Archaean to Proterozoic Crustal Evolution in the Central Zone of the Limpopo Belt (South Africa-Botswana): Constraints from Combined U-Pb and Lu-Hf Isotope Analyses of Zircon

A combined set of U–Pb and Lu–Hf in situ laser ablationICP-(MC)-MS zircon analyses were obtained from orthogneissesand granitoids in the Central Zone of the Limpopo Belt, whichcomprises the Beit Bridge and Mahalapye complexes. The resultsindicate that by combining the two isotope systems primary magmaticzircon domains can be distinguished from those formed duringlater metamorphic events, even if the distinct zircon domainsunderwent multiple Pb loss and the texture–age relationships,as obtained by cathodoluminescence images and U–Pb analyses,are ambiguous. Furthermore, the applied technique allows distinctionof zircon grains formed in juvenile magmas from those generatedby melting of older continental crust or affected by substantialcrustal contamination. The combined U–Pb and Lu–Hfdata reveal that the Sand River gneiss suite of the Beit BridgeComplex was emplaced at 3283 ± 8 Ma and formed from meltingof an older Archaean crust, which was derived from a depletedmantle source at around 3·65 Ga. The hafnium model age(T_DM^Hf) is significantly older than those obtained from zirconsfrom numerous Neoarchaean granitoids of the Beit Bridge Complex,comprising the Singelele gneiss (2647 ± 12 Ma), the Bulaigranite (2612 ± 7 Ma), the Regina gneiss (2649 ±9 Ma) and two samples of the Zanzibar gneiss (2613 ±6 Ma). These granitoids show initial Hf(t) values between +0·5 and –7·1, which correspond to initialT_DM^Hf between 3·46 and 3·01 Ga. These variableT_DM^Hf_initial and Hf(t)_initial values are interpreted to be theresult of different mixtures of reworked 3·65 Ga Palaeoarchaeancrust with juvenile magmas extracted from the depleted mantleduring the Neoarchaean at 2·65 Ga. This conclusion issupported by results obtained from the Mahalapye Complex, whichwas affected by migmatization and granite intrusions duringthe Palaeoproterozoic at 2·02–2·06 Ga. TheMokgware granite (2019 ± 9 Ma) contains zircon xenocrystswith Pb–Pb ages of 2·52–2·65 Ga and2·93 Ga and hafnium model ages of 3·0–3·4Ga, indicating that this granite is derived from remelting ofArchaean crust. In contrast, uniform T_DM^Hf_initial ages of 2·61–2·67Ga obtained from a diorite gneiss (2061 ± 6 Ma) of theMahalapye Complex indicate that its protolith may have beenformed from remelting of a Neoarchaean juvenile crust. VariableHf(t)_initial values from –3·7 to +6·3 ofzircon cores (2711 ± 11 Ma) in an adjacent leucosomealso support a model of mixing of juvenile mantle derived matterwith older crust in the Neoarchaean. KEY WORDS: Archaean; Palaeoproterozoic; Limpopo Belt; zircon, U–Pb dating; Lu–Hf isotopes; LA-ICP-MS     

The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks

Compositional fields for spinels from a wide variety of mafic–ultramaficigneous rock types and tectonic environments have been determinedfrom a global database of over 26 000 analyses. These fieldsare defined using contoured data density plots based on thespinel prism, and plots of T iO₂ vs ferric iron, for mantlexenoliths, ophiolitic rocks, continental layered intrusions,alkalic and lamprophyric rocks, tholeiitic basalts, Alaskanultramafic complexes and komatiites. Several trends appear regularlyin the various environments: a trend of widely variable Cr/(Cr+ Al) at low Fe²⁺/(Mg + Fe²⁺) (the Cr–Al trend); increasingFe³⁺, Fe²⁺/(Mg + Fe²⁺) and T iO₂ at constant Cr/(Cr + Al) (Fe–Ti trend); a trend found primarily in kimberlites, similar toFe–T i but at constant Fe²⁺/(Mg + Fe²⁺); and an unusualtrend of increasing Al found only in layered intrusions. TheCr–Al and Fe–T i trends are both found to varyingdegrees in tholeiitic basalts. The Cr–Al trend is prevalentin rocks that have equilibrated over a range of pressures, whereasthe Fe–T i trend is dominantly due to low-pressure fractionation.The most Cr-rich chromites found in nature occur in boninites,diamond-bearing kimberlites, some komatiites and ophioliticchromitites. Exceptionally reduced chromites are found in somekomatiites and in ophiolitic chromitites. Detrital chromitesfrom the Witwatersrand conglomerates are of komatiitic provenance. KEY WORDS: basalt; chromite; kimberlite; ophiolite; spinel     

Origin of Paleoproterozoic Komatiites at Jeesiorova, Kittila Greenstone Complex, Finnish Lapland

Komatiites from the 2 Ga Jeesiörova area in Finnish Laplandhave subchondritic Al₂O₃/TiO₂ ratios like those in Al-depletedkomatiites from Barberton, South Africa. They are distinct inthat their Al abundances are higher than those of the Al-depletedrocks and similar to levels in Al-undepleted komatiites. Moderatelyincompatible elements such as Ti, Zr, Eu, and Gd are enriched.Neither majorite fractionation nor hydrous melting in a supra-subductionzone setting could have produced these komatiites. Their highconcentrations of moderately incompatible elements may haveresulted from contamination of their parental melt through interactionwith metasomatic assemblages in the lithospheric mantle or enrichmentof their mantle source in basaltic melt components. Re–Osisotope data for chromite from the Jeesiörova rocks yieldan average initial ¹⁸⁷Os/¹⁸⁸Os of 0·1131 ± 0·0006(2), _Os(I) = 0·1 ± 0·5. These data, coupledwith an initial _Nd of +4, indicate that melt parental to thekomatiites interacted minimally with ancient lithospheric mantle.If their mantle source was enriched in a basaltic component,the combined Os–Nd isotopic data limit the enrichmentprocess to within 200 Myr prior to the formation of the komatiites.Their Os–Nd isotopic composition is consistent with derivationfrom the contemporaneous convecting upper mantle. KEY WORDS: Finnish Lapland; Jeesiörova; komatiites; mantle geochemistry; petrogenesis; redox state; Re/Os isotopes; Ti enrichment     

Assigning Dates to Thin Gneissic Veins in High-Grade Metamorphic Terranes: A Cautionary Tale from Akilia, Southwest Greenland

A granodiorite from Akilia, southwest Greenland, previouslysuggested to date putative life-bearing rocks to 3·84Ga, is re-investigated using whole-rock major and trace-elementgeochemistry, and detailed cathodoluminescence image-guidedsecondary ion mass spectrometer analyses of zircon U–Th–Pband rare earth elements. Complex zircon internal structure revealsthree episodes of zircon growth and/or recrystallization datedto c. 3·84 Ga, 3·62 Ga and 2·71 Ga. Rareearth element abundances imply a significant role for garnetin zircon generation at 3·62 Ga and 2·71 Ga. The3·62 Ga event is interpreted as partial melting of ac. 3·84 Ga grey gneiss precursor at granulite facieswith residual garnet. Migration of this 3·62 Ga magma(or melt–crystal mush) away from the melt source placesa maximum age limit on any intrusive relationship. These earlyArchaean relationships have been complicated further by isotopicreworking in the 2·71 Ga event, which could have includeda further episode of partial melting. This study highlightsa general problem associated with dating thin gneissic veinsin polyphase metamorphic terranes, where field relationshipsmay be ambiguous and zircon inheritance can be expected. KEY WORDS: Archaean; geochronology; Greenland; secondary ion mass spectrometry; zircon     

Crystal Accumulation and Shearing in a Megacrystic Quartz Monzonite: Bodoco Pluton, Northeastern Brazil

The Bodocó pluton, typical of numerous felsic intrusionsin northeastern Brazil that are characterized by blocky megacrystsof K-feldspar, consists mainly of porphyritic coarse-grainedquartz monzonite (SiO₂ 58–70 wt %) and is reversely zonedfrom a granitic margin to a quartz monzodioritic core. Thereis little variation in mineral composition throughout the pluton,despite a range of variation in mineral proportions. Isotopiccharacteristics also are homogeneous, with ¹⁸O_quartz between+9·3 and +9·8 and initial ⁸⁷Sr/⁸⁶Sr within limitsof 0·7056–0·7063. Petrogenetic modellingindicates that in situ crystal accumulation processes, accompaniedby the upward migration of a crystal-poor felsic melt, can accountfor many of the observed chemical and isotopic features, petrographictextures, and spatial relationships of rock types. Localizedshearing associated with regional ductile deformation producedextensive kilometre-wide bands of strongly foliated megacrysticquartz monzonite intruded by mafic dykes. Shear-related magmamingling and/or mixing were localized post-emplacement differentiationprocesses, particularly at the upper level of the intrusionand in quartz monzonite border units along the southeast margin. KEY WORDS: accumulation; Brazil; megacryst; petrogenesis; shearing     

The Origin and Evolution of the Kaapvaal Cratonic Lithospheric Mantle

A detailed petrological and geochemical study of low-temperatureperidotite xenoliths from Kimberley and northern Lesotho ispresented to constrain the processes that led to the magmaphileelement depletion of the Kaapvaal cratonic lithospheric mantleand its subsequent re-enrichment in Si and incompatible traceelements. Whole-rocks and minerals have been characterized forRe–Os isotope compositions, and major and trace elementconcentrations, and garnet and clinopyroxene for Lu–Hfand Sm–Nd isotope compositions. Most samples are characterizedby Archaean Os model ages, low Al, Fe and Ca contents, highMg/Fe, low Re/Os, very low (< 0·1 x chondrite) heavyrare earth element (HREE) concentrations and a decoupling betweenNd and Hf isotope ratios. These features are most consistentwith initial melting at 3·2 Ga followed by metasomatismby hydrous fluids, which may have also caused additional meltingto produce a harzburgitic residue. The low HREE abundances ofthe peridotites require that extensive melting occurred in thespinel stability field, possibly preceded by some melting inthe presence of garnet. Fractional melting models suggest that30% melting in the spinel field or 20% melting in the garnetfield followed by 20% spinel-facies melting are required toexplain the most melt-depleted samples. Garnet Nd–Hf isotopecharacteristics indicate metasomatic trace element enrichmentduring the Archaean. We therefore suggest a model includingshallow ridge melting, followed by metasomatism of the Kaapvaalupper mantle in subduction zones surrounding cratonic nuclei,probably during amalgamation of smaller pre-existing terranesin the Late Archaean (2·9 Ga). The fluid-metasomatizedresidua have subsequently undergone localized silicate meltinfiltration that led to clinopyroxene ± garnet enrichment.Calculated equilibrium liquids for clinopyroxene and their Hf–Ndisotope compositions suggest that most diopside in the xenolithscrystallized from an infiltrating kimberlite-like melt, eitherduring Group II kimberlite magmatism at 200–110 Ma (Kimberley),or shortly prior to eruption of the host kimberlite around 90Ma (northern Lesotho). KEY WORDS: Kaapvaal craton; lithospheric mantle; metasomatism; Nd–Hf isotopes; Re–Os isotopes     

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     

Phase Equilibria Constraints on the Chemical and Physical Evolution of the Campanian Ignimbrite

The Campanian Ignimbrite is a > 200 km³ trachyte–phonolitepyroclastic deposit that erupted at 39·3 ± 0·1ka within the Campi Flegrei west of Naples, Italy. Here we testthe hypothesis that Campanian Ignimbrite magma was derived byisobaric crystal fractionation of a parental basaltic trachyandesiticmelt that reacted and came into local equilibrium with smallamounts (5–10 wt%) of crustal rock (skarns and foid-syenites)during crystallization. Comparison of observed crystal and magmacompositions with results of phase equilibria assimilation–fractionationsimulations (MELTS) is generally very good. Oxygen fugacitywas approximately buffered along QFM + 1 (where QFM is the quartz–fayalite–magnetitebuffer) during isobaric fractionation at 0·15 GPa ( 6km depth). The parental melt, reconstructed from melt inclusionand host clinopyroxene compositions, is found to be basaltictrachyandesite liquid (51·1 wt% SiO₂, 9·3 wt%MgO, 3 wt% H₂O). A significant feature of phase equilibria simulationsis the existence of a pseudo-invariant temperature, 883 °C,at which the fraction of melt remaining in the system decreasesabruptly from 0·5 to < 0·1. Crystallizationat the pseudo-invariant point leads to abrupt changes in thecomposition, properties (density, dissolved water content),and physical state (viscosity, volume fraction fluid) of meltand magma. A dramatic decrease in melt viscosity (from 1700Pa s to 200 Pa s), coupled with a change in the volume fractionof water in magma (from 0·1 to 0·8) and a dramaticdecrease in melt and magma density acted as a destabilizingeruption trigger. Thermal models suggest a timescale of 200kyr from the beginning of fractionation until eruption, leadingto an apparent rate of evolved magma generation of about 10^–3km³/year. In situ crystallization and crystal settling in density-stratifiedregions, as well as in convectively mixed, less evolved subjacentmagma, operate rapidly enough to match this apparent volumetricrate of evolved magma production. KEY WORDS: assimilation; Campanian Ignimbrite; fractional crystallization; magma dynamics; phase equilibria     

The Proterozoic P-T-t Evolution of the Kemp Land Coast, East Antarctica; Constraints from Si-saturated and Si-undersaturated Metapelites

Integrated metamorphic and geochronological data place new constraintson the metamorphic evolution of a Neoproterozoic orogen in eastAntarctica. Granulite-facies rocks from a 150 km stretch ofthe Kemp Land coast reflect peak conditions involving T 870–990°Cat P 7·4–10 kbar, with pressure increasing westwardtowards an Archaean craton. Electron microprobe-derived (Th+ U)–Pb monazite ages from metapelitic assemblages indicatethat the major mineral textures in these rocks developed duringthe c. 940 Ma Rayner Orogeny. Complex compositional zoning inmonazite suggests high-T recrystallization over c. 25 Myr. Diversityin metapelitic reaction textures reflects silica and ferromagnesiancontent: Si-saturated Fe-rich metapelites contain garnet thatis partially pseudomorphed by biotite and sillimanite, whereasSi-saturated Mg-rich metapelites and Si-undersaturated metapeliticpods have reaction microstructures involving cordierite enclosingorthopyroxene, garnet and/or sapphirine, cordierite + sapphirinesymplectites around sillimanite and coarse-grained orthopyroxene+ corundum separated by sapphirine coronae. Interpretationsbased on P–T pseudosections provide integrated bulk-rockconstraints and indicate a clockwise P–T–t pathcharacterized by a post-peak P–T trajectory with dP/dT 15–20 bar/ °C. This moderately sloped decompressive-coolingP–T path is in contrast to near-isothermal decompressionP–T paths commonly cited for this region of the RaynerComplex, with implications for the post-collisional tectonicresponse of the mid- to lower crust within this orogenic belt. KEY WORDS: electron microprobe monazite dating; granulite facies; Rayner Complex; sapphirine; THERMOCALCMinerals abbreviations: q, quartz; g, garnet; sill, sillimanite; ky, kyanite; opx, orthopyroxene; cd, cordierite; ksp, alkali feldspar; pl, plagioclase; bi, biotite; sp, spinel; ilm, ilmenite; mt, magnetite; ru, rutile; sa, sapphirine; cor, corundum; osm, osumilite; liq, silicate melt; mnz, monazite     

Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada: Significance of Charnockitic Magmatism

Calc-alkaline batholiths of the Archaean Minto block, northeasternSuperior Province, Canada, have pyroxene- and hornblende-bearingmineral assemblages inferred to have crystallized from hot,water-undersaturated magmas at 2·729–2·724Ga. A regional amphibolite- to granulite-facies tectonothermalevent at 2·70 Ga resulted in mild to negligible metamorphiceffects on the dominantly granodioritic units. Geochemical,textural and thermobarometric studies define the crystallizationhistory in compositions ranging from cumulate pyroxenite throughquartz diorite, granodiorite, granite, and syn-magmatic gabbroicdykes. Early magmatic assemblages include orthopyroxene, clinopyroxene,plagioclase, biotite, Fe–Ti oxides and ternary feldspar,indicating crystallization from magmas containing <2 wt %H₂O at 1100–900°C. Water enrichment in the residualmelt induced hornblende crystallization at 5 ± 1 kbar,800–600°C. Characterized by a continuum of large ionlithophile element (LILE)-enriched, high field strength element(HFSE)-depleted compositions, the I-type suite resembles moderncontinental arc batholiths in composition and size but not primarymineralogy. Magmatic arcs produced between 2·75 and 1·85Ga commonly have charnockitic components, possibly because slab-derivedfluids interacted with mantle wedges at ambient temperatureshigher by 100°C than at present, producing large volumesof water-deficient magma. KEY WORDS: granitoid rocks; igneous pyroxenes; water-undersaturated magma; charnockite     

Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China

Picritic lava flows near Lijiang in the late Permian Emeishanflood basalt province are associated with augite-phyric basalt,aphyric basalt, and basaltic pyroclastic units. The dominantphenocryst in the picritic flows is Mg-rich olivine (up to 91·6%forsterite component) with high CaO contents (to 0·42wt %) and glass inclusions, indicating that the olivine crystallizedfrom a melt. Associated chromite has a high Cr-number (73–75).The estimated MgO content of the primitive picritic liquidsis about 22 wt %, and initial melt temperature may have beenas high as 1630–1690°C. The basaltic lavas appearto be related to the picritic ones principally by olivine andclinopyroxene fractionation. Age-corrected Nd–Sr–Pbisotope ratios of the picritic and basaltic lavas are indistinguishableand cover a relatively small range [e.g. _Nd(t) = –1·3to +4·0]. The higher _Nd(t) lavas are isotopically similarto those of several modern oceanic hotspots, and have ocean-island-likepatterns of alteration-resistant incompatible elements. Heavyrare earth element characteristics indicate an important rolefor garnet during melting and that the lavas were formed byfairly small degrees of partial melting. Rough correlationsof isotope ratios with ratios of alteration-resistant highlyincompatible elements (e.g. Nb/La) suggest modest amounts ofcontamination involving continental material or a relativelylow-_Nd component in the source. Overall, our results are consistentwith other evidence suggesting some type of plume-head originfor the Emeishan province. KEY WORDS: Emeishan; flood basalts; picrites; mantle plumes; late Permian     

Petrographic, Chemical and B-Isotopic Insights into the Origin of Tourmaline-Rich Rocks and Boron Recycling in the Martinamor Antiform (Central Iberian Zone, Salamanca, Spain)

Tourmaline in the Martinamor antiform occurs in tourmalinites(rocks with >15–20% tourmaline by volume), clasticmetasedimentary rocks of the Upper Proterozoic Monterrubio formation,quartz veins, pre-Variscan orthogneisses and Variscan graniticrocks. Petrographic observations, back-scattered electron (BSE)images, and microprobe data document a multistaged developmentof tourmaline. Overall, variations in the Mg/(Mg + Fe) ratiosdecrease from tourmalinites (0·36–0·75),through veins (0·38–0·66) to granitic rocks(0·23–0·46), whereas Al increases in thesame order from 5·84–6·65 to 6·22–6·88apfu. The incorporation of Al into tourmaline is consistentwith combinations of ^xAl(NaR)_–1 and AlO(R(OH))_–1exchange vectors, where ^x represents X-site vacancy and R is(Mg + Fe²⁺ + Mn). Variations in ^x/(^x + Na) ratios are similarin all the types of tourmaline occurrences, from 0·10to 0·53, with low Ca-contents (mostly <0·10apfu). Based on field and textural criteria, two groups of tourmaline-richrocks are distinguished: (1) pre-Variscan tourmalinites (probablyCadomian), affected by both deformation and regional metamorphismduring the Variscan orogeny; (2) tourmalinites related to thesynkinematic granitic complex of Martinamor. Textural and geochemicaldata are consistent with a psammopelitic parentage for the protolithof the tourmalinites. Boron isotope analyses of tourmaline havea total range of ¹¹B values from –15·6 to 6·8;the lowest corresponding to granitic tourmalines (–15·6to –11·7) and the highest to veins (1·9to 6·8). Tourmalines from tourmalinites have intermediate¹¹B values of –8·0 to +2·0. The observedvariations in ¹¹B support an important crustal recycling ofboron in the Martinamor area, in which pre-Variscan tourmaliniteswere remobilized by a combination of mechanical and chemicalprocesses during Variscan deformation, metamorphism and anatexis,leading to the formation of multiple tourmaline-bearing veinsand a new stage of boron metasomatism. KEY WORDS: tourmalinites; metamorphic and granitic rocks; mineral chemistry; whole-rock chemistry; boron isotopes     

The Gronnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility

The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcite–siderite carbonatite. Aegirine–augite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680–910°Cand silica activities of 0·28–0·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalite–magnetite–quartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (La_CN–Yb_CN = 10–70). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54–106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicate–carbonate liquid pairs. _Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and _Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has ¹⁸O values of 7·8 to 8·6and ¹³C values of –3·9 to –4·6. ¹⁸Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rock–water and mineral–water isotopefractionation to be 5·7 ± 0·4. Nd and C–Oisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in ¹⁸O betweencalculated syenitic melts and carbonatites, and the overlapin _Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite     

Magmatic Differentiation at an Island-arc Caldera: Okmok Volcano, Aleutian Islands, Alaska

Okmok volcano is situated on oceanic crust in the central Aleutianarc and experienced large (15 km³) caldera-forming eruptionsat 12 000 years BP and 2050 years BP. Each caldera-forming eruptionbegan with a small Plinian rhyodacite event followed by theemplacement of a dominantly andesitic ash-flow unit, whereaseffusive inter- and post-caldera lavas have been more basaltic.Phenocryst assemblages are composed of olivine + pyroxene +plagioclase ± Fe–Ti oxides and indicate crystallizationat 1000–1100°C at 0·1–0·2 GPain the presence of 0–4% H₂O. The erupted products followa tholeiitic evolutionary trend and calculated liquid compositionsrange from 52 to 68 wt % SiO₂ with 0·8–3·3wt % K₂O. Major and trace element models suggest that the moreevolved magmas were produced by 50–60% in situ fractionalcrystallization around the margins of the shallow magma chamber.Oxygen and strontium isotope data (¹⁸O 4·4–4·9,⁸⁷Sr/ ⁸⁶Sr 0·7032–0·7034) indicate interactionwith a hydrothermally altered crustal component, which led toelevated thorium isotope ratios in some caldera-forming magmas.This compromises the use of uranium–thorium disequilibria[(²³⁰Th/ ²³⁸U) = 0·849–0·964] to constrainthe time scales of magma differentiation but instead suggeststhat the age of the hydrothermal system is 100 ka. Modellingof the diffusion of strontium in plagioclase indicates thatmany evolved crystal rims formed less than 200 years prior toeruption. This addition of rim material probably reflects theremobilization of crystals from the chamber margins followingreplenishment. Basaltic recharge led to the expansion of themagma chamber, which was responsible for the most recent caldera-formingevent. KEY WORDS: Okmok; caldera; U-series isotopes; Sr-diffusion; time scales; Aleutian arc     

Trace Element and Sr-Pb-Nd-Hf Isotope Evidence for Ancient, Fluid-Dominated Enrichment of the Source of Aldan Shield Lamproites

A phase of Mesozoic extension associated with the terminationof continental collision at the southern margin of the AldanShield produced ultrabasic lamproites in a discontinuous belt500 km long and 150 km wide. The lamproites, locally poorlydiamondiferous, were emplaced as dykes, sills and pipes. AllAldan lamproites have primitive chemical characteristics (e.g.MgO up to 22·7 wt %) and are ultrapotassic (K₂O up to8·3 wt %) and peralkaline with K₂O + Na₂O/Al₂O₃ in therange 0·6–1·16. A distinctive feature ofthese rocks is their low TiO₂ content (0·5–1·4wt %). Aldan lamproites are moderately light rare earth element(LREE) enriched with (La/Yb)_N ranging from 10 to 47. Heavy rareearth element (HREE) abundances are lower than for all otherlamproites by up to a factor of five. Therefore, the combinedmajor and trace element characteristics of the Aldan samplesare not typical of other lamproite occurrences. Large ion lithophileelement concentrations are high (100–800 x Primitive Mantle)but the high field strength elements (HFSE; Nb, Ta, Ti) plusTh and U display unusually low concentrations for rocks of thistype. The style of trace element enrichment recorded by theAldan Shield lamproites is comparable with that of subduction-relatedmagmatism. The Aldan lamproites have among the most extremeinitial isotopic ratios yet recorded from mantle-derived magmas;_Ndi = –10·3 to –22·3, ⁸⁷Sr/⁸⁶Sr_i =0·7055–0·7079, _Hfi = –7·6 to–29·4 and ²⁰⁶Pb/²⁰⁴Pb_i = 16·6–17·4.When interpreted in terms of multi-stage Pb isotope evolution,the Pb isotope data require fractionation from a Bulk Earthreservoir at 3·0 Ga and subsequent evolution with second-stageµ values between 6·4 and 8·0. The inferredArchaean age of the lamproite source is consistent with Nd andHf model ages, which range from 1·5 to 3·0 Ga.Aldan lamproites have _Hf values that range from +3 to –7.Trace element and Sr–Nd–Pb–Hf isotopic ratiosshow coherent variations that suggest that Archaean source enrichmentproduced the negative _Hf as a result of metasomatism by slab-derivedhydrous melts that left rutile–garnet-bearing residua.We conclude that relatively large degrees of partial meltingproduced the lamproites (>5%), which explains the preservationof the isotope–trace element correlations and the lowREE contents. Although high-quality trace element data (e.g.HFSE) are not available for most lamproites, it appears thatmany of their source regions contain a component of recycledoceanic crust, possibly including subducted sediment. The sourcesof the Aldan and many other lamproites are distinct from oceanisland basalt mantle sources. This suggests that the long-termstorage of trace element enriched lamproite sources occurredin the sub-continental lithospheric mantle and not at depthwithin the convecting asthenosphere. KEY WORDS: potassic volcanism; isotope geochemistry; fluid enrichment     

Fluid--Rock Interaction during Low-Pressure Polymetamorphism of the Reynolds Range Group, Central Australia

Marbles and metapelites from the Reynolds Range Group (centralAustralia) were regionally metamorphosed at low pressure duringM₂ at 1.6 Ga, M₂ ranged in grade from greenschist to granulitefacies along the length of the Reynolds Range, and overprinted1.78 Ga granites and their contact aureoles in the ReynoldsRange Group metasediments. At all M₂ grades the marbles andmetapelites have highly variable oxygen isotope ratios [marbles:¹⁸O(carb) 14–20%; metapelites: ¹⁸O 6–14%). Similarly, 1.78 Ga granites have highly variable oxygen isotope ratios(¹⁸O 5–13%), with the lowest values occurring at thegranite margins. In all rock types, the lowest oxygen isotopevalues are consistent with the infiltration of channelled magmaticand/or meteoric fluids. The variable lowering of oxygen isotopevalues resulted from pre-M₂ contact metamorphism and fluid—rockinteraction around the 1.78 Ga granites. In contrast, mineralassemblages in the marbles define a trend of increasing X_CO2with increasing grade from <0.05 (greenschist facies) to0.7–1.0 (granulite facies). This, together with the lackof regionally systematic resetting of oxygen isotope ratios,implies that there was little fluid—rock interaction duringprograde regional metamorphism. KEY WORDS: low pressure; polymetamorphism; fluids; stable isotopes; petrology ^*Corresponding author Fax: 61–3–94791272. e-mail: geoisb{at}lure.latrobe.edu.au     

Volcan Popocatepetl, Mexico. Petrology, Magma Mixing, and Immediate Sources of Volatiles for the 1994-Present Eruption

Volcán Popocatépetl has been the site of voluminousdegassing accompanied by minor eruptive activity from late 1994until the time of writing (August 2002). This contribution presentspetrological investigations of magma erupted in 1997 and 1998,including major-element and volatile (S, Cl, F, and H₂O) datafrom glass inclusions and matrix glasses. Magma erupted fromPopocatépetl is a mixture of dacite (65 wt % SiO₂, two-pyroxenes+ plagioclase + Fe–Ti oxides + apatite, 3 wt % H₂O, P= 1·5 kbar, f_O2 = NNO + 0·5 log units) and basalticandesite (53 wt % SiO₂, olivine + two-pyroxenes, 3 wt % H₂O,P = 1–4 kbar). Magma mixed at 4–6 km depth in proportionsbetween 45:55 and 85:15 wt % silicic:mafic magma. The pre-eruptivevolatile content of the basaltic andesite is 1980 ppm S, 1060ppm Cl, 950 ppm F, and 3·3 wt % H₂O. The pre-eruptivevolatile content of the dacite is 130 ± 50 ppm S, 880± 70 ppm Cl, 570 ± 100 ppm F, and 2·9 ±0·2 wt % H₂O. Degassing from 0·031 km³ of eruptedmagma accounts for only 0·7 wt % of the observed SO₂emission. Circulation of magma in the volcanic conduit in thepresence of a modest bubble phase is a possible mechanism toexplain the high rates of degassing and limited magma productionat Popocatépetl. KEY WORDS: glass inclusions; igneous petrology; Mexico; Popocatépetl; volatiles     

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     

Comparison of Thermochronometers in a Slowly Cooled Granulite Terrain: Nagssugtoqidian Orogen, West Greenland

Uranium–Pb sphene and apatite, and ⁴⁰Ar/³⁹Ar hornblende,muscovite and K-feldspar ages from the core of the ProterozoicNagssugtoqidian orogen, West Greenland, are used to constrainthe timing of granulite-facies metamorphism and the subsequentcooling history. Metamorphic monazite growth occurred at 1858± 2, 1830 ± 1 and 1807 ± 2 Ma and definesthe peak of metamorphism. The uncertainty in the cooling rateshas to include the error in the decay constants of the systemsused. This source of uncertainty is, however, negligible ifa single decay scheme is used or when the age difference betweenthe chronometers is large (>100 m.y.). Over the last twodecades increasingly higher closure temperatures have been proposed.This trend reflects the difficulty of determining ‘absolute’closure temperatures and in using a limited number of closuretemperature estimates to infer closure temperatures of othergeochronometers. Cooling rates at Ussuit were 2·9 ±1·7°C/m.y. from 1762 Ma (670°C) to 1705 Ma (500°C),1·5 ± 1·1°C/m.y. from 1705 Ma to 1640Ma (410°C), and 0·9 ± 0·4°C/m.y.between 1640 and 1416 Ma (200°C). Between 1720 and 1645Ma cooling rates in Lersletten, 60 km north of Ussuit, are indistinguishablefrom those at Ussuit. After 1645 Ma, however, the area cooledto 200°C at a slightly faster rate of 2·6 ±1·2°C/m.y. KEY WORDS: ⁴⁰Ar/³⁹Ar and U–Pb geochronometers; granulite metamorphism; slow cooling; T–t path     

Crystallization of Chromite and Chromium Solubility in Basaltic Melts

The equilibrium between chromite and melt has been determinedon four basalts at temperatures of 1200–1400?C over arange of oxygen fugacity (f_o2) and pressures of 1 atm and 10kb. The Cr content of chromite-saturated melts at 1300?C and1 atm ranges from 0?05 wt.% Cr₂O₃ at a log f_o2= –3 to1?4 wt.% at a log f_o2=–12?8. The Cr²⁺/Cr³⁺ of melt increaseswith decreasing f_o2 and is estimated by assuming a constantpartitioning of Cr³⁺ between chromite and melt at constant temperature.The estimated values of Cr²⁺/Cr³⁺ in the melt are at f_o2 valuesof 4–5 orders of magnitude lower than the equivalent Fe²⁺/Fe³⁺values. The Cr/(Cr+Al) of chromite coexisting with melt at constanttemperature changes little with variation of f_o2 below log f_o2=–6.Five experiments at 10 kb indicate that Cr₂O₃ dissolved in themelt is slightly higher and the Cr/(Cr + Al) of coexisting chromiteis slightly lower than experiments at 1 atm pressure. Thus variationin total pressure cannot explain the large variations of Cr/(Cr+ Al) that are common to mid-ocean ridge basalt (MORB) chromite. Experiments on a MORB at 1 atm at f_o2 values close to fayalite-magnetite-quartz(FMQ) buffer showed that the Al₂O₃ content of melt is highlysensitive to the crystallization or melting of plagioclase,and consequently coexisting chromite shows a large change inCr/(Cr + Al). It would appear, therefore, that mixing of a MORBmagma containing plagioclase with a hotter MORB magma undersaturatedin plagioclase may give rise to the large range of Cr/(Cr +Al) observed in some MORB chromite.