Petrology of pelitic schists in the oligoclase-biotite zone of the Sanbagawa metamorphic terrain, Japan: phase equilibria in the highest grade zone of a high-pressure intermediate type of metamorphic belt

The oligoclase-biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (X_Ca= 0.10–0.28)-bearing assemblages in pelitic schists, and represents the highest-grade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet + biotite + chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming PH₂O = P_solid suggests that metamorphic conditions of this zone are about 610 ± 25°C and 10 ± 1 kbar.
Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase-biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high-pressure type of metamorphic belt and with those in a generalized'upper staurolite zone'as an example of a medium-pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as:     

Omphacite-bearing calcite marble and associated coesite-bearing pelitic schist from the meta-ophiolitic belt of Chinese western Tianshan

In the meta-ophiolitic belt of Chinese western Tianshan, marble (5–50 cm thick) is found interlayered with pelitic schist. The marble is mainly composed of calcite (>90% in volume) and accessory phases include omphacite, quartz, dolomite, albite, phengite, clinozoisite and titanite with or without rutile core. This is the first omphacite (Jd_35–50) reported from marble of Chinese western Tianshan. It mainly occurs in the calcite matrix, rarely as inclusion in albite. The presence of omphacite suggests that the layered marble was subjected to eclogite-facies metamorphism, consistent with the occurrence of high-Si phengite (Si a.p.f.u. up to 3.7) and aragonite relic in albite. The associated pelitic schist consists of quartz, white mica (phengite + paragonite), garnet, albite, amphibole (barroisite ± glaucophane) and rutile/titanite, as well as minor amounts of dolomite, tourmaline and graphite. Coesite is optically recognized within porphyroblastic pelitic garnet and is further confirmed via Raman spectroscopy. Thermodynamic models support the UHP metamorphism of calcite marble, similar to the associated pelitic schist. Shared UHP-LT history of calcareous and pelitic rocks in Chinese western Tianshan suggests that the supracrustal carbon-rich sediments have been carried to depths of >90 km during fast subduction and thus are potential sources for carbon recycled into arc crust.     

Metamorphic discontinuities in orogenic belts: example of the garnet–biotite–albite zone in the Otago Schist, New Zealand

A regional petrographic reconnaissance of psammitic and pelitic rocks in the Otago Schist, New Zealand, has revealed the presence of garnet (“grossalspite” with typical rim composition almandine₄₁, spessartine₂₅, grossular₃₃, pyrope₁) and biotite in 37 new samples, more than doubling the previously known number. A new garnet–biotite–albite zone can now be defined in the greenschist facies Otago Schist that is distinct from the better-known biotite, garnet and oligoclase zones in the along-strike Alpine Schist. The garnet–biotite–albite zone is in part metamorphically discontinuous with adjacent schists and does not support models of simple, continuous, progressive Jurassic regional metamorphism in Otago. The structurally higher (lower grade) boundary of the zone coincides in at least three places with previously mapped regional shear zones. The structurally lower (expected higher grade) boundary of the zone appears to be obliterated by a chlorite zone overprint which can be spatially related to Alpine Schist recrystallisation of ?Cretaceous age. The Otago situation serves as an example of the subtle metamorphic discontinuities that probably pervade many orogenic belts.     

Pressure-temperature path of Sanbagawa prograde metamorphism deduced from grossular zoning of garnet

The zonal structure of prograde garnet in pelitic schists from the medium-grade garnet zone and the higher-grade albite-biotite zone was examined to investigate the evolution of prograde P–T paths of the Sanbagawa metamorphism. The garnet studied shows a bell-shaped chemical zoning of the spessartine component, which decreases in abundance from the core towards the rim. Almandine and pyrope contents and X_Mg [=Mg/(Mg+Fe²⁺)] increase monotonously outwards. The general scheme of the zonal structure for grossular content [X_Grs=Ca/(Fe²⁺+Mn+Mg+Ca)] can be summarized as: (1) X_Grs increases outwards (inner segment) and reaches a maximum at an intermediate position between the crystal core and the rim, then decreases towards the outermost rim (outer segment) (2) the inner segment of garnet in the garnet zone samples tends to have a higher X_Grs/X_Sps values for a given X_Sps than those in the albite–biotite zone samples (3) average X_Sps at the maximum X_Grs position in the albite–biotite zone samples ranges from 0.02 to 0.12 and is lower than that in the garnet zone samples (0.13–0.32) (4) the maximum X_Grs in the albite–biotite zone samples (0.34–0.39 on average) tends to be higher than that in the garnet zone samples (0.26–0.36), and (5) differences of X_Grs between the maximum and rim in the albite–biotite zone samples are between 0.10 and 0.14 and higher than those in the garnet zone samples (< 0.11). These facts imply that albite–biotite zone materials (a) were recrystallized under lower dP/dT conditions at an early stage of the prograde metamorphism (b) began their exhumation under higher P–T conditions and (c) have been continuously heated during exhumation for a longer duration than the garnet zone materials. The systematic changes of prograde P–T paths can be interpreted as documenting the evolution of the Sanbagawa subduction zone.     

Low-grade Progressive Metamorphism of Pelitic Schists of the Sazare area, Sanbagawa Metamorphic Terrain in central Sikoku, Japan

The Sanbagawa metamorphic terrain in the Sazare area, centralSikoku, is divided into three zones of progressive metamorphism,A, B, and C, on the basis of mineral assemblages of peliticschists. The characteristic mineral assemblage of zone A isphengite+chlorite and that of zone C is phengite+chlorite+garnetwith biotite possibly at a higher-grade part. Zone B is transitionalbetween A and C. Variation of the chemistry of garnet rim andchlorite with increasing grade is conspicuous: the MnO and FeOcontents of chlorite decrease and the FeO content of garnetrims increase with grade. The Fe-Mg partition coefficient forthe chlorite—garnet rim pair also changes systematicallywith the grade defined by the assemblage suggesting that theprogressive metamorphism is primarily due to temperature increase.The difference in assemblages between the Barrovian biotitezone and zone C of the present area is due either to the commonoccurrence of clinozoisite in the latter or to the differentparagenetic relations induced by higher-pressure of equilibriumin the Sanbagawa belt as compared with the Scottish one. The thermal structure of the present area revealed by zonalmapping requires the presence of a large-scale overturned structure,which postdated the major mineral formation.     

The effect of whole-rock MnO content on the stability of garnet in pelitic schists during metamorphism

Garnet-bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions. Modelling of thermodynamic data for minerals in the system Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O, however, predicts that garnet should be observed only in rocks of a narrow range of very high Fe/Mg bulk compositions. Traditionally, the nearly ubiquitous presence of garnet in medium- to high-grade pelitic schists is attributed qualitatively to the stabilizing effect of MnO, based on the observed strong partitioning of MnO into garnet relative to other minerals. In order to quantify the dependence of garnet stability on whole-rock MnO content, we have calculated mineral stabilities for pelitic rocks in the system MnO–Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O for a moderate range of MnO contents from a set of non-linear equations that specify mass balance and chemical equilibrium among minerals and fluid. The model pelitic system includes quartz, muscovite. albite, pyrophyllite, chlorite, chloritoid, biotite, garnet, staurolite, cordierite, andalusite, kyanite. sillimanite, K-feldspar and H₂O fluid. In the MnO-free system, garnet is restricted to high Fe/Mg bulk compositions, and commonly observed mineral assemblages such as garnet–chlorite and garnet–kyanite are not predicted at any pressure and temperature. In bulk compositions with X_Mn= Mn/(Fe + Mg + Mn) > 0.01, however, the predicted garnet-bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes. Temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole-rock MnO content. The small MnO contents of normal pelitic schists (X_Mn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.     

Experimental investigation of the formation of cordierite-orthopyroxene parageneses in pelitic rocks

Cordierite and orthopyroxene (or orthoamphibole) are widespread in migmatitic terranes, and partial melting of pelitic rocks may be important in their production. In particular, the reaction quartz +albite+biotite+garnet+water vapor = cordierite +orthopyroxene or orthoamphibole+melt was among reactions discussed by Grant (1973) but poorly constrained in pressure-temperature space.This reaction involves too many phases to be readily studied experimentally. Therefore simpler melting and dehydration reactions involving quartzalbite-biotite-cordierite-orthopyroxene were investigated.In conjunction with the work of Hoffer (1976, 1978) these experiments place useful constraints on the above reaction and on the reaction quartz+albite+aluminosilicate+biotite+vapor = cordierite+garnet+melt. In pelitic rocks near the second illimanite isograd, cordierite and garnet may coexist with melt as low as 660° C and cordierite and orthopyroxene may coexist with melt at temperatures less than 675° C. In the absence of significant Mn or Ca, in pelitic rocks within the realm of melting, biotite+garnet assemblages are probably limited to pressures greater than 2kb and aluminosilicate+biotite assemblages to pressures greater than 3kb.     

Decompression-induced growth of albite porphyroblasts, Fleur de Lys Supergroup, western Newfoundland

Abstract Albite porphyroblasts are widely distributed in pelitic and semi-pelitic schists of the Fleur de Lys Supergroup, western Newfoundland. Textures and mineral assemblages indicate that albite grew during nearly isothermal decompression from P-T conditions of about 500° C, 9 kbar, to conditions of 550° C, 6.5 kbar. Three compositional varieties of albite-bearing schists, here termed PMAQ (paragonite-muscovite-albite-quartz), MMAQ (microcline-muscovite-albite-quartz), and PMMQ (paragonite-muscovite-margarite-quartz), can be distinguished on the basis of pre-porphyroblast mineral assemblages. Analysis of these assemblages in terms of the composition of the coexisting fluid [log a (Na⁺/H⁺) versus log a (K⁺/H⁺)] suggests that, as pressure and temperature changed, the stability field of albite expanded at the expense of coexisting matrix phyllosilicates. This promoted growth of albite on pre-existing or newly formed nuclei. Late oligoclase in PMAQ and PMMQ samples is associated with replacement of matrix garnet by plagioclase + mica ° Chlorite, particularly in strongly sheared samples.     

Peristerite compositions in quartzofeldspathic schists, Franz Josef-Fox Glacier Area, New Zealand

Plagioclase compositions vary from An_0.1–2.5 to An₃₂ with increasing grade in chlorite zone to oligoclase zone quartzofeldspathic schists, Franz Josef-Fox Glacier area, Southern Alps, New Zealand. This change is interrupted by the peristerite composition gap in rocks transitional between greenschist and amphibolite facies grade. Oligoclase (An_20-24) and albite (An_0.1–0.5) are found in biotite zone schists below the garnet isograd. With increasing grade, the plagioclase compositions outline the peristerite gap, which is asymmetric and narrows to compositions of An₁₂ and An₆ near the top of the garnet zone. In any one sample, oligoclase is the stable mineral in mica-rich layers above the garnet isograd, whereas albite and oligoclase exist in apparent textural equilibrium in adjacent quartz-plagioclase layers. The initial appearance of oligoclase in both layers results from the breakdown of epidote and possibly sphene. Carbonate is restricted to the quartz-plagioclase rich layers and probably accounts for the more sodic composition of oligoclase in these layers. The formation of more Ca-rich albite and more Na-rich oligoclase near the upper limit of the garnet zone coincides with the disappearance of carbonate and closure of the peristerite gap. Garnet appears to have only a localized effect on Ca-enrichment of plagioclase in mica-rich layers within the garnet zone. The Na-content of white mica increases sympathetically with increasing Ca-content of oligoclase and metamorphic grade. Comparison of the peristerite gap in the Franz Josef-Fox Glacier schists and schists of the same bulk composition in the Haast River area, 80 km to the S, indicates that oligoclase appears and epidote disappears at lower temperatures, and that the composition gap between coexisting albite and oligoclase is narrower in the Franz Josef-Fox Glacier area. It is suggested that a higher thermal gradient (38-40°C/km) and variations in Si/Al ordering during growth of the plagioclases between the two areas may account for these differences. In the Alpine schists the peristerite gap exists over a temperature and pressure interval of about 370-515°C and 5.5-7 kbar (550-700 MPa) P_H2O.     

Petrology of high-pressure granulites from the eastern Himalayan syntaxis

The eastern Himalayan syntaxis, situated at the eastern terminus of the Himalayas, is the least-known segment of the Himalayas. Recent research in this area has revealed that the syntaxis consists of the Gangdise, the Yarlung Zangbo, and the Himalayan units, each of which is bounded by faults. The Himalayan unit, the northernmost exposed part of the Indian plate, mainly contains amphibolite facies rocks, marked by the assemblages staurolite+kyanite+plagioclase+biotite+muscovite±sillimanite and garnet+amphibole+plagioclase, in the south; to the north, low- to medium-pressure granulite grade pelitic gneisses and marbles are present and are characterized by the assemblages garnet+sillimanite+K-feldspar+plagioclase or antiperthite+biotite+quartz±spinel±cordierite±orthopyroxene in gneisses, and anorthite+diopside±wollastonite and plagioclase+diopside+quartz+phlogopite+calcite in marbles. Within this unit, the Namula thrust system is a series of moderately north-dipping structures that displaced the granulite facies rocks southwards over the amphibolite facies rocks. High-pressure granulites occur as relics within these granulite facies rocks and contain garnet–kyanite granulite and garnet clinopyroxenite. The peak assemblage of the garnet–kyanite granulite includes garnet (core part)+kyanite+ternary feldspar+quartz+rutile. Sillimanite+garnet (rim part)+K-feldspar+ oligoclase+ilmenite+biotite and spinel+albite+biotite or spinel+cordierite±orthopyroxene, which are coronas around sillimanite and garnet, are retrograde products of this peak assemblage. Another peak assemblage includes very-high-Ca garnet (CaO 32–34 wt%, Alm₁₀±Grs_>80) and diopside (CaO 22–24 wt%), scapolite, meionite, quartz, and accessory Al-bearing titanite (Al₂O₃ 4–4.5 wt%). The diopside has kink bands. Partial or complete breakdown of Ca-rich garnet during post-peak metamorphism produced pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of hedenbergite and anorthite. Thermobarometric estimates in combination with reaction textures, mineral compositions, and recent experimental studies indicate that these peak assemblages were formed at P=c. 1.7–1.8 GPa, T =c. 890 °C, and the retrograde assemblages experienced near-isothermal decompression to P=0.5±0.1 GPa, T =850±50 °C. The whole-rock compositions indicate that marble and pelite are plausible candidates for the protoliths. These facts suggest the following (1) sedimentary rocks were transported to upper-mantle depths and equilibrated at those conditions to form these high-pressure granulites, which were then emplaced into the crust quickly. During the rapid exhumation of these rocks, the earlier high-pressure assemblages were overprinted by the later low- to medium-pressure assemblages, that is, the high-pressure granulite belt formed in the syntaxis. (2) The Namula thrust system is an important tectonic boundary in the syntaxis, or even in the Higher Himalaya more generally.     

K-Ar Age and Chemistry of Phengite from the Sanbagawa Schists in the Kanto Mountains, Central Japan, and their Implication for Exhumation Tectonics

EPMA analyses and K-Ar age determinations were carried out on phengite in pelitic schist from the Sanbagawa metamorphic belt of the Kanto Mountains, Central Japan.

Phengite from the Sanbagawa pelitic schist in the Kanto Mountains generally occurs as aggregates of fine-grained crystals. It is extremely fine-grained in domains adjacent to relatively rigid garnet and albite porphyroblasts. This suggests that deformation-induced grain-size reduction took place in phengite during the ductile deformation accompanying the exhumation of the host schists. EPMA analysis shows that phengite is chemically heterogeneous at the thin-section scale, suggesting that it formed during retrograde metamorphism in restricted equilibrium domains. The retrograde chemical reaction was promoted by the ductile deformation.

K-Ar ages of phengite get younger from the Southern Unit (82 Ma) to the Northern Unit (58 Ma) in the Kanto Mountains. The age range is similar to that in Central Shikoku. The older schists occur in the higher metamorphic grade zone in Central Shikoku and in the lower-grade zone in the Kanto Mountains. The thermal structures in Central Shikoku are inverted, so that the highest-grade zone occurs in the upper or middle parts of the apparent stratigraphic succession. In contrast, the Kanto Mountains have a normal thermal structure: the higher-grade zone is in the lower part of the apparent stratigraphic succession. The different tectonic features in exhumation produced the two contrasting age-temperature-structure relations at the western side of Sanbagawa belt in Central Shikoku and the eastern end of the Sanbagawa belt in the Kanto Mountains that are 800 km distant from each other. Namely, the western Sanbagawa belt in Central Shikoku underwent longer ductile deformation during the exhumation than the eastern Sanbagawa belt in the Kanto Mountains.     

Cordierite-bearing schists and gneisses from Timor, eastern Indonesia: P-T conditions of metamorphism and tectonic implications

In the Boi Massif of Western Timor the Mutis Complex, which is equivalent to the Lolotoi Complex of East Timor, is composed of two lithostratigraphical components: various basement schists and gneisses; and the dismembered remnants of an ophiolite. Cordierite-bearing pelitic schists and gneisses carry an early mineral assemblage of biotite + garnet + plagioclase + Al-silicate, but contain no prograde muscovite; sillimanite occurs in a textural mode which suggests that it replaced and pseudomorphed kyanite at an early stage and some specimens of pelitic schist contain tiny kyanite relics in plagioclase. Textural relations between, and mineral chemistries of, ferro-magnesian phases in these pelitic chists and gneisses suggest that two discontinuous reactions and additional continuous compositional changes have been overstepped, possibly with concomitant anatexis, as a result of decrease in P_load during high temperature metamorphism. The simplified reactions are: garnet and/or biotite + sillimanite + quartz + cordierite + hercynite + ilmenite + excess components. P-T conditions during the development of the early mineral assemblage in the pelitic gneisses are estimated to have been P + 10 kbar and T > 750°C, based upon the plagioclase-garnet-Al-silicate-quartz geobarometer and the garnet-biotite geothermometer. P-T conditions during the subsequent development of cordierite-bearing mineral assemblages in the pelitic gneisses are estimated to have been P + 5 kbar and T + 700°C with X_H2O < 0.5, based upon the Fe content of cordierite occurring in the assemblage quartz + plagioclase + sillimanite + biotite + garnet + cordierite coexisting with melt. Final equilibration between some of the phases suggests that conditions dropped to P > 2.3 kbar and T > 600°C. A similar exhumation P-T path is suggested for the pelitic schists with early metamorphic conditions of P > 6.2 kbar and T > 745°C and subsequent development of cordierite under conditions in the range P = 3-4 kbar and T = 600-700°C. The tectonic implications of these P-T estimates are discussed and it is concluded that the P-T path followed by these rocks was caused by decompression during rifting and synmetamorphic ophiolite emplacement resulting from processes during the initiation and development of a convergent plate junction located in Southeast Asia during late Jurassic to Cretaceous time.     

Conditions of metamorphism in an early Paleozoic blueschist,schist of Skookum Gulch,northern California

The Late Ordovician schist of Skookum Gulch, northern California, is one of only a few pre-Mesozoic blueschist localities in North America. Among these, the Skookum Gulch occurrence is noteworthy because it contains lawsonite and has had a relatively simple metamorphic and structural history.Numerous assemblages and a wide variation in the modal proportion of minerals are present due to a spectrum of bulk compositions. This is reflected in the composition of Na-amphibole, which varies from ferro-glaucophane, glaucophane, and crossite (±magnetite) to magnesio-riebeckite (+hematite). Application of the available experiments and empirically calibrated equilibria to the assemblages glaucophane+lawsonite+chlorite+quartz+albite and glaucophane+actinolite+epidote+chlorite+quartz+albite yield estimates of temperature and pressure near T= 275° C and P=7.0 kbar. Estimates of uncertainty are difficult to assess, but are no more than ±100° C and ±3.0 kbar, and are probably considerably smaller. Calcite +quartz+sphene and calcite+quartz+actinolite indicate an extremely H₂O-rich fluid (X(CO₂)<0.003).The absence of a greenschist facies overprint indicates that the schist of Skookum Gulch was uplifted soon after metamorphism. However, it was not exposed until the recent geologic past, having resided at shallow crustal levels for approximately 400 Ma.     

Eine Subfaziesfolge der Grünschieferfazies in den Mittleren Cévennen (Dép. Ardèche) mit Pyrophyllit aufweisenden Mineralparagenesen

In the middle part of the Cévennes, situated at the south-eastern border of the Massif Central, there are outcrops of micaschists, gneisses and granites in which the increase of metamorphic grade was investigated along the river Beaume. This paper presents the results of the petrographic-geochemical analysis of the micaschists of the greenschist facies.Chemical analyses of the micaschist-samples examined petrographically show their nearly constant composition, a fact which e.g., is expressed by similar CaO/Na₂O- and MgO/FeO-proportions. As the average values of the determined oxides approach the respective average values of analyses of graywackes recorded by Pettijohn (1957) the original sediment is likely to have been an argillaceous graywacke deficient in calcite.In enrichments of the phyllosilicates pyrophyllite was detected by X-ray diffraction; its amount is about 2 to 5 weight percent of the rocks.With increasing metamorphic grade the following parageneses were found in the metamorphic zones of the greenschist facies: Zone Ia: Quartz+chlorite+pyrophyllite+muscovite+clinozoisite. Zone Ib: Quartz+chlorite+pyrophyllite+muscovite+biotite+clinozoisite.The absence of biotite in Zone Ia, though the chemism of the rocks is practically the same, is obviously due to the different Al₂O₃-content of the chlorites of Zones Ia and Ib. The chlorite of Zone Ia is more deficient in Al₂O₃ than the one of Zone Ib. With passage from Zone Ia to Zone Ib the position of the tie line between chlorite and muscovite in the ACF-A'FK-diagram changes in such a way that in Zone Ia, because of purely chemical reasons, biotite cannot occur as coexisting mineral.The beginning of Zone II is characterized by the occurence of almandine, rich in spessartine. The following paragenesis is typical of this zone: Quartz+chlorite+pyrophyllite +muscovite+biotite+almandine+clinozoisite.Additionally the micaschists of these three zones display albite, the greater part of which is concentrated in mm-thin layers with associated minor amounts of quartz and micas. Paragonite whose formation by reaction between albite and pyrophyllite is to be expected based on experimental results (Winkler, 1967, p. 95) could not be proved by X-ray diffraction.In Zone III andalusite occurs instead of pyrophyllite. Furthermore, as chlorite and clinozoisite are absent and oligoclase occurs for the first time this zone is regarded as the first subfacies of the almandine-amphibolite-facies. The chemism and the observed mineral parageneses of the subfacies of the almandine-amphibolite-facies will be treated in a separate publication.     

Calcite–graphite isotope thermometry in amphibolite facies marble, Bancroft, Ontario

This study presents calcite–graphite carbon isotope fractionations for 32 samples from marble in the northern Elzevir terrane of the Central Metasedimentary Belt, Grenville Province, southern Ontario, Canada. These results are compared with temperatures calculated by calcite–dolomite thermometry (15 samples), garnet–biotite thermometry (four samples) and garnet–hornblende thermometry (three samples). Δ_cal‐gr values vary regularly across the area from >6.5‰ in the south to 4.0‰ in the north, which corresponds to temperatures of 525 °C in the south to 650 °C in the north. Previous empirical calibration of the calcite–graphite thermometer agrees very well with calcite–dolomite, garnet–biotite and garnet–hornblende thermometry, whereas, theoretical calibrations compare less well with the independent thermometry. Isograds in marble based on the reactions rutile + calcite + quartz =titanite and tremolite + calcite + quartz = diopside, span temperatures of 525–600 °C and are consistent with calculated temperature–X(CO₂) relations. Results of this study compare favourably with large‐scale regional isotherms, however, local variation is greater than that revealed by large‐scale sampling strategies. It remains unclear whether the temperature–Δ_cal‐gr relationship observed in natural materials below 650 °C represents equilibrium fractionations or not, but the regularity and consistency apparent in this study demonstrate its utility for thermometry in amphibolite facies marble.     

P–T–t evolution of granulite facies metamorphism and partial melting in the Winding Stair Gap,Central Blue Ridge,North Carolina,USA

The Winding Stair Gap in the Central Blue Ridge province exposes granulite facies schists, gneisses, granofelses and migmatites characterized by the mineral assemblages: garnet–biotite–sillimanite–plagioclase–quartz, garnet–hornblende–biotite–plagioclase–quartz ± orthopyroxene ± clinopyroxene and orthopyroxene–biotite–quartz. Multiple textural populations of biotite, kyanite and sillimanite in pelitic schists support a polymetamorphic history characterized by an early clockwise P–T path in which dehydration melting of muscovite took place in the stability field of kyanite. Continued heating led to dehydration melting of biotite until peak conditions of 850 ± 30 °C, 9 ± 1 kbar were reached. After equilibrating at peak temperatures, the rocks underwent a stage of near isobaric cooling during which hydrous melt ± K‐feldspar were replaced by muscovite, and garnet by sillimanite + biotite + plagioclase. Most monazite crystals from a pelitic schist display patchy zoning for Th, Y and U, with some matrix crystals having as many as five compositional zones. A few monazite inclusions in garnet, as well as Y‐rich cores of some monazite matrix crystals, yield the oldest dates of c. 500 Ma, whereas a few homogeneous matrix monazites that grew in the main foliation plane yield dates of 370–330 Ma. Culling and analysis of individual spot dates for eight monazite grains yields three age populations of 509 ± 14 Ma, 438 ± 5 Ma and 360 ± 5 Ma. These data suggest that peak‐temperature metamorphism and partial melting in the central Blue Ridge occurred during the Salinic or Taconic orogeny. Following near isobaric cooling, a second weaker thermal pulse possibly related to intrusion of nearby igneous bodies resulted in growth of monazite c. 360 Ma, coinciding with the Neoacadian orogeny.     

Chloritoid and barroisite-bearing pelitic schists from the eclogite unit in the Besshi district, Sanbagawa metamorphic belt

The Sanbagawa metamorphic rocks in the Besshi district, central Shikoku, are grouped into eclogite and noneclogite units. Chloritoid and barroisite-bearing pelitic schists occur as interlayers within basic schist in an eclogite unit of the Seba area in the Sanbagawa metamorphic belt, central Shikoku, Japan. Major matrix phases of the schists are garnet, chlorite, barroisite, paragonite, phengite, and quartz. Eclogite facies phases including chloritoid and talc are preserved only as inclusions in garnet. P–T conditions for the eclogite facies stage estimated using equilibria among chloritoid, barroisite, chlorite, interlayered chlorite–talc, paragonite, and garnet are 1.8 GPa/520–550 °C. Zonal structures of garnet and matrix amphibole show discontinuous growth of minerals between their core and mantle parts, implying the following metamorphic stages: prograde eclogite facies stage→hydration reaction stage→prograde epidote–amphibolite stage. This metamorphic history suggests that the Seba eclogite lithologies were (1) juxtaposed with subducting noneclogite lithologies during exhumation and then (2) progressively recrystallized under the epidote–amphibolite facies together with the surrounding noneclogite lithologies.

The pelitic schists in the Seba eclogite unit contain paragonite of two generations: prograde phase of the eclogite facies included in garnet and matrix phase produced by local reequilibration of sodic pyroxene-bearing eclogite facies assemblages during exhumation. Paragonite is absent in the common Sanbagawa basic and pelitic schists, and is, however, reported from restricted schists from several localities near the proposed eclogite unit in the Besshi district. These paragonite-bearing schists could be lower-pressure equivalents of the former eclogite facies rocks and are also members of the eclogite unit. This idea implies that the eclogite unit is more widely distributed in the Besshi district than previously thought.     

Evidence from mineral assemblages for infiltration of pelitic schists by aqueous fluids during metamorphism

Prograde mineral assemblages and compositions have been predicted for pelitic schist in the 10 component system Na₂O–K₂O–CaO–MnO–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O for three cases of prograde metamorphism and fluid-rock interaction: (1) increasing temperature (T) at constant pressure (P) and constant pore fluid volume (1%) without infiltration (no-infiltration case); (2) increasing T at constant P accompanied by sufficient fluid infiltration that fluid composition is at all times constant (large-flux case); and (3) increasing T at constantP accompanied by a timeintegrated fluid flux f 10⁴ cm^{3 cm 2} (intermediate-flux case). Stable mineral assemblages and compositions were calculated by solving a system of non-linear equations that specify mass balance and chemical equilibrium between minerals and fluid. The model pelitic system includes quartz, muscovite, plagioclasc, chlorite, ankerite, siderite, biotite, garnet, staurolite, andalusite, kyanite, sillimanite, K-feldspar, and a coexisting, binary H₂O–CO₂ fluid. Specifically, prograde thermal metamorphism was modelled for Shaw's (1956) average low-grade pelite and for a moderate range of bulk rock compositions at P=3, 5, and 7 kb and initial fluids with Xco ₂ ^o =0.02–0.40. The model predicts a carbonate-bearing mineral assemblage for average pelite under chlorite zone conditions composed of quartz, muscovite, albite, chlorite, ankerite, and siderite. The mineral assemblages predicted for the noinfiltration case are unlike those typically observed in regional metamorphic terranes. Simulations of metamorphism for the large-flux and intermediate-flux cases, however, reproduce the sequence of mineral assemblages observed in normal Barrovian regional metamorphic terranes. These results suggest that regional metamorphism of pelitic schists is typically associated with infiltration of significant quantities of aqueous fluid.     

Regression Modelling of Metamorphic Reactions in Metapelites, Snow Peak, Northern Idaho

The second of two periods of regional metamorphism that affectedpelitic rocks near Snow Peak caused complete re-equilibrationof mineral assemblages and resulted in a consistent set of metamorphicisograds. Metamorphic chlorite and biotite occur in the lowestgrade rocks. With increasing grade, garnet, staurolite, andkyanite join the assemblage, resulting in a transition zonecontaining all the above phases. At higher grade, chlorite,and finally staurolite disappear. Mass balance relations at isograds and among minerals of low-varianceassemblages have been modelled by a non-linear least-squaresregression technique. The progressive sequence can be describedin terms of schematic T-X_H2O relations among chlorite, biotite,garnet, staurolite, and kyanite at P_total above the KFMASH invariantpoint involving those phases. The first appearance of garnetwas the result of an Fe-Mg-Mn continuous reaction. As temperaturerose, the garnet zone assemblage encountered the stauroliteisograd reaction, approximated by the model reaction: 3?0 chlorite + 1?5 garnet + 3?3 muscovite + 05 ilmenite = 1?0staurolite + 3?1 biotite + 1?5 plagioclase + 3?3 quartz + 10?3H₂O. The staurolite zone corresponds to buffering along this reactionto the intersection where chlorite, biotite, garnet, staurolite,and kyanite coexist. The transition zone assemblage formed byreaction at this T–X _H2O intersection which migrates towardmore H2O-rich fluid composition with progressive reaction. Thenet reaction at the intersection is approximated by the transitionzone reaction: 1?0 chlorite +1?1 muscovite + 0?2 ilmenite = 2?7 kyanite + 1?0biotite + 0?4 albite + 4?2 H₂O. Chlorite was commonly the first phase to have been exhaustedand the remaining assemblage was buffered along a staurolite-outreaction, represented by the model reaction: 1?0 staurolite + 3?4 quartz + 0?4 anorthite + 1?4 garnet + 0?1ilmenite + 7?9 kyanite + 2?0 H₂O. Consumption of staurolite by this reaction resulted in the highestgrade assemblage, which contains kyanite, garnet, biotite, muscovite,quartz, plagioclase, ilmenite, and graphite.     

A mechanism of shape-transformation of quartz inclusions in albite of regional metamorphic rocks

The process of shape-transformation of quartz inclusions from polyhedral to spherical grains in albite single crystals during metamorphism is mainly controlled by the grain boundary diffusion of oxygen along the quartz/albite interface to reduce the interfacial free energy. The rate of the process, which is represented by the growth rate of the curvature of the edge surface of the grain, depends significantly on temperature and on the grain size of the quartz inclusion. The relations between temperature, T, the time, t_r, and the critical radius, R_c, which is equal to the radius of maximum spherical grains, are given by log R_c = −0.11E_b/RT + 0.25log t_r + C, in which E_b is the activation energy of the grain boundary diffusion of oxygen along the quartz/albite interface and C is a material constant.

The mean critical radius of spherical quartz inclusions in albite is 5 μm for the upper chlorite zone and garnet zone, 10 μm for the lower biotite zone, and 20 μm for the upper biotite zone in the Sambagawa metamorphic terrain. The mean values of the critical radii of spherical quartz inclusions in oligoclase of the Ryoke metamorphic rocks is about 5 μm for the chlorite zone and about 10–20 μm for the sillimanite zone.

Assuming temperatures of about 350°C for the upper chlorite and garnet zones, 400°C for the lower biotite zone, 550°C for the upper biotite zone, and 700°C for the sillimanite zone, the activation energy for the grain boundary diffusion of oxygen along the quartz/plagioclase interfase is estimated to be about 30 kcal/mol.