in the CaO‐Al2O3‐SiO2‐CO2‐H2O system with an internally consistent thermodynamic data base. High‐pressure metamorphism in the calc‐silicate rocks has been estimated at a minimum pressure of 31 kbar at a temperature of 630d̀ C with XH2, O ≥ 0.03. Small volumes of a CO2‐N2‐rich fluid whose composition was buffered on a local scale were present at peak‐metamorphic conditions. The P‐T conditions for the onset of the amphibolite facies overprint are about 10 kbar at the same temperature. XCo2 of the H2O‐rich fluid phase is regarded to have been <0.03 during amphibolite facies conditions. These P‐T estimates are interpreted as representing different stages of recrystallization during isothermal decompression. The presence of multiple generations of mineral phases and the preservation of very high‐pressure relics in single thin sections preclude pervasive post‐peak metamorphic fluid flow as a cause of a re‐equilibration within the calc‐silicates. The preservation of eclogite facies, very high‐pressure relics as well as amphibolite facies reactions textures in the presence of a fluid phase is in agreement with fast, tectonically driven unroofing of these rocks.
Evidence for pre‐regional metamorphic fluid infiltration of the Lower Calcsilicate Unit, Reynolds Range Group (central Australia)
Authors: BUICK I. S. , CARTWRIGHT I. , HAND M. , POWELL R.
Published: May 2007

Grandite garnet‐rich calcsilicate rocks from the Lower Calcsilicate Unit of the regionally metamorphosed Reynolds Range Group (central Australia) crop out along a strike‐parallel section in which a transition zone from M22 amphibolite to granulite facies rocks is exposed. Across this transition the grandite‐rich layers do not show systematic changes in mineral assemblages, compositions and modes, or stable isotope compositions. These layers are deformed by F22 folds that are associated with the peak of regional low‐pressure/high‐temperature metamorphism. Therefore, the grandite‐rich layers appear to pre‐date regional metamorphism and to have acted as closed chemical systems during prograde M22 metamorphism.

Mineral assemblages in the grandite‐rich layers are consistent with their formation through the infiltration of oxidized, water‐rich fluids (Xco2 < 0.1–0.3; log fo2 ‐16 to ‐14). The stable isotope values of calcite (Δ13C=‐4.2 to ‐0.8%0 PDB; Δ18O = 10.5–14.0%0 V‐SMOW) and bulk‐silicate fractions (Δ18O = 6.1 to 10.8%) of the grandite‐rich layers are most consistent with the infiltrating fluid being from a magmatic source. It is most likely that fluid infiltration occurred during the pre‐M22 contact metamorphism (M21) that affected much of the Reynolds Range Group. The preservation of these assemblages is probably due to their high variance and little pervasive fluid‐rock interaction having occurred during M22.

The clinopyroxene‐ and feldspar‐rich calcsilicate rocks that host the grandite‐rich layers contain poikiloblastic grandite garnet that formed during prograde M22 metamorphism. Thin marbles that locally occur with the grandite‐rich layers contain a third garnet generation that is post‐ or late M22. This grossular‐rich garnet occurs in coronas around calcite, plagioclase, clinopyroxene, wollastonite and scapolite. These coronas are consistent with cooling and/or compression. However, because the marble assemblages are themselves overprinted by M21 grandite‐rich layers the development of coronal garnet does not reflect a continuous P‐T‐t path. Rather, it more probably reflects the partial re‐equilibration of M21 contact metamorphic assemblages to post‐M22 conditions.
Low‐grade blueschist facies metamorphism of metagreywackes, Franciscan Complex, northern California
Authors: BRÖCKER M. , DAY H. W.
Published: May 2007

Abstract
Metagreywackes in the Eastern Belt of the Franciscan Complex contain the assemblage: Qtz + Ab + Lws + Chl + Ph + Pmp + Fgl + Hem ° Cal/Arg or compatible subassemblages. Blue amphibole first appears in the westernmost part of the belt and pumpellyite is absent in the eastern part. The compositions of the coexisting minerals and the nature of the continuous reactions in these low‐grade blueschists suggest that the distribution of blue amphibole and pumpellyite in the Eastern Belt of the Franciscan Complex reflects differences of effective bulk composition rather than differences in physical conditions of metamorphism. In rocks lacking pumpellyite, white mica may be essential to the growth of blue amphibole, but carbonate plays only a limited role. The continuous reaction that limits the appearance of blue amphibole and the disappearance of coexisting pumpellyite has the general form: Pmp + Chl + Ab + Qtz + Hem + H2O + FeMg‐1= Fgl + Lws. This reaction requires significant hydration as pressure increases in order to produce blue amphibole. Most of the Eastern Belt of the Franciscan Complex formed in limited ranges of temperature and pressure, which are estimated to be 240—280° C, 6.5‐7.5 kbar. Pressures in the westernmost part of the area were about 1 kbar lower than in the east. Pressures of about 8.5‐10 kbar are estimated for tectonic blocks that contain sodic clinopyroxene.
Qualitative thermobarometry of inverted metamorphism in the Pelona and Rand Schists, southern California, using calciferous amphibole in mafic schist
Authors: JACOBSON C. E.
Published: May 2007

Abstract
The Rand and Pelona Schists consist of eugeoclinal rock types overlain by continental basement along the Vincent‐Chocolate Mountains (VCM) faults. Both schists display inverted metamorphic zonation, defined in part by a systematic variation in composition of calcic to sodic‐calcic amphibole in mafic schist structurally upward. The compositional progressions include increase of total A1, A1IV and Ti, but decrease in the ratios of Na/(Na + Ca) to A1/(A1 + Si), and NaM4 to (A1VI+ Fe3++ Ti). These variations imply that structurally high rocks belong to a lower‐pressure metamorphic fades series than those at depth. This result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences.

Amphibole composition is dependent not only on structural position (i.e. P‐T), but also upon bulk‐rock composition. The important controls are whole‐rock Mg/(Mg + Fe2++ Mn) and Fe3+/Fe2+. The greatest impact of these factors, however, is on the absolute values of Na and Al, rather than their ratio. Thus, interpretation of facies series is not seriously hindered by compositional variability.

Sodic amphibole in epidote blueschists from the Rand Schist is extensively replaced by sodic‐calcic amphibole. Sodic‐calcic amphibole in the Rand Schist and Pelona Schist is, itself, rimmed by actinolitic amphibole. Similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation. In the Rand and Pelona Schists, the sequence probably formed during burial.
Pre‐Middle Jurassic accretionary metamorphism in the southern Klamath Mountains of northern California, USA
Authors: GOODGE J. W.
Published: May 2007

Abstract
High‐P/T metamorphic parageneses are preserved within two late Palaeozoic to early Mesozoic assemblages of the southern Klamath Mountains that show contrasting structural styles and mineral parageneses reflecting formation in different parts of a subduction‐zone regime. Blueschist facies tectonites of the Stuart Fork terrane represent a coherent subduction complex formed at relatively deep crustal levels, whereas the chaotic metasedimentary mélange of the eastern Hayfork terrane contains a diverse range of metamorphic parageneses reflecting complex structural mixing of metamorphic components at shallower levels. The convergent‐margin‐type accretionary metamorphism evident in both terranes pre‐dates Middle Jurassic low‐P/T metamorphism resulting from regional tectonic contraction and magmatism.

The epidote‐ to lawsonite‐zone Stuart Fork blueschists (and eclogites locally) formed at pressures of about 6‐11 kbar and temperatures of 250‐400° C. Deformed matrix material of the eastern Hayfork mélange formed at similar temperatures but lower pressures, on the order of 3‐6 kbar. The mélange contains a diverse assemblage of tectonic blocks that formed under a range of P‐T conditions, including those of the blueschist, pumpellyite‐actinolite, greenschist and upper greenschist to amphibolite facies.

The succession of mineral parageneses and inferred P‐T conditions of the eastern Hayfork blocks reflect those of igneous protolith formation, structural mixing, subduction‐zone metamorphism, olistolith transport, and tectonic and erosional denudation. Although temporal relations are not well constrained, the evolution of these terranes is consistent with formation within a single convergent‐margin system.
High‐pressure micro‐inclusions and development of an inverted metamorphic gradient in the Santiago Schists (Ordenes Complex, NW Iberian Massif, Spain): evidence of subduction and syncollisional decompression
Authors: ARENAS R. , PASCUAL F. J. RUBIO , GARCIA F. DIAZ , CATALAN J. R. MARTINEZ
Published: May 2007

Abstract
The Santiago Schists are located in the Basal Unit of the Ordenes Complex, one of the allochthonous complexes outcropping in the inner part of the Hercynian Belt in the north‐west of the Iberian Peninsula. Their tectonothermal evolution is characterized by the development of an eo‐Hercynian metamorphic episode (c. 374 Ma) of high‐P, low‐ to intermediate‐T. The mineral assemblage of the high‐P episode is preserved as a very thin Si= S1 foliation included in albite porphyroblasts, being composed of: albite + garnet‐I + white mica‐1 + chlorite‐1 + epidote + quartz + rutile ± ilmenite. The equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 ± 10 °C and 14.7 ± 0.7 kbar (probably minimum pressure). The later evolution (syn‐D2) of the schists defines a decompressive and slightly prograde P‐T path which reached its thermal peak at c. 525 ± 10 °C and 7 kbar. Decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient, so that the zones of garnet‐II, biotite (with an upper subzone with chloritoid) and staurolite developed from bottom to top of the formation.

The estimated P‐T path for the Santiago Schists suggests that the Basal Unit, probably a fragment of the Gondwana continental margin, was uplifted immediately after its subduction at the beginning of the Hercynian Orogeny. It also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension, probably related to the continued subduction of the Gondwana continental margin and the contemporaneous development of compensatory extension above it. The inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin, when it came into contact with the upper parts of the schists along a detachment, probably of extensional character.

The general metamorphic evolution of the Santiago Schists, with the development of high‐P assemblages with garnet prior to decompressive and prograde parageneses with biotite, is unusual in the context of the European Hercynian Belt, and shows a close similarity to the tectonothermal evolution of several high‐P, low‐ to intermediate‐T circum‐Pacific belts.
A multi‐stage pressure—temperature record in the Chilka Lake granulites: the epitome of the metamorphic evolution of Eastern Ghats, India?
Authors: SEN S. K. , BHATTACHARYA S. , ACHARYYA A.
Published: May 2007

Abstract
Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi‐stage P—T record. A high‐T decompression from above 10 kbar to 8 kbar around 1100°C has been determined from Mg‐rich metapelites (XMg>0.60) with quartz‐cordierite‐orthopyroxene‐sillimanite and cordierite—orthopyroxene—sapphirine—spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670°C at 8 kbar is evident. These changes are registered by the rim compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene—quartz—garnet—plagioclase—cordierite assemblage, and are further supported by the garnet + quartz ± orthopyroxene + cordierite and biotite‐producing reactions in sapphirine‐bearing metapelites. Another indication of isobaric cooling from 800 to 650°C at 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P—T values are obtained from metapelites with a quartz—plagioclase—garnet—sillimanite—cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700°C and the rims 5 kbar, 650°C, suggesting a third decompression.

The earliest deformation (F1) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P—T record. The effects of post‐crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking.

Two segments of the present P—T path replicate parts of the P—T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats.
Experimental study of phase relations involving osumilite in the system K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O at high pressure and temperature
Authors: AUDIBERT N. , BERTRAND P. , HENSEN B. J.
Published: May 2007

AbstractPhase relations and mineral chemistry for garnet (Grt), orthopyroxene (Opx), sapphirine (Spr), water‐undersaturated cordierite (Crd), osumilite (Osu), sillimanite (Sil), K‐feldspar (Kfs), quartz (Qtz) and a water‐undersaturated liquid (Liq) have been determined experimentally in the system KFMASH (K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O) under low PH2Oand fO2conditions. Four compositions have been studied with 100 [Mg/(Mg + Fe)] ranging from 65.6 to 89.7. Based on our experimental data, a P‐T grid is derived for the KFMASH system in the presence of quartz, orthopyroxene and liquid. Osumilite has been found in various mineral assemblages from 950 to 1100°C and 7.5 to 11 kbar. In the temperature range 1000‐1100°C, the pair Os‐Grt is stable over a pressure range of about 3kbar. The divariant reaction Os + Opx = Grt + Kfs + Qtz runs to the right with increasing pressure. Because osumilite is the most magnesian phase it is restricted to Mg‐rich compositions at high pressure. The reaction defining the upper pressure stability limit of Os‐Grt is located around 11 kbar with a nearly flat dP/dT slope over the temperature range 950–100°C. Over the entire temperature range investigated osumilite is not stable beyond 12 kbar. The data imply a restricted pressure range between 11 and 12 kbar for the stability of the assemblage Os‐Opx‐Sil‐Kfs‐Qtz. At 1050°C and above, osumilite occurs in various mineral assemblages together with the high‐T pair Spr‐Qtz.

When coexisting with garnet, orthopyroxene or sapphirine, osumilite is always the most magnesian phase. At 1050 and 1100°C, liquid is invariably the most Fe‐rich phase in the run product.

Our data support a theoretical P‐T grid for the KFMAS system in which osumilite is stable outside the field of the high‐T assemblage Spr‐Qtz. Moreover, our grid indicates that Os‐Opx‐Sil‐Kfs‐Qtz has a more restricted pressure and compositional stability domain than Os‐Grt, in agreement with natural occurrences. Osumilite is stable over a large pressure range, such that in Mg‐rich rocks, and at high temperature, it can occur at any depth in normal thickness continental crust.
Experimental diffusion of hydrogen into synthetic fluid inclusions in quartz
Authors: HALL D. L. , STERNER S. M.
Published: May 2007

Abstract
Quartz‐hosted, synthetic CO2‐H2O fluid inclusions behave as open systems with respect to diffusional transfer of hydrogen during laboratory‐simulated metamorphic re‐equilibration at 650, 750 and 825°C and 1.5 kbar total pressure with fO2 defined by the C‐CH4 buffer. Microthermometry and Raman spectroscopy show that the initial CO2‐H2O inclusions become CO2‐CH4‐H2‐H2O

inclusions after diffusive influx of hydrogen from the reducing confining medium. Measurable changes are observed in inclusion compositions after only 15 days of re‐equilibration, implying significant hydrogen mobility at still lower temperatures over geological time spans. Results of synthetic inclusion re‐equilibrium experiments have profound implications for the interpretation of natural fluid‐inclusion data; failure to account for potential hydrogen migration in inclusions from high‐temperature geological environments may lead to erroneous estimates of P‐T, and/or the compositions of metamorphic fluids.
Paragonite eclogites from Dabie Shan, China: re‐equilibration during exhumation?
Authors: OKAY A. I.
Published: May 2007

Abstract
Paragonite in textural equilibrium with garnet, omphacite and kyanite is found in two eclogites in the ultrahigh‐pressure metamorphic terrane in Dabie Shan, China. Equilibrium reactions between paragonite, omphacite and kyanite indicate a pressure of about 19 kbar at c. 700° C. However, one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet, suggesting minimum pressures of 27 kbar at the same temperature. The disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh‐pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh‐pressure assemblages. This hypothesis is tested by calibrating a garnet + zoisite/clinozoisite + kyanite + quartz/coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh‐ and high‐pressure terranes. These four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite/clinozoisite = grossular + kyanite + quartz/coesite + H2O. The results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts. For the paragonite eclogites in Dabie Shan the geobarometer indicates pressures in the quartz stability field, confirming that the former coesite‐bearing paragonite‐eclogite has re‐equilibrated at lower pressures. On the other hand, garnets from other coesite‐bearing but paragonite‐free kyanite‐zoisite eclogites show a very wide variation in grossular content, corresponding to a pressure variation from coesite into the quartz field. This wide variation, partly due to a rimward decrease in grossular component in garnet, is caused by partial equilibration of the mineral assemblage during the exhumation.
Tectonothermal evolution of high‐alumina rocks within the Protogine Zone, southern Sweden
Authors: ANDRÉASSON P. G. , DALLMEYER R. D.
Published: May 2007

Abstract
The Protogine Zone comprises a system of anastomosing deformation zones which approximately parallel the eastern boundary of the Sveconorwegian (1200–900 Ma) province in south‐west Sweden. Ages of granulite facies metamorphism in the Sveconorwegian province require exhumation from c. 30 to 35 km crustal depths after 920–880 Ma. 40Ar/39 Ar cooling ages are presented for muscovite from high‐alumina rocks formed by hydrothermal leaching associated with the Protogine Zone. Growth of fabric‐defining minerals was associated with a ductile deformational event; muscovite from these rocks cooled below argon retention temperatures (c. 375 ± 25° C) at c. 965–955 Ma. Muscovite from granofels in zones of intense alteration indicates that temperatures > 375 ± 25° C were maintained until c. 940 Ma. Textural relations of Al2SiO5 polymorphs and chloritoid suggest that dated fabrics formed during exhumation. The process of exhumation, brittle overprint on ductile structures and hydrothermal activity along faults within the Protogine Zone tentatively are interpreted as the peripheral effects of initial Neoproterozoic exhumation of the granulite region of south‐western Sweden.

Muscovite in phyllonites associated with the ‘Sveconorwegian thrust system’cooled below argon retention temperatures at c. 927 Ma. Exhumation associated with this cooling could have been related to extension and onset of brittle‐ductile deformation superimposed on Sveconorwegian contraction.
Strain‐related differences in the crystal growth of white mica and chlorite: a TEM and XRD study of the development of metapelitic microfabrics in the Southern Uplands thrust terrane, Scotland
Authors: MERRIMAN R. J. , ROBERTS B. , PEACOR D. R. , HIRONS S. R.
Published: May 2007

Abstract
TEM and XRD techniques were used to study crystal growth characteristics of the fabric‐forming phyllosilicates which developed in response to low‐grade metamorphism and tectonic imbrication in part of the Southern Uplands thrust terrane. Prograde regional metamorphism, ranging from late diagenesis through the anchizone to the epizone, was accompanied by the development of a slaty cleavage which is commonly bedding‐parallel. TEM‐measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with XRD‐measured crystallinity indices. Analytical TEM data show that prograde changes in composition lead to a net loss of Si, Ca and minor Fe from the fabric‐forming phyllosilicates. White micas are paragonite‐poor phengites with a mean b lattice parameter of 9.037 Å, and indicate an intermediate pressure series of metamorphism with a field gradient of <25° C km‐1. Chlorite compositions evolved from diabantite (with intergrown corrensite) to ripidolite over an estimated temperature range of 150–320° C. Field gradient and temperature estimates suggest that crystal growth and fabric development occurred at burial depths ranging from 6 km to at least 13 km in the thrust terrane. During late diagenesis, crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions, and resulted in similar size distributions which resemble typical Ostwald ripening curves. Under anchizonal and epizonal conditions, white mica grew more rapidly than chlorite because of its greater ability to store strain energy and recover from subgrain development; as a result crystal thickness distributions are not typical of Ostwald ripening. In contrast, chlorite crystals which grew under these conditions developed subgrain boundaries at high strain rates which were only partially recovered at low strain rates; these retained dislocations reduce the crystallite thicknesses detected by TEM and XRD, compared with those of white mica. These differences in strain‐induced crystal growth indicate that white mica (illite) and chlorite crystallinity indices are likely to show significant differences where low‐grade metamorphism is closely associated with tectonic fabric development.
Carbon isotope thermometry in marbles of the Adirondack Mountains, New York
Authors: KITCHEN N. E. , VALLEY J. W.
Published: May 2007

Abstract
Carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the Adirondack Mountains, New York. Eighty‐nine calcite‐graphite pairs from the amphibolite grade NW Adirondacks change systematically in temperature north‐westwards from 680 to 640 to 670° C over a 30‐km distance, reflecting transitions from amphibolite facies towards granulite facies to the north‐west and to the south‐east. Temperature contours based on calcite‐graphite thermometry in the NW Adirondacks parallel mineral isograds, with the orthopyroxene isograd falling above 675° C, and indicate that regional metamorphic temperatures were up to 75° C higher than temperatures inferred from isotherms based on cation and solvus thermometry (Bohlen et al. 1985). Fifty‐five calcite‐graphite pairs from granulite grade marbles of the Central Adirondacks give regional metamorphic temperatures of 670–780° C, in general agreement with cation and solvus thermometry.

Data for amphibolite and granulite grade marbles show a 12%oo range in δ13Ccal and δ13Cgr. A strong correlation between carbon isotopic composition and the abundance of graphite (Cgr/Crock) indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism. The trends seen in δ13C vs. Cgr/Crock and δ13Ccal vs. δ13Cgr could not have been preserved if significant amounts of CO2‐rich fluid had pervasively infiltrated the Adirondacks at any time. The close fit between natural data and calculated trends of δ13C vs. Cgr/Crock indicates a biogenic origin for Adirondack graphites, even though low δ13C values are not preserved in marble.

Delamination of 17 graphite flakes perpendicular to the c‐axis reveals isotopic zonation, with higher δ13C cores. These isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature, and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite. Samples located >2km from anorthosite show a decrease of 0.5‐0.8%oo in the outer 100 μ of the grains, while samples at distances over 8 km show smaller core‐to‐rim decreases of c.0.2%oo. Correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism. Core graphite compositions indicate contact metamorphic temperatures were 860–890° C within 1 km of the Marcy anorthosite massif. If samples with a significant contact metamorphic effect (Δ(cal‐gr) <3.2%oo) are not included, then the remaining 38 granulite facies samples define the relation Δ13C(cal‐gr) = 3.56 ± 106T‐2 (K).