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     

Early-Middle Jurassic Dolerite Dykes from Western Dronning Maud Land (Antarctica): Identifying Mantle Sources in the Karoo Large Igneous Province

A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New ⁴⁰Ar/³⁹Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO₂ and Zr contents, and reinforced by rare earth element (REE),⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO₂ and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO₂ and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, ⁸⁷Sr/⁸⁶Sr_i 0·7035, Nd_i 9). However, theyhave very high TiO₂ (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high Mg–Fe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO₂–Zr contents and are the most enriched magmagroup of the Karoo–Antarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENE–WSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNE–SSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke     

Genesis of Ultramafic Lamprophyres and Carbonatites at Aillik Bay, Labrador: a Consequence of Incipient Lithospheric Thinning beneath the North Atlantic Craton

Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite,damtjernite) and subordinate dolomite-bearing carbonatite withU–Pb perovskite emplacement ages of 590–555 Ma occurin the vicinity of Aillik Bay, coastal Labrador. The ultramaficlamprophyres principally consist of olivine and phlogopite phenocrystsin a carbonate- or clinopyroxene-dominated groundmass. Ti-richprimary garnet (kimzeyite and Ti-andradite) typically occursat the aillikite type locality and is considered diagnosticfor ultramafic lamprophyre–carbonatite suites. Titanianaluminous phlogopite and clinopyroxene, as well as comparativelyAl-enriched but Cr–Mg-poor spinel (Cr-number < 0.85),are compositionally distinct from analogous minerals in kimberlites,orangeites and olivine lamproites, indicating different magmageneses. The Aillik Bay ultramafic lamprophyres and carbonatiteshave variable but overlapping ⁸⁷Sr/⁸⁶Sr_i ratios (0·70369–0·70662)and show a narrow range in initial _Nd (+0·1 to +1·9)implying that they are related to a common type of parentalmagma with variable isotopic characteristics. Aillikite is closestto this primary magma composition in terms of MgO (15–20wt %) and Ni (200–574 ppm) content; the abundant groundmasscarbonate has ¹³C_PDB between –5·7 and –5,similar to primary mantle-derived carbonates, and ¹⁸O_SMOW from9·4 to 11·6. Extensive melting of a garnet peridotitesource region containing carbonate- and phlogopite-rich veinsat 4–7 GPa triggered by enhanced lithospheric extensioncan account for the volatile-bearing, potassic, incompatibleelement enriched and MgO-rich nature of the proto-aillikitemagma. It is argued that low-degree potassic silicate to carbonatiticmelts from upwelling asthenosphere infiltrated the cold baseof the stretched lithosphere and solidified as veins, therebycrystallizing calcite and phlogopite that were not in equilibriumwith peridotite. Continued Late Neoproterozoic lithosphericthinning, with progressive upwelling of the asthenosphere beneatha developing rift branch in this part of the North Atlanticcraton, caused further veining and successive remelting of veinsplus volatile-fluxed melting of the host fertile garnet peridotite,giving rise to long-lasting hybrid ultramafic lamprophyre magmaproduction in conjunction with the break-up of the Rodinia supercontinent.Proto-aillikite magma reached the surface only after coatingthe uppermost mantle conduits with glimmeritic material, whichcaused minor alkali loss. At intrusion level, carbonate separationfrom this aillikite magma resulted in fractionated dolomite-bearingcarbonatites (¹³C_PDB –3·7 to –2·7)and carbonate-poor mela-aillikite residues. Damtjernites maybe explained by liquid exsolution from alkali-rich proto-aillikitemagma batches that moved through previously reaction-lined conduitsat uppermost mantle depths. KEY WORDS: liquid immiscibility; mantle-derived magmas; metasomatism, Sr–Nd isotopes; U–Pb geochronology     

Experimental Constraints on the Role of Garnet Pyroxenite in the Genesis of High-Fe Mantle Plume Derived Melts

The anhydrous phase relations of an uncontaminated (primitive),ferropicrite lava from the base of the Early Cretaceous Paraná–Etendekacontinental flood basalt province have been determined between1 atm and 7 GPa. The sample has high contents of MgO (14·9wt %), FeO^* (14·9 wt %) and Ni (660 ppm). Olivine phenocrystshave maximum Fo contents of 85 and are in equilibrium with thebulk rock, assuming a of 0·32. A comparison of our results with previous experimental studiesof high-Mg rocks shows that the high FeO content of the ferropicritecauses an expansion of the liquidus crystallization field ofgarnet and clinopyroxene relative to olivine; orthopyroxenewas not observed in any of our experiments. The high FeO contentalso decreases solidus temperatures. Phase relations indicatethat the ferropicrite melt last equilibrated either at 2·2GPa with an olivine–clinopyroxene residue, or at 5 GPawith a garnet–clinopyroxene residue. The low bulk-rockAl₂O₃ content (9 wt %) and high [Gd/Yb]_n ratio (3·1)are consistent with the presence of residual garnet in the ferropicritemelt source and favour high-pressure melting of a garnet pyroxenitesource. The garnet pyroxenite may represent subducted oceaniclithosphere entrained by the upwelling Tristan starting mantleplume head. During adiabatic decompression, intersection ofthe garnet pyroxenite solidus at 5 GPa would occur at a mantlepotential temperature of 1550°C and yield a ferropicriteprimary magma. Subsequent melting of the surrounding peridotiteat 4·5 GPa may be restricted by the thickness of theoverlying sub-continental lithosphere, such that dilution ofthe garnet pyroxenite melt component would be significantlyless than in intra-oceanic plate settings (where the lithosphereis thinner). This model may explain the limited occurrence offerropicrites at the base of continental flood basalt sequencesand their apparent absence in ocean-island basalt successions. KEY WORDS: continental flood basalt; ferropicrite; mantle heterogeneity; mantle melting; phase relations; pyroxenite     

Liquidus Equilibria in the System K2O-Na2O-Al2O3-SiO2-F2O-1-H2O to 100 MPa: I. Silicate-Fluoride Liquid Immiscibility in Anhydrous Systems

Liquidus relations in the four-component system Na₂O–Al₂O₃–SiO₂–F₂O_–1were studied at 0· 1 and 100 MPa to define the locationof fluoride–silicate liquid immiscibility and outlinedifferentiation paths of fluorine-bearing silicic magmas. Thefluoride–silicate liquid immiscibility spans the silica–albite–cryoliteand silica–topaz–cryolite ternaries and the haplogranite-cryolitebinary at greater than 960°C and 0· 1–100 MPa.With increasing Al₂O₃ in the system and increasing aluminum/alkalication ratio, the two-liquid gap contracts and migrates fromthe silica liquidus to the cryolite liquidus. The gap does notextend to subaluminous and peraluminous melt compositions. Forall alkali feldspar–quartz-bearing systems, the miscibilitygap remains located on the cryolite liquidus and is thus inaccessibleto differentiating granitic and rhyolitic melts. In peralkalinesystems, the magmatic differentiation is terminated at the albite–quartz–cryoliteeutectic at 770°C, 100 MPa, 5 wt % F and cation Al/Na =0· 75. The addition of topaz, however, significantlylowers melting temperatures and allows strong fluorine enrichmentin subaluminous compositions. At 100 MPa, the binary topaz–cryoliteeutectic is located at 770°C, 39 wt % F, cation Al/Na 0·95, and the ternary quartz–topaz–cryolite eutecticis found at 740°C, 32 wt % F, 30 wt % SiO₂ and cation Al/Na 0· 95. Such location of both eutectics enables fractionationpaths of subaluminous quartz-saturated systems to produce fluorine-rich,SiO₂-depleted and nepheline-normative residual liquids. KEY WORDS: silicate melt; granite; rhyolite; fluorine; liquid immiscibility     

Geochemical and Isotopic Heterogeneities along an Island Arc-Spreading Ridge Intersection: Evidence from the Lewis Hills, Bay of Islands Ophiolite, Newfoundland

This study focuses on the origin of magma heterogeneity andthe genesis of refractory, boninite-type magmas along an arc–ridgeintersection, exposed in the Lewis Hills (Bay of Islands Ophiolite).The Lewis Hills contain the fossil fracture zone contact betweena split island arc and its related marginal oceanic basin. Threetypes of intrusions, which are closely related to this narrowtectonic boundary, have been investigated. Parental melts inequilibrium with the ultramafic cumulates of the PyroxeniteSuite are inferred to have high MgO contents and low Al₂O₃,Na₂O and TiO₂ contents. The trace element signatures of thesePyroxenite Suite parental melts indicate a re-enriched, highlydepleted source with 0·1 x mid-ocean ridge basalt (MORB)abundances of the heavy rare earth elements (HREE). Initial_Nd values of the Pyroxenite Suite range from -1·5 to+0·6, which overlap those observed for the island arc.Furthermore, the Pyroxenite Suite parental melts bear strongsimilarities to boninite-type equilibrium melts from islandarc-related pyroxenitic dykes and harzburgites. Basaltic dykessplit into two groups. Group I dykes have 0·6 x MORBabundances of the HREE, and initial _Nd values ranging from +5·4to +7·5. Thus, they have a strong geochemical affinitywith basalts derived from the marginal basin spreading ridge.Group II dykes have comparatively lower trace element abundances(0·3 x MORB abundances of HREE), and slightly lower initial_Nd values (+5·4 to +5·9). The geochemical characteristicsof the Group II dykes are transitional between those of GroupI dykes and the Pyroxenite Suite parental melts. Cumulates fromthe Late Intrusion Suite are similarly transitional, with _Ndvalues ranging from +2·9 to +4·6. We suggest thatthe magma heterogeneity observed in the Lewis Hills is due tothe involvement of two compositionally distinct mantle sources,which are the sub-island lithospheric mantle and the asthenosphericmarginal basin mantle. It is likely that the refractory, boninite-typeparental melts of the Pyroxenite Suite result from remeltingof the sub-arc lithospheric mantle at an arc–ridge intersection.Furthermore, it is suggested that the thermal-dynamic conditionsof the transtensional transform fault have provided the prerequisitefor generating magma heterogeneity, as a result of mixing relationshipsbetween arc-related and marginal basin-related magmas. KEY WORDS: Bay of Islands ophiolite; transform (arc)–ridge intersection; boninites; rare earth elements, Nd isotopes     

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     

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     

Thermal History of the Horoman Peridotite Complex: A Record of Thermal Perturbation in the Lithospheric Mantle

The ascent history of the Horoman peridotite complex, Hokkaido,northern Japan, is revised on the basis of a detailed studyof large ortho- and clinopyroxene grains 1 cm in size (megacrysts)in the Upper Zone of the complex. The orthopyroxene megacrystsexhibit distinctive M-shaped Al zoning patterns, which werenot observed in porphyroclastic grains less than 5 mm in sizedescribed in previous studies. Moreover, the Al and Ca contentsof the cores of the orthopyroxene megacrysts are lower thanthose of the porphyroclasts. The Upper Zone is inferred to haveresided not only at a higher temperature than previously suggestedbut also at a higher pressure (1070°C, 2·3 GPa) thanthe Lower Zone (950°C, 1·9 GPa), in the garnet stabilityfield, before the ascent of the two zones. The Horoman complexprobably represents a 12 ± 5 km thick section of lithosphericmantle with an 10 ± 8°C/km vertical thermal gradient.The current thickness of the Horoman complex is 3 km, whichis a result of shortening of the lithospheric mantle by 0·25± 0·1 during its ascent. The Upper Zone appearsto have experienced a heating event during its ascent throughthe spinel stability field, with a peak temperature as highas 1200°C. The effect of heating decreases continuouslytowards the base of the complex, and the lowermost part of theLower Zone underwent very minor heating at a pressure higherthan 0·5 GPa. The uplift and associated deformation,as well as heating, was probably driven by the ascent of a hotasthenospheric upper-mantle diapir into the Horoman lithosphere. KEY WORDS: Horoman; P–T trajectory; thermal history; Al diffusion in pyroxene; geothermobarometry     

Source of the Quaternary Alkalic Basalts, Picrites and Basanites of the Potrillo Volcanic Field, New Mexico, USA: Lithosphere or Convecting Mantle?

The <80 ka basalts–basanites of the Potrillo VolcanicField (PVF) form scattered scoria cones, lava flows and maarsadjacent to the New Mexico–Mexico border. MgO ranges upto 12·5%; lavas with MgO < 10·7% have fractionatedboth olivine and clinopyroxene. Cumulate fragments are commonin the lavas, as are subhedral megacrysts of aluminous clinopyroxene(with pleonaste inclusions) and kaersutitic amphibole. REE modellingindicates that these megacrysts could be in equilibrium withthe PVF melts at 1·6–1·7 GPa pressure. Thelavas fall into two geochemical groups: the Main Series (85%of lavas) have major- and trace-element abundances and ratiosclosely resembling those of worldwide ocean-island alkali basaltsand basanites (OIB); the Low-K Series (15%) differ principallyby having relatively low K₂O and Rb contents. Otherwise, theyare chemically indistinguishable from the Main Series lavas.Sr- and Nd-isotopic ratios in the two series are identical andvary by scarcely more than analytical error, averaging ⁸⁷Sr/⁸⁶Sr= 0·70308 (SD = 0·00004) and ¹⁴³Nd/¹⁴⁴Nd = 0·512952(SD=0·000025). Such compositions would be expected ifboth series originated from the same mantle source, with Low-Kmelts generated when amphibole remained in the residuum. ThreePVF lavas have very low Os contents (<14 ppt) and appearto have become contaminated by crustal Os. One Main Series picritehas 209 ppt Os and has a _Os value of +13·6, typical forOIB. This contrasts with published ¹⁸⁷Os/¹⁸⁸Os ratios for KilbourneHole peridotite mantle xenoliths, which give mostly negative_Os values and show that Proterozoic lithospheric mantle formsa thick Mechanical Boundary Layer (MBL) that extends to 70 kmdepth beneath the PVF area. The calculated mean primary magma,in equilibrium with Fo₈₉, has Na₂O and FeO contents that givea lherzolite decompression melting trajectory from 2·8GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modellingof REE abundances in Main Series Mg-rich lavas is successfulfor a model invoking decompression melting of convecting sub-lithosphericlherzolite mantle (Nd = 6·4; T_p 1400°C) between90 and 70 km. Nevertheless, such a one-stage model cannot accountfor the genesis of the Low-K Series because amphibole wouldnot be stable within convecting mantle at T_f 1400°C. Thesemagmas can only be accommodated by a three-stage model thatenvisages a Thermal Boundary Layer (TBL) freezing conductivelyonto the 70 km base of the Proterozoic MBL during the 20 Myrtectonomagmatic quiescence before PVF eruptions. As it grew,this was veined by hydrous small-fraction melts from below.The geologically recent arrival of hotter-than-ambient (T_p 1400°C) convecting mantle beneath the Potrillo area re-meltedthe TBL and caused the magmatism. KEY WORDS: western USA; picrites; Sr–Nd–Os isotopes; petrogenetic modelling; thermal boundary layer     

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     

Mineralogy and Petrology of the Aries Diamondiferous Kimberlite Pipe, Central Kimberley Block, Western Australia

Aries is a deeply weathered micaceous kimberlite pipe (820 Ma)consisting of four lobes: South, Central, North, and North Extension.It is the largest ( 18 ha) and most diamondiferous of the fewkimberlites currently known on the Australian continent, andis rich in country-rock (dolerite and quartzite) xenoliths.Three textural varieties of Aries kimberlites can be recognized,together with autoclastic breccias: (1) macrocrystalmedium-grained;(2) aphanitic (5 vol. % olivine macrocrysts); and (3) macrocrystalsegregated. The kimberlites contain two generations of olivinepseudomorphs (30–40 vol. %), and two of phlogopite (upto 60 vol. %), in a groundmass of apatite, calcite, diopside,sphene, spinels, serpentine, talc, and accessory groundmassminerals including aeschynite [(Ce, Ca) (Ti, Nb)₂O₆], barite,ilmenite, monazite, rutile, siderite, and unidentified Nb-Fe-titanates.Phlogopite zoning is complex and differs from lobe to lobe,but general compositions and trends resemble phlogopites fromkimberlites (TiO₂ 0–5–4 wt. %, A1₂O₃ 9–16%);tetraferriphlogopite substitution is indicated by low Al insome grains. Diopside is low in Cr, Al, Na, and Ti, with highmg-number [molecular Mg/(Mg + Fe²⁺) 93]. Apatite contains upto 17–5% SrO, calcite up to 1–7% SrO but littleMgO or FeO, sphene up to 1.5% Nb₂O₅, and ilmenite 2.6% Nb2O5and 16% MnO but no detectable MgO. Extremely complex moqftiological, textural, and compositionalvariations are present in spinels. They can be divided intofive textural-genetic types: cognate Groundmass chromian spinels(Type G); Inclusions of chromian spinels in olivine macrocrysts(Type I), probably representing either early phenocrysts ormantle xenocrysts: Macrocryst chromian spinels (Type M), probablyrepresenting xenocrysts; late-stage groundmass Fenian spinels(Type F), derived from serpentinization of olivine; Alterationferrian spinels (Type A), found as inclusions associated withsiliceous melt inclusions, in Types I and M, and probably representinginteraction of these earlier types with late-stage melts. Someof these, particularly Types M and F, show further texturalsub-types with no obvious genetic significance. The pipe formed from several magma-pulses. All four lobes maycontain at least one pulse in common, but Central and SouthLobes include additional pulse(s) which yielded distinctivephlogopite zoning, whereas North Lobe and North Extension includepulsc(s) which may have originated at higher mantle levels andyielded more evolved phlogopites. Aries most resembles South African Group II kimberlites mineralogically,certain West African micaceous kimberlites geochemically, andGroup I kimberlites isotopically. A distinctive mantle source-regionis implied by high Nb/U, Ce/Sr, Ce/P, Rb/Ba, and especiallyNb/Zr ratios. Similar anomalous geochemical signatures are sharedwith two other contemporaneous (800 Ma) lampro-phyric intrusionsin the east Kimberley (at Maude Creek and Bow Hill), suggestingthat a scattered alkaline province exists in the Kimberley Block,generated from a regionally anomalous mantle source.     

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     

Exhumation History of a Garnet Pyroxenite-bearing Mantle Section from a Continent-Ocean Transition (Northern Apennine Ophiolites, Italy)

Garnet clinopyroxenite and garnet websterite layers occur locallywithin mantle peridotite bodies from the External Liguride Jurassicophiolites (Northern Apennines, Italy). These ophiolites werederived from an ocean–continent transition similar tothe present-day western Iberian margin. The garnet clinopyroxenitesare mafic rocks with a primary mineral assemblage of pyrope-richgarnet + sodic Al-augite (Na₂O 2·5 wt %, Al₂O₃ 12·5wt %), with accessory graphite, Fe–Ni sulphides and rutile.Decompression caused Na-rich plagioclase (An_50–45) exsolutionin clinopyroxene porphyroclasts and extensive development ofsymplectites composed of secondary orthopyroxene + plagioclase(An_85–72) + Al-spinel ± clinopyroxene ±ilmenite at the interface between garnet and primary clinopyroxene.Further decompression is recorded by the development of an olivine+ plagioclase-bearing assemblage, locally under syn-kinematicconditions, at the expense of two-pyroxenes + Al-spinel. Mg-richgarnet has been also found in the websterite layers, which arecommonly characterized by the occurrence of symplectites madeof orthopyroxene + Al-spinel ± clinopyroxene. The enclosingperidotites are Ti-amphibole-bearing lherzolites with a fertilegeochemical signature and a widespread plagioclase-facies myloniticfoliation, which preserve in places a spinel tectonite fabric.Lu–Hf and Sm–Nd mineral isochrons (220 ±13 Ma and 186.0 ± 1·8 Ma, respectively) have beenobtained from a garnet clinopyroxenite layer and interpretedas cooling ages. Geothermobarometric estimates for the high-pressureequilibration have yielded T 1100°C and P 2·8 GPa.The early decompression was associated with moderate cooling,corresponding to T 950°, and development of a spinel tectonitefabric in the lherzolites. Further decompression associatedwith plagioclase–olivine growth in both peridotites andpyroxenites was nearly isothermal. The shallow evolution occurredunder a brittle regime and led to the superposition of hornblendeto serpentine veining stages. The garnet pyroxenite-bearingmantle from the External Liguride ophiolites represents a raretectonic sampling of deep levels of subcontinental lithosphereexhumed in an oceanic setting. The exhumation was probably accomplishedthrough a two-step process that started during Late Palaeozoiccontinental extension. The low-pressure portion of the exhumationpath, probably including also the plagioclase mylonitic shearzones, was related to the Mesozoic (Triassic to Jurassic) riftingthat led to continental break-up. In Jurassic times, the studiedmantle sequence became involved in an extensional detachmentprocess that resulted in sea-floor denudation. KEY WORDS: garnet pyroxenite; ophiolite; non-volcanic margin; mantle exhumation; Sm–Nd and Lu–Hf geochronology     

The 1982-83 Eruption at Galunggung Volcano, Java (Indonesia): Oxygen Isotope Geochemistry of a Chemically Zoned Magma Chamber

Mount Galunggung is a historically active volcano in southwesternJava that has erupted four times in the last two centuries.During the most recent event, which occurred during a 9–monthinterval in 1982– 83, some 305 10⁶ m³ of medium–K,calc–alkaline magma was erupted. This eruption was unusualbecause of its duration, the diversity of eruption dynamicsand products, and the range of lava compositions produced. Thecomposition of juvenile material changed gradually during thecourse of the eruption from initial plagioclase (An_60–75)and two–pyrozene bearing andesites with 58% SiO₂ to finalplagioclase (An_85–90), diopside, and olivine (Fo_85–90)bearing primitive magnesium basalts with 47% SiO₂ Mineralogicaland compositional relationships indicate a magmatic evolutioninvolving differentitation of high–Mg parental melt. Theeruptive volumes of 35 10⁶ m³ andesite, 120 10⁶ m³ maficandesite, and 150 10⁶ m³ basalt are consistent with the ideathat the 1982– 83 eruption progressively tapped and draineda magma chamber that had become chemically stratified throughextensive crystal fractionation. Separates of plagioclase and pyroxene have ¹⁸O( SMO W) rangesof + 5. 6 to + 6.0 and + 5.3 to + 5.6, respectively, with ¹⁸O_plag–pxvalues of + 0.4 to + 0.6o, indicating internal O–isotopeequiliburium at temperature of 1100–850 C. The magenesianbasalts have magmatic ¹⁸O/ ¹⁶O ratios similar to those of mid–oceanridge basalt, and the O–isotope ratios of compositionallyevolved derivative melts show no evidence for contaminationof the galunggung magmas by ¹⁸O–rich crust during differentiation.Andesites and transitional mafic and sites have a more variableO–isotope character, with laves and phenocrysts havingboth higher and lower ¹⁸O values than observed in the parentalmagnesium basalts. These features are interpreted to reflectintramagma chamber processes affecting the upper portions ofthe differentiating Galunggung magma body before the 1982–83eruption.     

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     

Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai'i

A diverse range of crustal xenoliths is hosted in young alkalibasalt lavas and scoria deposits (erupted 3–5 ka) at thesummit of Huallai. Leucocratic xenoliths, including monzodiorites,diorites and syenogabbros, are distinctive among Hawaiian plutonicrocks in having alkali feldspar, apatite, zircon and biotite,and evolved mineral compositions (e.g. albitic feldspar, clinopyroxeneMg-number 67–78). Fine-grained diorites and monzodioritesare plutonic equivalents of mugearite lavas, which are unknownat Huallai. These xenoliths appear to represent melt compositionsfalling along a liquid line of descent leading to trachyte—amagma type which erupted from Huallai as a prodigious lava flowand scoria cone at 114 ka. Inferred fractionating assemblages,MELTS modeling, pyroxene geobarometry and whole-rock norms allpoint to formation of the parent rocks of the leucocratic xenolithsat 3–7 kbar pressure. This depth constraint on xenolithformation, coupled with a demonstrated affinity to hypersthene-normativebasalt and petrologic links between the xenoliths and the trachyte,suggests that the shift from shield to post-shield magmatismat Huallai was accompanied by significant deepening of the activemagma reservoir and a gradual transition from tholeiitic toalkalic magmas. Subsequent differentiation of transitional basaltsby fractional crystallization was apparently both extreme—culminatingin >5·5 km³ of trachyte—and rapid, at 2·75x 10⁶ m³ magma crystallized/year. KEY WORDS: geothermobarometry; magma chamber; xenolith; cumulate; intensive parameters     

A Melt Inclusion Record of Volatiles, Trace Elements and Li-B Isotope Variations in a Single Magma System from the Plat Pays Volcanic Complex, Dominica, Lesser Antilles

Glass inclusions in plagioclase and orthopyroxene from daciticpumice of the Cabrits Dome, Plat Pays Volcanic Complex in southernDominica reveal a complexity of element behavior and Li–Bisotope variations in a single volcanic center that would gounnoticed in a whole-rock study. Inclusions and matrix glassesare high-silica rhyolite with compositions consistent with about50% fractional crystallization of the observed phenocrysts.Estimated crystallization conditions are 760–880°C,200 MPa and oxygen fugacity of FMQ + 1 to +2 log units (whereFMQ is the fayalite–magnetite–quartz buffer). Manyinclusion glasses are volatile-rich (up to 6 wt % H₂O and 2900ppm Cl), but contents range down to 1 wt % H₂O and 2000 ppmCl as a result of shallow-level degassing. Sulfur contents arelow throughout, with <350 ppm S. The trace element compositionof inclusion glasses shows enrichment in light rare earth elements(LREE; (La/Sm)_n = 2·5–6·6) and elevatedBa, Th and K contents compared with whole rocks and similaror lower Nb and heavy REE (HREE; (Gd/Yb)_n = 0·5–1·0).Lithium and boron concentrations and isotope ratios in meltinclusions are highly variable (20–60 ppm Li with ⁷Li= +4 to +15 ± 2; 60–100 ppm B with ¹¹B = +6 to+13 ± 2) and imply trapping of isotopically heterogeneous,hybrid melts. Multiple sources and processes are required toexplain these features. The mid-ocean ridge basalt (MORB)-likeHREE, Nb and Y signature reflects the parental magma(s) derivedfrom the mantle wedge. Positive Ba/Nb, B/Nb and Th/Nb correlationsin inclusion glasses indicate coupled enrichment in stronglyfluid-mobile (Ba, B) and less-mobile (Th, Nb) trace elements,which can be explained by fractional crystallization of plagioclase,orthopyroxene and Fe–Ti oxides. The ⁷Li and ¹¹B valuesare at the high end of known ranges for other island arc magmas.We attribute the high values to a ¹¹B and ⁷Li-enriched slabcomponent derived from sea-floor-altered oceanic crust and possiblyfurther enriched in heavy isotopes by dehydration fractionation.The heterogeneity of isotope ratios in the evolved, trappedmelts is attributed to shallow-level assimilation of older volcanicrocks of the Plat Pays Volcanic Complex. KEY WORDS: subduction; volcanic arcs; igneous processes; melt inclusions; SIMS; trace elements; lithium and boron isotopes; diffusion     

Correlations between U, Th Content and Metamorphic Grade in the Western Namaqualand Belt, South Africa, with Implications for Radioactive Heating of the Crust

The digital image of airborne radiometric data across SouthAfrica reveals that the largest anomaly, 100 nGy/h, is causedby the granulite-facies rocks of the Namaquan metamorphic complex,whereas most of the country is <60 nGy/h. This observationis consistent with geochemical data that show that the 1900± 100 Ma greenschist-facies Richtersveld Terrane nearNamibia (max. U = 3·4 ppm; Th = 20·1 ppm) andthe adjacent, 1100 ± 100 Ma, amphibolite-facies Aggeneys/SteinkopfTerranes (max. U 10 ppm; Th 52 ppm) are the least enrichedin U, Th and K. In contrast, the lower-T granulite-facies OkiepTerrane near Springbok hosts more enriched granites (max. U 17 ppm; Th 66 ppm) and noritic intrusions (max. U = 14 ppm;Th = 83 ppm). The most enriched rocks are found in the 1030Ma higher-T granulite-facies core of the Namaquan belt and includequartzo-feldspathic gneisses (max. U = 46 ppm; Th = 90 ppm)and charnockites (max. U = 52 ppm; Th = 400 ppm). Our findingscontradict the notion that granulite-facies terrains are characteristicallydepleted in U and Th. In this study we modeled the heat productionin the core of the Namaquan complex, where the granulites havehad a very unusual metamorphic history, and show that ultra-high-T(1000°C, P 10 kbar) metamorphic conditions could have beenachieved by radiogenic heating without invoking external heatsources. However, monazite-rich veins of charnockite and patchesof granulites mark the passage of CO₂-dominated melts and fluidsderived from fractionated noritic intrusions. KEY WORDS: charnockite; granulite; Namaqualand; thorium; uranium; radioactive heating; metamorphism     

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