Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific)

Macquarie Island is an exposure above sea-level of part of thecrest of the Macquarie Ridge. The ridge marks the Australia–Pacificplate boundary south of New Zealand, where the plate boundaryhas evolved progressively since Eocene times from an oceanicspreading system into a system of long transform faults linkedby short spreading segments, and currently into a right-lateralstrike-slip plate boundary. The rocks of Macquarie Island wereformed during spreading at this plate boundary in Miocene times,and include intrusive rocks (mantle and cumulate peridotites,gabbros, sheeted dolerite dyke complexes), volcanic rocks (N-to E-MORB pillow lavas, picrites, breccias, hyaloclastites),and associated sediments. A set of Macquarie Island basalticglasses has been analysed by electron microprobe for major elements,S, Cl and F; by Fourier transform infrared spectroscopy forH₂O; by laser ablation–inductively coupled plasma massspectrometry for trace elements; and by secondary ion mass spectrometryfor Sr, Nd and Pb isotopes. An outstanding compositional featureof the data set (47·4–51·1 wt % SiO₂, 5·65–8·75wt % MgO) is the broad range of K₂O (0·1–1·8wt %) and the strong positive covariation of K₂O with otherincompatible minor and trace elements (e.g. TiO₂ 0·97–2·1%;Na₂O 2·4–4·3%; P₂O₅ 0·08–0·7%;H₂O 0·25–1·5%; La 4·3–46·6ppm). The extent of enrichment in incompatible elements in glassescorrelates positively with isotopic ratios of Sr (⁸⁷Sr/⁸⁶Sr= 0·70255–0·70275) and Pb (²⁰⁶Pb/²⁰⁴Pb =18·951–19·493; ²⁰⁷Pb/²⁰⁴Pb = 15·528–15·589;²⁰⁸Pb/²⁰⁴Pb = 38·523–38·979), and negativelywith Nd (¹⁴³Nd/¹⁴⁴Nd = 0·51310–0·51304).Macquarie Island basaltic glasses are divided into two compositionalgroups according to their mg-number–K₂O relationships.Near-primitive basaltic glasses (Group I) have the highest mg-number(63–69), and high Al₂O₃ and CaO contents at a given K₂Ocontent, and carry microphenocrysts of primitive olivine (Fo_86–89·5).Their bulk compositions are used to calculate primary melt compositionsin equilibrium with the most magnesian Macquarie Island olivines(Fo_90·5). Fractionated, Group II, basaltic glasses aresaturated with olivine + plagioclase ± clinopyroxene,and have lower mg-number (57–67), and relatively low Al₂O₃and CaO contents. Group I glasses define a seriate variationwithin the compositional spectrum of MORB, and extend the compositionalrange from N-MORB compositions to enriched compositions thatrepresent a new primitive enriched MORB end-member. Comparedwith N-MORB, this new end-member is characterized by relativelylow contents of MgO, FeO, SiO₂ and CaO, coupled with high contentsof Al₂O₃, TiO₂, Na₂O, P₂O₅, K₂O and incompatible trace elements,and has the most radiogenic Sr and Pb regional isotope composition.These unusual melt compositions could have been generated bylow-degree partial melting of an enriched mantle peridotitesource, and were erupted without significant mixing with commonN-MORB magmas. The mantle in the Macquarie Island region musthave been enriched and heterogeneous on a very fine scale. Wesuggest that the mantle enrichment implicated in this studyis more likely to be a regional signature that is shared bythe Balleny Islands magmatism than directly related to the hypotheticalBalleny plume itself. KEY WORDS: mid-ocean ridge basalts; Macquarie Island; glass; petrology; geochemistry     

Continental Lithospheric Contribution to Alkaline Magmatism: Isotopic (Nd, Sr, Pb) and Geochemical (REE) Evidence from Serra de Monchique and Mount Ormonde Complexes

Isotopic results (Sr, Nd, Pb), as well as concentrations ofmajor and trace elements (REE) are reported for whole-rock samplesand mineral separates from the onland alkaline complex of Serrade Monchique (South Portugal) and the offshore alkali basaltvolcanic suite of Mount Ormonde (Gorringe Bank). These two geneticallyrelated alkaline complexes were emplaced at the east Atlanticcontinent–ocean boundary during the Upper Cretaceous,i.e. 66–72 m.y. ago. Taken together, Serra de Monchiqueand Mount Ormonde may be seen as one of the few examples ofwithin-plate magmatism that straddles the continent–oceanboundary. Major and trace element compositions fail to revealany significant differences between onland and offshore complexes.This is particularly true regarding less differentiated samples(mg-number 0.40) which show the same progressive and continuousenrichment of their trace element patterns, with no specificanomaly (e.g. negative Nb anomaly) being present in samplesfrom the onland complex. Initial Pb and Sr isotopic compositionsalso do not allow any distinction to be made between Serra deMonchique and Mount Ormonde samples. Initial Pb isotope ratiosare moderately high (19.1 < ²⁰⁶Pb/²⁰⁴Pb < 19.8; ²⁰⁷Pb/²⁰⁴Pb= 15.6) in both cases. Moreover, once the effects of Sr contaminationby seawater are taken into account and the most contaminatedsamples discarded using data from fresh clinopyroxene separatesand results of leaching experiments, the initial Sr isotopiccompositions of Mount Ormonde samples are found to be unradiogenic(⁸⁷Sr/⁸⁶Sr = 0.7031±1) and identical to those obtainedat Serra de Monchique (⁸⁷Sr/⁸⁶Sr = 0.7032±1). In contrast,a systematic mean difference of 2 _Nd units is observed betweenSerra de Monchique [_Nd(T) = +4.8] and Mount Ormonde [_Nd(T) =+6.6] whole-rock samples. Surprisingly, a variation is alsoobserved at Mount Ormonde between the whole-rock samples andone of the two analysed clinopyroxene separates. Whereas MountOrmonde whole-rock samples invariably yielded _Nd(T) = +6.6 (meanvalue), a value of +0.5 is obtained for one clinopyroxene separate,whereas another gives +6.0. The above geochemical and isotopicresults make it possible to assign respective roles to the asthenosphere,lithosphere and crust in the petrogenesis of Serra de Monchiqueand Mount Ormonde complexes. We propose that both complexesshare a common mantle source whose isotopic characteristicsare very similar to the source of oceanic island basalts. Continentalmantle lithosphere, already characterized isotopically by studiesof peridotite massifs within the Iberian peninsula, acts asa contaminant which is evident onland on the whole-rock scale,and also present offshore as discrete clinopyroxene xenocrysts.The continental crust appears to play no role in the petrogenesisof the Serra de Monchique alkaline rocks. KEY WORDS: alkaline complexes; continental lithosphere; isotope geochemistry; passive continental margin; within-plate volcanics     

Generation of Palaeocene Adakitic Andesites by Magma Mixing; Yanji Area, NE China

Palaeocene (c. 55–58 Ma) adakitic andesites from the Yanjiarea, NE China, are typically clinopyroxene-bearing sodic andesitescontaining 60· 9–62· 2% SiO₂ and 4·02–4· 36% MgO, with high Mg-number [100 Mg/(Mg+ Fe) atomic ratio] from 65· 5 to 70· 1. Whole-rockgeochemical features include high Cr (128–161 ppm) andNi (86–117 ppm) concentrations, extremely high Sr (2013–2282ppm), low Y (10–11 ppm) and heavy rare earth elements(HREE; e.g. Yb = 0· 79–1· 01 ppm), and mid-oceanridge basalt (MORB)-like Sr–Nd–Pb isotopic compositions[e.g. ⁸⁷Sr/ ⁸⁶Sr(i) = 0· 70298–0· 70316,_Nd(t) = +3· 8 to +6· 3 and ²⁰⁶Pb/ ²⁰⁴Pb = 17·98 – 18· 06], analogous to high-Mg adakites occurringin modern subduction zones. However, mineralogical evidencefrom clinopyroxene phenocrysts and microcrystalline plagioclaseclearly points to magma mixing during magma evolution. Iron-richclinopyroxene (augite) cores with low Sr, high Y and heavy REEcontents, slightly fractionated REE patterns and large negativeEu anomalies probably crystallized along with low-Ca plagioclasefrom a lower crustal felsic magma. In contrast, high Mg-numberclinopyroxene (diopside and endiopside) mantles and rims havehigher Sr and lower HREE and Y concentrations, highly fractionatedREE patterns (high La/Yb) and negligible Eu anomalies, similarto those found in adakites from subduction zones. The Yanjiadakitic andesites can be interpreted as a mixture between acrust-derived magma having low Mg-number and Sr, and high Yand HREE, and a mantle-derived high Mg-number adakite havinghigh Sr and low Y and HREE concentrations. During storage and/orascent, the mixed magma experienced further crustal contaminationto capture zircons, of a range of ages, from the wall rocks.The absence of coeval arc magmatism and an extensional tectonicregime in the Yanji area and surrounding regions suggest thatthese Palaeocene adakitic andesites were formed during post-subductionextension that followed the late Cretaceous Izanagi–Farallonridge subduction. Generation of these adakitic andesites doesnot require contemporaneous subduction of a young, hot oceanicridge or delamination of eclogitic lower crust as suggestedby previous models. KEY WORDS: magma mixing; adakitic andesites; Palaeocene; NE China     

Sources and Petrogenesis of Late Triassic Dolerite Dikes in the Liaodong Peninsula: Implications for Post-collisional Lithosphere Thinning of the Eastern North China Craton

A combination of major and trace element, whole-rock Sr, Ndand Hf isotope, and zircon U–Pb isotopic data are reportedfor a suite of dolerite dikes from the Liaodong Peninsula inthe northeastern North China Craton. The study aimed to investigatethe source, petrogenesis and tectonic setting of the dikes.Sensitive high-resolution ion microprobe U–Pb zircon analysesyield a Late Triassic emplacement age of 213 Ma for these dikes,post-dating the collision between the North China and Yangtzecratons and consequent ultrahigh-pressure metamorphism. Threegeochemical groups of dikes have been identified in the LiaodongPeninsula based on their geochemical and Sr–Nd–Hfisotope characteristics. Group 1 dikes are tholeiitic, withhigh TiO₂ and total Fe₂O₃ and low MgO contents, absent to weaknegative Nb and Ta anomalies, variable (⁸⁷Sr/⁸⁶Sr)_i (0·7060–0·7153),_Nd(t) (– 0·8 to –6·5) and _Hf(t) (–2·7to –7·8) values, and negative _Hf(t) (–1·1to –7·8). They are inferred to be derived frompartial melting of a relatively fertile asthenospheric mantlein the spinel stability field, with some upper crustal assimilationand fractional crystallization. Group 2 dikes have geochemicalfeatures of high-Mg andesites with (⁸⁷Sr/⁸⁶Sr)_i values of 0·7063–0·7072,and negative _Nd(t) (–3·0 to –9·5)and _Hf(t) (–3·2 to –10·1) values,and may have originated as melts of foundered lower crust, withsubsequent interaction with mantle peridotite. Group 3 dikesare shoshonitic in composition with relatively low (⁸⁷Sr/⁸⁶Sr)_ivalues (0·7061–0·7063), and negative _Nd(t)(–13·2 to –13·4) and _Hf(t) (–11·0to –11·5) values, and were derived by partial meltingof an ancient, re-enriched, refractory lithospheric mantle inthe garnet stability field. The geochemical and geochronologicaldata presented here indicate that Late Triassic magmatism occurredin an extensional setting, most probably related to post-orogeniclithospheric delamination. KEY WORDS: mafic dike; asthenospheric mantle; lithospheric mantle; delamination; North China Craton     

Early Cretaceous Basaltic and Rhyolitic Magmatism in Southern Uruguay Associated with the Opening of the South Atlantic

The Early Cretaceous volcanic rocks of southern Uruguay comprisemafic and felsic volcanics. The position of these outcrops atthe southern edge of the Paraná–Etendeka continentalflood basalt province provides an opportunity to investigatepossible lateral variations in both mafic and more evolved rocktypes towards the margins of such an area of plume-related magmatism.The mafic lavas are divided into two compositionally distinctmagma types. The more voluminous Treinte Y Trés magmatype is similar to the low-Ti basalts of the Paraná floodbasalt province. The Santa Lucía magma type is a distinctand rare basalt type with ocean-island basalt type asthenosphericaffinities (high Nb/La, low ⁸⁷Sr/⁸⁶Sr_i). The felsic volcanicsare divided into two series, the Lavalleja Series and the AigüaSeries. The Lavalleja Series are chemically and isotopicallysimilar to the Paraná–Etendeka low-Ti rhyolites,and are considered to be related to the Treinte Y Tréslavas by extensive fractionation and crustal assimilation. TheAigüa Series have low ¹⁴³Nd/¹⁴⁴Nd_i and low ⁸⁷Sr/⁸⁶Sr_i andunlike the rhyolites of the Paraná, are interpreted asmelts of pre-existing mafic lower crust that subsequently underwentextreme fractionation. The differences observed in the felsicsuites may be linked to differences in the volumes of the associatedbasalts and the amounts of extension. KEY WORDS: South America; flood basalts; felsic volcanics; crustal melts; plume     

Petrological, Geochemical and Isotopic Constraints on the Origin of the Harzburg Intrusion, Germany

We present mineralogical, petrological and geochemical datato constrain the origin of the Harzburg mafic–ultramaficintrusion. The intrusion is composed mainly of mafic rocks rangingfrom gabbronorite to quartz diorite. Ultramafic rocks are veryrare in surface outcrops. Dunite is observed only in deepersections of the Flora I drill core. Microgranitic (fine-grainedquartz-feldspathic) veins found in the mafic and ultramaficrocks result from contamination of the ultramafic magmas bycrustal melts. In ultramafic and mafic compositions cumulatetextures are widespread and filter pressing phenomena are obvious.The order of crystallization is olivine pargasite, phlogopite,spinel plagioclase, orthopyroxene plagioclase, clinopyroxene.Hydrous minerals such as phlogopite and pargasite are essentialconstituents of the ultramafic cumulates. The most primitiveolivine composition is Fo_89·5 with 0·4 wt % NiO,which indicates that the olivine may have been in equilibriumwith primitive mantle melts. Coexisting melt compositions estimatedfrom this olivine have mg-number = 71. The chemical varietyof the rocks constituting the intrusion and the mg-number ofthe most primitive melt allow an estimation of the approximatecomposition of the mantle-derived primary magma. The geochemicalcharacteristics of the estimated magma are similar to thoseof an island-arc tholeiite, characterized by low TiO₂ and alkalisand high Al₂O₃. Geochemical and Pb, Sr and Nd isotope data demonstratethat even the most primitive rocks have assimilated crustalmaterial. The decoupling of Sr from Nd in some samples demonstratesthe influence of a fluid that transported radiogenic Sr. Leadof crustal origin from two isotopically distinct reservoirsdominates the Pb of all samples. The ultramafic rocks and thecumulates best reflect the initial isotopic and geochemicalsignature of the parent magma. Magma that crystallized in theupper part of the chamber was more strongly affected by assimilatedmaterial. Petrographic, geochemical and isotope evidence demonstratesthat during a late stage of crystallization, hybrid rocks formedthrough the mechanical mixing of early cumulates and melts withstrong crustal contamination from the upper levels of the magmachamber. KEY WORDS: Harzburg mafic–ultramafic intrusion; Sr–Nd–Pb isotopes; magma evolution; crustal contamination     

Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from Petrology and U-Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS Dating

Metapelitic rock samples from the NE Shackleton Range, Antarctica,include garnet with contrasting zonation patterns and two agespectra. Garnet porphyroblasts in K-rich kyanite–sillimanite–staurolite–garnet–muscovite–biotite schistsfrom Lord Nunatak show prograde growth zonation, and give Sm–Ndgarnet, U–Pb monazite and Rb–Sr muscovite ages of518 ± 5, 514 ± 1 and 499 ± 12 Ma, respectively.Geothermobarometry and P–T pseudo-section calculationsin the model system CaO–Na₂O–K₂O– TiO₂–MnO–FeO–MgO–Al₂O₃–SiO₂–H₂Oare consistent with garnet growth during prograde heating from540°C/7 kbar to 650°C/7·5 kbar, and partial resorptionduring a subsequent P–T decrease to <650°C at <6kbar. All data indicate that rocks from Lord Nunatak were affectedby a single orogenic cycle. In contrast, garnet porphyroblastsin K-poor kyanite–sillimanite– staurolite–garnet–cordierite–biotite-schistsfrom Meade Nunatak show two growth stages and diffusion-controlledzonation. Two distinct age groups were obtained. Laser ablationplasma ionization multicollector mass spectrometry in situ analysesof monazite, completely enclosed by a first garnet generation,yield ages of c. 1700 Ma, whereas monazite grains in open garnetfractures and in most matrix domains give c. 500 Ma. Both agegroups are also obtained by U–Pb thermal ionization massspectrometry analyses of matrix monazite and zircon, which fallon a discordia with lower and upper intercepts at 502 ±1 and 1686 ± 2 Ma, respectively. Sm–Nd garnet datingyields an age of 1571 ± 40 Ma and Rb–Sr biotiteanalyses give an age of 504 ± 1 Ma. Integrated geochronologicaland petrological data provide evidence that rocks from MeadeNunatak underwent a polymetamorphic Barrovian-type metamorphism:(1) garnet 1 growth and subsequent diffusive garnet annealingbetween 1700 and 1570 Ma; (2) garnet 2 growth during the RossOrogeny at c. 500 Ma. During the final orogenic event the rocksexperienced peak P–T conditions of about 650°C/7·0kbar and a retrograde stage at c. 575°C/4·0 kbar. KEY WORDS: garnet microtexture; P–T pseudosection; geochronology; polymetamorphism; Shackleton Range; Antarctica     

Episodic Silicic Volcanism in Patagonia and the Antarctic Peninsula: Chronology of Magmatism Associated with the Break-up of Gondwana

New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and ⁴⁰Ar–³⁹Ardata are presented for the Jurassic silicic volcanic rocks andrelated granitoids of Patagonia and the Antarctic Peninsula.U–Pb is the only reliable method for dating crystallizationin these rocks; Rb–Sr is prone to hydrothermal resettingand Ar–Ar is additionally affected by initial excess ⁴⁰Ar.Volcanism spanned more than 30 My, but three episodes are definedon the basis of peak activity: V1 (188–178 Ma), V2 (172–162Ma) and V3 (157–153 Ma). The first essentially coincideswith the Karoo–Ferrar mafic magmatism of South Africa,Antarctica and Tasmania. The silicic products of V1 are lower-crustalmelts that have incorporated upper-crustal material. The geochemistryof V2 and V3 ignimbrites is more characteristic of destructiveplate margins, but the presence of inherited zircon still pointsto a crustal source. The pattern of volcanism corresponds inspace and in time to migration away from the Karoo mantle plumetowards the proto-Pacific margin of Gondwana during riftingand break-up. The heat required to initiate bulk crustal fusionmay have been supplied by the spreading plume-head, but thinningof the crust during continental dispersion would also have facilitatedanatexis. KEY WORDS: Antarctic Peninsula; ignimbrites; Jurassic; Patagonia; U–Pb; zircon     

The Plagioclase-Magma Density Paradox Re-examined and the Crystallization of Proterozoic Anorthosites

Intermediate-composition plagioclase (An_40–60) is typicallyless dense than the relatively evolved basaltic magmas fromwhich it crystallizes and the crystallization of plagioclaseproduces a dense residual liquid, thus plagioclase should havea tendency to float in these magmatic systems. There is, however,little direct evidence for plagioclase flotation cumulates eitherin layered intrusions or in Proterozoic anorthosite complexes.The layered series of the Poe Mountain anorthosite, southeastWyoming, contains numerous anorthosite–leucogabbro blocksthat constrain density relations during differentiation. Allblocks are more mafic than their hosting anorthositic cumulates,their plagioclase compositions are more calcic, and each blockis in strong Sr isotopic disequilibrium with its host cumulate.Associated structures—disrupted and deformed layering—indicatethat (1) a floor was present during crystallization and thatplagioclase was accumulating and/or crystallizing on the floor,(2) compositional layering and plagioclase lamination formeddirectly at the magma–crystal pile interface, and (3)the upper portions of the crystal pile contained significantamounts of interstitial melt. Liquid densities are calculatedfor proposed high-Al olivine gabbroic parental magmas and Fe-enrichedferrodioritic and monzodioritic residual magmas of the anorthositestaking into account pressure, oxygen fugacity, P₂O₅, estimatedvolatile contents, and variable temperatures of crystallization.For all reasonable conditions, calculated block densities aregreater than those of the associated melt. The liquid densities,however, are greater than those for An_40–60 plagioclase,which cannot have settled to the floor. Plagioclase must eitherhave been carried to the floor in relatively dense packets ofcooled liquid plus crystals or have crystallized in situ. Asloping floor, possibly produced by diapiric ascent of relativelylight plagioclase-rich cumulates, is required to allow for drainingand removal of the dense interstitial liquid produced in thecrystal pile and may be a characteristic feature during thecrystallization of many Proterozoic anorthosites and layeredintrusions. KEY WORDS: magma; density; Proterozoic anorthosites; blocks; plagioclase     

Potassic Mafic Lavas of the Bearpaw Mountains, Montana: Mineralogy, Chemistry, and Origin

The volcanic rocks of the Bearpaw Mountains are part of theMontana high-potassium province, emplaced through Archaean rocksof the Wyoming Craton between 54 and 50 Ma ago. Extrusive rocks,dominated by minettes and latites, have a volume of 825 km³.The minettes range in composition from 20 to 6% MgO. The moremagnesian varieties contain the phenocryst assemblage forsterite+ Cr-spinel + diopside phlogopite. More evolved rocks areolivine-free, with an assemblage of either salite + phlogopite+ pseudoleucite or salite + phlogopite + analcime. The analcimeis thought to be secondary after leucite, produced by loss ofpotassium from the minettes. Mineral chemistry and textures,especially of clinopyroxenes, indicate that mixing between minettemagmas of varying degrees of evolution was commonplace. Compositionalvariation was further extended by accumulation of olivine +spinel + clinopyroxene phenocrysts, and by the preservationof mantle xenocrysts in the minettes. The primary minette magmasare inferred to have had 12–14% MgO and to have been generatedat 30 kb from an olivine + diopside + phlogopite-bearing source.The primary magmas evolved dominantly by fractionation of olivine+ diopside. The minettes have high contents of large ion lithophile elements(LILE) and light rare earth elements (LREE), with K₂O up to6.18%, Ba 5491 ppm, Sr 2291 ppm, and Ce 99 ppm. (⁸⁷Sr/⁸⁶Sr)⁰ranges from 0.707 to 0.710 and Nd varies from –10 to–16. These data, plus high LILE/HFSE (high field strengthelements) values, are interpreted to show that the minettescontain at least three different mantle components. The lithospherewas initially depleted in Archaean times, but was metasomaticallyenriched in the Proterozoic and in the late Cretaceous and earlyTertiary. The latites have many chemical features in common with the minettes,such as potassic character and high LILE/HFSE values. They formedby fractional crystallization of minette magma in combinationwith assimilation of crustal rocks; this process enriched themagmas in SiO₂ and raised Na₂O/K₂O and ⁸⁷Sr/⁸⁶Sr values. Chemicaldata for phenocrysts and bulk rocks in minettes suggest mixingbetween minette and latite magmas.     

Petrogenesis of an Alkali Syenite-Granite-Rhyolite Suite in the Yanshan Fold and Thrust Belt, Eastern North China Craton: Geochronological, Geochemical and Nd-Sr-Hf Isotopic Evidence for Lithospheric Thinning

The Yanshan Fold and Thrust Belt in eastern China has been intrudedby a series of alkalic igneous rocks, ranging in compositionfrom granite and rhyolite to syenite and trachyte. Laser ablationinductively coupled plasma mass spectrometry U–Pb analysesof zircon from three alkaline suites yield Early Cretaceousages of 130–117 Ma. Three groups of rocks have been identifiedbased on their mineralogical, geochemical and Sr–Nd–Hfisotope characteristics. The alkali granites and rhyolites areferroan and have low Al₂O₃, MgO, CaO, Sr, Ba and Eu concentrationsand high SiO₂, total Fe₂O₃, K₂O, Nb, Ga, Ta, Th and heavy rareearth element abundances and Ga/Al ratios. Geochemical dataand Sr-, Nd- and zircon Hf-isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_i= 0·7050–0·7164, _Nd(t) = –8·4to –13·6 and _Hf(t) = –5·7 to –16·8]indicate that they were probably generated by shallow dehydrationmelting of biotite- or hornblende-bearing granitoid crustalsource rocks and then mixed with contemporaneous magma froma mantle and/or lower crustal source. Ferroan syenites havedistinct geochemical features from those of the alkaline granitesand rhyolites, suggesting that they were produced by clinopyroxeneand plagioclase fractionation of melt derived from an enrichedmantle source, mixed with lower and upper crustal-derived magmas.The magnesian syenites and trachytes have Sr-, Nd- and zirconHf-isotopic compositions that are distinct from those of theferroan syenites. They were mainly derived from partial meltingof lower crustal materials, mixed with enriched mantle-derivedalkali basaltic magma. The emplacement of an alkali syenite–granite–rhyolitesuite, coeval with the formation of metamorphic core complexesand pull-apart basins in eastern China, indicates they formedin an extensional setting, possibly as a result of lithosphericthinning. KEY WORDS: alkaline rocks; zircon U–Pb dating; petrogenesis; crustal extension; Yanshan Fold and Thrust Belt; North China Craton     

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     

Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe

We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension     

Effect of Carbon Dioxide on Dehydration Melting Reactions and Melt Compositions in the Lower Crust and the Origin of Alkaline Rocks

Dehydration melting experiments of alkali basalt associatedwith the Kenya Rift were performed at 0·7 and 1·0GPa, 850–1100°C, 3–5 wt % H₂O, and f_O2 nearnickel–nickel oxide. Carbon dioxide [X_CO2 = molar CO₂/(H₂O+ CO₂) = 0·2–0·9] was added to experimentsat 1025 and 1050°C. Dehydration melting in the system alkalibasalt–H₂O produces quartz- and corundum-normative trachyandesite(6–7·5 wt % total alkalis) at 1000 and 1025°Cby the incongruent melting of amphibole (pargasite–magnesiohastingsite).Dehydration melting in the system alkali basalt–H₂O–CO₂produces nepheline-normative tephriphonolite, trachyandesite,and trachyte (10·5–12 wt % total alkalis). In thelatter case, the solidus is raised relative to the hydrous system,less melt is produced, and the incongruent melting reactioninvolves kaersutite. The role of carbon dioxide in alkalinemagma genesis is well documented for mantle systems. This studyshows that carbon dioxide is also important to the petrogenesisof alkaline magmas at the lower pressures of crustal systems.Select suites of continental alkaline rocks, including thosecontaining phonolite, may be derived by low-pressure dehydrationmelting of an alkali basalt–carbon dioxide crustal system. KEY WORDS: alkali basalt; alkaline rocks; carbon dioxide; dehydration melting; phonolite     

Crustal Assimilation as a Major Petrogenetic Process in the East Carpathian Neogene and Quaternary Continental Margin Arc, Romania

Miocene to Pleistocene calc-alkaline volcanism in the East Carpathianarc of Romania was related to the subduction of a small oceanbasin beneath the continental Tisza–Dacia microlate. Volcanicproducts are predominantly andesitic to dadtic in composition,with rare basalts and rhyodacites (51–l71% SiO₂; mg-number0.65–0.26) and have medium- to high-K calcalkaline andshoshonitic affinities. Mg, Cr and Ni are low in all rock-types,indicating the absence of primary erupted compositions. Detailedtrace element and Sr, Nd, Pb and 0 isotope data suggest thatmagmas were strongly crustally contaminated. Assimilation andfractional crystallization (AFC) calculations predict the consumptionof 5–35% local upper-crustal metasediments or sedimentsfrom the palaeo-accretionary wedge. Variations in the isotopiccomposition of the contaminants and parental magmas caused variationsin the mixing trajectories in different parts of the arc Themost primitive isotopic compositions are found in low-K dacitesof the northern Cdlimani volcanic centre and are interpretedas largely mantle derived. A second possible mantle reservoirof lower ¹⁴⁹ Nd/¹⁴⁴ Nd and lower ²⁰⁶ Pb/²⁰⁴ Pb is identifiedfrom back-arc basic calc-alkaline rocks in the south of thearc Both magmatic reservoirs have elevated isotopic characteristics,owing either to source bulk mixing (between depleted or enrichedasthenosphere and <1% average subducted local sediment) orlower-crustal contamination. KEY WORDS: Carpathians; assimilation; calc-alkaline; Sr-Nd-Pb-0 isotopes; laser flurination     

Remelting of an Andesitic Crust as a Possible Origin for Rhyolitic Magma in Oceanic Arcs: an Example from the Izu-Bonin Arc

The Izu–Bonin volcanic arc is an excellent example ofan intra-oceanic convergent margin. A total of 1011 chemicalanalyses of 17 Quaternary volcanoes of the arc are reviewedto estimate relative proportions of magmas erupted. Basalt andbasic andesite (SiO₂ < 57 wt %) are the predominant eruptiveproducts of the Izu–Bonin arc, and rhyolite (SiO₂ >70 wt %) forms another peak in volume. Such rhyolites possesscompositions identical to those of partial melts produced bydehydration-melting of calc-alkaline andesites at low pressure(<7 kbar). Meanwhile, the major element variation of theShirahama Group Mio-Pliocene volcanic arc suite, Izu Peninsula,completely overlaps that of the Quaternary Izu–Bonin arcvolcanoes, and groundmasses of Shirahama Group calc-alkalineandesites have compositions similar to those of Izu–Boninrhyolites. Moreover, phenocryst assemblages of calc-alkalineandesites of the Shirahama Group resemble restite phase assemblagesof dehydration-melting of calc-alkaline andesite. These linesof evidence suggest that the rhyolite magmas may have been producedby dehydration-melting of calc-alkaline andesite in the upperto middle crust. If so, then the presence of large amounts ofcalc-alkaline andesite (3–5 times more abundant than therhyolites) within the oceanic arc crust would be expected, whichis consistent with a recently proposed structural model acrossthe Izu–Bonin arc. The calc-alkaline andesite magmas maybe water saturated, and would crystallize extensively and solidifywithin the crust. The model proposed here suggests that rhyoliteeruptions could be triggered by an influx of hot basalt magmafrom depth, reheating and partially melting the calc-alkalineandesite component of the crust. KEY WORDS: bimodal magmatism; calc-alkaline andesite; oceanic arcs; rhyolite     

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

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

The Mesozoic to Early Cenozoic Magmatism of the Benue Trough (Nigeria); Geochemical Evidence for the Involvement of the St Helena Plume

The Benue Trough is a continental rift related to the openingof the equatorial domain of the South Atlantic which was initiatedin Late Jurassic-Early Cretaceous times. Highly diversifiedand volumetrically restricted Mesozoic to Cenozoic magmaticproducts are scattered throughout the rift. Three periods ofmagmatic activity have been recognized on the basis of ⁴⁰ Ar-³⁹Ar ages: 147–106 Ma, 97–81 Ma and 68–49 Ma.Trace element and Sr, Nd and Pb isotope determinations, performedon selected basaltic samples, allow two groups of basaltic rocksto be identified: (1) a group with a tholeiitic affinity, withZr/Nb=7–11.1; La/Nb = 0.77–1; ⁸⁷Sr/⁸⁶Sr; =0.7042–0.7065¹⁴³Nd/¹⁴⁴Nd;_i = 0.5125–0.5127; ²⁰⁶Pb/²⁰⁴Pb_i = 17.59–18.48;(2) a group with an alkaline affinity, with Zr/Nb = 3.6–6.8;La/Nb=0.53–0.66; ⁸⁷Sr/⁸⁶ Sr_i=0.7029–0.7037; ¹⁴³Nd/¹⁴⁴Nd_i=0.5126–0.5129;²⁰⁶Pb/²⁰⁴Pb_i = 18.54–20.42. The geochemical data leadto the conclusion that three types of mantle sources were involvedin the genesis of the Mesozoic to Cenozoic basaltic rocks ofthe Benue, without significant crustal contamination: (1) anenriched subcontinental lithospheric mantle from which the tholeiiticbasalts were derived; (2) a HIMU-type (plume) component fromwhich the alkaline basalticrocks originated; (3) a depletedasthenospheric mantle (N-MORB-type source), which was involvedin the genesis of the alkaline basaltic magmas. According to(1) the postulated location of the St Helena hot spot in theEquatorial Atlantic at about 130 Ma and (2) the isotopic compositionof the alkaline basaltic rocks of the Benue Trough and theirgeochemical similarity with the basalts of St Helena, we concludethat the St Helena plume was involved in the genesis of thealkaline magmatism of the Benue at the time of opening of theEquatorial Atlantic. Moreover, the geochemical similarity betweenthe alkaline magmatism of the Benue Trough and that of the CameroonLine suggests that both magmatic provinces were related to theSt Helena plume. Finally, the temporal change of the mantlesources observed in the Benue Trough can be accounted for bythe recent models of plume dynamics, in the general frameworkof opening of the Equatorial Atlantic. KEY WORDS: Benue Trough; Mesozoic to Cenozoic magmatism; Equatorial Atlantic; mantle sources; St Helena plume ^*;Corresponding author.     

Petrology and Mineral Chemistry of Lower Crustal Intrusions: the Chilas Complex, Kohistan (NW Pakistan)

Mineral major and trace element data are presented for the mainrock units of the Chilas Complex, a series of lower crustalintrusions emplaced during initial rifting within the MesozoicKohistan (paleo)-island arc (NW Pakistan). Detailed field observationsand petrological analysis, together with geochemical data, indicatethat the two principal units, ultramafic rocks and gabbronoritesequences, originate from a common parental magma, but evolvedalong different mineral fractionation trends. Phase petrologyand mineral trace element data indicate that the fractionationsequence of the ultramafic rocks is dominated by the crystallizationof olivine and clinopyroxene prior to plagioclase, whereas plagioclaseprecedes clinopyroxene in the gabbronorites. Clinopyroxene inthe ultramafic rocks (with Mg-number [Mg/(Fe_tot + Mg] up to0·95) displays increasing Al₂O₃ with decreasing Mg-number.The light rare earth element depleted trace element pattern(Ce_N/Gd_N 0·5–0·3) of primitive clinopyroxenesdisplays no Eu anomaly. In contrast, clinopyroxenes from thegabbronorites contain plagioclase inclusions, and the traceelement pattern shows pronounced negative anomalies for Sr,Pb and Eu. Trace element modeling indicates that in situ crystallizationmay account for major and trace element variations in the gabbronoritesequence, whereas the olivine-dominated ultramafic rocks showcovariations between olivine Mg-number and Ni and Mn contents,pointing to the importance of crystal fractionation during theirformation. A modeled parental liquid for the Chilas Complexis explained in terms of mantle- and slab-derived components,where the latter component accounts for 99% of the highly incompatibleelements and between 30 and 80% of the middle rare earth elements.The geochemical characteristics of this component are similarto those of a low percentage melt or supercritical liquid derivedfrom subducted mafic crust. However, elevated Pb/Ce ratios arebest explained by additional involvement of hydrous fluids.In accordance with the crystallization sequence, the subsolidusmetamorphic reactions indicate pressures of 0·5–0·7GPa. Our data support a model of combined flux and decompressionmelting in the back-arc. KEY WORDS: Kohistan; Island arc; gabbro; trace element modelling; lower crustal intrusion     

Geochemical Evidence for Mantle Origin and Crustal Processes in Volcanic Rocks from Popocatepetl and Surrounding Monogenetic Volcanoes, Central Mexico

Elemental, isotopic, and mineral compositions as well as rocktextures were examined in samples from Popocatépetl volcanoand immediately surrounding monogenetic scoria cones of theSierra Chichinautzin Volcanic Field, central Mexico. Magma generationis strongly linked to the active subduction regime to the south.Rocks range in composition from basalt to dacite, but Popocatépetlsamples are generally more evolved and have mineral compositionsand textures consistent with more complicated, multi-stage evolutionaryprocesses. High-Mg calc-alkaline and more alkaline primitivemagmas are present in the monogenetic cones. Systematic variationsin major and trace element compositions within the monogeneticsuite can mostly be explained by polybaric fractional crystallizationprocesses in small and short-lived magmatic systems. In contrast,Popocatépetl stratovolcano has produced homogeneous magmacompositions from a shallow, long-lived magma chamber that isperiodically replenished by primitive basaltic magmas. The currenteruption (1994–present) has produced silicic dome lavasand pumice clasts that display mingling of an evolved daciticcomponent with an olivine-bearing mafic component. The longevityof the magma chamber hosted in Cretaceous limestones has fosteredinteraction with these rocks as evidenced by the chemical andisotopic compositions of the different eruptive products, contact-metamorphosedxenoliths, and fumarolic gases. Popocatépetl volcanicproducts display a considerable range of ⁸⁷Sr/⁸⁶Sr (0·70397–0·70463)and _Nd (+6·2 to +3·0) whereas Pb isotope ratiosare relatively homogeneous (²⁰⁶Pb/²⁰⁴Pb 18·61–18·70;²⁰⁷Pb/²⁰⁴Pb 15·56–15·60). KEY WORDS: Popocatépetl; Sierra Chichinautzin Volcanic Field; arc petrogenesis; radiogenic isotopes