bluuebunny/crossref_metadata_2025 huggingface dataset:
Porphyroblast nucleation, growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development
Authors: BELL T. H. , RUBENACH M. J. , FLEMING P. D.
Published: May 2007

Abstract
In regional metamorphic rocks, the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain‐rate gradients across the boundaries between the partitioned zones. These generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer. Phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain, because of their uniquely layered crystal structure. However, most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients, and hence are susceptible to dissolution and solution transfer.

As a consequence, zones of progressive shearing become zones of dissolution of most minerals, and of concentration of phyllosilicates and graphite. Exceptions are mylonites, where strain‐rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer. Porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing, as they would be dissolved by the effects of shearing strain on their boundaries. However, they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones, which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth. Zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation, this includes a build up of stored strain‐energy relative to zones of progressive shearing, in which dissolution is occuring.

Porphyroblast growth is generally syndeformational, and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account.

Porphyroblasts in a contact aureole do not grow statically either, as microfracturing, associated with emplacement, allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening.

The criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning, reactivation of deforming foliations, and a general lack of rotation of porphyroblasts, with the spectacular exception of genuinely spiralling garnet porphyroblasts.
Chromian omphacite from low‐grade metamorphic rocks, Nishisonogi, Kyushu, Japan
Authors: NISHIYAMA T. , UEHARA S. , SHINNO I.
Published: May 2007

Abstract
Chromian omphacite which contains up to 4 wt % Cr2O3 has been identified from low‐grade metamorphic rocks in Nishisonogi, Kyushu, Japan. It occurs as aggregates, forming a thin horizon ([20 mm thick) in alayered metagabbro within a serpentinite melange zone, together with Cr‐free omphacite, actino‐lite, epidote and sphene. It may have been formed by the metasomatic introduction of Cr into the metagabbro from the serpentinite rather than by reaction with chromite. The structural formula, based on EPMA analyses, and the optical absorption spectrum of the chromian omphacite show that the Cr is positioned in the octahedral site.
Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales
Authors: ROBINSON D. , BEVINS R. E.
Published: May 2007

Abstract
A diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the Lower Palaeozoic marginal basin of Wales by examination of variations in phyllo‐silicate mineralogy, illite crystallinity and bo parameter of white micas. This transition represents a temperature range from ∼ 150°C to ∼ 400°C and the metamorphism is of a low‐pressure facies series type, with a geothermal gradient of ∼ 40°Ckm‐1. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth‐related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn‐tectonic metamorphic event is apparent.

Correlation of pelite data with metabasite assemblages is variable, the most consistent relationship being between epizone crystallinity values andepidote‐actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnite‐pumpellyite assemblages in metabasites and it is suggested that the latter represent non‐buffered, and therefore non‐diagnostic, assemblages.
P–T conditions for the eclogitic re‐equilibration of the metaophiolites from Val d'Ala di Lanzo (internal Piemontese zone, Western Alps)
Authors: SANDRONE R. , LEARDI L. , ROSSETTI P. , COMPAGNONI R.
Published: May 2007

Abstract
A study has been made of the high‐pressure early‐Alpine re‐equilibration in the eclogites and metasedimentary cover of the Val d'Ala di Lanzo ophiolite. All of the main high‐pressure minerals have been analysed and their compositions used to determine re‐equilibration temperatures. The minimum conditions proposed (P= 1.3 GPa, T= 450–460°C) are also indicated by the presence of a jadeite+quartz‐bearing metagranite.

The temperatures are compared with those reported for similar eclogites from the Voltri Group, the Aosta Valley and the Valais. Comparison of recalculated temperatures shows that the temperature (and probably the pressure) of the eclogitic re‐equilibration increased in the Aosta Valley and the Valais, in keeping with what has been observed in the internal Penninic basement of the Gran Paradiso and Monte Rosa crystalline massifs.
The subsolidus segregation of layer‐parallel quartz‐feldspar veins in greenschist to upper amphibolite facies metasediments
Authors: SAWYER E. W. , ROBIN P.‐Y. F.
Published: May 2007

Abstract
Layer‐parallel (i.e. parallel to foliation or bedding) vein formation in the graywackes and pelites of the Quetico Metasedimentary Belt occurred during synchronous prograde metamorphism and regional (D2) compression. In a traverse across metasediments which change in metamorphic grade from greenschist to upper amphibolite (migmatite) facies, layer‐parallel veins show the following trends: (1) an increase in thickness and internal complexity, the latter due to successive boudinage; (2) low‐grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks, but at higher grades other sites are also used and (3) a systematic increase in plagioclase/quartz ratio in the veins towards higher grade, adjacent mafic selvedges first exhibit quartz depletion then, in the amphibolite facies, plagioclase depletion. Mineralogical zoning is often preserved in a single vein, older parts are more quartz‐rich than younger.

Mass balance calculations and whole‐rock geochemistry based on veins, mafic selvedges and country rock are consistent with a closedsystem subsolidus segregation origin. The layerparallel veins are syntectonic, and migration of the mobile components required to form their mineralogy is a stress‐induced mass transfer. The source of these components appears to be dominantly pressure solution of the same minerals in the host rocks, although metamorphic reactions may also have contributed. Veins nucleated first at those sites where initial sedimentary heterogeneites, such as fine‐scale graded bedding, provided gradients of normal stress across grain boundaries, and hence of chemical potential, necessary to drive the subsolidus segregation process. The earliest veins are thus parallel to bedding. Later, nucleation of the veins could also occur along more randomly distributed sites within the metasediments, and these veins grew parallel to the schistosity rather than bedding, if the two were distinct. Once formed, the veins themselves, which are more competent than the surrounding rock, provide the stress heterogeneity required for their further growth. The increasing plagioclase/quartz ratio in the veins may be due to a temperature dependent increase in plagioclase component mobility relative to quartz. Alternatively, the increasing transfer distances for silica, resulting from prior quartz depletion in the inner parts of the mafic selvedge, may increase the relative mobility of plagioclase component.
Contact aureoles as constraints on regional P‐T trajectories: an example from the Northern Alabama Piedmont, USA
Authors: GIBSON RICHARD G. , SPEER J. A.
Published: May 2007

Abstract
Contact aureoles of plutons emplaced into regionally metamorphosed terranes can provide indicators of physical conditions along a portion of regional metamorphic P‐T trajectories, thereby allowing reconstruction of more complete P‐T loops than would be otherwise possible. In the Northern Alabama Piedmont of the southern Appalachians, Wedowee Group metapelites preserve evidence for two regional metamorphic phases overprinted by contact metamorphism adjacent to the Blakes Ferry Trondhjemite. Textural evidence indicates that an early bt+st+grt assemblage was replaced by bt+chl+grt during the latter stages of regional metamorphism. Changes in AKFM topology, complex Fe‐Mg‐Ca garnet zoning, and the latestage appearance of epidote indicate that a sequence of continuous reactions (bt+st = grt+ ms followed by chl+ms+Ca‐grt+Ca‐pl=bt+ (Fe+Mg)‐grt+ep) occurred in response to increasing pressure and resulted in the observed changes in mineral assemblage. Pl‐ms‐bt‐grt thermobarometry indicates conditions of 580° 65°C, 8.5±0.8 kbar for equilibration of grt+ bt+chl. Pluton emplacement, subsequent to penetrative deformation, caused textural annealing and mineral re‐equilibration by the continuous reaction bt+(Fe+Mg)‐grt+ep = chl+ms+Ca‐grt+Ca‐pl within 50 m of the pluton. Conditions of 510±65°C, 5‐7 kbar are inferred. A reconstructed P‐T trajectory for this area is characterized by (1) early moderate‐T, moderate‐P metamorphism; (2) an increase in P to approximately 8.5 kbar; and (3) decompression and slight cooling prior to pluton emplacement. The compressional phase of this path is interpreted to result from underthrusting of the Wedowee metasediments to mid‐crustal levels during Palaeozoic crustal thickening. Late‐stage decompression prior to intrusion records uplift of these rocks in response to movement on structurally lower thrusts.
Metamorphism of high‐grade gneisses from the Moldanubian zone, Austria, with particular reference to the garnets
Authors: PETRAKAKIS K.
Published: May 2007

Abstract
Gneisses from the southern part of the Bunte Series have been studied in order to determine their metamorphic evolution. They all belong to the paragenetic facies type Bt+Sil+ Kfs+Qtz+Grt. The garnets of these rocks generally exhibit typical features of garnets from this high‐temperature facies type: diffusional homogenization throughout the bulk of the garnet and retrograde zoning patterns at the outer rim. Ca‐ and Mn‐richer relict cores in some garnets are evidence of the early metamorphism. These cores are enveloped by the high‐temperature homogenized garnet. The element distribution curves of the relict cores show evidence of partial element‐specific modification by volume diffusion.

Paragenetic and textural relations as well as temperature and pressure calculations suggest the following conditions of the main metamorphism: 700–770°C at 7‐9 kbar and PH2O Ptot

Despite the uncertainty due to the incomplete equilibration among garnet rims and other matrix minerals, an attempt is made to model the cooling and uplift history of the rocks using textural, paragenetic, chemical and literature data.

Gneise aus dem südlichen Teil der Bunten Serie wurden im Hinblick auf ihre Metamorphoseentwicklung untersucht. Sie gehören allgemein dem Faziestyp Bt+Sil+Kfs+Qtz+Grt an. Die Granate weisen alle für diesen Faziestyp charakteristischen Merkmale auf: weitgeheride Homogenisierung durch Diffusion und Abkühlungserscheinungen am äuβeren Rand. Ca‐ und Mn‐ reiche reliktische Kerne mancher Granate weisen auf ein älteres Ereignis hin. Diese Relikte werden im homogenisierten Granat höherer Bildungstem‐peratur eingeschlossen. Ihre Element‐ Verteilungskurven sind teilweise durch die intrakristalline Diffusion dem Kationenradius entsprechend abgeflacht.

Paragenetische Beziehungen und T‐ und P‐Berechnungen ergeben folgende Bedingungen für die Hauptmetamorphose: 700–770°C bei 7‐9 kbar und PH2OPtot.

Trotz der Unsicherheit, die auf eine unvoll‐ständige Einstellung des Gleichgewichtes zwischen Granaträndern und den übrigen Matrixmineralen zurückzuführen ist, wurde ver‐sucht, ein Modell über die Abkühlungs‐ und Hebungsgeschichte der untersuchten Gesteine zu erstelleu.
The Rayner Complex of East Antarctica: complex isotopic systematics within a Proterozoic mobile belt
Authors: BLACK L. P. , HARLEY S. L. , SUN S. S. , McCULLOCH M. T.
Published: May 2007

Abstract
New isotopic (Rb–Sr, U–Pb zircon and Sm–Nd) and petrological data are presented for part of an extensive Proterozoic mobile belt (locally known as the Rayner Complex) in East Antarctica. Much of the belt is the product of Mid‐Proterozoic (∼ 1800–2000 Ma) juvenile crustal formation. Melting of this crust at about 1500 Ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived. Deformation and transitional granulite‐amphibolite facies conditions (which peaked at 750 ± 50°C and 7–8 kbar (0.7–0.8 GPa) produced open to tight folding about E–W axes and syn‐tectonic granitoids about 960 Ma ago. Subsequent felsic magmatism occurred at about 770 Ma and not, as has been widely advocated, at 500–550 Ma, which appears to have been a time of widespread upper greenschist facies (400–500°C) metamorphism, localized shearing and faulting.

Sm‐Nd model ages of 1.65–2.18 Ga disprove a previously favoured hypothesis that the Rayner Complex mostly represents reworked Archaean rocks from the neighbouring craton (Napier Complex). Models that involve rehydration of the Napier Complex are no longer required, since the Rayner Complex was its own source of water. Two episodes of Proterozoic crustal growth are identified, the later of which occurred between about 1200 Ma and 1000 Ma, and was relatively minor. Sedimentation took place only shortly before Late Proterozoic orogenesis.

The multiphase history of the Rayner Complex has resulted in complex isotopic behaviour. Three temporally discrete episodes of Pb loss from zircon have been identified, the earliest two of which are responses to the c. 960 Ma and 540 Ma tectonothermal events. Fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth. Pb loss during the high‐grade event was probably governed by the same process or by lattice annealing. Some zircon suites also document recent Pb loss. Most lower concordia intercepts have no direct geological meaning and are explicable as mixed ages produced by incomplete Pb loss during two or more secondary events. Whereas all zircon separates from the orthogneisses produce U–Pb isotopic alignments, zircons from the only analysed paragneiss produce scattered data, in part reflecting a range of provenance. The 960 Ma event was also associated with the growth of a characteristically low U zircon (∼ 300 μg/g) in rocks of inferred high Zr content.

There is ubiquitous evidence for the resetting of Rb–Sr total‐rock isochrons. Even samples separated by up to 10 km fail to produce igneous crystallization ages. Minor mineralogical changes produced by the 540 Ma upper greenschist‐facies metamorphism were sufficient to almost completely reset some Rb–Sr isochrons and to produce open system conditions on outcrop scale, at least in one location.
Syntectonic porphyroblast growth in phyllites: textures and processes
Authors: PRIOR D. J.
Published: May 2007

Abstract
Porphyroblast textures in a Karakorum phyllite reveal that porphyroblast growth was syn‐tectonic with respect to a cleavage forming deformation. During and after porphyroblast growth it partitions the deformation such that zones of intensified cleavage are developed which wrap around the porphyroblast whilst the porphyroblast and its strain shadow undergo little deformation. Porphyroblast strain shadows comprise quartz, calcite and felspar with little mica, and are probably formed by solution transfer during deformation. Unless the deformation is so strongly partitioned that no deformation of the porphyroblasts and their immediate surrounds occurs, inequidimensional porphyroblasts will rotate. Porphyroblasts undergo some dissolution after they have finished growing.
Growth and concentration of fibrous sillimanite related to heterogeneous deformation in K‐feldspar‐sillimanite metapelites
Authors: VERNON R. H.
Published: May 2007

Abstract
In foliated K‐feldspar‐sillimanite metapelites, fibrous sillimanite is commonly concentrated in folia that anastomose between lenticular pods of coarser‐grained aggregates rich in quartz, feldspar and biotite, with or without garnet, cordierite and residual andalusite. Many of the folia appear to be limbs of crenulation microfolds. The sillimanite concentrations may be due largely to the ability of fibrous sillimanite aggregates to undergo strong non‐coaxial deformation by grain‐boundary sliding (‘fibre sliding’;) without appreciable build‐up of dislocations, whereas other minerals are unstable in these zones and so concentrate in lower‐strain interfolial zones. Initiation, and especially concentration of the sillimanite in folia, may be assisted by fluid flow and local base‐cation leaching, whereby minerals unstable in zones of strong non‐coaxial strain are dissolved and removed from these zones.
Late Proterozoic High‐pressure granulite facies meta‐morphism in the north‐east Ox inlier, north‐west Ireland
Authors: SANDERS I. S. , DALY J. S. , DAVIES G. R.
Published: May 2007

Abstract
High‐pressure granulite‐facies gneisses in the NE Ox inlier in NW Ireland have undergone extensive Caledonian retrogression. In the local area of Slishwood, however, reworking was negligible and the gneisses (psammites, semipelites, pelites, metabasites and ultramafites) preserve evidence of P–T changes at high grade which mainly post‐date pre‐Caledonian polyphase deformation. Temperatures reached 850–900°C (based on garnet‐clinopyroxene geothermometry and the presence of mesoperthite) during and after decompression from earlier eclogite‐facies conditions (inferred from textural evidence of plagioclase release in sieve‐textured augite). Subsequent cooling at high pressure is inferred from the unequivocal replacement of sillimanite by kyanite.

A Sm–Nd mineral isochron (gt–cpx–plag–WR) of 605 ± 37 Ma is taken to date a point on the cooling path, and confirms the hitherto suspected pre‐Caledonian age of the high‐grade metamorphism. Geochemical and Sm–Nd isotopic data indicate that the protoliths were probably late Proterozoic arkosic sediments and tholeiites. Following metamorphism they apparently came to reside near the base of the crust where they slowly cooled. The eventual exhumation of these gneisses is attributed to Caledonian crustal imbrication, followed by rapid isostatic recovery.
Models for the tectonothermal evolution of the eastern Dalradian of Scotland
Authors: BAKER A. J.
Published: May 2007

Abstract
The preserved array of pressures in the eastern Dalradian indicates that considerable syn‐ to post‐metamorphic differential uplift has occurred. This inferred differential uplift suggests that Buchan sillimanite zone rocks originally lay at higher structural levels than presently adjacent cooler kyanite zone rocks to the west. A number of features are believed to coincide with the western margin of the sillimanite zone. These are a maximum in temperature, sharp thermal features, a high strain zone, and a train of metabasites. These features are explained by invoking syn‐metamorphic movement between the Buchan sillimanite zone and the kyanite zone to its west, involving some horizontal component of movement. It is suggested that the lateral, now eroded, equivalents of the Buchan area once provided part of the required tectonic thickening for other parts of the Dalradian. Areas surrounding the Buchan area suffered tectonic burial followed by metamorphism during uplift relative to the Buchan area.
Metamorphism of carbonaceous material in the Tono contact aureole, Kitakami Mountains, Japan
Authors: OKUYAMA‐KUSUNOSE YASUKO , ITAYA TETSUMARU
Published: May 2007

Abstract
Optical and X‐ray studies of carbonaceous material in the Tono contact metamorphic aureole, Kitakami Mountains, northeast Japan, have revealed that metamorphic graphitization proceeded through two discontinuous changes: first, optically anisotropic domains develop within the coaly phytoclast, forming transitional material, and then, ordered graphite crystallizes by the decomposition of pre‐existing carbonaceous materials. Coaly material disappears in the uppermost chlorite zone. Transitional material appears in the middle of the lower chlorite zone. Graphite appears in the upper chlorite zone and its modal abundance increases across the andalusite iso‐grad to the cordierite isograd where all the carbonaceous materials have converted to graphite. The apparently continuous variation in the crystallographic parameters of the bulk carbonaceous material during graphitization is largely due to variation in the modal proportions of three types of carbonaceous materials. The temperature of graphitization in the present area is at least 100°C higher than the temperature in the Sanbagawa and New Caledonia high‐pressure metamorphic terrains, probably due to the slow reaction rate of metamorphic graphitization and to the short duration of contact metamorphism.
The effect of ductile deformation on the kinetics and mechanisms of the aragonite‐calcite transformation
Authors: SNOW ELEANOUR , YUND RICHARD A.
Published: May 2007

Abstract
The effect of ductile deformation (dislocation creep) on the kinetics of the aragonite‐calcite transformation has been studied at 1 atm (330° C and 360° C) and 900‐1500 MPa (500° C) using undeformed and either previously or simultaneously deformed samples (500° C and a strain rate of 10‐6 s). Deformation enhances the rate of the transformation of calcite to aragonite, but decreases the rate of transformation of aragonite to calcite. The difference results from a dependence of transformation rate on grain size, coupled with a difference in the accommodation mechanisms, climb versus recry‐stallization, of these minerals during dislocation creep. Dislocation climb is relatively easy in calcite and thus plastic strain results in high dislocation densities without significant grain size reduction. The rate of transformation to aragonite is enhanced primarily because of the increase in nucleation sites at dislocations and subgrain boundaries. In aragonite, on the other hand, dislocation climb is difficult and thus plastic strain produces extensive dynamic recry‐stallization resulting in a substantial grain size reduction. The transformation of aragonite is inhibited because the increase in calcite nucleation sites at dislocations and/or new grain boundaries is more than offset by the inability of calcite to grow across high angle grain boundaries. Thus the net effect of ductile deformation by dislocation creep on the kinetics of polymorphic phase transformations depends on the details of the accommodation mechanism.
The influence of deformation partitioning on dissolution and solution transfer in low‐grade tectonic mélange
Authors: HAMMOND R. L.
Published: May 2007

Abstract
Dissolution of quartz and the recryst‐allization and re‐equilibration of phyllosilicates produce the dark anastomosing seams that dominate microstructures of tectonic melange which occur in a low‐grade, imbricated and multiply deformed, mid‐Palaeozoic, intracra‐tonic fiysch sequence in northeastern Australia. Seams are composed of very closely spaced or coalesced cleavage lamellae, which are very thin layers of extremely fine‐grained phyllosilicates. Cleavage seams enclose lenses of silt‐stone or greywacke, which formerly occurred in continuous sedimentary layers, indicating extremely heterogeneous and disruptive deformation. Microphacoids enclosed by cleavage seams have subtle shape asymmetries analogous to those of porphyroclasts. Phyllosilicate‐preferred orientations within microphacoids commonly lie at a low angle to enclosing seams, and asymmetric relationships occur within seams. The shape and fabric asymmetries appear to be constant, and are regarded as analgous to S and C planes. The number and extent of seams, the amount of dissolution they indicate, and the efficiency of deformation partitioning imply some enhancement of chemical activity and substantial silica loss from the system. This, in turn, suggests the passage of large amount of silica‐undersaturated fluid, and melanges may be zones of high fluid flow. However, the microstructures and the disruptive nature of the fabrics may also reflect the influence of high bulk shear strains and suggest some relationship between the shearing component of deformation and the development of cleavages and foliations.
Evidence for a Variscan suture zone in the Vendée, France: a petrological study of blueschist facies rocks from Bois de Cené
Authors: GUIRAUD M. , BURG J.‐P. , POWELL R.
Published: May 2007

Abstract
In the Bois de Cené area, blueschist facies rocks, characterized by glaucophane and/ or chloritoid, provide evidence for a suture zone in the Variscan. This terrain is considered to be the eastern equivalent of the Ile de Groix high‐pressure metamorphic terrain. Petrological study of the two characteristic types of rocks found in the area shows that the primary high‐pressure paragenesis was modified during a retrogression which followed substantial decompression, probably at constant or decreasing temperature. The simplest interpretation is that this retrogression followed tectonic emplacement within a nappe pile.
Chloritoid‐bearing rocks associated with blueschists and eclogites, northern New Caledonia
Authors: GHENT EDWARD D. , STOUT MAVIS Z. , BLACK P. M. , BROTHERS R. N.
Published: May 2007

Abstract
Chloritoid‐bearing metasedimentary rocks occur in close proximity to blueschists and eclogites in the Tertiary high‐pressure metamorphic belt of northern New Caledonia. The typical assemblage of chloritoid‐bearing rocks in the epidote zone is quartzchlorite‐muscovite‐garnet‐chloritoid. In the omphacite zone, epidote is an additional member of the chloritoid‐bearing assemblage. Paragonite is rare, plagioclase was not detected, and rutile and ilmenite are the Fe‐Ti oxide phases. Chloritoid‐glaucophane is not a common assemblage. Chloritoid‐bearing rocks have relatively low (Ca+K+Na)/Al ratios and the chloritoids are relatively Mg‐rich with Mg/ (Mg+Fe) up to about 0.4. A comparison of the mineral assemblages and mineral chemistry with experimental and computed phase equilibria suggest an upper temperature limit near 560° C in the omphacite zone and a minimum temperature limit near 450° C at 10 kbar. An empirical garnet‐chlorite Fe‐Mg exchange thermometer does not yield consistent results for the higher‐grade rocks, suggesting Ts ranging from 390 to 535° C in the omphacite zone and 420–465° C in the epidote zone. The distribution coefficient KD= (Fe/Mg)ctd/(Fe/Mg)chl for chloritoid and chlorite ranges from 3.9 to 6.4, values which are lower than those (=10) from lower greenschist facies rocks, but are near those of upper greenschist facies and albite‐epidote amphibolite facies.
P‐Tgrids for silica‐undersaturated granulites in the systems MAS (n+ 4) and FMAS (n+ 3)‐tools for the derivation ofP‐Tpaths of metamorphism
Authors: HENSEN B. J.
Published: May 2007

AbstractConsidering the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO‐MgO‐Al2O3‐SiO2(FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa]. Constraints imposed by experimental data for the system MAS indicate that under lowPH2o conditions the invariant points occur at high temperature (> 900° C) and intermediate pressure (7‐10 kbar). This temperature is higher than that commonly advocated for granulite facies metamorphism. In granulites Fe‐Mg exchange geothermometers may yield temperatures of 100–150° C below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high‐temperature assemblages and by Al‐rich cores in orthopyroxene. Application of the FMAS grid to some well‐documented granulite occurrences introduces important constraints on theirP‐Thistories. Rocks of different bulk compositions, occurring in close proximity in the field, may record distinct segments of theirP‐Tpaths. This applies particularly to rocks with evidence for reaction in the form of coronas, symplectites and zoned minerals. Consideration of curved reaction boundaries andXMsisopleths may explain apparently contradictory results for the stability of cordierite obtained from low‐temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other.
Metamorphism in the Olary Block, South Australia: compression with cooling in a Proterozoic fold belt
Authors: CLARKE G.L. , GUIRAUD M. , POWELL R. , BURG J.P.
Published: May 2007

Abstract
The sedimentary and igneous rocks comprising the lower Proterozoic Olary Block, South Australia, were deformed and metamorphosed during the mid‐Proterozoic ‘Olarian’Orogeny. The area is divided into three zones on the basis of assemblages in metapelitic rocks, higher grade conditions occurring in the south‐east. Mineral assemblages developed during peak metamorphism, which accompanied recumbent folding, include andalusite in Zones I and II and sillimanite in Zone III. Upright folding and overprinting of mineral assemblages occurred during further compression, the new mineral assemblages including kyanite in Zone II and kyanite and sillimanite in Zone III. The timing relationships of the aluminosilicate polymorphs, together with the peak metamorphic and overprinting parageneses, imply an anticlockwise P–T path for the ‘Olarian’Orogeny, pressure increasing with cooling from the metamorphic peak.
Sapphirine parageneses from the Caraiba complex, Bahia, Brazil: the influence of Fe2+–Fe3+distribution on the stability of sapphirine in natural assemblages
Authors: ACKERMAND D. , HERD R.K. , REINHARDT M. , WINDLEY B.F.
Published: May 2007

AbstractSapphirine‐bearing rocks occur in three conformable, metre‐size lenses in intrusive quartzo‐feldspathic orthogneisses in the Curaçà valley of the Archaean Caraiba complex of Brazil. In the lenses there are six different sapphirine‐bearing rock types, which have the following phases (each containing phlogopite in addition):

A: Sapphirine, orthopyroxene;

B: Sapphirine, cordierite, orthopyroxene, spinel;

C: Sapphirine, cordierite;

D: Sapphirine, cordierite, orthopyroxene, quartz;

E: Sapphirine, cordierite, orthopyroxene, sillimanite, quartz;

F: Sapphirine, cordierite, K‐feldspar, quartz.

Neither sapphirine and quartz nor orthopyroxene and sillimanite have been found in contact, however. During mylonitization, introduction of silica into the three quartz‐free rocks (which represent relict protolith material) gave rise to the three cordierite and quartz‐bearing rocks. Stable parageneses in the more magnesian rocks were sapphirine–orthopyroxene and sapphirine–cordierite. In more iron‐rich rocks, sapphirine–cordierite, sapphirine‐cordierite–sillimanite, cordierite–sillimanite, sapphirine–cordierite–spinel–magnetite and quartz–cordierite–orthopyroxene were stable. The iron oxide content in sapphirine of the six rocks increases from an average of 2.0 to 10.5 wt % (total Fe as FeO) in the order: C,F–A,D–B,E. With increase in Fe there is an increase in recalculated Fe2O3in sapphirine.

The four rock types associated with the sapphirine‐bearing lenses are:

I: Orthopyroxene, cordierite, biotite, quartz, feldspar tonalitic to grandioritic gneiss;

II: Biotite, quartz, feldspar gneiss;

III: Orthopyroxene, clinopyroxene, hornblende, plagioclase meta‐norite;

IV: Biotite, orthopyroxene, quartz, feldspar, garnet, cordierite, sillimanite granulite gneiss.

The stable parageneses in type IV are orthopyroxene–cordierite–quartz, garnet–sillimanite–quartz and garnet–cordierite–sillimanite.

Geothermobarometry suggests that the associated host rocks equilibrated at 720–750°C and 5.5–6.5 kbar. Petrogenetic grids for the FMASH and FMAFSH (FeO–MgO–Al2O3–Fe2O3–SiO2–H2O) model systems indicate that sapphirine‐bearing assemblages without garnet were stabilized by a high Fe3+content and a highXMg= (Mg/ (Mg+Fe2+)) under theseP–Tconditions.
A pyroxene‐bearing meta‐ironstone and other pyroxene‐granulites from Tonagh Island, Enderby Land, Antarctica: further evidence for very high temperature (> 980°C) Archaean regional metamorphism in the Napier Complex
Authors: HARLEY SIMON L.
Published: May 2007

Abstract
A suite of granulites including a meta‐ironstone, pyroxenites, and spinel‐lherzolites from East Tonagh Island, Enderby Land, Antarctica, preserve exsolution‐recry‐stallization features consistent with a shared metamorphic evolution that involves marked cooling from initial metamorphic temperatures of nearly 1000°C. Reintegrated pre‐exsolution and pre‐reaction grain compositions in the meta‐ironstone indicate the former coexistence of metamorphic pigeonite (Wo12En38Fs50) and ferroaugite (Wo35En31Fs34) at temperatures in excess of 980°C for pressures of 7 kbar (0.7 GPa) using pyroxene quadrilateral thermometry (Lindsley, 1983). Intra‐grain lamellae relationships indicate the exsolution of a second pigeonite (Wo12En35Fs53) from the ferroaugite at temperatures in the range 930–970°C, prior to the c. 720–600°C exsolution of orthopyroxene and clinopyroxene (100) lamellae and later partial recrystallization at similar temperatures. Although pyroxenitic and iherzolitic granulites preserve a much less complete history, reintegrated porphyroclast compositions in these yield temperature estimates which approach those inferred from the metaironstone. Pyroxene thermometry based on neoblast compositions suggests that recrystallization post‐dating a late, low intensity, deformation phase (D3) occurred at temperatures greater than 600°C. These results are consistent with the independent evidence obtained from studies of metapelitic and felsic rock types for very high temperature metamorphism throughout the Napier Complex followed by near‐isobaric cooling and later deformation under lower‐grade granulite facies conditions. Comparison with similar pyroxene data from Fyfe Hills (Sandiford & Powell, 1986) demonstrates further the regional significance of these high temperatures, and implies broadly isothermal metamorphic conditions over a large area (∼ 5000 km2) and thickness (6–9 km) of lower crust at c. 3070 Ma.
Corona textures between kyanite, garnet and gedrite in gneisses from Errabiddy, Western Australia
Authors: BAKER JUDY , POWELL ROGER , SANDIFORD MICHAEL , MUHLING JAN
Published: May 2007

Abstract
Corona textures, which developed in alternating layers in rocks in a supracrustal belt at Errabiddy, Western Australia, involved:

(a) The production of staurolite, cordierite and quartz or sapphirine between Kyanite and/or sillimanite and gedrite; and

(b) The production of cordierite between garnet and gedrite.

These textures are inconsistent with development along the same pressure–temperature path in the system FeO–MgO–Al2O3–SiO2–H2O, but can be accounted for if CaO, mainly in garnet, is taken into account. The sapphirine‐bearing kyanite–gedrite textures are explained by lower a(SiO2) during their development. The assemblages indicate a consistent pressure–temperature (P–T) trajectory involving substantial uplift with only a slight decrease in temperature. The history of these rocks includes reheating of originally high‐grade rocks that had cooled to a stable conductive geotherm, followed by substantial, essentially isothermal uplift. The tectonic environment for this was presumably the one responsible for emplacement of the high‐grade terrain in the upper crust.
An example of high‐pressure low‐temperature metamorphic rocks from an island‐arc: the Paikon Series (Innermost Hellenides, Greece)
Authors: BAROZ F. , BEBIEN J. , IKENNE M.
Published: May 2007

Abstract
The Paikon Series is considered to be a volcanic arc sequence with a mainly neritic sedimentary sequence and bimodal tholeiitic volcanism of early Mesozoic age. The metamorphic assemblages are syn‐ to post‐kinematic with respect to a pre‐Tithonian tectonic phase and range from the lawsonite‐chlorite‐albite facies through transitional Na‐amphibole‐greenschist facies to the chlorite sub‐zone of the greenschist facies. The metamorphic imprint of the Paikon Series corresponds to a temperature range from less than 330° C to ± 450° C under a total pressure from 3 kbar to 6–7 kbar. The overprinting of these facies on an earlier blueschist assemblage, related either to a subduction zone or to a tectonic overpressure caused by thrusting, is suspected.
Clinopyroxene–a mineral telescoped through the processes of blueschist facies metamorphism
Authors: MARUYAMA S. , LIOU J.G.
Published: May 2007

Abstract
Textural evolution and compositional variation of clinopyroxenes in Ward Creek metabasites are described. Pyroxenes change, with increasing grade, from finegrained aggregates through fan‐shaped medium‐grained prisms to blocky coarse crystals. Characteristic features of metamorphic pyroxenes include: (1) the occurrence of coexisting pyroxene pairs, the compositions of which are used to delineate compositional gaps; (2) the existence of large compositional variations of pyroxenes, within a single specimen, which record a considerable span of P and/or T for crystallization; and, (3) the development of compositional trends in single specimens and in three metamorphic zones which are progressive in nature.

The first formed clinopyroxene (Jd20Aug65Ac15) in the lower lawsonite zone mimics the composition of relict igneous augite. It changes continuously, with increasing grade, at nearly constant low XJd content towards acmite. At a composition around Jd20Aug30Ac50, the trend turns towards jadeite and intersects a solvus to form two coexisting clinopyroxenes in the middle lawsonite zone. At higher grade, the compositional gap becomes restricted towards the jadeite‐omphacite join and clinopyroxene increases in XJd toward jadeite. A reversed compositional trend occurs at higher grade; clinopyroxenes decrease in jadeite component at nearly constant Aug/Ac ratio of 50/50 and finally become omphacite in the uppermost pumpellyite and epidote zones. The Na–Ca pyroxenes, close to the binary join Jd–Ac, occur in the lawsonite‐ and pumpellyite‐zones, ranging from XJd= 1.0–0.30 together with Ab and Qz. The ubiquitous occurrence of aragonite at temperature estimates of 170–240° C by Taylor & Coleman (1968) for these zones does not support the low‐temperature extrapolation of the Jd–Ab–Qz curve by Holland (1980).

The estimated metamorphic field gradient indicates an inflection point at 7 kbar, 200° C. Below this, blueschist facies metamorphism proceeded under dominant pressure‐increase from 4 to 7 kbar at nearly constant temperature, about 150–200° C, whereas at higher grade recrystallization, above the inflection point, the metamorphic temperature increased from 200 to 350° C at nearly constant pressure, about 7–8 kbar. Such an inflection point suggests the depth of underplating of either seamounts or accretionary packages in a subduction zone.
Shallow‐level metamorphic fluids in a high uplift rat metamorphic belt; Alpine Schist, New Zealand
Authors: CRAW D.
Published: May 2007

Abstract
Fluids, some of which are CO2‐rich (up to 40 mol.% CO2) and some of which are highly saline (up to 18 wt% NaCl equivalent), are trapped as fluid inclusions in quartz‐calcite (∼ metallic minerals) veins which cross‐cut the pumpellyite‐actinolite to amphibolite facies rocks of the Alpine Schist. Fluids were commonly trapped as immiscible liquid‐vapour mixes in quartz and calcite showing open‐space growth textures. Fluid entrapment occurred at fluid pressures near 500 bars (possibly as low as 150 bars) at temperatures ranging from 260 to 330° C. Saline fluids may have formed by partitioning of dissolved salts into an aqueous phase on segregation of immiscible fluids from a low‐density CO2‐rich fluid. Calcite deposited by these fluids has δ13C ranging from – 8.4 to – 11.5 and δ18O from + 4 to + 13. Isotopic data, fluid compositions and mode of occurrence suggest that the fluids are derived from high‐grade metamorphic rocks. Fluid interaction with wall‐rock has caused biotite crystallization and/or recrystallization in some rocks and retrogression of biotite to chlorite in other rocks.

Fluid penetration through the rock is almost pervasive in many areas where permeability, probably related to Alpine Fault activity, has focussed fluids on a regional scale into fractured rocks. The fluid flow process is made possible by high uplift‐rates (in excess of 10 mm/year) bringing hot rocks near to the surface.
Compositional controls on coexisting prehnite‐actinolite and prehnite‐pumpellyite facies assemblages in the Tal y Fan metabasite intrusion, North Wales: implications for Caledonian metamorphic field gradients
Authors: BEVINS R.E. , MERRIMAN R.J.
Published: May 2007

Abstract
The Tal y Fan Intrusion is a 110 m thick sub‐concordant metabasite sheet intruded into volcaniclastic and pyroclastic rocks of Ordovician age in North Wales. Despite low grade metamorphism, primary textural zones resulting from initial cooling of the sheet are preserved and retain primary mineralogical and chemical variations which influenced the nature and extent of metamorphic recrystallization. This has resulted in a vertical sequence of secondary mineral assemblages through the intrusion. During early hydrothermal alteration K‐feldspar replaced plagioclase micropheno‐crysts in the marginal and contact zones, and olivine in the central zone was replaced by saponite. Subsequent regional metamorphism resulted in the development of (metastable) prehnite‐pumpellyite‐epidote assemblages in two sub‐zones characterized by high Fe2O3. Elsewhere the assemblage prehnite‐actinolite‐epidote developed except in the contact and marginal zones where activity of CO2 suppressed both prehnite and pumpellyite. Both assemblages contain excess albite, quartz and chlorite and, on the basis of uniform mineral compositions over the area of an individual thin section, are considered to represent buffered equilibrium assemblages indicative of prehnite‐pumpellyite and prehnite‐actinolite facies conditions. A metamorphic temperature of 310° C at 1.85 kbar is obtained using the P‐T‐X grid of Liou, Maruyama & Cho (1985), which implies a field gradient of ∼ 44° C km‐1. Assuming that metamorphism relates to burial, an overburden thickness of ∼ 7 km is indicated. Total maximum thicknesses, however, of Ordovician, Silurian and Lower Devonian strata, in the area, do not exceed 6 km indicating a field gradient of 52° C km‐1. These relatively high gradients may possibly be related to concealed late Caledonian intrusions, or alternatively may result from high heat flow as a consequence of crustal thinning, rapid sedimentation and intense magmatic activity in a marginal basin setting.
Crystallization of melts, pegmatite intrusion and the Inverian retrogression of the Scourian complex, north‐west Scotland
Authors: CARTWRIGHT I.
Published: May 2007

Abstract
Partial melting of tonalitic gneisses in the 2.7 Ga Badcallian granulite facies metamorphic episode in the Scourian complex of north‐west Scotland produced a suite of granitic to trondhjemitic liquids. On cooling and excavation of the complex, these melts underwent fractional crystallization and the residual liquids eventually became water saturated. Comparison with experimental data suggests that water saturation would have occurred in these melts at around 620–700°C. From the retrograde P–T‐time path followed by the complex it is estimated that H2O‐dominated fluids were exsolved from these melts at c. 2.5 Ga. It is proposed that these fluids were the cause of the 2.5 Ga Inverian retrogression of the Scourian complex and that water‐saturated melts formed during the crystallization of the leucogneisses were intruded as a suite of pegmatites. The timing of pegmatite intrusion is consistent with this proposition as are the temperature estimates, timing, distribution and nature of the Inverian phase of metamorphism. It is likely that the crystallization of melts is an important process in bringing about hydrous retrogressive metamorphic episodes in a number of other basement terrains, such as West Greenland and Australia.
High‐grade metamorphic processes which influence Archaean gold deposits, with particular reference to Big Bell, Australia
Authors: PHILLIPS G. NEIL , NOOY DORRIT DE
Published: May 2007

Abstract
Mineralogical and geochemical evidence indicates that partial melting and desulphidation have occurred in the Big Bell gold deposit. Decarbonation may also have occurred, to account for the lack of a carbonate alteration halo; this is compatible with the present data, but difficult to test.

The Big Bell deposit consists of auriferous sulphide‐bearing (‘lode’;) schists with muscovite and K‐feldspar, and surrounding biotite schists, all derived by intense premetamorphic alteration of rocks of mafic composition. Assemblages which include cordierite‐sillimanite‐K‐feldspar‐garnet‐biotite‐quartz suggest peak metamorphic conditions of 4–5 kbar, and 650–700° C, based on phase relations, geobaro‐meters and garnet‐biotite Fe‐Mg exchange partitioning. Partial melting occurred at peak metamorphism, particularly in the altered mafic rocks in and around the deposit, and its occurrence may have been essential to the preservation of the deposit. Melting greatly limited the importance of devolatilization reactions, resulting in negligible aqueous fluids and no means of removing appreciable gold. Minor gold loss may have accompanied desulphidation. A diversity of complex metamorphic assemblages occurs around the mine, compared to the assemblages developed regionally; variable bulk rock composition influences this contrast, but there is no evidence of higher metamorphic grades at the mine, nor that this might have been the prime control on the different assemblages in this narrow belt.

It is suggested that the Big Bell and Hemlo deposits are the higher metamorphic grade equivalents of the more abundant greenschist facies gold deposits within Archaean greenstone belts. This interpretation is favoured by the host rock setting and geochemical characteristics of Big Bell. Alternative models that suggest that this class of deposit is a new type must account for the absence of high‐grade equivalents of the greenschist facies deposits and also the lack of low‐grade equivalents of the Big Bell/Hemlo type.

Archaean gold deposits in high‐grade metamorphic terrains have undergone a series of processes that are not recorded in the more typical gold deposits of the greenschist facies.
A polycyclic two‐stage corona growth in the Iforas Granulitic Unit (Mali)
Authors: BOULLIER A.‐M. , BARBEY P.
Published: May 2007

Abstract
Retrograde and prograde mineral assemblages from metapelitic and metabasic rocks of the Iforas Granulitic Unit (Mali) were generated by the superimposition of two granulite facies metamorphic events. They clearly result from a polycyclic evolution and can be related to a late Eburnean unroofing followed by a Pan‐African burial.

Thermobarometry on Pan‐African garnet‐bearing assemblages yields (P, T) estimates of 620±50°C and 5± Ikbar. The nearly anhydrous conditions produced in the Eburnean appear to be the direct cause of the unusually lowtemperature granulite‐facies metamorphism in the Pan‐African. These P, T estimates are compared with those obtained on the underlying unit (Kidal Assemblage) upon which the Iforas Granulitic Unit was thrust. A P‐T‐t path, during the Pan‐African orogeny, is proposed and discussed for both the Iforas Granulites and Kidal Assemblage.