Polybaric Evolution of Calc-alkaline Magmas from Nisyros, Southeastern Hellenic Arc, Greece

The lavas of Nisyros were erupted between about 0?2 m.y B.P.and 1422 A.D., and range in composition from basaltic andesiteto rhyodacite. Most were erupted prior to caldera collapse (exactdate unknown), and the post-caldera lavas are petrographically(presence of strongly resorbed phenocrysts) and chemically (lowerTiO₂ K₂O, P₂O₅, and LIL elements) distinct from the pre-calderalavas. The pre-caldera lavas do not form a continuous seriessince lavas with SiO₂ contents between 60 and 66 wt.% are absent.Nevertheless, major element variations demonstrate that fractionalcrystalliz ation (involving removal of olivine, dinopyroxene,plagioclase, and Fe-Ti oxide from the basaltic andesites andandesites and plagioclase, clinopyroxene, hypersthene, Ti-magnetite,ilmenite, apatite, and zircon from the dacites and rhyodacites)played a major role in the evolution of the pre-caldera lavas.Several lines of evidence indicate that other processes werealso important in magma evolution: (1) Quantitative modelingof major element data shows that phenocryst phases of unlikelycomposi tion or unrealistic assemblages of phenocryst phasesare required to relate the dacites and rhyodacites to the basalticandesites and andesites; (2) The proportions of olivine andclinopyroxene required in quantitative models for the initialstages of evolution differ from those observed petrographicallyand this is not likely to reflect either differential ratesof crystal settling or the curvature of cotectics along whichliquids of basaltic andesite to andesite composition lie; (3)The concentrations of Rb, Cs, Ba, La, Sm, Eu, and Th in therhyod.acites are too high for these lavas to be related to thedacites by fractional crystallization alone; and (4) ⁸⁷Sr/⁸⁶Srratios for the andesites and rhyodacites are higher than thosefor the basaltic andesites and dacites, respectively. It isshown that fractional crystallization was accompanied by assimilation,and that magma mixing played a minor role (if any) in the evolutionof the pre-caldera lavas. Trace element and isotopic data indicatethat the andesites evolved from the basaltic andesites by AFCinvolving average crust or upper crust, whereas the rhyodacitesevolved from the dacites by AFC involving lower crust. Additionalevidence for polybaric evolution is provided by the occurrenceof distinct Ab-rich cores of plagioclase phenocrysts in thedacites and rhyodacites, which record a period of high pressurecrystallization, and by the occurrence of both normal and reverse-zonedphenocrysts in the basaltic andesites and andesites. Furthermore,calculated pressures of crystallization are {small tilde}8 kbfor the dacites and rhyodacites and 3?5–4 kb for the basalticandesites and andesites. It is concluded that the dacites andrhyodacites evolved via AFC from basaltic andesites and andesiteslargely in chambers sited near the base of the crust whereasthe basaltic andesites and andesites mostly evolved in chamberssited at mid-crustal levels. Eruption from different chambersexplains the compositional gap in the chemistry of the pre-calderalavas since eruptive products represent a more or less randomsampling of residual liquids which separate (via filter pressing)from bodies of crystallizing magma at various depths. Magmamixing was important in the evolution of the post-caldera lavas,but geochemical data require that these magmas evolved fromparental magmas which were derived from a more refractory sourcethan the parental magmas to the pre-caldera lavas. ^*Present address: Netherlands Energy Research Foundation (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands     

Compositional and Dynamic Controls on Mafic--Silicic Magma Interactions at Continental Arc Volcanoes: Evidence from Cordn El Guadal, Tatara-San Pedro Complex, Chile

Heterogeneous andesitic and dacitic lavas on Cordn El Guadalbear on the general problem of how magmas of differing compositionsand physical properties interact in shallow reservoirs beneathcontinental arc volcanoes. Some of the lavas contain an exceptionallylarge proportion (<40%) of undercooled basaltic andesiticmagma in various states of disaggregation. Under-cooled maficmagma occurs in the silicic lavas as large (<40 cm) basalticandesitic magmatic inclusions, as millimeter-sized crystal-clotsof Mg-rich olivine phenocrysts plus adhering Carich plagioclasemicrophenocrysts (An_50–70), and as uniformly distributed,isolated phenocrysts and microphenocrysts. Compositions andtextures of plagioclase phenocrysts indicate that inclusion-formingmagmas are hybrids formed by mixing basaltic and dacitic melts,whereas textural features and compositions of groundmass phasesindicate that the andesitic and dacitic lavas are largely mechanicalmixtures of dacitic magma and crystallized basaltic andesiticmagma. This latter observation is significant because it indicatesthat mechanical blending of undercooled mafic magma and partiallycrystallized silicic magma is a possible mechanism for producingthe common porphyritic texture of many calc-alkaline volcanicrocks. The style of mafic-silicic magma interaction at CordonEl Guadal was strongly dependent upon the relative proportionsof the endmembers. Equally important in the Guadal system, however,was the manner in which the contrasting magmas were juxtaposed.Textural evidence preserved in the plagioclase phenocrysts indicatesthat the transition from liquid-liquid to solid-liquid mixingwas not continuous, but was partitioned into periods of magmachamber recharge and eruption, respectively. Evidently, duringperiods of recharge, basaltic magmas rapidly entrained smallamounts of dacitic magma along the margins of a turbulent injectionfountain. Conversely, during periods of eruption, dacitic magmagradually incorporated small parcels of basaltic andesitic magma.Thus, the coupled physical-chemical transition from mixed inclusionsto commingled lavas is presumably not coincidental. More likely,it probably provides a partial record of the dynamic processesoccurring in shallow magma chambers beneath continental arevolcanoes. KEY WORDS: Chile; commingling; magma mixing; magmatic inclusions ^*Present address: Department of Earth Sciences, Montana State University, Bozeman, MT 59717, USA     

Geochemical evidence for arc-type volcanism in the Aegean Sea: the blueschist unit of Siphnos, Cyclades (Greece)

Blueschists, eclogites, chlorite–actinolite rocks and jadeite-gneisses of the blueschist unit of Siphnos have been investigated for their geochemical composition. Their protolith nature is characterised and a geodynamic model for the pre-metamorphic evolution of these metavolcanic rocks is proposed on the basis of immobile elements, especially trace elements and rare earth elements (REE).

The protoliths of the eclogites are characterised as calc-alkaline basalts, andesites and Fe-rich tholeiites evolving in an island-arc setting. Trace element data indicate that subducted marine sediments were assimilated in the magma chamber, enriching the protoliths in LILE and Pb. Produced in the early stage of back-arc basin opening, a protolith with affinities to both island-arc and MORB formed the precursor of the chlorite–actinolite rocks. They were created by low degrees of partial melting of very primitive magmas, akin to spinel-peridotites and have affinities to boninites, probably through melting of the peridotitic mantle wedge. Tholeiitic basalts and andesites with N-MORB affinity, especially in their REE-patterns, were then produced by partial melting, possibly in an embryonic back-arc basin. These rocks were the protoliths of the blueschists of Siphnos. Their enrichment in some LILE and Pb indicates a N-MORB source contaminated by marine sediments, probably shales or other Pb-rich sediments. Because the jadeite-gneisses show affinities to MOR-granites and volcanic arc granites, intrusion of their protoliths in a back-arc environment is likely. The protoliths of the quartz-jadeite gneisses are rhyodacites/dacites and rhyolites, those of the glaucophane-jadeite gneisses were andesites.

The proposed geodynamic model, solely based on geochemical data, is consistent with geochemical data from neighbouring islands, though those rock units show much higher chemical variability. Consistent with geotectonic models, which are based on structural and geophysical data, the volcanic protoliths of the Siphnos blueschist unit reflect the transition from subduction to spreading environment and record in detail: subduction, formation of an island-arc, and the evolution of a back-arc basin.     

辽宁赛马碱性岩体层硅铈钛矿化学成分变化及其对碱性岩浆演化的指示意义

辽宁赛马岩体是我国典型的产铀碱性杂岩体,但其稀土矿化机制尚不明确.通过光学显微镜、扫描电镜和电子探针分析,得知该岩体从霞石正长岩经霓霞正长伟晶岩至晚期异霞正长岩,代表性稀土矿物层硅铈钛矿[Na₂Ca₄REETi（Si₂O₇）₂OF_3]不断富集,Nb、Zr和REE（特别是HREE）等高场强元素含量不断升高,部分颗粒具Zr、REE等元素成分环带,以上成分变化与稀土等不相容元素性质、碱性岩浆成分和岩浆结晶分异过程密切相关.此外,层硅铈钛矿经历了一系列的热液蚀变,蚀变部分Ti、Ca、Sr、Na含量增加而Zr、REE含量降低,最终形成由残余层硅铈钛矿+方解石+萤石+铈硅磷灰石组成的假晶,可能与富碱质、F和CO₂的自交代流体作用有关.该研究揭示了碱性岩浆演化过程中,层硅铈钛矿成分变化及热液蚀变组合对指示岩浆结晶分异程度、探究稀土元素分馏及其热液活动性的具有重要意义.      

Post-caldera dacites from the Santorini volcanic complex,Aegean Sea,Greece: an example of the eruption of lavas of near-constant composition over a 2,200 year period

The post-caldera Kameni islands of the Santorini volcanic complex, Aegean Sea, Greece are entirely volcanic and were formed by eleven eruptions between 197 B.C. and 1950. Petrographic, mineral chemical and whole-rock major and trace element data are presented for samples of lava collected from the products of seven eruptive cycles which span the entire period of activity. The main phenocryst phases are plagioclase, clinopyroxene, orthopyroxene and titaniferous magnetite, which are weakly zoned (e.g. plagioclase — An₅₅ to An₄₂). The lavas are typical calc-alkaline dacites and show a restricted range of composition (from 64.1 to 68.4 wt. % SiO₂). The phenocrysts were in equilibrium with the melts at temperatures of 960–1012 °C, pressures of 800–1500 bars and oxygen fugacities of 10^–9.6-10^–9.9 bars. The pre-eruptive water content of the magmas was 3–4 wt. % but since the lavas contain only 0.1–0.4 wt. % H₂O, a considerable amount (about 0.01–0.015 km³) of water was lost prior to or during eruption. This indicates that the magmas rose to the surface gradually allowing the (largely) non-explosive loss of volatiles. The lavas were probably extruded initially from more or less cylindrical conduits which developed into fissures as the eruptions proceeded. The post-caldera lavas evolved from more mafic parental magmas (basalt-andesite) via fractional crystallization. The small range of compositional variation shown by these lavas can be explained in terms of near-equilibrium crystallization. Analyses of samples of lavas belonging to single eruption cycles and to individual flows indicate that the underlying magma chamber is compositionally zoned. The average composition of erupted magma has remained approximately constant since 1570 A.D. but that fact that the 197 B.C. magma was sligthly richer in SiO₂ provides additional evidence that the magma chamber is compositionally zoned. Crystal settling has not affected the composition of the magma over a 2,200 year period of time which indicates that the melts do not behave as Newtonian fluids. Zonation was thus probably established prior to the 197 B.C. eruption though it is possible that it is developed and maintained by crystal-liquid differentiation processes other than crystal settling (e.g. boundary layer crystallization). The data indicate that there has been no significant cooling during 2,200 years; the maximum amount of cooling is <50 °C and is probably less than 30 °C. Two hypotheses are considered to explain the thermal and chemical buffering of the post-caldera magma chamber: (i) The magma chamber is large and heat losses due to conduction are largely compensated by latent heat supplied by thick, partially crystalline cumulate sequences. (ii) Periodic influx of hot mafic magma, which does not mix with the dacitic magma, inhibits cooling. The second alternative is favored because the post-caldera lavas differ geochemically from the pre-caldera lavas which signifies that a new batch of magma was formed and/or emplaced after the catastrophic eruption of 1390 B.C., and hence that mafic magmas may still be reaching upper crustal levels.     

Aragonite stromatolitic buildups from Santorini (Aegean Sea,Greece): Geochemical and palaeontological constraints of the caldera palaeoenvironment prior to the Minoan eruption (ca 3600 yr bp)

The pyroclastic deposits of the Minoan eruption (ca 3600 yr bp ) in Santorini contain abundant xenoliths. Most of these deposits are calcareous blocks of laminated‐botryoidal, stromatolite‐like buildups that formed in the shallow waters of the flooded pre‐Minoan caldera; they consist of (i) light laminae, of fibrous aragonite arranged perpendicular to layering, and (ii) dark laminae, with calcified filaments of probable biological origin. These microstructures are absent in the light laminae, suggesting a predominant inorganic precipitation of aragonite on substrates probably colonized by microbes. Internal cavities contain loose skeletal grains (molluscs, ostracods, foraminifera and diatoms) that comprise taxa typical of shallow marine and/or lagoon environments. Most of these forms are typical of warm water environments, although no typical taxa from hydrothermal vents have been observed. Past gasohydrothermal venting is recorded by the occurrence of barite, pyrolusite and pyrite traces. The most striking features of the stable isotopic data set are: (i) an overall wide range in δ¹³C_PDB (0·16 to 12·97‰) with a narrower variation for δ¹⁸O_PDB (?0·23 to 4·33‰); and (ii) a relatively uniform isotopic composition for the fibrous aragonite (δ¹³C = 12·40 ± 0·43‰ and δ¹⁸O = 2·42 ± 0·77‰, n = 21). The δ¹³C and δ¹⁸O values from molluscs and ostracods display a covariant trend, which reflects a mixing between sea water and a fluid influenced by volcano‐hydrothermal activity. Accordingly, ⁸⁷Sr/⁸⁶Sr from the studied carbonates (0·708758 to 0·709011 in fibrous aragonite and 0·708920 to 0·708991 in molluscs) suggests that the aragonite buildups developed in sea water under the influence of a hydrothermal/volcanic source. Significant differences in trace elements have been detected between the fibrous aragonite and modern marine aragonite cements. The caldera water from which the fibrous aragonite crusts formed received an input from a volcano‐hydrothermal system, probably producing diffuse venting of volcanogenic CO₂ gas and of a fluid enriched in Ca, Mn and Ba, and depleted in Mg and probably in Sr.     

Evolution of Calc-alkaline Magmas in Continental Arc Volcanoes: Evidence from Alicudi, Aeolian Arc (Southern Tyrrhenian Sea, Italy)

Major, trace element, and Sr isotopic data are reported forvolcanic rocks from the island of Alicudi, Aeolian Arc, SouthernTyrrhenian Sea. The island is constructed of basalt, basalticandesite to high-K andesite lavas, and pyroclastites, whichshow a continuum in the variation of many major and trace elements.Total iron, MgO, CaO, Ni, Co, Sc, and Cr decrease with increasingsilica, whereas incompatible elements Rb, Ba, Th, and LREE displaythe opposite tendency. Very significant positive correlationsare defined by incompatible elements on interelemental variationdiagrams. Sr isotopic ratios vary from 0–70352 to 0–70410.Overall, basalts (0–70352–O-70410) and basalticandesltes (0–70356–0–70409) are enriched in⁸⁷Sr compared with high-K andesites (O–70352–O–70367),which display the lowest Sr isotopic ratios within the entireAeolian archipelago. Overall negative relationships exist between⁸⁷Sr/⁸⁶Sr and several incompatible trace element abundancesand ratios, such as Th, U, LREE, Zr, La/Yb, and Th/Hf. Otherelemental ratios such as La/Rb, Ba/Rb, and Sr/Rb show more complexbehaviour, even though negative correlations with Sr isotopicratios are observed in the basalts. The observed compositional variations are best explained interms of a model in which primitive calc-alkaline magmas evolvedby crystal-liquid fractionation to give a series of variouslydifferentiated liquids, which underwent different degrees ofinteraction with crustal material. The more mafic and hotterbasaltic liquids appear to have assimilated higher amounts ofmetamorphic wall rocks than did the cooler late erupted andesiticmagmas. This process produced significant variations of Sr isotopicratios, Rb, Cs, Rb/Sr ratios, and LILE/Rb ratios in mafic magmas,but had only minor effects on the abundances and ratios of otherincompatible elements such as Th, LREE, La/Yb, and Th/Hf. When compared with mafic rocks from other Aeolian islands, theAlicudi basalts are more primitive geochemically and isotopically.Going eastward, there is a decrease in Ni and Cr abundances,mg-number and Nd isotopic ratios which parallels an increaseof Sr isotopic ratios in basaltic rocks along the arc. Thesecompositional variations are typical of volcanic series whichhave undergone interaction with upper-crustal material, andsuggest that this process may have contributed significantlyto the regional geochemical and isotopic trends observed inthe Aeolian arc.     

Zircon in amphibolites from Naxos,Aegean Sea,Greece: origin,significance and tectonic setting

We report U–Pb zircon ages of c. 700–550 Ma, 262–220 Ma, 47–38 Ma and 15–14 Ma from amphibolites on Naxos Island in the Aegean extensional province of Greece. The zircon has complex internal structures. Based on cathodoluminescence response, zoning and crosscutting relationships a minimum of four zircon growth stages are identified: inherited core, magmatic core, inner metamorphic (?) rim and an outer metamorphic rim. Trace element compositions of the amphibolites suggest igneous differentiation and crustal assimilation. Zircon solubility as a function of saturation temperatures, Zr content and melt composition indicates that the zircon did not originally crystallize in the mafic bodies but was inherited from felsic precursor rocks, and subsequently assimilated into the mafic intrusives during emplacement. Zircon inheritance is corroborated by the complex, xenocrystic nature of the zircon in one sample. Ages of c. 700–550 Ma and 262–220 Ma are assigned to inherited zircon. Available geochemical data suggest that the 15–14 Ma metamorphic rims grew in situ in the amphibolites, corresponding to a high‐grade metamorphic event at this time. However, the geochemical data cannot conclusively establish if the c. 40 Ma zircon rims also grew in situ, or whether they were inherited along with the xenocrystic cores. Two scenarios for emplacement of the mafic intrusives are discussed: (i) Intrusion during late‐Triassic to Jurassic ocean basin development of the Aegean realm, in which case the 40 Ma zircon rims would have grown in situ, and (ii) emplacement in the Miocene as a result mafic underplating during large‐scale extension. In this case, only the 15–14 Ma metamorphic outer rims would have formed in situ in the amphibolitic host rocks.     

Petrological Study of the Myoko and Kurohime Volcanoes, Japan: Crystallization Sequence and Evidence for Magma Mixing

Quaternary volcanic rocks of the Myoko and Kurohime volcanoes,central Japan, are divided into two types, R-type and N-type,according to the presence or absence of reversely zoned maficphenocrysts. Disequilibrium phenocryst assemblages, such asMg-rich olivine with Fe-rich orthopyroxene and augite, Mg-richnormally zoned pyroxenes with Fe-rich reversely zoned ones,and a wide (commonly bimodal) compositional variation of plagioclasephenocrysts are commonly observed in the R-type rocks. Whole-rock bulk chemical compositions of the R-type rocks showgood linear correlations with the ratio of the number of An-rich(An 80) phenocrysts to the number of Ab-rich phenocrysts (An60). The chemical trends and petrographic features of R-typerocks result from mixing of magmas. The N-type rocks, whichare free from the disequilibrium phenocryst assemblages, cannotresult from magma mixing and constitute a single differentiationseries. Rocks of the pigeonitic rock series (P-series) and those ofthe hypersthenic rock series (H-series) (Kuno, 1950) roughlycorrespond to the N-type rocks and R-type rocks respectively.Therefore, some rocks of the H-series (equivalent to the calc-alkalinerock series) in Japan represent magmas formed by mixing.     

Xenoliths of lawsonite-ferroglaucophane rocks from a Quaternary volcano of Milos (Aegean Sea,Greece)

Xenoliths of metamorphic rocks with ferroglaucophane + albite + stilpnomelane + muscovite ± lawsonite ± pumpellyite are described fromn the rhylolitic tephra deposits of the Late Quaternary Phyriplaka Volcano, Milos, Aegean Island, Greece. The chemical bulk analyses reveal metabasaltic parent rocks with strongly modified chemical compositions. Unusual for common fresh basalt types is extreme Na₂O combined with low K₂O and rather low CaO. These are chemical characteristics of spilitic basalts. High iron in the bulk rock and very high Fe/Mg is probably the cause of extreme ferroglaucophane compositions. The metamorphic assemblages point to a high-p/low-T metamorphism in the pressure range 4–8 kb and with temperatures of about 350°C. Phyriplaka Volcano is likely to have only a shallow explosive center and ferroglaucophane rocks were ejected from the immediate basement of the island, which was uplifted after the metamorphic event.     

Zoned Pyroxenes from Shoshonite Lavas of Lesbos, Greece: Inferences concerning Shoshonite Petrogenesis

Samples from several stratigraphic levels in the Miocene volcanicsuccession on Lesbos, Greece, contain complexly zoned clinopyroxeneand feldspar phenocrysts. Electron microprobe analyses havebeen made of zoned clinopyroxenes and feldspars, and their inclusions;and phenocryst mineralogy is compared with host-rock chemicalcomposition. The commonest pyroxene zoning sequence is coresof greenish salite or augite mantled by diopside which, in turn,may be rimmed by augite, with each type of pyroxene in opticaland chemical discontinuity with the next. None of the pyroxeneshas high pressure characteristics. The Ts content of phenocrystcores is correlated with the K-content of the host rock; andthe most diopsidic overgrowths also occur in the most K-richrocks. Augite rims occur only in Al-rich rocks. Most of the features of the pyroxenes can be accounted for bya hypothetical fractional crystallization scheme. However, twoobservations suggest that mixing of small magma batches mayalso be important. The earliest crystallized diopsides thatrim salite are rich in Cr₂O₃, suggesting precipitation froman unfractionated magma. Anorthoclase (implying advanced crystallizationof feldspars) occurs as cores to feldspar phenocrysts and asinclusions in clinopyroxene cores in several samples.     

Mixed Magmas, Mush Chambers and Eruption Triggers: Evidence from Zoned Clinopyroxene Phenocrysts in Andesitic Scoria from the 1995 Eruptions of Ruapehu Volcano, New Zealand

Juvenile ejecta from the September and October 1995 eruptionsof Ruapehu volcano, New Zealand, indicate that mixing occurredbetween relatively higher- and lower-temperature (high-T andlow-T) andesitic magmas. Compositional zonations in clinopyroxenephenocrysts provide direct evidence for a pre-eruption crystal–meltmush chamber containing low-T magma, and elucidate the processesof magma mixing and eruption, following the injection of high-Tmagma. Many phenocrysts with Fe-rich cores derived from low-Tmagma have extremely reverse zoned mantles around slightly resorbedcores. Mg-value [100Mg/(Mg + Fe)] increases from 65–70to     

Evidence for Multiple Mechanisms of Crustal Contamination of Magma from Compositionally Zoned Plutons and Associated Ultramafic Intrusions of the Alaska Range

Models of continental crustal magmagenesis commonly invoke theinteraction of mafic mantle-derived magma and continental crustto explain geochemical and petrologic characteristics of crustalvolcanic and plutonic rocks. This interaction and the specificmechanisms of crustal contamination associated with it are poorlyunderstood. An excellent opportunity to study the progressiveeffects of crustal contamination is offered by the compositeplutons of the Alaska Range, a series of nine early Tertiary,multiply intruded, compositionally zoned (Peridotite to granite)plutons. Large initial Sr and Nd isotopic contrasts betweenthe crustal country rock and likely parental magmas allow evaluationof the mechanisms and extents of crustal contamination thataccompanied the crystallization of these ultra-mafic throughgranitic rocks. Three contamination processes are distinguishedin these plutons. The most obvious of these is assimilationof crustal country rock concurrent with magmatic fractionalcrystallization (AFC), as indicated by a general trend towardcrustal-like isotopic signatures with increasing differentiation.Second, many ultramafic and mafic rocks have late-stage phenocrystreaction and orthocumulate textures that suggest interactionwith felsic melt. These rocks also have variable and enrichedisotopic compositions that suggest that this felsic melt wasisotopically enriched and probably derived from crustal countryrock. Partial melt from the flysch country rock may have reactedwith and contaminated these partly crystalline magmas followingthe precipitation and accumulation of the cumulus phenocrystsbut before complete solidification of the magma. This suggeststhat in magmatic mush (especially of ultramafic composition)crystallizing in continental crust, a second distinct processof crustal contamination may be super imposed on AFC or magmamixing involving the main magma body. Finally, nearly all rocks,including mafic and ultramafic rocks, have (⁸⁷Sr/⁸⁶Sr)_i thatare too high, and (T) _Nd that are too low, to represent theexpected isotopic composition of typical depleted mantle. However,gabbro xenoliths with typical depicted-mantle isotopic compositionsare found in the plutons. This situation requires either anadditional enriched mantle component to provide the parentalmagma for these plutons, or some mechanism of crustal contaminationof the parent magma that did not cause significant crystallizationand differentiation of the magma to more felsic compositions.Thermodynamic modeling indicates that assimilation of alkali-andwater-rich partial melt of the metapelite country rock by fractionating,near-liquidus basaltic magma could cause significant contaminationwhile suppressing significant crystallization and differentiation. KEY WORDS: crustal contamination; Alaska Range; isotope geochemistry; zoned plutons; assimilation ^*Corresponding author. e-mail: preiners{at}u.washington.edu; fax: (206) 543-3836.     

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     

Geochemical Constraints from Zoned Hydrothermal Tourmalines on Fluid Evolution and Sn Mineralization: an Example from Fault Breccias at Roche, SW England

Hydrothermal fluid evolution north of the St Austell granite,southwest England, has been studied through geochemical analysisof tourmaline from a fault breccia of <2 cm width withinmassive quartz–tourmaline rocks at Roche. Brecciated tourmalinegrains have overgrowths of <400 µm width [Fe/(Fe +Mg) = 0·31–0·99] with four chemically distinctzones (1–4, towards the margins). Variations in overgrowthcomposition were caused by episodic mixing between Mg-, Al-richmagmatic hydrothermal fluids (dominant in zone 1), with an increasingcomponent of more oxidizing, Fe-rich formation waters (zones2 and 4). More oxidizing conditions are supported by high Sncontents in zone 2 (<0·35 wt %), with Sn probablypresent as Sn⁴⁺ rather than Sn²⁺, the usual form in hydrothermalfluids. From X-ray maps, zones 1 and 3 occur exclusively asovergrowths on pre-existing grains, indicating that overgrowthformation was kinetically favoured over tourmaline nucleation.In zones 2 and 4, nucleation and growth occurred, possibly asa result of supersaturation with respect to tourmaline duringincreased mixing with formation waters. Tourmaline is associatedwith the main episode of mineralization in many important mineraldeposits, often unaffected by alteration. This method of studyinghydrothermal fluid evolution may therefore have uses in exploration,particularly for tourmaline-breccia-hosted ores in Cu-porphyrydeposits. KEY WORDS: breccia; Cornwall; hydrothermal; tin; tourmaline     

Magma Mixing and the Production of Compositional Variation within Granite Suites: Evidence from the Granites of Southeastern Australia

Granite suites are groups of plutons possessing characteristicfeatures that are a result of their derivation from source materialof a specific composition. Variation within suites has beenascribed to a variety of processes. Magma mixing or minglingis a popular hypothesis, generally proposed in terms of blendingbetween a crustal melt and mafic material from the mantle thatcaused that melting. When the compositions of pairs of suitesfrom the Bega Batholith of southeastern Australia are compared,any differences seen at either end of the range in compositionare also seen at the other limit, so that both the most maficand most felsic rocks show similar relative abundances of particularelements. Similar relationships are seen for other granitesin the region. These observations are not consistent with large-scalemagma mixing or mingling and, although those processes may operateon a small scale, they cannot have been responsible for themajor compositional variations. Likewise, assimilation of countryrocks had no significant role in producing variation in thegranites of southeastern Australia. The production of variationby differential separation of melt from residual solid sourcematerial, or restite, must be favoured for many of the granitesuites of this region. KEY WORDS: assimilation; enclaves; granite suites; magma mixing; restite     

The tectonic significance of a porphyroblastic blueschist facies overprint during Alpine orogenesis: Sifnos, Aegean Sea, Greece

The MacArgon program^∗ has been used to model published apparent age spectra for white micas from the island of Sifnos, Cyclades, Greece. These micas formed during a period of porphyroblastic mineral growth (M₂) in the epidote-blueschist facies, at > ~ 460 °C and < ~ 14 kbar. M₂ marked the onset of a major period of deformation (D₃) during which kilometre-scale ductile shear zones formed and widespread recumbent folding took place. The modelling experiments suggest that flat apparent age spectra observed in the eclogite-blueschist domain (EBD) can only be obtained if the rocks cooled rapidly below ~ 350 °C after M₂ at ~ 42 Ma. Cooling rates must be > 50 °C/m.y., and since M₂ is followed by D₃ we infer that these are the cooling rates during D₃. Modelling experiments for micas from the greenschist domain (GSD) structurally underneath the EBD suggest that the GSD cooled rapidly, but at ~ 32 Ma. One tectonic model that might explain the rapidity of cooling rates inferred for the EBD model supposes that ambient temperatures were at ~ 500 °C when M₂ took place, but thereafter the high pressure rocks were thrust to the north and rapidly cooled as the result of their juxtaposition against cooler, shallower levels of the crust.     

The Turiy Massif, Kola Peninsula, Russia: Isotopic and Geochemical Evidence for Multi-source Evolution

The Turiy Massif, lying within the Kandalaksha Graben, and onthe southern coast of the Kola Peninsula, contains carbonatites,phoscorites, melilitolites, ijolites and pyroxenites withinone central and four surrounding satellite complexes. Sr–Ndisotopic data from the central complex phoscorites and carbonatites,and the nearby Terskii Coast kimberlites, combined with otherrecently published data on the Devonian Kola Alkaline Province,allow us to redefine the position of the Kola Carbonatite Line(KCL) of Kramm (European Journal of Mineralogy 5, 985–989,1993). We propose that the revised-KCL mantle sources includea lower-mantle plume, and a second enriched source, which alsocontributed to the Terskii Coast and Archangelsk kimberlites.The Turiy Massif silicate rocks and northern complex carbonatiteshave more enriched isotopic signatures than the distinct, anddepleted signatures of the central complex phoscorites and carbonatites,particularly with respect to     

新疆阿尔泰可可托海3号伟晶岩脉岩浆-热液过程:来自电气石化学组成演化的证据

新疆阿尔泰可可托海3号脉不同结构带及其蚀变围岩中电气石化学组成电子探针(EMPA)分析结果显示,蚀变围岩、外接触带和内接触带中电气石为是富钙-铁的镁电气石,电气石化伟晶岩中电气石为富镁-铁的锂电气石,伟晶岩早期结构带(Ⅰ-Ⅳ带)中电气石为黑电气石-锂电气石系列,而伟晶岩晚期结构带(Ⅴ-Ⅶ带)中电气石为锂电气石。蚀变围岩、外接触带和内接触带中电气石以Y位中低Al,高Mg、Ca以及显著低的Al/(Al+Fe)、Fe/(Fe+Mg)比值为特征,主要存在R3++O2-=R2++OH-的置换;伟晶岩内部结构带(Ⅰ-Ⅶ带)中电气石以Y位上极低Mg、Ca,高Fe、Al、Li以及显著高的Al/(Al+Fe)、Fe/(Fe+Mg)比值为特征,主要存在+Al3+=Na++Fe(Mg)2+和Li++Al3+=Fe(Mn)2++Mg2+的置换。蚀变围岩、外接触带和内接触带中电气石化学组成特征指示存在围岩-流体、围岩-熔体相之间的相互作用。伟晶岩早期结构带(Ⅰ-Ⅳ带)中电气石无明显组成分带,为岩浆成因;晚期结构带(Ⅴ-Ⅶ带)中电气石显示振荡环带,形成于岩浆-热液过渡阶段体系。随着伟晶岩由外向里固结,温度逐渐降低,伟晶岩内部结构带中电气石显示Y位上Al、Li、Mn及Fe/(Fe+Mg)比值逐渐增大的演化趋势;内部结构带中电气石几乎不含Mg,指示3号伟晶岩脉岩浆-热液演化是在相对封闭体系中进行的。     

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