Mineralogy and Petrology of Erta Ale and Boina Volcanic Series, Afar Rift, Ethiopia

Erta Ale and Boina are two volcanic areas in northern Afar riftwhere a complete suite of products from mildly alkalic transitionalbasalts to peralkaline rhyolites have been erupted in very recentQuaternary time. Subaphyric lava samples of both localities,representing the entire sequence of erupted magmas, have beenselected for a mineralogical study and all the main mineralphases analysed with the microprobe. Results confirm that ErtaAle and Boina rocks can be explained as suites of liquids producedby fractional crystallization of transitional basalts at shallowcrustal levels. The crystallization history is reconstructedand temperature and oxygen fugacity are evaluated. The differentiationprocesses are quantitatively reconstructed using the compositionsof analysed microphenocrysts to compute the fraction of thesolids separating at each step of fractionation. Slight butclear chemical differences in the parent basalt are reflectedin the two series particularly in the late stages of fractionation.The slightly more alkalic Boina series was produced under fO²controlled by QFM buffer; olivine was stable all along the crystallizationinterval and clinopyroxenes have compositions intermediate betweenthose considered typical of respectively tholeiitic and alkalicsuites. The main differences between the two series are a shiftin the appearance of oxides towards more evolved liquids, crystallizationat Erta Ale of more calcic plagioclase and more magnesian olivineand clinopyroxene than those observed at Boina from liquidswith the same Ca/(Na + Ca) and Mg/(Fe + Mg) and the lack ofalkali feldspar in Erta Ale rhyolites. Together with the lowerCa content of Erta Ale clinopyroxenes, they are the expressionof the more tholeiitic affinity of Erta Ale rocks. Departuresfrom the typical mineralogy of the series are observed for someErta Ale basalt and rhyolite and variously explained as an expressionof a slight difference in the primary nature of the magma, asa liquid-xenocrysts mixing and as fO₂ changes in an open system. Present address: Bureau de Reacherches Géologiques et Minières, Département Géothermie, Orléans, France.     

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.     

Petrology of Santorini Volcano, Cyclades, Greece

The Pliocene to Recent lavas, dyke rocks, and cognate xenolithsof Santorini island group belong to four distinct series, eachof high-alumina basalt-andesite-dacite type. The oldest seriesincludes hornblende dacites and minor basaltic andesites. Theformer contain hornblende-rich cognate xenoliths of basalticcomposition, which consist essentially of crystals ‘floating’in residual acid liquid (glass). The chemical variation of theseries, like that of lavas of volcanic centres north-west ofSantorini, is of ‘calc-alkali’ type. The second and third series consist of a range of lavas frombasalt to rhyodacite. No hydrous mineral occurs as a stablephase. Augite is the phenocrystal pyroxene of basalts; augiteand hypersthene of andesites and dacites. The groundmass pyroxenesof basalts and most andesites are augite and pigeonite, whiledistinctive hornblende xenocryst-bearing andesites of the secondseries, and acid lavas of both second and third, carry augiteand hypersthene in the groundmass. Interstitial glass increasesin proportion from basalts to andesites, and forms a major componentof acid lavas. The second series, like the oldest, lacks absoluteiron enrichment. The third, however, shows weak iron enrichmentof andesitic relative to basaltic compositions. Of the youngest (historic) series, only the acid members (hyalodacites)have been extruded as lavas. The more basic members are representedby non-cumulate xenoliths of basaltic to andesitic compositionwhich, like those of the oldest series, consist of a mesh ofcrystals set in abundant glass. This modern series also displaysfeeble absolute iron enrichment. The compositional range of minerals other than plagioclase isvery limited in the two xenolithic series, but much greaterin the two lava series. Glass compositions are virtually constantwithin individual series. Estimates of temperatures and oxygenfugacities of Fe-Ti oxide mineral equilibration, and deductionsfrom liquid compositional trends indicate that the oldest serieswas characterized by higher f_O2, and f_H2O, and lower temperaturesthan the three younger, ‘dry’ series. Its silicaenrichment trend appears to have been controlled chiefly byfractionation of silica-poor hornblende, rather than magnetiteas in the younger series. The presence, in all series, of xenolithsof gabbroic cumulates, and the constancy of glass compositionssuggests that each series was generated by the tapping of adifferentiating highalumina basalt magma in a high level magmachamber.     

Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilimaussaq Complex, South Greenland

The 1·13 Ga Ilímaussaq intrusive complex, SouthGreenland, is composed of various types of alkali granite andsilica-undersaturated alkaline to agpaitic nepheline syenitesrelated to three subsequently intruded magma batches. Mineralchemistry indicates continuous fractionation trends within eachrock type, but with distinct differences among them. The last,peralkaline magma batch is the most fractionated in terms ofX_Fe^{mafic mineral}, feldspar composition and mineral assemblage.This indicates that an evolving magma chamber at depth discontinuouslyreleased more highly fractionated alkaline melts. Fluid inclusionsin some sodalites record a pressure drop from 3·5 to1 kbar indicating that crystallization started during magmaascent and continued in the high-level magma chamber. On thebasis of phase equilibria and preliminary fluid inclusion data,crystallization temperature drops from >1000°C (augitesyenite liquidus) to <500°C (lujavrite solidus) and silicaactivity decreases from     

Phase Relations of Peralkaline Silicic Magmas and Petrogenetic Implications

The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO₂relative to the Ni–NiO solid buffer), between 800 and660°C and for melt H₂O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckite–arfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H₂O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H₂O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 50–60°C,and that of amphibole by 30°C. Riebeckite–arfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40–100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO₂ lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H₂O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H₂O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO₂ at or below the fayalite–quartz–magnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H₂O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria     

Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions

Many basaltic flood provinces are characterized by the existenceof voluminous amounts of silicic magmas, yet the role of thesilicic component in sulphur emissions associated with trapactivity remains poorly known. We have performed experimentsand theoretical calculations to address this issue. The meltsulphur content and fluid/melt partitioning at saturation witheither sulphide or sulphate or both have been experimentallydetermined in three peralkaline rhyolites, which are a majorcomponent of some flood provinces. Experiments were performedat 150 MPa, 800–900°C, fO₂ in the range NNO –2 to NNO + 3 and under water-rich conditions. The sulphur contentis strongly dependent on the peralkalinity of the melt, in additionto fO₂, and reaches 1000 ppm at NNO + 1 in the most stronglyperalkaline composition at 800°C. At all values of fO₂,peralkaline melts can carry 5–20 times more sulphur thantheir metaluminous equivalents. Mildly peralkaline compositionsshow little variation in fluid/melt sulphur partitioning withchanging fO₂ (D_S 270). In the most peralkaline melt, D_S risessharply at fO₂ > NNO + 1 to values of >500. The partitioncoefficient increases steadily for S_bulk between 1 and 6 wt% but remains about constant for S_bulk between 0·5 and1 wt %. At bulk sulphur contents lower than 4 wt %, a temperatureincrease from 800 to 900°C decreases D_S by 10%. These results,along with (1) thermodynamic calculations on the behaviour ofsulphur during the crystallization of basalt and partial meltingof the crust and (2) recent experimental constraints on sulphursolubility in metaluminous rhyolites, show that basalt fractionationcan produce rhyolitic magmas having much more sulphur than rhyolitesderived from crustal anatexis. In particular, hot and dry metaluminoussilicic magmas produced by melting of dehydrated lower crustare virtually devoid of sulphur. In contrast, peralkaline rhyolitesformed by crystal fractionation of alkali basalt can concentrateup to 90% of the original sulphur content of the parental magmas,especially when the basalt is CO₂-rich. On this basis, we estimatethe amounts of sulphur potentially released to the atmosphereby the silicic component of flood eruptive sequences. The peralkalineEthiopian and Deccan rhyolites could have produced 10¹⁷ and10¹⁸ g of S, respectively, which are comparable amounts to publishedestimates for the basaltic activity of each province. In contrast,despite similar erupted volumes, the metaluminous Paraná–Etendekasilicic eruptives could have injected only 4·6 x 10¹⁵g of S in the atmosphere. Peralkaline flood sequences may thushave greater environmental effects than those of metaluminousaffinity, in agreement with evidence available from mass extinctionsand oceanic anoxic events. KEY WORDS: silicic flood eruptions; sulphur; experiment; Ethiopia; Deccan     

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.     

Synthesis and Stability Relations of Epidote, Ca2Al2FeSi3O12 (OH)

Hydrothermal investigation of the bulk composition 2CaO·Al₂O₃·l/2Fe₂O₂·3SiO₂+excessH₂O (Ps 33 +excess H₂O) has been conducted using conventionalapparatus and solid oxygen buffer techniques. Coarse-grainedepidotes (over 150 microns in some cases) were readily synthesizedfrom oxide mixtures with a 98 per cent yield as well as fromtheir high temperature equivalents at 600–700 °C and5 kb P_fluid and over a range of oxygen fugacities. Electronmicroprobe analyses show that maximum Fe⁺³ content of syntheticepidotes varies as a function of fo₂. Epidote is most iron-rich(Ps 33 ± 2) at high (HM and CCO) oxygen buffers and becomesprogressively more aluminous (Ps 25 ± 3) with decreasingfo₂ values and temperatures. Such variation is consistent withthe change of refractive indices and cell dimensions. The meanrefractive indices and cell dimensions for synthetic epidote(Ps 33) are N = 1.745 ± 0.005, N = l.786±0.005,a = 8.920±0.005 Å, b = 5.645±0.004 Å,c = 10.190 ű0.006 Å, and ß = 115°31'±4' and for epidote (Ps 25) are N = 1.735±0.005,N = 1.775±0.005, a = 8.891±0.005 Å, b =5.625±0.004 Å, c = 10.177±0.006 Å,and ß = 115° 30'±3'. Mössbauer spectraindicate synthetic epidotes are relatively disordered. Garnets of intermediate composition in the grossular-andraditeseries were synthesized and the cell dimensions and refractiveindices vary linearly with composition. With successive decreasein fo₂, garnet synthesized on the Ps 33 bulk composition movestoward the grossular end member with simultaneously increasingalmandine component; concomitantly the hercynite component ofthe coexistent magnetite increases. The fo₂-T-P_fluid relations were determined by employing mineralmixtures of synthetic epidote and its high temperature equivalentin subequal proportions. Equilibrium was demonstrated for thereactions (1) epidote (Ps 33) = anorthite+grandite+FeO_x+quartz+ fluid, and (2) epidote (Ps 25) (+quartz) = garnet₃₈+anorthite+magnetitc+fluid.With fo₂ defined by the HM buffer, epidote (Ps 33) is stableup to 748 °C, 5 kb, 678 °C, 3 kb, and 635 °C, 2kb P_fluid. With fo₂ defined by the NNO buffer, the epidote (Ps25) high temperature stability limit is reduced about 100 °Cat 5kb P_fluid. At slightly lower fo₂, than defined by the QFMbuffer, epidote is not stable at any temperatures; the assemblagehedenbergite+anorthite+garnet₃₈+fluid replaces epidote in thepresence of excess quartz. Combined with previously determined equilibria for prehnite,andradite, and hedenbergite, isobaric fo,-T relations were furtherinvestigated by chemographic analysis interrelating the phasesprehnite, epidote, grandite, hedenbergite, wollastonite, anorthite,and magnetite in the system CaO-Fe₂O₃-Al₂O₃-SiO₂-H₂0. Such analysisallowed the construction of a semi-quantitative petrogeneticgrid applicable to natural parageneses in low µCO₂ environments,and the delineation of the low temperature stability limit ofepidote as a function of fo₂. Enlargement of the epidote stabilityrange toward both high and low temperatures with increasingfo₂, is consistent with widespread occurrences of epidote inlow- and mediumgrade metamorphic rocks.     

Subduction Style Magmatism in a Non-subduction Setting: the Colville Igneous Complex, NE Washington State, USA

The Colville Igneous Complex is located within the Eocene MagmaticBelt of the North American Cordilleran interior. It straddlesthe US–Canadian border in northeast Washington and southernBritish Columbia. The complex consists of three intrusive andtwo extrusive phases, the first extrusive phase being contemporaneouswith the latter two intrusive phases. As a consequence of sub-solidusre-equilibration in the plutonic rocks, this study concentrateson the two extrusive phases, the Sanpoil Volcanic Formationand the Klondike Mountain Formation. The Sanpoil Volcanic Formationconsists of andesites, dacites and rare trachyandesites (SiO₂= 55–70 wt %) exhibiting a slight decrease in total alkalis(Na₂O + K₂O) with increasing silica. The Klondike Mountain Formationconsists of basalts, basaltic andesites, andesites, dacitesand rhyolites (SiO₂ = 51–75 wt %) with total alkalis increasingwith increasing silica. The calc-alkaline affinity of the rocksof the Colville Igneous Complex, coupled with the presence ofa ‘subduction signature’ of enriched large ion lithophileelements (LILE) and depleted high field strength elements (HFSE),has traditionally been attributed to petrogenesis in a subduction-relatedmagmatic arc, the ‘Challis Arc’. New trace and rareearth element and isotopic data (⁸⁷Sr/⁸⁶Sr_i,     

Mafic Mineralogy of Ferroaugite Syenite from the Coldwell Alkaline Complex, Ontario, Canada

A 1500 m thick sheet-like body of ferroaugite syenite is divisiblemineralogically into an upper and lower series of syenites.The lower syenites are characterized by well developed igneouslayering defined by mafic cumulus minerals. The syenites aresaturated to oversaturated and contain as cumulus phases alkalifeldspar, olivine (Fa_83–93), ferroaugite (Di₅₀Hd₄₅Ac₅–Di₅Hd₉₀Ac₅)and ilmeno-magnetite. Amphiboles which crystallized from theintercumulus liquid range in composition from ferrohastingsitichornblende to ferroedenitic hornblende to ferroactinolitic edenite.The upper series are coarse grained cumulates with poorly definedlayering and abundant patch pegmatites. Cumulus phases are alkalifeldspar, olivine (Fa₉₃), and acmitic-hedenbergite (Di₅Hd₅₀Ac₅–Ac₅₀Hd₅₀).Intercumulus liquids are peralkaline and crystallized to aenigmatiteand amphiboles which range in composition from ferrorichteritickatophorite to ferrorichterite, Patch pegmatites are peralkalinerocks composed of ferrorichterite, ferroactinolite, alkali feldspar,aenigmatite, quartz and zircon. Extreme differentiation of ferroaugitesyenite magma generates residua which are ironrich, oversaturatedand peralkaline. Initial and final temperatures of crystallizationare estimated from mineral stability data to be 800–900°C to 500–550 °C respectively. Thermodynamic dataand mineral compositions indicate that during crystallizationthe oxygen fugacity of the magma decreased from approximately10^–15 to 10^–23–10^–24 bars. Ferroaugitesyenite pyroxene compositional trends are similar to those ofundersaturated peralkaline syenites (llimaussaq) and demonstratethat acmite enrichment trends are independent of silica activityand take place under decreasing oxygen fugacities.     

Almandine in Thermal Aureoles

The concept that almandine-rich garnet is an anomalous phasein thermal metamorphic assemblages is based partly upon therarity of almandine in hornfelses, and partly upon evidenceof breakdown of ‘regionally’ formed garnets whentheir host-rocks have been thermally metamorphosed. Where amphibolitefacies regional gneisses have been re-metamorphosed within thepyroxene-hornfels facies conditions of the late Caledonian Lochnagaraureole, garnet of composition Alm₈₀Py₁₁p⁶ (Gr+And)₄ has reactedwith biotite, sillimanite, and muscovite to give cordierite-richpseudomorphs; where armoured from reaction by immersion in quartzor feldspar, no signs of dissolution are seen. In some totallyreconstituted hornfelses, almandine garnet of composition AIm₈₀P₁₃Sp₄(Gr+And)3 appears to have coexisted stably with cordierite andorthoclase. It is concluded that the ‘unstable’garnet was breaking down not because its P/T stability fieldhad been exceeded, but because the alman-dine-bearing rock-compositionfield in the thermal (pyroxene-hornfels) facies was more restrictedthan that in the regional (amphibolite) facies. Chemical datafrom the hornfelses suggest that this restriction was mainlydue to the stability of cordierite—the plane spinel-cordierite-quartzrestricts garnet to those rocks with effective mol. (FeO+MgO-t-MnO)A12O3>1, while within that range cordierite-biotite tie-linesrestrict garnet to rocks of high (FeO+MnO)/MgO ratio (Fig. 4b). Recorded instances of garnet ‘instability’ in thermalaureoles show similar features to the Lochnagar aureole—garnetbreaks down by reaction; unequivocal instances of isochemicalbreakdown are rare. This, combined with the widespread occurrenceof almandine in volcanic and plutonic igneous rocks, suggeststhat almandine is a physically stable phase not only in thehornblende-hornfels facies, but also in the pyroxene-hornfelsfacies and possibly in portion of the sanidinite facies as well.The rarity of almandine in thermal aureoles is the result ofits very narrow rock composition field under the P/T conditionsof such environments. Comparison of thermal and granulite facies garnet-cordieriteassemblages suggests that P and T modify the almandine-bearingrock composition field mainly by modifying the limiting Mg/Feratio of garnet and the limiting Fe/Mg ratio of cordierite (Fig.10). The wide rock-composition field of almandine in the amphibolitefacies may be contingent upon the inhibition of cordierite byhydrous minerals under relatively high partial pressures ofwater.     

The Petrology and Origin of the Tertiary Lundy Granite (Bristol Channel, UK)

The British Tertiary Volcanic Province (BTVP) comprises within-platecentral igneous complexes associated with plateau lavas andregional dyke swarms. Lundy is the southernmost complex of theBTVP and comprises granite ({small tilde}90%) emplaced intodeformed Devonian sedimentary rocks within the Hercynian Cornubiangranite province of southwest England. The complex is intrudedby a northwest-southeast trending dyke swarm. In common withother BTVP igneous complexes, Lundy is associated with positivegravity and magnetic anomalies which are interpreted in termsof the presena of an underlying basic intrusion at shallow depth,with a volume exceeding that of the overlying granite. The Lundy intrusion is a coarse-grained megacrystic granitecontaining up to 20% alkali feldspar megacrysts in a coarse-grainedgroundmass composed of alkali feldspar, quartz, lithium-bearingmuscovite, and ‘biotite’ (lithian siderophyllite),with a range of aaxssory minerals. The main granite has a coarse-grained(locally miarolitic) pegmatitic facies and is intruded by thinsheets and veins of fine-grained aplite and microgranite. Themineralogy indicates crystallization of the Lundy granite froma highly fractionated H₂O- and halogen-rich magma at a relativelyshallow crustal level. The main Lundy granite is a peraluminous leucogranite with Na₂O=3–4%,K₂O{small tilde}5%, low TiO₂, MeO, CaO, Zr, and Sr, and highRb and Rb/Sr in comparison with many other peralurninous granites,including those from the Cornubian batholith and the BTVP. Anew Rb-Sr whole-rock isochron for the granite yields an ageof 58?7?1?6 Ma with an initial ⁸⁷Sr/86Sr of 0?715?0?006. _Ndvalues for the granite (–0?9 to –1?9) plot betweencontemporaneous mantle (positive _Nd and Cornubian granites (_Nd=ca.–11). The trace element data (Rb, Y, Nb) show affinities with syn-collisionand within-plate granites. As the Sr isotope data indicate amajor crustal component, and the Nd isotope data suggest bothmantle and crustal components, we propose that the Lundy graniteis derived from a parental magma comprising crustal components(derived from a similar source to that of the Cornubian granitebatholith) and a mantle-derived component (derived from a differentiateof contemporaneous basaltic magma This magma experienced fractionalcrystallization of plagioclase, alkai feldspar, Fe-Mg minerals,and REE-bearing accessory minerals before emplacement, and theLundy granite experienced further in situ fractional crystallization,associateded with crustal contamination by the Devonian shaleafter emplacement.     

Quantification of Magmatic and Hydrothermal Processes in a Peralkaline Syenite-Alkali Granite Complex Based on Textures, Phase Equilibria, and Stable and Radiogenic Isotopes

The Puklen complex of the Mid-Proterozoic Gardar Province, SouthGreenland, consists of various silica-saturated to quartz-bearingsyenites, which are intruded by a peralkaline granite. The primarymafic minerals in the syenites are augite ± olivine +Fe–Ti oxide + amphibole. Ternary feldspar thermometryand phase equilibria among mafic silicates yield T = 950–750°C,a_SiO2 = 0·7–1 and an f_O2 of 1–3 log unitsbelow the fayalite–magnetite–quartz (FMQ) bufferat 1 kbar. In the granites, the primary mafic minerals are ilmeniteand Li-bearing arfvedsonite, which crystallized at temperaturesbelow 750°C and at f_O2 values around the FMQ buffer. Inboth rock types, a secondary post-magmatic assemblage overprintsthe primary magmatic phases. In syenites, primary Ca-bearingminerals are replaced by Na-rich minerals such as aegirine–augiteand albite, resulting in the release of Ca. Accordingly, secondaryminerals include ferro-actinolite, (calcite–siderite)_ss,titanite and andradite in equilibrium with the Na-rich minerals.Phase equilibria indicate that formation of these minerals tookplace over a long temperature interval from near-magmatic temperaturesdown to     

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

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

Bulk-rock Major and Trace Element Compositions of Abyssal Peridotites: Implications for Mantle Melting, Melt Extraction and Post-melting Processes Beneath Mid-Ocean Ridges

This paper presents the first comprehensive major and traceelement data for 130 abyssal peridotite samples from the Pacificand Indian ocean ridge–transform systems. The data revealimportant features about the petrogenesis of these rocks, mantlemelting and melt extraction processes beneath ocean ridges,and elemental behaviours. Although abyssal peridotites are serpentinized,and have also experienced seafloor weathering, magmatic signaturesremain well preserved in the bulk-rock compositions. The betterinverse correlation of MgO with progressively heavier rare earthelements (REE) reflects varying amounts of melt depletion. Thismelt depletion may result from recent sub-ridge mantle melting,but could also be inherited from previous melt extraction eventsfrom the fertile mantle source. Light REE (LREE) in bulk-rocksamples are more enriched, not more depleted, than in the constituentclinopyroxenes (cpx) of the same sample suites. If the cpx LREErecord sub-ridge mantle melting processes, then the bulk-rockLREE must reflect post-melting refertilization. The significantcorrelations of LREE (e.g. La, Ce, Pr, Nd) with immobile highfield strength elements (HFSE, e.g. Nb and Zr) suggest thatenrichments of both LREE and HFSE resulted from a common magmaticprocess. The refertilization takes place in the ‘cold’thermal boundary layer (TBL) beneath ridges through which theascending melts migrate and interact with the advanced residues.The refertilization apparently did not affect the cpx relicsanalyzed for trace elements. This observation suggests grain-boundaryporous melt migration in the TBL. The ascending melts may notbe thermally ‘reactive’, and thus may have affectedonly cpx rims, which, together with precipitated olivine, entrappedmelt, and the rest of the rock, were subsequently serpentinized.Very large variations in bulk-rock Zr/Hf and Nb/Ta ratios areobserved, which are unexpected. The correlation between thetwo ratios is consistent with observations on basalts that D_Zr/D_Hf< 1 and D_Nb/D_Ta < 1. Given the identical charges (5⁺ forNb and Ta; 4⁺ for Zr and Hf) and essentially the same ionicradii (R_Nb/R_Ta = 1·000 and R_Zr/R_Hf = 1·006–1·026),yet a factor of 2 mass differences (M_Zr/M_Hf = 0·511 andM_Nb/M_Ta = 0·513), it is hypothesized that mass-dependentD values, or diffusion or mass-transfer rates may be importantin causing elemental fractionations during porous melt migrationin the TBL. It is also possible that some ‘exotic’phases with highly fractionated Zr/Hf and Nb/Ta ratios may existin these rocks, thus having ‘nugget’ effects onthe bulk-rock analyses. All these hypotheses need testing byconstraining the storage and distribution of all the incompatibletrace elements in mantle peridotite. As serpentine containsup to 13 wt % H₂O, and is stable up to 7 GPa before it is transformedto dense hydrous magnesium silicate phases that are stable atpressures of 5–50 GPa, it is possible that the serpentinizedperidotites may survive, at least partly, subduction-zone dehydration,and transport large amounts of H₂O (also Ba, Rb, Cs, K, U, Sr,Pb, etc. with elevated U/Pb ratios) into the deep mantle. Thelatter may contribute to the HIMU component in the source regionsof some oceanic basalts. KEY WORDS: abyssal peridotites; serpentinization; seafloor weathering; bulk-rock major and trace element compositions; mantle melting; melt extraction; melt–residue interaction; porous flows; Nb/Ta and Zr/Hf fractionations; HIMU mantle sources     

The McIntosh Layered Troctolite-Olivine Gabbro Intrusion, East Kimberley, Western Australia

The well-preserved ?lower Proterozoic McIntosh intrusion consistsof 96 macro-layers with a total stratigraphic thickness of about6 km. The lowermost rocks in this possible cone-shaped intrusionare hidden, and the roof and the upper layers were removed byerosion. The layered sequence is dominated by 40 bimodal cyclicunits of troctolite and olivine gabbro. Minor gabbronorite layersoccur throughout the sequence, and are more abundant and morefractionated higher in the sequence. Six imperfect megacycicunits are developed in the upper 2700 m, each unit consistingof several troctolite-olivine gabbro cyclic units followed bya Fe-Ti oxide-bearing gabbronorite. The overall cumulus crystallizationorder in each megacyclic unit was plagioclase first, closelyfollowed by olivine, then augite, orthopyroxene, and magnetitesuccessively. Cryptic composition data for troctolites and olivine gabbrosshow a slight overall decrease of 10 mol per cent An and Fofrom the base to the top of the layered sequence (approximateranges An_80–70 and Fo_78–68). Several major fluctuationsoccur however, and are generally associated with the oxide gabbronorites,which are significantly more fractionated than the adjacentlayers (plagioclase An_53–60, orthopyroxene Mg_52–69Each fluctuation comprises a marked progressive discontinuity(rapid normal fractionation) followed by a gradual to rapidregressive discontinuity (or reversal) in the overlying troctolitesand olivine gabbros. Apparently, such marked progressive discontinuitieshave not been described in layered intrusions. A chilled margin and the overall composition of the intrusionsuggest an olivine tholeiite parent magma, inferred to havecrystallized at P 6 kb, relatively low _PH2O and high f_O2 (>NNO buffer). The troctolite-olivine gabbro cyclic units areinferred to have formed by fractional crystallization of periodicadditions of new magma. However, the oxide gabbronorites seemtoo fractionated relative to the underlying layers to have formedby conventional crystal fractionation mechanisms, and they couldhave resulted from a ‘liquid fractionation’ processin which fractionated residual magma, instead of rising, periodicallybecame denser and ponded on the temporary floor (a density crossover).Gradual, reversed cryptic trends in the cyclic units above theoxide gabbronorite layers may reflect mixing of this fractionatedmagma with successive magma additions.     

Geochemical and Isotopic (O, Nd, Pb and Sr) Constraints on A-type Granite Petrogenesis Based on the Topsails Igneous Suite, Newfoundland Appalachians

The voluminous, bimodal, Silurian Topsails igneous suite consistsmainly of ‘A-type’ peralkaline to slightly peraluminous,hypersohnis to subsolvus granites with subordinate syenite,onzonite and diabase, plus consanguineous basalts and highsilicarhyolites. _Nd(T) values from the suite range from –1.5to +5.4; most granitoid components exhibit positive _Nd(T) values(+1.1 to +3.9). Granitoid initial ⁸⁷Sr/⁸⁶Sr and most ¹⁸ O valuesare in the range expected for rocks derived from mantle-likeprotoliths (0.701–0.706 and +6 to +80/). Restricted ²⁰⁷Pb/²⁰⁴Pbvariation is accompanied by significant dispersion of ²⁰⁶Pb/²⁰⁴Pband ²⁰⁸Pb/²⁰⁴Pb. Superficially, petrogenesis by either direct(via fractionation from basalt) or indirect (via melting ofjuvenile crust) derivation from mantle sources appears plausible.Remelting of the granulitic protolith of Ordovician are-typegranitoids can be ruled out, because these rocks exhibit negative_Nd(T) and a large range in ²⁰⁷Pb/²⁰⁴Pb. Geochemical and isotopicrelationships are most compatible with remelting of hybridizedlithospheric mantle generated during arc-continent collision.A genetic link is suggested among collision-related delaminationor slab break-off events and emplacement of ‘post-erogenic’granite suites. A-type granites may recycle previously subductedcontinental material, and help explain the mass balance notedfor modern arcs. However, they need not represent net, new,crustal growth. KEY WORDS: A-type granites; juvenile crust; isotopes; Newfoundland ^*Telephone: (613) 995-4972. Fax: (613) 995-7997. e-mail: jwhalen{at}gsc.emr.ca     

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.     

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     

Mineralogy and Petrogenesis of the Deccan Trap Lava Flows around Mahabaleshwar, India

Electron microprobe analyses of minerals of thirteen DeccanTrap lava flows at Mahabaleshwar have been carried out in thepresent study. All of these flows have tholeiitic bulk compositionsand all, except one (represented by MB-81-17 of Mahoney et al.,1982) contain olivine, plagioclase, two pyroxenes, and Fe-Tioxide minerals. Olivine and plagioclase appear as distinct phenocrystsin all but one flow, and Ca-rich pyroxene joins as a phenocrystphase in the younger flows. Pigeonite and Fe-Ti oxide minerals(titanomagnetite and ilmenite) occur in the groundmass. Olivineoccurs as both groundmass and phenocryst phase in MB-81-17 (whichis the only flow without low-Ca pyroxene phase); in all otherflows olivine appears only as phenocryst phase. In all but one(MB-81-17) flow olivine is completely altered. MB-81-17 olivinegrains are only partly altered, and in this rock the cores ofphenocrysts are rounded and have a composition of Fo₇₇ whereastheir euhedral rims have a composition around Fo₆₇. The groundmassolivine grains in MB-81-17 are Fo_41–32. Substantial Fe-enrichmentand zoning trends are shown by the pyroxenes in individual rocks.The cores of Ca-rich pyroxene phenocrysts of some of the flowshave as much as 4 wt. per cent A1₂O₃ and may have crystallizedat higher (crustal) pressures. Pigeonite thermometry (Ishii,1975) suggests an average of 1050?C for crystallization of thegroundmass pigeonite (eruption temperature?). Fe-Ti oxide mineralsare mostly altered in the older flows. In the younger flows,coexisting unaltered titanomagnetite and ilmenite yield maximumtemperature estimates for the crystallization of these phaseof about 1025?C and an oxygen fugacity of 10^–11.5 atm.The T-fo₂ path followed by these flows seems to have been consistentlysomewhat lower than that defined by the 1 atm. fayalite-magnetitequartz curve. All of the lavas examined have experienced extensivefractional crystallization of olivine and some clinopyroxeneat relatively higher pressures. These lavas were saturated orclose to being saturated with olivine+plagioclase+clinopyroxeneduring eruption. Plagioclase accumulation, although it appearsto have occurred, has not been significant. It is suggestedthat MB-81-17 magma was contaminated by a calcite-rich rock(limestone?) whereas the lower Group 1 magmas may have beenselectively contaminated by quartz-bearing contaminant. Alternately,parental magma of MB-81-1 (with the highest Mg-number and 8= -16) may have been produced in the upper mantle into whichminor masses of old crust was well mixed. Magma mixing, crystalfractionation, and contamination processes of Mahabaleshwarbasalts and possible genetic relationships with other DeccanTrap lavas are discussed.