Garnet Chlorite Research Articles

Prograde Barrovian metamorphism along Darjeeling–Tista transect, Eastern Himalaya, India: constraints from textural relationship, phase equilibria and geothermobarometry

The Darjeeling–Tista transect, located in Lesser Himalaya, is a part of the North‐Eastern Himalaya. Along the transect, polyphase deformation and prograde Barrovian metamorphism have been delineated. The time relations between the phases of deformations (D1, D2 and D3) and metamorphic crystallization reveal a single major prograde metamorphic event that initiated with the D1 deformation and finally outlasted it. The earlier phase of this metamorphism is essentially regional syntectonic low grade, which may be designated (M1a). This was followed by regional static metamorphism (M1b) in the post‐tectonic phase between the D1 and D2 deformations. This M1 metamorphism is superposed by later retrogressive metamorphism (M2) during the D2 and D3 deformations. The different parageneses of pelitic rocks containing chlorite, muscovite, biotite, garnet, staurolite, kyanite, sillimanite, K‐feldspar and plagioclase show various textures that resulted from the continuous and discontinuous reactions in the different zones. The metamorphic zones and isograds delineated on the basis of specific metamorphic reactions, namely reaction isograd or isoreaction grad, and by critical mineral assemblages are as follows: (a) chlorite–biotite zone, (b) garnet–chlorite zone, (c) staurolite–biotite–chlorite zone, (d) kyanite–biotite–staurolite zone and (e) sillimanite–biotite–staurolite zone. Chemographic relations and inferred reactions for different zones are portrayed in the AKF and AFM projections. A sequence of metamorphic reactions at different isograds has been deduced through textural relations. The prograde P‐T evolution of the study area has been constrained through the use of internally consistent winTWQ programme and Perple_X software in the MnNCKFMASHTO model system. The study has the potential to instigate future researches focusing on Himalayan metamorphic evolutionary trends using modern approaches. Copyright © 2017 John Wiley & Sons, Ltd.

Ocean floor and subduction record in the Zermatt‐Saas rodingites, Valtournanche, Western Alps

AbstractMultiscale structural analysis and petrological modelling were used to establish the pressure‐peak mineral assemblages and pressure–temperature (P–T) conditions recorded in the rodingites of the upper Valtournanche portion of the oceanic Zermatt‐Saas Zone (ZSZ; Western Alps, northwestern Italy) during Alpine subduction. Rodingites occur in the form of deformed dykes and boudins within the hosting serpentinites. A field structural analysis showed that rodingites and serpentinites record four ductile deformation stages (D1–D4) during the Alpine cycle, with the first three stages associated with new foliations. The most pervasive fabric is S2 that is marked by mineral assemblages in serpentinite indicating pressure‐peak conditions, involving mostly serpentine, clinopyroxene, olivine, Ti‐clinohumite and chlorite. Three rodingite types can be defined: epidote‐bearing, garnet–chlorite–clinopyroxene‐bearing and vesuvianite‐bearing rodingite. In these, the pressure‐peak assemblages coeval with S2 development involve: (i) epidoteII + clinopyroxeneII + Mg‐chloriteII + garnetII ± rutile ± tremoliteI in the epidote‐bearing rodingite; (ii) Mg‐chloriteII + garnetII clinopyroxeneII ± vesuvianiteII ± ilmenite in the garnet–chlorite–clinopyroxene‐bearing rodingite; (iii) vesuvianiteII + Mg‐chloriteII + clinopyroxeneII + garnetII ± rutile ± epidote in vesuvianite‐bearing rodingite. Despite the pervasive structural reworking of the rodingites during Alpine subduction, the mineral relicts of the pre‐Alpine ocean floor history have been preserved and consist of clinopyroxene porphyroclasts (probable igneous relicts from gabbro dykes) and Cr‐rich garnet and vesuvianite (relicts of ocean floor metasomatism). Petrological modelling using thermocalc in the NCFMASHTO system was used to constrain the P–T conditions of the S2 mineral assemblages. The inferred values of 2.3–2.8 GPa and 580–660 °C are consistent with those obtained for syn‐S2 assemblages in the surrounding serpentinites. Multiscale structural analysis indicates that some ocean floor minerals remained stable under eclogite facies conditions suggesting that minerals such as vesuvianite, which is generally regarded as a low‐P phase, could also be stable in favourable chemical systems under high‐P/ultra‐high‐pressure (HP/UHP) conditions. Finally, the reconstructed P–T–d–t path indicates that the P/T ratio characterizing the D2 stage is consistent with cold subduction as estimated in this part of the Alps. The estimated pressure‐peak values are higher than those previously reported in this part of ZSZ, suggesting that the UHP units are larger and/or more abundant than those previously suggested.

The metahyaloclastitic matrix of a unique metavolcanic block reveals subduction in the Somozas Mélange (Cabo Ortegal Complex, NW Iberia): tectonic implications for the assembly of Pangea

AbstractThe allochthonous Cabo Ortegal Complex (NW Iberian Massif) contains a ~500 m thick serpentinite‐matrix mélange located in the lowest structural position, the Somozas Mélange. The mélange occurs at the leading edge of a thick nappe pile constituted by a variety of terranes transported to the East (present‐day coordinates; NW Iberian allochthonous complexes), with continental and oceanic affinities, and represents a Variscan suture. Among other types of metaigneous (calcalkaline suite dated at 527–499 Ma) and metasedimentary blocks, it contains close‐packed pillow‐lavas and broken pillow‐breccias with a metahyaloclastitic matrix formed by muscovite–paragonite–margarite–garnet–chlorite–kyanite–hematite–epidote–quartz–rutile. Pseudosection modelling in the MnCNTKFMASHO system indicates metamorphic peak conditions of ~17.5–18 kbar and ~550 °C followed by near‐isothermal decompression. This P–T evolution indicates subduction/accretion of an arc‐derived section of peri‐Gondwanan transitional crust. Subduction below the Variscan orogenic wedge evolved to continental collision with important dextral component. Closure of the remaining oceanic peri‐Gondwanan domain and associated release of fluid led to hydration of the overlying mantle wedge and the formation of a low‐viscosity subduction channel, where return flow formed the mélange. The submarine metavolcanic rocks were deformed and detached from the subducting transitional crust and eventually incorporated into the subduction channel, where they experienced fast exhumation. Due to the cryptic nature of the high‐P metamorphism preserved in its tectonic blocks, the significance of the Somozas Mélange had remained elusive, but it is made clear here for the first time as an important tectonic boundary within the Variscan Orogen formed during the late stages of the continental convergence leading to the assembly of Pangea.

Formation age and genesis of the Gongchangling Neoarchean banded iron deposit in eastern Liaoning Province: Constraints from geochemistry and SHRIMP zircon U–Pb dating

The Gongchang iron deposit, a typical Algoma-type banded iron formation (BIF) deposit located in the Eastern Block of the North China Craton (NCC), is hosted in Neoarchean metamorphic rocks, including amphibolite, biotite leptynite, garnet biotite schist, plagioclase-hornblende schist, garnet chlorite schist, muscovite quartz schist and actinolite magnetite quartzite. Geochemical data suggest that protoliths of the amphibolites and biotite leptynite should be basalt and dacite, respectively. SHRIMP U–Pb dating of zircons from the plagioclase-hornblende schist shows that the Gongchangling iron deposit was formed at 2554±12Ma. Major elements and REE data of the ores indicate that the Gongchangling BIF precipitated from seawater, and the ultimate iron and silica origins may have been related to nearby volcanic activity. The δ18O values of quartz from the BIFs range between 9.1 and 12.9‰, suggesting that exotic high δ18O fluids were involved during metamorphism. The pyrite have δ34S values of range between −3.5‰ and 14.7‰, suggesting that mineralization were originally from metamorphism. Comparison of the mineralization age of the Gongchangling BIF with those of other local BIFs in North China and other cratons throughout the world indicates that the Gongchangling BIF formed coevally with the global BIF mineralization at ∼2.5Ga. Taken together, these data suggest that the Gongchangling iron deposit was formed in a volcanic submarine environment in which hydrothermal fluids associated with volcanic activity supplied both iron and silica for the banded iron formations.

Reprint of "The Gongchangling BIFs from the Anshan–Benxi area, NE China: Petrological–geochemical characteristics and genesis of high-grade iron ores"

The Gongchangling BIFs are located in the Anshan–Benxi area, northeastern part of the North China Craton, of which the No. 2 mining area is the largest and most typical high-grade iron ore producing area in China. In this study, the mineralogy, petrology and chemistry of iron ores (BIFs and high-grade iron ores) and their wall-rocks (amphibolite and garnet–chlorite schist) from the Gongchangling No. 2 mining area were examined to constrain the genesis of high-grade iron ore.The BIFs are composed essentially of magnetite and quartz, in addition to actinolite in a few samples; while the high-grade iron ores are mainly composed of magnetite and the quartz is relatively rare. The low contents of Al2O3, TiO2 and HFSE in the BIFs and high-grade iron ores indicate that they are marine chemical precipitates with little input of terrigenous clastic sediments. The magnetite and quartz from BIFs and high-grade iron ores exhibit the similar chemical composition. The REY patterns of BIFs and high-grade iron ores suggest their precipitation after mixing of hydrothermal component with seawater in certain proportions under relatively low oxygen level, and the obvious positive Eu anomalies indicate that high-temperature hydrothermal fluids which percolate and dissolve the submarine volcanic rock may supply the ore-forming material to the Gongchangling iron deposit.The amphibolite as interlayer of BIFs recorded the metamorphic temperature of 567±25°C, moreover, the garnet from wall-rock of high-grade iron ore exhibits the growth zoning produced during prograde metamorphism, indicating that metamorphic hydrothermal fluids played an important role in the genesis of high-grade iron ore. It was proposed that the Gongchangling iron deposit was formed in an arc-related tectonic setting at the late Neoarchean, then it underwent lower amphibolite facies metamorphism, and the high-grade iron ore is the reformed product of BIFs by metamorphic hydrothermal fluids.

The Gongchangling BIFs from the Anshan–Benxi area, NE China: Petrological–geochemical characteristics and genesis of high-grade iron ores

The Gongchangling BIFs are located in the Anshan–Benxi area, northeastern part of the North China Craton, of which the No. 2 mining area is the largest and most typical high-grade iron ore producing area in China. In this study, the mineralogy, petrology and chemistry of iron ores (BIFs and high-grade iron ores) and their wall-rocks (amphibolite and garnet–chlorite schist) from the Gongchangling No. 2 mining area were examined to constrain the genesis of high-grade iron ore.The BIFs are composed essentially of magnetite and quartz, in addition to actinolite in a few samples; while the high-grade iron ores are mainly composed of magnetite and the quartz is relatively rare. The low contents of Al2O3, TiO2 and HFSE in the BIFs and high-grade iron ores indicate that they are marine chemical precipitates with little input of terrigenous clastic sediments. The magnetite and quartz from BIFs and high-grade iron ores exhibit the similar chemical composition. The REY patterns of BIFs and high-grade iron ores suggest their precipitation after mixing of hydrothermal component with seawater in certain proportions under relatively low oxygen level, and the obvious positive Eu anomalies indicate that high-temperature hydrothermal fluids which percolate and dissolve the submarine volcanic rock may supply the ore-forming material to the Gongchangling iron deposit.The amphibolite as interlayer of BIFs recorded the metamorphic temperature of 567±25°C, moreover, the garnet from wall-rock of high-grade iron ore exhibits the growth zoning produced during prograde metamorphism, indicating that metamorphic hydrothermal fluids played an important role in the genesis of high-grade iron ore. It was proposed that the Gongchangling iron deposit was formed in an arc-related tectonic setting at the late Neoarchean, then it underwent lower amphibolite facies metamorphism, and the high-grade iron ore is the reformed product of BIFs by metamorphic hydrothermal fluids.

Metamorphosed Pb–Zn–(Ag) ores of the Keketale VMS deposit, NW China: Evidence from ore textures, fluid inclusions, geochronology and pyrite compositions

The Keketale Pb–Zn deposit is located in the Devonian volcanic-sedimentary Maizi basin of the Altay orogenic belt. The mineralization at Keketale is hosted in marbles and deformed volcanic tuffs and biotite–garnet–chlorite schists, folded into a series of overturned synclines formed in multiple deformation events. Keketale contains economic amounts of Pb (0.89Mt @ 1.51wt.%), Zn (1.94Mt @ 3.16wt.%) and Ag (650t @ 40g/t).Detailed petrographic studies have defined two main generations of sulfide development. The banded pyrite of the early Stage A is commonly stratiform, with minor galena, sphalerite and chalcopyrite. Stage B is characterized by a large amount of polymetallic sulfides including pyrrhotite, chalcopyrite, sphalerite and galena, with minor pyrite hosted in quartz veins.Three types of fluid inclusions (FIs), including mixed carbonic-aqueous (C-type), pure carbonic (PC-type) and aqueous (W-type), have been recognized in quartz of stage B. The C-type FIs have homogenization temperatures of 150–326°C and salinities of 0.2–16.6wt.% NaCl equivalent. The PC-type FIs are dominated by CO2 with minor CH4 and N2 and have initial ice-melting temperatures of −57.5 to −56.7°C, CO2 homogenization temperatures of 11–14.1°C. The W-type primary FIs were completely homogenized at temperatures of 124–359°C with salinities of 5.0–14.6wt.% NaCl equivalent. Such CO2-rich fluid inclusions are consistent with those discovered in orogenic-type deposits in the Altay area and elsewhere.Muscovite separates from the polymetallic quartz veinlets of stage B yield a well-defined 40Ar/39Ar isotopic plateau age of 259.33±2.56Ma, with an isochron age of 259.62±2.65Ma. This age is coeval with the closure of the Paleo-Asia Ocean and reactivation of the Ertix Fault system.LA-ICP-MS analyses of two generations of pyrite indicate that the banded pyrite of stage A is relatively depleted in metallic elements and contains low contents of Cu (0.39ppm), Ag (0.20ppm), Au (below the detection limits), Pb (17.43ppm) and Zn (14.38ppm); whereas the pyrite in quartz–polymetallic sulfide veinlets of the stage B is relatively rich in metallic elements, e.g., Cu (2.56ppm), Ag (3.07ppm), Au (0.01ppm), Pb (1047ppm) and Zn (1136ppm). The trace amounts of Cu, Pb, Zn, Au and Ag are interpreted to have been initially locked in the lattice of type-A pyrite, and then liberated and precipitated as micromineral inclusions with type-B pyrite during subsequent metamorphism and deformation.Two key factors are considered vital to the formation of economic ores of the Keketale Pb–Zn deposit, namely the original Devonian banded pyrite formed in a VMS system and subsequent Permian deformation and metamorphic processes that liberated Cu, Pb, Zn, Au and Ag from the lattice of type-A pyrite to form galena, sphalerite and chalcopyrite with minor muscovite in quartz veinlets. The model provides a new interpretation of VMS Pb–Zn deposit occurring in back-arc basin environments followed by collision, and new insights into the unique regional Fe–Cu–Pb–Zn–Au mineralization in the Altay orogenic belt.

Stability of chloritoid + biotite-bearing assemblages in some metapelites from the Palaeoproterozoic Singhbhum Shear Zone, eastern India and their implications

Abstract The arcuate Singhbhum Shear Zone (SSZ) of the East Indian Shield area, fringing the Archaean Singhbhum Craton, exposes a mélange-like ensemble of polymetamorphosed and highly tectonized siliciclastic sediments, mafic–ultramafic extrusive, tourmaline-rich rocks, magnetite–apatite rocks, granites and Cu–Fe–U sulphide ores of Meso- to Palaeoproterozoic age (1.5–1.77 Ga). Metapelites from two localities in the SSZ developed the enigmatic assemblage chloritoid–biotite–garnet–chlorite, which are the first reported from India and the eighth in world occurrence. Textural studies and algebraic analyses of the phase compositions in the KFMASH system indicate the operation of the reaction, chloritoid+biotite→garnet+chlorite+H 2 O and this reaction has a negative slope in pressure–temperature ( P – T ) space. Quantitative geothermobarometry and pseudosections in the NCMnKFMASHO system indicate that this reaction occurred during prograde metamorphism that culminated at 6.3±1 kbar and 490±40 °C. The stability of the chloritoid+biotite is also sensitive to the MnO content of the bulk rock, and to the chemical potential of H 2 O and oxygen ( μ H 2 O and μ O 2 , respectively), of the ambient fluid phase. Thrusting of continental crust in a collisional setting is invoked to explain the peak metamorphic temperatures and the clockwise P – T trajectory is construed from the petrological study of the chloritoid–biotite schist.

First find of cerianite in zircons from metasomatites of the Terskii greenstone belt, Baltic shield

) is one of the most scarce rareearth minerals. Because of difficulties in discovery andidentification of this mineral, E.I. Semenov proposedconsidering cerianite as an object of nano (not micro)mineralogy [1]. Since its discovery in the 1950s, nomore than 20 reliable finds of this mineral (mainly inthe form of microscopic grains and microinclusions inother minerals) have been documented. Cerianite wasdiscovered for the first time in Sudbury (Canada) inthe carbonate dikes cutting across nepheline syenites[2]. In addition to Ce, cerianite contains isomorphicadmixtures of other LREE (La, Pr, Nd). Significantamounts of Th and other elements (Xe, Cs, Pb, Rh)were found in cerianite from pegmatites of East Antarctica [3]. Supergene cerianite occurs as a weatheringproduct of bastnaesite [1] and fluocerite [4], as well asin the weathering crust of different compositions [1,5–7]. In the oceanic Fe–Mn ores, cerianite isobserved as submicron films on apatite [8]. The formation of cerianite in granites is considered to berelated to hydrothermal alteration, accompanied byfluid leaching of Ce from monazite [9]. Cerianite associates with minerals and ores of noble metals: Au, Pt(Sukhoi Log deposit [10]); Au, Pd, and REE (oremanifestations of the Polar Urals [11] and others]).Overgrowths of cerianite of a few microns thick ondiamond phase were found in the carbonado fromBrasilia [12].Cerianite is a sensitive indicator of conditions ofmineral formation, because Ce is the only compatibleREE, which in natural conditions can occur not onlyin trivalent but also in tetravalent form. Cerianite isformed only in strongly oxidizing conditions, mainlyin alkaline solutions (fluids) [13].Submicron inclusions of cerianite were found by usin the zircon from twomica garnet–chlorite–quartz–plagioclase metasomatite (sample 2508) in the Terskiigreenstone belt in the southern framing of the Imandra–Varzuga structure (southeastern part of the KolaPeninsula). Complex isotopic–geochemical study ofzircons from metasomatites of the Terskii greenstonebelt allowed us to distinguish two stages of metamorphism (2680 and 2025 Ma) and two stages of metasomatism (2600 and 1800 Ma) [14]. Practically all zircons of different ages were completely or partiallymetasomatized, which caused their enrichment inREE (especially LREE), Th, U, Sr, and Ba, and formation of flat REEs with a reduced Ce anomaly.The structural features, majorelement composition of the zircons, and the composition of foreignmineral inclusions, if present, were studied in theregime of compositional contrast on a JEOL JSM6460LV scanning electron microscope equipped withan Oxford INCA detector at the St. Petersburg MiningInstitute. U–Pb dating of zircons from metasomatiteswas conducted on a SHRIMPII ion microprobe atthe Center for Isotopic Research, Karpinskii AllRussia Research Institute of Geology. The REE and traceelement composition of zircons was analyzed at thesame grains and points as U–Pb dating, on a CamecaIMS4f ion microprobe at the Yaroslavl Branch of thePhysicoTechnical Institute of the Russian Academyof Sciences.Zircon containing cerianite differs in the heterogeneous structure. Its weakly altered right upper part(point 2.1 in Fig. 1) retains traces of faceting and primary growth zoning. At the final stage of the Svecofennian metasomatism, most zircon was transformed with the formation of “patches” of irregularreniform shape, which form a band up to 100 µm insize with a characteristic dark tint in backscatteredelectrons (BSE) (point 2.2 and inset

Classification and provenance of soapstones and garnet chlorite schist artifacts from Medieval sites of Tuscany (Central Italy): insights into the Tyrrhenian and Adriatic trade

Soapstones (talc-bearing schists) and garnet chlorite schist artifacts found in Medieval archaeological sites of Tuscany (Central Italy) were classified, in order to define provenance of the different lithotypes. In Italy and throughout the Central Europe, these greenschist facies metamorphic rocks are generally known, among the archaeologists, as the pietra ollare from the Alps. The investigated Tuscan archaeological sites are between 6 and 13th century AD and were strictly linked, in that period, to the well defined network trade running along Tyrrhenian coast. Samples come from little containers used for cooking and preserving food and showing traces of lathe manufacturing at their sidewalls. According to modal mineralogy, petrographic texture, XRD, SEM-EDS and whole rock chemistry we recognised, among the 18 studied findings, three different petrographic groups of the Alpine pietra ollare. (i) Fine grained magnesite talc schists (i.e. soapstones) from outcrops of the Central Alps located in the Valchiavenna area. (ii) Garnet chlorite schists from the Valle d'Aosta region. (iii) Amphibole talc schists (i.e. soapstones) with a provenance in the Ticino area. It is worth noting that artifacts of pietra ollare lithotypes from the Western Alps (i.e. garnet chlorite schists and amphibole talc schists) were not detected in the archaeological sites of the Middle Adriatic coast of the Central Italy, belonging to the same Medieval time interval. This emphasises that the petrographic groups of pietra ollare from the Alps spread to the south of the Po Plain according to Western and Eastern trade along the Italian Peninsula, using respectively, the Tyrrhenian and the Adriatic Sea commercial routes.

Metamorphic evolution of the northwestern Ogcheon metamorphic belt, South Korea

The Ogcheon metamorphic belt (OMB) comprises Late Proterozoic to Paleozoic metasedimentary and metavolcanic sequences which are intruded by Mesozoic granitoid plutons. To delineate the metamorphic evolution of the OMB, we have investigated mineral parageneses, metamorphic reactions and P–T conditions of pelitic and mafic schists in the Chungju area, northwestern part of the OMB. The regional metamorphic grade increases north-westward from biotite to garnet zones, although low-pressure assemblages have developed in contact aureoles around Jurassic granites. Garnet crystals show chemical zoning, typical for prograde metamorphism, with decreasing Mn, and increasing Fe and Mg from core to rim. Compositions of amphiboles constituting the common assemblage in the mafic schist are consistent with those of biotite- and garnet-zones documented in the medium-pressure metabasites. P–T conditions of the garnet zone, estimated from garnet–biotite, garnet–chlorite and amphibole–plagioclase geothermometers together with garnet–plagioclase–biotite–muscovite/quartz geobarometers, are in the range of 5–8 kbar and 520–590°C. Retrograde P–T path based on fluid inclusion studies suggests that the exhumation of the OMB has passed through the P–T range of 1–3 kbar and 350–500°C, following the isochore curves of the CO 2 inclusions. In conjunction with structural and geochronologic data, we conclude that the OMB has experienced a polycyclic P–T evolution characterized by (1) crustal thickening during the Middle Paleozoic time and (2) regional retrograde metamorphism in the Triassic. Our result further suggests that the Triassic collision belt in east-central China does not pass through the OMB.

Evolution of a kyanite‐bearing belt within a HT‐LP orogen: the case of NW Variscan Iberia

Kyanite replaces andalusite in a belt of Ordovician and Silurian pelitic rocks that form a narrow synform pinched between high‐grade antiforms in NW Variscan Iberia. Kyanite occurs across the belt in Al‐rich, black pelites in assemblages I: kyanite–chloritoid–chlorite–muscovite and II: kyanite–staurolite– chlorite–muscovite. In I, kyanite occurs in the matrix and in kyanite–muscovite aggregates that pseudomorph earlier andalusite porphyroblasts. The aggregates are found across the belt and can still be recognized in assemblage II and even in III: andalusite–staurolite–biotite–muscovite, this latter being a hornfelsic Silurian schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed inside new massive pleochroic andalusite. KFMASH and MnKFMASH pseudosections have been constructed using Thermocalc for Al‐rich and Al‐poorer compositions from the belt. Chloritoid zoning in Al‐rich rocks containing assemblage I, plus chloritoid–chlorite thermometry complemented with garnet–chlorite thermometry in Al‐poorer lithologies, mean that the path is one of increasing pressure and temperature. Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite–chloritoid– chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims. The passage from assemblage I to II implies a prograde path within the kyanite field. Assemblage III represents peak conditions, indicating a prograde staurolite‐consuming reaction across a KFMASH field, leading eventually to a locally found andalusite–biotite–muscovite hornfels. The lowest pressure stages are recorded by cordierite–biotite in Al‐poor pelites. Garnet‐bearing MnKFMASH assemblages in Al‐poorer pelites record conditions similar to assemblages II and III. The replacement of andalusite by kyanite in assemblage I is attributed to downdragging of andalusite‐bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage. Prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the Vivero fault.

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 XMn= 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 (XMn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.

Early Archaean Age for the Isua Iron Formation, West Greenland

SOME 150 km north-east of Godthaab, on the edge of the inland ice cap, a refolded syncline of mainly basic rocks containing banded ironstones and other rocks of supracrustal origin is enclosed by granitic gneisses (Fig. 1). These gneisses have yielded an Rb-Sr whole-rock isochron age of 3,700 ± 140 m.y., indistinguishable from that of the Amitsoq gneisses of the Godthaab area1. The present metamorphic grade of both the gneisses and the iron formation is upper greenschist to amphibolite facies. There is no published map of the area, but the following geological details are taken from reports by Keto2. The total succession of supracrustal rocks is 2 to 3 km thick. The lower part of the succession consists of quartzites and meta-greywackes. These are overlain by garnet–chlorite schists and banded iron formation, including both Fe-oxides and carbonates. The top of the succession consists of greenschist facies metabasites, Bridgwater et al.3 suggest that the Isua supracrustals may represent a shallow-water shelf facies.