bluuebunny/crossref_metadata_2025 huggingface dataset:
Metamorphic and structural diachroneity in the Finnmarkian nappes of north Norway
Authors: RICE A. H. N.
Published: May 2007

Abstract
Two periods of garnet growth (Gt1 and Gt2) have been found in the Finnmarkian nappes of north Norway. In the Kolvik Nappe (the lowest nappe) Gt1 has preserved an S2 syntectonic spiral inclusion fabric; in the Olderfjord Nappe an earlier S1 fabric and an interkinematic inter‐D1–D2 fabric have been preserved in Gt1 whilst only the S1 fabric has been found in Gt1 in the Brennsvik Nappe (the highest nappe). In each nappe Gt2 overgrew a penetrative fabric (S2) wrapped around Gt1. In the Kolvik Nappe inclusion fabrics may be continuous from Gt1 into Gt2 but in the higher nappes there is a distinct break. Gt2 may have been partially syntectonic with D3 in the Brennsvik Nappe.

Chemically Gt1 in the Kolvik Nappe and in parts of the Olderfjord and Brennsvik Nappes has antithetic Fe‐Mn zoning. In all nappes XCa and XMg are weakly zoned in Gt1; XMg increases outwards and is greater in the higher nappes in Gt1 suggesting higher nucleation temperatures. In the Olderfjord and Brennsvik Nappes Gt2 is marked by increasing XCa, probably due to changing garnet‐plagioclase equilibria, although the Fe/Mg ratio remains constant. XMg is higher in Gt2 than Gt1.

Basement rocks within the nappe pile have an early pre‐Finnmarkian growth (Gt1) and a later Finnmarkian growth (GtH) correlated with Gt2 on the basis of chemical zoning patterns.

The diachroneity of Gt1 is ascribed to progressively earlier (compared to the structural development) cessation of overstepping of garnet‐forming reactions before peak metamorphism in the higher nappes, resulting in earlier structural events being preserved.
Migmatite formation at subsolidus conditions–an alternative to anatexis
Authors: LINDH ANDERS , WAHLGREN CARL‐HENRIC
Published: May 2007

Abstract
This contribution discusses the formation of stromatic high‐grade migmatites. Volume considerations require that most of the leucosome material is not added from outside the system. A segregation mechanism is necessary except in those cases where the protolith of the migmatite already had a banded structure. Although partial melting is most often advocated to provide the segregation mechanism, several arguments can be raised against high degrees of melting: mineral compositions and even zoning patterns are similar in both mesosomes and leucosomes; sufficient degrees of melting at reasonable temperatures require more than the available amounts of water; the leucosomes do not always approximate to a minimum melt composition; high degrees of melting cannot occur without an appreciable volume increase; etc. Diffusion works as a segregation mechanism at low temperatures. As diffusion rates increase exponentially with temperature diffusion must become still more important as a segregation mechanism at high temperatures. A model is suggested based on the diffusion of components in response to the gradient δσ/δx, where σ= 1/3∑3i=1 σi is the mean pressure. In homogeneously strained rocks, σ3 is larger in rock parts rich in incompetent phases than in rock parts depleted in incompetent phases. Accordingly, mechanically competent but chemically incompetent high‐volume phases like quartz and feldspars stressed in micadominated parts of a rock (high σσ) migrate to parts of the rock that are depleted in mica (low σ¯). It is suggested that hornblende occurring in many leucosomes may be premigmatitic or early syn‐migmatitic and due to its mechanical competency it initiates the segregation. Diffusion occurs along grain boundaries and is enhanced by small amounts of ‘intergranular fluid’;. At the best, semiquantitative estimates of diffusion rates and distances indicate that the process should work over geological times.
Continuous reactions between biotite, garnet, staurolite, kyanite‐sillimanite‐andalusite and P‐T‐time‐deformation path in micaschists from the estuary of the river Vilaine, South Brittany, France
Authors: TRIBOULET CLAUDE , AUDREN CLAUDE
Published: May 2007

Abstract
Microprobe analysis of the continuous chemical evolution of coexisting biotite‐garnet and biotite‐garnet‐staurolite has been undertaken from interbedded micaschists of the volcanodetrital group of the Vilaine. A thermobarometric study using pertinent mineralogical equilibria reveals a complex P‐T evolution, continuous throughout time, from high pressure, medium temperature (kyanite zone) to medium pressure, high temperature (sillimanite zone), then low pressure, medium temperature (andalusite zone). The T, P, fH2o and XH2o variations have been calculated from coexisting biotite‐garnet pairs, and from the equilibria: paragonite (in white mica) + quartz ± albite (in plagioclase) + Al silicate + H2O; and, 3 anorthite ± grossular + 2 Alsilicate + quartz. The P‐T evolution is correlated with the continuous change in composition of minerals (using P–XMg and T–XMg diagrams) and with the evolution of assemblages. This continuous P‐T‐time evolution, correlated with the successive formation of S1‐S2 foliations, allows us to propose a P‐T‐time‐deformation path for the micaschists and to relate the growth of its mineral components to tectonic processes.
Plagioclase relations in pelites, central Menderes Massif, Turkey. I. The peristerite gap with coexisting kyanite
Authors: ASHWORTH J. R. , EVIRGEN M. M.
Published: May 2007

Abstract
In pelites of the central Menderes Massif, albite and oligoclase with only slight chemical zoning coexist in apparent textural equilibrium in the garnet zone, staurolite zone, and staurolite + kyanite transition zone. The metamorphic temperature range is estimated as approximately 440–550°C (from the Hodges‐Spear calibration of the garnet‐biotite geo‐thermometer), or 440–500°C (Ganguly‐Saxena calibration). While oligoclase composition at the peristerite gap changes from An22 to An14, albite also becomes more sodic (An1,5–An0.6). The slope of the albite limb is thus the reverse of that reported in other areas, and may not be a true equilibrium feature. Occurrence of kyanite, at temperatures below the crest of the gap, is due to low water activity in the presence of graphite: aH2o is estimated at approximately 0.1–0.2 from the Na content of muscovite coexisting with albite + kyanite + quartz.
Alpine metamorphism in the south‐east Tauern Window, Austria: 2. Rates of heating, cooling and uplift
Authors: CLIFF R. A. , DROOP G. T. R. , REX D. C.
Published: May 2007

Abstract
Existing geochronological data are reviewed and new Rb‐Sr, K‐Ar and 39Ar–40Ar ages are presented, including a suite of 33 mica ages from a 20 km north–south tunnel section. These data are discussed in relation to the thermal history from the overthrusting of the Autroalpine nappes c. 65 Myr ago to the present. The earliest phase of metamorphism, involving lawsonite crystallization, is associated with emplacement of these nappes. Subsequently, temperatures in the rocks beneath rose, at a mean rate of 3–6°C/Myr, until the climax of metamorphism.

At high structural levels, published data indicate an age > 35 Myr for the metamorphic climax. In contrast, a new 39Ar–40Ar step‐heating age of 23.8 ± 0.8 Myr on amphibole, from near the base of Peripheral Schieferhülle, closely approximates the age of metamorphism and provides the first clear indication that the climax of metamorphism occurred later at deeper structure levels. Following the climax, near‐isothermal uplift and erosion reduced pressure to c. 1 kbar before white mica closure at 19 Myr; this implies uplift at >3 mm/yr.

Along the tunnel section, white mica K‐Ar ages vary systematically from 24 Myr to 16.5 Myr with position relative to a late 4 km amplitude dome whereas biotite Rb‐Sr ages are uniform at 16.5 Myr across the whole profile; doming is thus dated at 16.5 Myr with transient uplift rates >5 mm/yr. At other times uplift rates were <1 mm/yr.
Glaucophane and pyroxene breakdown reactions in the Pennine units of the Eastern Alps
Authors: HOLLAND T. J. B. , RAY N. J.
Published: May 2007

Abstract
New occurrences of crossite and jadeitic pyroxene are described from a thick metabasite unit within the upper levels of the Peripheral Schieferhülle in the Tauern Window, Austria. Unusual textures are preserved which provide evidence for the reactions and mechanisms involved in the breakdown of crossite and jadeitic pyroxene. Zones of albite and chlorite, produced by reaction between crossite and paragonite, have been preserved due to sluggish reaction kinetics during decompression from the blueschist to the greenschist facies. The zonal sequence is interpreted in terms of chemical potential gradients in Na, Mg and Al, which have been established by overstepping the equilibrium boundary. Breakdown textures of jadeite‐acmite pyroxene to a symplectite of albite + hematite + actinolite, and of crossite to talc and actinolite are also described.

The occurrence of crossite and jadeitic pyroxene at high levels within the Peripheral Schieferhülle implies that even upper levels of the structural sequence have undergone blueschist facies metamorphism with pressures in excess of 8 kbar during the Alpine collisional event.
Large‐ion lithophile element characteristics of an amphibolite facies to granulite facies transition at Gruinard Bay, North‐west Scotland
Authors: FOWLER M. B.
Published: May 2007

Abstract
Lewisian grey gneisses from Gruinard Bay, North‐west Scotland retain mineralogical and geochemical evidence for Scourian horn‐blende‐granulite facies metamorphism, and they may be used to assess current models of elemental depletion at granulite grade. Their ‘immobile’major and trace element geochemistry is indistinguishable from that of Lewisian amphibolite and pyroxene‐granulite facies counterparts. The K, Rb, Th and U contents of the Gruinard Bay gneisses are depleted relative to amphibolite facies gneisses, but generally the abundances of these elements are above those of comparable pyroxene granulites. U and Th have reached an advanced stage of depletion, but allanite appears to be crucial in maintaining significantly higher U and Th abundances at Gruinard Bay than in pyroxene granulites. K and Rb loss is less extreme, and depends on the stability of the rock‐forming minerals: K‐feldspar; biotite; and, amphibole. Early removal of K and Rb has resulted in a small rise in K/Rb, but further preferential Rb loss would have been required to generate the characteristically high K/Rb ratios of Lewisian pyroxene granulites.

The residence of U and Th in the accessory minerals of granulite facies gneisses, which are often correlated with the residua of intracrustal partial melting, renders unlikely their extreme incompatibility required by such models. Even if such phases are ignored, high mineral‐melt partition coefficients for silicic melts argue against partial fusion as an efficient depletion mechanism. On the other hand, the advanced stage of U and Th depletion reached in Gruinard Bay gneisses, which were still partly hydrous, severely restricts the role played by CO2‐dominated fluids and a hydrous medium is preferred.
Fluid inclusions in skarns (carbonate replacement deposits)
Authors: KWAK T. A. P.
Published: May 2007

Abstract
Fluid inclusions in ore skarn minerals reflect the physiochemical nature of the solutions present during the skarn‐forming process. Because of the dense nature of skarn minerals and the dynamic processes operative during skarn genesis, sufficient primary fluid inclusions are usually present. Ore skarn solutions, as opposed to metamorphic skarn or ore vein solutions, have much higher CaCl2 contents and usually very high formation temperatures (>500°C) and salinities (>40 wt % T.D.S.). Temperatures and salinities generally decrease away from the solution source, both in time and space. The gradients found at greater distances from the source in distal (far from contact) skarns tend to be less (e.g. 210–350°C) for a particular skarn stage to that in proximal (near contact) skarns (e.g. 400–650°C). This information is useful for delineating the parts of such a hydrothermal system. Temperatures also tend to decrease with time, which is reflected by the superimposition of various overprinting, retrograde mineral stages. In a few areas (e.g. Naica, Mexico) intermittent boiling of ore solutions occurs, periodically elevating both temperatures and salinities, but commonly boiling only occurs early in skarn genesis just after an early, commonly lower temperature, phase.

Most fluid inclusions represent a mix of‘exhaust’or reacted solutions with minor unreacted or new (pre‐) ore solution components. Limited data on the distribution of elements present in fluid inclusions that do not normally take part in skarn genesis (Na, K and Cl) indicate that their proportions reflect the nature of the associated pluton. High KC1 contents are found in skarns adjacent to high K granitoids, whereas high NaCl contents are found in skarns adjacent to calcic granitoids.

In many examples, daughter minerals present in minor proportions in opened fluid inclusion cavities reflect the metal characteristics of the ore solution. Small rare‐earth metal, tungsten, zinc and copper daughter(?) minerals have been identified. The temperature and (or) salinity data for skarns of different metal or geological type is not particularly useful to delineate whether a skarn locality is part of a more complex, as yet unexplored system. Solutions in Pb–Zn skarns tend to be lower‐temperature (150–400°C) and more dilute (<30 wt % T.D.S.) than in other skarn types, but exceptions occur.
The occurrence of deerite in highly oxidizing conditions within the ‘schistes lustrés’of eastern Corsica
Authors: VERNIÉ P. , KIENAST J. R. , MÉVEL C.
Published: May 2007

Abstract
A deerite‐bearing rock occurs at the boundary between quartzite and metabasites within the ‘schistes lustrés’of eastern Corsica. It contains the typomorphic assemblage pyroxene, blue amphibole, hematite and magnetite. Pyroxene shows homogeneous composition close to the aegirine end‐member and blue amphibole is zoned from crossite core to riebeckite rim. The bulk chemical analysis of the rock is remarkable by its very high iron content and the presence of an unusually large amount of Zn which is concentrated in both deerite and amphibole. Electron microprobe analyses of the Corsican deerite are compared with those published in the literature; as shown by deerite from the Fransciscan iron formation, the principal substitution for Fe2+ is Mn whereas the amount of substitution for Fe3+ is low. In the system SiO2‐FeO‐Fe2O3‐Al2O3‐Na2O‐MgO‐H2O the typomorphic paragenesis can be described by an univariant reaction interpreted as the result of a pressure decrease. P‐T conditions of metamorphism, previously estimated to be 8 kbar and 300°C, are in good agreement with present knowledge of the deerite stability field. The occurrence of hematite and magnetite in equilibrium permits an estimation of the oxygen fugacity (log fo2= ‐29.41 bar). Oxidation conditions are higher than those previously mentioned in the literature for similar assemblages.
A newly discovered high‐pressure terrane in eastern Hokkaido, Japan
Authors: SAKAKIBARA MASAYUKI
Published: May 2007

Abstract
The upper Jurassic Nikoro greenstone complex of eastern Hokkaido suffered high‐pressure intermediate type metamorphism. Characteristic minerals include lawsonite, aragonite, sodic pyroxene of the aegirinejadeite series, winchite. sodic amphibole of the glaucophane‐riebeckite series, pumpellyite, epidote and actinolite.

High‐pressure metamorphism of the Nikoro greenstone complex is related to subduction of the Kula plate toward the Palaeo‐Okhotsk Land during Cretaceous time.
The origin of strain patterns resulting from contemporaneous deformation and metamorphism in the Sambagawa metamorphic belt
Authors: TORIUMI MITSUHIRO , NODA HARUTAKA
Published: May 2007

Abstract
The magnitudes of plastic strains of 104 metacherts were determined from the deformed shape of initially spherical radiolarians in the Sambagawa high‐P type metamorphic belt of Western Shikoku, Japan. The strain magnitude increases with increasing metamorphic temperature from several per cent to 250%. The a2/a3 ratio of strain ellipsoids in the higher metamorphic grades decreases with increasing metamorphic grade while the a1/a2 ratio increases rapidly. The long axis of the strain ellipsoid for every grade is nearly parallel to the length of the metamorphic belt, suggesting that the flow direction of the synmetamorphic deformation was uniform along the belt. A map of strain zones within the Sambagawa high‐P type metamorphic belt reveals that the metamorphic belt underwent a progressive bulk inhomogeneous shear deformation and that the high‐grade zones represent a deep‐seated boundary shear zone on the accretionary wedge between a subducting oceanic plate and the immobile rigid continental plate.
Mode of occurrence and significance of jadeite in the Kamuikotan metamorphic rocks, Hokkaido, Japan
Authors: TAKAYAMA MASASHI
Published: May 2007

Abstract
In the Kamuikotan zone, jadeite occurs in pelitic rocks, in metaplagiogranites, in veins in amphibolites and mafic sedimentary rocks, and in jadeite‐albite rocks. In the first and second types, jadeite is associated with quartz, and is often in direct contact with it. However, such rock‐types never occur as part of the coherent metamorphic sequence, but are found only as exotic blocks enclosed in serpentinite. Thus, jadeite + quartz‐bearing assemblages are not regarded as representative of the Kamuikotan metamorphism. Lawsonite and aragonite, however, commonly do occur in the Kamuikotan metamorphic rocks, and this metamorphism belongs to a subfacies of the lawsonite‐albite facies, in which aragonite is stable. The serpentinite matrix which carried jadeite + quartz‐bearing pelites and metaplagiogranites into the metamorphic sequence is interpreted as a tectonic rather than a sedimentary melange.
Wollastonite and scapolite in Precambrian calc‐silicate granulites from Australia and Antarctica
Authors: WARREN R.G. , HENSEN B.J. , RYBURN R.J.
Published: May 2007

Abstract
Scapolite, wollastonite, calcite, diopside, grossular‐andradite garnet and sphene occur in calc‐silicate rocks in the granulite terrain of the Arunta Block, central Australia. This assemblage buffers the CO2 activity at a low value, so that any coexisting fluid phase must be H2O rich and CO2 poor (Xco2= 0.2‐0.3). In contrast, the H2O activity in the surrounding felsic and mafic granulites was low. Thus fluid activities during granulite facies metamorphism were locally buffered in various rock units and fluid flow appears to have been restricted or fluid may have been absent. Late retrograde rims of garnet and garnet‐quartz separate phases formed in the high‐grade stage. Formation of these rims would have required either an influx of water‐rich fluid or a decrease in pressure. Evidence from the surrounding granulites shows that in one locality, the calc‐silicate rocks had undergone late isobaric hydration; in another locality, minor uplift had occurred soon after peak P‐T conditions. In both, scapolite had partly broken down to plagioclase‐calite. A calc silicate rock from the granulite terrain of Enderby Land, Antarctica, contains scapolite, wollastonite, calcite, diopside, quartz and sphene; this assemblage also indicates low CO2 activities. In this rock, wollastonite has broken down to calcite‐quartz, to indicate isobaric cooling without influx of hydrous fluid.
Timing of porphyroblast growth in the Fleur de Lys Supergroup, Newfoundland
Authors: JAMIESON R. A. , VERNON R. H.
Published: May 2007

Abstract
In the Fleur de Lys Supergroup, western Newfoundland, inclusion trails in garnet and albite porphyroblasts indicate that porphyroblasts overgrew a crenulation foliation, without rotation, probably during the deformation event that produced the crenulations. Further deformation of the matrix resulted in strong re‐orientation and retrograde metamorphism of the matrix foliation, which is consequently highly oblique to the crenulation foliation preserved in the porphyroblasts. The resulting matrix foliation locally preserves relics of the early crenulations, and also has itself been crenulated later in places. Thus the porphyroblasts grew before the later stages of deformation, rather than during the final stage, as had been suggested previously. The new interpretation is consistent with available 40Ar/39Ar cooling ages which indicate a late Ordovician‐early Silurian metamorphic peak, rather than the Devonian peak suggested by previous workers. The inclusion patterns and microprobe data indicate normal outward growth of garnet porphyroblasts from a central nucleus, rather than as a series of veins as proposed by de Wit (1976a, b). However, the observations presented here support growth of porphyroblasts without rotation, which is implied by the de Wit model.
High pressure metamorphism in the nappes of the Valle dell'Orco traverse (Western Alps collisional belt) *
Authors: POGNANTE U. , TALARICO F. , RASTELLI N. , FERRATI N.
Published: May 2007

Abstract
A detailed field and petrological study of rocks from nappes cut by the Valle dell'Orco (Italian Western Alps), in particular the Sesia–Lanzo composite unit, has revealed geological and metamorphic histories which started in pre‐alpine times and lasted up to the alpine subduction‐collisional processes. During these processes the nappes sustained an early high P–low T stage and a later low P greenschist facies stage, but followed partly distinctive P–T–time trajectories. This paper discusses the kinematic evolution and the thermal history of the alpine belt from the early subduction/underthrust to the later exhumation stage. The metamorphic crystallization is often governed by incomplete and/or local equilibrium, and the pervasive syn‐metamorphic deformation and the composition of the syn‐metamorphic fluid phase (if present) have exerted an effective local control on reaction kinetics.
Chloritoid–hornblende assemblages in quartz–muscovite pelitic rocks of the Central Metasedimentary Belt, Grenville Province, Canada
Authors: THOMPSON P.H. , LECLAIR A.D.
Published: May 2007

Abstract
Chloritoid–hornblende occurs in quartz–muscovite pelitic schist derived from sediment in a volcaniclastic sequence of the Grenville Supergroup and from reworked sedimentary and regolithic material above the unconformity at the base of the Flinton Group. Comparison of these samples with other pelitic rocks on triangular composition diagrams and in the ACNF and ACFM tetrahedra indicates that the presence of hornblende cannot be explained by unusually high CaO content. The rare assemblage is attributed to a combination of relatively low Al2O3 and high K2O with high CaO/(CaO+Na2O) and FeO/(FeO+MgO).

On two qualitative reaction grids derived from AFM diagrams projected through CaO and plagioclase, respectively, the P–T stability field of chloritoid–hornblende overlaps the first appearance of staurolite–biotite in normal pelitic rocks in the kyanite field. Staurolite–hornblende overlaps chloritoid–hornblende and extends to the higher temperatures and pressures of the kyanite–hornblende field.

The phase relations in these rocks provide a link between the conventional hornblende‐absent grids for pelitic rocks and those for K2O‐poor (muscovite‐absent) pelitic and mafic amphibolitic rocks.
Cordierite‐anthophyllite rocks from north‐west Queensland, Australia: metamorphosed magnesian pelites
Authors: REINHARDT J.
Published: May 2007

AbstractCordierite‐anthophyllite rocks and related cordierite‐rich, talc‐rich and chlorite‐rich rocks occur in the Rosebud Syncline, north‐west Queensland, Australia, as part of a Proterozoic metasedimentary sequence. Field relations and rock compositions attest the sedimentary origin of these rather unusual metamorphic rocks. Their chemical composition is comparable to that of unmetamorphosed, alkali‐ and Ca‐poor pelites, which are associated with some evaporite deposits.

Other occurrences of cordierite‐anthophyllite rocks have commonly been interpreted as metamorphosed chloritic alteration products derived from mafic or felsic volcanics. A comparative chemical study, using analyses of cordierite‐anthophyllite rocks from such alteration zones and analyses of unmetamorphosed magnesian pelites, demonstrates the general chemical similarity between these two rock groups of entirely different origin. However, distinct differences in major element relations help to distinguish these two genetic groups. Particularly useful are Al2O3–FeO–MgO plots, in which evaporitic pelites occupy the Fe‐poor side.

The highly magnesian metamorphic rocks from the Rosebud Syncline fall entirely into the compositional field of evaporitic clays and shales. Furthermore, analyses of relatively immobile trace elements give supporting evidence for the sedimentary origin of these cordierite‐anthophyllite rocks. The correlation with trace element ranges of clays and shales is very good. However, the correlation with trace element ranges of mafic and felsic volcanics is poor, and major discrepancies occur with Cr, Ni, Co, Nb, Sc, Th and Ti.

Thus, the magnesian metamorphics of the Rosebud Syncline appear to be derived from evaporitic clays rich in magnesian clay minerals, such as palygorskite, sepiolite, chlorite or corrensite. The complete metamorphic rock assemblage of interlayered calcareous, aluminous and magnesian rocks is interpreted as a metamorphosed carbonate‐evaporite‐pelite sequence.
Evidence for an inverted metamorphic gradient associated with a Precambrian suture, southern Wyoming
Authors: DUEBENDORFER E.M.
Published: May 2007

Abstract
An inverted metamorphic gradient associated with the northern mylonite zone of the Cheyenne belt, a deeply eroded Precambrian suture in southern Wyoming, has been documented within metasedimentary rocks of the Early Proterozoic Snowy Pass Supergroup. Metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite, garnet, and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone. A minimum temperature increase of approximately 100° C over a km‐wide zone is required for this transition. Parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the Snowy Pass Supergroup.

Field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite‐grade rocks over rocks of the Snowy Pass Supergroup along the northern mylonite zone. Mineral equilibria and garnet‐biotite geothermometry on synkinematic mineral assemblages within the Snowy Pass metasedimentary rocks indicate deformation at minimum temperatures of 480° C and pressures of 350–400 MPa (3°5–4°0 kbar). This implies tectonic burial or upper plate thickness of 13–15 km.

The narrow character of metamorphic zonation and microtextures within the Snowy Pass Supergroup which indicate late synkine‐matic growth of garnet and staurolite, preclude rotation of pre‐existing isograds by folding as a mechanism for development of the inverted gradient. Conductive transport of heat from the upper into the lower plate across the originally low‐angle thrust is insufficient to produce the necessary temperatures in the lower plate. Shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated. Up‐dip advection of metamorphic fluids is a feasible, but unproven, mechanism for heat transport. The possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present.
Controls on P–T–t deformation path from amphibole zonation during progressive metamorphism of basic rocks (estuary of the River Vilaine, South Brittany, France)
Authors: TRIBOULET Cl. , AUDREN Cl.
Published: May 2007

Abstract
Edenite/tremolite and edenite/magnesio‐hornblende in equilibrium with plagioclase, chlorite, epidote, quartz and vapour involve several types of reactions for which KD can be related to T and P. Thermodynamic calculation of these equilibria leads to isopleth systems. Given knowledge of the progressive changes of end‐member activities in zoned Ca–Mg amphiboles (based on microprobe analyses), it is possible to construct precise pressure–temperature–time paths (P–T–t paths) which have been followed by metabasites during polyphase metamorphism. When applied to basic rocks from the River Vilaine area, this method allows us to construct a P–T–t path that can be compared directly to the P–T–t path constructed from interbedded acid rocks (aluminous micaschists) in the same structural unit. Through time, both basic and acid rocks underwent the same complex deformation history that can be described conveniently in the L–S fabric system of Flinn. This allows us to construct a P–T–t deformation path for this structural unit.

These paths are interpreted in terms of an under/overthrusting continental collision belt (the Hercynian belt), and represent an illustration of the time delay caused by stacking of more than two crustal units.
Partial melting and the formation of granulite facies assemblages in Namaqualand, South Africa
Authors: WATERS D.J.
Published: May 2007

Abstract
Dehydration‐melting reactions, in which water from a hydrous phase enters the melt, leaving an anhydrous solid assemblage, are the dominant mechanism of partial melting of high‐grade rocks in the absence of externally derived vapour. Equilibria involving melt and solid phases are effective buffers of aH2,o. The element‐partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which, for most cases, aH2o is driven to lower values. The mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at T± 850°C is relatively small (10–20%), and probably insufficient to mobilize a partially melted rock body.

Granulite facies metapelite, biotite gneiss and metabasic gneiss in Namaqualand contain coarse‐grained, discordant, unfoliated, anhydrous segregations, surrounded by a finer grained, foliated matrix that commonly includes hydrous minerals. The segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration‐melting reactions: Bt + Sil + Qtz + PI = Grt ° Crd + Kfs + L (metapelite), Bt + Qtz + Pl = Opx + Kfs + L (biotite gneiss), and Hbl + Qtz = Opx + Cpx + Pl + L (metabasic gneiss). The size, shape, distribution and modes of segregations suggest only limited migration and extraction of melt. Growth of anhydrous poikiloblasts in matrix regions, development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity. Also, they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles. The preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that H2O in the melt was released as vapour on crystallization.

This model, combined with the natural observations, suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt.
Evolution of structurally contrasting anatectic migmatites in the 3‐kbar Ballachulish aureole, Scotland
Authors: PATTISON D.R.M. , HARTE B.
Published: May 2007

Abstract
Anatectic migmatites of contrasting structural style are found adjacent to the contacts of the Ballachulish Igneous Complex, Argyllshire, Scotland. On the east flank, evidence for migmatization is largely restricted to the local development of millimetre‐centimetre scale Kfs + Qtz‐rich leucocratic segregations, which accompany fragmentation of brittle hornfels layers and ductile deformation of mm‐cm scale semipelitic layers. Large volumes of semipelitic rock rich in feldspar and quartz on the east flank show no migmatitic features, and bedding is usually preserved undisturbed right up to the contact. On the west flank, in contrast, similar semipelitic rocks show widespread migmatitic features and disruption of layering is substantial and widespread over a 400 m wide zone. Within the west‐flank migmatites, 1–100 cm scale rigid bedding fragments (schollen) may be suspended and disoriented in a semipelitic matrix that underwent ductile deformation. The P‐T conditions on both flanks are in the same range: 3 kbar and 650–700°C.

The contrast in gross structural style is believed to result from differences in the volumes of melt produced and differences in the proportion of rock in which the critical melt fraction of the rocks was exceeded. On the east flank, only on a mm‐cm scale was enough melt locally accumulated to cause disruption of some layers and segregation of melt. On the west flank, melting proceeded substantially in a broad tract of semipelitic rocks, resulting in larger scale contrasts in rheology that led to the present chaotic structures in this zone.

Because migmatization occurred at a pressure too low for muscovite dehydration melting, and at temperatures too low for substantial biotite dehydration melting, the different amounts of melting on the east and west flanks most probably resulted from the introduction of differing amounts of externally derived water. On the east flank, and throughout most of the aureole, the absence of melting even in quartzofeldspathic protoliths indicates that there was no substantial movement of fluid towards or away from the igneous complex during migmatization. The contrasting situation on the west flank may have resulted from devolatilization of underlying quartz diorite magma (˜ 690–710°C), which released heat and fluids into the overlying quartz‐ and feldspar‐rich semipelites (solidus temperature ˜ 650–680°C).
Microstructural evidence of rotation and non‐rotation of mica porphyroblasts
Authors: VERNON R.H.
Published: May 2007

Abstract
Mica porphyroblasts in schists from several regions show nearly planar inclusion trails that are parallel over areas much larger than the wavelengths of later folds. This indicates that the porphyroblasts have not rotated, with respect to geographical co‐ordinates, during deformation. Instead, the matrix has rotated, as suggested by Ramsay (1962). Even in zones of marked shortening in the matrix adjacent to large rigid porphyroblasts (e.g. of cordierite or staurolite), small biotite porphyroblasts have not rotated, but have become thinned by solution, as indicated by parallelism of inclusion trails in separate biotite grains and by evidence of truncation of inclusion trails by the matrix foliation. Less common are biotite porphyroblasts that have single asymmetrical microfolds in the matrix adjacent to the porphyroblasts and so appear to have rotated; these porphyroblasts are characterized by kinking.
Definition of low‐grade metamorphic zones using illite crystallinity
Authors: BLENKINSOP T.G.
Published: May 2007

Abstract
Two major problems which exist in the use of illite crystallinity to define low‐grade metamorphic zones are the variety of values chosen for the zone boundaries and the persistent use of three different indices of crystallinity. Although measurement techniques, which cause much of the interlaboratory variation, can be standardized, it is shown that there is, nevertheless, significant additional variation which demands calibration on standards. The greatest variations are due to choices of different absolute values of crystallinity to define zone boundaries. The problem of relating measurements between different indices is approached by fitting mathematical relationships to pairs of measurements from the same sample. A power–law relationship is a satisfactory fit to the Kubler–Weaver and Weaver–Weber pairs, while the Kubler–Weber indices are linearly related. These relationships are used to transform definitions of the diagenetic zone, anchizone and epizone from one index to the others, although they apply strictly only to the data set from which they are derived. This results in compatibility between the three zones and shows that previous definitions to the anchizone in different indices have been chosen at incompatible values. The boundaries of Kubler's anchizone (0.42 and 0.25 Δ2θ) are 0.4 and 0.215 Δ2θ in this study, which become 5.1 and 14.6 in the Weaver index and 278 and 149 in the Weber index. An error analysis shows that percentage errors in both Kubler and Weaver indices increase with crystallinity; the Kubler measurements are marginally preferred at all grades.
Endogreisen, brecciation and fluid activity at the Mount Bischoff Sn‐deposit, north‐west Tasmania, Australia
Authors: WRIGHT J.H. , KWAK T.A.P.
Published: May 2007

Abstract
Endogreisens which replace K‐feld‐spar‐quartz dykes in a Devonian (360 Ma) tin deposit at Mt Bischoff, north‐west Tasmania, formed from the interaction of unusual solutions, probably derived from an underlying leucogranite pluton, porphyry dykes and limited quantities of local dolomitic country rock components. The intensity of greisenization and pH of the solutions increase inward to the greisenized dykes’cores and downward. The following types of greisen assemblages indicate increasing degrees of greisenization: ‘sericite’muscovite + quartz ± tourmaline ± fluorite, topaz + quartz ± tourmaline ± fluorite, weberite, prosopite, ralstonite, Ca‐ralstonite; and quartz ± topaz ± fluorite. Where the solutions interacted with dolomite, exogreisens consisting of topaz‐ or tourmaline‐bearing assemblages were formed. The greisens were subsequently overprinted to varying degrees by siderite, sulphides and hydrous silicates (talc, serpentine, chlorite, micas).

The temperature during greisenization ranged from 180 to 414°C, based on fluid inclusions in topaz, quartz, fluorite, sellaite and cassiterite. The main greisen‐forming event occurred at temperatures of 360±20°C. The fluids boiled intermittently. Their salinities ranged from 31.5 to 38.9 wt% total dissolved salts, consisting of Ca–K–Na–Fe–Cl±hydrocarbon species. Fluid inclusion data indicate that only 0.5–1.5 km of cover were present above this deposit at the time of formation.

The greisenized dykes were intruded by and intrude different stages of breccias. The breccias consist mainly of country rock and greisenized dyke fragments, with rock‐flour and later tourmaline alteration. The Mt Bischoff greisen system is possibly part of a ‘porphyry tin’style deposit formed at near‐surface conditions (0.5–1.0 km).
Domal structures and high‐grade metamorphism in the Higher Himalayan Crystalline, Zanskar Region, north‐west Himalaya, India
Authors: KUNDIG R.
Published: May 2007

ABSTRACT In the main Himalayan range in the Ladakh‐Zanskar area, domal structures have been observed at structurally deeper levels in the tectonic unit of the Higher Himalayan Crystalline. Their formation occurred during a second, temperature‐dominated phase (M2) of high‐grade regional metamorphism, characterized by the semipelitic paragenesis of sillimanite‐K‐feldspar and incipient anatexis. The doming event reveals a local system of synmetamorphic uplift superimposed on a regional system of northeast‐southwest trending compression. In the main Himalayan range the development of the dominant S2 foliation is related to deformation during the doming phase, which started early in the M2 event. The deformation propagated continuously north‐east and south‐west with time. In the north‐east, on the northern slopes of the main Himalayan range, this deformation is expressed by extensional shear movements of the upper tectonic levels finally leading to the late‐ to postmetamorphic normal fault system of the Zanskar shear zone. Towards the south‐west, deformation is expressed by compressional movements, e.g. at the Main Central Thrust (MCT) in the Kishtwar window area. The observed compression and extension is inferred to relate to an increased uplift of the domal bulges of the tectonic Kishtwar window and of the whole main Himalayan range.
Synthetic fluid inclusions ‐ VI. Quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure
Authors: BODNAR R. J. , BINNS P. R. , HALL D. L.
Published: May 2007

ABSTRACT The decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure H2O synthetic inclusions formed by healing fractures in natural quartz. Three different modes of non‐elastic deformation, referred to as stretching, leakage or partial decrepitation, and total decrepitation have been observed. The internal pressure required to initiate non‐elastic deformation is inversely related to inclusion size according to the equation:

internal pressure (kbar) = 4.26 D‐0.423

where D is the inclusion diameter in microns. Regularly shaped inclusions require a higher internal pressure to initiate non‐elastic deformation than do irregularly shaped inclusions of similar size. Heating inclusions through the α/β quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion.

Long‐term heating experiments (∼2 years) suggest that the internal pressure required to initiate non‐elastic deformation does not decrease significantly with time and indicates that short‐lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature. Inclusions that do exhibit decreased density (higher homogenization temperature) are, however, always accompanied by a change in shape from irregular to that of a negative crystal.

Observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals. These results indicate that an inclusion will not be influenced by a neighbouring inclusion, or other defect in the host phase, as long as the distance between the two is >2–4 diameters of the larger of the two inclusions.
Evidence for deformation‐induced K‐feldspar replacement by myrmekite
Authors: SIMPSON C. , WINTSCH R. P.
Published: May 2007

ABSTRACT Several examples of deformation‐induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths, namely a muscovite‐poor S‐C mylonite and a single foliation, muscovite‐poor mylonitic gneiss. Back‐scattered SEM and conventional optical microscopy show that in both rock types, syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of K‐feldspar grains that faced the local inferred incremental shortening direction for the mylonite. Myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction.

The replacement of K‐feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains. We suggest that the ambient temperature, pressure and chemical activities were such that the replacement reaction was favoured, but the addition of extra strain energy along the high‐pressure sides of the grains localized the reaction at these sites. This energy could arise from elastic strain, or strain associated with tangled dislocations or twin boundaries. The relative roles of stress and strain energy concentrations in driving the replacement reaction are not known, but both were probably important.
Low‐pressure granulite facies metapelitic assemblages and corona textures from MacRobertson Land, east Antarctica: the importance of Fe2O3 and TiO2 in accounting for spinel‐bearing assemblages
Authors: CLARKE G. L. , POWELL R. , GUIRAUD M.
Published: May 2007

Low‐pressure granulite facies metasedimentary gneisses exposed in MacRobertson Land, east Antarctica, include hercynitic spinel‐bearing metapelitic gneisses. Peak metamorphic mineral assemblages include spinel + rutile + ilmenite + sillimanite + garnet, spinel + ilmenite + sillimanite + garnet + cordierite, ortho‐pyroxene + magnetite + ilmenite + garnet, spinel + cordierite + biotite + ilmenite and orthopyroxene + cordierite + biotite, each with quartz, K‐feldspar and melt. The presence of garnet + biotite‐ and cordierite + orthopyroxene‐bearing assemblages implies crossing tie‐lines in AFM projection for the K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O (KFMASH) system. This apparent contradiction, and the presence of spinel, rutile and ilmenite in the assemblages, is acounted for by using the KFMASH‐TiO2‐O2 system, i.e. AFM + TiO2+ Fe2O3. We derive a petrogenetic grid for this system, applicable to low‐pressure granulite facies metamorphic conditions. Retrograde assemblages are interpreted from corona textures on hercynitic spinel and Fe‐Ti oxides. The relative positions of the peak and retrograde metamorphic assemblages on the petrogenetic grid suggest that corona development occurred during essentially isobaric cooling.
Reaction history of garnet‐sapphirine granulites and conditions of Archaean high‐pressure granulite‐facies metamorphism in the Central Limpopo Mobile Belt, Zimbabwe
Authors: DROOP G. T. R.
Published: May 2007

ABSTRACTSequential reaction textures in Archaean garnet‐corundum‐sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse‐grained, high‐pressure (P> 9.5 kbar) granulite‐facies assemblages (M1) to late, low‐pressure (P<6 kbar) granulite‐facies sub‐assemblages (M2).

The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late‐M1 boron‐free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine‐grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel).

The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time‐sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)‐discontinuous reactions in the FMASH multisystem sapphirine‐garnet‐corundum‐spinel‐cordierite‐gedrite‐kornerupine, the facies‐sequence indicates decompression at essentially constantTassuming constanta(H2O).

Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the oppositedPfrom that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction ofP‐Tvectors.

Garnet‐biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures ofca.850°C for the M1 stage. A minimum late‐M1 pressure ofca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early‐M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high‐pressure granulite‐facies metamorphism was followed by an almost isothermal pressure decrease of > 5 kbar, indicative of rapid uplift. TheP‐Tpath is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite‐facies terranes.