bluuebunny/crossref_metadata_2025_split huggingface dataset:
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.
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 PtotDespite 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.ZusammenfassungGneise 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.
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.
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.
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.
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.
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.
Empirical garnet–muscovite geothermometry in low‐grade metapelites, Selwyn Range (Canadian Rockies)
Authors: HYNES A. , FOREST R.C. 
Published: May 2007

Abstract
Partitioning of Fe and Mg between garnet and phengitic muscovite was calibrated as a geothermometer by Green & Hellman (1982) using experimental data at 25–30 kbar. When the thermometer is applied to pelites regionally metamorphosed at pressures of between 3 and 7 kbar it yields temperatures much higher than those from the garnet–biotite thermometer. A new empirical calibration is proposed for use with such rocks, with particular application where garnet occurs at lower grades than biotite. The new calibration is where K is given by: In K= In Kd and Xii are mole fractions in the garnets.The calibration was derived from comparison with the garnet–biotite thermometer of Ferry & Spear (1978), assuming no pressure‐dependence for the partitioning between garnet and muscovite, no ferric iron partitioning, ideal mixing in muscovite, and the garnet mixing model of Ganguly & Saxena (1984) modified for a non‐linear Ca effect. This latter garnet mixing model was selected because it gave the geologically most reasonable results. It has not proved possible to distinguish a pressure effect from a ferric‐iron effect.Despite the simplifying assumptions used to derive the calibration, it yields temperatures generally within 15°C of those given by the garnet–biotite thermometer, and has been used to supply thermometric data in a low‐grade region of the Canadian Rockies.
Coexisting amphiboles in an eclogite from the Western Alps: new constraints on the miscibility gap between sodic and calcic amphiboles
Authors: REYNARD B. , BALLÈVRE M. 
Published: May 2007

Abstract
Crystal‐chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the Aosta Valley, Western Alps. Textural analysis gives evidence of three successive high‐pressure parageneses:1. Pre‐kinematic high‐grade blueschist assemblages, preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite (stage A).2. Synkinematic eclogite assemblages, composed of garnet + omphacite + glaucophane ± actinolite ± white mica ° Clinozoisite + quartz + rutile (stage B).3. Post‐kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite (stage C). P–T conditions of the eclogitic metamorphism have been estimated at around 500–550°C, 16 kbar.Glaucophane and actinolite coexist as discrete grains in stage A and B assemblages. This texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles (from NaM4= 0.80 in actinolite to NaM4= 1.70 in glaucophane at T= 500–550°C). A comparison with published analyses allows a new solvus along the glaucophane–actinolite join to be drawn.The later barroisitic amphibole (stage C) exhibits strong chemical zonation indicating disequilibrium growth. This amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end‐members.