bluuebunny/crossref_metadata_2025_split huggingface dataset:
Metamorphism and granite genesis in the Hidaka Metamorphic Belt, Hokkaido, Japan
Authors: OSANAI Y. , KOMATSU M. , OWADA M. 
Published: May 2007

The Main Zone of the Hidaka Metamorphic Belt is an uplifted crustal section of island‐arc type. The crust was formed during early Tertiary time, as a result of collision between two arc–trench systems of Cretaceous age. The crustal metamorphic sequence is divided into four metamorphic zones (I–IV), in which zone IV is in the granulite facies.A detailed study of the evolution of the Hidaka Belt, based on a revised P–T–t analysis of the metamorphic rocks, notably a newly found staurolite‐bearing granulite, confirms a prograde isobaric heating path, after a supposed event of tectonic thickening of accretionary sedimentary and oceanic crustal rocks. During the peak metamorphic event (c. 53 Ma), the regional geothermal gradient attained 33–40° C km−1, and the highest P–T condition obtained from the lowest part of the granulite unit is 830° C, 7 kbar. In this part, XH2O of Gt–Opx–Cd gneiss is about 0.15 and that of Gt–Cd–Bt gneiss is 0.4. The P–T–XH2O condition of the granulite unit is well within a field where fluid‐present partial melting of pelitic and greywacke metamorphic rocks takes place. This is in harmony with the restitic nature of the Gt–Opx–Cd gneiss in the lowest part of the granulite unit.The possibility that partial melting took place in the Main Zone is significant for the genesis of the peraluminous (S‐type) granitic rocks within it. The S‐type granitic rocks in this zone are Opx–Gt–Bt tonalite in the granulite zone, Gt–Cd–Bt tonalite in the amphibolite zone, and Cd–Bt–Mus tonalite in the Bt–Mus gneiss zone. The mineralogical and chemical nature of these strongly peraluminous tonalitic rocks permit them to be regarded as having been derived from S‐type granitic magma generated by crustal anatexis of pelitic metamorphic rocks in deeper crust.
Chronology of the high‐pressure metamorphism of Norwegian garnet peridotites/pyroxenites
Authors: JAMTVEIT B. , CARSWELL D. A. , MEARNS E. W. 
Published: May 2007

Eclogite facies mineral assemblages are variably preserved in mafic and ultramafic rocks within the Western Gneiss Region (WGR) of Norway. Mineralogical and microstructural data indicate that some Mg–Cr‐rich, Alpine‐type peridotites have had a complex metamorphic history. The metamorphic evolution of these rocks has been described in terms of a seven‐stage evolutionary model; each stage is characterized by a specific mineral assemblage. Stages II and III both comprise garnet‐bearing mineral assemblages. Garnet‐bearing assemblages are also present in Fe–Ti‐rich peridotites which commonly occur as layers in mafic complexes. Sm–Nd isotopic results are reported for mineral and whole rock samples from both of these types of peridotites and related rocks.The partitioning of Sm and Nd between coexisting garnet and clinopyroxene is used to assess chemical equilibrium. One sample of Mg–Cr‐type peridotite shows non‐disturbed partitioning of Sm and Nd between Stage II garnet and clinopyroxene pairs and yields a garnet–clinopyroxene–whole‐rock date of 1703 ± 29 Ma (I= 0.51069, MSWD = 0.04). This is the best estimate for the age of the Stage II high‐P assemblage. Other Stage II garnet–clinopyroxene pairs reflect later disturbance of the Sm–Nd system and yield dates in the range 1303 to 1040 Ma. These dates may not have any geological significance. Stage III garnet–clinopyroxene pairs typically have equilibrated Sm–Nd partitioning and two samples yield dates of 437 ± 58 and 511 ± 18 Ma. This suggests that equilibration of the Stage III high‐P assemblage is related to the Caledonian orogeny and is more or less contemporaneous with high‐P metamorphism of ‘country‐rock’eclogites in the surrounding gneisses. The Sm–Nd mineral data for the Fe–Ti‐rich garnet peridotites and for a superferrian eclogite, which occurs as a dyke within the Gurskebotn Mg–Cr‐type peridotite, are consistent with a Palaeozoic high‐P metamorphism.Finally a synoptic P–T–t path is proposed for the Mg–Cr‐type peridotites which is consistent with the petrological and geochronological data.
Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization
Authors: POWELL R. , WILL T. M. , PHILLIPS G. N. 
Published: May 2007

An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (XCO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al2O3–SiO2–H2O–CO2, using a new, expanded, internally consistent dataset.Greenstone metamorphism cannot have involved much crustal over‐thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low‐P high‐T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper‐earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper‐later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid‐crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes.Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower XCO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.
Initial tectonothermal evolution within the Scandinavian Caledonide Accretionary Prism: constraints from 40Ar/39Ar mineral ages within the Seve Nappe Complex, Sarek Mountains, Sweden
Authors: DALLMEYER R. D. , ANDRÉASSON P. G. , SVENNINGSEN O. 
Published: May 2007

In the Caledonide orogen of northern Sweden, the Seve Nappe Complex is dominated by rift facies sedimentary and mafic rocks derived from the Late Proterozoic Baltoscandian miogeocline and offshore‐continent–Iapetus transition. Metamorphic breaks and structural inversions characterize the nappe complex. Within the Sarek Mountains, the Sarektjåkkå Nappe is composed of c. 600‐Ma‐old dolerites with subordinate screens of sedimentary rocks. These lithological elements preserve parageneses which record contact metamorphism at shallow crustal levels. The Sarektjåkkå Nappe is situated between eclogite‐bearing nappes (Mikka and Tsäkkok nappes) which underwent high‐P metamorphism at c. 500 Ma during westward subduction of the Baltoscandian margin. 40Ar/39Ar mineral ages of c. 520–500 Ma are recorded by hornblende within variably foliated amphibolite derived from mafic dyke protoliths within the Sarektjåkkå Nappe. Plateau ages of 500 Ma are displayed by muscovite within the basal thrust of the nappe and are consistent with metamorphic evidence which indicates that the nappe escaped crustal depression as a result of detachment at an early stage of subduction. Cooling ages recorded by hornblende from variably retrogressed eclogites in the entire region are in the range of c. 510–490 Ma and suggest that imbrication of the subducting miogeocline was followed by differential exhumation of the various imbricate sheets. Hornblende cooling ages of 470–460 Ma are recorded from massive dyke protoliths within the Sarektjåkkå Nappe. These are similar to ages reported from the Seve Nappe Complex in the central Scandinavian Caledonides. Probably these date imbrication and uplift related to Early Ordovician arrival of outboard terranes (e.g. island‐arc sequences represented by structurally lower horizons of the Köli Nappes).Metamorphic contrasts and the distinct grouping of mineral cooling ages suggest that the various Seve structural units are themselves internally imbricated, and were individually tectonically uplifted through argon closure temperatures during assembly of the Seve Nappe Complex. The cooling ages of 520–500 Ma recorded within Seve terranes and along terrane boundaries of the Sarek Mountains provide evidence of significant accretionary activity in the northern Scandinavian Caledonides in the Late Cambrian–Early Ordovician.
On the formation of granulites
Authors: BOHLEN S. R. 
Published: May 2007

The tectonic settings for the formation and evolution of regional granulite terranes and the lowermost continental crust can be deduced from pressure–temperature–time (P–T–time) paths and constrained by petrological and geophysical considerations. P–T conditions deduced for regional granulites require transient, average geothermal gradients of greater than 35°C km−1, implying minimum heat flow in excess of 100 mWm−2. Such high heat flow is probably caused by magmatic heating. Tectonic settings wherein such conditions are found include convergent plate margins, continental rifts, hot spots and at the margins of large, deep‐seated batholiths. However, particular P–T–time paths do not allow specific tectonic settings to be distinguished at this time. Under different conditions, both clockwise, CW (Pmax attained before Tmax), and anticlockwise, ACW (Pmax attained slightly after Tmax), paths are possible in the same tectonic setting. Both CW and ACW end‐member paths can yield nearly isobaric cooling, IBC, paths. Such cooling paths are clearly not an artefact of thermobarometry, but can be constrained by solid–solid and devolatilization equilibria and geophysical modelling.In terms of understanding the evolution of the deep crust, a potentially significant group of regional granulite terranes are those that show evidence for ACW‐IBC paths. Such paths are the likely result of: (i) episodic igneous activity resulting in intrusions within all levels of the crust, (ii) thickening of the crust by magmatic underplating, (iii) slow uplift as a result of the formation of a deep, garnet‐rich crustal root and (iv) excavation resulting from a later tectonic event unrelated to that resulting in the formation of the granulites. The later event might be triggered by the delamination of the garnet‐rich, lowermost crust.
Two‐stage decompression in garnet‐bearing mafic granulites from Sostrene Island, Prydz Bay, East Antarctica
Authors: THOST D. E. , HENSEN B. J. , MOTOYOSHI Y. 
Published: May 2007

Mafic garnet‐bearing granulites from Sostrene Island, 150 km southwest of Davis Station on the coast of Prydz Bay, East Antarctica, exhibit two‐stage symplectic coronas on garnet, formed after peak metamorphic conditions (M1). An outer corona of Opx (Mg66) + Pl (An94–97) + minor Hbl mantles a finer‐grained inner corona of Opx (Mg67) + Pl (An95–96) + Spl (Mg36). Both symplectites contain minor ilmenite–magnetite intergrowths. The finer‐grained symplectite also occurs along a fracture cleavage in the garnet.The outer corona originated during a second metamorphic event (M2) via the reaction Grt + Cpx (Hbl) + SiO2= Opx + Pl (1), whereas the inner corona formed later in response to decompression and minor deformation, resulting in the fracture cleavage in the garnet, according to the reaction Grt = Opx + Pl + Spl (2). The grossular content of the garent (XGrs= 0.168) is almost exactly that which is required for the stoichiometric breakdown by reaction (2) (calculated XGrs= 0.167). The mafic rocks are silica undersaturated, and the SiO2 for reaction (1) was most probably derived externally from the surrounding felsic gneisses.Preferred P–T estimates for M1 based on garnet core (Prp40Alm42Grs17Sps1)–matrix Opx–Cpx–Hbl pairs are c. 10 kbar at 980°C. The fine‐grained symplectite formed post‐peak M2 at c. 7 kbar and 850°C. The enclosing felsic gneisses yield pressure estimates of between 5 and 7 kbar, which compare with conditions of c. 6 kbar and 775°C in the nearby Bolingen Islands. These lower P–T estimates are considered to be representative of the widespread 1100‐Ma metamorphic event recognized in outcrops along the Prydz Bay coast. The high‐P, high‐T estimates derived from the garnet relics provide evidence for an earlier, possibly Archaean, high‐grade metamorphic event.
Late Sveconorwegian (Grenville) high‐pressure granulite facies metamorphism in southwest Sweden
Authors: JOHANSSON L. , LINDH A. , MÖLLER C. 
Published: May 2007

Mafic granulite, garnet amphibolite and charnockite occur in the southwest Swedish part of the Baltic Shield. This part is generally considered to be the continuation of the Grenville collisional belt in Canada. The area with granulite facies rocks, the Southwest Swedish Granulite Region (SGR), is considerably larger than previously thought. The SGR is bounded to the east and west by two major tectonic zones. The first quantitative age data and P–T determinations for the high‐grade metamorphism in the SGR are presented.Conventional geothermobarometry was applied to mafic granulites from five localities. The estimated P–T conditions for the peak of metamorphism range from 705°C and 8.1 kbar at Hallandsås in the south, to 770°C and 10.5 kbar at Ullared in the north (medium‐ to high‐P granulite facies conditions). Sm–Nd geochronology on minerals from the mafic granulites at Hallandsås and Ullared give late Sveconorwegian (Grenville) ages of 907 ± 12 and 916 ± 11 Ma for the high‐grade metamorphism, which is considerably younger than previously thought.Our results stress the hitherto underestimated importance of the late Sveconorwegian high‐grade metamorphism in the southwestern part of the Baltic Shield.
On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling
Authors: SPEAR F. S. 
Published: May 2007

Abstract
Finite difference models of Fe‐Mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet‐biotite Fe‐Mg exchange thermometry. For rocks cooled from high grades where the garnet was initially homogeneous, the calculated temperature (Tcalc) using garnet core and matrix biotite depends on the size of the garnet, the ratio of garnet to biotite in the rock (Vgarnet/Vbiotite) and the cooling rate. For garnets with radii of 1 mm and Vgarnet/Vbiotite<1, Tcalc is 633, 700 and 777°C for cooling rates of 1, 10 and 100°C/Ma. For Vgarnet/Vbiotite= 1 and 4 and a cooling rate of 10° C/Ma, Tcalc is approximately 660 and 610° C, respectively. Smaller and larger garnets have lower and higher Tcalc, respectively. These results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below Tcalc of the garnet core, provided that Vgarnet/Vbiotite is sufficiently small (<0.1) and that the composition of the biotite at the metamorphic peak has not been altered during cooling.Numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550–600° C generate a ‘well’in Fe/(Fe + Mg) near the rim during cooling. Maximum calculated temperatures for the assemblage garnet + chlorite + biotite + muscovite + plagioclase + quartz using the Fe/(Fe + Mg) at the bottom of the ‘well’with matrix biotite range from 23–43° C to 5–12° C below the peak metamorphic temperature for cooling rates of 1 and 100° C/Ma, respectively. Maximum calculated temperatures for the assemblage garnet + staurolite + biotite + muscovite + plagioclase + quartz are approximately 70° C below the peak metamorphic temperature and are not strongly dependent on cooling rate. The results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet‐biotite Fe‐Mg exchange thermometry on amphibolite facies rocks (Tmax > 550° C) because the rim composition of the garnet, which is required to calculate the peak temperature, is that most easily destroyed by diffusion.
Allochemical retrograde metamorphism in shear zones: an example in metapelites, Virginia, USA
Authors: GATES A. E. , SPEER J. A. 
Published: May 2007

Abstract
Metapelites in the Altavista area, southwest Virginia Piedmont, USA, underwent allochemical hydrothermal retrograde metamorphism in synmetamorphic shear zones. The metapelites of the Evington Group were metamorphosed in a prograde sequence of chlorite, staurolite, and sillimanite zones. Garnet–biotite geothermometry and phase relations support eastward increasing metamorphic grade, ranging from 570° C in the staurolite zone to 650° C in the sillimanite zone at c. 5.8 kbar. Sillimanite‐zone rocks later underwent progressive retrogression around shear zones which acted as fluid conduits. Retrograde assemblages are successively zoned around the shear zones with staurolite‐, chloritoid‐ and kyanite‐bearing assemblages. The shear zones commonly contain kyanite or tourmaline veins. Applicable phase equilibria indicate that retrogression occurred during isobaric cooling through c. 200–270° C. Rock compositional changes with retrogression occurred in steps: SiO2 was gained in the early stages of the retrogression but lost in the late stages; Al2O3, K2O, and H2O were increasingly gained through the sequence; CaO was increasingly lost. Addition of H2O and decreasing temperatures resulted in new ferromagnesian minerals (staurolite, chloritoid, chlorite) and changes in H2O, SiO2, Al2O3, K2O, and CaO contents produced muscovite and sodic plagioclase.Subsequent to prograde metamorphism, deeply derived fluids migrated upwards along shear zones, providing fluid and energy for the retrograde reactions. The sheared rocks underwent fluid infiltration with fluid fluxes of 1.8 × 107–4.3 × 107 cm3/cm2 corresponding to minimum estimated fluid‐to‐rock ratios of 7.5–21 as a function of position within the shear zone. Fluid flow was from high to low temperature early and low to high temperature later in the retrogression.
Chlorite crystallinity: an empirical approach and correlation with illite crystallinity, coal rank and mineral facies as exemplified by Palaeozoic and Mesozoic rocks of northeast Hungary
Authors: ÁRKAI P. 
Published: May 2007

Abstract
Fairly strong (r= 0.75–0.85) positive linear correlations were found between crystallinity indices (peak widths) measured on the first two basal reflections of chlorite and those of illite–muscovite in <2‐μm fractions of a representative shale–slate–phyllite series from Palaeozoic and Mesozoic formations of northeast Hungary. The metamorphic grade ranges from late or deep diagenesis through anchizone to epizone conditions. Chlorite crystallinity values measured on air‐dried and ethylene‐glycol‐solvated samples suggest that the effects of expandable interlayers are negligable, especially in the higher grade (∼temperature) part of the series. However, the greater scattering of crystallinity values for the chlorite 001 reflection compared to those of the 002 reflection may be related to the effects of minor amounts of interlayered and/or discrete smectite and/or vermiculite. With increasing metamorphic grade and advancing equilibrium recrystallization, the chlorite compositions in different samples become more homogenous. No correlation exists between crystallinity and changes in chlorite composition as estimated from the intensity ratios of basal reflections. Hence an increase of domain size and a decrease of lattice distortion with increasing grade (∼temperature) may be decisive factors affecting chlorite crystallinity.Chlorite crystallinity can be applied as a reliable regional, statistical technique complementary with, or instead of, the illite crystallinity method. The illite and chlorite crystallinity scales used here are related to Kübler's epi‐, anchi‐ and diagenetic zones and correlated with coal rank, conodont colour alteration and mineral facies data. As the effects of the detrital white mica can be observed even in the <2‐μm fractions of anchizonal metapelites, the anchizone boundaries determined solely on the base of ‘fixed’illite crystallinity values may vary with amounts of detrital and newly formed muscovite–illite. Hence a complex approach utilizing more than one method for determination of grade is preferred for petrogenetic purposes, even if relationships between crystallinity scales, coal rank and mineral facies also vary strongly in different tectonic settings and lithologies.
Development of slaty cleavage and degree of very‐low‐grade metamorphism: a review
Authors: KISCH H. J. 
Published: May 2007

Abstract
Illite crystallinity (IC) and other indicators of the grade of very‐low‐grade metamorphism associated with the appearance of various stages of slaty cleavage in phyllosilicate‐rich rocks have been compiled from a wide variety of terranes. IC values have been converted to a Kübler‐equivalent standard scale, but the diverse characterizations of the cleavage fabrics in published descriptions do not always allow an unequivocal identification of equivalent stages of cleavage development.Nevertheless, there exists a distinct relationship between grade and the appearance of various stages of cleavage development.(1) Indications of incipient slaty cleavage, such as S0–S1 pencil structure, appearance of primary (S1) crenulation cleavage and of closely‐spaced cleavage without parallel fabric in the microlithons, is associated with a wide range of mostly medium‐ and high‐grade diagenetic IC values.(2) The appearance of smooth cleavage with a strong parallel fabric in the microlithons and/or quartz–mica ‘beards’and the chlorite–mica stacks shortened at a high angle to (001), and of irregular cleavage in sandy beds is associated with a much narrower range of predominantly low‐ and medium‐grade anchimetamorphic grades (rarely high‐grade diagenetic). The first appearance of these stages of cleavage development with higher grades can often be related to post‐kinematic magmatic heating, polymetamorphism (pre‐cleavage metamorphism), or ‘static’recrystallization without cleavage formation, for example in low‐strain zones.There exists a relationship between finite strain, fabric and metamorphic grade in mudstones and slates; in coarser clastic rocks the same finite strain–fabric relationship occurs at appreciably higher grades. A relationship between finite strain in carbonate rocks and IC in the nearby rocks has been reported from the Helvetic zone of the Swiss Alps.The earlier stages of cleavage formation are associated with little improvement in IC; the narrow range of IC associated with smooth cleavage is concluded to represent recrystallization and grain growth concurrent with cleavage formation.