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 Cretaceous Igneous Province of Madagascar: Geochemistry and Petrogenesis of Lavas and Dykes from the Central-Western Sector

The Cretaceous lava sequence and associated mafic dyke swarmin central–western Madagascar (Mailaka and Bemaraha areas)range in composition from picrite basalts to cordierite–orthopyroxene-bearingrhyodacites (MgO from 14 to 0·6 wt %). Petrographic andchemical data indicate the presence of both tholeiitic and transitionalmagma series, with variable degree of rare earth element enrichment[(La/Nd)_n = 1–1·4 for tholeiites vs (La/Nd)_n =0·65–1 for transitional rocks]. Initial (at 88Ma) ⁸⁷Sr/⁸⁶Sr and     

Volcanism in the Vitim Volcanic Field, Siberia: Geochemical Evidence for a Mantle Plume Beneath the Baikal Rift Zone

The Baikal Rift is a zone of active lithospheric extension adjacentto the Siberian Craton. The 6–16 Myr old Vitim VolcanicField (VVF) lies approximately 200 km east of the rift axisand consists of 5000 km³ of melanephelinites, basanites, alkaliand tholeiitic basalts, and minor nephelinites. In the volcanicpile, 142 drill core samples were used to study temporal andspatial variations. Variations in major element abundances (e.g.MgO = 3·3–14·6 wt %) reflect polybaric fractionalcrystallization of olivine, clinopyroxene and plagioclase. ⁸⁷Sr/⁸⁶Sr_i(0·7039–0·7049), ¹⁴³Nd/¹⁴⁴Nd_i (0·5127–0·5129)and ¹⁷⁶Hf/¹⁷⁷Hf_i (0·2829–0·2830) ratiosare similar to those for ocean island basalts and suggest thatthe magmas have not assimilated significant amounts of continentalcrust. Variable degrees of partial melting appear to be responsiblefor differences in Na₂O, P₂O₅, K₂O and incompatible trace elementabundances in the most primitive (high-MgO) magmas. Fractionatedheavy rare earth element (HREE) ratios (e.g. [Gd/Lu]_n > 2·5)indicate that the parental magmas of the Vitim lavas were predominantlygenerated within the garnet stability field. Forward major elementand REE inversion models suggest that the tholeiitic and alkalibasalts were generated by decompression melting of a fertileperidotite source within the convecting mantle beneath Vitim.Ba/Sr ratios and negative K anomalies in normalized multi-elementplots suggest that phlogopite was a residual mantle phase duringthe genesis of the nephelinites and basanites. Relatively highlight REE (LREE) abundances in the silica-undersaturated meltsrequire a metasomatically enriched lithospheric mantle source.Results of forward major element modelling suggest that meltingof phlogopite-bearing pyroxenite veins could explain the majorelement composition of these melts. In support of this, pyroxenitexenoliths have been found in the VVF. High Cenozoic mantle potentialtemperatures (1450°C) predicted from geochemical modellingsuggest the presence of a mantle plume beneath the Baikal RiftZone. KEY WORDS: Baikal Rift; mafic magmatism; mantle plume; metasomatism; partial melting     

Geochemistry and Intrusive History of the Ashland Pluton, Klamath Mountains, California and Oregon

The Ashland pluton is a calc-alkaline plutonic complex thatintruded the western Paleozoic and Triassic belt of the KlamathMountains in late Middle Jurassic time. The pluton comprisesa series of compositionally distinct magma pulses. The oldestrocks are hornblende gabbro and two-pyroxene quartz gabbro withinitial ⁸⁷Sr/⁸⁶Sr = 0{dot}7044, ¹⁸O = 8{dot}7%, and REE patternswith chondrite normalized La/Lu = 7. These units were followedby a suite of tonalitic rocks (La_N/Lu_N = 7) and then by a suiteof K₂O- and P₂O₅ rocks of quartz monzodioritic affinity (La_N/Lu_N= 13–21; La_N/Sm_N = 2{dot}4–3{dot}) The quartz monzodioriticrocks were then intruded by biotite granodiorite and granitewith lower REE abundances but more fractionated LREE(La_N/Lu_N= 13–19; La_N/Sm_N = 4{dot}3–6 and they, in turn,were host to dikes and bosses of hornblende diorite. The latestintrusive activity consisted of aplitic and granitic dikes.Combined phase equilibria and mineral composition data, indicateemplacement conditions of approximately P_total = 2{dot}3kb,P_H2O between 1{dot}5 and 2{dot}2 kb, and f_O2 between the nickel-nickeloxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO₂ togetherwith increasing ¹⁸O and decreasing initial ⁸⁷Sr/⁸⁶Sr. The isotopicdata are consistent with either (1) combined fractional crystallizationof andesitic magma and concurrent assimilation of crustal materialcharacterized by low Sr₁ and high (¹⁸O or, more probably, (2)a series of partial melting events in which sources were successivelyless radiogenic but richer in ¹⁸O Each intrusive stage displaysevidence for some degree of crystal accumulation and/or fractionalcrystallization but neither process adequately accounts fortheir compositional differences. Consequently, each stage appearsto represent a distinct partial melting or assimilation event. The P₂O₅-rich nature of the quartz monzodiorite suite suggestsaccumulation of apatite. However, the suite contains abundantmafic microgranitoid enclaves and most apatite in the suiteis acicular. These observations suggest that magma mixing affectedthe compositional variation of the quartz monzodiorite suite.Mass balance calculations are consistent with a simple mixingprocess in which P₂O₅-rich alkalic basalt magma (representedby the mafic microgranitoid enclaves) was combined with a crystal-poorfelsic magma (represented by the tonalite suite), yielding aquartz monzodioritic magma that then underwent differentiationby crystal fractionation and accumulation.     

The 26{middle dot}5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body

The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km³ of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO₂ and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO₂. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and ⁸⁷Sr/⁸⁶Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO₂)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO₂) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO₂ content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics     

A North-South Transect across the Central Mexican Volcanic Belt at ~100{degrees}W: Spatial Distribution, Petrological, Geochemical, and Isotopic Characteristics of Quaternary Volcanism

Within the Zitácuaro–Valle de Bravo (ZVB) regionof the central Mexican Volcanic Belt (MVB), three lava serieshave erupted during the Quaternary: (1) high-K₂O basaltic andesitesand andesites; (2) medium-K₂O basaltic andesites, andesitesand dacites; (3) high-TiO₂ basalts and basaltic andesites. Thedominant feature of the first two groups is the lack of plagioclaseaccompanying the various ferromagnesian phenocrysts (olivine,orthopyroxene, augite, and hornblende) in all but the dacites.This absence of plagioclase in the phenocryst assemblages ofthe high-K₂O and medium-K₂O intermediate lavas is significantbecause it indicates high water contents during the stage ofphenocryst equilibration. In contrast, the high-TiO₂ group ischaracterized by phenocrysts of plagioclase and olivine. Thespatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280–330 km fromthe Middle America Trench (MAT), is characterized by high-K₂Omelts that are relatively enriched in fluid-mobile elementsand have the highest ⁸⁷Sr/⁸⁶Sr ratios. Medium-K₂O basaltic andesiteand andesite lavas are present throughout the transect, butthose closest to the MAT are MgO-rich (3·5–9·4wt %) and have phenocryst assemblages indicative of high magmaticwater contents (3·5–6·5 wt % water) andrelatively low temperatures (950–1000°C). In markedcontrast, the northern section of the ZVB transect (380–480km from the MAT) has high-TiO₂, high field strength element(HFSE)-enriched magmas that have comparatively dry (< 1·5wt % magmatic water) and hot (1100–1200°C) phenocrystequilibration conditions. The central section of the ZVB transect(330–380 km from the MAT) is a transition zone and producesmoderately light rare earth element (LREE) and large ion lithophileelement (LILE)-enriched, medium-K₂O lavas with phenocryst assemblagesindicative of intermediate (1·5–3·5 wt %)water contents and temperatures. The high-K₂O series compositionsare the most enriched in LILE and LREE, with a narrow rangeof radiogenic ⁸⁷Sr/⁸⁶Sr from 0·704245 to 0·704507,¹⁴³Nd/¹⁴⁴Nd values ranging from 0·512857 to 0·512927(_Nd = 4·27–5·63), and ²⁰⁸Pb/²⁰⁴Pb valuesfrom 38·248 to 38·442, ²⁰⁷Pb/²⁰⁴Pb values from15·563 to 15·585, and ²⁰⁶Pb/²⁰⁴Pb values from18·598 to 18·688. The medium-K₂O series compositionsare only moderately enriched in the LILE and LREE, with a broaderrange of ⁸⁷Sr/⁸⁶Sr, but similar ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁸Pb/²⁰⁴Pbvalues to those of the high-K₂O series. In contrast, the high-TiO₂series compositions have little enrichment in LILE or LREE andinstead are enriched in the HFSE and heavy rare earth elements(HREE). The high-TiO₂ lavas are isotopically distinct in theirlower and narrower range of ¹⁴³Nd/¹⁴⁴Nd. The isotopic variationsare believed to reflect the upper mantle magma source regionsas the low content of phenocrysts in most lavas precludes significantupper crustal assimilation or magma mixing, other than thatrepresented by the presence of quartz xenocrysts (< 2 vol.%) with rhyolitic glass inclusions, which are found in manyof these lavas. The systematic spatial variation in compositionof the three lava series is a reflection of the underlying subduction-modifiedmantle and its evolution. KEY WORDS: central Mexico; geochemistry; isotopes; Quaternary volcanism; hydrous lavas     

Kistufell: Primitive Melt from the Iceland Mantle Plume

This paper presents new geochemical data from Kistufell (64°48'N,17°13'W), a monogenetic table mountain situated directlyabove the inferred locus of the Iceland mantle plume. Kistufellis composed of the most primitive olivine tholeiitic glassesfound in central Iceland (MgO 10·56 wt %, olivine Fo_89·7).The glasses are interpreted as near-primary, high-degree plumemelts derived from a heterogeneous mantle source. Mineral, glassand bulk-rock (glass + minerals) chemistry indicates a low averagemelting pressure (15 kbar), high initial crystallization pressuresand temperatures (10–15 kbar and 1270°C), and eruptiontemperatures (1240°C) that are among the highest observedin Iceland. The glasses have trace element signatures (La_n/Yb_n<1, Ba_n/Zr_n 0·55–0·58) indicative ofa trace element depleted source, and the Sr–Nd–Pbisotopic ratios (⁸⁷Sr/⁸⁶Sr 0·70304–0·70308,¹⁴³Nd/¹⁴⁴Nd 0·513058–0·513099, ²⁰⁶Pb/²⁰⁴Pb18·343–18·361) further suggest a long-termtrace element depletion relative to primordial mantle. HighHe isotopic ratios (15·3–16·8 R/R_a) combinedwith low ²⁰⁷Pb/²⁰⁴Pb (15·42–15·43) suggestthat the mantle source of the magma is different from that ofNorth Atlantic mid-ocean ridge basalt. Negative Pb anomalies,and positive Nb and Ta anomalies indicate that the source includesa recycled, subducted oceanic crustal or mantle component. PositiveSr anomalies (Sr_n/Nd_n = 1·39–1·50) furthersuggest that this recycled source component involves lower oceaniccrustal gabbros. The     

Lamproitic Rocks from a Continental Collision Zone: Evidence for Recycling of Subducted Tethyan Oceanic Sediments in the Mantle Beneath Southern Tibet

Major and trace element, Sr–Nd–Pb isotope and mineralchemical data are presented for newly discovered ultrapotassiclavas in the Tangra Yumco–Xuruco graben in southern Tibet.The ultrapotassic lavas are characterized by high MgO, K₂O andTiO₂, low Al₂O₃ and Na₂O contents, and also have high molarK₂O/Al₂O₃, molar (K₂O + Na₂O)/Al₂O₃ and K₂O/Na₂O ratios. Theirhigh abundances of incompatible trace elements such as largeion lithophile elements (LILE) and light rare earth elements(LREE) reach the extreme levels typical of lamproites. The lamproitesshow highly radiogenic ⁸⁷Sr/⁸⁶Sr (0· 7166–0·7363) and unradiogenic ¹⁴³Nd/¹⁴⁴Nd (0· 511796–0·511962), low ²⁰⁶Pb/²⁰⁴Pb (18· 459–18· 931),and elevated radiogenic ²⁰⁷Pb/²⁰⁴Pb (15· 6732–15·841) and ²⁰⁸Pb/²⁰⁴Pb (39· 557–40· 058) ratios.On the basis of their geochemical and isotopic systematics,the lamproites in south Tibet have a distinct magma source thatcan be differentiated from the sources of potassic lavas inthe east Lhasa and Qiangtang blocks. Their high Nb/Ta ratios(17· 10–19· 84), extremely high Th/U ratios(5· 70–13· 74) and distinctive isotope compositionsare compatible with a veined mantle source consisting of partialmelts of subducted Tethyan oceanic sediments and sub-continentallithospheric depleted mantle. Identification of the lamproitesand the delineation of their mantle source provide new evidencerelevant for models of the uplift and extension of the Tibetanplateau following the Indo-Asia collision. Metasomatism by partialmelts from isotopically evolved, old sediment subducted on theyoung Tethyan slab is an alternative explanation for PrecambrianNd and Pb model ages. In this model, differences in isotopiccomposition along-strike are attributed to differences in thetype of sediment being subducted, thus obviating the need formultiple metasomatic events over hundreds of million years.The distribution of lamproites, restricted within a north–south-trendinggraben, indicates that the initiation of east–west extensionin south Tibet started at 25 Ma. KEY WORDS: lamproites; subducted oceanic sediment; Tibetan active continental collision belt     

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.     

Dissolution of Quartz, Albite, and Orthoclase in H2O-Saturated Haplogranitic Melt at 800{degrees}C and 200 MPa: Diffusive Transport Properties of Granitic Melts at Crustal Anatectic Conditions

We have conducted experiments on dissolution of quartz, albite,orthoclase, and corundum into H₂O-saturated haplogranite meltat 800°C and 200 MPa over a duration of 120–1488 hwith the aim of ascertaining the diffusive transport propertiesof granitic melts at crustal anatectic temperatures. Cylindersof anhydrous starting glass and a single mineral phase (quartzor feldspar) were juxtaposed along flat and polished surfacesinside gold or platinum capsules with 10 wt % added H₂O. Concentrationprofiles in glass (quenched melt) perpendicular to the mineral–glassinterfaces and comparison with relevant phase diagrams suggestthat melts at the interface are saturated in the dissolvingphases after 384 h, and with longer durations the concentrationprofiles are controlled only by diffusion of components in themelt. The evolution of the concentration profiles with timeindicates that uncoupled diffusion in the melt takes place alongthe following four linearly independent directions in oxidecomposition space: SiO₂, Na₂O, and K₂O axes (Si-, Na-, and K-eigenvectors,respectively), and a direction between the Al₂O₃, Na₂O, andK₂O axes (Al-eigenvector), such that the Al/Na molar ratio isequal to that of the bulk melt and the Al/(Na + K) molar ratiois equal to the equilibrium ASI (= mol. Al₂O₃/[Na₂O + K₂O])of the melt. Experiments in which a glass cylinder was sandwichedbetween two mineral cylinders—quartz and albite, quartzand K-feldspar, or albite and corundum—tested the validityof the inferred directions of uncoupled diffusion and exploredlong-range chemical communication in the melt via chemical potentialgradients. The application of available solutions to the diffusionequations for the experimental quartz and feldspar dissolutiondata provides diffusivities along the directions of the Si-eigenvectorand Al-eigenvector of (2·0–2·8) x 10^–15m²/s and (0·6–2·4) x 10^–14 m²/s, respectively.Minimum diffusivities of alkalis [(3–9) x 10^–11m²/s] are orders of magnitude greater than the tetrahedral componentsof the melt. The information provided here determines the rateat which crustal anatexis can occur when sufficient heat issupplied and diffusion is the only mass transport (mixing) processin the melt. The calculated diffusivities imply that a quartzo-feldspathicsource rock with initial grain size of 2–3 mm undergoinghydrostatic, H₂O-saturated melting at 800°C (infinite heatsupply) could produce 20–30 vol. % of homogeneous meltin less than 1–10 years. Slower diffusion in H₂O-undersaturatedmelts will increase this time frame. KEY WORDS: chemical diffusion; haplogranite; mineral dissolution experiments; crustal anatexis     

Formation of Wollastonite by Chemically Reactive Fluid Flow During Contact Metamorphism, Mt. Morrison Pendant, Sierra Nevada, California, USA

Quartz–calcite sandstones experienced the reaction calcite+ quartz = wollastonite + CO₂ during prograde contact metamorphismat P = 1500 bars and T = 560°C. Rocks were in equilibriumduring reaction with a CO₂–H₂O fluid with XCO₂ = 0·14.The transition from calcite-bearing, wollastonite-free to wollastonite-bearing,calcite-free rocks across the wollastonite isograd is only severalmillimeters wide. The wollastonite-forming reaction was drivenby infiltration of quartz–calcite sandstone by chemicallyreactive H₂O-rich fluids, and the distribution of wollastonitedirectly images the flow paths of reactive fluids during metamorphism.The mapped distribution of wollastonite and modeling of an O-isotopeprofile across a lithologic contact indicate that the principaldirection of flow was layer-parallel, directed upward, withany cross-layer component of flow <0·1% of the layer-parallelcomponent. Fluid flow was channeled at a scale of 1–100m by pre-metamorphic dikes, thrust and strike-slip faults, foldhinges, bedding, and stratigraphic contacts. Limits on the amountof fluid, based on minimum and maximum estimates for the displacementof the wollastonite reaction front from the fluid source, are(0·7–1·9) x 10⁵ cm³ fluid/cm² rock. Thesharpness of the wollastonite isograd, the consistency of mineralthermobarometry, the uniform measured ¹⁸O–¹⁶O fractionationsbetween quartz and calcite, and model calculations all arguefor a close approach to local mineral–fluid equilibriumduring the wollastonite-forming reaction. KEY WORDS: contact metamorphism, fluid flow, wollastonite, oxygen isotopes, reaction front     

Sources and Fluids in the Mantle Wedge below Kamchatka, Evidence from Across-arc Geochemical Variation

Major and trace element and Sr–Nd–Pb isotopic variationsin mafic volcanic rocks hve been studied in a 220 km transectacross the Kamchatka arc from the Eastern Volcanic Front, overthe Central Kamchatka Depression to the Sredinny Ridge in theback-arc. Thirteen volcanoes and lava fields, from 110 to 400km above the subducted slab, were sampled. This allows us tocharacterize spatial variations and the relative amount andcomposition of the slab fluid involved in magma genesis. TypicalKamchatka arc basalts, normalized for fractionation to 6% MgO,display a strong increase in large ion lithophile, light rareearth and high field strength elements from the arc front tothe back-arc. Ba/Zr and Ce/Pb ratios, however, are nearly constantacross the arc, which suggests a similar fluid input for Baand Pb. La/Yb and Nb/Zr increase from the arc front to the back-arc.Rocks from the Central Kamchatka Depression range in ⁸⁷Sr/⁸⁶Srfrom 0·70334 to 0·70366, but have almost constantNd isotopic compositions (¹⁴³Nd/¹⁴⁴Nd 0·51307–0·51312).This correlates with the highest U/Th ratios in these rocks.Pb-isotopic ratios are mid-ocean ridge basalt (MORB)-like butdecrease slightly from the volcanic front to the back-arc. Theinitial mantle source ranged from N-MORB-like in the volcanicfront and Central Kamchatka Depression to more enriched in theback-arc. This enriched component is similar to an ocean-islandbasalt (OIB) source. Variations in (CaO)_6·0–(Na₂O)_6·0show that degree of melting decreases from the arc front tothe Central Kamchatka Depression and remains constant from thereto the Sredinny Ridge. Calculated fluid compositions have asimilar trace element pattern across the arc, although minordifferences are implied. A model is presented that quantifiesthe various mantle components (variably depleted N-MORB-mantleand enriched OIB-mantle) and the fluid compositions added tothis mantle wedge. The amount of fluid added ranges from 0·7to 2·1%. The degree of melting changes from     

Magmatic Evolution of the La Pacana Caldera System, Central Andes, Chile: Compositional Variation of Two Cogenetic, Large-Volume Felsic Ignimbrites

La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km³. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (76–77 wt % SiO₂) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (66–70 wt % SiO₂), dense (40–60% vesicularity)and crystal rich (30–40 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770–790°C with melt water contents of 3·1–4·4wt %. The pre-eruptive Toconao magma was cooler (730–750°C)and its melt more water rich (6·3–6·8 wt% H₂O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (⁸⁷Sr/⁸⁶Sr_i 0·7094–0·7131;¹⁴³Nd/¹⁴⁴Nd 0·51222–0·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H₂O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber     

The Petrology and Geochemistry of Oto-Zan Composite Lava Flow on Shodo-Shima Island, SW Japan: Remelting of a Solidified High-Mg Andesite Magma

The Oto-Zan lava in the Setouchi volcanic belt is composed ofphenocryst-poor, sparsely plagioclase-phyric andesites (sanukitoids)and forms a composite lava flow. The phenocryst assemblagesand element abundances change but Sr–Nd–Pb isotopiccompositions are constant throughout the lava flow. The sanukitoidat the base is a high-Mg andesite (HMA) and contains Mg- andNi-rich olivine and Cr-rich chromite, suggesting the emplacementof a mantle-derived hydrous (7 wt % H₂O) HMA magma. However,Oto-Zan sanukitoids contain little H₂O and are phenocryst-poor.The liquid lines of descent obtained for an Oto-Zan HMA at 0·3GPa in the presence of 0·7–2·1 wt % H₂Osuggest that mixing of an HMA magma with a differentiated felsicmelt can reasonably explain the petrographical and chemicalcharacteristics of Oto-Zan sanukitoids. We propose a model wherebya hydrous HMA magma crystallizes extensively within the crust,resulting in the formation of an HMA pluton and causing liberationof H₂O from the magma system. The HMA pluton, in which interstitialrhyolitic melts still remain, is then heated from the base byintrusion of a high-T basalt magma, forming an H₂O-deficientHMA magma at the base of the pluton. During ascent, this secondaryHMA magma entrains the overlying interstitial rhyolitic melt,resulting in variable self-mixing and formation of a zoned magmareservoir, comprising more felsic magmas upwards. More effectiveupwelling of more mafic, and hence less viscous, magmas througha propagated vent finally results in the emplacement of thecomposite lava flow. KEY WORDS: high-Mg andesite; sanukitoid; composite lava; solidification; remelting     

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

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

Granitoid Compositional Zoning by Side-wall Boundary Layer Differentiation: Evidence from the Palisade Crest Intrusive Suite, Central Sierra Nevada, California

Compositionally zoned plutons are an important feature of theSierra Nevada batholith, California. Two such plutons have beenexamined to determine the mechanism by which crystals separatefrom a magma. The Tinemaha pluton shows continuous compositionalvariation from 58 to 67% SiO₂, whereas the McMurry Meadows plutonis bimodal, with an outer margin of mafic granodiorite (59–60%SiO₂) and an inner core of granite (66–69% SiO₂). Extremedifferentiates also occur as small isolated masses within thesuite and may contain up to 76% SiO₂. Both plutons are uniformin strontium isotopic composition but are different from eachother, with initial ⁸⁷Sr/⁸⁶Sr values of 0?70719 and 0?70651respectively. The Tinemaha pluton is both horizontally and vertically( 1000 m) zoned, with fractionation occurring both inward fromthe contacts and upward. The vertical trends in relative mineralproportions are not consistent with crystal settling. Both thevertical and horizontal variations in the chemical compositionof 50 elements, in mineralogy, and in accessory mineral lightrare-earth element zoning, are all directly relatable to side-wallcrystallization which created a less-dense melt that buoyantlymoved upward along the wall towards the top of the magma chamber.The different rates for diffusive heat exchange and compositionaldiffusion within the magma initiated the double-diffusive gradientin the magma chamber. Compositional variations in the side-wallcrystal accumulation zone occur as boundary layer melts evolve,reflecting changes in the bulk convecting magma. The compositionalgap in the McMurry Meadows pluton is the result of a similarbut more efficient side-wall fractionation process, relatedto a higher proportion of melt to crystals in the initial magmaand a slower rate of side-wall solidification as a result ofthe thermal blanket created by the enclosing Tinemaha pluton.     

Petrology of the Blue Mountain Complex, Marlborough, New Zealand

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

Evidence of Two Different Components in a Hercynian Peraluminous Cordierite-bearing Granite: the San Basilio Intrusion (Central Sardinia, Italy)

Detailed petrographic and geochemical data and Sr and Nd isotopecompositions of enclaves and host-granite are reported for oneof the largest strongly peraluminous cordierite-bearing intrusionsof the Hercynian Sardinia-Corsica Batholith: the San BasilioGranite. Compared with other peraluminous series, the San BasilioGranite has a ‘non-minimum melt’ composition andshows variations primarily owing to fractionation of early-crystallizedplagioclase, quartz and biotite. Crystallization age is constrainedat 305 Ma, by Rb-Sr whole-rock age [30523 Ma with (⁸⁷Sr/⁸⁶Sr)_i= 0.711050.00041], and occurred during late Hercynian tectonicevents. _Nd(305Ma) values range from –7.8 to –7.5.The San Basilio Granite contains both magmatic and metamorphicenclaves. Magmatic enclaves, similar to mafic microgranularenclaves common in calc-alkaline granitoids, are tonalitic incomposition and show a variation in silica content from 60.3to 67.7 wt % correlating with a variation in (⁸⁷Sr/⁸⁶ Sr) _(305Ma)and _{Nd (305 Ma)} from 0.7092 to 0.7109 and from –6.6 to–7.4, respectively. Together with petrographic and othergeochemical data, the Sr and Nd isotopic data record differentstages in a complex homogenization process of an unrelated maficmagma with a crustal melt. A process of simple mixing may accountfor the variations of nonalkali elements and, to some extent,of Sr and Nd isotopes, whereas the distribution of alkali elementsrequires diffusioncontrolled mass transfer. Petrographic andmineralogical data on metamorphic enclaves and geochemical modellingfor trace elements in granite indicate melt generation by high-degreepartial melting involving biotite breakdown of a dominantlyquartzo-feldspathic protolith at about T>750–800Cand P>6 kbar leaving a granulite facies garnet-bearing residue,followed by emplacement at 3 kbar. _Nd(305Ma) values of thegranite fall within the range defined by the pre-existing metamorphicrocks but (⁸⁷Sr/⁸⁶Sr) _(305Ma) ratios are lower, indicating involvementof at least two distinct components: a dominant crustal componentand a minor well-mixed mafic end-member. These data point toa decoupling between the Sr-Nd isotope systematics and majorand trace element compositions, suggesting that the effect ofthe mafic component was minor on granite major and trace elementconcentrations, but significant on Sr and Nd isotopes. The studyof the magmatic enclaves and the isotopic evidence demonstratethat unrelated mafic magmas, probably derived from the mantle,had a close spatial and temporal association with the productionof ‘on-minimum melt’ strongly peraluminous granites,and support the proposal that heat from the mafic magma contributedto crustal melting. KEY WORDS: cordierite-bearing granite; enclaves; felsic-mafic interaction; Sardinia-Corsica Batholith; Sr and Nd isotopes ^*Corresponding author.     

The Role of Continental Crust and Lithospheric Mantle in the Genesis of Cameroon Volcanic Line Lavas: Constraints from Isotopic Variations in Lavas and Megacrysts from the Biu and Jos Plateaux

We present a combined Sr, Nd, Pb and Os isotope study of lavasand associated genetically related megacrysts from the Biu andJos Plateaux, northern Cameroon Volcanic Line (CVL). Comparisonof lavas and megacrysts allows us to distinguish between twocontamination paths of the primary magmas. The first is characterizedby both increasing ²⁰⁶Pb/²⁰⁴Pb (19·82–20·33)and ⁸⁷Sr/⁸⁶Sr (0·70290–0·70310), and decreasing_Nd (7·0–6·0), and involves addition of anenriched sub-continental lithospheric mantle-derived melt. Thesecond contamination path is characterized by decreasing ²⁰⁶Pb/²⁰⁴Pb(19·82–19·03), but also increasing ⁸⁷Sr/⁸⁶Sr(0·70290–0·70359), increasing ¹⁸⁷Os/¹⁸⁸Os(0·130–0·245) and decreasing _Nd (7·0–4·6),and involves addition of up to 8% bulk continental crust. Isotopicsystematics of some lavas from the oceanic sector of the CVLalso imply the involvement of a continental crustal component.Assuming that the line as a whole shares a common source, wepropose that the continental signature seen in the oceanic sectorof the CVL is caused by shallow contamination, either by continent-derivedsediments or by rafted crustal blocks that became trapped inthe oceanic lithosphere during continental breakup in the Mesozoic. KEY WORDS: crustal contamination; CVL; megacrysts; ocean floor; osmium isotopes     

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