Staurolite Stability and Related Parageneses: Theory, Experiments, and Applications

The results of recent investigations on the stability limitsof staurolite have been combined together with those of thepresent study to develop a semi-quantitative model of the P–T–f_o2–Xrelations of staurolite±quartz±magnetite. Theproblem with respect to the hydroxyl content of staurolite hasbeen analysed; it is concluded that no evidence has yet beenmustered to discount the idealised stoichiometry proposed byNaray-Szabó & Sasvari (1958), at least as a limitingcomposition. The stability limits of staurolite±magnetitehave been calculated from the experimental data for the equilibriainvolving quartz. Also the conditions over which the assemblagecordierite+magnetite+quartz could be stable, as well as a quantitativemodel for the fo₂-P stability of almandine ± quartz havebeen deduced theoretically. An analysis is presented of the paragenetic relations of staurolitein common pelitic schists. It is suggested that the formationof staurolite at the expense of either chloritoid or chlorite,rather than the unqualified first appearance of staurolite asproposed by Winkler (1970), should define a ‘staurolite-in’isograd in the range of 500–575 °C. In regional metamorphism,chloritoid, staurolite, and aluminum silicates should, underequilibrium conditions, be unstable relative to almandine ingraphitic pelitic schists involving magnetite (chloritoid/staurolite/Al₂SiO₆+magnetite+quartzalmandine+O₂+H₂O).The limits of P-T conditions over which staurolite and cordieritemay coexist in natural assemblages have been deduced; it isrestricted, almost entirely within the field of andalusite,between 500–700 °C, and 2–6 kbars, thus definingthe range of P-T conditions for the ‘low-pressure intermediate’—or ‘Buchan’–type amphibolite facies discussedby Miyashiro (1961). In assemblages involving staurolite andandalusite, cordierite rather than almandine should usuallybe stable; the reverse holds for assemblages involving stauroliteand sillimanite.     

Cordierite-Cummingtonite Facies Rocks from the Gold Brick District, Colorado

The unusual association of cordierite and cummingtonite (? gedrite+ chlorite + biotite + ilmenite + plagioclase + quartz) definesa metamorphic facies within aluminous, low-Ca amphibolites fromthe Proterozoic rocks of the Gold Brick District, east of Gunnison,Colorado. More Fe-rich bulk chemistries in the same facies arecharacterized by assemblages consisting of cordierite+-gedrite+ garnet + chlorite + biotite + ilmenite + plagioclase + quartz,whereas more Mg-rich compositions are characterized by cordierite+ anthophyllite + chlorite + biotite + ilmenite ? plagioclase+ quartz. The assemblage gedrite 4- cummingtonite + chlorite+ biotite + ilmenite + plagioclase + quartz was also observed.Coexisting cordierite+ anthophyllite + cummingtonite was notobserved in any rocks, apparently because this assemblage isstable over only a very narrow range of bulk compositions. Metamorphosedpelitic rocks are more iron rich than the assemblage cordierite+ gedrite + garnet + chlorite + biotite + ilmenite + plagioclase+ quartz and consist of garnet ?cordierite ?staurolite ? chlorite? andalusite + biotite + ilmenite + plagioclase + quartz? microclineor muscovite. Mineral rim compositions from cordierite-bearing amphibolitesand metapelites determined by electron microprobe analysis showsystematic Fe/Mg partitioning and define assemblages that occupynon-overlapping regions of the compositional system SiO₂-TiO₂-Al₂O₃-MnO-FeO-MgO-CaO-Na₂O-K₂O-H₂Oas determined by algebraic and statistical methods developedby Braun & Stout (1975) and Fisher (1989). Graphical methods(projections) produced spurious overlaps not confirmed by themore rigorous algebraic tests. The spurious overlaps were generatedbecause standard projective analysis was not able simultaneouslyto account for the important effects of the components Na₂O,CaO, and MnO on the AFM topologies. The results of algebraicand statistical analysis are consistent with an equilibriumorigin at constant values of temperature and pressure. The cordierite-cummingtonite facies encompasses the relativelylow-pressure and moderate-temperature conditions associatedwith the stability field of andalusite. Garnet-biotite geothermo-metry,and garnet, aluminosilicate, silica, plagioclase (GASP) geobarometrysuggest that temperatures and pressures were nearly constantacross the study area at 550( ? 70) ?C and 3 kb, respectively,near the peak of metamorphism. Other geothermometers and geobarometers,and independent pressure and temperature estimates, are compatiblewith garnet-biotite thermometry and GASP geo-barometry. Gradientsin fO₂ or H₂O are not required to explain the compatibilityof these assemblages at constant T and P. Cordierite + cummingtonite-bearingrocks can apparently be derived from anthophyllite +garnet-bearingrocks by increasing temperature or decreasing pressure.     

High-Grade Fluid Metasomatism on both a Local and a Regional Scale: the Seward Peninsula, Alaska, and the Val Strona di Omegna, Ivrea-Verbano Zone, Northern Italy. Part I: Petrography and Silicate Mineral Chemistry

K-feldspar–plagioclase–quartz mineral textures aswell as biotite and hornblende compositions are compared forsuites of metamorphosed mafic rocks from two widely separatedtraverses. A portion of either traverse has experienced a high-gradedehydration event transforming it from an H₂O-rich, hornblende-bearingzone to an H₂O-poor, hornblende-free, orthopyroxene-bearing,‘granulite facies’ zone at 700–800°C and7–8 kbar. In the Kigluaik Mountains, Seward Peninsula,Alaska, dehydration took place over an 85 cm thick layer ofmetatonalite in contact with a marble during regional metamorphismand involved a CO₂-rich fluid, whereas for the Val Strona diOmegna traverse, Ivrea–Verbano Zone, northern Italy, dehydrationtook place over a 3–4 km thick sequence of metabasitesinterlayered with metapelites in a contact metamorphic eventinvolving basaltic magmas intruded at the base of the sequence.Orthopyroxene-bearing samples from both dehydration zones showmicro-veins of K-feldspar along quartz and plagioclase grainboundaries as well as replacement antiperthite in plagioclase.K came primarily from the breakdown of hornblende + quartz toorthopyroxene ± clinopyroxene, feldspar and fluid. Biotiteeither was stabilized or formed in the dehydration zones andis enriched in Ti, Mg, F and Cl relative to biotite in the amphibolitefacies zone. KEY WORDS: KCl–NaCl brines; metasomatism; granulite facies metamorphism; charnockite–enderbite; orthopyroxene; K-feldspar; biotite; hornblende     

Metamorphic Evolution of the Ribeira Belt: Evidence from Outcrops in the Rio de Janeiro Area, Brazil

Migmatitic granulites and arc-related felsic intrusives of Pan-Africanage form the bedrock in the Rio de Janeiro area, SE Brazil.These rocks preserve a partial record of three parageneses.The earliest assemblage (M₁) grew during fabric formation inthe rocks (D₁) and is characterized by the mineral assemblagePl + Bt + Sil + Kfs + Qtz. Peak metamorphic conditions (M₂)are characterized by the assemblage Bt + Crd + Kfs + Pl + Grt+ liq + Qtz and are inferred to have developed during D₂ foldingof the rocks at T = 750–800°C and P = 7 kbar. M₃ reactiontextures overprint the M₂ assemblage and comprise symplectiticintergrowth of cordierite(II) and quartz that formed after garnet,whereas secondary biotite formed as a result of reactions betweengarnet and K-feldspar. By comparing the observed modal abundanceswith modal contours of garnet, cordierite and quartz on therelevant pseudosection a post M₂ P–T vector indicatingcontemporaneous cooling and decompression can be deduced. Theinferred equilibrium assemblage and reaction textures are interpretedto reflect a clockwise P–T path involving heating followedby post-peak decompression and associated cooling. We inferthat metamorphism occurred in response to advective heatingby the abundant syn-collisional (arc-related) I-type granitoidsin the region, consistent with the unusually high peak T/P ratio. KEY WORDS: advective heating; Ribeira belt; granulite; partial melting; P–T pseudosection     

Talc-, Magnesite- and Ti-Clinohumite-Bearing Ultrahigh-Pressure Meta-Mafic and Ultramafic Complex in the Dabie Mountains, China

The Bixiling mafic-ultramafic metamorphic complex is a 1•5km² tectonic block within biotite gneiss in the southern Dabieultrahigh-pressure terrane, central China. The complex consistsof banded eclogites that contain thin layers of garnet-bearingcumulate ultramafic rock. Except for common eclogitic phases(garnet, omphacite, kyanite, phengite, zoisite and rutilc),banded eclogites contain additional talc and abundant coesiteinclusions in omphacite, zoisite, kyanite and garnet. Some metaultramaficrocks contain magnesite and Ti-clinohumite. Both eclogites andmeta-ultramafic rocks have undergone multi-stage metamorphism.Eclogite facies metamorphisrn occurred at 610–700C andP>27 kbar, whereas amphibolite facies retrograde metamorphismis characterized by symplectites of plagioclase and hornblendeafter omphacite and replacement of tremolite after talc at P<6–15kbar and T <600C. The meta-ultramafic assemblages such asolivine + enstatite + diopside + garnet and Ti-clinohumite +diopside + enstatite + garnet + magnesite olivine formed at700–800C and 47–67 kbar. Investigation of the phaserelations for the system CaO-MgO-SiO₂-H₂O-CO₂ and the experimentallydetermined stabilities of talc, magnesite and Ti-clinohumiteindicate that (1) UHP talc assemblages are restricted to Mg-Algabbro composition and cannot be an important water-bearingphase in the ultramafic mantle, and (2) Ti-clinohumite and magnesiteare stable H₂O-bearing and CO₂-bearing phases at depths >100km. The mafic-ultramafic cumulates were initially emplaced atcrustal levels, then subducted to great depths during the Triassiccollision of the Sine-Korean and Yangtze cratons. KEY WORDS: eclogite; magnesite; meta-ultramafics; talc; ultrahigh-P metamorphism ^*Corresponding author     

Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral-Fluid Equilibria and Petrogenetic Grids

The role of clinopyroxene in producing grandite garnet is evaluatedusing data from an ultrahigh-temperature metamorphosed calc-silicategranulite occurrence in the Eastern Ghats Belt, India. ‘Peak’pressure–temperature conditions of metamorphism were previouslyconstrained from associated high Mg–Al granulites as c.0·9 GPa, >950°C, and the rocks were near-isobaricallycooled to c. 750°C. Grandite garnet of variable compositionwas produced by a number of reactions involving phases suchas clinopyroxene, scapolite, plagioclase, wollastonite and calcite,in closely spaced domains. Compositional heterogeneity is preservedeven on a microscale. This precludes pervasive fluid fluxingduring either the peak or the retrograde stage of metamorphism,and is further corroborated by computation of fluid–rockratios. With the help of detailed textural and mineral compositionalstudies leading to formulation of balanced reactions, and usingan internally consistent thermodynamic dataset and relevantactivity–composition relationships, new petrogenetic gridsare developed involving clinopyroxene in the system CaO–Al₂O₃–FeO–SiO₂–CO₂–O₂in T–aCO₂–fO₂ space to demonstrate the importanceof these factors in the formation of grandite garnet. Two singularcompositions in garnet-producing reactions in this system arededuced, which explain apparently anomalous textural relations.The possible role of an esseneite component in clinopyroxenein the production of grandite garnet is evaluated. It is concludedthat temperature and fO₂ are the most crucial variables controllinggarnet composition in calc-silicate granulites. fO₂, however,behaves as a dependent variable of CO₂ in the fluid phase. Externalfluid fluxing of any composition is not necessary to producechemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidation–decarbonation equilibria     

An Experimentally Constrained Petrogenetic Grid in the Silica-Saturated Portion of the System KFMASH at High Temperatures and Pressures

Experiments in the quartz-saturated part of the system KFMASHunder fO₂ conditions of the haematite–magnetite bufferand using bulk compositions with X_Mg of 0·81, 0·72,0·53 define the stability limits of several mineral assemblageswithin the P–T field 9–12 kbar, 850–1100°C.The stability limits of the mineral assemblages orthopyroxene+ spinel + cordierite ± sapphirine, orthopyroxene + garnet+ sapphirine, sapphirine + cordierite + orthopyroxene and garnet+ orthopyroxene + spinel have been delineated on the basis ofP–T and T–X pseudosections. Sapphirine did not appearin the bulk composition of X_Mg = 0·53. A partial petrogeneticgrid applicable to high Mg–Al granulites metamorphosedat high fO₂, developed in our earlier work, was extended tohigher pressures. The experimental results were successfullyapplied to several high-grade terranes to estimate P–Tconditions and retrograde P–T trajectories. KEY WORDS: KFMASH equilibria; experimental petrogenetic grid at high fO₂     

The Graphical Analysis of Greenschist to Amphibolite Facies Mineral Assemblages in Metabasites

The mineral assemblages of greenschist to amphibolite faciesmetabasites may usually be represented in a system of principalcomponents: CaO-Al₂O₃-(Fe₂O₃)-FeO-MgO-Na₂O-SiO₂-C0₂-H₂O Assemblages co-existing with quartz, ‘albite’, ‘epidote’and a fluid of restricted composition, may be shown by projectionin a CAFM subsystem from ‘epidote’ onto an extendedAFM plane. This projection is analogous to the Thompson projectionfor pelites and is particularly useful in displaying the effectsof Fe/Mg and Al substitution in the silicates as well as incorporatingCaO; it is illustrated by plotting assemblages from the SouthernAlps of New Zealand and the Scottish Highlands and demonstrateschanges occurring with grade in the assemblages. Some commonisograds and facies boundaries are seen to be strongly dependenton bulk rock composition. In some cases MnO must be consideredas an additional component. A model of P_solid=P_fluid, where the fluid is composed of CO₂+H₂Ois consistent with many greenschist to amphibolite facies metabasicassemblages. Natural assemblages indicate this fluid phase tohave restricted mobility. Theoretical consideration of mineralreactions resulting from increasing X_CO2 in conjunction withdata from natural mineral assemblages, leads to the distinctionof five principal types of assemblage which may be expectedas a function of varying X_CO2. Recognition of these assemblagetypes provides a useful guide to relative X_CO2 during metamorphism.     

Petrology and Mineralogy of Eclogite and Garnet Amphibolite from Puerto Cabello, Venezuela

Occurrences of eclogite, eclogite-amphibolite, and garnet amphibolitefrom Puerto Cabello, Venezuela are described. Chemical analysesof thirteen rocks, eight garnets, six clinopyroxenes, and nineamphiboles are presented. Field evidence shows that the eclogitesand associated mafic rocks occur as sheets or lenses conformablewith pelitic and calcareous rocks and have been metamorphosedin situ. The associated country rocks have attained a metamorphicmineral assemblage typical of the epidote-amphibolite facies.The chemical composition of the eclogite and associated rocksis variable but is essentially that of tholeiitic basalt. Clino-pyroxenesfrom the eclogites are omphacites containing 34–51 moleper cent jadeite and the remainder largely diopside; coexistinggarnets contain approximately 50 mole per cent almandine, 24mole per cent pyrope, 25 mole per cent grossular+andradite,and 1 mole per cent spessartine. Determinations for the temperatureof metamorphism of these rocks give approximately 525±50°C. Determinations of total pressure of metamorphism, basedon the jadeite content of omphacite, on glaucophane II, kyanite,and calcite stability fields, and on stratigraphic reconstructionsgive a range of values between 5 and 10 kb with a most probablevalue near 7 kb. These data suggest a moderate geothermal gradientduring metamorphism of approximately 20 °C/km. An evaluationof the role of volatiles during metamorphism indicates thatthe original basalts were converted to eclogites in an environmentclosed to water and carbon dioxide. The field relationshipsat Puerto Cabello demonstrate the general instability of eclogite;the great majority of mafic rocks are amphibolites, garnet amphibolites,and eclogite-amphibolites. Eclogites, sensu stricto, are preservedonly in random ‘dry’ pockets within the stratigraphicsection containing the metabasalts. The transformation of basaltto eclogite to amphibolite is considered to have been a continuousparagenetic sequence during a single pseriod of metamorphism.     

Reaction Textures and Metamorphic Evolution of Sapphirine-bearing Granulites from the Gruf Complex, Italian Central Alps

Mineral chemistries and textures are described from a suiteof sapphirine-bearing granulites from the Gruf Complex of theItalian Central Alps. The granulites contain combinations ofgarnet, orthopyroxene, sapphirine, sillimanite, cordierite,biotite, quartz, spinel, corundum, staurolite, plagioclase,K-feldspar, ilmenite and rutile, in assemblages with low (usuallynegative) variance. They are outstanding in that they preservea textural and chemical record of a protracted metamorphic evolution. Reaction textures are common and include: (i) pseudomorphs (e.g.of sillimanite after kyanite); (ii) relatively coarse-grainedmonomineralic reaction rims (e.g. of cordierite between sapphirineand quartz); (iii) fine-grained symplectitic coronas (e.g. oforthopyroxene + sapphirine round garnet); (iv) inclusions, ingarnet cores, of minerals (e.g. staurolite) not found elsewherein the rocks. Detailed microprobe study has revealed large chemical variationswithin each phase. Different textural types of each phase havedifferent compositions, and strong zoning is preserved in garnet(Mg/(Mg + Fe) from 0.30 to 0.61) and coarse sapphirine. Inclusionpopulations in garnet correlate with host composition. The textural and chemical features are interpreted in termsof successive equilibrium assemblages and reactions. Metamorphicconditions operative at each stage in the evolution are calculatedusing published geothermometers and geobarometers as well asthermodynamically calibrated MAS and FASH equilibria. The resultsare used to construct a P—T-time path for the sapphirine-granulites,which can be summarized as follows: (i) Increasing T at high P (>7 kb). Partial melting. (ii) A maximum T of 830 ?C attained at 10 kb. (iii) Almost isothermal decompression, reaching 750 ?C at 5kb, under conditions of low µ_H2O. (iv) Further cooling, and decompression. Localized hydration.Rocks exposed. The P—T-time path is interpreted as the product of a singlemetamorphic cycle (the tertiary ‘Lepontine’ event)and is extrapolated to the Gruf Complex as a whole. When combinedwith published geochronological data, the results indicate anaverage uplift rate in excess of 2 mm/yr for the Gruf Complexbetween 38 and 30 Ma ago. An in situ partial melting origin for the sapphirine-granulitesis favoured. Extraction of an iron-rich granitic liquid froma normal pelitic palaeosome could generate a refractory residuewith the required Mg, Al-rich composition. The change in bulksolid composition during partial melting is thought to accountfor the extraordinarity strong zoning in the garnets.     

The Genesis of Zoned Skarns in the Sierra Nevada, California

Zoned skarns occur at plutonic-metamorphic contacts, in veinscutting marble, and at contacts between marble and interlayeredamphibolite and biotite-rich rocks. For P = 2 kb, fluid inclusionsand P-T-X_CO2 stability relations of calc-silicate assemblagessuggest T< 650 °C and a H₂O-rich fluid (X_CO2 < 0.1).Small-scale, Ca-rich endoskarns are common near exoskarns. Massbalance calculations suggest that: (a) the formation of exoskarnrequires the influx of solute in an aqueous solution from uncontaminatedmagma in addition to material derived from the endoskarn, (b)some ‘limestone assimilation’ is required to formendoskarns, and (c) skarn formation was essentially a constant-volumeprocess. Applying chromatographic theory, compositional profilesof garnet and pyroxene across zoned skarns suggest that infiltrationmetasomatism was an important process, although diffusion metasomatismappears to have produced local compositional gradients at theinfiltration ‘fronts’. Fluid flow calculations showthat thick exoskarns could readily form by intergranular infiltration of aqueous solutions. Reciprocal diffusional exchangeis suggested as a dominant mechanism in the formation of zonedskarns formed at contacts between interlayered metamorphic lithologies.     

Polymetamorphic history of a relict Permian hornfels from the central Gran Paradiso Massif (Western Alps,Italy): a microstructural and thermodynamic modelling study

A petrological and thermobarometric study of the Lago Teleccio hornfelses was undertaken to reconstruct the polymetamorphic evolution and constrain the P–T conditions of Permian contact metamorphism. The Lago Teleccio metasedimentary rocks record a Variscan regional metamorphism characterized by amphibolite facies mineral assemblages including quartz, plagioclase, K‐feldspar (Kfs 1), biotite, garnet (Grt 1) and staurolite; this was followed by a late‐Variscan mylonitization event. Metamorphism of the Variscan metamorphic rocks at the contact with a Permian granitic intrusion produced static recrystallization and/or new growth of quartz, garnet (Grt 2), plagioclase, K‐feldspar (Kfs 2), cordierite, green spinel, biotite and prismatic sillimanite (Contact 1). This thermal event, which occurred at a peak pressure of 0.23–0.35 GPa, temperature of 670–700 °C and a_H2O of 0.75–1, was followed either during post‐contact metamorphism cooling or, more likely, during the early‐Alpine metamorphism by the breakdown of cordierite into an anhydrous kyanite + orthopyroxene + quartz assemblage. The poorly developed early‐Alpine eclogite facies metamorphism (Alpine 1) was characterized by relatively anhydrous mineral associations and low strain, which locally produced coronitic and pseudomorphous microstructures in metasedimentary rocks, with scanty formation of jadeite, zoisite and a new high‐pressure garnet (Grt 3). Greenschist facies retrogression (Alpine 2) was characterized by the local development of a chlorite‐ and muscovite‐bearing mineral association, suggestive of aqueous fluid incursion. In the hornfelses, the limited extent of metamorphic overprinting is suggested by the fine grain size of the Alpine mineral associations, which formed at the expense of the Permian contact metamorphic associations, and was favoured by the anhydrous mineralogy of the hornfelses.     

Petrology of thermally metamorphosed pelitic rocks in the Champawat Area,Kumaun Himalaya

In the Champawat area, Kumaun Himalaya, greenschist facies regionally metamorphosed rocksviz chlorite-phyllite and schist have been subjected to thermal metamorphism due to emplacement of batholithic granite/granodiorite body. As a consequence, biotite, garnet, andalusite, fibrolite, sillimanite and perthite minerals have formed in the contact rocks. The conspicuous absence of cordierite and staurolite reported from such aureole rocks is due to higher FeO/MgO ratio of the bulk rock composition in the former while the absence of staurolite is due to low Al₂O₃/FeO+MgO ratio in the schists. AFM diagram demonstrates that in muscovite-bearing schist, the bulk composition of chlorite- and cordierite-bearing rocks are restricted to low FeO/MgO side and thus the restricted occurrence of former and the absence of latter in the contact rocks of the area. This is further evident from the common occurrence of almandine-rich garnet in the rocks.     

The Graphical Analysis of Greenschist to Amphibolite Facies Mineral Assemblages in Metabasites

The mineral assemblages of greenschist to amphibolite faciesmetabasites may usually be represented in a system of principalcomponents: CaO–Al₂O₃–(Fe₂O₃)–FeO–MgO–Na₂O–SiO₂–CO₂–H₂O Assemblages co-existing with quartz, ‘albite’, ‘epidote’and a fluid of restricted composition, may be shown by projectionin a CAFM subsystem from ‘epidote’ onto an extendedAFM plane. This projection is analogous to the Thompson projectionfor pelites and is particularly useful in displaying the effectsof Fe/Mg and Al substitution in the silicates as well as incorporatingCaO; it is illustrated by plotting assemblages from the SouthernAlps of New Zealand and the Scottish Highlands and demonstrateschanges occurring with grade in the assemblages. Some commonisograds and facies boundaries are seen to be strongly dependenton bulk rock composition. In some cases MnO must be consideredas an additional component. A model of P_solids=P_fluid, where the fluid is composed of CO₂+H₂Ois consistent with many greenschist to amphibolite facies metabasicassemblages. Natural assemblages indicate this fluid phase tohave restricted mobility. Theoretical consideration of mineralreactions resulting from increasing X_co2, in conjunction withdata from natural mineral assemblages, leads to the distinctionof five principal types of assemblage which may be expectedas a function of varying X_Co2. Recognition of these assemblagetypes provides a useful guide to relative X_Co2 during metamorphism. ^* Present Address: Department of Geology, University of California, 405 Hilgard Avenue, Los Angeles, California 90024.     

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 lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur 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, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. X_Mg in the coexisting phasesdecreases in the order cordierite–biotite–sapphirine–orthopyroxene–spinel–garnet–(Fe–Ti)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving 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 petrogeneticgrid for the system FeO–MgO–Al₂O₃–SiO₂–K₂O–H₂Oat high fo₂, has been constructed and the effects of ZnO andFe₂O₃ on this grid have been explored Combining available experimentaland natural occurrence data, the high fo₂ invariant points inthe partial grid have been located in P–T space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise P–T trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry Corresponding author     

Petro-tectonic Imprints in the Sapphirine Granulites from Anantagiri, Eastern Ghats Mobile Belt, India

Sapphirine granulite occurring as lenses in charnockite at Anantagiri,Eastern Ghat, India, displays an array of minerals which developedunder different P-T-X conditions. Reaction textures in conjunctionwith mineral chemical data attest to several Fe-Mg continuousreactions, such as

1. spinel+rutile+quartz+MgFe_–1=sapphirine+ilmenite
2. cordierite=sapphirine+quartz+MgFe_–1
3. sapphirine+quartz=orthopyroxene+sillimanite+MgFe_–1
4. orthopyroxene+sapphirine+quartz=garnet+MgFe_–1
5. orthopyroxene+sillimanite=garnet+quartz+MgFe_–1
6. orthopyroxene+sillimanite+quartz+MgFe_–1=cordierite.

Calculated positions of the reaction curves in P-T space, togetherwith discrete P-T points obtained through geothermobarometryin sapphirine granulite and the closely associated charnockiteand mafic granulite, define an anticlockwise P-T trajectory.This comprises a high-T/P prograde metamorphic path which culminatedin a pressure regime of 8?3 kb above 950?C, a nearly isobariccooling (IBC) path (from 950?C, 8?3 kb, to 675?C, 7?5kb) anda terminal decompressive path (from 7?5 to 4?5 kb). Spinel,quartz, high-Mg cordierite, and sapphirine were stabilized duringthe prograde high-T/P metamorphism, followed by the developmentof orthopyroxene, sillimanite, and garnet during the IBC. Retrogradelow-Mg cordierite appeared as a consequence of decompressionin the sapphirine granulite. Deformational structures, reportedfrom the Eastern Ghat granulites, and the available geochronologicaldata indicate that prograde metamorphism could have occurredat 30001?00 and 2500?100 Ma during a compressive orogeny thatwas associated with high heat influx through mafic magmatism. IBC ensued from P_max and was thus a direct consequence of progrademetamorphism. However, in the absence of sufficient study onthe spatial variation in P-T paths and the strain historiesin relation to time, the linkage between IBC and isothermaldecompression (ITD) has remained obscure. A prolonged IBC followedby ITD could be the consequence of one extensional mechanismwhich had an insufficient acceleration at the early stage, orITD separately could be caused by an unrelated extensional tectonism.The complex cooled nearly isobarically from 2500 Ma. It sufferedrapid decompression accompanied by anorthosite and alkalinemagmatism at 1400–1000 Ma.     

Petrology of some Glaucophane Schists and Related Rocks from Taiwan

Detailed laboratory study has been made on pre-Tertiary coarse-grainedglaucophane schist, garnet-epidote amphibolite, and epidoteamphibolite in the eastern slope of the Central Mountain Range,Taiwan. These petrotectonic assemblages are considered to beexotic tectonic blocks emplaced within the feebly metamorphosedin situ graphite and quartzose schists of the Yuli belt. Thinlenses of Mn-rich metamorphosed tuff are intercalated withinthe metabasaltic rocks. Such high MnO (2 wt. per cent) and lowMgO (3–4 wt. per cent) tuffaceous rocks are similar inbulk composition to some volcanic clays collected in deep oceanbasins. They consist of the characteristic assemblage Mn-bearinggarnet (5–7 wt. per cent MnO and 30 volume per cent inthe rock)+muscovite+epidote+hornblende+quartz+ albite+rutile?pyrite. Successive stages of conversion of garnet-epidote amphiboliteto blueschist assemblages were noticed. The most recrystallizedschists display abundant Mn-bearing garnet, zoned amphibole,phengite, zoned epidote, stilpnomelane, chlorite, quartz, minoralbite, magnetite, and sphene. The recrystallization processis nearly isochemical except the glaucophane schists appearto be more oxidized and contain more Na₂O than the relict amphibolites.Intimately associated amphibolites of basaltic composition,in contrast, contain the assemblage hornblende+paragonite+epidote+chlorite+quartz+albite+rutile. Microprobe analyses of the coexisting minerals in glaucophaneschists, garnet-epidote amphibolites and epidote amphibolitesyield the following results: (1) garnets, consisting of almandine,spessartine, and grossular components, are less Mn and Mg-richcompared to those in in situ metabasalts of the Franciscan;(2) rim epidotes of the glaucophane schists are more pistastic(X_Fe=0?27–0?30) than that of the garnet-epidote amphibolite(0?2–0?22) implying higher fO₂ values for the glaucophanization;(3) phengitic micas of the glaucophane schist have less Al₂O₃content (29 wt. per cent) than those of the garnet-epidote amphibolite(32 wt. per cent) whereas micas of epidote amphibolites areparagonites with K/(K+Na) ratio of 0?04; (4) the zoned amphibolesshow glaucophane occurring marginal to cores of calcic amphibole.Sodic amphiboles with Al₂O₃ of 6-? to 10?4 wt. per cent arecrossite-glaucophane whereas all calcic amphiboles analyzedare barroisite-pargasite (Al₂O₃ greater than 10 wt. per cent). The garnet-epidote-rutile bearing glaucophane schist of Taiwanprobably recrystallized at temperatures above 350 ?C (the epidotezone) whereas the lawsonite-sphene glaucophane schists of theFranciscan equilibrated below 350 ?C (the lawsonite zone). TheMn-rich basaltic tuffs and their associated flows appear tohave been metamorphosed at profound depths and at the relativelyhigh temperatures of the epidote amphibolite facies, succeededlater by glaucophane schist facies metamorphism at lower temperatures.     

Mineral Assemblages of Amphibolites Associated with Alpine-Type Ultramafics in the Dinaride Ophiolite Zone (Yugoslavia)

Amphibolites are frequently and characteristically associatedwith alpine-type ultramafics within ophiolite zones of differentgeological ages. Ultramafics of the Dinaridic ophiolite zoneare predominantly lherzolite, and are associated with differentvarieties of gabbro, dolerite, diabase, spilite, and amphibolite.Amphibolites commonly form narrow, interrupted zones aroundlarger ultramafic massifs, and, in some areas, they exceed theultramafics in size. All these rocks are members of the Jurassic(?)volcanic-sedimentary complex (‘Diabas-Hornstein Formation’)consisting predominantly of graywacke type sandstone and shale. The amphibolites characteristically have crystalloblastic texturesand are commonly banded and foliated, which distinguishes themfrom associated gabbros and dolerites. Different kinds and varietiesof metamorphic rocks can be found within the Dinaridic amphibolitecomplexes: amphibolite schists containing plagioclase, pyroxeniteschists containing diopside and plagioclase, monomineralic amphiboleschists, and eclogites. Samples of each of these rock typeswere collected from several localities and monomineralic fractionsof amphiboles, plagioclases, garnets, and clinopyroxenes wereseparated and examined in detail. Amphiboles range from kaersutite, pargasite, and edenite throughtransitional varieties enriched in tschermakite and ‘common’hornblende molecules. Plagioclase varies from anorthite to sodicoligoclase. Associated garnets are enriched either in pyropeor in almandine. Clinopyroxene is hedenbergitic diopside withabout 10–20 per cent of jadeite molecule and hypersthenecontains about 40 per cent FeSiO₃. Variations in mineral composition of the amphibolites demonstratestheir metamorphic origin. Although there is a continuous changein chemical composition, three mineral assemblages can be distinguished.Data from experimental petrology indicate that the amphibolitesassociated with alpine-type ultramafics can be stable underupper mantle PT conditions.     

The Occurrrence of Garnet in the Granulite-Facies Terrane of the Addirondack Highlands and Elsewhere, an Amplification and a Reply

Elaborating on the theme of a previous paper (1965) and in responseto Buddington‘s criticism (1966) the present discussionis primarily concerned with the high-grade meta-morphic reactionsand regional-metamorphic zoning in the Adirondacks. The hypersthene isograd, delineated by the first appearanceof orthopyroxene as a product of P_w-T-controlted reactions involvinghornblende and biotite in quartz-buartz-bearing rocks. separatesthe almandite-amphibolite-facies terrane of the lowlands fromthe granulite-facies region of the highlands. In the almandite-amphibolite facies of the lowlands at leastone isograd, the orthoclase-garnet isograd, (Ruddington's lstgarnet isograd), can be recognized which is delineated by theP_w-T-controlled appearance of these minerals at the expenseof muscovite and part of the biotite and quartz. The granulite-facies terrane of the highlands is characterizedby the predominance of assemblages of the hornblende-granulitefacies in which orthopyroxene cocxists with hydrous mineralssuch as biotite and hornblende in apparent equilibrium. In theAdlrondacks there is no sizeable, regional development of thepyroxene-granulite facies where hydrous minerals are virtuallyabsent. Within the hornblende-granulite-facies terrane two isogradscan be recognized. The garent-clinopyroxene isograd (Buddington's3rd garnet isograd) delineates the regional development of garnet,clinopyroxene, and quartz by P₁-T-controlled reaction betweenorthopyroxene and plagioclase. The isograd defines in the easternAdirondack a region of the hornblende-clinopyroxene-almanditesubfacies of the honblende-granulite facies. In the westernAdirondacks, near the hypersthene isograd, a yet uncharted cordieriteisograd may define the areal extent of the biotite-cordierite-almanditesubfacies of the hornblende-granulite facies. The central portionof the Adirondacks, where cordierite is absent and garnet andclinopyroxene do not coexist except in silica-deficient rocks,is the region of the hornblende-orthopyroxene-plagioclase subfaciesof the hornblende-granulite facies.     

High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications

Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitic–trondhjemitic–granodioriticgneisses in the Hengshan Complex. Petrological evidence fromthese rocks indicates four distinct metamorphic assemblages.The early prograde assemblage (M₁) is preserved only in thehigh-pressure granulites and represented by quartz and rutileinclusions within the cores of garnet porphyroblasts, and omphacitepseudomorphs that are indicated by clinopyroxene + sodic plagioclasesymplectic intergrowths. The peak assemblage (M₂) consists ofclinopyroxene + garnet + sodic plagioclase + quartz ±hornblende in the high-pressure granulites and orthopyroxene+ clinopyroxene + garnet + plagioclase + quartz in the medium-pressuregranulites. Peak metamorphism was followed by near-isothermaldecompression (M₃), which resulted in the development of orthopyroxene+ clinopyroxene + plagioclase symplectites and coronas surroundingembayed garnet grains, and decompression-cooling (M₄), representedby hornblende + plagioclase symplectites on garnet. The THERMOCALCprogram yielded peak (M₂) P–T conditions of 13·4–15·5kbar and 770–840°C for the high-pressure granulitesand 9–11 kbar and 820–870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The P–T conditions of pyroxene+ plagioclase symplectite and corona (M₃) were estimated at