Metamorphic Conditions and Fluid Compositions of Scapolite-Bearing Rocks from the Lapis Lazuli Deposit at Sare Sang, Afghanistan

Scapolite and other halogen-rich minerals (phlogopite, amphibole,apatite, titanite and clinohumite) occur in some high-pressureamphibolite facies calc-silicates and orthopyroxene-bearingrocks at Sare Sang (Sar e Sang or Sar-e-Sang), NE Afghanistan.The calc-silicates are subdivided into two groups: garnet-bearingand garnet-free, phlogopite-bearing. Besides garnet and/or phlogopite,the amphibolite facies mineral assemblages in the calc-silicatesinclude clinopyroxene, calcite, quartz and one or more of theminerals scapolite, plagioclase, K-feldspar, titanite, apatiteand rarely olivine. Orthopyroxene-bearing rocks consist of clinopyroxene,garnet, plagioclase, scapolite, amphibole, quartz, calcite andaccessory dolomite and alumosilicate (kyanite?). Retrogradephases in the rocks are plagioclase, scapolite, calcite, amphibole,sodalite, haüyne, lazurite, biotite, apatite and dolomite.The clinopyroxene is mostly diopside and rarely also hedenbergite.Aegirine and omphacite with a maximum jadeite content of 29mol % were also found. Garnet from the calc-silicates is Grs_45–95Py_0–2and from the orthopyroxene-bearing rocks is Grs_10–15Py_36–43.Peak P–T metamorphic conditions, calculated using availableexchange thermobarometers and the TWQ program, are 750°Cand 1·3–1·4 GPa. Depending on the rock type,the scapolite exhibits a wide range of composition (from Eq_An= 0·07, X_Cl =0·99 to Eq_An = 0·61, X_Cl =0·07).Equilibria calculated for scapolite and coexisting phases atpeak metamorphic conditions yield X_CO2 = 0·03–0·15.X_NaCl (fluid), obtained for scapolite, ranges between 0·04and 0·99. Partitioning of F and Cl between coexistingphases was calculated for apatite–biotite and amphibole–biotite.Fluorapatite is present in calc-silicates, but orthopyroxene-bearingrocks contain chlorapatite. Cl preferentially partitions intoamphibole with respect to biotite. All these rocks have sufferedvarious degrees of retrogression, which resulted in removalof halogens, CO₂ and S. Halogen- and S-bearing minerals formedduring retrogression and metasomatism are fluorapatite, sodalite,amphibole, scapolite, clinohumite, haüyne, pyrite, andlazurite, which either form veins or replace earlier formedphases. KEY WORDS: scapolite; fluid composition; high-pressure; amphibolite facies; Western Hindukush; Afghanistan     

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     

The Klokken Gabbro-Syenite Complex, South Greenland: Quantitative Interpretation of Mineral Chemistry

The layered syenite series in the Klokken stock formed by continuousin situ fractionation of a trachytic magma in a chamber linedby gabbro with 3000 m of cover rocks. The following mineralsand reactions are assessed as geothermometers and barometers:two feldspars; hypersolvus ternary feldspars; ferrohedenbergite-ß-wollastonite;clinopyroxene-olivine Fe-Mg exchange; Fe-Ti oxides; sanidine-magnetite-annite;ferroedenite stability. Estimates of silica activity are obtainedfrom the silica-magnetite-fayalite assemblage. The gabbros ended magmatic crystallization at > 1000–1050°C.The less fractionated members of the syenite range probablycrystallized with P_H2O < P_total, at T > 870°C and,P_H2O 800 bars. In the more fractionated syenites P_H2O = P_totalduring intercumulus feldspar growth, and all magmatic phasescrystallized within the interval 940–830°C and P_H2O< 1100 bars. Magmatic f_O2 (bars) was 1 log unit below theQFM buffer. a_SIO2 in gabbros was slightly above the albite-nephelinebuffer, but rose suddenly to just <1 in the syenites, a jumpmirrored by minor elements in pyroxenes and opaque oxides. Biotitegrew subsolidus in most rocks, at f_O2 < QFM, except in intermediaterocks when f_O2 > QFM and was defined by the sanidine-magnetite-biotiteassemblage. In these rocks P_H2O of 450 bars at 760°C isobtained using existing experimental data, but application ofthis data to Fe-rich biotites in the layered series (where biotiteis an intercumulus phase) requires P > 10 kb at magmatictemperatures. High TiO₂ or F: OH probably accounts for increasedT stability of natural annites at low P. The syenitic liquid fractionated down a low temperature zonein a multicomponent system precipitating alk fsp + ol + cpx+ mt and the more fractionated members of the layered serieshad a negligible crystallization interval, a prerequisite forthe development of the unique Klokken-type inversely gradedmineral layering.     

Phase Relations and Stability of Magnetoplumbite- and Crichtonite-Series Phases under Upper-Mantle P-T Conditions: an Experimental Study to 15 GPa with Implications for LILE Metasomatism in the Lithospheric Mantle

High-pressure–high-temperature experiments were performedin the range 7–15 GPa and 1300–1600°C to investigatethe stability and phase relations of the K- and Ba-dominantmembers of the crichtonite and magnetoplumbite series of phasesin simplified bulk compositions in the systems TiO₂–ZrO₂–Cr₂O₃–Fe₂O₃–BaO–K₂Oand TiO₂–Cr₂O₃–Fe₂O₃–BaO–K₂O. Both seriesof phases occur as inclusions in diamond and/or as constituentsof metasomatized peridotite mantle xenoliths sampled by kimberlitesor alkaline lamprophyres. They can accommodate large ion lithophileelements (LILE) and high field strength elements (HFSE) on awt % level and, hence, can critically influence the LILE andHFSE budget of a metasomatized peridotite even if present onlyin trace amounts. The Ba and K end-members of the crichtoniteseries, lindsleyite and mathiasite, are stable to 11 GPa and1500–1600°C. Between 11 and 12 GPa, lindsleyite breaksdown to form two Ba–Cr-titanates of unknown structurethat persist to at least 13 GPa. The high-pressure breakdownproduct of mathiasite is a K–Cr-titanate with an idealizedformula KM₇O₁₂, where M = Ti, Cr, Mg, Fe. This phase possessesspace group P6₃/m with a = 9·175(2) Å, c = 2·879(1)Å, V = 209·9(1) ų. Towards high temperatures,lindsleyite persists to 1600°C, whereas mathiasite breaksdown between 1500 and 1600°C to form a number of complexTi–Cr-oxides. Ba and K end-members of the magnetoplumbiteseries, hawthorneite and yimengite, are stable in runs at 7,10 and 15 GPa between 1300 and 1400°C coexisting with anumber of Ti–Cr-oxides. Molar mixtures (1:1) of lindsleyite–mathiasiteand hawthorneite–yimengite were studied at 7–10GPa and 1300–1400°C, and 9–15 GPa and 1150–1400°C,respectively. In the system lindsleyite–mathiasite, onehomogeneous Ba–K phase is stable, which shows a systematicincrease in the K/(K + Ba) ratio with increasing pressure. Inthe system hawthorneite–yimengite, two coexisting Ba–Kphases appear, which are Ba rich and Ba poor, respectively.The data obtained from this study suggest that Ba- and K-dominantmembers of the crichtonite and magnetoplumbite series of phasesare potentially stable not only throughout the entire subcontinentallithosphere but also under conditions of an average present-daymantle adiabat in the underlying asthenosphere to a depth ofup to 450 km. At still higher pressures, both K and Ba may remainstored in alkali titanates that would also be eminently suitablefor the transport of other ions with large ionic radii. KEY WORDS: crichtonite; magnetoplumbite; high-P–T experiments; phase relations; upper mantle     

Fractionation and Assimilation Processes in the Alkaline Augite Syenite Unit of the Ilimaussaq Intrusion, South Greenland, as Deduced from Phase Equilibria

The early augite syenite unit in the 1·13-Ga-old Ilímaussaqintrusive complex, South Greenland, consists of a magmatic assemblageof ternary alkali feldspar + fayalitic olivine + augite + titanomagnetite+ apatite + baddeleyite ± nepheline ± quartz ±ilmenite ± zircon. Feldspar, nepheline and QUILF thermometryyield T = 1000–700°C, at P = 1 kbar, which is derivedfrom fluid inclusion data from other parts of the complex. Ternaryfeldspar was the first major liquidus phase. It crystallizedat temperatures between 950 and 1000°C from a homogeneousmagma with a_SiO2 = 0·8 and f_O2 about 1·5–2log units below the fayalite–magnetite–quartz (FMQ)buffer. Later, closed system fractionation produced nepheline-bearingassemblages with a_SiO2 = 0·4 and log f_O2 = FMQ –3 to FMQ – 5. Assimilation of wall rocks produced localvariations of melt composition. Four traverses through the unitwere sampled parallel to the assumed direction of crystallization.They exhibit significant differences in their mineral assemblagesand compositions. The chemical zoning and calculated intensiveparameters of four sample suites reflect both closed systemfractional crystallization and local assimilation of wall rocks. KEY WORDS: alkaline magmatism; assimilation; fractionation; redox equilibria; QUILF     

Petrology of the Blue Mountain Complex, Marlborough, New Zealand

Blue Mountain is a central-type alkali ultrabasic-gabbro ringcomplex (lxl7middot;5 km) introducing Upper Jurassic sediments,Marlborough, New Zealand. The ultrabasic-gabbroic rocks containlenses of kaersutite pegmatite and sodic syenite pegmatite andare intruded by ring dykes of titanaugite-ilmenite gabbro andlamprophyre. The margin of the intrusion is defined by a ringdyke of alkali gabbro. The plutonic rocks are cut by a swarmof hornblendebiotite-rich lamprophyre dykes. Thermal metamorphismhas converted the sediments to a hornfels ranging in grade fromthe albite-epidote hornfels facies to the upper limit of thehornblende hornfels facies. The rocks are nepheline normative and consist of olivine (Fo_82–74),endiopside (Ca₄₅Mg₄₈Fe₇–Ca₃₆Mg₅₅Fe₉), titanaugite (Ca₄₀Mg₅₀Fe₁₀–Ca₄₄Mg₃₉Fe₁₇),plagioclase (An_73–18), and ilmenitetitaniferous magnetite,with various amounts of titaniferous hornblende and titanbiotite.There is a complete gradation between endiopside and titanaugitewith the coupled substitution R_y⁺²+Si;;(Ti⁺⁴+Fe⁺³+Al⁺³ and asympathetic increase in CaAl₂SiO₆ (0·2–10·2percent) and CaTiAl₂O₆ (2·1–8·1 per cent)with fractionation. Endiopside shows a small, progressive Mgenrichment along a trend subparallel to the CaMgSi₂O₆–Mg₂Si₂O₆boundary, and titanaugite is enriched in Ca and Fe⁺²+Fe⁺³ withdifferentiation. Oscillatory zoning between endiopside and titanaugiteis common. Exsolved ilmenite needles occur in the most Fe-richtitanaugites. The amphiboles show the trend: titaniferous hornblende(1·0–57middot;7 per cent TiO₂) kaersutite (6·4per cent TiO2) Fe-rich hastingsite (18·0–19·1per cent FeO as total Fe). Biotite is high in TiO₂ (6·6–7·8per cent). Ilmenite and titaniferous magnetite (3·5–10·6per cent TiO₂) are typically homogeneous grains; their compositioncan be expressed in terms of R⁺²RO₃:R⁺²O:R₂⁺³O₄. The intrusion of igneous rocks was probably controlled by subterraneanring fracturing. Subsidence of the country rock within the ringfracture provided space for periodic injections of magma froma lower reservoir up the initial ring fracture to form the BlueMountain rocks at a higher level. Downward movement of the floorof the intrusion during crystallization caused inward slumpingof the cumulates which affected the textural, mineralogical,and chemical evolution of the rocks in different parts of theintrusion. The order of mineral fractionation is reflected by the chemicalvariation in the in situ ultrabasic-gabbroic rocks and the successiveintrusions of titanaugite-ilmenite gabbro and lamprophyre ringdykes, marginal alkali gabbro and lamprophyre dyke swarm. Aninitial decrease, then increase in SiO2; a steady decrease inMgO, CaO, Ni, and Cr: an initial increase, then decrease inFeO+Fe₂O₃, TiO₂, MnO, and V; almost linear increase in A1₂O₃and late stage increase in alkalis and P₂O₃, implies fractionationof olivine and endiopside, followed by titanaugite and Fe-Tioxides, followed by plagioclase, hornblende, biotite, and apatite.Reversals in the composition of cumulus olivine and endiopsideand Solidification Index, indicate that the ultrabasic-gabbroicsequence is composed of four main injections of magma. The ultrabasic rocks crystallized under conditions of high P_H2Oand fairly high, constant     

Mantle Xenoliths from Tenerife (Canary Islands): Evidence for Reactions between Mantle Peridotites and Silicic Carbonatite Melts inducing Ca Metasomatism

Mantle xenoliths from Tenerife show evidence of metasomatismand recrystallization overprinting the effects of extensivepartial melting. The evidence includes: recrystallization ofexsolved orthopyroxene porphyroclasts highly depleted in incompatibletrace elements into incompatible-trace-element-enriched, poikiliticorthopyroxene with no visible exsolution lamellae; formationof olivine and REE–Cr-rich, strongly Zr–Hf–Ti-depletedclinopyroxene at the expense of orthopyroxene; the presenceof phlogopite; whole-rock CaO/Al₂O₃ >> 1 (Ca metasomatism) inrecrystallized rocks; and enrichment in incompatible elementsin recrystallized rocks, relative to rocks showing little evidenceof recrystallization. The ‘higher-than-normal’ degreeof partial melting that preceded the metasomatism probably resultsfrom plume activity during the opening of the Central AtlanticOcean. Sr–Nd isotopic compositions are closely similarto those of Tenerife basalts, indicating resetting from theexpected original mid-ocean ridge basalt composition by themetasomatizing fluids. Metasomatism was caused by silicic carbonatitemelts, and involved open-system processes, such as trappingof elements compatible with newly formed acceptor minerals,leaving residual fluids moving to shallower levels. The compositionsof the metasomatizing fluids changed with time, probably asa result of changing compositions of the melts produced in theCanary Islands plume. Spinel dunites and wehrlites representrocks where all, or most, orthopyroxene has been consumed throughthe metasomatic reactions. KEY WORDS: Canary Islands; Tenerife; mantle xenoliths; geochemistry; Ca metasomatism; open-system processes; lithosphere; ocean islands     

A Clinopyroxene Paragenesis of Albite--Epidote--Amphibolite Facies in Meta-Syenites from the South-East Tauern Window, Austria

Mineral assemblages and textures are described from clinopyroxene-bearingmeta-syenites and related rocks from a small area in the PenninicBasement Complex of the south-east Tauern Window. Evidence from mineral textures, mineral compositions and geobarometryindicate that the clinopyroxene, a sodic salite, crystallizedas part of an equilibrium albite-epidote-amphibolite faciesparagenesis in the 35–40 Ma meso-Alpine metamorphic event.Phase relations in co-facial quartz + albite + K-feldspar +sphene-bearing meta-syenites and meta-granites are examinedusing a projection from these minerals onto the plane (A1₂O₃+ Fe₂O₃)-CaO-(MgO + FeO + MnO). The projection demonstratesthat salitic clinopyroxene can only be a stable phase in suchrocks if the bulk-rock Al/Na + K ratios are low. This is confirmedby comparing the whole-rock analyses of clinopyroxene-bearingmeta-syenites with those of clinopyroxene-free meta-syenitesand meta-granites. Mineral assemblages in a variety of lithologies from the south-eastTauern Window are used to construct a generalized AKM diagramfor magnesian albite + epidote + quartz-bearing rocks of thealbite-epidote-amphibolite facies. Thermochemical calculations indicate that the meta-syeniteswere metamorphosed at temperatures close to 500 C and at a pressureof 6+2 –4 kb. Fluids in equilibrium with meta-syeniteand meta-granite mineral assemblages had X_H2O values of 0–95,assuming X_H2O + X_CO2O= 1.0.     

The Mineralogy and Geochemistry of the Metamorphosed Basic and Ultrabasic Rocks of the Jijal Complex, Kohistan, NW Pakistan

The Jijal complex, covering more than 150 sq. km in the extremenorth of Pakistan, is a tectonic wedge of garnet granulitesintruded in the south by a 10 x 4 km slab of ultramafic rocks.The granulites are divisible into plagioclase-bearing (basicto intermediate) and plagioclase-free (ultrabasic to basic)types, the two types reflecting differences in bulk chemistry.Garnet + plagioclase + clinopyroxene + quartz + rutile ±hornblende ± epidote is the most common assemblage. Theplagioclase-free rocks are composed mainly of two or three ofthe minerals garnet, amphibole, clinopyroxene and epidote. Orthopyroxeneoccurs in websteritic rocks devoid of epidote. Much of the amphiboleand some epidote appear to be prograde products. Although variationdiagrams do not reveal a genetic link between the two typesof granulite, it is considered that they are comagmatic ratherthan the products of two or more unrelated magmas. The compositions of garnet (Py_28–46 Alm _27–43Gro_16–28),clinopyroxene (Mg_44–34Fe_5–17Ca_51–49, Al₂O₃3·0–9·9 per cent), orthopyroxene (with upto 5·5 per cent Al₂O₃), amphibole (with up to 16·3per cent Al₂O₃ and high Al^vi/Al^iv), and the abundance of garnetsuggest a high-pressure origin for the granulites. The rocksappear to have differentiated from a tholeiitic magma of oceanicaffinity or they may be genetically related to the pyroxenegranulites of Swat considered to have originally crystallizedfrom a calc-alkaline magma of island arc or continental marginaffinity. They probably crystallized in the ancient Tethyancrust/upper mantle (or less likely in a continental margin),later to be metamorphosed to granulites (670–790 °C,12–14 kb) during the collision of the Indian-Asian landmasses,and carried upwards during later Himalayan orogenic episodes. The ultramafic rocks are alpine-type in nature and devoid ofgarnet. They are dominated by diopsidites; dunites, peridotites,and harzburgites together form <50 per cent of the area ofoutcrop. The chemistry of the rocks, and their olivines (Fo_92–89)and clinopyroxenes (Mg_49.5–48Fe_2.8–5.2Ca_47.4–46.8)are similar to those of alpine complexes of the harzburgitesubtype. It is not clear whether they represent a faulted slabof suboceanic crust/upper mantle, mantle diapirs in deep orogenicroots, or dismembered ultramafic rocks differentiated from abasaltic magma. They seem to have a complex history; their presentmineralogy is suggestive of high grade metamorphism (800–850°C, 8–12 kb). They are magmatically unrelated to thegarnet granulites and were probably intruded into the latteras plastic crystalline material after both had been independentlymetamorphosed, but before the entire complex was carried tectonicallyinto its present surroundings. The abundances of the diopsiditesis in marked contrast to other alpine-type complexes and thepossibility of Ca and Si metasomatism during or before theirmetamorphism should not be totally ruled out.     

Nepheline--Alkali Feldspar Equilibria: I. Experimental Data and Thermodynamic Calculations

Nepheline-alkali feldspar equilibria with alkali chloride aqueoussolutions have been determined for the temperature range 400to 700 °C at 1000 bars pressure. Nepheline-alkali feldsparequilibria with alkali chloride melts have been determined forthe temperature range 800 to 1100 °C at approximately 6bars pressure. (1) NaAlSiO₄ + KCl(aq) = NaCl(aq) + KAlSiO₄ (2) NaAlSiO₄ + KCl(melt) = NaCl(melt) + KAlSiO₄ (3) NaAlSi₃O₈(high) + KCl(aq) = NaCl(aq) + KAlSi₃O₈(San) (4) NaAlSi₃O₈(low) + KCl(aq) = NaCl(aq) + KAlSi₃O₈(Mic) (5) NaAlSi₃O₈(high) + KCl(melt) = NaCl(melt) + KAlSi₃O₄(San) (6) NaAlSi₃O₈(low) + KCl(melt) = NaCl(melt) + KAlSi₃O₈(Mic) From these, two diagrams of phase relationships were derivedfor the following exchange equilibria: (7) NaAlSiO₄ + KAlSi₃O₈(San) = NaAlSi₃O₈(high) + KAlSiO₄; (8) NaAlSiO₄ + KAlSi₃O₈(Mic) = NaAlSi₃O₈(low) + KAlSiO₄. The effect of pressure on these equilibria has been determinedby comparing the experimental data for 1000 and 5000 bars (t= 500 °C) and thermodynamic calculations. It has also beenshown that the effect of excess silica in nepheline solid solutionon the K—Na distribution between nepheline and alkalifeldspar is substantial and opposite to that of temperature.In the high temperature region an increase in silica contentin nepheline of 2 wt. per cent eliminates the effect on theredistribution of a temperature increase of 100 °C. Thesecation exchange data and unit cell data for the crystal phasesare used to calculate thermodynamic mixing properties of nephelinesolid solution and alkali feldspar solid solution for a widerange of temperature and pressure.     

The System Na2O-Al2O3-Fc2O3-SiO2 at 1 Atmosphere, and the Petrogenesis of Alkaline Rocks

Equilibria involving acmite, albite, nepheline, quartz, anda liquid phase constitute the petrologically important partof the system Na₂O–Al₂O₃–Fe₂O₂–SiO₂, and theunivariant and invariant relations provide useful analogiesfor a wide variety of alkaline igneous rocks. These relationsare dominated by the incongruent melting behaviour of acmite,which does not appear on the liquidus of the join acmite-nepheline-silica;instead, a broad field of hematite is present and acmite crystallizesonly from liquids containing potential sodium silicate. Consequently,the oversaturated and undersaturated eutectics, correspondingto granitic and nepheline syenitic liquids, are rich in sodiumsilicate and distinct from those found in Petrogeny's Residuasystem: the temperatures of the eutectics are 7285C and 7155C, respectively. Survival of peralkaline granite in the aluminouscontinental crust can be explained by the strongly peralkalinecomposition of the oversaturated eutectic. Magma of this typemay be the primitive granite of the non-orogenic zones. Theubiquitous alkali metasomatism around alkaline complexes canalso be interpreted in terms of residual liquids enriched inalkali silicates. Transition from undersaturated to oversaturatedliquids is possible by fractionation of hematite and a new processfor achieving the reverse transition has been found. This dependson the substitution of Fe³ for Al³ in feldspar and suggestsa more important role for syenite in any scheme of petrogenesis. Each of the two eutectics is linked to a corresponding peritecticat which hematite reacts to give acmite. The liquid at the undersaturated,quaternary reaction point is of ijolitic type, providing thefirst intimation that ijolite may represent a low-melting fractionin nature. The system Na₂O–Al₂O₃–Fe₂O₃–SiO₂thus constitutes the peralkaline residua system and on thisbasis a coherent picture of stable continental magmatism canbe constructed. Ijolite is seen as the low-melting fractionfrom a range of peralkaline compositions and from rocks suchas melilite basalt, while the frequently associated carbonatiteis considered to be the volatile-rich, fugitive material fromthe mantle. Such a relationship is consistent with the dualassociation of carbonatite with either ijolite or kimberliteunder different tectonic conditions. The more common syenite,nepheline syenite, and alkaline granite of the non-orogenicregions are regarded as low-melting fractions from basalticmaterials in the deep crust. Most of this activity, involvingmagmas of residual type, could thus be explained in terms ofpartial melting in the deep crust and upper mantle. A possiblemechanism for this would be arching of the rigid continentalcrust, the consequent relief of lithostatic load giving riseto melting, and the concentration of fugitive constituents,in the underlying zones.     

Processes and Conditions During Contact Anatexis, Melt Escape and Restite Formation: the Huntly Gabbro Complex, NE Scotland

The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic,mid-crustal, layered, mafic intrusions, emplaced into Proterozoicmetaclastic rocks of NE Scotland soon after the thermal peakof static, high-T, low-P regional metamorphism. This gabbroand its associated contact metamorphic rocks illustrate a varietyof processes operating during contact anatexis and subsequentmelt segregation and extraction. These processes may closelymirror those occurring at much larger scales in the deep crustduring high-grade regional metamorphism and the generation ofgranitic magmas. The emplacement of the Huntly mafic magma resultedin high-grade contact metamorphism and, locally, anatexis ofmetapelites, leading to the formation of migmatites. The migmatitesand country-rock schists were studied to establish the physicalconditions of metamorphism and anatexis, the nature of the meltingreactions, the compositions of the melts produced, and the extentto which melting was a closed- or open-system process. The country-rockschists immediately to the south of the Huntly Complex containmineral assemblages characteristic of the regional andalusitezone. Thermobarometry of an andalusite schist yields regionalmetamorphic conditions of 537 ± 42°C and 0·27± 0·12 GPa, consistent with previously publishedP–T estimates. The contact metamorphic rocks include sillimanitehornfelses, metatexites and diatexites. The metatexites consistof cordierite–K-feldspar hornfels melanosomes and K-feldspar-richgarnetiferous leucosomes. The diatexites consist of schollenof fine-grained granoblastic hornfels and metatexite suspendedin igneous-textured matrix rocks composed of abundant sub/euhedralgarnet, cordierite, plagioclase and, locally, orthopyroxene,with minor interstitial biotite, K-feldspar and quartz. Thehornfels melanosomes and schollen retained their structuralintegrity during partial melting, but the matrix rocks did not.In the highest-grade diatexites, the assemblage Grt + Opx +Crd + Hc + Pl characterizes both the hornfels schollen and thesub/euhedral minerals of the matrix rocks. Application of phaseequilibria to Opx-bearing rocks yields estimated peak-metamorphicconditions of 900 ± 50°C, 0·45 ± 0·1GPa and a_H2O < 0·3. The pressure estimate impliesan emplacement depth of     

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     

Solid-Fluid Equilibria in the System KAlSi3O8-NaAlSi3O8-Al2SiO5-SiO2-H2O-HCl

Activity diagrams in the system KAlSi₃O₈-NaAlSi₃O₈-Al₂SiO₅-SiO₂-H₂O-HClhave been calculated in terms of a_K+/a_H+ and a_N+/a_H+ from existingexperimental data. They show the effect of temperature, pressure,and a_H2O on the stability fields of the alkali feldspars, micas,and aluminium silicate. These activity diagrams are useful in revealing the bufferingcapacity of mineral assemblages and the chemical potential gradientsestablished by changes in T, P, a_H2O, and mineral assemblage.An analysis of mineral paragenesis in terms of these diagramssuggests that mosaic equilibrium, allowing limited metasomatismand internal buffering of chemical potentials, best describemetamorphic systems. Thus the dehydration reaction: muscovite+quartz=K-feldspar+Al₂SiO₅+H₂O which is most important in closed systems, probably fails todescribe in detail the mechanism of natural muscovite decomposition.Rather the decomposition of muscovite is more likely representedby ionic reactions. The replacement of muscovite by feldspar: muscovite+6 SiO₂+2 K⁺=3 K-feldspar+2 H⁺ muscovite+6 SiO₂+3 Na⁺=3 Albite+K⁺+2 H⁺ is favored at high temperature and low pressure, and may accountfor the crystallization of some feldspars in metamorphic rocks.The reaction involving aluminium silicate replacement of muscovite: 2 muscovite+2 H⁺=3 Al₂SiO₅+3 SiO₂+3 H₂O+2 K⁺ is favored at high temperature and pressure and low a_H2O, andcould contribute to the development of the aluminium silicates.It is concluded that both activity diagrams and AKNa projectionsshould be used together to more completely evaluate mineralparagenesis in terms of mosaic equilibria.     

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     

A Biotite Isograd in South-Central Maine, U.S.A.: Mineral Reactions, Fluid Transfer, and Heat Transfer

The biotite isograd in pelitic schists of the Waterville Formationinvolved reaction of muscovite + ankerite + rutile + pyrite+graphite + siderite or calcite to form biotite + plagioclase+ ilmenite. There was no single reaction in all pelites; eachrock experienced a unique reaction depending on the mineralogyand proportions of minerals in the chlorite-zone equivalentfrom which it evolved. Quartz, chlorite, and pyrrhotite werereactants in some rocks and products in others. All inferredbiotite-forming reactions involved decarbonation and desulfidation;some were dehydration reactions and others were hydration reactions.P-T conditions at the biotite isograd were near 3500 bars and400 °C. C-O-H-S fluids in equilibrium with the pelitic rockswere close to binary CO₂-H₂O mixtures with X_CO2 = 0.02–0.04.During the biotite-forming reaction, pelitic rocks (a) decreasedby 2–5 percent in volume, (b) performed – (4–11)kcal/liter P-V work on their surroundings, (c) absorbed 38–85kcal/liter heat from their surroundings, and (d) were infiltratedby at least 0.9–2.2 rock volumes H₂O fluid. The biotite isograd sharply marks the limit of a decarbonationfront that passed through the terrane during regional metamorphism.Decarbonation converted meta-shales with 6–10 per centcarbonate to carbonate-free pelitic schists. One essential causeof the decarbonation event was pervasive infiltration of theterrane by at least 1–2 rock volumes H₂O fluid early inthe metamorphic event under P-T conditions of the biotite isograd.Average shale contains 4–13 per cent siderite, ankerite,and/or calcite, but average pelitic schist is devoid of carbonateminerals. If the Waterville Formation serves as a general modelfor the metamorphism of pelitic rocks, it is likely that worldwidemany pelitic schists developed by decarbonation of shale caused,in part, by pervasive infiltration of metamorphic terranes byseveral rock volumes of aqueous fluid during an early stageof the metamorphic event.     

Fenitizing Processes Induced by Ferrocarbonatite Magmatism at Swartbooisdrif, NW Namibia

The southeastern margin of the anorthositic Kunene IntrusiveComplex, NW Namibia, has been subsequently invaded by Mesoproterozoicsyenite, nepheline syenite and ferrocarbonatite dykes alongNE- and SE-trending faults. The first generation of carbonatiteintrusions frequently contains fenitized anorthositic wall-rockfragments set in a ferrocarbonatite matrix; later, subordinateveins of massive ferrocarbonatite are almost xenolith-free andcut through the main carbonatite dykes. A mantle source forboth carbonatite generations is constrained by their respectiveoxygen and carbon isotope compositions of ankerite (¹⁸O_SMOW8·91–9·73; ¹³C_PDB –6·98 to–6·76). Na-rich fluids, released from the meltparental to the ferrocarbonatites, caused the fenitization ofboth the incorporated anorthosite xenoliths and the borderinganorthosite, syenite and nepheline syenite. This process ismainly characterized by the progressive transformation of Ca-richplagioclase, K-feldspar and nepheline into albite and/or sodalite.The changing mineral modes indicate that the fenitizing fluidswere sodium-rich and strongly Si-deficient solutions, whichalso contained significant amounts of Sr, Ba, Nb and the lightrare earth elements. On the basis of mineral equilibria studies,it is possible to reconstruct the temperature conditions forcarbonatite emplacement (c. 830 ± 200°C) and recrystallization(c. 480 ± 130°C), and for the metasomatic formationof sodalite (c. 700 ± 70°C). KEY WORDS: anorthosite; fenitization; ferrocarbonatite; sodalite; stable isotopes     

Crystallization Conditions of the Sybille Monzosyenite, Laramie Anorthosite Complex, Wyoming

The Sybille Monzosyenite, associated with the Laramie AnorthositeComplex, consists of rocks ranging in composition from monzogabbroto monzosyenite. There is a continuous range of mineral compositionswith plagioclase varying from An₄₅ to An₂₅ and olivine fromFa₇₅ to Fa₉₈ Strongly ternary (Or, Ab, An all > 10 mol%)feldspars–presently mesoperthites–are found in allrock types and define a continuous trend also in the feldsparternary system. The mineral compositions suggest that the rocktypes of the Sybille Monzosyenite could be part of a singledifferentiation sequence; contamination of the later units byassimilation of, or admixture of partial melts from, countryrock is also likely. Original magmatic temperatures of approximately950–1050?C are indicated by estimated original compositionsfor pyroxenes and feldspars; pressure was near 3 kb, as indicatedby the most magnesian olivine that coexists with quartz. Oxygenfugacity of crystallization is estimated as 1.5 to 2.0 log unitsbelow FMQ by using the displaced equilibrium: SiO₂ + 2Fe₂TiO₃= 2FeTiO₃ + Fe₂SiO₄. Such oxygen fugacities are consistent withthe occurrence of graphite and CO₂-rich fluid inclusions, whichsuggest that crystallization took place in the presence of aCO₂ vapor phase. Temperatures indicated by the present mineralassemblages show that all geothermometers used were reset duringcooling, first by intergrain and then by intragrain processes.     

Selected Phase Relationships in the System Al-Mn-Fe-Si-O-H: A Model for Garnet Equilibria

The bulk compositions 3FeO_x.Al₂O₃.3SiO₂ $ excess H₂O and 3MnO.Al₂O₃.3SiO₂$ excess H₂O were investigated employing conventional hydrothermaltechniques. Almandine and spessartine were synthesized and stabilityrelationships determined in terms of temperature, fluid pressure,and oxygen fugacity. Synthetic almandine has unit cell edge, a₀ = 11.528 0.001 index of refraction, N_D = 1.829 0.003. No systematic variationsof these values with respect to temperature, fluid pressure,and oxygen fugacity were observed. Spessartine, synthesizedat high temperatures, has average values of a₀ = 11.614 0.001 and N_D = 1.799 0.003. However, below about 600 C a₀ graduallyincreases to 11.635 0.001 and N_D decreases to 1.772 0.003with decreasing temperature, irrespective of fluid pressureand oxygen fugacity. These changes appear to reflect the productionof hydrospessartine below about 600 C. The stability of almandine strongly depends on the oxygen fugacity.It is stable up to the vicinity of oxygen fugacities definedby the fayalite–magnetite$quartz buffer; the low f_o2,range has not been determined, but lies at oxygen fugacitiesless than those defined by the ironquartz–fayalite buffer.The stability field of almandine$fluid is bounded by the followingP_fluid-T values. At low oxidation states, the low temperature hydrous assemblageof equivalent composition consists of quartz$iron chlorite ($magnetite)$fluidand the high temperature equivalent assemblage consists of fayalite$ironcordierite$hercynite₈₈$fluid. Where f_O2 approximates or is inexcess of that defined by the fayalite–magnetite$quartzbuffer the low temperature hydrous assemblages consist of quartz$ironchlorite$magnetite$fluid, iron chlorite$pyrophyllite$magnetite$fluid,magnetite$mullite$pyrophyllite$fluid, and hematite$mullite$pyrophyllite$fluid;the anhydrous equivalent assemblages consist of quartz$hercynite₈₈,$magnetite₈₈$fluid, quartz$mullite$magnetite$fluid, and quartz$mullite$hematite$fluid,both in order of increasing oxygen fugacity. The stability of spessartine, in contrast to that of almandine,is essentially independent of oxygen fugacity at least up tothat defined by the magnetite-hematite buffer. Spessartine isstable up to the highest temperature, 930 C, employed in thisinvestigation at P_fluid = 500 bars. However, it decomposes toa hydrous assemblage consisting of quartz$manganese chlorite$fluidat the following P_fluid-T values: 414 5 C and 3000 bars;405 5C and 2000 bars; 386 10 C and 1000 bars; 3645C and 500 bars. Garnets are rare constituents of igneous rocks; those whichdo occur are predominantly spessartine-rich, and are virtuallyconfined to felsic magmas. Garnets are absent from mafic igneousrocks because the thermal stability ranges of iron-rich membersare below the solidus. The near absence of almandine in contactmetamorphosed pelitic rocks may reflect a relatively high oxidationstate in the aureoles rather than inappropriate P-T conditions.It is argued that the compositions of pyralspite garnets inpelitic schists are subject to various physical and chemicalfactors, including f_O2. With appropriate provisions, the Mn/Feratios of garnet coexisting with chlorite and quartz might beused as a temperature indicator. The rarity of spessartine in igneous and metamorphic rocks apparentlystems from the departure of rock bulk composition from Mn-richvalues rather than from the absence of appropriate physicalconditions.     

Ultrahigh-temperature Metamorphism (1150{degrees}C, 12 kbar) and Multistage Evolution of Mg-, Al-rich Granulites from the Central Highland Complex, Sri Lanka

Mg- and Al-rich granulites of the central Highland Complex,Sri Lanka preserve a range of reaction textures indicative ofa multistage P–T history following an ultrahigh-temperaturemetamorphic peak. The granulites contain a near-peak assemblageof sapphirine–garnet–orthopyroxene–sillimanite–quartz–K-feldspar,which was later overprinted by intergrowth, symplectite andcorona textures involving orthopyroxene, sapphirine, cordieriteand spinel. Biotite-rims, kornerupine and orthopyroxene-rimson biotite are considered to be late assemblages. Thermobarometriccalculations yield an estimated P–T of at least 1100°Cand 12 kbar for the near-peak metamorphism. Isopleths of Al₂O₃in orthopyroxene are consistent with a peak temperature above1150°C. The P–T path consists of four segments. Initialisobaric cooling after peak metamorphism (Segment A), whichproduced the garnet–sapphirine–quartz assemblage,was followed by near-isothermal decompression at ultrahigh temperature(Segment B), which produced the multiphase symplectites. Furtherisobaric cooling (Segment C) resulted in the formation of biotiteand kornerupine, and late isothermal decompression (SegmentD) formed orthopyroxene rims on biotite. This evolution canbe correlated with similar P–T paths elsewhere, but thereare not yet sufficient geochronological and structural dataavailable from the Highland Complex to allow the tectonic implicationsto be fully assessed. KEY WORDS: central Highland Complex; granulites; multistage evolution; Sri Lanka; UHT metamorphism