bluuebunny/crossref_metadata_2025 huggingface dataset:
Conditions of Archean granulite metamorphism in the Godthab‐Fiskenaesset region, southern West Greenland
Authors: RICIPUTI L. R. , VALLEY J. W. , McGREGOR V. R.
Published: May 2007

The Nordlandet peninsula (Akia terrane) and the Tasiusarsuaq terrane in southern West Greenland were metamorphosed to granulite facies at 3.0 and 2.8 Ga respectively. Temperatures of metamorphism are estimated using magnetite + ilmenite, garnet + orthopyroxene, garnet + clinopyroxene and garnet + biotite thermometers. Barometry has been carried out in the two terranes using eight different garnet barometers. A uniform set of activity models for all minerals, including the garnet activity model of Newton et al. (1986), is applied to each barometer in order to permit comparison. Pressure estimates using the different barometers are generally quite consistent (±1.5 kbar). Use of the Newton et al. (1986) garnet activity data results in pressures similar to those obtained using other garnet activity models.

Peak metamorphic conditions on the Nordlandet peninsula are estimated to have been 800 ± 50°C, 7.9 ± 1.0 kbar. Values of logfO2 are estimated to have been 1.1 to 2.0 above the quartz + magnetite + fayalite buffer from assemblages of magnetite + ilmenite and quartz + magnetite + ferrosilite. Peak metamorphic conditions in the Tasiusarsuaq terrane are estimated to have been 780 ± 50°C, 8.9 ± 1.0 kbar. Estimates of fH2O and fCO2 using biotite, amphibile, grossular + anorthite and grossular + scapolite equilibria are low in both terranes. These results suggest that granulite metamorphism was fluid absent in both terranes, and that the metamorphism in the Akia terrane and possibly also in the Tasiusarsuaq terrane was initiated by the injection of large volumes of magma into the lower crust.
Compositional characteristics and phase equilibria in manganiferous iron formations from a high‐grade terrain near Satnuru, Karnataka, India
Authors: BHATTACHARYA A. , SPIERING B. , SEN S. K. , NATARAJAN R. , MAZUMDAR A. C.
Published: May 2007

Manganese‐rich and manganese‐poor iron formations which occur as thin layers in the Halaguru‐Satnuru area, south of Kabbaldurga, Karnataka, India are chemically intermediate between the ‘Algoma’and ‘Lake Superior’types, but higher in their MnO and TiO2contents. The rocks are of four petrographic varieties: (a) quartz‐magnetite‐orthopyroxene‐clinopyroxene, (b) quartz‐magnetite‐orthopyroxene‐clinopyroxene‐garnet, (c) quartz‐magnetite‐clinopyroxene‐garnet, and (d) quartz‐magnetite‐clinopyroxene‐garnet‐plagioclase.

In the orthopyroxene‐clinopyroxene pairs, Mn‐Mg and Mn‐Fe exchange is ideal irrespective of the MnSiO3contents of orthopyroxenes (0.6–1.8 mol. % in Mn‐poor and 15–25 mol. % in Mn‐rich compositions). Mg‐Fe exchange in the same pair is however non‐ideal. Mn‐Fe exchange in orthopyroxene‐garnet pairs is ideal. The distribution patterns in the other binaries are inconclusive regarding ideality of exchange. Orthopyroxene‐garnet and clinopyroxene‐garnet geothermometers, modified for high spessartine contents, give temperatures of 800 ± 30° C. A modified version of the Harley (1984) geothermometer registers 740 ± 60° C, in agreement with the consensus temperature value.

The equilibrium logffo2values in the iron formations, as calculated from the reaction 6FeSiO3+ O2= 2Fe3O4+ 6SiO2are in the range of −14.2 to −15.5. Algebraic analysis of variations offo2with composition of phases indicates buffering of O2in the rocks. The absence of grunerite in these assemblages is compatible withXH2Obeing less than 0.3 in the ambient fluid. Computations from volatile equilibria in the C‐O‐H system, however, predict high XH2Ovalues (>0.7) atac= 1.0, implying that the activity of graphite must have been greatly reduced—this is in accordance with the absence of graphite in these rocks.
A model for garnet and plagioclase growth in pelitic schists: implications for thermobarometry and P‐T path determinations
Authors: SPEAR F. S. , KOHN M. J. , FLORENCE F. P. , MENARD T.
Published: May 2007

Numerical models of the progressive evolution of pelitic schists in the NCMnKFMASH system with the assemblage garnet + biotite + chlorite ± staurolite + plagioclase + muscovite + quartz + H2O are presented with the goal of predicting compositional changes in garnet and plagioclase along different P‐T paths. The numerical models support several conclusions that should prove useful for interpreting the P‐T paths of natural parageneses: (i) Garnet may grow along P‐T vectors ranging from heating with decompression to cooling with compression. P‐T paths deduced from garnet zoning that are inconsistent with these growth vectors are self‐contradictory. (ii) There is a systematic relation between garnet and plagioclase composition and growth such that for most P‐T paths, garnet growth requires plagioclase consumption. Furthermore, mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic. Inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores. Complex plagioclase textures may arise from the local variability of growth and precipitation kinetics. (iii) A decrease of Fe/(Fe + Mg) in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled. However, there is no single reliable ΔP monitor and inferences about ΔP can only be made by considering plagioclase and garnet together. (iv) Consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and P‐T conditions impossible. Low ‘peak’temperatures typically recorded by staurolite‐bearing assemblages may reflect this phenomenon. (v) Diffusional homogenization of garnet affects the computed P‐T path and results in a clockwise rotation of the computed P‐T vector relative to the true P‐T path.
Prograde and retrograde metamorphism of hematitebearing basic schists in the Sanbagawa belt in central Shikoku
Authors: OTSUKI M. , BANNO S.
Published: May 2007

The mineral assemblages of hematite‐bearing basic schists in intermediate high‐pressure metamorphism are temperature dependent. For assemblages with excess hematite, albite, muscovite and quartz, the paragenetic relations can be dealt with in terms of a four‐component system, without omitting or grouping major components.

In the Sanbagawa belt in central Shikoku, the dominant amphibole in the hematite‐bearing basic schists changes from winchite, via crossite and barroisite to hornblende. The stability of amphibole is described chemographically within a pseudoternary system with another excess phase, epidote. Many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low‐T/P amphibole around the prograde amphibole. The examination of amphibole zoning makes it possible to draw a retrograde P‐T trajectory which passes on the lower pressure side of the prograde one.
Petrological constraints on the eclogite‐ and blueschistfacies metamorphism and P‐T‐t paths in the Western Alps*
Authors: POGNANTE U.
Published: May 2007

Phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high‐P rocks in the nappes of the Western Alps: very high‐P eclogite‐facies (including kyanite eclogites and coesite‐pyrope assemblages), eclogite‐facies (paragonite‐zoisite eclogites), high‐T blueschist‐facies (glaucophane‐garnet ± lawsonite assemblages) and low‐T blueschist‐facies (glaucophane‐lawsonite ± pumpellyite assemblages). The blueschist‐facies‐eclogite‐facies transition is promoted chiefly by increasing T, low bulk XMg and relatively low μH2O. The variety of assemblages and the heterogeneous approach to equilibrium observed in the Alpine rocks are not only constrained by the intersection of the reaction surfaces in P‐T‐X space, but also by the effect and timing of the processes which control kinetics (i.e. pervasive deformation and fluid infiltration). The faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures (and times), all may have induced μH2O gradients and contributed to the heterogeneous distribution of deformation through a process of reaction‐enhanced ductility. Also mass‐transfer may have been an effective process in determining the type of high‐P assemblage in particular rock volumes.

As regards the P‐T‐t paths, only the post‐climax histories are recorded well in the Alpine nappes. Post‐eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early‐Alpine (Cretaceous) event. In contrast, more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early‐Alpine event and the subsequent meso‐Alpine (Palaeogene) collision between the ‘European’and ‘African’plates. In the Western Alps, prevalent eclogite‐facies conditions were attained during the metamorphic climax of the early‐Alpine subduction, while blueschist‐facies recrystallization characterizes the early‐Alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso‐Alpine continental collision.
Isocon analysis of migmatization in the Front Range, Colorado, USA
Authors: OLSEN S. N. , GRANT J. A.
Published: May 2007

Isocon analysis has been applied to five sets of leucosome, mafic selvages and immediately adjacent mesosome in the migmatites from a 15‐m outcrop in the Colorado Front Range. The results show: (i) mafic selvages formed from the adjacent mesosome by loss of felsic components and therefore the mesosomes are indeed palaeosomes or protoliths; (ii) the leucosomes did not form in a closed system from the palaeosome (in which case the material lost from the palaeosome during selvage formation would become the leucosome). The observed volumes and compositions of leucosomes require that the present leucosome must contain some material in addition to the felsic components lost from the selvages. The materials that must be added are leucotonalitic to granitic in composition, varying greatly in K/(Na + Ca) ratio. The trend in leucosome composition can be reproduced by assuming that a metasomatic exchange, KNa + Ca, modified originally leucotonalitic leucosomes to more K‐rich compositions. These leucosomes most likely formed by injection of silicate melts accompanied, or followed, by metasomatism. The trend of leucosome compositions in this study reflects the general trend in the leucosome compositions which have been published from other areas, indicating that the proposed mechanism can be applicable to other regional migmatites.
Proterozoic granulite facies metamorphism in the southeastern Reynolds Range, central Australia: geological context, P–T path and overprinting relationships
Authors: CLARKE G. L. , POWELL R.
Published: May 2007

In the southeastern Reynolds Range, central Australia, a low‐P granulite facies metamorphism affected two sedimentary sequences: the Lander Rock Beds and the Reynolds Range Group. In the context of the whole of the Reynolds Range and the adjacent Anmatjira Range, this metamorphism is M3 in a sequence M1–4 that occurred over a period of 250 Ma. In particular, M1 affected the Lander Rock Beds prior to the deposition of the Reynolds Group. M3 has an areally restricted, high‐grade area in the southeastern Reynolds Range, affecting both the Reynolds Range Group and the underlying Lander Rock Beds. The effects of M3 are characterized by spinel + quartz‐bearing peak metamorphic assemblages in metapelites, which imply peak conditions of ≥750°C and 4.5 ± 1 kbar, and involved isobaric cooling or compression with cooling. It is concluded that one of a series of thermal perturbations caused by thinning of mantle lithosphere contemporaneous with crustal thickening was responsible for M3. In the southeastern Reynolds Range, evidence of both the unconformity between the two rock groups and previous metamorphism/deformation has been completely erased by recrystallization during M3–D3.
Infiltration‐driven dehydration and anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada
Authors: PATTISON D. R. M.
Published: May 2007

Dark hornblende + garnet‐rich, quartz‐absent metagabbro boudins from the Seguin subdomain, Ontario Grenville Province, are transected by anastomosing light‐coloured veins rich in orthopyroxene, clinopyroxene, plagioclase and sometimes quartz. The veins vary in texture from fine‐grained diffuse veins and patches that overprint the metagabbro, to coarse tonalitic leucosomes with sharp borders. The diffuse veins and patches are suggestive of channellized subsolidus dehydration of the metagabbro, while the tonalitic leucosomes are suggestive of local internally‐derived anatexis. All vein types grade smoothly into each other, with the tonalitic leucosomes being the latest.

Relative to the host metagabbro, the veins have higher Si, Na, Ba & Sr, lower Fe, Mg, Ca & Ti, and similar Al. The coarser veins are enriched in K. Plagioclase becomes steadily enriched in Na in the transition from host metagabbro (An47) to the veins (An35), and in the coarsest veins it is antiperthitic. Differences in composition of the other minerals between host metagabbro and vein are minor. Pressure–temperature estimates are scattered, but indicate a minimum temperature during vein formation of 700°C at about 8 kbar.

Mass balance constraints indicate that the veins formed from the metagabbro in an open system. The transecting veins are interpreted to represent pathways of Si + Na + Ba + Sr ± K ± Al‐enriched, low aH2O fluids that metasomatized the host metagabbro to form the anhydrous veins. An initial period of localized solid‐state dehydration of the metagabbro, represented by the diffuse veins, was followed by a transition to localized anatexis, represented by the tonalitic leucosomes. The change to anatexis may have been due to the addition of K to the infiltrating fluid. The source and delivery mechanism of the fluids is unknown.
Decompressional coronas and symplectites in granulites of the Musgrave Complex, central Australia
Authors: CLARKE G. L. , POWELL R.
Published: May 2007

Abstract
In granulite facies metapelitic rocks in the Musgrave Complex, central Australia, reaction between S1 garnet and sillimanite involves the development in S2 of both garnet + cordierite + hercynitic spinel + biotite and hercynitic spinel + cordierite + sillimanite + biotite. The S2 assemblages occur either in coronas and symplectites, mainly around garnet, or, in rocks in which S2 is more strongly developed, as recrystallized assemblages. Ignoring the presence of biotite and ilmenite, the mineral textures can be accounted for qualitatively by a consideration of the model system FeO‐MgO‐Al2O3‐SiO2 (FMAS); the textural relationships accord with decompression accompanying the change from S1 to S2. However, since biotite and ilmenite are involved in the assemblages, the parageneses are better accounted for in terms of equilibria in the expanded model system K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2‐TiO2‐Fe2O3 (KFMASHTO), i.e. AFM + TiO2+ Fe2O3. The coronas reflect the tectonic unroofing of at least part of the Musgrave Complex from peak S1 conditions of about 8 kbar to S2 conditions of about 4 kbar.
A new petrogenetic grid for low‐grade metabasites
Authors: FREY M. , CAPITANI C. DE , LIOU J. G.
Published: May 2007

Abstract
We have used internally‐consistent thermodynamic data to present calculated phase equilibria for the system Na2O‐CaO‐MgO‐Al2O3‐SiO2‐H2O (NCMASH), in the range 0–500° C and 0.1–10 kbar, involving the phases anorthite, glaucophane, grossular, heulandite, jadeite, laumontite, lawsonite, paragonite, prehnite, pumpellyite, stilbite, tremolite, wairakite, zoisite with excess albite, clinochlore, quartz and pure water. Average activity terms derived from published mineral chemical data were included for clinochlore, glaucophane, prehnite, pumpellyite, tremolite, and zoisite. The new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite, prehniteactinolite, prehnite‐pumpellyite, pumpellyite‐actinolite, blueschist and greenschist facies metabasites. The stability fields of mineral assemblages containing prehnite, pumpellyite, epidote, actinolite (+ albite + chlorite + quartz) were analysed in some detail, using activity data calculated from five specific samples. For example, the prehnite‐actinolite facies covers a P‐T field ranging from about 220 to 320° C at pressures below 4.5 kbar. The transition from the prehnite‐actinolite and pumpellyite‐actinolite to greenschist facies occurs at about 250–300° C at 1–3 kbar and at about 250–350° C at 3–8 kbar. P‐T fields of individual facies overlap considerably due to variations in chemical composition.
The P–T–deformation path for a mid‐Proterozoic, low‐pressure terrane: the Reynolds Range, central Australia
Authors: DIRKS P. H. G. M. , HAND M. , POWELL R.
Published: May 2007

Abstract
The Proterozoic low‐pressure, high‐temperature (LPHT) terrane of the Reynolds Range occurs in a 130‐km‐long, NW‐trending belt in the central part of the Arunta Block, central Australia. The Reynolds Range has been affected by two mid‐Proterozoic tectonic cycles, DI and DII, associated with two metamorphic events, MI and MII. DI–MI effects are restricted to the older of two sedimentary successions, the Lander Rock beds, which are separated from the younger Reynolds Range Group by an angular unconformity. The dominant structural–metamorphic features formed during DII–MII affected both sedimentary successions and the various granites that intruded them, and reworked most DI–MI effects. The DII deformation history can be subdivided into one prograde, two peak, and one retrograde stage. Average P–T calculations in the southeastern half of the range indicate a peak‐metamorphic pressure of 4.1 ± 0.3 kbar. Because the calculated values are derived from the same stratigraphic level corresponding to the base of the Reynolds Range Group, which is exposed throughout the area, it is likely that pressures were similar in the entire range. In fact, however, the peak‐metamorphic temperature shows a dramatic increase from greenschist facies (c. 400° C) in the northwest to granulite facies (740 ± 60° C) in the southeast, indicating that MII was associated with anomalously high heat flows. The P–T path is anticlockwise, with isobaric cooling from the metamorphic peak indicated by corona textures. However, the evidence of a prograde increase in pressure is indirect and based on the compressional nature of the structures. Peak‐metamorphic mineral assemblages and retrograde mineral assemblages in amphibolite facies shear zones show the same metamorphic zonation, suggesting they formed in response to the same thermal event. If this is true, the implication is that a thermal perturbation external to the crust was maintained for a considerable period of time (110 Ma, based on zircon dating). As it is not clear whether Proterozoic, asthenosphere‐active, thermal perturbations operated for this long, the alternative interpretation must be considered, namely that the peak‐metamorphic events are separate from the shear zone event associated with reheating of the area.
Integrated TEM, XRD and electron microprobe investigation of mixed‐layer chlorite–smectite from the Point Sal ophiolite, California
Authors: BETTISON‐VARGA L. , MACKINNON I. D. R. , SCHIFFMAN P.
Published: May 2007

Abstract
Five basalt samples from the Point Sal ophiolite, California, were examined using HRTEM and AEM in order to compare observations with interpretations of XRD patterns and microprobe analyses. XRD data from ethylene‐glycol‐saturated samples indicate the following percentages of chlorite in mixed‐layer chlorite–smectite identified for each specimen: (i) L2036 ± 50%, (ii) L2035 ± 70 and 20%, (iii) 1A‐13 ± 70%, (iv) 1B‐42 ± 70%, and (v) 1B‐55 = 100%. Detailed electron microprobe analyses show that ‘chlorite’analyses with high Si, K, Na and Ca contents are the result of interlayering with smectite‐like layers. The Fe/(Fe + Mg) ratios of mixed‐layer phyllosilicates from Point Sal samples are influenced by the bulk rock composition, not by the percentage of chlorite nor the structure of the phyllosilicate.

Measurements of lattice‐fringe images indicate that both smectite and chlorite layers are present in the Point Sal samples in abundances similar to those predicted with XRD techniques and that regular alternation of chlorite and smectite occurs at the unit‐cell scale. Both 10‐ and 14‐Å layers were recorded with HRTEM and interpreted to be smectite and chlorite, respectively. Regular alternation of chlorite and smectite (24‐Å periodicity) occurs in upper lava samples L2036 and 1A‐13, and lower lava sample 1B‐42 for as many as seven alternations per crystallite with local layer mistakes. Sample L2035 shows disordered alternation of chlorite and smectite, with juxtaposition of smectite‐like layers, suggesting that randomly interlayered chlorite (<0.5)–smectite exists. Images of lower lava sample 1B‐55 show predominantly 14‐Å layers. Units of 24 Å tend to cluster in what may otherwise appear to be disordered mixtures, suggesting the existence of a corrensite end‐member having thermodynamic significance.
Thermobarometry and granite genesis: the Hercynian low‐P, high‐T Velay anatectic dome (French Massif Central)
Authors: MONTEL J. M. , MARIGNAC C. , BARBEY P. , PICHAVANT M.
Published: May 2007

The Velay dome (French Massif Central) offers a quasi‐continuous section across an anatectic domain comprising low‐ to high‐grade schists, gneisses and granites. Two main tectonometamorphic events, and their related generation of granitic material, were recognized in addition to a major Barrrovian tangential event (D2) attributed to intracontinental collision tectonics: (i) a medium‐ to low‐P, high‐T event (D3) which gave rise to migmatites and syntectonic monzonitic granites and granodiorites, and (ii) a widespread melting event (D4) which led to the generation of migmatities, the Velay granite and post‐anatectic granites.

Thermobarometry on samples collected from both the metamorphic envelope and the granitic core distinguishes two distinct geotherms: (i) a first, associated with the D3 event, characterized by P > 5 kbar, T≤ 750° C and water‐present melting (biotite remains stable) which led to large‐scale migmatization but minor amount of granites; (ii) a second, associated with the D4 event and characterized by vapour‐absent melting (P= 4–5 kbar, T= 760–850° C) which gave rise to the Velay granites and late‐migmatitic granites. The temperature increase during the D4 event is attributed to the intrusion of hot mafic magmas within the crust.

The time‐integrated features of the different granitic rocks in the Velay dome can be directly related to aH2O in the source region and illustrate the progressive dehydration of a middle to lower crustal segment over 60 Ma.
P–T–X relationships in the Precambrian Al–Mg‐rich granulites from In Ouzzal, Hoggar, Algeria
Authors: BERTRAND P. , OUZEGANE KH. , KIENAST J. R.
Published: May 2007

The Al–Mg‐rich granulites from the In Ouzzal craton, Algeria, show a great diversity of mineral reactions which correspond to continuous equilibria as predicted by phase relationships in the FeO–MgO–Al2O3–SiO2 system. The sequence of mineral reactions can be subdivided into three distinct stages: (1) a high‐P stage characterized by the growth of coarse mineral assemblages involving sapphirine and the disappearance of early corundum and spinel‐bearing assemblages; (2) a high‐T stage characterized by the development of Sa–Qz‐bearing assemblages; and (3) a later stage, in which garnet‐bearing assemblages are replaced by more or less fine symplectites involving cordierite.

During the course of early mineral reactions, the distribution coefficient, Kd, between the various ferromagnesian phases decreased significantly whereas Al2O3 in pyroxene increased concomitantly. These observations, when combined with topological constraints, clearly indicate that the high‐P stage 1 was accompanied by a significant rise in temperature (estimated at 150 ± 50° C) under near isobaric conditions, in agreement with the reaction textures. By stage 2, pressure and temperature were extreme as evidenced by the low Kd value between orthopyroxene and garnet (Kd= 2.06–1.99), the high alumina content in pyroxene (up to 11.8%) and the high magnesium content in garnet [100 Mg/(Mg + Fe) = 60.6]. Mineral thermometry based on Fe–Mg exchange between garnet and pyroxene and on Al‐solubility in pyroxene gives temperatures close to 970 ± 70° C at 10 ± 1.5 kbar. These results are in agreement with the development of Sa–Qz assemblages on a local scale.

Late mineral reactions have been produced during a decompression stage from about 9 to 6 kbar. Except for local re‐equilibration of Mg and Fe at grain boundaries, there is no evidence for further reactions below 700° C.

We interpreted the whole set of mineral reactions as due to changes in pressure and temperature during a tectonic episode located at c. 2 Ga. Because of the lack of evidence for further uplift after the thermal relaxation which occurred at c. 6 kbar, it is possible however that the exhumation of this granulitic terrane occurred in a later tectonic event unrelated to its formation.
Charnockitic alteration: evidence for CO2infiltration in granulite facies metamorphism
Authors: NEWTON R. C.
Published: May 2007

Charnockitic alteration (arrested orthopyroxene formation in biotite‐ and amphibole‐bearing rocks) occurs in high‐grade terranes of all ages. Three criteria are used to show that this alteration was produced in many locations by a migrating fluid phase: (i) diffuseness of the alteration—the alteration zones are often quite unlike discrete migmatitic veins; (ii) relation to deformation—most occurrences show alteration closely associated with warping of foliation or dilation cracks; (iii) open‐system alteration—whilst some occurrences represent nearly isochemical alteration, slight changes in bulk composition, often loss of mafic constituents and gain of Na and Si, are evident in detailed mass‐balance analysis. Y and sometimes Rb are characteristically depleted. Partial melting sometimes accompanied volatile infiltration, as evidenced by more discrete veins and euhedral orthopyroxene. It is quite unlikely, however, that open‐system alteration was produced by escape of viscous quartzo‐feldspathic melts. Pervasive migration of low‐Tlamprophyric (mafic–alkaline, CO2‐charged) interstitial liquids is a possibility by virtue of their extreme fluidity, but CO2infiltration was needed to generate these liquids. Vapour‐deficient dehydration melting is another feasible mechanism of orthopyroxene formation which may have operated in conjunction with CO2infiltration.

Characteristic development of charnockitic alteration in some prograde amphibolite to granulite facies transitions, as in the Dharwar Craton of South India, suggests that the alteration is a fundamental feature of the granulite facies metamorphism, implying active and causal participation of migrating fluids. In other high‐grade terranes like the Adirondack Mountains of New York, this kind of alteration is rare, and fluid action does not seem to have been important in the metamorphism.

A vapour phase participating in charnockitic metamorphism was necessarily one of relatively low H2O, therefore presumably rich in CO2. Consideration of possible large CO2sources leads to the conclusion that emanations from volatile‐rich basalts emplaced in the lower crust are the most probable source of charnockitizing fluids. The ultimate source would therefore be enriched subcontinental lithosphere or asthenosphere. The Rb‐depleted pyroxene gneiss (charnockitic) terranes may be characteristic of zones of large‐scale transcurrent or oblique‐motion faults which tap such great depths.
Tertiary deep crustal ultrametamorphism in the Hidaka metamorphic belt, northern Japan
Authors: OSANAI Y. , OWADA M. , KAWASAKI T.
Published: May 2007

The Main Zone of the Hidaka metamorphic belt is an imbricate stack of crustal material derived from an island arc in which a sequence of units with increasing metamorphic grade from low to high structural levels is exposed. The basal part of the metamorphic sequence underwent granulite facies metamorphism with peak P–T conditions of 7kbar, 870°C. In this zone pelitic granulite includes leucosomes which consist mainly of orthopyroxene‐plagioclase‐quartz.

To test whether the leucosome was derived by partial melting of the surrounding pelite, melting experiments of the pelitic granulite were carried out for water‐saturated and dry systems at 7 kbar and 850°C. The chemical composition of the leucosome produced during these runs shows a peraluminous S‐type tonalitic affinity and is located very close to the tie‐line between the average melts produced in water‐saturated systems and the average composition of the residual orthopyroxene + plagioclase. This therefore suggests that the lecosome in pelitic granulite was formed by incipient anatexis at close to the highest P–T condition of the Main Zone.

The age of the crustal anatexis is determined by the Rb‐Sr whole rock isochron method for garnet‐cordierite‐biotite gneiss (host rock), garnet‐orthopyroxene‐cordierite gneiss (restite) and S‐type tonalite (melt). This gives an age of 56.0 Ma with an initial 87Sr/86Sr ratio of 0.705711. The S‐type tonalite magmas that form large intrusive masses in the Main Zone were probably generated by crustal anatexis in deeper parts of the crust at the same time (late Palaeocene).
Calcite–graphite isotope thermometry: a test for polymetamorphism in marble, Tudor gabbro aureole, Ontario, Canada
Authors: DUNN S. R. , VALLEY J. W.
Published: May 2007

Graphitization and coarsening of organic material in carbonate‐bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory, pressure‐independent geothermometer. Comparison of observed isotopic fractionations between calcite and graphite (δ13CCal–Gr) with independent petrological thermometers provides the following empirical calibration over the range 400–800°C: δ13CCal–Gr= 5.81 times 106×T–2(K) ‐ 2.61. This system has its greatest potential in marbles where calcite + graphite is a common assemblage and other geothermometers are often unavailable. The temperature dependency of this empirical calibration differs from theoretical calibrations; reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications.

Both calcite‐graphite isotopic thermometry and calcite‐dolomite solvus thermometry are applied to marble adjacent to the Tudor gabbro in the Grenville Province of Ontario, Canada. The marble has undergone two metamorphic episodes, early contact metamorphism and later regional metamorphism. Values of δ13CCal–Gr decrease regularly from c. 8‰ in samples over 2 km from the pluton to values of 3–4‰ within 200 m of the contact. These samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer, and indicate temperatures of 450–500° C away from the intrusion and 700–750°C near the gabbro. This thermal profile around the gabbro is consistent with conductive heat flow models. In contrast, the distribution of Mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c. 500°C, even at the contact.

Graphite textures are important for interpreting the results of the calcite–graphite thermometer. Coarsening of graphite approaching the Tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion. In contrast to isotopic exchange during prograde metamorphism, which is facilitated by graphitization, retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally. Resistance of the calcite–graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems.
The metamorphic history of the Vredefort dome at approximately 2 Ga as revealed by coesite‐stishovite‐bearing pseudotachylites
Authors: MARTINI J. E. J.
Published: May 2007

The Vredefort dome (2.0 Ga) represents the central uplift of a very large impact structure. This uplift exposed a nearly complete cross‐section through the continental crust in the region, which is 25–30 km thick. Two metamorphic events took place at about the same time as the impact. The first event, so‐called static metamorphism, is pre‐impact and produced lithologies varying from low‐grade shale to high‐grade hornfels. It resembles contact metamorphism by its lack of schistosity, but is more regional as it extends over a large area and is not associated with large intrusions.

The second event, the post‐shock metamorphism, is responsible for the recrystallization of the shock features. The investigation of this event has been focused on the degree of alteration of the coesite‐stishovite‐bearing pseudotachylite veins that formed during the transit of the shock wave. These high‐pressure silica polymorphs are only present in the upper part of the stratigraphic sequence; downward they have been converted to fibrous quartz. At the highest grade, the fibrous quartz is in turn replaced by triple‐junctioned mosaic quartz. The post‐shock metamorphism was generated by the heat of the rock before shock, plus the heat released by the shock wave. The isograds, plotted on a map, can be translated into depth of burial and therefore provide valuable information regarding the geological setting immediately before impact. At the time of impact, the rocks were relatively cool and the static metamorphism had ceased with several tens of millions of years separating the two metamorphic events. The static metamorphism was probably caused by continental crustal extension in a stress‐free environment and the lack of deformation is probably due to rapid uplift during the later stages of the impact event.
Mineralogy and pressure–temperature–time path of Cretaceous granulite gneisses, south‐eastern San Gabriel Mountains, southern California
Authors: BARTH A. P. , MAY D. J.
Published: May 2007

Mid‐Cretaceous granulite gneisses crop out in a narrow belt in the Cucamonga region of the south‐eastern foothills of the San Gabriel Mountains, southern California. Interlayered mafic granulites and pelitic, carbonate, calc‐silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700–800°C. Regional deformation and formation of banded gneisses ceased by c. 108 Ma. although mafic‐intermediate magmatism and high‐grade metamorphism continued locally as late as c. 88 Ma. Garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near‐isobaric cooling, but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo‐chronological data. It is more likely that the entire terrane remained at granulite facies P–T conditions until 88 Ma, followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes. Uplift occurred between c. 88 and 78 Ma at rates of approximately 1–2 km Ma‐1. The geotectonic evolution of the Cucamonga granulites is similar to mid‐Cretaceous high‐P granulites in the Sierra Nevada and Salinian block of central California. Late Cretaceous uplift common to these granulites may provide an important tectonic link between dismembered Mesozoic batholithic terranes in the California Cordillera.
Experimental study of the stability of sudoite and magnesiocarpholite and calculation of a new petrogenetic grid for the system FeO–MgO–Al2O3–SiO2–H2O
Authors: VIDAL O. , GOFFÉ B. , THEYE T.
Published: May 2007

The solid‐solid reaction magnesiocarpholite = sudoite + quartz has been bracketed between 350 and 500°C, 6.3 and 7.8 kbar. Because it is impossible to synthesize end‐member sudoite, all experiments were carried out using natural minerals as starting materials. Although mineral compositions were very close to those of the end‐members, the effect of the fluorine content in carpholite was significant. Particularly in those experiments where sudoite grows at the expense of carpholite, electron microprobe analysis of the run products shows that a more stable F‐rich carpholite crystallizes too, and consumes the fluorine released in solution by the breakdown of the original carpholite.

Our experimental results are combined, through a thermodynamic analysis, with a previous data set and with previous experimental data concerning the relative stability of chlorite, talc and magnesiocarpholite with excess of quartz and water as a function of P–T and AlAl(SiMg)‐1 substitutions in phyllosilicates. This allows us to constrain the feasible thermodynamic parameters (H°f, sud; S° sud) and (H°f,car; S°car) for the Mg end‐members. Using the partition coefficients calculated from natural parageneses, we have computed a petrogenetic grid for the system FeO–MgO–Al2O3–SiO2–H2O. It demonstrates that parageneses involving sudoite and carpholite can be used as indicators of P–T conditions, up to 600° C, 8 kbar for sudoite, and at higher pressure for carpholite.
Stable isotopic and petrological constraints on scapolitization of the Whitestone meta‐anorthosite, Grenville Province, Ontario*
Authors: MOECHER D. P. , ESSENE E. J. , VALLEY J. W.
Published: May 2007

The Whitestone Anorthosite (WSA), located in the Central Gneiss Belt of the south‐western Grenville Province, Ontario, exhibits a nearly concentric metamorphic envelope characterized by an increase in modal scapolite, hornblende, epidote and garnet, developed around a core of granulite facies clinopyroxene ± orthopyroxene ± garnet meta‐anorthosite. Scapolite‐ and hornblende‐bearing assemblages develop mainly at the expense of plagioclase and pyroxene within the envelope.

Stable isotopic and petrological data for scapolite‐bearing mineral assemblages within meta‐anorthosite constrain the source of carbon responsible for CO3‐scapolite formation and the extent of fluid/rock interaction between the anorthosite and adjacent lithologies. Stable isotopic data indicate increasing δ18O and δ13C from core to margin of the meta‐anorthosite and for samples from the southern extension of the WSA, where it is ductilely deformed within the Parry Sound Shear Zone (PSSZ). The average δ18OSMOW value (whole rock) for the WSA core is 6.9‰, increasing to 11.5‰ where the WSA is in tectonic contact with marble breccia. The average δ13CPBD value of scapolite in meta‐anorthosite from the centre of the WSA is ‐3.4‰, increasing to ‐0.5‰ at the eastern (marble) contact. Average values of δ13C for scapolite and whole‐rock δ18O for samples from the shear zone are ‐1.0 and 8.0‰, respectively. Marbles have average δ18O and δ13C values of 19.2 and ‐0.4‰, respectively.

The sulphate content of texturally primary scapolite decreases from the core of the WSA (XSO4= 0.48) to the eastern contact (≤0.05). Texturally late scapolite after plagioclase and garnet tends to be CO3‐rich relative to texturally primary scapolite, and some scapolite grains show zoning in the anion site with CO3‐enriched rims. Scapolite composition may vary at any scale from a single grain to outcrop.

The pattern of isotopic enrichment in 13C and 18O preserved in the eastern margin of the WSA is consistent with marble as the major source of fluid contributing to the formation of the metamorphic envelope. The decrease in XSO4 and increase in XCO3 in scapolite toward the margin of the WSA indicate that the volatile content was reset by, or developed from, a CO2‐bearing fluid. Assuming derivation of fluid from marble, minimum fluid/rock values at the margin of the WSA range from 0.03 for the least enriched, to 0.30 for the most isotopically enriched samples. Although marble is not found in immediate contact with samples of sheared meta‐anorthosite from the PSSZ, a marble source is also consistent with the C and O isotope composition and anion chemistry of scapolite within these samples.
Granulite facies metamorphism, palaeo‐isotherms and disturbance of the U‐Pb systematics of zircon in anorogenic plutonic rocks from the Adirondack Highlands
Authors: CHIARENZELLI J. R. , McLELLAND J. M.
Published: May 2007

Abstract
In the Adirondack Highlands of New York State, the effect of granulite facies metamorphism on the physical and isotopic characteristics of zircon from anorogenic plutonic rocks has a distinct geographical pattern. The location of zircon populations which appear to have been altered describes a roughly circular area where metamorphic palaeotemperatures have been determined to be in excess of 750° C. Zircons from anorogenic plutonic rocks outside this area were undisturbed during metamorphism and yield well constrained ages.

Granitic, charnockitic and mangeritic anorogenic plutonic rocks peripheral to the Marcy anorthosite massif have large, euhedral, prismatic zircons that display fine, internal, magmatic growth zonations and abundant, randomly orientated, mineral inclusions. Co‐genetic zircon fractions yield linear discordant arrays and well constrained upper intercepts of 1125–1157 Ma. Metamorphic zircon is limited to sporadically developed and volumetrically insignificant, clear, low‐U overgrowths or protuberances.

In marked contrast, zircons from petrographically and geochemically identical rocks adjacent to, or within, the Marcy anorthosite massif are typically large, limpid, anhedral to subhedral crystals or crystal fragments lacking internal features except for tubular cavities and CO‐2‐rich inclusions. Co‐genetic zircon fractions yield nearly concordant, non‐linear clusters with 207Pb/206Pb minimum ages of 1073–1095 Ma. Metamorphic overgrowths cannot be readily identified by optical or cathodoluminscence techniques; however, many grains show complex and unusual external boundaries suggestive of post‐crystallization modification.

These data indicate that temperatures as low as 750° C, in combination with other factors, may have been sufficient to facilitate recrystallization, and diffusion of radiogenic Pb from the zircon crystal structure, during the complex, protracted metamorphism of the Adirondack Highlands.
U–Pb isotopic dating of fluid infiltration and metasomatism during Dalradian regional metamorphism in Connemara, western Ireland
Authors: CLIFF R. A. , YARDLEY B. W. D. , BUSSY F.
Published: May 2007

Abstract
A metasomatic diopside rock occurs at the top of the dolomitic Connemara Marble Formation of western Ireland and contains titanite and K‐feldspar in addition to around 90% diopside (XMg= 0.90–0.97). U–Pb isotopic measurements on this mineral assemblage show that the titanite is both unusually uranium‐rich and isotopically concordant, with the result that a precise U–Pb age of 478 ± 2.5 Ma can be determined. The age is identical within error to a less precise Rb–Sr age of diopside–K‐feldspar of 483 ± 6 Ma. Petrological evidence indicates that the assemblage crystallized at c. 620° C close to or below the closure temperature of titanite. The age thus provides a precise estimate of the time of metamorphism; this age is 11 ± 3 Ma younger than the 490 Ma age for nearby gabbroic plutons which has previously been used to constrain the peak metamorphic age. This difference accords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak. Analysis of minerals with high closure temperature from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism. The success of this approach is demonstrated by the results from Connemara.
C‐O‐H‐S fluid composition and oxygen fugacity in graphitic metapelites
Authors: CONNOLLY J. A. D. , CESARE B.
Published: May 2007

Abstract
C‐O‐H fluid produced by the equilibration of H2O and excess graphite must maintain the atomic H/O ratio of water, 2:1. This constraint implies that all thermodynamic properties of the fluid are uniquely determined at isobaric‐isothermal conditions. The O2, H2O and CO2 fugacities (fo2, fH2O and fCO2) of such fluids have been estimated from equations of state and fit as a function of pressure and temperature. These fugacities can be taken as characteristic for graphitic metamorphic systems in which the dominant fluid source is dehydration, e.g. pelitic lithologies. Because there are no compositional degrees of freedom for graphite‐saturated fluids produced entirely by dehydration, the variance of the dehydration process is not increased in comparison with that in non‐graphitic systems. Thus, compositional ‘buffering’of C‐O‐H fluids by dehydration equilibria, a common petrological model, requires that redox reactions, decarbonation reactions or external, H/O ± 2, fluid sources perturb the evolution of the metamorphic system. Such perturbations are not likely to be significant in metapelitic environments, but their tendency will be to increase the fO2 of the fluid phase. At high metamorphic grades, pyrite desulphidation reactions may cause a substantial reduction of fH2O and slight increases in fO2 and fCO2 relative to sulphur‐free fluid. At low metamorphic grade, sulphur solubility in H/O ± 2 fluids is so low that pyrite decomposition must occur by sulphur‐conserving reactions that cause iron depletion in silicates, a common feature of sulphidic pelites. With increasing temperature and sulphur solubility, pyrite desulphidation may be driven by dehydration reactions or infiltration of H2O‐rich fluids. The absence of magnetite and the assemblages carbonate + aluminosilicate or pyrite + pyrrhotite + ilmenite from most graphitic metapelites is consistent with an H/O = 2 model for GCOH(S) fluid. For graphitic rocks in which such a model is inapplicable, a phase diagram variable that defines the H/O ratio of GCOH(S) fluid is more useful than the conventional fO2 variable.