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1.
TAMURA  Y. 《Journal of Petrology》1995,36(2):417-434
The Mio-Pliocene Shirahama Group, Izu Peninsula, Central Japan,a well-exposed submarine volcanic arc complex of lava flows,pyroclastic rocks and associated shallow intrusives, is characterizedby a tholeiitic series (basalt to dacite) and a calc-alkalineseries (andesite to dacite). Chemical variations in the tholeiiticseries and calc-alkaline series are consistent with crystalfractionation from basalt and magnesian andesite (boninite),respectively. Crystal–liquid phase relations of thesemagmas have been investigated by study of sample suites fromthese two series. Compositions of liquids in equilibrium withphenocrysts were determined by microprobe grid analyses, inwhich 49 points were averaged in 03 mm 03 mm groundmassareas. The liquid compositions, coupled with the phenocrystmineralogy of the same samples, define the liquid lines of descentof these volcanic arc magmas. Major findings include the following:(1) Crystallization of the tholeiitic series magma is consistentwith early stage crystallization in the simple system Fo–Di–Silica–H2O,with olivine having a reaction relation to augite and the tholeiiticliquid. (2) The later stage products of the tholeiitic seriesmagma are, however, crystal-poor (<10%) dacites with no maficminerals, suggesting that tholeiitic liquids, hypersthene andaugite were no longer on the cotectic (3) A characteristic ofthe calc-alkaline series magmas is the development of rhyoliticliquids. Hypersthene, augite, plagioclase and Fe–Ti oxideoccur in most calc-alkaline rocks studied, and hornblende andquartz can be found in about half of these. However, their differentiationpaths show that the cotectic relation between quartz and liquidended at a later stage, resulting in the resorption of quartzphenocrysts and ultimately in the formation of quartz-free magmas.(4) The late-stage liquids of both the tholeiitic and calc-alkalineseries have deviated from their cotectics, which cannot be explainedby fractional crystallization alone. The addition of H2O froman outside system is probably required to explain the differentiationpaths. (5) The formation of chilled margins, the in situ crystallizationof a magma chamber in the solidification zone, and/or the migrationof groundwater into the magma chamber are thought to be likelyprocesses affecting magmas during their migration and intrusioninto the crust. An extreme effect of H2O addition would be tolower the liquidus temperatures of all precipitating silicatephases far below their restorable range before eruption, resultingin the production of aphyric magmas. Even when a temperaturedecrease in the magma chamber causes a liquid to intersect theliquidus of a pre-existing phase, the addition of H2O shiftsthe cotectic toward SiO2, resulting in quartz being the lastphase to crystallize. The resorption of quartz is interpretedto be the result of a liquidus boundary shift caused by theaddition of H2O. The genesis of aphyric rhyolites is thereforeinferred to result from fractional crystallization followingaddition of H20. KEY WORDS: Shirahama Group; Japan; island arc; rhyolite; magma series  相似文献   

2.
Phenocryst zoning patterns are used to identify open-systemmagmatic processes in the products of the 2001 eruption of ShiveluchVolcano, Kamchatka. The lavas and pumices studied are hornblende–plagioclaseandesites with average pre-eruptive temperatures of 840°Cand fO2 of 1·5–2·1 log units above nickel–nickeloxide (NNO). Plagioclase zoning includes oscillatory and patchyzonation and sieve textures. Hornblendes are commonly unzoned,but some show simple, multiple or patchy zoning. Apatite microphenocrystsdisplay normal and reverse zoning of sulphur. The textural similarityof patchy hornblende and plagioclase, together with Ba–Srsystematics in patchy plagioclase, indicate that the cores ofthese crystals were derived from cumulate material. Plagioclase–liquidequilibria suggest that the patchy texture develops by resorptionduring H2O-undersaturated decompression. When H2O-saturatedcrystallization recommences at lower pressure, reduced pH2Oresults in lower XAn in plagioclase, causing more Al-rich hornblendeto crystallize. Plagioclase cores with diffuse oscillatory zoning,and unzoned hornblende crystals, probably represent a populationof crystals resident in the magma chamber for long periods oftime. In contrast, oscillatory zoning in the rims of plagioclasephenocrysts may reflect eruption dynamics during decompressioncrystallization. Increasing Fe/Al in oscillatory zoned rimssuggests oxidation as a result of degassing of H2O during decompression.A general lack of textural overlap between phenocryst typessuggests that different phenocryst populations were spatiallyor temporally isolated during crystallization. We present evidencethat the host andesite has mixed with both more felsic and moremafic magmas. Olivine and orthopyroxene xenocrysts with reactionor overgrowth rims and strong normal zoning indicate mixingwith basalt. Sieve-textured plagioclase resulted from mixingof a more felsic magma with the host andesite. The mineralogyand mineral compositions of a mafic andesite enclave are identicalto those of the host magma, which implies efficient thermalquenching, and thus small volumes of intruding magma. Mixingof this magma with the host andesite results in phenocryst zoningbecause of differences in dissolved volatile contents. We suggestthat small magma pulses differentiated at depth and ascendedintermittently into the growing magma chamber, producing incrementalvariations in whole-rock compositions. KEY WORDS: patchy zoning; magma mixing; Shiveluch  相似文献   

3.
The Proterozoic (950 Ma) Lyngdal granodiorite of southern Norwaybelongs to a series of hornblende–biotite metaluminousferroan granitoids (HBG suite) coeval with the post-collisionalRogaland Anorthosite–Mangerite–Charnockite (AMC)suite. This granitoid massif shares many geochemical characteristicswith rapakivi granitoids, yet granodiorites dominate over granites.To constrain both crystallization (P, T, fO2, H2O in melt) andmagma generation conditions, we performed crystallization experimentson two samples of the Lyngdal granodiorite (with 60 and 65 wt% SiO2) at 4–2 kbar, mainly at fO2 of NNO (nickel–nickeloxide) to NNO + 1, and under fluid-saturated conditions withvarious H2O–CO2 ratios for each temperature. Comparisonbetween experimental phase equilibria and the mineral assemblagein the Lyngdal granodiorite indicates that it crystallized between4 and 2 kbar, from a magma with 5–6 wt % H2O at an fO2of NNO to NNO + 1. These oxidized and wet conditions sharplycontrast with the dry and reduced conditions inferred for thepetrogenesis of the AMC suite and many other rapakivi granitesworldwide. The high liquidus temperature and H2O content ofthe Lyngdal granodiorite imply that it is not a primary magmaproduced by the partial melting of the crust but is derivedby the fractionation of a mafic magma. Lyngdal-type magmas appearto have volcanic equivalents in the geological record. In particular,our results show that oxidized high-silica rhyolites, such asthe Bishop Tuff, could be derived via fractionation of oxidizedintermediate magmas and do not necessarily represent primarycrustal melts. This study underlines the great variability ofcrystallization conditions (from anhydrous to hydrous and reducedto oxidized) and petrogenetic processes among the metaluminousferroan magmas of intermediate compositions (granodiorites,quartz mangerites, quartz latites), suggesting that there isnot a single model to explain these rocks. KEY WORDS: ferroan granitoids; crystallization conditions; experiments; Norway; Sveconorwegian; Bishop Tuff  相似文献   

4.
Petrographical and geochemical characteristics of calc-alkalineandesites on Shodo-Shima Island, SW Japan, having bulk compositionslargely identical to the continental crust, are presented. Thefollowing petrographic observations suggest a role for magmamixing in producing such andesite magmas: (1) two types of olivinephenocrysts and spinel inclusions, one with compositions identicalto those in high-Mg andesites and the other identical to thosein basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe3+relations, respectively; (2) the presence of orthopyroxene phenocrystswith mg-number >90 suggests the contribution of an orthopyroxene-bearinghigh-Mg andesite magma to production of calc-alkaline andesites;(3) reversely zoned pyroxene phenocrysts may not be in equilibriumwith Mg-rich olivine, suggesting the involvement of a differentiatedandesite magma as an endmember component; (4) the presence ofvery Fe-rich orthopyroxene phenocrysts indicates the associationof an orthopyroxene-bearing rhyolitic magma. Contributions fromthe above at least five endmember magmas to the calc-alkalineandesite genesis can also provide a reasonable explanation ofthe Pb–Sr–Nd isotope compositions of such andesites. KEY WORDS: calc-alkaline andesites; high-Mg andesites; magma mixing; continental crust; SW Japan  相似文献   

5.
AUDETAT  A.; PETTKE  T. 《Journal of Petrology》2006,47(10):2021-2046
The magmatic processes leading to porphyry-Cu mineralizationat Santa Rita are reconstructed on the basis of petrographicstudies, thermobarometry, and laser-ablation inductively-coupled-plasmamass-spectrometry analyses of silicate melt and sulfide inclusionsfrom dikes ranging from basaltic andesite to rhyodacite. Combinedresults suggest that magma evolution at Santa Rita is similarto that of sulfur-rich volcanoes situated above subduction zones,being characterized by repeated injection of hot, mafic magmainto an anhydrite-bearing magma chamber of rhyodacitic composition.The most mafic end-member identified at Santa Rita is a shoshoniticbasaltic andesite that crystallized at 1000–1050°C,1–3 kbar and log fO2 = NNO + 0·7 to NNO + 1·0,whereas the rhyodacite crystallized at 730–760°C andlog fO2 = NNO + 1·3 to NNO + 1·9. Mixing betweenthe two magmas caused precipitation of 0·1–0·2wt % magmatic sulfides and an associated decrease in the Cucontent of the silicate melt from 300–500 ppm to lessthan 20 ppm. Quantitative modeling suggests that temporal storageof ore-metals in magmatic sulfides does not significantly enhancethe amount of copper ultimately available to ore-forming hydrothermalfluids. Magmatic sulfides are therefore not vital to the formationof porphyry-Cu deposits, unless a mechanism is required thatholds back ore-forming metals until late in the evolution ofthe volcanic–plutonic system. KEY WORDS: porphyry-Cu; sulfur; sulfides; magma mixing; LA-ICP-MS  相似文献   

6.
High-magnesian andesite occurs at Hachimantai, northern Honshu,Japan. Disequilibrium zoning features indicate that the phenocrystminerals were derived from three different magmas. Chemicalcompositions and zoning profiles are accounted for by two-stagemagma mixing: the first mixing occurred between a crystal-freebasalt magma and a more differentiated olivine basalt magma;the second stage occurred by mixing between the resultant ofthe first-stage mixing and a hypersthene–augite andesitemagma. Mass balance of phenocryst crystals shows that end-membercompositions were c. 52·0 wt % SiO2 and 10·1 wt% MgO for the mafic end-member and 57·0 wt % SiO2 forthe felsic end-member of the second-stage mixing. Phenocrystminerals of the first-stage mixing end-member indicate the similarityof the end-member composition to that of basalts from nearbyvolcanoes. The counterpart aphyric magma in the first-stagemixing was more magnesian than the estimated mafic end-member.Calculations of the phase equilibria of similar basalts fromnearby volcanoes and comparison of results with previous phaseequilibrium experiments showed that the olivine basalt end-memberof the first stage was hydrous and situated at a depth wherethe pressure was less than 2 kbar. Two-pyroxene thermometryestimates are about 1050°C for the pyroxenes derived fromthe felsic end-member of the second-stage mixing, and about1180°C for groundmass pyroxenes. Crystallization temperaturesof 1170–1230°C are estimated for minerals from themafic end-member of the second-stage mixing based on phase equilibriumcalculations. These similar temperature estimates between thegroundmass and the mafic end-member imply achievement of thermalequilibrium between end-members preceding crystallization. Themagma plumbing system of the eastern Hachimantai is illustratedby a recent volcanic event, involving lateral dike intrusiontoward a pressure source. The encounter of a laterally migratingbasalt dike and an andesite magma chamber triggered the magmamixing that produced the high-magnesian andesite. The modelcan account for the relation between the petrological modeland surface distribution of volcanic rocks. The infrequencyof such mixing-derived high-magnesian andesite stems from therarity of high-magnesian basalt as a potential mixing end-memberin northern Honshu. KEY WORDS: high-magnesian andesite; Hachimantai; Northern Honshu; high-magnesian basalt; two-stage magma mixing  相似文献   

7.
A diverse assemblage of small mafic and ultramafic xenolithsoccurs in alkalic lava from Davidson and Pioneer seamounts locatedat the continental margin of central California. Based on mineralcompositions and textures, they form three groups: (1) mantlexenoliths of lherzolite, pyroxenite, and dunite with olivineof >Fo90; (2) ocean crust xenoliths of dunite with olivine<Fo90, troctolite, pyroxene-gabbro, and anorthosite withlow-K2O plagioclase; (3) cumulates of seamount magmas of alkalicgabbro with primary amphibole and biotite and anorthosites withhigh-K2O plagioclase. The alkalic cumulates are geneticallyrelated to, but more evolved than, their host lavas and probablycrystallized at the margins of magma reservoirs. Modeling andcomparison with experimentally derived phases suggest an originat moderate pressures (0·5–0·9 GPa). Thehigh volatile contents of the alkalic host lavas may have pressurizedthe magma chambers and helped to propel the xenolith-bearinglavas directly from deep storage at the base of the lithosphereto the eruption site on the ocean floor, entraining fragmentsof the upper mantle and ocean crust cumulates from the underlyingabandoned spreading center. KEY WORDS: basaltic magmatism; continental margin seamounts; geothermobarometry; mineral chemistry; xenoliths  相似文献   

8.
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  相似文献   

9.
The effects of small amounts of H2O (<4 wt % in the melt)on the multiply saturated partial melting of spinel lherzolitein the system CaO–MgO–Al2O3–SiO2 ±Na2O ± CO2 have been determined at 1·1 GPa inthe piston-cylinder apparatus. Electron microprobe analysisand Fourier transform infrared spectroscopy were used to analysethe experimental products. The effects of H2O are to decreasethe melting temperature by 45°C per wt % H2O in the melt,to increase the Al2O3 of the melts, decrease MgO and CaO, andleave SiO2 approximately constant, with melts changing fromolivine- to quartz-normative. The effects of CO2 are insignificantat zero H2O, but become noticeable as H2O increases, tendingto counteract the H2O. The interaction between H2O and CO2 causesthe solubility of CO2 at vapour saturation to increase withincreasing H2O, for small amounts of H2O. Neglect of the influenceof CO2 in some previous studies on the hydrous partial meltingof natural peridotite may explain apparent inconsistencies betweenthe results. The effect of small amounts of H2O on multiplysaturated melt compositions at 1·1 GPa is similar tothat of K2O, i.e. increasing H2O or K2O leads to quartz-normativecompositions, but increasing Na2O produces an almost oppositetrend, towards nepheline-normative compositions. KEY WORDS: H2O; CO2; FTIR; hydrous partial melting; mantle melting; spinel lherzolite; system CaO–MgO–Al2O3–SiO2 ± H2O ± CO2 ± Na2O  相似文献   

10.
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/86Sri 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  相似文献   

11.
Volcán Popocatépetl has been the site of voluminousdegassing accompanied by minor eruptive activity from late 1994until the time of writing (August 2002). This contribution presentspetrological investigations of magma erupted in 1997 and 1998,including major-element and volatile (S, Cl, F, and H2O) datafrom glass inclusions and matrix glasses. Magma erupted fromPopocatépetl is a mixture of dacite (65 wt % SiO2, two-pyroxenes+ plagioclase + Fe–Ti oxides + apatite, 3 wt % H2O, P= 1·5 kbar, fO2 = NNO + 0·5 log units) and basalticandesite (53 wt % SiO2, olivine + two-pyroxenes, 3 wt % H2O,P = 1–4 kbar). Magma mixed at 4–6 km depth in proportionsbetween 45:55 and 85:15 wt % silicic:mafic magma. The pre-eruptivevolatile content of the basaltic andesite is 1980 ppm S, 1060ppm Cl, 950 ppm F, and 3·3 wt % H2O. The pre-eruptivevolatile content of the dacite is 130 ± 50 ppm S, 880± 70 ppm Cl, 570 ± 100 ppm F, and 2·9 ±0·2 wt % H2O. Degassing from 0·031 km3 of eruptedmagma accounts for only 0·7 wt % of the observed SO2emission. Circulation of magma in the volcanic conduit in thepresence of a modest bubble phase is a possible mechanism toexplain the high rates of degassing and limited magma productionat Popocatépetl. KEY WORDS: glass inclusions; igneous petrology; Mexico; Popocatépetl; volatiles  相似文献   

12.
Alaskan-type platinum-bearing plutons and potassium-enrichedmafic to ultramafic volcanic rocks are temporally and spatiallyassociated within the Late Cretaceous–Paleocene Achaivayam–Valaginskiiintra-oceanic palaeo-arc system, allochthonously present inthe Koryak Highland and Kamchatka Peninsula (Far East Russia).The compositions of the parental magmas to the Alaskan-typecomplexes are estimated using the Galmoenan plutonic complexas an example. This complex, composed of dunites, pyroxenitesand minor gabbros, is the largest (20 km3) in the system andthe best studied owing to associated platinum placer deposits.The compositions of the principal mineral phases in the Galmoenanintrusive rocks [olivine (Fo79–92), clinopyroxene (1–3·5wt % Al2O3, 0·1–0·5 wt % TiO2), and Cr-spinel(5–15 wt % Al2O3 and 0·3–0·7 wt %TiO2)] are typical of liquidus assemblages in primitive island-arcmagmas in intra-oceanic settings, and closely resemble the mineralcompositions in the Achaivayam–Valaginskii ultramaficvolcanic rocks. The temporal and spatial association of intrusiveand extrusive units, and the similarity of their mineral compositions,suggest that both suites were formed from similar parental magmas.The composition of the parental magma for the Galmoenan plutonicrocks is estimated using previously reported data for the Achaivayam–Valaginskiiultramafic volcanic rocks and phenocryst-hosted melt inclusions.Quantitative simulation of crystallization of the parental magmain the Galmoenan magma chamber shows that the compositions ofthe cumulate units are best modelled by fractional crystallizationwith periodic magma replenishment. The model calculations reproducewell the observed mineral assemblages and the trace elementabundances in clinopyroxene. Based upon the estimated compositionof the parental magmas and their mantle source, we considerthat fluxing of a highly refractory mantle wedge (similar tothe source of boninites) by chlorine-rich aqueous fluids isprimarily responsible for both high degrees of partial meltingand the geochemical characteristics of the magmas, includingtheir enrichment in platinum-group elements. KEY WORDS: subduction; platinum-group elements; clinopyroxene; trace elements; fractional crystallization; Alaskan-type plutons  相似文献   

13.
The late Archaean Panozero pluton in Central Karelia (BalticShield) is a multi-phase high-Mg, high-K intrusion with sanukitoidaffinities, emplaced at 2·74 Ga. The magmatic historyof the intrusion may be subdivided into three cycles and includesmonzonitic and lamprophyric magmas. Compositional variationsare most extreme in the monzonite series and these are interpretedas the result of fractional crystallization. Estimates of thecomposition of the parental magmas to the monzonites and lamprophyresshow that they are enriched in light rare earth elements, Sr,Ba, Cr, Ni and P but have low contents of high field strengthelements. Radiogenic isotope data indicate a low U/Pb, highTh/U, high Rb/Sr, low Sm/Nd source. The magmatic rocks of thePanozero intrusion are also enriched in H2O and CO2; carbonisotope data are consistent with mantle values, indicating afluid-enriched mantle source. The similarity in trace elementcharacter of all the Panozero parental magmas indicates thatall the magmas were derived from a similar mantle source. Thepattern of trace element enrichment is consistent with a mantlesource enriched by fluids released from a subducting slab. Nd-isotopedata suggest that this enrichment took place at c. 2·8Ga, during the main episode of greenstone belt and tonalite–trondhjemite–granodioriteformation in Central Karelia. Sixty million years later, at2·74 Ga, the subcontinental mantle melted to form thePanozero magmas. Experimental studies suggest that the monzoniticmagmas originated by the melting of pargasite–phlogopitelherzolite in the subcontinental mantle lithosphere at 1–1·5GPa. The precise cause of the melting event at 2·74 Gais not known, although a model involving upwelling of asthenosphericmantle following slab break-off is consistent with the geochemicalevidence for the enrichment of the Karelian subcontinental mantlelithosphere by subduction fluids. KEY WORDS: Archaean; sanukitoid; monzonite; Karelia; mantle metasomatism  相似文献   

14.
The Bandas del Sur Formation preserves a Quaternary extra-calderarecord of central phonolitic explosive volcanism of the LasCañadas volcano at Tenerife. Volcanic rocks are bimodalin composition, being predominantly phonolitic pyroclastic deposits,several eruptions of which resulted in summit caldera collapse,alkali basaltic lavas erupted from many fissures around theflanks. For the pyroclastic deposits, there is a broad rangeof pumice glass compositions from phonotephrite to phonolite.The phonolite pyroclastic deposits are also characterized bya diverse, 7–8-phase phenocryst assemblage (alkali feldspar+ biotite + sodian diopside + titanomagnetite + ilmenite + nosean–haüyne+ titanite + apatite) with alkali feldspar dominant, in contrastto interbedded phonolite lavas that typically have lower phenocrystcontents and lack hydrous phases. Petrological and geochemicaldata are consistent with fractional crystallization (involvingthe observed phenocryst assemblages) as the dominant processin the development of phonolite magmas. New stratigraphicallyconstrained data indicate that petrological and geochemicaldifferences exist between pyroclastic deposits of the last twoexplosive cycles of phonolitic volcanism. Cycle 2 (0·85–0·57Ma) pyroclastic fall deposits commonly show a cryptic compositionalzonation indicating that several eruptions tapped chemically,and probably thermally stratified magma systems. Evidence formagma mixing is most widespread in the pyroclastic depositsof Cycle 3 (0·37–0·17 Ma), which includesthe presence of reversely and normally zoned phenocrysts, quenchedmafic glass blebs in pumice, banded pumice, and bimodal to polymodalphenocryst compositional populations. Syn-eruptive mixing eventsinvolved mostly phonolite and tephriphonolite magmas, whereasa pre-eruptive mixing event involving basaltic magma is recordedin several banded pumice-bearing ignimbrites of Cycle 3. Theperiodic addition and mixing of basaltic magma ultimately mayhave triggered several eruptions. Recharge and underplatingby basaltic magma is interpreted to have elevated sulphur contents(occurring as an exsolved gas phase) in the capping phonoliticmagma reservoir. This promoted nosean–haüyne crystallizationover nepheline, elevated SO3 contents in apatite, and possiblyresulted in large, climatologically important SO2 emissions. KEY WORDS: Tenerife; phonolite; crystal fractionation; magma mixing; sulphur-rich explosive eruptions  相似文献   

15.
This paper characterizes late Holocene basalts and basaltic andesites at Medicine Lake volcano that contain high pre-eruptive H2O contents inherited from a subduction related hydrous component in the mantle. The basaltic andesite of Paint Pot Crater and the compositionally zoned basaltic to andesitic lavas of the Callahan flow erupted approximately 1000 14C years Before Present (14C years b.p.). Petrologic, geochemical and isotopic evidence indicates that this late Holocene mafic magmatism was characterized by H2O contents of 3 to 6 wt% H2O and elevated abundances of large ion lithophile elements (LILE). These hydrous mafic inputs contrast with the preceding episodes of mafic magmatism (from 10,600 to ∼3000 14C years b.p.) that was characterized by the eruption of primitive high alumina olivine tholeiite (HAOT) with low H2O (<0.2 wt%), lower LILE abundance and different isotopic characteristics. Thus, the mantle-derived inputs into the Medicine Lake system have not always been low H2O, primitive HAOT, but have alternated between HAOT and hydrous subduction related, calc-alkaline basalt. This influx of hydrous mafic magma coincides temporally and spatially with rhyolite eruption at Glass Mountain and Little Glass Mountain. The rhyolites contain quenched magmatic inclusions similar in character to the mafic lavas at Callahan and Paint Pot Crater. The influence of H2O on fractional crystallization of hydrous mafic magma and melting of pre-existing granite crust beneath the volcano combined to produce the rhyolite. Fractionation under hydrous conditions at upper crustal pressures leads to the early crystallization of Fe-Mg silicates and the suppression of plagioclase as an early crystallizing phase. In addition, H2O lowers the saturation temperature of Fe and Mg silicates, and brings the temperature of oxide crystallization closer to the liquidus. These combined effects generate SiO2-enrichment that leads to rhyodacitic differentiated lavas. In contrast, low H2O HAOT magmas at Medicine Lake differentiate to iron-rich basaltic liquids. When these Fe-enriched basalts mix with melted granitic crust, the result is an andesitic magma. Since mid-Holocene time, mafic volcanism has been dominated primarily by hydrous basaltic andesite and andesite at Medicine Lake Volcano. However, during the late Holocene, H2O-poor mafic magmas continued to be erupted along with hydrous mafic magmas, although in significantly smaller volumes. Received: 4 January 1999 / Accepted: 30 August 1999  相似文献   

16.
The major element chemical compositions of lava from four eruptionson the east rift zone of Kilauea between August 1968 and October1971 reflect three petrologic processes:
  1. Production of chemically distinct batches of magma in the mantle.
  2. Separation of olivine, augite, and plagioclase from liquidduringflow in the rift-zone conduits.
  3. Mixing of differentmagmas during ascent to the surface.
Chemically none of the four Kilauea east-rift eruptions matchesthe preceding summit eruption in Halemaumau that ended in July1968. The Mauna Ulu eruption, May 1969 to October 1971 (thelast of flie east-rift eruptions), can be divided into fiveolivine-controlled and chemically distinct variants. Three ofthese characterize the first seven months of the eruption andare closest in composition to the 1967–8 Halemaumau eruption.Variants 4 and 5 were erupted later and have compositions thatare distinctly different from that of the 1967–8 eruption.Major differences are higher Al2O3 (0?15–0?23 per cent),and lower K2O (0?07–0?10 per cent) and TiO2 (0?12–0?23per cent) in variants 4 and 5 at the same MgO content. Somelavas from eruptions in August and October 1968 and February1969, have olivine-controlled magma compositions that are identicalto mixtures of Mauna Ulu variants 1–3 and the 1967–8composition. This observation fits an hypothesis advanced earlierby T. L. Wright and R. S. Fiske that magmas in the central magmachamber become mixed with magmas in the rift zone and can beidentified as mixing components of rift eruption magmas beforethey appear as distinctive magmas in summit eruptions. Lavas representing mixing of olivine-controlled magma with differentiatedmagma were erupted in October 1968, February 1969, and in Mayand December 1969. The changes in amount of K2O and TiO2 during the latter partof the 1969–71 Mauna Ulu eruption are the reverse of theoverall secular change in composition of Kilauea summit lavasfrom pre-1750 through 1967–8. The K2O and TiO2 contentsof the latest overflows during the 1969–71 Mauna Ulu eruption(April 1971) are comparable to that of lava erupted at Kilaueasummit prior to 1750. The changing chemistry of Kilauea magma is found to be of useas a ‘tracer’ in the complex Kilauea conduit system.Application of these data to older lava sequences is difficulbecause of the complexity of the processes controlling lavacomposition and the absence of detailed information about thetime-space chemical variation during individual eruptions.  相似文献   

17.
Phase equilibria simulations were performed on naturally quenchedbasaltic glasses to determine crystallization conditions priorto eruption of magmas at the Mid-Atlantic Ridge (MAR) east ofAscension Island (7–11°S). The results indicate thatmid-ocean ridge basalt (MORB) magmas beneath different segmentsof the MAR have crystallized over a wide range of pressures(100–900 MPa). However, each segment seems to have a specificcrystallization history. Nearly isobaric crystallization conditions(100–300 MPa) were obtained for the geochemically enrichedMORB magmas of the central segments, whereas normal (N)-MORBmagmas of the bounding segments are characterized by polybariccrystallization conditions (200–900 MPa). In addition,our results demonstrate close to anhydrous crystallization conditionsof N-MORBs, whereas geochemically enriched MORBs were successfullymodeled in the presence of 0·4–1 wt% H2O in theparental melts. These estimates are in agreement with direct(Fourier transform IR) measurements of H2O abundances in basalticglasses and melt inclusions for selected samples. Water contentsdetermined in the parental melts are in the range 0·04–0·09and 0·30–0·55 wt% H2O for depleted and enrichedMORBs, respectively. Our results are in general agreement (within±200 MPa) with previous approaches used to evaluate pressureestimates in MORB. However, the determination of pre-eruptiveconditions of MORBs, including temperature and water contentin addition to pressure, requires the improvement of magma crystallizationmodels to simulate liquid lines of descent in the presence ofsmall amounts of water. KEY WORDS: MORB; Mid-Atlantic Ridge; depth of crystallization; water abundances; phase equilibria calculations; cotectic crystallization; pressure estimates; polybaric fractionation  相似文献   

18.
Calc-alkaline batholiths of the Archaean Minto block, northeasternSuperior Province, Canada, have pyroxene- and hornblende-bearingmineral assemblages inferred to have crystallized from hot,water-undersaturated magmas at 2·729–2·724Ga. A regional amphibolite- to granulite-facies tectonothermalevent at 2·70 Ga resulted in mild to negligible metamorphiceffects on the dominantly granodioritic units. Geochemical,textural and thermobarometric studies define the crystallizationhistory in compositions ranging from cumulate pyroxenite throughquartz diorite, granodiorite, granite, and syn-magmatic gabbroicdykes. Early magmatic assemblages include orthopyroxene, clinopyroxene,plagioclase, biotite, Fe–Ti oxides and ternary feldspar,indicating crystallization from magmas containing <2 wt %H2O at 1100–900°C. Water enrichment in the residualmelt induced hornblende crystallization at 5 ± 1 kbar,800–600°C. Characterized by a continuum of large ionlithophile element (LILE)-enriched, high field strength element(HFSE)-depleted compositions, the I-type suite resembles moderncontinental arc batholiths in composition and size but not primarymineralogy. Magmatic arcs produced between 2·75 and 1·85Ga commonly have charnockitic components, possibly because slab-derivedfluids interacted with mantle wedges at ambient temperatureshigher by 100°C than at present, producing large volumesof water-deficient magma. KEY WORDS: granitoid rocks; igneous pyroxenes; water-undersaturated magma; charnockite  相似文献   

19.
Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (87Sr/86Srinitial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy  相似文献   

20.
Volcán San Pedro in the Andean Southern Volcanic Zone(SVZ) Chile, comprises Holocene basaltic to dacitic lavas withtrace element and strontium isotope ratios more variable thanthose of most Pleistocene lavas of the underlying Tatara–SanPedro complex. Older Holocene activity built a composite coneof basaltic andesitic and silicic andesitic lavas with traceelement ratios distinct from those of younger lavas. Collapseof the ancestral volcano triggered the Younger Holocene eruptivephase including a sequence of lava flows zoned from high-K calc-alkalinehornblende–biotite dacite to two-pyroxene andesite. Notably,hornblende–phlogopite gabbroic xenoliths in the daciticlava have relatively low 87Sr/86Sr ratios identical to theirhost, whereas abundant quenched basaltic inclusions are moreradiogenic than any silicic lava. The latest volcanism rebuiltthe modern 3621 m high summit cone from basaltic andesite thatis also more radiogenic than the dacitic lavas. We propose thefollowing model for the zoned magma: (1) generation of hornblende–biotitedacite by dehydration partial melting of phlogopite-bearingrock similar to the gabbroic xenoliths; (2) forceful intrusionof basaltic magma into the dacite, producing quenched basalticinclusions and dispersion of olivine and plagioclase xenocryststhroughout the dacite; (3) cooling and crystallization–differentiationof the basalt to basaltic andesite; (4) mixing of the basalticandesite with dacite to form a small volume of two-pyroxenehybrid andesite. The modern volcano comprises basaltic andesitethat developed independently from the zoned magma reservoir.Evolution of dacitic and andesitic magma during the Holoceneand over the past 350 kyr reflects the intrusion of multiplemafic magmas that on occasion partially melted or assimilatedhydrous gabbro within the shallow crust. The chemical and isotopiczoning of Holocene magma at Volcán San Pedro is paralleledby that of historically erupted magma at neighboring VolcánQuizapu. Consequently, the role of young, unradiogenic hydrousgabbro in generating dacite and contaminating basalt may beunderappreciated in the SVZ. KEY WORDS: Andes; dacite; gabbro; Holocene; strontium isotopes  相似文献   

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