bluuebunny/crossref_metadata_2025 huggingface dataset:
A tectonic model for the metamorphic evolution of the Basal Gneiss Complex, Western South Norway
Authors: CUTHBERT S. J. , HARVEY M. A. , CARSWELL D. A.
Published: May 2007

A review of currently available information relevant to the Basal Gneiss Complex (BGC) of Western South Norway, combined with the authors’own observations, leads to the following conclusions.

1. Most of the BGC consists of Proterozoic crystalline rocks and probably subordinate Lower Palaeozoic cover.

2. The last major deformation of these rocks was during the Caledonian orogeny and involved large‐scale thrusting, recumbent folding and doming. The structural development of the BGC is closely tied in with that of the Caledonian allochthon.

3. The whole eclogite‐bearing part of the BGC has suffered a high pressure metamorphism with conditions of between 550°C, 12.5 kbar (Sunnfjord) and about 750°C, 20 kbar (Møre og Romsdal) at the metamorphic climax.

4. This metamorphism was of Caledonian age, probably rather early in the Caledonian tectonic history of the BGC and is considered to have been a rather transient event.

By setting these conclusions in a framework provided by geophysical evidence for the deep structure of the crust in southern Norway we have constructed a geotectonic model to explain the recorded metamorphic history of the BGC. It is suggested that considerable crustal thickening was caused by imbrication of the Baltic plate margin during continental collision with the Greenland plate. This resulted in high pressure metamorphism in the resulting nappe stack. Progradation of the suture caused underthrusting of the Baltic foreland below the eclogite‐bearing terrain causing it to emerge at the Earth's surface, aided by tectonic stripping and erosion.

Application of isostacy equations to the model shows that eclogites can be formed by in‐situ metamorphism in crustal rocks and reappear at the land surface above a normal thickness of crust in a single orogenic episode of approximately 65‐70 Ma duration.
High pressure‐low temperature metamorphism in the Gran Paradiso basement, Western Alps
Authors: VEARNCOMBE J. R.
Published: May 2007

The Gran Paradiso basement complex of the French and Italian Alps is composed of metasediments, termed the gneiss minuti, and metabasic rocks, both of which are intruded by a late Hercynian granite. The Bonneval gneiss, which crops out at the western edge of the complex, is composed of highly deformed metasediments, volcanics and volcaniclastic rocks. Eclogites, now highly altered, occur in the metabasic rocks. Kyanite and blue‐green amphibole are locally present in the gneiss minuti and aegirine plus riebeckite occur in the Bonneval gneiss. A moderately high pressure ‐ low temperature metamorphic event of probable Alpine age occurred in the basement complex. This metamorphic event differs from that in the overlying Sesia unit and ophiolites of the Schistes lustrés nappe in being at lower pressures (below the ab = jd100+ qz transition) and post‐dating the major (D2A) deformation. The origin of the metamorphism is discussed and interpreted as a probable consequence of the overlying nappe pile which was emplaced during the D2A event. Subsequent greenschist facies metamorphism in the basement complex is a consequence of thermal relaxation during uplift.
Deformation and metamorphism in the Bard area of the Sesia Lanzo Zone, Western Alps, during subduction and uplift
Authors: WILLIAMS P. F. , COMPAGNONI R.
Published: May 2007

The structure, microstructure and petrology of a small area close to the village of Bard in Val d'Aosta (Italy) has been studied in detail. The area lies across the contact between the Gneiss Minuti (GM) and the Eclogitic Micaschist (EMS) Complexes of the Lower element of the Sesia portion of the Sesia‐Lanzo Zone (Western Alps). Both complexes have undergone high‐pressure metamorphism, but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the GM to the EMS. Therefore, we interpret the contact as a thrust dividing the lower element of the Sesia into two sub‐elements. This interpretation is supported by structural evidence.

The early Alpine (90‐70 Ma) metamorphic history is best preserved in the EMS and is one of increasing pressure associated with thrusting. The maximum P/T recorded in the EMS is >1500 MPa (>15kbar) and 550°C and in the GM is < 1500‐1300 MPa (< 15‐13 kbar) and 500‐550°C. We suggest that the rocks were probably in an active Benioff zone during this time.

From then on the histories of the GM and EMS are the same. Deformation continued and the thrust and thrust slices were folded during decreasing pressure. We interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the Benioff zone had become inactive and a new Benioff zone had developed further to the north‐west.

A still later deformation and the Lepontine metamorphism (38 Ma) are related to continued uplift. Much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface.
Petrology of pelitic schists in the oligoclase‐biotite zone of the Sanbagawa metamorphic terrain, Japan: phase equilibria in the highest grade zone of a high‐pressure intermediate type of metamorphic belt
Authors: ENAMI M.
Published: May 2007

The oligoclase‐biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (XCa= 0.10–0.28)‐bearing assemblages in pelitic schists, and represents the highest‐grade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet + biotite + chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming PH2O = Psolid suggests that metamorphic conditions of this zone are about 610 ± 25°C and 10 ± 1 kbar.

Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase‐biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high‐pressure type of metamorphic belt and with those in a generalized‘upper staurolite zone’as an example of a medium‐pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as:
Metamorphism of the Scourian Complex, NW Scotland
Authors: BARNICOAT A. C.
Published: May 2007

The rocks of the Scourian Complex have been intensively studied, but there is still no consensus as to the conditions of the granulite‐facies metamorphism preserved in these rocks. Recent estimates of these conditions fall into two groups, one at 820‐920°C and ca. 11 kbar and the second at ca. 1000°C and >12 kbar. Investigation of a variety of rocks shows that the recorded conditions vary with grain‐size, with higher‐grade conditions recorded by the cores of coarser (ca. 10 mm) crystals, and lower‐grade conditions recorded by the rims of coarser grains and by finer grains. This observation suggests that re‐equilibration during recovery of these rocks to the surface has been important which may account for the discrepancy in estimated P‐T conditions. Revised estimates of the equilibration conditions of the Scourian Complex of T > 1000°C and P > 8.5 kbar are presented. The conditions suggested for the peak of metamorphism mean that the role of anatexis in the genesis of these rocks must be considered and the nature of the fluid phase thoroughly investigated.
Cordierite‐bearing schists and gneisses from Timor, eastern Indonesia: P‐T conditions of metamorphism and tectonic implications
Authors: BROWN MICHAEL , EARLE MICHAEL M.
Published: May 2007

In the Boi Massif of Western Timor the Mutis Complex, which is equivalent to the Lolotoi Complex of East Timor, is composed of two lithostratigraphical components: various basement schists and gneisses; and the dismembered remnants of an ophiolite. Cordierite‐bearing pelitic schists and gneisses carry an early mineral assemblage of biotite + garnet + plagioclase + Al‐silicate, but contain no prograde muscovite; sillimanite occurs in a textural mode which suggests that it replaced and pseudomorphed kyanite at an early stage and some specimens of pelitic schist contain tiny kyanite relics in plagioclase. Textural relations between, and mineral chemistries of, ferro‐magnesian phases in these pelitic chists and gneisses suggest that two discontinuous reactions and additional continuous compositional changes have been overstepped, possibly with concomitant anatexis, as a result of decrease in Pload during high temperature metamorphism. The simplified reactions are: garnet and/or biotite + sillimanite + quartz + cordierite + hercynite + ilmenite + excess components. P‐T conditions during the development of the early mineral assemblage in the pelitic gneisses are estimated to have been P + 10 kbar and T > 750°C, based upon the plagioclase‐garnet‐Al‐silicate‐quartz geobarometer and the garnet‐biotite geothermometer. P‐T conditions during the subsequent development of cordierite‐bearing mineral assemblages in the pelitic gneisses are estimated to have been P + 5 kbar and T + 700°C with XH2O < 0.5, based upon the Fe content of cordierite occurring in the assemblage quartz + plagioclase + sillimanite + biotite + garnet + cordierite coexisting with melt.

Final equilibration between some of the phases suggests that conditions dropped to P > 2.3 kbar and T > 600°C. A similar exhumation P‐T path is suggested for the pelitic schists with early metamorphic conditions of P > 6.2 kbar and T > 745°C and subsequent development of cordierite under conditions in the range P = 3‐4 kbar and T = 600‐700°C. The tectonic implications of these P‐T estimates are discussed and it is concluded that the P‐T path followed by these rocks was caused by decompression during rifting and synmetamorphic ophiolite emplacement resulting from processes during the initiation and development of a convergent plate junction located in Southeast Asia during late Jurassic to Cretaceous time.
Contrasting textures in metamorphic and anatectic migmatites: an example from the Scottish Caledonides
Authors: McLELLAN E. L.
Published: May 2007

Abstract.A method for the quantitative analysis of the spatial relations of minerals is described. Dispersed distributions are formed by annealing and destroyed in post‐tectonic migmatization. Aggregate distributions characterize solid‐state differentiation, whereas leucosomes formed in systems of high fluid:rock ratio (in the examples studied, anatectic melts) show random distributions.

Quantitative textural analysis can be used to indicate whether migmatization was post‐tectonic or earlier, though caution is necessary if post‐migmatite cooling is slow or if there is some minor deformation. More importantly, it can be used to discriminate melt‐present from melt‐absent leucosomes; this is exemplified by a suite of metamorphic and anatectic migmatites from the Scottish Caledonides.

The textural evolution of anatexites with increasing melt percentage is traced. Initial feldspar porphyroblastesis occurs by Ostwald ripening via grain boundary melts; subsequently ophthalmites develop with fabrics and chemistry inherited from the palaeosome. At greater than 30% melt these inherited fabrics are wholly destroyed. Deformation prompts segregation into melanosome and leucosome; resultant leucosomes contain no inherited crystals. The scale of anatectic systems is fixed at the point at which segregation begins; ophthalmites provide evidence for melt and crystal transfer beyond original palaeosome boundaries. In contrast, metamorphic migmatites are necessarily small‐scale systems because of diffusive constraints, and melanosomes are invariably produced.
Mineral parageneses in metamorphic rocks exposed along Tailuko Gorge, Central Mountain Range, Taiwan*
Authors: ERNST W. G.
Published: May 2007

Field, petrographic and microprobe investigations of metaclastic rocks, calcareous schists, marbles, chloritic calcareous meta‐volcanic units and schists/paragneisses which crop out along the eastern portion of the Central East‐West Cross Island Highway in Taiwan demonstrate that metamorphic intensity gradually increases eastward. The lower greenschist facies Slate Formation on the W contains completely recrystallized, pure albitic plagioclase, but at least some of the white micas (± chlorites) probably represent relict detrital flakes. Neo‐blastic biotite and epidote occur sporadically in the Pihou(?) Formation, and increase dramatically eastward; concomitantly the abundance of carbonaceous matter decreases to zero in the eastern Tailuko zone, and the amount of chlorite + white mica diminishes somewhat. Epidote becomes more aluminous at higher metamorphic grade. Eastward, phengites change progressively to more muscovitic compositions as the proportion of biotite increases.

A close approach to chemical equilibrium for the pre‐Cenozoic, complexly deformed metamorphic basement assemblages is suggested by regular, systematic, major and minor element partitioning between analysed coexisting phases. Fractionation is less pronounced on the E, reflecting higher temperatures. Estimated physical conditions of recrystallization with αH2O and αCO2 moderate, are: T > 325 ± 75°C, P > 3 kbar (W); T > 425 ± 75°C, P > 4kbar(E).

The gradual eastward increase in metamorphic intensity from the Slate Formation through the Pihou(?) Formation and the three Tailuko zones, as well as the relict precursor textures in the pre‐Cenozoic layered basement rocks indicate that the observed paragenetic sequence could represent a synchronous Neogene recrystallization event, probably accompanying the Plio‐Pleistocene collision of the Asiatic continental margin and the Luzon (Coastal Range) andesitic arc.
A new geobarometer for the assemblage biotite‐muscovite‐chlorite‐quartz
Authors: POWELL ROGER , EVANS JANE A.
Published: May 2007

The equilibrium thermodynamics of the reaction:

And the equilibrium constant is composed of activities formulated using ideal mixing on sites. Consideration is given to the evaluation of uncertainties in pressures calculated using the geobarometer. Preliminary testing suggests that the geobarometer has considerable potential. Much wider testing is now required.
Bulk composition and mineral parageneses of sapphirine‐bearing rocks along a gabbro‐lherzolite contact at Finero, Ivrea Zone, N Italy
Authors: SILLS JANE D. , ACKERMAND DIETRICH , HERD RICHARD K. , WINDLEY BRIAN F.
Published: May 2007

Sapphirine occurs in a 3‐5 m wide zone between amphibole‐lherzolite and garnetiferous metagabbro at Finero in the Ivrea Zone, NW Italian Alps. Layers consisting of plag + hb + sa + cpx + opx + sp + gt are interbanded with spinel pyroxenites, which may contain sapphirine replacing spinel. All minerals are very magnesian, with XMgbetween 0.78 and 0.92. Bulk rock analyses suggest that precursors to the sapphirine‐bearing rocks were igneous cumulates of plagioclase + olivine + hornblende + spinel. Up to 16wt% CaO does not inhibit sapphirine formation and it is the unusually Mg‐rich nature of the host rocks which allows sapphirine development. The early igneous assemblage was replaced by one of cpx + sa + hb +± plag at a pressure of 9 ± 1 kbar and temperatures of 900 ± 50°C. Subsequent rapid uplift caused the instability of gt, gt + hb, hb and sa + cpx to form opx + plag ± sp ± sa symplectites.
Element mobility associated with syn‐metamorphic shear zones near Scotchport, NW Mayo, Ireland
Authors: WINCHESTER J. A. , MAX M. D.
Published: May 2007

Abstract
Three major blastomylonitic synmetamorphic (epidote amphibolite to mid amphibolite facies) shear zones are seen on the NW coast of the Mullet Peninsula in NW Mayo. These shear zones occur at the contacts of major structural units and in an imbricated slice where rocks of the Erris Complex are deformed and chemically modified. Chemical changes associated with individual shear zones have been deduced by comparing the compositions of various gneisses both within and adjacent to the shear zones. Compositional changes are different in the constituent rock‐types within each unit and many elements normally considered immobile have been selectively mobilized within the shear zones. Little evidence of wholesale metasomatic introduction of components into these shear zones was found to accompany the selective mobilization.
Inverted isograds at Sulitjelma, Norway: the result of shear‐zone deformation
Authors: MASON ROGER
Published: May 2007

Abstract
At Sulitjelma, Norway, there is a major inversion of metamorphic isograds beneath an inverted but undisrupted ophiolite. The flysch‐like Furulund schist in which the inverted isograds occur is also inverted and the early folds in it are downward facing. The isograds cut across the axial surfaces of early folds and across the schistosity. These relationships are explained as the consequence of metamorphism during the progressive development of a large overfold. The inverted limb of the overfold is regarded as a major, thick, gently‐dipping shear zone, separating the lower‐grade, lower part of the Caledonian allochthon below from the higher‐grade upper part of the allochthon above. The association between stratigraphical inversion, downward‐facing of syn‐schistosity folds and metamorphic inversion is explained by the progressive development of the shear zone. It is suggested that the presence of such shear zones is a common feature of orogenic belts formed by continental collision.
Alpine metamorphism of Hercynian hornblende granodiorite beneath the blueschist facies schistes lustrés nappe of NE Corsica
Authors: GIBBONS WES , HORAK JANA
Published: May 2007

Abstract
The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle‐to‐late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies (schistes lustrés) nappe, was confined to a major ductile shear zone (c. 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca‐amphibole + albite + sphene, and biotite to actinolitic Ca‐amphibole + albite + phengite + Ti‐ore + epidote. Zone 3: Hornblende to crossite + low Ti‐biotite + phengite + sphene, and biotite to crossite + low Ti‐biotite + phengite + Ti‐ore + sphene ± epidote. P‐T conditions at the base of the shear zone are estimated to have been 390‐490°C at 600‐900 M Pa (6‐9kbar) and the Corsican basement is therefore deduced to have been buried to 20‐30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction.
Reaction pathways during the progressive deformation of a blueschist metabasite: the role of chemical disequilibrium and restricted range equilibrium
Authors: RIDLEY JOHN , DIXON JOHN E.
Published: May 2007

Abstract
There are discrete masses of un‐deformed metabasite within the blueschist series of the island of Syros. Greece. Around the margins of these masses are zonal sequences through rocks showing intracrystalline deformation but without a geometric fabric, to rocks with discrete and anastomosing shear zones, and finally to penetratively foliated rocks with isolated relics of the original undeformed texture. Textural relics suggest that this spatial sequence is at least qualitatively also a temporal sequence.

This progressive shear zone deformation took place concurrently with a glaucophane‐epidote to eclogite reaction. The reaction pathways in the rocks that underwent the shear zone deformation can be compared with those in rocks of a similar composition that suffered a longer deformation history and show no relics of an undeformed parent. Although the final assemblages are in both cases the same, the pathways are different. These differences are in part related to reactions promoted by the change from local to bulk equilibrium on the onset of deformation in the rocks. They are also related to the crystallization and later breakdown during the sequence of progressive equilibration of a metastable phase, in this case an impure glaucophane.
Fluid‐rock interaction in some shear zones from the Pyrenees
Authors: McCAIG A. M.
Published: May 2007

Abstract
Mylonites from shear zones cutting Hercynian gneisses in the central Pyrenees have been studied in thin section and using the electron microprobe. The shear zones contain retrogressive greenschist facies assemblages implying introduction of an aqueous fluid during deformation in the zones. Textural evidence suggests that fluid‐rock interaction occurred throughout the active life of the shear zones.

Whole‐rock chemical changes during deformation are documented in a variety of mylonitic lithologies and retrogressed country rocks. The overall effect was to reduce chemical differences between lithologies. Activity diagrams show that this would be expected if a hydrous fluid was circulating between different lithologies during deformation. In most cases fluid/rock ratios were relatively small resulting in gradual chemical changes and repeated recrystallization. ‘Open‐system’behaviour with reduction in the number of phases is seen in some granite mylonites, suggesting focusing of fluid movement in parts of the shear zones. Continual fluid‐rock interaction may have led to reaction‐enhanced ductility in the shear zones over a long period of time. The source of fluid is uncertain, but may be related to underthrusting of material beneath the area investigated.
P‐T‐X (CO2) conditions in mafic and calc‐silicate hornfelses from Oberon, New South Wales, Australia
Authors: ANDREW A. S.
Published: May 2007

Abstract
The Rockley Volcanics from near Oberon, New South Wales occur within the aureole of the Carboniferous Bathurst Batholith and have been contact metamorphosed at P ∼ 100 ± 50MPa (10.5kbar) and a maximum T ∼ 565°C in the presence of a C–O–H fluid. Prior to contact metamorphism the volcanics were regionally metamorphosed and altered with the extensive development of actinolite, chlorite, plagioclase, quartz and calcite. The contact metamorphosed equivalents of these rocks have been subdivided into: Ca‐poor (cordierite + gedrite), Mg‐rich (amphibole + olivine + spinel), mafic (amphibole + plagioclase) and Ca‐rich (amphibole + garnet + diopside; diopside + plagioclase; garnet + diopside + wollastonite) rocks.

The chemistry of the minerals in the hornfelses was controlled by the bulk rock chemistry and fluid composition. Pargasites and hastingsites as well as an unusual phlogopite with blue green pleochroism, are found in Ca‐rich hornfelses. A comparison of the assemblages with experimentally derived equilibria suggests that the fluid phase associated with the Ca‐rich hornfelses was water‐rich (Xco2= 0.1 to 0.3) while that associated with the Mg‐rich hornfelses was enriched in CO2 (Xco2 > 0.7). The different hornfels types have reacted to contact metamorphism independently in both their solid and fluid chemistries.
Phase equilibria in the hornblende‐bearing basic gneisses of the Uvete area, central Kenya
Authors: MIYAKE AKIRA
Published: May 2007

Abstract
The hornblende‐bearing basic gneisses in the Uvete area, central Kenya, were metamorphosed under a narrow range of P and T (6.5 ± 0.5kbar and 530 ± 40°C) of the staurolitekyanite zone in the Mozambique metamorphic belt. They show a wide variety of divariant and trivariant mineral assemblages consisting of hornblende, cumminatonite, gedrite, anthophyllite, chlorite, garnet, epidote, clinopyroxene, plagio‐clase and quartz. The bulk and mineral chemistries and the graphical representation of phase relations show that each mineral assemblage approaches chemical equilibrium and defines a unique composition volume in the A′(Al + Fe3+− (13/7)Na)‐F(Fe2+)‐M′(Mg)‐C′(Ca‐(3/7)Na) tetrahedron. The composition volumes are distributed quite regularly and do not overlap each other.

The phase relations in the Uvete area are in contrast with those in the staurolite‐kyanite zone amphibolites in the Mt. Cube quadrangle, Vermont. The amphibolites there contain low‐variance mineral assemblages formed under different values of μH2O and μCO2. These assemblages define overlapping composition volumes in the A′‐F′‐M′‐C’tetrahedron.

The mineral assemblages in the Uvete area are interpreted as having formed in equilibrium with fluid at a high and nearly constant μH2O value. Such a fluid composition was externally controlled by the supply of H2O‐rich fluid expelled from the surrounding pelitic and psammitic rocks. The body size of the basic gneisses in the Uvete area (less than 400m in thickness) was small enough for the fluid to migrate completely.
Two‐pyroxene thermometry of Precambrian granulites from Cape Riche, Albany‐Fraser Province, Western Australia
Authors: STEPHENSON N. C. N.
Published: May 2007

Coexisting Ca‐poor and Ca‐rich pyroxenes in granulites at Cape Riche, in the Precambrian Albany‐Fraser Province, Western Australia, are dominantly chemically homogeneous within individual samples, suggesting a major episode of equilibration. However, occasional grains in a few samples contain exsolved domains interpreted as relics of an earlier, higher‐T assemblage. Pyroxene pairs in ten, presumably isothermal, samples from a restricted area are used to (i) assess the suitability of several versions of the two‐pyroxene thermometer for application to metamorphic rocks, and (ii) determine the thermal history of the Cape Riche pyroxenes.

The various versions of the two‐pyroxene thermometer applied to the well‐equilibrated homogeneous pyroxene grains show poor to good precision and yield mean temperatures varying widely from 683° to 893°C, in the following order of increasing T: Lindsley (1983; opx version), 683°± 11°C; Kretz (1982;KDversion), 705°± 19°C; Ross & Huebner (1975), 709°± 30°C; Kretz (1982; solvus version), 735°± 24°C; Fonarev & Graphchikov (1982; opx version), <750°C; Lindsley (1983; cpx version), 784°± 40°C; Fonarev & Graphchikov (1982; cpx version), ~820°± 30°C; Wood & Banno (1973), 849°± 16°C; Powell (1978), 854°± 23°C; Wells (1977), 893°± 10°C. Independent T estimates, based on mafic assemblages and garnet‐biotite thermometry, suggest that the major episode of metamorphism occurred at 700‐800°C (P ~ 5 kbar). Therefore the Wells, Powell, Wood & Banno and Fonarev & Graphchikov (cpx) temperatures are almost certainly too high. In the absence of a more precise independent T estimate it is difficult to assess the relative merits of the results obtained from the remaining versions of the two‐pyroxene thermometer, none of which can be unequivocally demonstrated to be seriously in error, though the Lindsley (opx) T is probably too low. Other significant shortcomings evident in the results include the relatively poor precision obtained from the three methods based on purely graphical representation of the augite limb of the solvus (i.e., the Ross & Huebner, Fonarev & Graphchikov (cpx) and Lindsley (cpx) versions), and the apparent dependence of derived T on Mg/Fe2+ratio for the Powell, Wood & Banno and Lindsley (cpx) methods.

For the bulk compositions of exsolved domains, the different versions of the two‐pyroxene thermometer yield mean temperatures 23° to 82°C (overall mean, 65°C) higher than for homogeneous grains in the same samples. These exsolved domains are interpreted as relics of a higher‐T (peak?) metamorphic assemblage, rather than an igneous precursor.
The pumpellyite‐actinolite and contiguous facies in part of the Phyllite‐Quartzite Series, central Northern Peloponnesus, Greece
Authors: BALTATZIS E. G. , KATAGAS C. G.
Published: May 2007

Variations in assemblage and composition of the constituent minerals in basic and intermediate metavolcanics encountered in the Zarouchla Group of the Phyllite‐Quartzite Series are consistent with a progressive sequence, corresponding to temperature conditions estimated at 290‐380°C (minimum values) under a total pressure greater than 3°5kbar and possibly as high as 5 kbar. In the absence of more critical evidence, the parageneses recorded in the metavolcanic rocks are interpreted as belonging to a prograde facies series from the lawsonite‐albitechlorite facies through the pumpellyite‐actinolite facies to the greenschist facies. The present distribution of mineral assemblages does not show a simple increase of metamorphic grade in a given direction but is apparently related to the tectonic evolution of the metamorphic sequence.
Petrology and geothermometry of retrograded Hercynian charnockites and host gneisses, Agly Massif, French Pyrenees
Authors: PAQUET JACQUES
Published: May 2007

Abstract
A Hercynian charnockite occurs within high‐grade gneisses in the Agly Massif, French Pyrenees. Its thermal history has been evaluated using the Fe‐Mg distribution coefticient (KD) between garnet and biotite. These minerals have different origins but similar compositions in the charnockites and host gneisses. In the charnockite, the Bi–Ga pairs are the retrograde products of Opx alteration. This Opx reaction with feldspar can be written. Opx + PI + Fluid 1(H2O + Al + K + Fe + Ti) = Bi + Ga + Q + Fluid 2(H2O + Na). The garnets are relatively Ca poor (4–2.5% grossular); they are automorphic and zoned in the gneisses and poikiloblastic in the charnockites. Both types show a retrograde rim (of few hundred microns’width) across which Fe and Mn increase as Mg decreases. The biotites show a good correlation between the octahedral cations (Ti4++ Fe2+) and (Mg2++ Al3+VI); Ti and Fe both increase, whereas Mg and AlVI decrease. There is an inverse linear correlation between Fe2+ and Mg2+ and the Fe/Mg ratio increases as Ti increases. The relation between Ti and KGa‐BiDFe‐Mg is less clear: it seems that KD slightly decreases as Ti increases. The equilibration temperatures of Ga–Bi pairs are discussed: the charnockite Ga‐Bi pairs have equilibrated between 550°C and 600°C; whereas those of the gneisses have equilibrated between 550°C and 650°C. Two main thermal steps appear: one in the gneisses between 600‐650°C and a second one in both the gneisses and the charnockites between 550°C and 600°C.
Development of P‐T prograde and P‐retrograde, T‐prograde isogradic surfaces during blueschist to eclogite regional deformation/metamorphism in New Caledonia, as indicated by progressively developed porphyroblast microstructures
Authors: BELL T. H. , BROTHERS R. N.
Published: May 2007

Abstract
The mid‐Tertiary blueschists, eclogites and eclogitic gneisses of northern New Caledonia are the products of four phases of regional metamorphism and deformation (D1–D4). Omphacite, lawsonite and Mn‐rich garnet isogradic surfaces were developed during the second deformation (D2) under prograde pressure and temperature conditions. Subsequent deformations (D3–D4) folded these D2 isogradic surfaces. However, within the P‐retrograde, T‐prograde metamorphic environment of the D4 phase, omphacite altered to albite and chlorite; as a result, a late‐stage sub‐horizontal isogradic surface developed for omphacite‐out where this mineral preserved as relics within syn‐D4 albite porphyroblasts. Other minerals that crystallized for the first time (epidote) or had rim additions (almandine phengite) during D4, also form nearly horizontal isogradic surfaces. Porphyroblastic garnet and albite contain inclusion trails, which allow their microstructural development and crystallization of the matrix to be traced from D2 to D4.

Late syn‐D4 the temperature increased markedly in association with an extensive exothermic decarbonation, even though the rocks were in a state of pressure retrogression. This caused considerable neocrystallization, recrystallization and growth of mattix and porphyroblasts such that, although S2 foliation crenulated by D3 and D4 is readily observable, almost all signs of stored strain due to D3 and D4 have been removed, and the deeper schists and eclogitic gneisses superficially appear to have undergone a drastic annealing recrystallization, post‐dating deformation.
The greenschist metamorphism of Archaean synvolcanic stocks near Wawa, Ontario, Canada
Authors: STUDEMEISTER PAUL A.
Published: May 2007

Abstract
Two Archaean synvolcanic stocks with contact aureoles occur in the Wawa greenstone belt near Wawa, Ontario, Canada. The Gutcher Lake and Jubilee stocks consist mainly of granitoid trondhjemite with feldspar laths mottled by white mica + calcite + epidote and rimmed by clear albite. Biotite is partly or wholly pseudomorphosed by chlorite + sphene; some epidote is partly altered to calcite + chlorite. The granitoid phase grades into a foliated phase of quartz + albite + white mica + calcite + chlorite near fracture zones traversing the stocks.

The alteration of the Gutcher Lake stock along its foliated margin involved addition of K2O, H2O + CO2, MnO, plus Rb; loss of CaO plus Sr; and a shift in Fe+2/Fet from 0.66 to 0.81. The alteration of the Jubilee stock along the Darwin Shear involved addition of H2O + CO2; loss of Sr; and no significant shift in Fe+2/Fet. The greenschist alteration also modified the contact aureoles bordering both stocks.

One interpretation is that regional metamorphism in the Archaean overprinted a greenschist assemblage on both stocks. The alteration was intense near fracture zones and sporadic remote from fractures. Lower integrated water to rock ratios along the Darwin Shear compared to the margin of the Gutcher Lake stock may explain the comparatively lower perturbation of the element abundances and redox state of iron.
Deformation partitioning and porphyroblast rotation in meta‐morphic rocks: a radical reinterpretation
Authors: BELL T. H.
Published: May 2007

Abstract
Most porphyroblasts never rotate during ductile deformation, provided they do not internally deform during subsequent events, with the exception of relatively uncommon but spectacular examples of spiralling garnets. Instead, the surrounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast. Consequently, porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails, even where these structures have been rotated or obliterated in the matrix due to subsequent deformation. These relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subsequent deformation. They can also be used to determine the deformation history, as porphyroblasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component.
The metamorphic evolution of granulites at Fyfe Hills; implications for Archaean crustal thickness in Enderby Land, Antarctica
Authors: SANDIFORD MICHAEL
Published: May 2007

AbstractGranulites at Fyfe Hills in Enderby Land, Antarctica, crystallized at temperatures in excess of 850°C, and possibly as high as 1000°C, and at pressures of 8‐10kbar during the mid to late Archaean. A number of features, including repeated retrograde metamorphism at 5.5‐8kbar, retrograde reaction textures, and rimward zoning in pressure sensitive systems, suggest that following peak metamorphism the granulites stabilized at a depth of 18‐26 km. After stabilization, the granulites cooled near‐isobarically to temperatures of 600‐700°C. Assuming a total crustal thickness of 35‐40 km during this late Archaean interval of isobaric cooling, the peak metamorphic crustal thickness is estimated at 35‐56 km. This estimate is significantly less than the 60‐70 km obtained by summing the depths of the present levels of exposure (26‐34 km) and the thickness of the crust presently beneath Fyfe Hills (approxi‐mately 35km) and is, therefore, consistent with independent evidence for extensive post‐Archaean thickening of the Enderby Land crust.
Regression diagnostics and robust regression in geothermometer/geobarometer calibration: the garnet‐clinopyroxene geothermometer revisited
Authors: POWELL ROGER
Published: May 2007

Abstract
The calibration of geothermometers and geobarometers should involve not only the determination of the parameters in the equation used, but also the uncertainties on, and the correlations between, these parameters. This necessitates the use of a technique such as least squares. Given the poor performance of least squares in the presence of outliers in the data, techniques for identifying outliers for exclusion—regression diagnostics, and techniques for handling data which include outliers—robust regression and jackknifing, are essential. These techniques are summarized and their importance is emphasized, and they are applied to the calibration of the garnet‐clinopyroxene Fe‐Mg exchange geothermometer.

The experimental data of Raheim & Green (1974) and Ellis & Green (1979) are explored using regression diagnostics to discover outliers in the data. After exclusion of the two influential outliers found, a new geothermometer equation for garnet‐clinopyroxene Fe‐Mg exchange is derived using robust regression and based on all the data: thus, T(K) = 2790 + 10P+ 3140xca,g/1.735 + In KD where T is in Kelvin and P is in kbar. This equation, as might be hoped, is essentially identical to that of Ellis & Green (1979). Equations for calculating the uncertainty in a calculated temperature, contributed by uncertainties in the calibration, are also derived.
Effect of metamorphism and partial melting of host rocks on zircons
Authors: GUPTA L. N. , JOHANNES W.
Published: May 2007

AbstractZircons have been studied from different layers of migmatites (from Arvika, western Sweden and Nelaug, southern Norway) and from a paragneiss (from Arvika) associated with one of the migmatites. The main purpose of the investigation is to establish whether or not information about zircons can help in the elucidation of the parentage and rock‐forming processes of migmatites.

The elongation ratio of zircons from all layers is small and characteristic of sedimentary zircons. Further, the absence of characteristic colours and the growth trends of the zircons (indicated by the reduced major axes) observed in the various samples both support a sedimentary parentage for these rocks. The zircons of all layers exhibit secondary growth (overgrowth, outgrowth and multiple growth) due to metamorphism. Compared with the zircons from the paragneiss, those of the migmatite layers are more clouded and less rounded, some of them becoming opaque or even skeletal; this is especially true of the zircons from the leucosomes. These observations indicate an alteration of the original sedimentary zircons in the migmatite, especially in the leucosomes, in response to the migmatization process, previously interpreted as partial melting.
An internally consistent thermodynamic dataset with uncertainties and correlations: 1. Methods and a worked example
Authors: POWELL ROGER , HOLLAND T. J. B.
Published: May 2007

Abstract
This, the first two papers, sets out the philosophy and methods of determining an internally consistent thermodynamic dataset for minerals using the least squares method. The applicability of the least squares method is discussed, and it is applied to a small set of experimental equilibria in the system Na2O–Al2O3–SiO2–H2O. The importance is stressed of defining not only the enthalpies of formation of minerals, but also the uncertainties and the correlations among them. The system which has been used as an illustration for this paper serves as a visual guide to the method, as it is small enough to represent graphically in two dimensions. In the paper which follows, we extend the method to a system of 60 equations (experimentally determined equilibria) involving 34 unknowns (enthalpies of formation of mineral end‐members).
An internally consistent thermodynamic dataset with uncertainties and correlations: 2. Data and results
Authors: HOLLAND T. J. B. , POWELL ROGER
Published: May 2007

Abstract
This, the second of two papers, represents the application of a least squares approach, discussed in the previous paper, to the generation of an internally consistent thermodynamic dataset involving 60 reactions among 43 phases, in the system K2O–Na2O–CaO–MgO–Al2O3–SiO2–H2O–CO2. We make the assumption that all the thermodynamic data, with the exception of enthalpies of formation of the phases, are well known, and solve for an internally consistent set of enthalpies which reproduces the 60, experimentally determined, phase equilibrium reactions. An important difference between our dataset and that of previous alternatives in the literature is that we are able to determine the uncertainties on, and correlations between, the enthalpies of formation for all phases in the set, and hence are able to apply simple error propagation techniques to determine the uncertainties in any phase equilibrium calculations performed using this dataset. Selection of reactions, for geothermometry and geobarometry, may be more readily made by choosing equilibria with small uncertainties in their thermodynamics. Our data are in reasonably close agreement with the high temperature molten oxide calorimetry results on silicate minerals where available, a fact which lends a degree of confidence to the results.
Metamorphic P–T Paths from pelitic schists and greenstones from the south‐west Tauern Window, Eastern Alps
Authors: SELVERSTONE JANE , SPEAR FRANK S.
Published: May 2007

Abstract
Petrological data from intercalated pelitic schists and greenstones are used to construct a pressure–temperature path followed by the Upper Schieferhülle (USH) series during progressive metamorphism and uplift in the south‐west Tauern Window, Italy. Pseudomorphs of Al–epidote + Fe‐epidote + albite + oligoclase + chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite‐albite‐chlorite subfacies of the blueschist facies at P± 7–8 kbar. Geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475±25°C, 5–6 kbar; calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of ∼ 7.5 kbar. Garnet zoning patterns are complex and reversals in zoning can be correlated between samples. Thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity. Fluid inclusion data from coexisting immiscible H2O–CO2–NaCl fluids constrain the uplift path to have passed through temperatures of 380 + 30°C at 1.3 + 0.2 kbar.

There is no evidence for metamorphism of USH at pressures greater than ∼ 7.5 kbar in this area of the Tauern Window. This is in contrast to pressures of ± 10 kbar recorded in the Lower Schieferhülle only 2–3 km across strike. A history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the P–T data obtained from these adjacent tectonic units.
Garnet evolution in pre‐Variscan pelitic rocks from the Lake Emosson area, Aiguilles Rouges Massif, Western Alps
Authors: VON RAUMER JÜRGEN F. , SCHWANDER HANS W.
Published: May 2007

Abstract
The chemical evolution of garnets from pelitic rocks of probable Palaeozoic age corresponds to a complex metamorphic evolution of the host rocks.

Among the almandine‐rich garnets (Alm60–80), two main types of evolution can be distinguished. Early Mn‐rich garnets coexisting with kyanite may be replaced by plagioclase and then, during a late stage, by biotite and/or sillimanite. The second type of evolution corresponds to an overgrowth of Mn‐poor late‐stage garnet on older Mn‐rich garnets which corresponds to a thermal peak with sillimanite‐type of metamorphism. This new garnet may appear either as an overgrowth with a strong discontinuity, or as small, new euhedral garnet or as skeletal garnet.

This chemical evolution of garnet corresponds to an early collisional stage of metamorphism (of high pressure type with high Mn values) of probable Ordovician age followed by uplift and a thermal peak (low Mn values) in Devonian times.
Volcanogenic Cu–Pb–Zn bodies in granulites of the central Arunta Block, central Australia
Authors: WARREN R. G. , SHAW R. D.
Published: May 2007

Abstract
Small unexploited copper‐lead‐zinc deposits, characterized by a distinctive wall‐rock association of cordierite quartzite, silica‐undersaturated rocks, calc‐silicate rocks and impure marbles, occur in quartzofeldspathic gneisses and mafic granulites of the Strangways Metamorphic Complex, central Arunta Block, central Australia. Available data support the hypothesis that these are metamorphosed volcanogenic ore bodies. The chemical compositions of the quartzofeldspathic gneisses are comparable with those of less metamorphosed felsic igneous rocks, particularly the felsic igneous rocks emplaced in the North Australian Orogenic Province in the interval 1880–1800 Ma; and the mafic granulites are chemically similar to basalts (olivine‐normative tholeiites). The wall‐rock suite can be correlated from chemistry and lithological association with the suites of wall rocks found in unmetamorphosed volcanogenic ore deposits. That the protolith of the cordierite quartzites may well have been leached tuff, similar to the illite‐chlorite‐quartz tuff found in volcanogenic ore deposits, is also shown by retrogression of the granulitefacies assemblage: cordierite‐garnet‐ortho‐pyroxene‐biotite‐quartz in the cordierite quartzites to cordierite‐anthophyllite‐bearing assemblages and thence to chlorite‐muscovite‐quartz assemblages. Lenses of silica‐undersaturated rocks with spinel and, less commonly, sapphirine are interpreted as the metamorphosed equivalents of chlorite‐rich pods found within leached tuffs in volcanogenic ore deposits. The wall rocks form sheet‐like bodies; this suggests that they were deposited in relatively shallow water, thus precluding the formation of massive sulphides.
P‐Tpaths from high temperature shear zones beneath ophiolites
Authors: JAMIESON R. A.
Published: May 2007

AbstractMctamorphic rocks of the St Anthony Complex of north‐western Newfoundland are best interpreted in terms of a high‐temperature shear zone formed between down‐going continental margin rocks and overriding oceanic lithosphere in a subduction zone. High‐grade rocks, immediately beneath the oceanic lithosphere peridotite, display retrograde meta‐morphism in high‐strain zones, whereas lower grade rocks, near the base of the metamorphic complex, display prograde metamorphism in high‐strain zones. Mylonite zones in meta‐basitcs at all levels in the complex contain the assemblage epidote‐hornblende‐albite‐sodic oligoclase. These observations suggest that the ‘inverted metamorphic gradient’within the St Anthony Complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone. The degree of re‐equilibration is strongly dependent on the degree of strain, and is best achieved in synmetamorphic mylonite zones. This interpretation of the St Anthony Complex can be extended to other sub‐ophiolite metamorphic sheets, which show very similar relationships. It is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones, theP‐Tpaths of which preserve records of burial and exhumation in subduction zones.