Petrogenetic Grid Research Articles

Petrology and microthermometry of aluminous rocks in the Botswanan Limpopo Central Zone: evidence for isothermal decompression and isobaric cooling

Phase analysis in the model K2O‐poor aluminous rock system (FMASH) illustrates the following sequence of reactions during retrograde metamorphism in the Botswanan Limpopo Central Zone:Opx+Sil+Qtz=Crd ,Opx+Sil=Spr+Crd ,Grt+Qtz=Opx+Crd ;Opx+Crd+W=Ged+Qtz ,Grt+Opx+Crd+W=Ged ;andGrt+Qtz+W=Ged+Crd .A quantitative petrogenetic grid with phase relations shows that sapphirine results from nearly isothermal decompression in the quartz‐undersaturated portions of the grid, and that gedrite formation by reactions (4)–(6) records isobaric cooling from high temperature (c. 800° C) after the decompression. Conditions for hydration in the western part of the area were 700–800° C and c. 6 kbar, based on microthermometric data and the available garnet–cordierite geothermometer. On the basis of these conditions and predicted thermodynamic properties of gedrite, phase relations in T–XMg space were constructed to investigate the isobaric cooling event. The results are in good agreement with the hydration P–T path. Further, the T–XMg topologies show that hydration of orthopyroxene in the central part of the area (reaction 4) occurred at about 800° C and c. 5 kbar. Therefore, we conclude that the Botswanan Limpopo Central Zone has suffered isothermal decompression, similar to the Central Zone in South Africa and Zimbabwe, followed by isobaric cooling. The isobaric cooling event in the western (at c. 6 kbar) and central (at c. 5 kbar) parts of the area commenced at nearly the same temperature (c. 800° C), and appear to be consistent with a tectonic model that involved westward movement (thrusting) of the Central Zone.

Probable anticlockwise P,–T evolution in extending crust: Hlinsko region, Bohemian Massif

ABSTRACT In the Hlinsko region (Variscan Bohemian Massif, Czech Republic) a major extensional shear zone separates low‐grade metasedimentary series (Hlinsko schists) and high‐grade rocks of the Moldanubian terrane (Svratka Crystalline Unit). During late‐Variscan extension, a tonalite intruded syntectonically into the normal ductile shear zone, and caused contact metamorphism of the overlying schists. Concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists. In order to decipher the detailed petrological evolution of the Hlinsko unit situated in the hangingwall of this tectonic contact, a phase diagram approach and petrogenetic grids, calculated with the thermocalc computer program, were used.The crystallization/deformation relationships and the paragenetic analysis of the Hlinsko schists define a P–T path with an initial minor increase in pressure followed by cooling. Calculated pseudosections constrain this anticlockwise P‐T evolution to the upper part of the andalusite field between 0.36 and 0.40 GPa for temperatures ranging from 570 to 530°C. A low aH2O is required to explain the presence of andalusite‐biotite‐bearing assemblages, and could be related to the presence of abundant graphite.In contrast, the footwall rocks of the Svratka Crystalline Unit record decompression from around 0.8 GPa at a relatively constant temperature, followed by cooling. Thus, the footwall and the hangingwall units display opposite, but convergent P–T histories. Decompression in the footwall rocks is related to a rapid exhumation. We propose that the inverse, anticlockwise P–T path recorded in the hangingwall pelites is related to the rapid, extension‐controlled sedimentation of the overlying flysch series.

Ultrametamorphism in Precambrian granulite terranes: Evidence from Mg‐AI granulites and calc‐silicate granulites of the Eastern Ghats, India

AbstractHigh Mg‐Al granulites and calc‐silicate granulites provide evidence for ultra‐high temperatures of metamorphism (ca. 1000°C) at moderate pressures (9–10 kbar) in the Eastern Ghats Belt, India. Lack of proper geochronological data prevents the dating of this extreme metamorphism. High Mg‐Al granulites contain different subsets of mineral assemblages involving spinel, quartz, sapphirine, cordierite, orthopyroxene, garnet and sillimanite coexisting with either rutile‐ilmenite or titanohaematite‐ferrianilmenite. These high Mg‐AI rocks are poor in Zn and Cr, as reflected primarily in the composition of spinel. Evidence of ultra‐high temperature metamorphism comes from (i) textural interpretation of the former coexistence of spinel‐cordierite‐quartz and sapphirine‐quartz and stabilization of the assemblages orthopyroxene‐sillimanite‐cordierite and spinel‐quartz‐sapphirine‐garnet and (ii) the high Al2O3 content of orthopyroxene coexisting with garnet and/or cordierite. Consideration of the sequence of deduced mineral reactions in petrogenetic grids in the system FMAS attests to an anticlockwise P‐T path of evolution for the granulites. In calc‐silicate granulites stabilization of nearly pure meionite and of the wollastonite‐plagioclase‐andradite‐rich garnet, wollastonite‐scapolite‐grandite garnet‐calcite association corroborate high temperatures of metamorphism. Conventional mineralogical geothermobarometry in all the rocks record lower temperatures (maximum 950°C) at 9–10 kbar pressures, attributed to resetting of the mineral compositions during cooling. Following peak metamorphism, the rocks firstly experienced near‐isobaric cooling followed by near‐isothermal decompression. On the basis of the available evidence it appears that non‐extensional lithospheric thinning and/or heat input from basic/enderbitic magma are the causes of such ultra‐high temperature metamorphism on an anticlockwise path in the Eastern Ghats Belt.

The Effect of Mn on the Phase Relations of Medium-Grade Pelites: Constraints from Natural Assemblages on Petrogenetic Grid Topology

The Effect of Mn on the Phase Relations of Medium-Grade Pelites: Constraints from Natural Assemblages on Petrogenetic Grid Topology

Petrogenetic grid for ultrahigh‐pressure metamorphism in the model system CaO‐MgO‐SiO2‐CO2‐H2O

Abstract Petrogenetic grids for ultrahigh‐pressure (UHP) metamorphism were calculated at different Xco2 conditions in the model system CaO‐MgO‐SiO2‐CO2‐H2O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P‐T grids at Xco2= 0.01 and 0.5 are described. In the calculated P‐T grid at Xco2= 0.01, four out of 10 stable invariant points, Co‐En‐Ms‐Tc, Co‐Di‐En‐Tc‐Tr, Co‐Di‐Ms‐Tc‐Tr and Di‐En‐Ms‐Tc‐Tr lie within the stability field of coesite. If the fluid phase has Xco2= 0.5, no invariant point is stable under UHP conditions. Some magnesite‐bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO2, Ms + Tr = Do + Fo + CO2+ H2O and Ms + Tc = Fo+ CO2+ H2O at Xco2= 0.01 and by reaction Ms + Tc = Fo + CO2+ H2O together with these three at Xco2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low Xco2 conditions include the following: Co‐Do‐Ms, Co‐Di‐Ms, Co‐Di‐Tc, Di‐Ms‐Tc, Di‐En‐Tc‐, Di‐En‐Ms, Co‐Di‐En, Di‐En‐Fo, Di‐Fo‐Ms, Di‐Do‐Fo. Among them, talc‐bearing assemblages are restricted to Xco2 < 0.02 and their high‐P limit is 31.7 kb (749°C) at Xco2= 0.01. Dolomite‐magnesite‐silica assemblages have large P‐T stability fields even if Xco2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite‐dolomite assemblage is restricted only to a calc‐silicate rock interlayered with marble where Xco2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low Xco2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of Xco2 on the stability of coesite‐dolomite‐magnesite, diopside‐enstatite‐magnesite, diopside‐talc assemblages is examined and the occurrence of coesite‐dolomite, magnesite‐bearing and talc‐bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P‐T grids.

Microstructure, metamorphism, thermochronology and P‐T‐t‐deformation history of the Port aux Basques gneisses, south‐west Newfoundland, Canada*

Abstract The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Basques Complex; and the Harbor le Cou Group. A similar regionally developed polyphase history of penetrative deformation characterizes each of these units. Thickening during D1 produced rare recumbent folds (F1) and an axial planar schistosity (S1), overprinted by D2 recumbent folds (F2), and transposed during development of a locally penetrative, differentiated crenulation cleavage (S2). In western sectors of the area, D2 was associated with NW‐directed reverse shearing. The NE‐trending structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upright‐to‐steeply inclined, periclinal folds (F3) and a variably penetrative schistosity (S3).Amphibolite facies metamorphism increases in grade from west to east across the area. Microstructures, including porphyroblast‐matrix foliation relations and internal textural unconformities in garnet, indicate episodic porphyroblast nucleation and growth, which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism. Garnet, kyanite and staurolite porphyroblasts are wrapped by the S2 foliation, but each may contain trails of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequential reaction out of kyanite, staurolite and muscovite in favour of sillimanite, garnet, biotite and K‐feldspar, and the development of an increasing volume of anatectic migmatite in south‐eastern sectors of the area record syn‐ to late‐D2 peak metamorphic conditions. Microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in P‐T space for units of the Port aux Basques gneisses. Peak metamorphic conditions are estimated to have been 620–650° C at ≤8kbar in the west and 700–750° C at ≤8 kbar in the east.Titanite from an upper amphibolite facies calc‐silicate gneiss yields U‐Pb ages of c. 420 Ma, interpreted to date cooling shortly after the thermal peak in these gneisses. Variable D3 strain was associated with some recrystallization of hornblende and micas. 40Ar/39Ar hornblende plateau isotope correlation ages range from c. 419 to c. 393 Ma, from east to west across the area, and are interpreted to record cooling through c. 500° C coeval with or soon after D3 deformation. The range in ages may record the effects of heterogeneous D3 deformation and differential uplift from south‐east to north‐west associated with displacement on major shear zones. 40Ar/39Ar muscovite plateau ages cluster at c. 390 Ma, and date cooling through c. 375° C during regional exhumation. Cooling rates are moderate to fast and may indicate a component of tectonic exhumation.The Port aux Basques gneisses are a product of Silurian collisional tectonics. The higher grade of metamorphism in comparison with adjacent areas of the Canadian Appalachians is interpreted to reflect greater thickening due to juxtaposition of the St Lawrence promontory (Laurentian margin) with the Cabot promontory (Avalonian margin) during closure of the Iapetus Ocean.

The stability of osumilite in metapelitic granulites

Abstract A petrogenetic grid and related diagrams derived from KFMASH‐system experiments demonstrate that osumilite is stable in relatively magnesian bulk rock compositions (XMg > 0.6) at temperatures in excess of 875° C and pressures less than 11 kbar. The experiments, involving the dehydration melting of biotite in synthetic metapelites, were conducted in the range 850–1000° C. Both the mineral assemblages and phase compositions reported from well‐documented natural examples of osumilite‐bearing rocks are reproduced by the experiments at P‐T conditions similar to those previously estimated for these occurrences. Peak metamorphic P‐T conditions can be reliably inferred from distinctive osumilite‐bearing assemblages identified in the phase diagrams, thereby avoiding the problems of diffusional re‐equilibration that often prohibits conventional geothermobarometry from recovering peak conditions. Integration of the experimental data with recent independent experiments, after correcting the latter for an underestimated friction correction, allows extension of the petrogenetic grid to higher temperatures. The extended grid is applied to assess and refine the metamorphic history of the Napier Complex, East Antarctica: the high‐P stability limit for osumilite in the Napier Complex is 9–10 kbar, the prograde P‐T‐t path is not necessarily anticlockwise and isobaric cooling in the Scott and Tula mountains occurred, respectively, at pressures greater and less than reactions in the range 8–9 kbar. The stability range for osumilite predicted by the KFMASH‐system petrogenetic grid overlaps many more metamorphic terranes than osumilite is found in. Whilst osumilite is not distinctive in thin section and is prone to retrogression, it is possible that carbon dioxide present in the natural system stabilizes cordierite at the expense of osumilite.

Partial melting and phase relations in high-grade metapelites: an experimental petrogenetic grid in the KFMASH system

Partial melting and phase relations in high-grade metapelites: an experimental petrogenetic grid in the KFMASH system

Partial melting and phase relations in high-grade metapelites: an experimental petrogenetic grid in the KFMASH system

Petrogenetic grids are a powerful tool for understanding metamorphic terrains and many theoretical grids have been suggested for the process of granulite formation in metapelitic rocks, via fluid-absent biotite melting reactions. However, application of these grids has been difficult due to the lack of suitable experimental constraints. We present here an experimentally determined and tightly constrained petrogenetic grid for KFMASH system metapelites which extends from 840–1000°C and 5.0–12.5 kbar. Sixty four experiments on three KFMASH, mineral-mix, bulk compositions (XMg=0.62, 0.74, 0.86) provide phase composition and assemblage data from which a grid can be derived and constrained. Reversal experiments and consideration of the phase composition data show the experiments to be close to equilibrium. The KFMASH univariant fluid-absent biotite melting reactions occur between 850 and 870°C at 5 kbar and between 900 and 915°C at 10 kbar. These reactions are connected to equilibria beyond the stability of biotite to develop a fixed framework within which the phase assemblage evolution of metapelitic rocks can be interpreted. The effect of minor components on phase equilibria is evaluated using the experimentally determined grid as a simple-system reference. The temperature at which melting occurs in metapelites is strongly controlled by the concentrations of titanium and fluorine in biotite. Pressure-temperature pseudosections presented for each of the experimental compositions show both the univariant and divariant reactions available to a particular bulk composition, clearly illustrating the possible evolution of the phase assemblage. The pseudosections also constrain the stability limits of

Reaction Textures in a Suite of Spinel Granulites from the Eastern Ghats Belt, India: Evidence for Polymetamorphism, a Partial Petrogenetic Grid in the System KFMASH and the Roles of ZnO and Fe2O3

Spinel granulites, with or without sapphirine, occur as lenses in garnetiferous quartzofeldspathic gneisses (leptynites) near Gokavaram in the Eastern Ghats Belt, India. Spinel granulites are mineralogically heterogeneous and six mineral associations occur in closely spaced domains. These are (I) spinel-quartz-cordierite, (II) spinel-quartz-cordierite-garnet-orthopyroxene-sillimanite, (III) spinel-cordierite-orthopyroxene-sillimanite, (IV) spinel-quartz-sapphirine-sillimanite-garnet, (V) spinel-quartz-sapphirine-garnet and (IV) rhombohedral (Fe-Ti) oxide-cordierite-orthopyroxene-sillimanite. Common to all the associations are a porphyroblastic garnet (containing an internal schistosify defined by biotite, sillimanite and quartz), perthite and plagioclase. Spinel contains variable amounts of exsolved magnetite and is distinctly Zn rich in the sapphirine-absent associations. X Mg in the coexisting phases decreases in the order cordierite-biotite-sapphirine-orthopyroxene-spinel-garnet-(Fe-Ti) oxides. Textural criteria and compositional characteristics of the phases document several retrograde mineral reactions which occurred subsequent to prograde dehydration melting reactions involving biotite, sillimanite, quartz, plagioclase and spinel. The following retrograde mineral reactions are deduced: (1) spinel + quartz → cordierite, (2) spinel + quartz → garnet + sillimanite, (3) garnet + quartz → cordierite + orthopyroxene, (4) garnet + quartz + sillimanite → cordierite, (5) spinel + cordierite → orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz → sapphirine, (7) spinel + sapphirine + quartz → garnet + sillimanite, and (8) spinel + quartz sapphirine + garnet. A partial petrogenetic grid for the system FeO-MgO-Al 2 O 3 -SiO 2 -K 2 O-H 2 O at high fo 2 , has been constructed and the effects of ZnO and Fe 2 O 3 on this grid have been explored Combining available experimental and natural occurrence data, the high fo 2 invariant points in the partial grid have been located in P-T space. Geothermobarometric data and consideration of the deduced mineral reactions in the petrogenetic grid show that the spinel granulites evolved through an anticlockwise P-T trajectory reaching peak metamorphic conditions >9 kbar and 950 °C, followed by near-isobaric cooling (dT/dP = 150 °C/kbar). This was superimposed by an event of near-isothermal decompression (dT/dP = 15 °C/kbar). The studied spinel granulites, therefore, preserve relic prograde mineral associations and reaction textures despite being metamorphosed at very high temperatures, and bear evidence of polymetamorphism.

Granulite-facies metamorphism in the Anatectic Complex of Toledo, Spain: late Hercynian tectonic evolution by crustal extension

The Anatectic Complex of Toledo consists of a heterogeneous suite of high-grade metamorphic rocks, mainly granulitic migmatites of pelitic composition and orthogneisses, and different types of syn-orogenic granitoids with minor associated basic rocks. None of the metamorphic rock types records evidence of the early prograde history, and only the peak and retrograde P–T conditions can be estimated. The intrusion of the syn-orogenic granitoids is either slightly before or synchronous with the metamorphic climax. These rock types thus can be used to make P–T estimates of the peak and retrograde conditions. Microtextural analysis of reaction textures in conjunction with a petrogenetic grid, has enabled construction of an incomplete ‘clockwise’ P–T path that is characterized by an isothermal decompression after or during the metamorphic peak temperatures. The peak estimates from different geo-thermobarometers are 800° ± 50 °C and 4–6 Kbar, the highest temperatures recorded in this part of the Iberian Hercynian Belt. It is proposed that this high- T /low-P metamorphism is at least partially related to an extensional process, possibly due to gravitational collapse of previously thickened crust.

Metamorphism of cale-silicate and associated rocks in the Pan-African Kaaimans Group, Saldania Belt, South Africa

Phase relations and mineral compositions indicate a polymetamorphic history of carbonate and siliciclastic sediments constituting the Kaaimans Group of the Pan-African Saldania Belt along the south coast of South Africa. Regional metamorphism during Pan-African orogeny did not exceed greenschist facies conditions but was superimposed locally onto low- to medium-grade contact metamorphism related to the intrusion of syn-orogenic granitoids. Petrogenetic grids, constructed for the System CaO-MgO Al2O3-SiO2-H2O-CO2 and based on available thermodynamic data, reveal that vesuvianite, grossular-rich garnet and wollastonite bearing minerai assemblages, found in cale-silicate rocks adjacent to the syn-orogenic Rooiklip granite gneiss sheet, were formed at temperatures possibly as low as 450°C and pressures between 2 and 3 kbar. These mineral assemblages were in equilibrium with an H2O-CO2 fluid that was characterized by a CO2/(CO2 + H2O) mole fraction of less than 0.02. The fluid composition in the cale-silicate rocks was buffered entirely externally through magmatic/hydrothermal fluids related to the granite intrusion. The fluid flux was enhanced by syn-intrusive bedding-parallel shearing in the sedimentary rocks. Vesuvianite, which is of highly variable but generally Fe-rich composition, contains up to 2.1 wt.% F and hence acted as sink for fluorine. A regional increase in Pan-African metamorphic grade from the tectonically lowest to highest formation suggests an overturned position of the whole Kaaimans Group. Later metamorphism during the Cape Orogeny caused only very low- to low-grade retrogression.

Phase diagram methods for graphitic rocks and application to the system C−O−H−FeO−TiO2−SiO2

Carbon-saturated C−O−H (GCOH) fluids have only one compositional degree of freedom. This degree of freedom is specified by the variable X o that expresses the atomic fraction of oxygen relative to oxygen+hydrogen. The only valid constraint on the maximum in the activity of GCOH fluid species is related to the bulk composition of the fluid, as can be expressed by X o. In fluid-saturated graphitic rocks, mineral devolatilization reactions are the dominant factor is determining the redox state of the metamorphic environment. X o is directly proportional to the fo2 of GCOH fluid, and because its value can only be affected by fluid-rock interaction, it is an ideal measure of the redox character and composition of GCOH fluid. Phase diagrams as a function of X o are analogous to the P-T-Xco2 diagrams used for binary H2O−CO2 fluids; this analogy can be made rigorously if the C−O−H fluid composition is projected through carbon into the O−H subcomposition. After projection, the fluid is described as a binary fluid with the components O and H, and the compositional variable X o. Description of GCOH fluids in this manner facilitates construction of phase diagram projections that define the P-T stability of mineral assemblages for all possible fluid compositions as well as fluid-absent conditions. In comparison to phase diagrams with variables based on the properties of fluid species, P-T-X o diagrams more clearly constraint accessible fluid compositions and fluid evolution paths. Calculated P-T-X o projections are presented for the C−O−H−FeO−TiO2−SiO2 system, a limiting model for the stability of Fe−Ti oxides in graphitic metapelites and phase relations in metamorphosed iron-formations. With regard to the latter, the stability of the assemblage qtz+mag+gph has been a source of controversy. Both the calculated C−O−H−FeO−TiO2−SiO2 system petrogenetic grid and natural examples, suggest that this assemblage has a large P-T stability field. Discrepancies between earlier C−O−H−FeO−SiO2 system phase diagram topologies are reconciled by the qtz+mag+gph=sid+fa phase field, a barometric indicator for metamorphosed-iron formations. A more general implication of calculated P-T-X o phase relation is that few inorganic mineral-fluid equilibria appear to be capable of generating hydrogen-rich, fO2, GCOH fluids at crustal metamorphic conditions. The utility of P-T-X o diagrams derives from the use of a true compositional variable to describe fluid composition, this approach can be extended to the treatment of carbon-undersaturated systems, and provides a simple means of understanding metasomatic processes of graphite precipitation.

PT Paths from modal proportions: application to the Koralm Complex, Eastern Alps

Thermodynamic pseudosections portray those parts of a petrogenetic grid that are relevant to a given bulk composition and the reactions appearing on them can therefore be used directly to infer the PT path that the rock followed. However, for many ‘normal’ bulk compositions the use of pseudosections is hampered by the fact that they display only few large fields of high thermodynamic variance in the PT range of interest. Here it is discussed how modal information on reaction progress within these fields can be used to determine PT path information for thermodynamically high variant metamorphic assemblages. We use this information on reaction progress to contour pseudosections for modal proportions of minerals using the software package THERMOCALC. The approach is applied to di- tri- and quadrivariant assemblages from the Koralm complex in the eastern Alps. A PT path for these rocks is derived from modal considerations and compared with interpretations of mineral composition contours on the same pseudosection and with conventional thermobarometry. It is shown that at least part of the complex must have cooled initially near isobarically from prevalent peak conditions around 700°C and 14 kbar before the rocks commenced a Barroviantype decompression path.

Garnet coronas in scapolite‐wollastonite calc‐silicates from East Antarctica: the application and limitations of activity‐corrected grids

ABSTRACTCalc‐silicate boudins within Proterozoic granulite facies gneisses of the northern Prince Charles Mountains, East Antarctica, preserve a number of reaction textures including garnet coronas between calcite and scapolite; garnet‐quartz coronas between scapolite and wollastonite and between plagioclase and wollastonite; calcite‐quartz intergrowths in wollastonite; and calcite‐plagioclase symplectites in scapolite. These textures have been modelled using petrogenetic grids for reactions in the CaO‐Al2,O3‐SiO2‐CO2 system, but with reduced mineral activities to account for additional components in real mineral compositions. Such fixed‐composition reduced‐activity grids are strictly valid only at the point in P‐T‐aCO2 space where an assemblage last equilibrated, and do not show the true positions of reactions away from this point because mineral compositions change with reaction progress. In this case, however, mineral compositions close to end‐member values and low extents of reaction progress mean that compositional change was limited and the grids are good approximations to true pseudosections over the entire P‐T‐aco2 range of interest.The grids show that the textures are consistent with near‐isobaric cooling from about 850 to 700d̀ C at 7 kbar, a P‐T path compatible with thermobarometric studies of other lithologies from the area. Phase relationships indicate that CO2 activities were buffered by the local mineral assemblage during peak and retrograde metamorphism, either under fluid‐absent conditions or within a non‐pervasive fluid phase. Previous studies of garnet coronas in scapolite‐wollastonite calc‐silicates have used qualitative grids based on limited experimental data to invoke garnet growth during water infiltration at high temperature, but the grids used here show that garnet coronas can form on cooling, without any need for water influx.

Contact metamorphism around the Stawell granite, Victoria, Australia

ABSTRACTThe contact metamorphosed metapelitic and metapsammitic rocks surrounding the Stawell granite, western Victoria, Australia, are divided into three zones: the low‐grade zone, the medium‐grade zone and the high‐grade zone. Detailed petrological study shows consistency of element distributions, implying that equilibrium was widely attained in the rocks, although equilibrium volumes are generally small (millimetre scale) and considerable mineral chemical variations exist between adjacent domains. The metamorphic mineral assemblages are generally of high variance (KFMASH variance ≤ 2). Consequently, the chemical evolution of assemblages is controlled largely by bulk composition and metamorphic temperature, the former factor being more important in most rocks. The chemographic relations of mineral assemblages in low‐ and medium‐ to high‐grade zones are presented in compatibility diagrams projected from biotite, quartz and H2O, and biotite, K‐feldspar and H2O, respectively. These compatibility diagrams have the advantage of showing both quartz‐bearing and quartz‐absent assemblages. The metamorphic reactions are modelled successfully by a calculated petrogenetic grid that combines both KFASH and KMASH equilibria. Based on petrographic observations and with constraints from the calculated petrogenetic grid, the following KFMASH reactions, in the order of increasing metamorphic grade, are responsible for producing the various mineral assemblages in the Stawell rocks:chl + mu + q = bi + cd + V,chl + q + cd = g + V,mu + bi + q = ksp + cd + V,mu + q = ksp + and + cd + V (or KASH mu + q = ksp + and + V),mu + cd = ksp + and + bi + V,mu + bi + and = ksp + sp + V,and + bi = ksp + sp + cd + V,mu + bi = ksp + cor + sp + V,mu = ksp + cor + and + sp + V (or KASH mu = ksp + cor + V),bi + cd + q = g + ksp + V.The combined KFASH and KMASH grid provides constraints on reaction coefficients in the above sequence of reactions and on temperature and pressure of metamorphism.

Metamorphic evolution of granulites in the Minto Block, northern Québec: extraction of peak P‐T conditions taking account of late Fe‐Mg exchange

ABSTRACTIn the central Minto Block of northern Québec, the Lake Minto and Goudalie domains are dominated, respectively, by orthopyroxene‐bearing plutonic suites (granite‐granodiorite and diatexite) and a tonalitic gneiss complex, both of which contain scattered remnant paragneisses. Two main granulite‐grade mineral assemblages are observed in the paragneiss: garnet (Grt)‐orthopyroxene (Opx)‐plagioclase‐quartz (GOPQ) and garnet (Grt)‐cordierite (Crd)‐sillimanite‐plagioclase‐quartz (GCSPQ). These show distinct lithological associations, with the GCSPQ assemblages occurring exclusively within the diatexite in the Lake Minto domain. Petrogenetic grid considerations demonstrate that the GOPQ rocks are higher grade than the GCSPQ rocks. Maximum temperatures for GOPQ rocks, obtained from equilibria based on Al solubility in orthopyroxene in equilibrium with garnet, range from 950 to 1000d̀ C, significantly higher than garnet‐orthopyroxene Fe‐Mg exchange temperatures of 700 ± 50d̀ C, the latter probably representing a closure temperature below peak conditions. The Al temperatures were corrected for late cation exchange by adjusting the Fe/(Fe + Mg) ratios in garnet and orthopyroxene, to achieve internal consistency between the GOPQ thermometers and barometers. Grt‐Crd thermometry records temperatures of 750±50d̀ C. Peak P‐T conditions range from 5‐6 kbar and 750‐800d̀ C in the Goudalie and eastern Lake Minto domains, to 7‐10 kbar and 950‐1000d̀ C in the western and central Lake Minto domain. This variability contrasts with the uniform crustal pressures of 5 ± 1 kbar recorded by the GCSPQ assemblages in the diatexites and the hornblende granodiorites (c. 4‐5 kbar) across the same area. The GOPQ rocks are inferred to record earlier P‐T conditions that prevailed before the formation of GCSPQ assemblages and the intrusion of the granodiorites. Partial P‐T paths in GOPQ rocks from both domains, based on net transfer equilibria corrected for Fe‐Mg resetting, document cooling of 100‐250d̀ C from thermal‐peak conditions, concomitant with a modest pressure decrease of 2‐3 kbar. Although textures diagnostic of isobaric cooling are not developed, the paths are consistent with a tectonic model in which granulite metamorphism and crustal thickening in the Minto Block were consequences of magmatic underplating. The progression from higher P‐T conditions recorded by GOPQ assemblages to lower P‐T conditions recorded by GCSPQ assemblages is attributed to variable amounts of synmagmatic uplift and cooling in a single, continuous thermal event in the Minto crust, associated with protracted crustal magmatism. In the Goudalie and eastern Lake Minto domains, where GOPQ and GCSPQ rocks and Hbl granodiorites have similar P‐T conditions of equilibration, the crust may not have been thickened as much as further west, where GOPQ P‐T conditions are significantly higher than those of the hornblende granodiorites and the GCSPQ rocks.

Fluid‐mineral equilibria in prehnite‐pumpellyite to greenschist facies metabasites near Flin Flon, Manitoba, Canada: implications for petrogenetic grids

ABSTRACTA sequence of regional metamorphic isograds indicating a range from prehnite‐pumpellyite to lower amphibolite facies was mapped in metabasites near Flin Flon, Manitoba. The lowest grade rocks contain prehnite + pumpellyite and are cut by younger brittle faults containing epidote + chlorite + calcite. Isobaric temperature‐XCO2 and pressure‐temperature (constant XCO2) diagrams were calculated to quantify the effects of CO2 in the metamorphic fluid on the stability of prehnite‐pumpellyite facies minerals in metabasites containing excess quartz and chlorite. Prehnite and, to a lesser extent, pumpellyite are stable only in fluids with Xco2 <0.002. For Xco2>0.002, epidote + chlorite + calcite assemblages are stable. Our calculated phase relations are consistent with regional metamorphism in the Flin Flon area in the presence of an H2O‐rich fluid and a more CO2‐rich fluid in the later fault zones. We believe that the potential effects of small amounts of CO2 in the metamorphic fluid should be assessed when considering the pressure‐temperature implications of mineral assemblages in low‐grade metabasites.

Petrology of lawsonite‐, pumpellyite‐ and sodic amphibole‐bearing metabasites from north‐east Oman

ABSTRACTThe assemblages chlorite‐pumpellyite‐lawsonite‐albite‐quartz, chlorite‐lawsonite‐quartz‐epidote and chlorite‐epidote‐albite‐quartz occur in metabasaltic blocks and veins in a metamorphosed tectonic mélange in the structurally highest unit of the autochthonous and parautochthonous section underlying the Semail ophiolite in Saih Hatat, north‐east Oman. The pre‐Permian basement of this section contains mafic units characterized by the assemblage crossite‐epidote‐chlorite‐quartz‐albite /el actinolite. These assemblages indicate a down‐section increase in metamorphic grade from ‘lawsonite‐albite facies' conditions in the mélange to ‘epidote‐blueschist’ facies conditions in the basement.Application of empirically and experimentally based thermobarometers as well as petrogenetic grids calculated for a model basaltic system indicates that the P‐T conditions of metamorphism ranged from 3 to 6 kbar and 250 to 300d̀ C for the mélange and P > 6.8 kbar, T > 310d̀ C for the basement units. Textural relations interpreted in the context of petrogenetic grids indicate that these units followed clockwise P‐T paths of evolution. The estimated P‐T conditions and down‐section increase in metamorphic grade in central, western and northern Saih Hatat are consistent with the hypothesis relating metamorphism to the Late Cretaceous tectonic loading of the continental margin by an ophiolite slab < 18 km in thickness. These results contrast with field and petrological observations documented for blueschists and eclogites exposed along the eastern coast of Saih Hatat which may have formed at an earlier stage in response to an Early Cretaceous collisional event.

A calculated petrogenetic grid for the system K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O, with particular reference to contact‐metamorphosed pelites

ABSTRACTA petrogenetic grid is presented for the system KFMASH (K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O), including biotite, muscovite, K‐feldspar, chlorite, chloritoid, staurolite, cordierite, garnet, orthoamphibole, orthopyroxene, spinel, andalusite, sillimanite, kyanite, quartz and corundum with H2O in excess, which was calculated using the computer program THERMOCALC and the Powell and Holland internally consistent thermodynamic dataset. By removing the normal constraint of having quartz in excess, both quartz‐bearing and quartz‐absent equilibria are shown. Quartz‐absent equilibria are particularly relevant at high‐T and low‐P conditions, because of their common occurrence at these conditions. The calculated mineral assemblage and mineral compositional variations in terms of FeMg‐1 and (Fe, Mg)SiAl‐2 exchange vectors are broadly compatible with observations on natural rocks, particularly when non‐KFMASH components are taken into account.