Pumpellyites and coexisting minerals in different low-grade metamorphic facies of Liguria, Italy

Pumpellyite from four-phase assemblages (pumpellyite + epidote + prehnite + chlorite; pumpellyite + epidote + actinolite + chlorite; pumpellyite + epidote + Na-amphibole + chlorite, together with common excess phases), considered to be low variance in a CaO-(MgO + FeO)-Al₂O₃-Fe₂O₃ (+Na₂O + SiO₂+ H₂O) system, have been examined in areas which underwent metamorphism in the prehnite-pumpellyite, pumpellyite-actinolite and low-temperature blueschist facies respectively. The analysed mineral assemblages are compared for nearly constant (basaltic) chemical composition at varying metamorphic grade and for varying chemical composition (basic, intermediate, acidic) at constant metamorphic conditions (low-temperature blueschist facies). In the studied mineral assemblages, coexisting phases approached near chemical equilibrium. At constant (basaltic) bulk rock composition the MgO content of pumpellyite increases, and the X_Fe3+ of both pumpellyite and epidote decreases with increasing metamorphic grade, the Fe³⁺ being preferentially concentrated in epidote. Both pumpellyite and epidote compositions vary with the bulk rock composition at isofacial conditions; pumpellyite becomes progressively enriched in Fe and depleted in Mg from basic to intermediate and acidic bulk rock compositions. The compositional comparison of pumpellyites from high-variance (1–3 phases) assemblages in various bulk rock compositions (basic, intermediate, acidic rocks, greywackes, gabbros) shows that the compositional fields of both pumpellyite and epidote are wide and variable, broadly overlapping the compositional effects observed at varying metamorphic grade in low-variance assemblages. The intrinsic stability of both Fe- and Al-rich pumpellyites extends across the complete range of the considered metamorphic conditions. Element partitioning between coexisting phases is the main control on the mineral composition at different P-T conditions.     

Mineral assemblages and phase equilibria of metabasites from the prehnite–pumpellyite to amphibolite facies,with the Flin Flon Greenstone Belt (Manitoba) as a type example

An exceptionally well-exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan) is used to characterize the mineral assemblage evolution associated with prehnite–pumpellyite through amphibolite facies metamorphism of basalts. Data from these rocks are combined with a large literature data set to assess the ability of current thermodynamic models to reproduce natural patterns, evaluate the use of metabasic rocks at these grades to estimate pressure–temperature (P–T) conditions of metamorphism, and to comment on the metamorphic devolatilization that occurs. At Flin Flon, five major isograds (actinolite-in, prehnite- and pumpellyite-out, hornblende-in, oligoclase-in, and actinolite-out) collectively represent passage from prehnite–pumpellyite to lower amphibolite facies conditions. The evolution in mineral assemblages occurs in two narrow (~1,000 m) zones: the prehnite–pumpellyite to greenschist facies (PP-GS) transition and greenschist to amphibolite facies (GS-AM) transition. Across the GS-AM transition, significant increases in the hornblende and oligoclase proportions occur at the expense of actinolite, albite, chlorite, and titanite, whereas there is little change in the proportions of epidote. The majority of this mineral transformation occurs above the oligoclase-in isograd within the hornblende–actinolite–oligoclase zone. Comparison with thermodynamic modelling results suggests data set 5 (DS5) of Holland and Powell (1998, Journal of Metamorphic Geology, 16 (3):309–343) and associated activity–composition (a–x) models is generally successful in reproducing natural observations, whereas data set 6 (DS6) (Holland & Powell, 2011, Journal of Metamorphic Geology, 29 (3):333–383) and associated a–x models fail to reproduce the observed mineral isograds and compositions. When the data from Flin Flon are combined with data from the literature, two main pressure-sensitive facies series for metabasites are revealed, based on prograde passage below or above a hornblende–albite bathograd at ~3.3 kbar: a low-pressure ‘actinolite–oligoclase type’ facies series, characterized by the appearance of oligoclase before hornblende, and a moderate- to high-pressure ‘hornblende–albite type’ facies series, characterized by the appearance of hornblende before oligoclase. Concerning the PP-GS transition, the mineral assemblage evolution in Flin Flon suggests it occurs over a small zone (<1,000 m), in which assemblages containing true transitional assemblages (prehnite and/or pumpellyite coexisting with actinolite) are rare. This contrasts with thermodynamic modelling, using either DS5 or DS6, which predicts a wide PP-GS transition involving the progressive appearance of epidote and actinolite and disappearance of pumpellyite and prehnite. Patterns of mineral assemblages and thermodynamic modelling suggest a useful bathograd (‘CHEPPAQ bathograd’), separating prehnite–pumpellyite-bearing assemblages at low pressures and pumpellyite–actinolite-bearing assemblages at higher pressures, occurs at ~2.3 to 2.6 kbar. Observations from the Flin Flon sequence suggests devolatilization across the GS-AM transition (average: ~1.8 wt% H₂O) occurs over a very narrow interval within the actinolite–hornblende–oligoclase zone, associated with the loss of >75% of the total chlorite. By contrast, modelling of the GS-AM transition zone predicts more progressive dehydration of ~2 wt% H₂O over a >50°C interval. Observations from the field suggest devolatilization across the PP-GS transition occurs over a very narrow interval given the rarity of transitional assemblages. Modelling suggests fluid release of 1.0–1.4 wt% resulting from prehnite breakdown over a ~10°C interval. This fluid may not be entirely lost from the rock package due to involvement in the hydration of igneous mineralogy across the PP-GS transition as observed in the Flin Flon sequence.     

A Petrographic and Mineralogical Study of Volcanic Rocks from the Mayaxueshan Area, North Qilian Fold Belt, NW China

The Ordovician volcanic rocks in the Mayaxueshan area have been pervasively altered or metamorphosed and contain abundant secondary minerals such as albite, chlorite, epidote, prehnite, pumpellyite, actinolite, titanite, quartz, and/or calcite. They were denoted as spilites or spilitic rocks in terms of their petrographic features and mineral assemblages. The metamorphic grades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. This indicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area have formed as a result of Caledonian regional metamorphism. We suggest that the previously denoted spilitic rocks or altered volcanic rocks should be re-denoted as metabasalts or metabasaltic rocks. The metamorphic grade of the volcanic rocks increases with their age: prehnite-pumpellyite facies for the upper part of the Middle Ordovician volcanic rocks, prehnite-pumpeilyite to lower greenschist facies for the lower part of the Middle Ordovician vol     

Tectonothermal history of the western Lachlan Fold Belt, Australia: insights from white mica studies

Detailed b lattice parameter and illite crystallinity (IC) studies of K-white micas in slates from the Stawell and Ballarat-Bendigo Zones (SZ, BBZ) in the western Lachlan Fold Belt of Victoria, Australia, reveal a metamorphic pattern characterized by regional metamorphism associated with crustal thickening and younger contact metamorphism accompanied by deformation. The IC data indicate that rocks regionally metamorphosed prior to the intrusion of the Early and Late Devonian granitoids, vary in grade from epizonal (greenschist facies) to diagenetic (zeolite facies) and that most are of epizonal to anchizonal (prehnite–pumpellyite facies) grade. In the BBZ, a decrease in grade from west to east occurs. Across fault zones, IC values show little change, indicating that limited vertical displacement has occurred. This is in accord with the thin skinned deformation model proposed for the western Lachlan Fold Belt. The b lattice parameters (x=9.022 Å; n=137; σ_n=0.009) indicate baric conditions intermediate between those of New Hampshire (P=Al₂SiO₅ triple point) and Otago (intermediate P ). Thus, a moderately low geothermal gradient existed 450–430 Ma ago, when these rocks were deformed. K_D Fe/Mg (actinolite)/Fe/Mg (chlorite) values (0.52–0.70) obtained from coexisting actinolite and chlorite in metabasites from fault zones support the moderately high-P (c. 4 kbar) metamorphism suggested by the b cell parameter values. The metamorphic conditions indicated by these data are contrary to the low-P/high-T conditions proposed by previous authors, who inferred an intimate association between deformation, granitoid intrusion and gold mineralization. The b lattice parameter of white micas in slates adjacent to Early Devonian (c. 400 Ma) granitoids with schist bearing aureoles in the north-eastern part of the BBZ (x=9.002 Å; n=27; σ_n=0.007), indicate pressures in the order of c. 2.5 kbar which are in accord with those obtained from andalusite–cordierite and zoisite–garnet bearing assemblages observed in the higher grade metapelitic and calcareous rocks. This contrasts with the higher pressure (c. 4 kbar) existing during regional metamorphism and implies that c. 6.5–8 km of metasedimentary rocks in the BBZ were removed before the emplacement of the Early Devonian granitoids. Metamorphic assemblages in hornfelses associated with Late Devonian granitoids indicate a further 5–6 km of metasediment were removed in the next 40 Ma prior to their emplacement. This study shows the value of white mica studies in elucidating the tectonothermal history of a low-grade metamorphic terrane dominated by metapelitic rocks.     

Pumpellyite–actinolite facies of the Sanbagawa metamorphism

The pumpellyite–actinolite facies proposed by Hashimoto is defined by the common occurrence of the pumpellyite–actinolite assemblage in basic schists. It can help characterize the paragenesis of basic and intermediate bulk compositions, which are common constituents of various low-grade metamorphic areas. The dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite–actinolite and prehnite–pumpellyite facies in P–T space. In the Sanbagawa belt in Japan, the mineral parageneses of hematite-bearing and -free basic schists, as well as pelitic schists have been well documented. The higher temperature limit of this facies is defined by the disappearance of the pumpellyite+epidote+actinolite+chlorite assemblage in hematite-free basic schists with X_Fe3+ of epidote around 0.20–0.25 and the appearance of epidote+actinolite+chlorite assemblage with X^Ep_Fe3+≤0.20. In hematite-bearing basic schists, there is a continuous change of paragenesis to higher grade, epidote–glaucophane or epidote–blueschist facies. In pelitic schists, the albite+lawsonite+chlorite assemblage does occur but only rarely, and its assemblage cannot be used to determine the regional thermal structure. The lower temperature equivalence of the pumpellyite–actinolite assemblage is not observed in the field. The Mikabu Greenstone complex and the northern margin of the Chichibu complex, which are located to the south of the Sanbagawa belt, are characterized by clinopyroxene+chlorite or lawsonite+actinolite assemblages, which are lower temperature assemblages than the pumpellyite+actinolite assemblage. These three metamorphic complexes belong to the same subduction-metamorphic complex. The pumpellyite–actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes     

The greenschist facies in part of eastern Otago,New Zealand

Rocks of the greenschist facies in eastern Otago, New Zealand, have been investigated in an area some thirteen to sixteen kilometers wide and sixty-five kilometers long extending northeastwards approximately normal to the boundary of the schist with lower grade rocks. Quartzo-feldspathic schists predominate but greenschists and metacherts occur sporadically throughout the area. At the southwestern edge of the area schists are in the chlorite zone, slightly above the high-grade limit of pumpellyite. Metamorphic grade increases toward the northeast into the biotite zone which occupies about half the terrane studied and is believed to be everywhere little advanced in metamorphic grade past that of the biotite isograd. Some 130 mineral specimens have been partially analysed with the electron probe. Results derived from these data as well as other mineralogical investigation are as follows: Albite contains a maximum of 1% anorthite plus orthoclase in epidote-bearing rocks from all parts of the area.Compositions of epidotes range from 12% to 32% Ca₂Fe₃(SiO₄)₃(OH), but most lie between 15% and 20%, a compositional field thought by Strens (1965) and Holdaway (1965) to occupy a miscibility gap in the epidote series. Zoning in some epidotes suggests a history of early growth of small, sparse iron-rich epidotes, and later growth of relatively large amounts of iron-poor epidote probably caused by breakdown of prehnite and/or pumpellyite. Muscovites vary widely in celadonite content; but the composition shows little if any dependence on metamorphic grade within the area studied. Most tend to be celadonite-rich, and in this respect are similar in composition to muscovites from rocks of the glaucophane-schist facies.Chlorites range widely in Mg/Fe; but Al/Mg+Fe is relatively uniform. Chlorites associated with actinolite tend to have higher Mg/Fe than those associated with stilpnomelane. Following the classification of Foster (1962) most chlorites are brunsvigite and some are ripidolite. Textural and chemical relations between biotite and coexisting minerals demonstrate that, contrary to some previous suggestions, biotite is not a relict mineral. An alteration product of chlorite bears strong resemblance to biotite, and previous misidentification of this mineral as biotite has caused much confusion regarding the distribution and metamorphic significance of biotite in Otago schists.An attempt to determine the reaction producing biotite is not successful. Possibly biotitebearing rocks have slightly higher biotite component than rocks of the chlorite zone. All newly formed amphibole found in eastern Otago is pale green, Al- and Na-poor actinolite. One of the chemical conditions necessary for the formation of actinolite in schists of eastern Otago is a relatively high Mg/Fe+Al ratio.Stilpnomelane is an integral part of assemblages in which it occurs, being developed under conditions of relatively low and in rocks with a high Fe/Mg + Al ratio. The present highly oxidized state of all stilpnomelane observed in this study is probably not a primary feature of the mineral but developed after metamorphism.Porphyroblastic garnets are accessory constituents in about half the quartzo-feldspathic schists collected from the biotite zone but are extremely rare in specimens of the same lithology from the chlorite zone. Either a garnet-producing reaction began in quartzo-feldspathic schists at about the biotite isograd, or rocks of biotite zone tend to have slightly higher garnet component than those of the chlorite zone. Composition of the garnets ranges widely, extremes being: 77% spess., 18% gross., 5% alm.; 25% spess., 50% gross., 25% alm.; 15% spess., 30% gross., 55% alm. Most of the variation in composition is controlled by host rock composition, but garnets at higher grade tend to have lower spessartine content. The garnets are zoned; generally Mn decreases and Fe increases from core to rim.For the most part chemical equilibrium among different grains and minerals was closely approached over distances of at least a few millimeters. However, profound disequilibrium exists within some individual grains, such as a zoned garnet which over a distance of only 15 microns ranges in spessartine content from 77% in the core to 35% on the rim.This report is a condensed version of part of the author's Ph.D. thesis (Brown, 1966), University of California, Berkeley.     

The effect of Mn on mineral stability in metapelites revisited: new a–x relations for manganese‐bearing minerals

The a–x relations recently presented in White et al. ( 2014 , Journal of Metamorphic Geology, 32, 261–286) are extended to include MnO. This provides a set of internally consistent a–x relations for metapelitic rocks in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (MnNCKFMASHTO) system. The mixing parameters for the Mn‐bearing minerals were estimated using the micro‐? approach of Powell et al. ( 2014 , Journal of Metamorphic Geology, 32, 245–260). Then the Mn‐end‐member thermodynamic properties were calibrated using a database of co‐existing minerals involving literature data from rocks and from experiments on natural materials. Mn‐end‐members were calibrated for orthopyroxene, cordierite, staurolite, chloritoid, chlorite, biotite, ilmenite and hematite, assuming known properties for the garnet end‐member spessartine. The addition of MnO to phase diagram calculations results in a marked expansion of the stability of garnet‐bearing assemblages. At greenschist facies conditions garnet stability is extended down temperature. At amphibolite facies conditions, the garnet‐in boundary shifts to lower pressure. While the addition of MnO greatly influences the stability of garnet, it has relatively little effect on the stability of other common metapelitic minerals, with the resultant diagrams being topologically very similar to those calculated without MnO. Furthermore, the addition of MnO in the amounts measured in most metapelites has only a small effect on the mode of garnet, with calculated garnet modes remaining smaller than 1% in the P–T range outside its predicted Mn‐free P–T range.     

P–T–X controls on phase stability and composition in LTMP metabasite rocks – a thermodynamic evaluation

The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H₂O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe₂O₃ bulk contents on the stability of these phases is illustrated using P–T and P–X phase diagrams. This approach allowed P–T conditions of ～4.0 kbar and ～260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe₂O₃ contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous–Triassic New England Orogen. P–T and P–X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe₂O₃‐saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO‐rich (10.30 wt%) rocks. With intermediate Fe₂O₃ (～3.50 wt%) and CaO (～8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P–T conditions. For Fe₂O₃‐saturated rocks, compositional isopleths for sodic amphibole (Al³⁺ and Fe³⁺ on the M2 site), epidote (Fe³⁺/Fe³⁺ + Al³⁺) and chlorite (Fe²⁺/Fe²⁺ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al³⁺ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher‐P stability fields where albite is absent, Fe²⁺‐Mg ratios in chlorite may be dependent on pressure. The Fe³⁺/Al and Fe²⁺/Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages.     

Mineralogy and petrology of a piemontite-bearing schist, western Otago, New Zealand

Abstract. Pink piemontite-spessartine-bearing and grey-green spessartine-bearing manganiferous quartzose schists derived from siliceous pelagites, and green quartzofeldspathic schists, are described from the greenschist facies of the Haast Schist terrane, near Arrow Junction, western Otago. Electron microprobe data are reported for sphene, spessartine-rich garnet, manganoan epidote, piemontite, tourmaline, phengitic muscovite, chlorite, albite, haematite, rutile, manganoan calcite and chalcopyrite. Metamorphism occurred at about 6.4kbar, 400°C. X_co2 was above the quartz-rutile-calcite-sphene buffer (X_co2± 0.02) throughout the recorded metamorphic history of the piemontite schists. It dropped from above to below this critical buffering value in a spessartine-rich schist and it was close to or below the buffering value in the quartzofeldspathic schists. Production of piemontite required high f_O2, believed to be inherited from MnO_x in the parent pelagite. Substantial loss of O₂ (e.g. minimum of 0.19% by weight in one rock) during diagenesis and/or metamorphism is inferred. In the grey-green schists this inhibited piemontite formation. Slight loss of O₂ and Ca²⁺ accompanied minor late-stage replacement of piemontite by second generation spessartine. Observed zoning and mineral replacements indicate rise of temperature, drop in pressure, or invasion by solutions of lower f_O2 and X_CO2 equilibrated with surrounding schists. The detailed chemistry of the minerals studied correlates with available Mn and with bulk-rock (Fe³⁺ x 100)/(Fe²⁺+ Fe³⁺). The oxidation ratio ranges from 24 in average green quartzofeldspathic schist, through 78 in average grey-green manganiferous quartzose schist, to almost 100 in some piemontite-bearing schists. As Fe²⁺ gives way to Fe³⁺, Mg/Fe ratios tend to rise in chlorite, phengite, tourmaline, spessartine, and calcite, Mn increases and Ti decreases in haematite, Mn increases in spessartine and calcite, and Fe increases in rutile. Available divalent cations are depleted relative to Al; chlorite is more aluminous, and phengite more paragonitic than in typical Haast schists.     

Dehydration veins in diagenetic and very-low-grade metamorphic rocks: features of the crustal seismogenic zone and their significance to mineral facies

Abstract Declining temperatures during decay of a hydrothermal system, or during uplift and erosion, tend to result in veins involving progressive hydration reactions, e.g. veins with laumontite cutting prehnitepumpellyite facies rocks, and stilbite veins cutting laumontite veins. In contrast, examples are described of analcime replacement of heulandite along fractures in heulanditized vitric tuff, of replacement of analcime by albite along fractures in quartz-analcime rock, of joint-controlled replacement of heulandite in tuff by laumontite + quartz + (Na, K)-feldspars, of replacement of laumontite by prehnite + quartz along fractures in alumontitized vitric tuff, and of laumontitebearing feldspathic sandstones cut by vein assemblages of quartz and prehnite ° Calcite. The vein mineral assemblage, sometimes with pumpellyite and/or epidote in the prehnite-bearing veins, tends to spread as a zone of dehydration into the adjacent country rock. Except perhaps for albite replacement of analcime, and for laumontite replacement of heulandite, these open-system reactions involve cation activity ratios in the fluid. All involve dehydration. They are favoured by an increase in temperature, and except under certain situations where P-T equilibrium curves have negative slopes, are favoured by a fall in P_H2O. Evidence indicates that in at least some cases the triggering mechanism was a drop in P_H2O; this may be a widespread phenomenon associated with brittle fracture in the seismogenic upper crust. This may cause fluid pressure to drop from values approaching lithostatic to nearer hydrostatic, and equilibrium may be displaced to yield a less hydrous assemblage that appears as a dehydration vein and vein verge. The dehydration vein assemblage may be diagnostic of a higher grade mineral facies and adds to the mineral complexity attributable to varying permeabilities and fluid pressures in upper crustal strata. Mineral facies are likely to be more uniformly distributed in higher grade rocks from below the brittle-ductile transition zone. Reactions involving complex solid solutions are inappropriate as facies boundaries.     

Thermal modelling in shallow subduction: an application to low P/T metamorphism of the Cretaceous Shimanto Accretionary Complex, Japan

This paper presents the results of numerical modelling to investigate the regional occurrence of prehnite‐bearing metamorphic rocks at shallow levels in subduction zones. The modelling assumes a simple geometrical configuration in which the thermal structure in a prism is controlled by boundary conditions at the top and base of the prism. It is expected that the predominant metamorphic facies in a prism will change with decreasing age of the descending slab. The results of thermal modelling show that the facies boundary between pumpellyite–actinolite and prehnite–actinolite facies (including prehnite–pumpellyite facies) overlaps with an array of P–T conditions in the prism when the age of a descending slab is younger than 10 Myr. This implies that the change of the predominant metamorphic facies from pumpellyite–actinolite to prehnite–actinolite facies will switch drastically. The critical age of the switch depends on subduction parameters. In particular, the critical age of the descending slab is <5 Myr in the case of no shear heating, with a subduction rate of v=75–200 mm y^?1 and subduction angle of θ=5–15°. For shear heating (constant shear stress=30 MPa) with a subduction rate of v=75 mm y^?1 and subduction angle of θ=10° the critical age is 7 Myr. To test this switching behaviour in the development of prehnite–actinolite facies in the prism, petrologic data from the Cretaceous Shimanto Accretionary Complex (CSAC) in Kyushu, Japan were compiled. The regional occurrence and mineral assemblages of prehnite‐bearing metamorphic rocks suggest that the most of CSAC was metamorphosed under prehnite–actinolite facies. This conclusion is consistent with subduction of a young, hot slab, as has been proposed based on other geological observations. This suggests that the regional extent of the prehnite–actinolite facies metamorphic rocks may be a unique evidence for the subduction of a young, hot slab.     

Low‐grade metamorphism of mafic lavas,Upper Permian Broughton Formation,Sydney Basin

Coexisting primary minerals and hydrous alteration minerals in basalt lavas of the Upper Permian Broughton Formation of the Sydney Basin are indicative of the involvement of a hydrothermal fluid phase during low‐grade metamorphism. Variation and zonation of alteration phases in vesicles and vugs indicate that the alteration minerals developed in response to several episodes of precipitation, with early CO₂‐rich fluids producing assemblages rich in calcite and chlorite‐smectite while later CO₂‐poor fluids precipitated Ca‐zeolites, prehnite and pumpellyite. Vesicular parts of flows typically show much higher contents of alteration minerals than more massive parts of the same flow, but no systematic increase in either the style or intensity of alteration with increasing depth in the lava pile is evident. The presence of Ca‐zeolites, prehnite, pumpellyite and rare epidote suggests uppermost zeolite facies to lowermost prehnite‐pumpellyite facies metamorphism. Stability relationships of the metamorphic phases based on experimental and theoretical studies, used in conjunction with measured parameters for modern geothermal systems, indicate a peak metamorphic temperature of ～200–230°C while the extant stratigraphy indicates that the maximum depth of burial was ～ 1200 m. Alteration developed in response to circulation of hot, aqueous fluids generated by thermal convection cells associated with the Permian lavas and/or a large buried intrusion.     

Comparison of diagenetic and low-grade metamorphic evolution of chlorite in associated metapelites and metabasites: an integrated TEM and XRD study

Chlorite is a common sheet silicate that occurs in various lithologies over a wide grade range involving diagenesis and low‐grade metamorphism. Thus, the reaction progress of chlorite offers a unique opportunity for direct correlation of zonal classification of metasedimentary rocks based on illite crystallinity with metabasite mineral facies. To provide such correlation, chlorite crystallinity indices, apparent mean crystallite sizes and lattice strains, crystallite size distributions and compositions of chlorite from coexisting metapelites and metabasites were determined by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), analytical electron microscopy (AEM) and electron microprobe (EMP) methods. Samples were from Palaeozoic and Mesozoic formations of the Bükkium (innermost Western Carpathians, Hungary) that underwent Alpine (Cretaceous) orogenic metamorphism. Metapelites range in grade from late diagenesis to epizone, whereas metabasites vary from prehnite–pumpellyite through pumpellyite–actinolite to greenschist facies. Despite significant differences in composition, mineral assemblages and textures, reaction progress, as measured in part by chlorite crystallinity, in metapelites paralleled that in metabasites. Chlorite crystallinity and mean crystallite size increase and the proportion of mixed layers in chlorite decreases, whereas the calculated lattice strain does not change significantly with increasing metamorphic grade. Similar trends, but (especially at higher grades) significant differences, were found in mean crystallite size values using various methods for XRD line profile analyses. The increase in crystallite size with increasing grade was demonstrated also by direct TEM measurements on ion‐milled whole‐rock samples, but with a larger scatter of data at higher grades. In spite of the different kinds of mixed layering in chlorite (Mg‐rich smectitic, mostly random, local corrensite‐like units in metabasites, and Fe‐rich berthierine and dioctahedral smectite in metapelites), XRD‐calculated and TEM‐measured parameters were found to be reliable tools for measuring reaction progress and metamorphic grade of the same degree in both lithotypes.     

Prehnite-Pumpellyite to Greenschist Fades Transition in the Karmutsen Metabasites, Vancouver Island, B.C.

Mineral paragenescs in the prehnite-pumpellyite to greenschistfades transition of the Karmutsen metabasites are markedly differentbetween amygdule and matrix, indicating that the size of equilibriumdomain is very small. Characteristic amygdule assemblages (+chlorite + quartz) vary from: (1) prehnite + pumpeUyite + epidote,prehnite + pumpellyite + calcite, and pumpellyite + epidote+ calcite for the prehnite-pumpellyite facies; through (2) calcite+ epidote + prehnite or pumpellyite for the transition zone;to (3) actinolite + epidote + calrite for the greenschist facies.Actinolite first appears in the matrix of the transition zone.Na-rich wairakites containing rare analcime inclusions coexistwith epidote or Al-rich pumpellyite in one prehnite-pumpellyitefacies sample. Phase relations and compositions of these wairakite-bearingassemblages further suggest that pumpellyite may have a compositionalgap between 0.10 and 0.15 X_Fe?. Although the facies boundaries are gradational due to the multi-varianceof the assemblages, several transition equilibria are establishedin the amygdule assemblages. At low X_co2, pumpellyite disappearsprior to prehnite by a discontinuous-type reaction, pumpellyite+ quartz + CO₂ = prehnite + epidote + calcite + chlorite + H₂O,whereas prehnite disappears by a continuous-type reaction, prehnite+ CO₂ = calcite + epidote + quartz-l-H₂O. On the other hand,at higher X_CO2 a prehnite-out reaction, prehnite + chlorite+ H₂O + CO₂ = calcite + pumpellyite + quartz, precedes a pumpellyiteoutreaction, pumpellyite + CO₂ = calcite + epidote + chlorite +quartz + H₂O. The first appearance of the greenschist faciesassemblages is defined at both low and high XCOj by a reaction,calcite + chlorite + quartz = epidote + actinolite+ H₂O + CO₂.Thus, these transition equilibria are highly dependent on bothX_Fe³⁺ + of Ca-Al silicates and X_H20 of the fluid phase. Phaseequilibria together with the compositional data of Ca-Al silicatesindicate that the prehnite-pumpellyite to greenschist faciestransition for the Karmutsen metabasites occurred at approximately1.7 kb and 300?C, and at very low X_co2, probably far less than0.1.     

Reactions leading to the formation and breakdown of stilpnomelane in the Otago Schist, New Zealand

Semi‐pelitic rocks ranging in grade from the prehnite–pumpellyite to the greenschist facies from south‐eastern Otago, New Zealand, have been investigated in order to evaluate the reactions leading to formation and breakdown of stilpnomelane. Detrital grains of mica and chlorite along with fine‐grained authigenic illite and chlorite occur in lower‐grade rocks with compactional fabric parallel to bedding. At higher grades, detrital grains have undergone dissolution, and metamorphic phyllosilicates have crystallized with preferred orientation (sub)parallel to bedding, leading to slaty cleavage. Stilpnomelane is found in metapelites of the pumpellyite–actinolite facies and the chlorite zone of the greenschist facies, but only rarely in the biotite zone of the greenschist facies. Illite or phengite is ubiquitous, whereas chlorite occurs only rarely with stilpnomelane upgrade of the pumpellyite‐out isograd. Chemical and textural relationships suggest that stilpnomelane formed from chlorite, phengite, quartz, K‐feldspar and iron oxides. Stilpnomelane was produced by grain‐boundary replacement of chlorite and by precipitation from solution, overprinting earlier textures. Some relict 14 Å chlorite layers are observed by TEM to be in the process of transforming to 12 Å stilpnomelane layers. The AEM analyses show that Fe is strongly partitioned over Mg into stilpnomelane relative to chlorite (K_D≈2.5) and into chlorite relative to phengite (K_D≈1.9). Modified A′FM diagrams, projected from the measured phengite composition rather than from ideal KAl₃Si₃O₁₀(OH)₂, are used to elucidate reactions among chlorite, stilpnomelane, phengite and biotite. In addition to pressure, temperature and bulk rock composition, the stilpnomelane‐in isograd is controlled by variations in K, Fe³⁺/Fe²⁺, O/OH and H₂O contents, and the locus of the isograd is expected to vary in rocks of different oxidation states and permeabilities. Biotite, quartz and less phengitic muscovite form from stilpnomelane, chlorite and phengite in the biotite zone. Projection of bulk rock compositions from phengite, NaAlO₂, SiO₂ and H₂O reveals that they lie close to the polyhedra defined by the A′FM minerals and albite. Other extended A′FM diagrams, such as one projected from phengite, NaAlO₂, CaAl₂O₄, SiO₂ and H₂O, may prove useful in the evaluation of other low‐grade assemblages.     

Prehnite–pumpellyite to greenschist facies transition, Smartville Complex, near Auburn, California

The Smartville Complex is a late Jurassic, rifted volcanic arc in the northern Sierra Nevada, California. Near Auburn, California, it consists of a lower volcanic unit, dominated by basaltic flows, and an upper volcanic unit of andesitic volcaniclastic rocks, both of which have been intruded by dykes and irregular bodies of diabase. These rocks contain relict igneous minerals, and the metamorphic minerals albite, chlorite, quartz, pumpellyite, prehnite, epidote, amphibole, titanite, garnet, biotite, K-feldspar, white mica, calcite, and sulphide and oxide minerals.
Prehnite–pumpellyite (PrP), prehnite–actinolite (PrA), and greenschist (GS) zones have been identified. The pumpellyite-out isograd separates the PrP and PrA zones, and the prehnite-out isograd separates the PrA and GS zones. The minerals Ab + Qtz + Mt + Tn are common to most assemblages in all three zones. The MgO/(MgO + FeO) ratio of the effective bulk composition has an important and systematic effect on the observed mineral assemblages in the PrP zone. Prehnite-bearing assemblages contain the additional minerals, Pmp + Amp + Ep + Chl in MgO-rich rocks, and either Pmp + Ep + Chl or Amp + Ep + Chl in less magnesian rocks. Subcalcic to calcic amphibole is common in the PrP zone. The mineral assemblage Prh + Act + Ep + Chl, without Pmp, characterizes the PrA zone, and the mineral assemblage Act + Ep + Chl, without Prh or Pmp, characterizes the GS zone. The disappearance of pumpellyite and prehnite occurred by continuous reactions.
The sequence of mineral assemblages was produced by burial metamorphism at P–T conditions of 300° 50°C at approximately 2.5 ± 0.5 kbar. During metamorphism, the composition of the fluid phase was nearly 100% H₂O and the oxygen fugacity was between the hematite–magnetite and quartz–fayalite–magnetite buffers.     

Die Glaukophangesteine,ihre stofflichen Äquivalente und Uniwandlungsprodukte in Nordcalabrien (Süditalien)

In the southern Apennin (= northern part of the region dealt with) and the Coasta Chain (= southern part) there are metabasalts wich are classified in the northern part as:

1. Glaucophane rocks of the albite-lawsonite-glaucophane-subfacies with the assemblage glaucophane + pumpellyite + lawsonite ±albite ±aragonite ±muscovite (7 rock analyses, 8 mineral analyses). These rocks are conceived as relics of an older burial metamorphism.
2. Rocks with pumpellyite and chlorite or also chlorite alone, that are interpreted as reaction rims between the metastable glaucophane rocks and the country rock (phyllites, quartzites). The assemblages pumpellyite + chlorite and chlorite alone are to be found (2 rock analyses and 2 mineral analyses).
3. Rocks with lawsonite and/or epidote belong to the same mineral facies as the country rock: a facies similar to the greenschist facies (called “lawsonite-albite-chlorite-subfacies”) which is characterized by the assemblages lawsonite + albite + chlorite ±calcite and also epidote ±lawsonite + albite + chlorite ± muscovite. These types are attributed to a younger dynamo-metamorphism (2 rock analyses).

In the southern part, the metabasalts can be found only as rocks with epidote and/or lawsonite, a metamorphism with more than one event cannot be proved petrologically (3 rock analyses). Equations of the observed mineral reactions are given. The transitions of one facies into another are represented in the pseudo-quaternary system Al₂O₃-CaO-Na₂O · Al₂O₃-2 Fe₂O₃ + FeO + MnO + MgO-(H₂O). The pressure-temperature conditions are estimated on the basis of published experimental data (300° C and 6–7 kb for the glaucophane rocks; 400° C and about 6 kb for the rocks with lawsonite and/or epidote) and are compared with geologic facts.     

Very low grade metamorphism of the Taveyanne formation of western Switzerland

The andesitic early Oligocene Taveyanne metagreywacke of the Helvetic nappes of western Switzerland shows an increase of metamorphic grade from zeolite facies through lower greenschist facies. Electron microprobe analysis, fluid inclusion thermometry, stable isotope analysis, coal rank, illite and chlorite crystallinity and thermodynamic calculations were carried out to determine metamorphic conditions. Evaluation of all techniques used in this study suggest that only combinations of different parameters yield reliable information to constrain very low-grade metamorphic conditions. Electron microprobe analyses are presented for actinolite, chlorite, epidote, phengite, laumontite, prehnite, pumpellyite, and titanite. With increasing metamorphic grade, chlorite is enriched in tetrahedral Al, pumpellyite becomes poorer in Fe_tot and more homogeneous in chemical composition, and titanite tends to incorporate Ti at the expense of Al and Fe³⁺. Metamorphic P-T conditions were determined by a combination of fluid inclusion microthermobarometry, stable isotope thermometry on quartz-calcite veins, chlorite “geothermometry” and thermodynamic calculations. Peak temperatures range from 210–250 °C for zeolite facies to 270–300 °C for prehnite-pumpellyite facies to 300–360 °C for pumpellyite-actinolite facies. An evaluation of 289 chlorite analyses indicates that the tetrahedral Al content is negatively correlated with the saponite component. Temperatures derived from chlorite “geothermometry” match maximum temperature conditions mentioned above. Illite crystallinity data for shales and slates intercalated with the Taveyanne metagreywacke indicate that the diagenetic zone correlates with the zeolite facies, the upper anchizone with the prehnite-pumpellyite facies, and the lower epizone with the pumpellyite-actinolite facies. A comparison of coal rank and illite crystallinity data (n=12,r=0.91) yielded R _max values of 2.9 and 5.5% for the lower and upper boundary of the anchizone, respectively. Received: 2 August 1996 / Accepted: 16 July 1997     

Deformation‐driven,regional‐scale metasomatism in the Hamersley Basin,Western Australia

Mafic volcanic rocks of the Fortescue Group form the lowermost stratigraphic unit of the 100,000 km² Hamersley Basin on the southern margin of the Archean Pilbara Craton, Western Australia. A regional burial metamorphic gradient extends across the basin from prehnite–pumpellyite facies in the north to greenschist facies in the south. Phase equilibria modelling of mafic rocks with the computer program thermocalc , in subsets of the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–Fe₂O₃, successfully reproduces observed metamorphic mineral assemblages, giving conditions of ~210 °C, 2 kbar in the north and 335 °C, 3.2 kbar in the south. Superimposed on this metamorphic gradient, regional‐scale metasomatism in the Fortescue Group progressively produces a suite of prehnite‐bearing and pumpellyite–quartz/epidote–quartz‐dominated assemblages. Further modelling of variably metasomatized samples consistently estimates conditions of 260–280 °C, 2.5–3 kbar across the basin. All modelled samples were likely metasomatized at approximately the same structural level, following regional deformation during the Ophthalmian orogeny. Folding during the Ophthalmian orogeny produced topographic and/or tectonic driving forces for regional‐scale fluid flow, pushing metasomatic fluid northwards across the Hamersley Basin. These new phase equilibria calculations support previous interpretations linking the Ophthalmian orogeny, fluid flow and upgrading of Hamersley iron ore deposits. We propose an extension of this fluid flow to the Fortescue Group, the metasomatism of which may have contributed a source of Fe to the Hamersley iron ore deposits.     

Seafloor hydrothermal alteration at an Archaean mid-ocean ridge

A hydrothermally metamorphosed/altered greenstone complex capped by bedded cherts exposed in the North Pole, Pilbara Carton, Western Australia, is interpreted as an accretionary complex. It is distinctive in being characterised by both duplex structure and an oceanic crust stratigraphy. This complex is shown to represent an Archaean upper oceanic crust with a mid‐ocean ridge hydrothermal metamorphism that increases in grade stratigraphically downward. Three mineral zones have been defined; Zone A of the zeolite facies, the prehnite‐pumpellyite facies or the lower‐greenschist facies at high‐X_CO2 condition, Zone B of the greenschist facies, and Zone C of the greenschist/amphibolite transition facies. In Zone A metabasites, Ca‐Al silicates including Ca‐zeolites, prehnite and pumpellyite are absent and epidote/clinozoisite is extremely rare. Instead, abundant carbonates are present with chlorite suggesting high‐X_CO2 composition in the fluid. On the other hand, in Zones B and C metabasites, where Ca‐amphibole + epidote/clinozoisite + chlorite + Ca‐Na plagioclase are the dominant assemblages, carbonate is not identified. The metamorphic conditions boundary of Zones B/C were estimated to be about 350 °C at a pressure of <0.5 kbar. Fluid compositions coexisting with Archaean greenstones at the transition between Zones B and C were estimated by thermodynamic calculation in the CaFMASCH system (T = 350–370 °C, P = 150–1000 bar) at X_CO2 of 0.012–0.140, such values are higher than present‐day vent fluids collected near mid‐ocean ridges with low‐X_CO2 values, up to 0.005. The Archaean seawater depth at the mid‐ocean ridge was estimated to be 1600 m at X_CO2 = 0.06 using a depth‐to‐boiling point curve for a fluid. The carbonation due to high‐X_CO2 hydrothermal fluids occurred near the ridge‐axis before or was coincident with ridge metamorphism.