Iron-Titanium Oxide Minerals in the Bjerkrem-Sogndal Massif, South-western Norway

Ilmenite and magnetite are investigated from the point of viewof their distribution, microtexture, and chemical composition(major and minor elements) in the Bjerkrem-Sogndal massif (Egersundarea, South-Rogaland, SW. Norway). This massif is an igneouslayered synkinematic lopolith made up of cumulates of the anorthosite-mangeritesuite. The lower part of the massif presents a rhythmic structure. The microtextures of ilmenite result from simple exsolutionof ilmenite-hematite solid solutions. Magnetite contains intergrowthsof ilmenite formed by oxidation-exsolution of ulv?spinel-magnetitesolid-solutions. In the stratigraphic sequence, on a large scale, ilmenite appearsfirst alone, and is then accompanied by magnetite; its hematitecontent decreases towards the top of the massif, while the titaniumcontent of the magnetite increases. On the scale of the rhythms,similar trends but of lesser amplitude are also observed. Evidence of deuteric readjustment of the orthomagmatic compositionof the two oxides is provided (1) by the observation of microtexturesat the contact between grains (zoning of primary ilmenite andrim of secondary ilmenite) (2) by the existence of differencesin chemical composition between isolated grains and grains incontact, and (3) by the determination of the equilibrium temperatureby means of the Buddington and Lindsley geothermometer. Reconstitution of the T-fo₂ orthomagmatic conditions in twoparticular levels of the massif shows that the reducing characterof the magma increases during differentiation. The sudden changesin the oxide assemblage at the base of the rhythms reflect asudden increase in the fo₂ of the magma. These increases, asshown by variation in Cr, Ni, and Co, are due to recurrencesof the basic character of the magma. The variations of the minor elements Mn, V, Ga, and Zn are interpretedin terms of the influence of the deuteric readjustment. It followsthat the ratios Mn/Fe²⁺, Ga/Fe³⁺, and Zn/Fe²⁺ increase and thatthe ratio V/Fe³⁺ decreases in the magma in the course of differentiation.The distribution of Mn between ilmenite and magnetite is discussed. Intermittent supplies of undifferentiated magma are proposedas the geological mechanism controlling the chemical recurrencesassociated with the rhythmic structure.     

Melting and Thermal Reconstitution of Pelitic Xenoliths, Wehr Volcano, East Eifel, West Germany

Pelitic xenoliths derived from amphibolite grade basement rocksoccur within a Pleistocene, trachytic, pyroclastic unit of theWehr volcano, East Eifel, West Germany: With increasing temperatureand/or prolonged heating at high temperature, quartz-plagioclaseand micaceous layers of the xenoliths have undergone meltingto form buchites and thermal reconstitution by dehydration reactions,melting and crystallization to form restites respectively. Thexenoliths provide detailed evidence of melting, high temperaturedecomposition of minerals, nucleation and growth of new phasesand P-T-f_o2 conditions of contact metamorphism of basement rocksby the Wehr magma. Melting begins at quartz-oligoclase (An_17·3Ab_82·3Or_0·4-An_20·0Ab_78·1Or_1·9)grain boundaries in quartz-plagioclase rich layers and the amountof melting is controlled by H₂O and alkalis released duringdehydroxylation/oxidation of associated micas. Initially, glasscompositions are heterogeneous, but with increasing degreesof melting they become more homogeneous and are similar to S-typegranitic minimum melts with SiO₂ between 71 and 77 wt. per cent;A/(CNK) ratios of 1·2–1·4; Na₂O < 2·95and normative corundum contents of 1·9–4·0per cent. Near micas plagioclase melts by preferential dissolutionof the NaAlSi₃O₈ component accompanied by a simultaneous increasein CaAl²Si²O⁸ (up to 20 mol. per cent An higher than the bulkplagioclase composition) at the melting edge. With increasingtemperature the end product of fractional melting is the formationand persistence of refractory bytownite (An_78–80) in thosexenoliths where extensive melting has taken place. Initial stage decomposition of muscovite involves dehydroxylation(H₂O and alkali loss). At higher temperatures muscovite breaksdown to mullite, sillimanite, corundum, sanidine and a peraluminousmelt. Mullite (40–43 mol. per cent SiO₂) and sillimanite(49 mol. per cent SiO₂) are Fe₂O₃ and TiO₂ rich (up to 6·1–0·84and 3·6–0·24 wt. per cent respectively).Al-rich mullite (up to 77 wt. per cent Al₂O₃) occurs with corundumwhich has high Fe₂O₃ and TiO₂ (up to 6·9 and 2·1wt. per cent respectively). Annealing at high temperatures andreducing conditions results in the exsolution of mullite fromsillimanite and ilmenite from corundum. Glass resulting fromthe melting of muscovite in the presence of quartz is peraluminous(A/(CNK) = 1·3) with SiO² contents of 66–69 percent and normative corundum of 4 per cent. Sanidine (An_1·9Ab_26·0Or_72·1-An_1·3Ab_15·9Or_82·9)crystallized from the melt. Dehydroxylation and oxidation of biotite results in a decreaseof K₂O from 8·6 to less than 1 wt. per cent and oxidetotals (less H₂O + contents) from 96·5 to 88·6,exsolution of Al-magnetite, and a decrease in the Fe/(Fe + Mg)ratio from 0·41 to 0·17. Partial melting of biotitein the presence of quartz/plagioclase to pleonaste, Al-Ti magnetite,sanidine(An_2·0Ab_34·9Or63·1) and melt takesplace at higher temperatures. Glass in the vicinity of meltedbiotite is pale brown and highly peraluminous (A/CNK = 2·1)with up to 6 wt. per cent MgO+FeO_{(total iroq)} and up to 10 percent normative corundum. Near liquidus biotite with higher Al₂O₃and TiO₂ than partially melted biotite crystallized from themelt. Ti-rich biotites (up to 6 wt. per cent TiO₂) occur withinthe restite layers of thermally reconstituted xenoliths. Meltingof Ti-rich biotite and sillimanite in contact with the siliceousmelt of the buchite parts of xenoliths resulted in the formationof cordierite (100 Mg/(Mg+Fe+Mn) = 76·5–69·4),Al-Ti magnetite and sanidine, and development of cordierite/quartzintergrowths into the buchite melt. Growth of sanidine enclosedrelic Ca-plagioclase to form patchy intergrowths in the restitelayers. Cordierite (100 Mg/(Mg+Fe+Mn) = 64–69), quartz,sillimanite, mullite, magnetite and ilmenite, crystallized fromthe peraluminous buchite melt. Green-brown spinels of the pleonaste-magnetite series have awide compositional variation of (mol. per cent) FeAl₂O₄—66·6–45·0;MgAl₂O₄—53·0–18·7; Fe₃O₄—6·9–28·1;MnAl₂O₄—1·2–1·5; Fe₂TiO₄—0·6–6·2.Rims are generally enriched in the Fe₃O₄ component as a resultof oxidation. Compositions of ilmenite and magnetite (single,homogeneous and composite grains) are highly variable and resultfrom varying degrees of high temperature oxidation that is associatedwith dehydroxylation of micas and melting. Oxidation mainlyresults in increasing Fe³⁺, Al and decreasing Ti⁴⁺, Fe²⁺ inilmenite, and increasing Fe²⁺, Ti⁴⁺ and decreasing Fe³⁺ in associatedmagnetite. A higher degree of oxidation is reached with exsolutionof rutile from ilmenite and formation of titanhematite and withexsolution of pleonaste from magnetite. Ti-Al rich magnetite(5·1–7·5 and 8·5–13·5wt. per cent respectively) and ilmenite crystallized from meltsin buchitic parts of the xenoliths. Chemical and mineralogic evidence indicates that even with extensivemelting the primary compositions of individual layers in thexenoliths remained unmodified. Apparently the xenoliths didnot remain long enough at high temperatures for desilicationand enrichment in Al₂O₃, TiO₂, FeO, Fe₂O₃, and MgO that resultsby removal of a ‘granitic’ melt, and/or by interactionwith the magma, to occur. T °C-f_o2 values calculated from unoxidized magnetite/ilmenitegive temperatures ranging from 615–710°C for contactmetamorphism and the beginning of melting, and between 873 and1054°C for the crystallization of oxides and mullite/sillimanitefrom high temperature peraluminous melts. f_o2 values of metamorphismand melting were between the Ni-NiO and Fe₂O₃-Fe₃O₄ buffer curves.The relative abundance of xenolith types, geophysical evidenceand contact metamorphic mineralogy indicates that the xenolithswere derived from depths corresponding to between 2–3kb P_load = P_fluid. The xenoliths were erupted during the latestphreatomagmatic eruption from the Wehr volcano which resultedin vesiculation of melts in partially molten xenoliths causingfragmentation and disorientation of solid restite layers.     

Fe--Al Oxides: Phase Relationships below 1,000?C

The subsolidus phase relationships of magnetite, hercynite,hematite, corundum, wostite, and iron are described. The phaseswere synthesized from chemical mixtures. Reactions and solidsolution between them were induced under controlled conditionsof composition, temperature, total vapor pressure, and partialpressure of oxygen. Reaction rates are slow, so that the experimentslasted from 1 to 40 days, and quenching is completely successful. A solvus was determined which limits solid solution along themagnetitc-hercynite join at temperatures below 860^o?15^oC. Compositionsof the spinel solid solutions were determined by measuring theshift of the (440) reflection, using a powder X-ray diffractometer.The calibration curve, 20 vs. composition, was made from measurementsof spinel solid solutions synthesized in the one-phase region.The cell edge a_o changes from 8–391?0.002 A (magnetic,Fe⁺²Fe₂⁺²O₄OJ to 8.150?0.004 (hercynite, Fe⁺²Al₂O₄)by a_o?8.391–0.00190x- 0.5X²10^-5 (X is mol per cent FeAl₂O₄ in solid solution). In the system Fe-Al₂-O₃-O there are five univariant assemblages: 1. Hematite-corundum+magnetite +V (vapor) 2. Corundum+magnetite+hercynite+V 3. Magnetite+hercynite+w?stite+V 4. Hercynite+wilstite+iron+V 5. Hercynite+iron+corundum+V The lines were located by determining the composition of themagnetite, hercynite, hematite, and corundum solid solutionsfor each assemblage. The diagrams provide a basis for the discussion of the paragenesisof the oxide minerals. The progressive metamorphism of lateritedeposits can be represented by (1) laterites and bauxites: hematiteH+hydratedaluminum oxides; (2) diasporites: hematite+diaspore+corundum,with magnetite as a rare accessory; (3) emery: corundum+magnetite,with hematite as an accessory. The path of these mineral changeson the diagrams shows the decrease in oxygen content of thesolids with decrease in the partial pressure of oxygen and relatesthe aluminum content of the magnetite to temperature. The occurrences of hercynite are discussed. It is a rare mineralbecause it requires unusual conditions to grow, i.e. relativelylow oxygen pressure and an extremely Fe-Al-rich environment.     

Experimentally Determined Conditions in the Fish Canyon Tuff, Colorado, Magma Chamber

The Fish Canyon Tuff, Colorado, forms one of the largest (3000km³ known silicic eruptions in Earth history. The tuff is ahomogeneous quartz latite consisting of 40% phenocrysts (plagioclase,sanidine, biotite, hornblende, quartz, magnetite, apatite, sphene,and ilmenite) in equilibrium with a highly evolved rhyoliticmelt now represented by the matrix glass. Melt inclusions trappedin hornblende and quartz phenocrysts are identical to the newlyanalyzed matrix glass composition indicating that hornblendeand quartz crystallized from a highly evolved magma that subsequentlyexperienced little change. This study presents experimentalphase equilibrium data which are used to deduce the conditions(P, T, f_O2, f_H2O, etc.) in the Fish Canyon magma chamber priorto eruption. These new data indicate that sanidine and quartzare not liquidus phases until 780?C temperatures are achieved,consistent with Fe-Ti oxide geothermometry which implies thatthe magmatic temperature prior to eruption was 760?30?C. NaturalFe-Ti oxide pairs also suggest that log f_O2 was -12.4 (intermediatebetween the Ni-NiO and MnO-Mn₃O₄ oxygen buffers) in the magmachamber. This f_O2.102 is supported by the experimentally determinedvariations in hornblende and melt Mg-numbers as functions off_O2 A new geobarometer based on the aluminum content of hornblendesin equilibrium with the magmatic assemblage hornblende, biotite,plagioclase, quartz, sanidine, sphene, ilmenite or magnetite,and melt is calibrated experimentally, and yields pressuresaccurate to ?0.5 kb. Total pressure in the Fish Canyon magmachamber is inferred to have been 2.4 kb (equivalent to a depthof 7.9 km) based on the Al-content of natural Fish Canyon hornblendesand this new calibration. This depth is much shallower thanhas been proposed previously for the Fish Canyon Tuff. Variationsin experimental glass (melt) composition indicate that the magmawas water-undersaturated prior to eruption. X_H2O in the fluidphase that may have coexisted with the Fish Canyon magma isestimated to have been 0.5 by comparing the An-content of naturalplagioclases to experimental plagioclases synthesized at differentX_H2O and P_totals. This ratio corresponds to about 5 wt.% waterin the melt at depth. The matrix glass chemistry is reproducedexperimentally under these conditions: 760?C, 2.4 kb, X_H2O=0.5,and log f_o2=NNO+2 log units. The fugacity of SO₂ (91 b) is calculatedfrom the coexistence of pyrrhotite and magnetite. Maximum CO₂fugacity (2520 b) is inferred assuming the magma was volatilesaturated at 2.4 kb.     

Sillimanite and Ilmenite from High-grade Metamorphic Rocks of Antarctica and Other Areas

Sillimanite from a variety of high-grade metamorphic rocks containsfrom 0.13 to 1.82 weight per cent Fe₂O₃ and less than 0.1 weightper cent TiO₂. The iron is trivalent and substitutes for Alonly. Ilmenite associated with the sillimanite contains no morethan 0.4 weight per cent Al₂O₃, SiO₂, CaO, and MnO; and MgOdoes not exceed 1.6 weight per cent. It ranges in compositionfrom Ilm₉₉Hem₁ to Ilm₈₅Hem₁₅. A least squares fit of precision unit cell data on 10 analyzedsillimanites gives the following cell dimensions for iron-freesillimanite: a = 7.4830 Á, b = 7.6708 Á, c = 5.7694Á and V = 331.15 Á³. The projected increase incell volume with substitution of 10 mole per cent Fe₂SiO₃ is1.66 per cent. A regular increase in the Fe₂O₃ content of sillimanite withincreasing Fe₂O₃ content of associated ilmenite in 15 of 21samples analyzed suggests that sillimanite and ilmenite crystallizedin equilibrium in the 15 samples. The compositions of the tensillimanite-ilmenite pairs analyzed by the author fit the followingempirical curve (sol;(X_Fe2O3)Il = 1.110 x 10^–3. This regularincrease in Fe₂O₃ contents fits a model of Fe³⁺ substitutionfor Al on two independent sites in sillimanite and a coupledsubstitution of for Fe²⁺ Ti on two sites in ilmenite. Sillimaniteand ilmenite are behaving as ideal solutions over the compositionalrange 0 < X_Fe2SIO3 < 0.013 in sillimanite and 0 < X_Fe2O3< 0.15 in ilmenite. Equations have been derived for expressing the variation inFe₂O₃ content of sillimanite associated with quartz and ilmeniteor hematite as a function of pressure, temperature, and Fe₂O₃content of the oxide minerals. For example, the Fe₂O₃ contentof a sillimanite with 1.5 mole per cent Fe₂SiO₃ coexisting withTi-free hematite is calculated to decrease 11 per cent witha 5 kb increase in pressure. The rate of increase with temperatureof the Fe₂O₃ content of sillimanite is greater in hematite-bearingassemblages than in ilmenite-bearing assemblages.     

Mineralogy and Petrology of the Metamorphosed Wabush Iron Formation, Southwestern Labrador

The Wabush Iron Formation, of late Precambrian (Proterozoic)age is part of the Labrador Trough in southwestern Labrador,Canada. It is the regionally metamorphosed equivalent of lowgrade metamorphic (chlorite zone) iron-rich sediments of thecentral part of the Labrador Trough. The metamorphic grade iskyanite-staurolite zone, as concluded from conformably underlyingpelitic schist assemblages. Sedimentary textural features suchas very pronounced banding and a very rare occurrence of relicgranules are still preserved. The iron formation consists mainly of quartz, specularite, magnetite,cummingtonite-grunerite, and ferrodolomite-ankerite. Less commonare actinolite, anthophyllite, riebeckitetremolite, magnesioriebeckite,ferrosalite, orthopyroxene, aegirine-augite, aegirine, rhodonite,garnet (almandine, spessartine, calderite), siderite, rhodochrosite,calcite, and kutnahorite. Conventional wet chemical analyses or electron microprobe analyseshave been made of thirty-four phases belonging to the abovelist. Six additional electron probe analyses have been madeof phases from the underlying pelitic schists. All conventionallyanalyzed phases are characterized by complete optical, unitcell parameter, and density measurments. The analyzed assemblages from the silicate and silicate-carbonateiron formation include grunerite-ferrosalite, grunerite-eulite-siderite,grunerite-actinolite, grunerite-almandine, cummingtonite-spessartine,rhodonite-kutnahorite-calderite, aegirine-augite-riebeckite-tremolite,magnesioriebeckite-cummingtonite-rhodonite, aegirine-augite-rhodonite-rhodo-chrosite,and aegirine-rhodonite-calderite-rhodochrosite. The assemblages are concluded to be equilibrium assemblages.Of the volatile components, O₂, CO₂, and H₂O, O₂, is concludedto have behaved as an inert (buffered) component. Variationsin the activity of CO₂ are concluded to have existed betweensilicate-oxide and carbonate-oxide members of the iron formation.It is not clear, however, whether CO₂ has acted as a perfectlymobile component with strong aco₂ gradients throughout the area,or as an inert component in some parts of the area. H₂O is consideredto have been perfectly mobile. An increase in Mg/(Mg+Fe) ratioin ferromagnesian silicates is correlated with an increase inthe oxidation state of the assemblage. A similar increase in(Mg+Mn)/(Mg+Mn+Fe) is found in manganoan ferromagnesian silicateswith increasing activity of O₂. A number of ferromagnesian silicatescontain large amounts of Na⁺ and Fe³⁺ as a result of the verylow Al₂O₃ content of the iron formation. The P and T conditionsof metamorphism are deduced from experimental studies applicableto the underlying pelitic schists.     

Fe-Ti-oxides in metamorphic basites from the Eastern Alps,Austria: a contribution to the formation of solid solutions of natural Fe-Ti-oxide assemblages

The development of Fe-Ti oxide assemblages in basic rocks from the Penninic series of the southern Venediger rea, Austria, during polyphase Alpine metamorphism has been studied. Textural and compositional relations indicate thorough reequilibration of the opaque mineral assemblages during late Barrovian metamorphism at essentially static conditions of lower amphibolite to greenschist facies. In contrast, silicate mineralogy of the preceeding blueschist to eclogite facies metamorphism might still be preserved to a large extent. Chemical adjustment of the Fe-Ti oxide minerals to decreasing P-T conditions is characterized by (1) formation of complex intergrowths of ilmenite and hematite solid solutions (<550° C), (2) the decomposition of hemo-ilmenite ₁ to ferrianilmenite₂+magnetite+rutile and of ilmeno-hematite₁ to titanhematite₂+rutile±magnetite (<450° C), and (3) low-grade oxidation of ferrianilmenite₂ to magnetite+hematite-rutile intergrowths or hematite +rutile and of titanhematite₂ to hematite-rutile intergrowths (≦400° C). Chemical equilibrium is suggested by the regular partitioning of Cr, V, Mg and Mn between coexisting hemo-ilmenite, ilmeno-hematite, and magnetite. The hematite-ilmenite miscibility gap has been delimited on the basis of the bulk compositions of the exsolved phases and the temperature estimates obtained from Fe-Ti oxide thermometry.     

Superdeep diamonds from the Juina area, Mato Grosso State, Brazil

Alluvial diamonds from the Juina area in Mato Grosso, Brazil, have been characterized in terms of their morphology, syngenetic mineral inclusions, carbon isotopes and nitrogen contents. Morphologically, they are similar to other Brazilian diamonds, showing a strong predominance of rounded dodecahedral crystals. However, other characteristics of the Juina diamonds make them unique. The inclusion parageneses of Juina diamonds are dominated by ultra-high-pressure ("superdeep") phases that differ both from "traditional" syngenetic minerals associated with diamonds and, in detail, from most other superdeep assemblages. Ferropericlase is the dominant inclusion in the Juina diamonds. It coexists with ilmenite, Cr-Ti spinel, a phase with the major-element composition of olivine, and SiO₂. CaSi-perovskite inclusions coexist with titanite (sphene), "olivine" and native Ni. MgSi-perovskite coexists with TAPP (tetragonal almandine-pyrope phase). Majoritic garnet occurs in one diamond, associated with CaTi-perovskite, Mn-ilmenite and an unidentified Si-Mg phase. Neither Cr-pyrope nor Mg-chromite was found as inclusions. The spinel inclusions are low in Cr and Mg, and high in Ti (Cr₂O₃<36.5 wt%, and TiO₂>10 wt%). Most ilmenite inclusions have low MgO contents, and some have very high (up to 11.5 wt%) MnO contents. The rare "olivine" inclusions coexisting with ferropericlase have low Mg# (87-89), and higher Ca, Cr and Zn contents than typical diamond-inclusion olivines. They are interpreted as inverted from spinel-structured (Mg, Fe)₂Si₂O₄. This suite of inclusions is consistent with derivation of most of the diamonds from depths near 670 km, and adds ilmenite and relatively low-Cr, high-Ti spinel to the known phases of the superdeep paragenesis. Diamonds from the Juina area are characterized by a narrow range of carbon isotopic composition ('¹³C=-7.8 to -2.5‰), except for the one majorite-bearing diamond ('¹³C=-11.4‰). There are high proportions of nitrogen-free and low-nitrogen diamonds, and the aggregated B center is predominant in nitrogen-containing diamonds. These observations have practical consequences for diamond exploration: Low-Mg olivine, low-Mg and high-Mn ilmenite, and low-Cr spinel should be included in the list of diamond indicator minerals, and the role of high-Cr, low-Ti spinel as the only spinel associated with diamond, and hence as a criterion of diamond grade in kimberlites, should be reconsidered.     

Mineral Compositions and Equilibria in the Metamorphosed Iron Formation of the Gagnon Region, Quebec, Canada

Forty-seven specimens of the Wabush Iron Formation were collectedfrom ten outcrop areas. Twenty-five specimens contain the assemblage(1), quartz+clinopyroxene+calcite with or without orthopyroxene,grunerite, magnetite, ankerite, and siderite. Five specimenscontain assemblage (2), quartz+clinopyroxene+actinolite+calcite+magnetite+hematite,and two contain assemblage (3), quartz+orthopyroxene+actinolite+magnetite+hematite.In three specimens of assemblage (1), graphite occurs in theabsence of magnetite; pyrrhotite and pyrite occur separatelyor together in specimens with assemblage (1). Thirty-nine clinopyroxenes, 38 orthopyroxenes, 18 grunerites,7 actinolites, 16 calcites, 1 ankerite, and 1 siderite wereanalyzed for iron, manganese, and calcium by X-ray emissionspectrography. Magnesium contents were estimated by assumingstoichiometric proportions. Minerals occurring with hematite show low Fe/(Fe+Mg) ratios,and those in the other assemblages show higher values with awide range of variation. In orthopyroxene, Fe/(Fe+ Mg) rangesfrom 0·17 (with hematite) to 0·77. Regularity in the distributions of Fe, Mn, and Ca between pairsof coexisting minerals shows that equilibrium was attained inmost of the rocks studied. This regularity is also accomplishedin the distribution of Mn between calcite and coexisting silicatesas well as between the silicates themselves. Small differencesin the distributions of Ca and Fe depend on both outcrop areaand mineral assemblage. Phase rule considerations suggest that the specimens with dolomite-ankeriteor magnesitesiderite do not represent equilibrium assemblages.Variations in orthopyroxene compositions in assemblages withpyrite or pyrrhotite, or both, and magnetite indicate non-equilibrationof sulfides with silicates. The presence of the oxygen buffer,magnetite+hematite, attests to the immobility of oxygen duringmetamorphism. Within each outcrop area, over which the temperature and pressureare assumed to have been uniform, variations in the compositionsof the silicates in the sub-assemblages quartz+ orthopyroxene+gruneriteand quartz+orthopyroxene+clinopyroxene+calcite indicate gradientsof µ_H2O µ_CO2 and respectively. As characterizedby the composition of orthopyroxene, both gradients are relativelylow along strike, and high across strike. The direction of gradientsacross strike is almost without reversals, which is consistentwith intergranular diffusion of H₂O and CO₂. Phase rule restrictionsfor a majority of assemblages are not in accord with the simultaneousimposition of µ_H2O and µ_CO2 gradients on the rocks,nor the formation of an H₂O-CO₂ fluid phase during metamorphism.     

The Stability of Andradite, Hedenbergite, and Related Minerals in the System Ca--Fe--Si--O--H

Phase relations for the bulk compositions 3CaO·2FeO_x·3SiO₂+excessH₂O and CaO·FeO_x·2SiO₂+excess H₂O were determinedusing conventional hydrothermal techniques with solid phaseoxygen buffers to control fO₂. Andradite, Ca₃Fe³⁺₂Si₃O₁₂, synthesized above 550 °C hasan average unit cell edge, a_o, of 12.055±0.001 Å,and an index of refraction, n, of 1.887±0.003. Belowthis temperature, a_o increases whereas n decreases, indicatingthe formation of a member of the andradite-hydroandradite solidsolution. At 2000 bars P_fluid andradite is stable above an f_O2of 10¹⁵ bar at 800 °C and 10-³² bar at 400 °C. At lowerf_O2 andradite+fluid gives way at successively lower temperaturesto the condensed assemblages magnetite+wollastonite, kirschsteinite(CaFe²⁺SiO₄)+ wollastonite and kirschsteinite+xonotlite (Ca₆Si₆O₁₇(OH)₂). Synthetic hedenbergite, CaFe²⁺Si₂O₆, has average unit cell dimensionsof a_o = 9.857± 0.004 Å, b_o = 9.033±0.002Å, c_o = 5.254±0.002 Å and ß = 104.82°±0.03°,and refractive indices of n = 1.731±0.003 and n = 1.755±0.005.At 2000 bars P_fiuid, hedenbergite is stable below an f_O2 of10-¹³ bar at 800 °C and 10-²⁸ bar at 400 °C. Above thesef_O2 values, hedenbergite+O₂ breaks down to andradite+magnetite+quartz. The mineral pair andradite +hedenbergite thus limit the f_O2range possible for their joint formation under equilibrium conditions. The hydration of wollastonite to xonotlite occurs at much lowertemperatures than previous experimental work indicated. A tentativehigh temperature limit for this reaction is set at 185°±15°C and 5000±25 bars and 210°±15 °Cand 2000±20 bars. Inasmuch as the growth of xonotlitefrom wollastonite + H₂O was never accomplished, this high temperaturelimit does not represent an equilibrium univariant curve. Nine phases were encountered in the study of andradite and hedenbergite.They are andradite, hedenbergite, magnetite, wollastonite, kirschsteinite,xonotlite, quartz, ilvaite, and vapor (fluid). An invariantpoint analysis using the method of Schreinemakers shows thetopologic relations of the reactions involved. The resultinggrid can be used to interpret natural occurrences.     

Iron-Titanium Oxide Minerals and Synthetic Equivalents

Phase-equilibrium studies in the system FeO-Fe₂O₃—TiO₂permit determination of the temperature and oxygen fugacityof formation of coexisting pairs of titaniferous magnetite andilmenite in many rocks. Temperatures thus obtained are probablyaccurate to ?50?C. Temperatures indicated for most igneous andmetamorphic rocks for which data are available are generallyconsistent with temperatures inferred by other methods. Temperaturesfor certain gabbroic rocks are too low for magmatic crystallizationand probably reflect the migration of ilmenite from titaniferousmagnetite to form separate granules upon cooling. The experimentally determined solubility of ilmenite in magnetiteis much too small to account for most ilmeno-magnetites by simpleexsolution. Subsolidus oxidation of magnetite-ulv?spinelg_SSto yield ilmenite-magnetite intergrowths has been experimentallyverified and probably takes place during cooling of many igneousand perhaps some metamorphic rocks. Oxidation at surface orhypabyssal conditions may produce metastable titanomaghemites. In order of increasing intensity of oxidation, the followingFe—Ti oxide pairs occur in plutonic rocks: ulv?spinel-richmagnetite_SS+ilmenite_SS, ulv?spinel-poor magnetite_SS+ilmenite_SS,ulv?spinel-poor magnetite_SS+hematite_SS, hematite_SS+rutile.     

Magma storage prior to the 1912 eruption at Novarupta, Alaska

New analytical and experimental data constrain the storage and equilibration conditions of the magmas erupted in 1912 from Novarupta in the 20th century's largest volcanic event. Phase relations at H₂O+CO₂ fluid saturation were determined for an andesite (58.7 wt% SiO₂) and a dacite (67.7 wt%) from the compositional extremes of intermediate magmas erupted. The phase assemblages, matrix melt composition and modes of natural andesite were reproduced experimentally under H₂O-saturated conditions (i.e., P_H2O=P_TOT) in a negatively sloping region in T-P space from 930 °C/100 MPa to 960 °C/75 MPa with fO₂~NNO+1. The H₂O-saturated equilibration conditions of the dacite are constrained to a T-P region from 850 °C/50 MPa to 880 °C/25 MPa. If H₂O-saturated, these magmas equilibrated at (and above) the level where co-erupted rhyolite equilibrated (~100 MPa), suggesting that the andesite-dacite magma reservoir was displaced laterally rather than vertically from the rhyolite magma body. Natural mineral and melt compositions of intermediate magmas were also reproduced experimentally under saturation conditions with a mixed (H₂O + CO₂) fluid for the same range in P_H2O. Thus, a storage model in which vertically stratified mafic to silicic intermediate magmas underlay H₂O-saturated rhyolite is consistent with experimental findings only if the intermediates have X_H2O^fl=0.7 and 0.9 for the extreme compositions, respectively. Disequilibrium features in natural pumice and scoria include pristine minerals existing outside their stability fields, and compositional zoning of titanomagnetite in contact with ilmenite. Variable rates of chemical equilibration which would eliminate these features constrain the apparent thermal excursion and re-distribution of minerals to the time scale of days.     

(Ruby--Sapphire)--Chromian Mica--Tourmaline Rocks from Westland, New Zealand

Boulders of the assemblage ruby—sapphire corundum, chromianmuscovite, margarite, tourmaline (chromian chlorite, Zn—Mnchromite and Mn—Ti magnetite) occur in glacial moraineand rivers of north Westland, South Island of New Zealand. Thelocation, Cr-rich composition of the boulders and the presenceof rare serpentinite rinds indicate that they are derived fromultramafic rocks (Pounamu Ultramafics) that occur within AlpineSchist of the Southern Alps. The largest sample is progressivelyzoned outwards from a corundum—margarite core, throughan intermediate zone of Cr-muscovite, to an outer zone of Cr-chloritethat is in contact with serpentinite. Most finds consist oferosion-resistant corundum-rich cores. In the corundum, Cr₂O₃content ranges from 0.5 to 13%, with red coloration becomingmore intense with increasing Cr. In addition to the dominantCr³⁺ Al³⁺ substitution, those of (Fe, V)³⁺ Cr³⁺ and (Ti⁴⁺+Fe²⁺) 2Cr³⁺ result in spectacular colour zoning from colourlessto deep ruby red-carmine and pale blue to dark blue—violet.Corundum has grown by replacement of the micaceous matrix thatconsists of chromian muscovite (0.10–4.10% Cr₂O₃) andchromian margarite (0.46–1.20% Cr₂O₃). Both micas containa significant paragonite component (up to 21.5% in muscoviteand up to 40.8% in margarite). Late phase muscovite is Ba richwith up to 4.77% BaO, and margarite has up to 0.66% SrO. Tourmalineoccurs as veins, vein outgrowths and larger poikilitic crystalsthat replace the mica matrix. Chromium content ranges between0.82 and 3.6% Cr₂O₃. High bulk rock Al (up to 78% Al₂O₃), K,Ca, Cr and Na, and low Si (14.5–23.1% SiO₂), suggest thatthe corundum—Cr-silicate rocks are the products of extrememetasomatic alteration of quartzofeldspathic schist enclavesin serpentinite. Isocon analysis indicates that conversion ofthe schist to the micaceous matrix of the corundum rocks involvesconservation of Ca, Al, K, volatiles and Sr, a mass loss of59% and a volume reduction of 69% consequent on removal of 70–80%Si and all other elements (most >80%), with enrichment ofbetween 900 and 1800% Cr. The formation of corundum from themica matrix involved a further mass—volume reduction anddecrements in Si, Ca, K, volatiles and Sr from reaction sites.Concentric mineral zonation in single rock samples and zoning—replacementin minerals, e.g. Cr in corundum and chromite, Ti, Fe²⁺ in corundum,Ba in muscovite, Sr in margarite, and Mn and Zn in chromiteand magnetite, imply element redistribution during metasomatism.Experimental reaction between quartzofeldspathic schist andserpentinite at 450C and 2 kbar produced reaction sequencescontaining newly formed Ca-plagioclase—phlogopitic micachloriteand muscovite—chlorite that in terms of composition areanalogous with the observed (corundum—margarite)—muscovite—chloritezonation. The temperature of metamorphism of garnet zone rocks(45020C) that contain the corundum—Cr-silicate rocksis well below that of the breakdown of muscovite and margariteto form corundum and indicates the importance of fluid composition,particularly the cation—hydrogen variables a_Ca²⁺/H⁺, aK⁺/H₊and a_S1O2. Introduction of boron into the schist (from serpentinite),and boron released from the breakdown of original tourmalinein the schist, resulted in tourmaline veining and reaction ofthe mica matrix to form tourmaline that invoved both a massand volume increase and addition of Fe, Mg together with B. KEY WORDS: corundum—Cr-silicate rocks; metasomatism; New Zealand; Southern Alps ^*Corresponding author.     

The Crossite Content of Ca-Amphibole as a Guide to Pressure of Metamorphism

A correlation between the crossite component (NaM₄) in Ca-amphiboleand pressure of metamorphism has long been recognized (Shido& Miyashiro, 1959), but only recently has the reaction beenidentified which buffers this aspect of amphibole composition(Brown, 1974): Ca-amphibole+iron oxide+albite+chloriteI+H₂O (±stilp,qtz) = crossite+epidote (±muscovite, qtz). The exact stoichiometry of the reaction depends on compositionalvariables in the minerals, especially Fe²⁺/Mg and Fe³⁺/Al. Ca-amphiboleshould have fixed NaM₄, at any given T and P, where it coexistswith iron oxide, albite, and chlorite. Comparison of Ca-amphibole composition with mineral assemblage,in rocks from Otago, N.Z., and elsewhere, supports this hypothesis.In any terrane NaM₄ is nearly constant at a particular metamorphicgrade where amphibole exists in the buffering assemblage, butvaries widely outside of this assemblage. Variations in Fe²⁺/Mgand Fe³⁺/Al in the amphibole have relatively little effect onNaM₄, but in high pressure amphiboles NaM₄ varies inverselywith Al^iv. Ca-amphiboles from high pressure areas have substantially moreNaM₄ (Otago, 0.6 of 2.0) than those from lower pressure areas(Sierra contact aureoles, 0.1). These relations suggest thatin the buffering assemblage, the NaM₄ content of Ca-amphiboleshould be a useful relative barometer for low to medium grademetamorphic rocks.     

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     

An Experimental Study of the Influence of Oxygen Fugacity on Fe-Ti Oxide Stability, Phase Relations, and Mineral--Melt Equilibria in Ferro-Basaltic Systems

Equilibrium crystallization experiments at atmospheric pressureand over a range of oxygen fugacity (fO₂) have been carriedout on a ferro-basaltic composition similar to liquids proposedto have been parental to much of the exposed portion of theSkaergaard intrusion. Before Fe-Ti oxide saturation the liquidline of descent is little affected by fO₂. However, the appearancetemperatures of the magnetite-ulvspinel solid solution (Mt)and the ilmenite-haematite solid solution (Ilm) depend stronglyon fO₂. Above the fayalite-magnetite-quartz (FMQ) buffer Mtis the first oxide phase to appear on the liquidus, but belowthe FMQ buffer Ilm is the first oxide to crystallize. The appearancetemperature of Mt is 1100C at FMQ and the Mt liquidus slopeis 30C/log fO₂ unit between FMQ–;2 and FMQJ+1. The Ilmliquidus is at 1100C between FMQ and FMQ–2, but movesto lower temperature at higher fO₂ where Mt is the first oxidephase. The results indicate that the ferric iron content ofMt-saturated melts varies linearly with inverse temperature,and that Ilm saturation is closely related to melt TiO₂ content.Mt saturation produces an immediate enrichment of SiO₂ and depletionin FeO^* in the melt phase, whereas Ilm saturation produces similarenrichment in SiO², but inn enrichment may continue for 10Cbelow the ilmenite liquidus. The experimental liquids reacha maximum of 18 wt% FeO^*, at 48 wt% SiO₂ for ilmenite-saturatedmelts at low f_O2, more differentiated melts having lower ironand higher silica. Cotectic proportions, derived from mass balancecalculations, are in good agreement with data from natural samplesand other experimental studies. Olivine resorption is inferredat all fO₂, with the onset of resorption occurring 10C higherthan the appearance of magnetite. The effect of fO₂ on silicatemineral compositions, and partitioning of elements between coexistingmineral-melt pairs, is small. Thermodynamic considerations suggestthat variations of Fe-Mg partitioning between the iron-richolivines, pyroxenes and melts produced in this study may beexplained by known non-idealities of Fe-Mg mixing in the crystallinephases, rather than nonidealities in the coexisting melts. Theseexperiments also provide insights into many features commonto natural tholeiitic series of volcanic and plutonic rocks,and provide experimental data required for modelling of fractionalcrystallization and crystallization closed to oxygen, processeswhich are not easily investigated experimentally. KEY WORDS: ferro-basalt; Fe-Ti oxides; oxygen fugacity; Skaergaard intrusion; iron enrichment ^*Corresponding author. Present address: Bayerisches Geoinstitut, Univerritt Bayreuth, D-95440 Bayreuth, Germany     

Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet

The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock ⁸⁷Sr/⁸⁶Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial ⁸⁷Sr/⁸⁶Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An₇₂ plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An₄₃; Mg# 5 clinopyroxene; Fo₁ olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V₂O₅ of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO₂ and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO₂ and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation     

Oxide and sulphide isograds along a Late Archean, deep-crustal profile in Tamil Nadu, south India

Oxide–sulphide–Fe–Mg–silicate and titanite–ilmenite textures as well as their mineral compositions have been studied in felsic and intermediate orthogneisses across an amphibolite (north) to granulite facies (south) traverse of lower Archean crust, Tamil Nadu, south India. Titanite is limited to the amphibolite facies terrane where it rims ilmenite or occurs as independent grains. Pyrite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade. Pyrrhotite is confined to the high‐grade granulites. Ilmenite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade and occurring primarily as hemo‐ilmenite in the high‐grade granulite facies rocks. Magnetite is widespread throughout the traverse and is commonly associated with ilmenite. It decreases in abundance with increasing metamorphic grade. In the granulite facies zone, reaction rims of magnetite + quartz occur along Fe–Mg silicate grain boundaries. Magnetite also commonly rims or is associated with pyrite. Both types of reaction rims represent an oxidation effect resulting from the partial subsolidus reduction of the hematite component in ilmenite to magnetite. This is confirmed by the presence of composite three oxide grains consisting of hematite, magnetite and ilmenite. Magnetite and magnetite–pyrite micro‐veins along silicate grain boundaries formed over a wide range of post‐peak metamorphic temperatures and pressures ranging from high‐grade SO₂ to low‐grade H₂S‐dominated conditions. Oxygen fugacities estimated from the orthopyroxene–magnetite–quartz, orthopyroxene–hematite–quartz, and magnetite–hematite buffers average 2.5 log units above QFM. It is proposed that the trends in mineral assemblages, textures and composition are the result of an external, infiltrating concentrated brine containing an oxidizing component such as CaSO₄ during high‐grade metamorphism later acted upon by prograde and retrograde mineral reactions that do not involve an externally derived fluid phase.     

A Complex Petrogenesis for an Arc Magmatic Suite, St Kitts, Lesser Antilles

St Kitts forms one of the northern group of volcanic islandsin the Lesser Antilles arc. Eruptive products from the Mt Liamuigacentre are predominantly olivine + hypersthene-normative, low-Kbasalts through basaltic andesites to quartz-normative, low-Kandesites. Higher-Al and lower-Al groups can be distinguishedin the suite. Mineral assemblages include olivine, clinopyroxene,orthopyroxene, plagioclase and titanomagnetite with rarer amphibole,ilmenite and apatite. Eruptive temperatures of the andesitesare estimated as 963–950°C at fO₂ NNO + 1 (whereNNO is the nickel–nickel oxide buffer). Field and mineralchemical data provide evidence for magma mixing. Glass (melt)inclusions in the phenocrysts range in composition from andesiteto high-silica rhyolite. Compositional variations are broadlyconsistent with the evolution of more evolved magmas by crystalfractionation of basaltic parental magmas. The absence of anycovariation between ⁸⁷Sr/⁸⁶Sr or ¹⁴³Nd/¹⁴⁴Nd and SiO₂ rulesout assimilation of older silicic crust. However, positive correlationsbetween Ba/La, La/Sm and ²⁰⁸Pb/²⁰⁴Pb and between ²⁰⁸Pb/²⁰⁴Pband SiO₂ are consistent with assimilation of small amounts (<10%)of biogenic sediments. Trace element and Sr–Nd–Pbisotope data suggest derivation from a normal mid-ocean ridgebasalt (N-MORB)-type mantle source metasomatized by subductedsediment or sediment melt and fluid. The eruptive rocks arecharacterized by ²³⁸U excesses that indicate that fluid additionof U occurred <350 kyr ago; U–Th isotope data for mineralseparates are dominated by melt inclusions but would allow crystallizationages of 13–68 ka. However, plagioclase is consistentlydisplaced above these ‘isochrons’, with apparentages of 39–236 ka, and plagioclase crystal size distributionsare concave-upwards. These observations suggest that mixingprocesses are important. The presence of ²²⁶Ra excesses in twosamples indicates some fluid addition <8 kyr ago and thatthe magma residence times must also have been less than 8 kyr. KEY WORDS: Sr–Nd–Pb isotopes; U-series isotopes; crystal size distribution; petrogenesis     

Effects of P, f(O2) and Mg/Fe Ratio on Dehydration Melting of Model Metagreywackes

We present results of dehydration melting experiments [3–15kbar, 810–950C f(O₂) QFM (quartz-fayalite-magetite)and Ni-NiO] on two Fe-rich mixtures of biotite (37%), plagioclaseAn₃₈ (27%), quartz (34%) and ilmenite (2%), which differ onlyin their biotite compositions (mg-number 23 and 0.4). Dehydrationmelting of metagreywackes of constant modal composition generatesa wide range of melt fractions, melt compositions and residualassemblages, through the combined effects of pressure, Fe/Mgratio and f(O₂). Crystallization of garnet is the chief controlon melting behavior, and is limited by two reactions: (1) thebreakdown of garnet + quartz to orthopyroxene + plagioclaseat low P, and (2) the oxidation of garnet to magnetite + anorthite+ quartz (enstatite), which is sensitive to both f(O₂) andP. Because of these reactions, melting of Mg-rich metagreywackesis rather insensitive to f(O₂) but strongly sensitive to P;the converse is true for Fe-rich metagreywackes. Garnet crystallizationrequires that plagioclase break down incongruently, liberatingalbite. This increases the Na₂O content of the melts and enhancesmelt production. Thus, melting of metagreywacke in a reducingdeep-crustal environment (with garnet stable) would producemore, and more sodic, melt than would garnet-absent meltingof the same source material in a relatively oxidizing, shallow-crustalenvironment. KEY WORDS: anatexis; metasediments; gneisses; granites; garnet ^*Corresponding author. Telephone: 706-542-2394; fax: 706-542-2425; e-mail: alpatino{at}uga.cc.uga.edu