bluuebunny/crossref_metadata_2025_split huggingface dataset:
Endogreisen, brecciation and fluid activity at the Mount Bischoff Sn‐deposit, north‐west Tasmania, Australia
Authors: WRIGHT J.H. , KWAK T.A.P. 
Published: May 2007

Abstract
Endogreisens which replace K‐feld‐spar‐quartz dykes in a Devonian (360 Ma) tin deposit at Mt Bischoff, north‐west Tasmania, formed from the interaction of unusual solutions, probably derived from an underlying leucogranite pluton, porphyry dykes and limited quantities of local dolomitic country rock components. The intensity of greisenization and pH of the solutions increase inward to the greisenized dykes’cores and downward. The following types of greisen assemblages indicate increasing degrees of greisenization: ‘sericite’muscovite + quartz ± tourmaline ± fluorite, topaz + quartz ± tourmaline ± fluorite, weberite, prosopite, ralstonite, Ca‐ralstonite; and quartz ± topaz ± fluorite. Where the solutions interacted with dolomite, exogreisens consisting of topaz‐ or tourmaline‐bearing assemblages were formed. The greisens were subsequently overprinted to varying degrees by siderite, sulphides and hydrous silicates (talc, serpentine, chlorite, micas).The temperature during greisenization ranged from 180 to 414°C, based on fluid inclusions in topaz, quartz, fluorite, sellaite and cassiterite. The main greisen‐forming event occurred at temperatures of 360±20°C. The fluids boiled intermittently. Their salinities ranged from 31.5 to 38.9 wt% total dissolved salts, consisting of Ca–K–Na–Fe–Cl±hydrocarbon species. Fluid inclusion data indicate that only 0.5–1.5 km of cover were present above this deposit at the time of formation.The greisenized dykes were intruded by and intrude different stages of breccias. The breccias consist mainly of country rock and greisenized dyke fragments, with rock‐flour and later tourmaline alteration. The Mt Bischoff greisen system is possibly part of a ‘porphyry tin’style deposit formed at near‐surface conditions (0.5–1.0 km).
Synthetic fluid inclusions ‐ VI. Quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure
Authors: BODNAR R. J. , BINNS P. R. , HALL D. L. 
Published: May 2007

ABSTRACT The decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure H2O synthetic inclusions formed by healing fractures in natural quartz. Three different modes of non‐elastic deformation, referred to as stretching, leakage or partial decrepitation, and total decrepitation have been observed. The internal pressure required to initiate non‐elastic deformation is inversely related to inclusion size according to the equation:internal pressure (kbar) = 4.26 D‐0.423where D is the inclusion diameter in microns. Regularly shaped inclusions require a higher internal pressure to initiate non‐elastic deformation than do irregularly shaped inclusions of similar size. Heating inclusions through the α/β quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion.Long‐term heating experiments (∼2 years) suggest that the internal pressure required to initiate non‐elastic deformation does not decrease significantly with time and indicates that short‐lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature. Inclusions that do exhibit decreased density (higher homogenization temperature) are, however, always accompanied by a change in shape from irregular to that of a negative crystal.Observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals. These results indicate that an inclusion will not be influenced by a neighbouring inclusion, or other defect in the host phase, as long as the distance between the two is >2–4 diameters of the larger of the two inclusions.
Evidence for deformation‐induced K‐feldspar replacement by myrmekite
Authors: SIMPSON C. , WINTSCH R. P. 
Published: May 2007

ABSTRACT Several examples of deformation‐induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths, namely a muscovite‐poor S‐C mylonite and a single foliation, muscovite‐poor mylonitic gneiss. Back‐scattered SEM and conventional optical microscopy show that in both rock types, syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of K‐feldspar grains that faced the local inferred incremental shortening direction for the mylonite. Myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction.The replacement of K‐feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains. We suggest that the ambient temperature, pressure and chemical activities were such that the replacement reaction was favoured, but the addition of extra strain energy along the high‐pressure sides of the grains localized the reaction at these sites. This energy could arise from elastic strain, or strain associated with tangled dislocations or twin boundaries. The relative roles of stress and strain energy concentrations in driving the replacement reaction are not known, but both were probably important.
Low‐pressure granulite facies metapelitic assemblages and corona textures from MacRobertson Land, east Antarctica: the importance of Fe2O3 and TiO2 in accounting for spinel‐bearing assemblages
Authors: CLARKE G. L. , POWELL R. , GUIRAUD M. 
Published: May 2007

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

ABSTRACTSequential reaction textures in Archaean garnet‐corundum‐sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse‐grained, high‐pressure (P> 9.5 kbar) granulite‐facies assemblages (M1) to late, low‐pressure (P<6 kbar) granulite‐facies sub‐assemblages (M2).The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late‐M1 boron‐free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine‐grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel).The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time‐sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)‐discontinuous reactions in the FMASH multisystem sapphirine‐garnet‐corundum‐spinel‐cordierite‐gedrite‐kornerupine, the facies‐sequence indicates decompression at essentially constantTassuming constanta(H2O).Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the oppositedPfrom that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction ofP‐Tvectors.Garnet‐biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures ofca.850°C for the M1 stage. A minimum late‐M1 pressure ofca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early‐M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high‐pressure granulite‐facies metamorphism was followed by an almost isothermal pressure decrease of > 5 kbar, indicative of rapid uplift. TheP‐Tpath is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite‐facies terranes.
Low‐pressure granulite facies metamorphism in the Larsemann Hills area, East Antarctica; petrology and tectonic implications for the evolution of the Prydz Bay area
Authors: STÜWE K. , POWELL R. 
Published: May 2007

ABSTRACT Thermobarometric studies on various granulite facies areas along the Prydz Bay coast, East Antarctica (73°‐79°E, 68°‐70°S), show that, at around 1100 Ma, during a late Proterozoic orogeny, the rocks of the Larsemann Hills suffered a lower pressure metamorphic peak than the surrounding areas. Along the Prydz Bay coast, the rocks affected by this event include parts of the Vestfold Hills block plus all of the Rauer Group, the Larsemann Hills and the Munro Kerr Mountains. The dykes in the south‐west corner of the Vestfold Hills were recrystallized during this event with little deformation at temperatures not quite as high as in the areas further south‐west (650°C, 6.5 kbar) (Collerson et al., 1983), the Rauer Group was metamorphosed at 800°C and 7.5 kbar (Harley, 1987a), the Larsemann Hills at 750°C and 4.5 kbar, and the Munro Kerr Mountains probably at around 850°C and 5 kbar. Retrograde equilibration in the different areas occurred during decompression to about 10 km depth in all areas, followed by isobaric cooling at this depth.This paper shows that the peak metamorphism in the Larsemann Hills occurred at a pressure which is too low to have been the consequence of thermal relaxation of overthickened crust with normal mantle heat flow. Although other areas in Prydz Bay were metamorphosed at sufficiently high pressures so that their decompression paths are not inconsistent with a continental collision model, the inferred pre‐metamorphic peak histories and the requirement of consistency with the Larsemann Hills, make it unlikely that collision followed by erosion‐driven decompression is an appropriate model. We suggest that the thermal regime of the crust in the Larsemann Hills region was controlled by a perturbation in the asthenosphere, with magma invasion of the crust. We suggest that the 500 Ma event, represented in Prydz Bay by granitic outcrops at Landing Bluff and by several K/Ar ages from the Larsemann Hills area, was responsible for the final excavation of the terrane.
Metamorphism of an ophiolitic tectonic mélange, northern California Klamath Mountains, USA
Authors: DONATO M. M. 
Published: May 2007

ABSTRACT Mineral assemblages in pelitic, mafic, calcareous and ultramafic rocks within a metamorphosed tectonic mélange indicate that the Marble Mountain terrane and adjacent Western Hayfork subterrane (northern California) underwent regional low‐ to medium‐pressure amphibolite facies metamorphism. Metamorphic conditions estimated by comparison of observed assemblages with experimentally‐determined reaction boundaries and by geothermometry constrain metamorphic temperatures between about 500° and 570°C. The occurrence of andalusite in regionally metamorphosed pelites indicates pressures below about 370 MPa. Metabasite amphibole compositions also suggest low to intermediate metamorphic pressures.Metaserpentinites containing the upper amphibolite facies assemblage (olivine + enstatite + anthophyllite) are found locally within the study area and have been reported previously by other workers elsewhere in the Marble Mountain terrane. These assemblages may reflect higher temperatures of recrystallization than assemblages in surrounding rocks and may represent vestiges of an earlier high‐temperature metamorphic event undergone by the ultramafic rocks prior to incorporation in the mélange.Although the age of the low‐ to intermediate‐pressure metamorphism is poorly constrained, cross‐cutting plutons indicate that metamorphism must be older than about 162 Ma. Therefore this regional metamorphic event, which probably marks the accretion of these terranes to the North American continental margin, is older than the currently accepted 151–147 Ma age of the Nevadan event in the Klamath Mountains. The inferred low to intermediate pressures of metamorphism and the lithologies of the protoliths suggest a near‐arc tectonic setting and refute a subduction zone model for this event.
Amphibolites from Panama: anticlockwise P‐T paths from a Pre‐upper Cretaceous metamorphic basement in Isthmian Central America
Authors: TOURNON J. , TRIBOULET C. , AZEMA J. 
Published: May 2007

ABSTRACTRocks from the metamorphic basement of the Azuero and Sona peninsulas, Panama, consist of schistose amphibolites and minor amounts of metasediment. In the Sona peninsula, strongly zoned amphiboles indicate that the amphibolites followed a progressive anticlockwsie P‐T path prograde from low T/low P to medium T/high P, and are retrograded into the greenschist facies. In contrast, the amphibolites of the Azuero peninsula are affected by a low to medium T/low P metamorphism.The metamorphic events of the Sona amphibolites occurred prior to the intra‐Senonian tectonic phase which affects the Mesozoic formations along the Pacific coast of Costa Rica and Panama. The regional significance of such a basement in Isthmian Central America is discussed.
Calibration of the anchizone: a critical comparison of illite ‘crystallinity’scales used for definition
Authors: KISCH H. J. 
Published: May 2007

Instrumental settings for determination of illite/muscovite ‘crystallinity’(half‐height width of the 10‐Å X‐ray diffraction peak) and the limits of the anchimetamorphic zone adopted by various authors fall into several groups.The variation in the limiting peak widths between the authors that have adopted Kubler's Neuchâtel boundary values of 0.42° and 0.25°Δ2θ can be interpreted in terms of variation in the instrumental settings. The choice of time constants higher than those given by the formula causes peak broadening; this effect is particularly marked at high scan rates. The peak broadening is by constant increments that are virtually independent of the absolute peak width.The differences between the Kubler (Neuchâtel)‐derived limiting values and some other scales are appreciably greater than can be accounted for by these differences in instrumental settings: many of these scales are not equivalent. In particular, the limits adopted by Dunoyer de Segonzac (1969) and subsequent workers at Strasbourg are too broad; their anchizone represents a range of grades of metamorphism lower than that of Kubler, widely overlapping the latter's ‘diagenetic’zone. Those adopted by some other, mainly French, authors are too narrow.The limits of the anchizone should be calibrated by inter‐laboratory standards, and the instrumental settings should be specified in full.
Sequential porphyroblast growth during deformation in a low‐pressure metamorphic terrain, Orrs Island–Harpswell Neck, Maine
Authors: LANG H. M. , DUNN G. R. 
Published: May 2007

Poikiloblastic index minerals in pelitic rocks from the Orrs Island–Harpswell Neck area of coastal Maine contain inclusion textures that indicate sequential growth of progressively higher grade metamorphic minerals during development of a near‐vertical crenulation foliation. The sequence of zones in the field is garnet, staurolite, staurolite–andalusite, staurolite–sillimanite and sillimanite. Inclusion fabrics characteristic of different stages in crenulation cleavage development indicate that index minerals nucleated and grew sequentially: biotite began to grow before deformation, garnet began to grow during early stages of crenulation cleavage development, staurolite grew during intermediate stages, and andalusite grew relatively late, when transposition of the foliation was nearly complete. Muscovite pseudomorphs and sillimanite were mainly post‐kinematic. The fact that metamorphic index minerals grew sequentially in individual rocks in the same order in which they appear across the field area indicates that the high temperature part of the pressure–temperature path was similar to the metamorphic field gradient.Metamorphism in the Orrs Island–Harpswell Neck area is consistent with the magmatic heating model that has been proposed for western Maine. Sequential development of index minerals in pelitic rocks in the Orrs Island–Harpswell Neck area apparently resulted from sequential nucleation after substantial overstepping of mineral‐forming reactions. Once nucleation of an index mineral had taken place, initial growth was rapid and poikiloblasts preserved inclusion trails characteristic of the prevailing stage of crenulation cleavage development. Because nucleation of sillimanite may have required more overstepping of the andalusite–sillimanite reaction than nucleation at dehydration reactions, determination of metamorphic conditions for rapidly heated rocks such as these by comparison with a petrogenetic grid is problematic. Garnet zoning patterns in these rocks should reflect the fact that growth of garnet interiors occurred early during metamorphism in equilibrium with a low‐grade assemblage. Only garnet rims would be expected to record the subsequent pressure–temperature path.
Glaucophane‐bearing assemblage overprinted by greenschist‐facies metamorphism in the Variscan Kaczawa complex, Sudetes, Poland
Authors: KRYZA R. , MUSZYNSKI A. , VIELZEUF D. 
Published: May 2007

An Early Palaeozoic (Ordovician ?) metamudstone sequence near Wojcieszow, Kaczawa Mts, Western Sudetes, Poland, contains numerous metabasite sills, up to 50 m thick. These subvolcanic rocks are of within‐plate alkali basalt type. Primary igneous phases in the metabasites, clinopyroxene (salite) and kaersutite, are veined and partly replaced by complex metamorphic mineral assemblages. Particularly, the kaersutite is corroded and rimmed by zoned sodic, sodic–calcic and calcic amphiboles. The matrix is composed of actinolite, pycnochlorite, albite (An ≤ 0.5%), epidote (Ps 27–33), titanite, calcite, opaques and, occasionally, biotite, phengite and stilpnomelane.The sodic amphiboles are glaucophane to crossite in composition with NaBfrom 1.9 to 1.6. They are rimmed successively by sodic–calcic and calcic amphiboles with compositions ranging from magnesioferri‐winchite to actinolite. No compositions between NaB= 0.92 and NaB= 1.56 have been ascertained.The textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist (or blueschist–greenschist transitional) assemblage. The presence of glaucophane and no traces of a jadeitic pyroxene + quartz association indicate pressures between 6 and 12 kbar during the high‐pressure episode. Temperature is difficult to assess in this metamorphic event. The replacement of glaucophane by actinolite + chlorite + albite, with associated epidote, allows restriction of the upper pressure limit of the greenschist recrystallization to <8 kbar, between 350 and 450°C. The mineral assemblage representing the greenschist episode suggests theP–Tconditions of the high‐pressure part of the chlorite or lower biotite zone. The latest metamorphic recrystallization, under the greenschist facies, may have taken place in the Viséan.
Biotite and garnet compositional variation and mineral equilibria in Grenville gneisses of the Otter Lake area, Quebec
Authors: KRETZ RALPH 
Published: May 2007

Garnet‐biotite gneisses, some of which contain sillimanite or hornblende, are widespread within the Otter Lake terrain, a portion of the Grenville Province of the Canadian Shield. The metamorphic grade is upper amphibolite to, locally, lower granulite facies.The atomic ratio Fe2+/(Fe2++ Fe3+) in biotite ranges from 0.79 to 0.89 (ferrous iron determinations in 10 highly pure separates), with a mean of 0.86. Mg and Fe2+ atoms occupy 67–78% of the octahedral sites, the remainder are occupied by Fe3+, Ti, and Al, and some are vacant. Mg/(Mg + Fe2+), denoted X, in the analysed samples ranges from 0.32 to 0.65. Garnet contains 1–24% grossular, 1–12% spessartine and X ranges from 0.07 to 0.34.Compositional variation in biotite and garnet is examined in relation to three mineral equilibria:(I) biotite + sillimanite + quartz = garnet + K‐feldspar + H2O;(II) pyrope + annite = almandine + phlogopite;(III) anorthite = grossular + sillimanite + quartz.Measurements of X (biotite) and X (garnet) are used to construct an illustrative model for equilibrium (I) which relates the observed variation in X to a temperature range of 70°C or a range in H2O activity of 0.6; the latter interpretation is preferred.In sillimanite‐free gneisses, the distribution of Mg and Fe2+ between garnet (low in Ca and Mn) and biotite is adequately described by a distribution coefficient (KD) of 4.1 (equilibrium II). The observed increase in the distribution coefficient with increasing Ca in garnet is ln KD= 1.3 + 2.5 × 10−2 [Ca]where [Ca] = 100 Ca/(Mg + Fe2++ Mn + Ca). The distribution coefficient is apparently unaffected by the presence of up to 12% spessartine in garnet.In several specimens of garnet‐sillimanite‐plagioclase gneiss, the Ca contents of garnet and of plagioclase increase in unison, as required by equilibrium (III). The mean pressure calculated from these data (n= 17) is 5.9 kbar, and the 95% confidence limits are ±0.5 kbar.