bluuebunny/crossref_metadata_2025_split huggingface dataset:
Metamorphism of the Witwatersrand gold fields: conditions during peak metamorphism
Authors: PHILLIPS G.N. 
Published: May 2007

Abstract
Pelitic assemblages from all major Witwatersrand gold fields record metamorphic conditions of the greenschist facies, with minimal regional grade changes over at least 200 km strike length. Diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites, the actively‐exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons. Bulk rock composition has been a major control on assemblage development.Key metapelitic assemblages include pyrophyllite, chloritoid, chlorite and muscovite in each gold field, with less common metamorphic biotite. Accessory minerals are pyrite, tourmaline, rutile and zircon. The abundance of chloritoid and pyrophyllite in thin shaly units, together with their minor, but widespread, distribution in quartzites and conglomerates, indicate that metamorphic temperatures reached 350°C ± 50°C in all the gold fields. Pressures are less‐well constrained, 1–2 kbar being inferred. Outside the gold fields, higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions.The temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to (re)mobilize gold and may explain its secondary textural features.
Metamorphic evolution of the southern part of the Hidaka belt, Hokkaido, Japan
Authors: SHIBA M. 
Published: May 2007

Abstract
The Hidaka metamorphic terrane in the Meguro‐Shoya area, Hokkaido, Japan is divided into four progressive metamorphic zones: A—biotite zone; B—cordierite zone; C—cordierite–K‐feldspar zone; and, D—sillimanite–K‐feldspar zone of the andalusite–sillimanite facies series type of metamorphism. The metamorphic grade ranges from the higher temperature part of the greenschist facies (zone A) through the amphibolite facies (zones B and C) to the lower temperature part of the granulite facies (zone D). The zone boundaries intersect the bedding planes at high angles. P–T conditions estimated are 450–550°C and 2 kbar for zone A, 550–600°C and 2–2.5 kbar for zone B, 600–650°C and 2.5–3 kbar for zone C and 650–750°C and 3–4 kbar for zone D. The metapelites of zone D were partially melted.At the later stage of the regional metamorphism which is early Oligocene to early Miocene in age, cordierite tonalite and biotite tonalite intrusives associated with segments of the highest grade rocks (zone D) were emplaced into the lower temperature part of the regional metamorphic rocks, giving rise to a contact metamorphic aureole. The thermally metamorphosed terrain (zone C') belongs to the amphibolite facies and its P–T conditions are estimated to have been 550–700°C and 2 kbar.The P–T–t paths of the Hidaka metamorphism show a thickening–heating–uplifting process. The metamorphism is inferred to have taken place beneath an active island arc accompanied by partial melting of the crust.
What controls partial melting in migmatites?
Authors: JOHANNES W. 
Published: May 2007

Abstract
The layers of six stromatic migmatites from Northern, Western, and Central Europe display small but systematic chemical and mineralogical differences. At least five of these migmatites do not show any signs of largescale metamorphic differentiation, metasomatism, or segregation of melts. It is concluded, therefore, that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks. Almost isochemical partial melting seems to be the most probable process transforming layered paragneisses, metavolcanics, or schists into migmatites.The formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers. The neosomes have less biotite and more K‐feldspar, if K‐feldspar is present at all, than the adjacent mesosomes. These differences are small but systematic and seem to control the access of different amounts of water to the various rock portions. Petrographical observations, chemical data, and theoretical considerations indicate a close relationship between rock composition, rock deformation, transport of water, partial melting, and formation of layered migmatites.
Experimental deformation of partially melted granitic aggregates
Authors: DELL'ANGELO L.N. , TULLIS J. 
Published: May 2007

Abstract
The effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions (900°C, 1500 MPa, 10‐4 to 10‐6/s) where melt‐free samples deform by dislocation creep, with microstructures approximately equivalent to those of upper greenschist facies. Experiments were performed on samples of various grain sizes, including an aplite (150 μm) and sintered aggregates of quartz‐albitemicrocline (10–50 and 2–10 μm). Water was added to the samples to obtain various amounts of melt (1–15% in the aplite, 1–5% in the sintered aggregates). Optical and TEM observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels, while the melt in the aplites is inhomogeneously distributed.The effect of partial melt on deformation depends an melt amount and distribution, grain size and strain rate. For samples deformed with ˜ 1% melt, all grain sizes exhibit microstructures indicative of dislocation creep. For samples deformed with 3–5% melt, the 150 μm and 10–50 μm grain size samples also exhibit dislocation creep microstructures, but the 2–10 μm grain size samples exhibit abundant TEM‐scale evidence of dissolution‐precipitation and little evidence of dislocation activity, suggesting a switch in deformation mechanism to predominantly melt‐enhanced diffusion creep. At natural strain rates melt‐enhanced diffusion creep would predominate at larger grain sizes, although probably not for most coarse‐grained granites.The effects of melt percentage and strain rate have been studied for the 150 μm aplites. For samples with ˜ 5 and 10% melt, deformation at 10–6/s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep. Conversely, deformation at 10‐5/s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results. For samples with 15% melt, deformation at both strain rates produces cataclasis, presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains, which would produce high stress concentrations at point contacts. At natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate. However, if the melt is confined and cannot migrate, a high pore fluid pressure should promote brittle deformation.
Silicate and sulphide thermobarometry of low‐ to medium‐grade metamorphic rocks from Holkham Bay, south‐east Alaska
Authors: STOWELL H. H. 
Published: May 2007

ABSTRACT The western metamorphic belt of the Coast Plutonic Complex, south‐east Alaska and adjacent British Columbia, contains strongly deformed rocks and a prominent topographic low: the Coast Range megalineament. Near Holkham Bay, south‐east Alaska, the lineament separates the western metamorphic belt into: a western low‐grade (greenschist facies) terrane, and an eastern medium‐grade (amphibolite facies) terrane.Sphalerite compositions of grains in direct contact with pyrite and pyrrhotite in chlorite‐muscovite zone rocks in the low‐grade terrane give pressures of about 8 kbar; compatible with pressures of 8‐10 kbar at 500°C calculated from plagioclase‐biotite‐garnet‐muscovite assemblages adjacent to the Windham Bay pluton about 15 km away. A pressure of 4.8 ± 0.7 kbar was calculated from sphalerite compositions in staurolite zone rocks east of the Coast Range megalineament. This is indistinguishable from pressures of 4.8 ± 1 kbar at 585°C and 5.1 ± 1 kbar at 680°C (plagioclase‐garnet‐aluminum silicate‐quartz equilibria), and 4.1 ± 1 kbar at 585°C (plagioclase‐biotite‐garnet‐muscovite equilibrium) determined for the medium‐grade terrane. An identical pressure of 4.8 ± 0.7 kbar was calculated from sphalerite compositions in biotite zone rocks adjacent to the lineament; this is considerably higher than a pressure of 3.1 ± 1 kbar at 525°C obtained using plagioclase‐biotite‐garnet‐muscovite geobarometry from shear zones within the lineament. The discrepancy may be explained by later equilibration of mineral phases within the shear zones.The geothermobarometry suggests relatively low temperatures and high pressures for the low‐grade terrane (6‐10 kbar), and intermediate temperatures and pressures for the medium‐grade terrane to the east (4‐6 kbar). Comparison of the barometers indicate that sphalerite can be used to estimate metamorphic pressures, similar to those estimated from silicate mineral chemistry when pyrrhotite‐sphalerite‐pyrite assemblages are used.
Metamorphic evolution of the Bunger Hills, East Antarctica: evidence for substantial post‐metamorphic peak compression with minimal cooling in a Proterozoic orogenic event
Authors: STÜWE K. , POWELL R. 
Published: May 2007

ABSTRACT The Bunger Hills, East Antarctica, experienced a low‐pressure granulite facies orogenic event during the Proterozoic. The stable coexistence of the S1 foliation‐parallel M1 assemblages, garnet‐cordierite‐spinel‐ilmenite and garnet‐sillimanite‐spinel‐ilmenite‐rutile, in quartz‐bearing pelitic gneisses is evidence for metamorphic peak pressures of around 4 kbar during M1, at temperatures of about 800°C. The growth of massive reaction coronas of garnet and cordierite around hercynitic spinel and iron‐titanium oxides during M2 is evidence for the destabilization of the M1 assemblages during compression. Thermodynamic calculations on the M2 assemblages indicate formation pressures of 6–7 kbar at temperatures of about 750°C. Thus, the gneisses from the Bunger Hills indicate about 2 kbar or more of compression during minimal cooling. Such a P‐T path is different from that of many other Proterozoic terranes which are characterized by isobaric cooling or decompression. A large charnockite body, which is undeformed, was intruded at ∼950°C, towards the end of compression.The low pressures during M1 can be best explained by metamorphism at mid‐crustal levels in thin continental crust in thin lithosphere above a thermal perturbation in the underlying asthenosphere. We suggest that the compression during cooling was a result of gravitational backflow in which the action of body forces between adjacent normal thickness crust and the thin crust of the Bunger Hills is 'switched on’by the thermal perturbation. Within such a model, the timing of intrusion of the charnockite exposed in the Bunger Hills is consistent with its generation by partial melting during the metamorphic maximum of the lowermost crust.
Statistical microanalysis applied to garnet growth zoning and to cordierite fluid content
Authors: PAQUET J. , DELAY F. , LEMAGUER D. 
Published: May 2007

ABSTRACTAn analytical study to evaluate quantitatively weak zoning of a garnet from a high‐grade kinzigite has been performed with an electron microprobe. The technique consists of the reconstruction of a profile step‐by‐step by successive analyses performed during relatively long counting times (30 s), along a radial profile of 2,500 μm length. The successive analytical data along this profile are statistically treated by Fisher's test and compared with the χ2 values (Pearson's law). These statistical tests were applied to assess microprobe stability and analysis homogeneity, and as a consequence to assure high credibility of the radial variations of the garnet. From core to rim, and for each element, zoning appears as the radial juxtaposition of stationary Poissonian samples. These samples being associated, the garnet appears to be constituted of successive concentric domains with stationary compositions. Different substitutions between Mg, Fe, Mn and Ca are evidenced. Such an analytical approach to chemical zoning can be useful for understanding growth mechanisms, and the possible diffusion reactions with the environment at each growth step. In addition, such a procedure can be used to evaluate accurately the fluid content of cordierite, and to appreciate the nature of the fluids concerned. As an example, the fluid content of a cordierite from a similar high‐grade kinzigite has been evaluated.
Reviews
Authors: Brodie K. 
Published: May 2007

INTRODUCTION TO METAMORPHIC TEXTURES AND MICROSTRUCTURES. By A. J. Barker. Blackie, Glasgow, 1989. ECLOGITE FACIES ROCKS. Edited by D. A. Carswell. Blackie, Glasgow, 1990. EVOLUTION OF METAMORPHIC BELTS. Edited by J. S. Daly, R. A. Cliff & B. W. D. Yardley. AN INTRODUCTION TO METAMORPHIC PETROLOGY. By B. W. D. Yardley.
Thermobarometry and P–T–t paths: the granulite to eclogite transition in lower crustal xenoliths from eastern Australia
Authors: PEARSON N. J. , O'REILLY S. Y. 
Published: May 2007

‘Lower crustal’ suite xenoliths in basaltic and kimberlitic magmas are dominated by mafic granulites and may also include eclogites and garnet pyroxenites. Pressures of up to 25 kbar obtained from such xenoliths are well in excess of an upper value of c. 12 kbar for exposed granulite terranes. Palaeogeotherms constructed from xenoliths for the lower crust beneath the Phanerozoic fold belts of eastern Australia (SEA) and beneath the eastern margin of the Australian craton (EMAC) indicate two distinct thermal regimes. The two geotherms have similar form, with the EMAC curve displaced c. 150°C to lower temperatures. Reaction microstructures show the partial re‐equilibration of primary igneous assemblages to granulite and eclogite assemblages and are interpreted to reflect the cooling from magmatic temperatures. Variations in mineral compositions and zoning are used to constrain further the history of several EMAC xenoliths to near‐isobaric trajectories.Detailed graphical models are constructed to predict compositional changes for isobaric P–T paths (at 7, 14 & 21 kbar) to transform an SEA‐type geotherm to a cratonic geotherm. The models show that for the assemblage grt + cpx ± ky + plag + qtz, the changes associated with falling temperature in Xgr, Xjd (increase) and Xan (decrease) will be greater at higher pressures. These results indicate that discernible zoning is more likely to be preserved in the higher pressure xenoliths. The zoning recorded in clinopyroxene from mafic granulite xenoliths over the pressure range c. 12–22 kbar suggests isobaric cooling of a large crustal thickness (30–35 km).An isobaric cooling path is consistent with magma accretion models for the transition of a crust–mantle boundary from an SEA‐type geotherm to a cratonic geotherm. The coexistence of granulite and eclogite over the depth range 35–75 km beneath the EMAC indicates that the granulite to eclogite transition in the lower crust is controlled by P–T conditions, bulk chemistry and kinetic factors. At shallower crustal levels, typified by exposed granulite terranes, isobaric cooling may not result in the transition to eclogite.
Episodic metamorphic reactions during orogenesis: the control of deformation partitioning on reaction sites and reaction duration
Authors: BELL T. H. , HAYWARD N. 
Published: May 2007

Abstract
Textural ‘unconformities’or truncations are common in porphyroblasts with complex inclusion trails. They reflect cycles of successive foliations that develop against competent porphyroblasts during orogenesis and are preserved by successive growth increments. Their truncational character results from shear and dissolution along a particular foliation generating a differentiated crenulation cleavage. The increment of porphyroblast growth that follows a textural ‘unconformity’may or may not mark a significant compositional change, depending on the amount of movement of the rock through P–T space between cleavage‐forming events.Although historically interpreted to result from a significant metamorphic hiatus, most textural unconformities indicate that the reactions involved in the formation of these minerals are episodic during continuous prograde metamorphism, starting and stopping as a function of the stage of crenulation of the matrix foliation and the pattern of deformation partitioning. Such episodic reaction behaviour can only occur for multivariant reactions, or successive but different univariant reactions. The reason why garnet is the most common porphyroblast to exhibit evidence for episodic reactions is probably the fact that it grows by multivariant reactions over a much wider P–T range than most other common porphyroblast phases.Porphyroblast growth is micrometasomatic. It is episodic because a significant reduction of strain occurs within domains of progressive shortening each time continuous progressive shearing domains form on their margins. This stops microfracture development across the progressive shortening domains, thereby preventing rapid access and interaction of fluid, ions and complexes with porphyroblast boundaries. Shifting patterns of deformation partitioning and resulting small‐scale juxtaposition of different compositional layers spreads the duration and location of multivariant reactions and causes differential timing of porphyroblast growth along a particular stratigraphic horizon. It may also locally preserve metastable metamorphic assemblages.In regionally metamorphosing/deforming pelites, near‐simultaneous cessation of porphyroblast growth on all rims, once continuous differentiated progressive shearing domains have formed nearby, precludes fluid recirculation as a significant process for removal of material during cleavage development. Alternatively, diffusion of simple molecules and dissociated ions along actively shearing and micro‐gaped phyllosilicates, with recomplexing in fluid‐filled microfractures, readily explains the control of deformation partitioning on reaction site and reaction duration.
The smectite–chlorite transition in drillhole NJ‐15, Nesjavellir geothermal field, Iceland: XRD, BSE and electron microprobe investigations
Authors: SCHIFFMAN P. , FRIDLEIFSSON G. O. 
Published: May 2007

Abstract
In well NJ‐15 of the Nesjavellir geothermal field, Iceland, the transition of discrete smectite into discrete chlorite has been studied from drill cuttings recovered at depths of less than 1714 m and over a continuous range of temperatures between 60 and 300° C. At temperatures below 180° C, the clay fractions contain mixtures of di‐ and trioctahedral smectites, whose layer charge increases with depth. Between 200 and 240° C, discrete smectites have transformed into smectite‐rich, randomly interstratified chlorite and smecite (R0 C/S). Because the abundance of chlorite interlayers in this C/S is generally <20%, its presence can be detected only by electron microprobe techniques and not by X‐ray diffraction. Between 245 and 265° C, both regularly (R1) and randomly interstratified C/S are the predominant layer silicates. Discrete chlorite first appears at approximately 270° C and coexists with minor amounts of R0 C/S at higher temperatures. R0 and R1 C/S form a nearly complete compositional series between trioctahedral saponite and discrete chlorite end‐members. The interlayer cation and Si content of smectites and C/S decrease with increasing temperature. The Mg/(Mg + Fe) content of smectite, C/S, and chlorite is unrelated to temperature. The percentage of chlorite in C/S, as determined by electron microprobe analyses, increases continuously with increasing temperature, except for occurrences of smectite‐rich C/S in fresh basaltic dykes which have not thermally equilibrated with the higher grade country rocks.
Proterozoic low‐pressure/high‐temperature metamorphism and an anticlockwise P–T–t path for the Hazeldene area, Mount Isa Inlier, Queensland, Australia
Authors: RUBENACH M. J. 
Published: May 2007

In the Hazeldene area, situated in the Mount Isa Inlier, Queensland, the metamorphic grade changes from chlorite zone, through biotite and cordierite zones, to sillimanite/K‐feldspar zone.Microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation (S2), was replaced by biotite, andalusite and sillimanite at the metamorphic peak late in S2, and in turn by kyanite + chlorite adjacent to localized small post‐D2 shear zones. Although the anticlockwise P–T–t path is well defined, the precise P–T conditions are uncertain because of problems with experimental and thermodynamic data. The best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600° C at 4 kbar.The metamorphism has been dated at 1544 Ma, and was synchronous with a major crustal shortening event. Because proposed extensional events occurred more than 60 Ma earlier, their contribution to the peak metamorphic thermal perturbation would have been insignificant. The syn‐metamorphic Mica Creek Pegmatites, the abundance of high heat‐producing elements in the nearby pre‐D2 Sybella Granite, and advective heat by fluids which caused considerable metasomatism in the Hazeldene area, may have each contributed to the thermal budget. However, the metamorphic thermal gradient may be 80°C km‐1 or higher, strongly suggesting a local magmatic control. As none are known in the area, such syn‐metamorphic plutons would have to lie beneath the exposed high‐grade rocks.
Tectonothermal evolution of the High Himalayan Crystalline Sequence, Langtang Valley, northern Nepal
Authors: INGER S. , HARRIS N. B. W. 
Published: May 2007

The High Himalayan Crystalline Sequence in north‐central Nepal is a 15‐km‐thick pile of metasediments that is bound by the Main Central Thrust to the south and a normal fault to the north. The Langtang section through the metasediments shows an apparent inversion of metamorphic isograds with high‐P, kyanite‐grade rocks exposed beneath low‐P, sillimanite‐grade rocks. Textural evidence confirms that the observed inversion is a result of a polyphase metamorphic history and phase equilibria studies indicate that thermal decoupling has occurred within a mechanically coherent section of crust. Rocks now exposed at the base of the High Himalayan thrust sheet underwent Barrovian regional metamorphism (M1) prior to 34 Ma in the early stages of the Himalayan orogeny, recording metamorphic conditions of T= 710 ± 30° C, P= 9 ± 1 kbar. After the activation of the Main Central Thrust, which emplaced these metapelites southwards onto the lower grade Lesser Himalayan formations, the upper part of the thrust sheet was overprinted by a second heating event (M2), resulting in sillimanite‐grade metamorphism and anatexis of metapelites at T= 760 ± 30° C, P= 5.8 ± 0.4 kbar between 17 and 20 Ma. Crustally derived, leucogranite magmas have been emplaced into low‐grade Tethyan sediments on the hangingwall of the normal fault that bounds the northern limit of the metapelitic sequence.The cause of the selective heating of the upper section of the metasediments during M2 cannot be reconciled with either post‐thrusting thermal relaxation or advection models. The cause of M2 remains problematical but it is suggested that heat focusing has occurred at the top of the High Himalayan Crystalline Sequence as a result of movement on the normal fault blanketing metapelites of high heat productivity with low‐grade sediments of low thermal conductivity. This model implies that the normal fault was active before M2, consistent with decompression textures that formed during, or shortly after, sillimanite‐grade metamorphism.