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1.
Compositional evolution of the zoned calcalkaline magma chamber of Mount Mazama,Crater Lake,Oregon 总被引:34,自引:10,他引:34
The climactic eruption of Mount Mazama has long been recognized as a classic example of rapid eruption of a substantial fraction of a zoned magma body. Increased knowledge of eruptive history and new chemical analyses of 350 wholerock and glass samples of the climactic ejecta, preclimactic rhyodacite flows and their inclusions, postcaldera lavas, and lavas of nearby monogenetic vents are used here to infer processes of chemical evolution of this late Pleistocene — Holocene magmatic system. The 6845±50 BP climactic eruption vented 50 km3 of magma to form: (1) rhyodacite fall deposit; (2) welded rhyodacite ignimbrite; and (3) lithic breccia and zoned ignimbrite, these during collapse of Crater Lake caldera. Climactic ejecta were dominantly homogeneous rhyodacite (70.4±0.3% SiO2), followed by subordinate andesite and cumulate scoriae (48–61% SiO2). The gap in wholerock composition reflects mainly a step in crystal content because glass compositions are virtually continuous. Two types of scoriae are distinguished by different LREE, Rb, Th, and Zr, but principally by a twofold contrast in Sr content: High-Sr (HSr) and low-Sr (LSr) scoriae. HSr scoriae were erupted first. Trace element abundances indicate that HSr and LSr scoriae had different calcalkaline andesite parents; basalt was parental to some mafic cumulate scoriae. Parental magma compositions reconstructed from scoria wholerock and glass data are similar to those of inclusions in preclimactic rhyodacites and of aphyric lavas of nearby monogenetic vents.Preclimactic rhyodacite flows and their magmatic inclusions give insight into evolution of the climactic chamber. Evolved rhyodacite flows containing LSr andesite inclusions were emplaced between 30000 and 25000 BP. At 7015±45 BP, the Llao Rock vent produced a zoned rhyodacite pumice fall, then rhyodacite lava with HSr andesite inclusions. The Cleetwood rhyodacite flow, emplaced immediately before the climactic eruption and compositionally identical to climactic rhyodacite (volatile-free), contains different HSr inclusions from Llao Rock. The change from LSr to HSr inclusions indicates replenishment of the chamber with andesite magma, perhaps several times, in the latest Pleistocene to early Holocene.Modeling calculations and wholerock-glass relations suggest than: (1) magmas were derived mainly by crystallization differentiation of andesite liquid; (2) evolved preclimactic rhyodacite probably was derived from LSr andesite; (3) rhyodacites contain a minor component of partial melt from wall rocks, and (4) climactic and compositionally similar rhyodacites probably formed by mixing of evolved rhyodacite with HSr derivative liquid(s) after replenishment of the chamber with HSr andesite magma. Density considerations permit a model for growth and evolution of the chamber in which andesite recharge magma ponded repeatedly between cumulates and rhyodacite magma. Convective cooling of this andesite resulted in rapid crystallization and upward escape of buoyant derivative liquid which mixed with overlying, convecting rhyodacite. The evolved rhyodacites were erupted early in the chamber's history and(or) near its margins. Postcaldera andesite lavas may be hybrids composed of LSr cumulates mixed with remnant climactic rhyodacite. Younger postcaldera rhyodacite probably formed by fractionation of similar andesite and assimilation of partial melts of wallrocks.Uniformity of climactic rhyodacite suggests homogeneous silicic ejecta from other volcanoes resulted from similar replenishment-driven convective mixing. Calcalkaline pluton compositions and their internal zonation can be interpreted in terms of the Mazama system frozen at various times in its history. 相似文献
2.
Petrographic and microprobe investigations of calc-alkaline (CA) rocks from the High Cascade Range (i.e., Mt. St. Helens, Mt. Jefferson, Crater Lake and Mt. Shasta) of western North America show that crystal clots represent primary igneous phase assemblages and are not products of amphibole reactions with melt. For each eruptive complex, crystal clots display diverse modal proportions even within a single eruptive unit. Nevertheless, in all cases the crystal-clot minerals are also represented in the rock as phenocrysts or microphenocrysts. Basalts contain clots of ol+plag+mgt, ol+mgt, cpx+ plag+mgt, cpx+mgt and plag+mgt; andesites, clots of cpx+mgt, opx+mgt, cpx+opx+plag+mgt, cpx+plag+mgt, opx+plag+mgt and plag±mgt; and dacites, clots of opx+mgt, cpx+opx+plag+ mgt, opx+plag+mgt, amph+plag+mgt±ilm, amph+mgt±ilm and plag±mgt. The bulk compositions of most of these clot assemblages could not have been derived from amphibole percursors. Although some amphiboles in dacitic rocks display a breakdown reaction of amph=plag+cpx+opx +mag, these mineral clusters, unlike those of clots, typically have a relict amphibole crystal outline and a fine-grained metamorphic texture. Plagioclase grains in the mineral clusters lack oscillatory zoning which is typical of crystal clot plagioclase grains. The euhedral to subhedral shapes of most clot minerals and the oscillatory zoning present in most clot plagioclase grains are not likely to have formed from the breakdown of amphibole. Crystal clots are also observed in Hawaiian and ocean floor basalts, although amphibole fractionation has not been proposed for those lavas. Magnetite fractionation may be the controlling process limiting iron enrichment in CA magmas rather than amphibole fractionation. Textural evidence indicates that magnetite is an early-forming phase in CA magmas. V, which is concentrated in magnetite, shows a strong decrease with increasing silica in many CA rocks, supporting a magnetite fractionation model.Hawaii Institute of Geophysics Contrib. No. 969 相似文献
3.
JANE D. SILLS DIETRICH ACKERMAND RICHARD K. HERD BRIAN F. WINDLEY 《Journal of Metamorphic Geology》1983,1(4):337-351
Sapphirine occurs in a 3-5 m wide zone between amphibole-lherzolite and garnetiferous metagabbro at Finero in the Ivrea Zone, NW Italian Alps. Layers consisting of plag + hb + sa + cpx + opx + sp + gt are interbanded with spinel pyroxenites, which may contain sapphirine replacing spinel. All minerals are very magnesian, with XMg between 0.78 and 0.92. Bulk rock analyses suggest that precursors to the sapphirine-bearing rocks were igneous cumulates of plagioclase + olivine + hornblende + spinel. Up to 16wt% CaO does not inhibit sapphirine formation and it is the unusually Mg-rich nature of the host rocks which allows sapphirine development. The early igneous assemblage was replaced by one of cpx + sa + hb +± plag at a pressure of 9 ± 1 kbar and temperatures of 900 ± 50°C. Subsequent rapid uplift caused the instability of gt, gt + hb, hb and sa + cpx to form opx + plag ± sp ± sa symplectites. 相似文献
4.
Minor- and trace-element zoning in plagioclase: implications for magma chamber processes at Parinacota volcano, northern Chile 总被引:6,自引:1,他引:6
Catherine Ginibre Gerhard Wörner Andreas Kronz 《Contributions to Mineralogy and Petrology》2002,143(3):300-315
Textural and compositional zoning in plagioclase phenocrysts in a sample from Parinacota volcano (Chile) was investigated using backscattered electron images and electron microprobe analysis of major and trace elements. Large (2 mm) oscillatory zoned crystals (type I) with resorption surfaces of moderate An discontinuities (Ⲓ% An) and decreasing trace-element contents (Sr, Mg, Ti) towards the rim reflect melt differentiation and turbulent convection in the main magma body. Early recharge with a low-Sr mafic magma is seen in the core. Small-scale Sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years. Smaller crystals (type II) with low trace-element/An ratio reflect the influence of an H2O-rich melt probably from a differentiated boundary layer. Closed-system in-situ crystallisation, mafic magma recharge and the role of a water-rich differentiated boundary layer can be distinguished from the An-trace element relationships. Crystals apparently move relatively freely between different parts and regimes in the magma chamber, evidence for "convective crystal dispersion". High-Sr type II crystals indicate an earlier input of Sr-rich mafic magma. Recharge of two distinct mafic magma types is thus identified (high-Sr and low-Sr), which must have been present - at increasing recharge rates with time - in the plumbing system throughout the volcano's history. 相似文献
5.
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 crystalmeltmush 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 6570to 相似文献
6.
Hornblende dehydration reactions during magma ascent at Soufrière Hills Volcano, Montserrat 总被引:1,自引:1,他引:0
V. J. E. Buckley R. S. J. Sparks B. J. Wood 《Contributions to Mineralogy and Petrology》2006,151(2):121-140
Hornblende phenocrysts in recent andesites of the Soufrière Hills Volcano display reaction rims of microcrystalline plagioclase,
pyroxene, Fe-oxides and interstitial glass, formed by decompression during magma ascent. Mass balance calculations give the
following reactions with mineral proportions in agreement with modal abundances:
These reactions require an open chemical system with exchange of selected components with surrounding melt. Volatiles, TiO2 and alkalis are expelled and SiO2 and FeOT are consumed. Matrix glasses fall into two compositional groups. Glasses in pumice are relatively rich in CaO and poor in
K2O and Na2O compared to glasses in dome samples. The former glasses formed by moderate amounts of groundmass crystallisation of plagioclase,
associated with rapid magma ascent in explosive eruptions. The later glasses evolved in response to hornblende breakdown,
groundmass crystallisation and mixing of melts from different levels during slow magma ascent and extended residence time
in the dome. Interstitial glass compositions in reaction rims reflect the compositions of the surrounding matrix glasses,
but show variable compositional differences mostly consistent with the proposed open-system reactions. 相似文献
7.
Magmatic inclusions in phenocrystals from andesitic lavas,Krakatau volcano,Indonesia 总被引:1,自引:0,他引:1
There is a great number of magmatic inclusions in different types of phenocrystals in andesitic lavas from the Krakatau volcano,
Indonesia. Sequence of crystallization (from early to late) has been established for the phenocrystals on the basis of homogenization
temperatures and chemical compositions of the magmatic inclusions: the cantral phase of plagioclase phenocrystals (An 83.2–72.6),
olivine→ the intermediate phase of plagioclase phenocrystals (An 61.3), clinopyroxene, titanomagnetite→ the peripheral phase
of plagioclase phenocrystals (An 54.4–42.6). The crystallization of the central phase of plagioclase phenocrystals takes place
in the depth of magma chamber at about 5 Kbar. High pressures and high titanium contents favor the silicate-liquid immiscibility
in the magmatic inclusions of early plagioclase phenocrystals. This is typical of many andesitic lavas generated in subduction
regions. 相似文献
8.
Heather Michelle Wright Charles R. Bacon Jorge A. Vazquez Thomas W. Sisson 《Contributions to Mineralogy and Petrology》2012,164(6):1027-1052
The well-documented eruptive history of Mount Mazama, Oregon, provides an excellent opportunity to use pre-eruptive volatile concentrations to study the growth of an explosive silicic magmatic system. Melt inclusions (MI) hosted in pyroxene and plagioclase crystals from eight dacitic–rhyodacitic eruptive deposits (71–7.7?ka) were analyzed to determine variations in volatile-element concentrations and changes in magma storage conditions leading up to and including the climactic eruption of Crater Lake caldera. Temperatures (Fe–Ti oxides) increased through the series of dacites, then decreased, and increased again through the rhyodacites (918–968 to ~950 to 845–895?°C). Oxygen fugacity began at nickel–nickel-oxide buffer (NNO) +0.8 (71?ka), dropped slightly to NNO +0.3, and then climbed to its highest value with the climactic eruption (7.7?ka) at NNO +1.1 log units. In parallel with oxidation state, maximum MI sulfur concentrations were high early in the eruptive sequence (~500?ppm), decreased (to ~200?ppm), and then increased again with the climactic eruption (~500?ppm). Maximum MI sulfur correlates with the Sr content (as a proxy for LREE, Ba, Rb, P2O5) of recharge magmas, represented by basaltic andesitic to andesitic enclaves and similar-aged lavas. These results suggest that oxidized Sr-rich recharge magmas dominated early and late in the development of the pre-climactic dacite–rhyodacite system. Dissolved H2O concentrations in MI do not, however, correlate with these changes in dominant recharge magma, instead recording vapor solubility relations in the developing shallow magma storage and conduit region. Dissolved H2O concentrations form two populations through time: the first at 3–4.6 wt% (with a few extreme values up to 6.1 wt%) and the second at ≤2.4 wt%. CO2 concentrations measured in a subset of these inclusions reach up to 240?ppm in early-erupted deposits (71?ka) and are below detection in climactic deposits (7.7?ka). Combined H2O and CO2 concentrations and solubility models indicate a dominant storage region at 4–7?km (up to 12?km), with drier inclusions that diffusively re-equilibrated and/or were trapped at shallower depths. Boron and Cl (except in the climactic deposit) largely remained in the melt, suggesting vapor–melt partition coefficients and gas fractions were low. Modeled Li, F, and S vapor–melt partition coefficients are higher than those of B and Cl. The decrease in maximum MI CO2 concentration following the earliest dacitic eruptions is interpreted to result from a broadening of the shallow storage region to greater than the diameter of subjacent feeders, so that greater proportions of reservoir magma were to the side of CO2-bearing vapor bubbles ascending vertically from the locus of recharge magma injection, thereby escaping recarbonation by streaming vapor bubbles. The Mazama melt inclusions provide a picture of a growing magma storage region, where chemical variations in melt and magma occur due to changes in the nature and supply rate of magma recharge, the timing of degassing, and the possible degree of equilibration with gases from below. 相似文献
9.
Origin of Phenocrysts and Compositional Diversity in Pre-Mazama Rhyodacite Lavas, Crater Lake, Oregon 总被引:3,自引:3,他引:0
Phenocrysts in porphyritic volcanic rocks may originate in avariety of ways in addition to nucleation and growth in thematrix in which they are found. Porphyritic rhyodacite lavasthat underlie the eastern half of Mount Mazama, the High Cascadeandesite/dacite volcano that contains Crater Lake caldera, containevidence that bears on the general problem of phenocryst origin.Phenocrysts in these lavas apparently formed by crystallizationnear the margins of a magma chamber and were admixed into convectingmagma before eruption. About 20 km3 of pre-Mazama rhyodacite magma erupted during arelatively short period between400 and 500 ka; exposed pre-Mazamadacites are older and less voluminous. The rhyodacites formedas many as 40 lava domes and flows that can be assigned to threeeruptive groups on the basis of composition and phenocryst content.Phenocryst abundance decreases (from 32 to 8 vol.%) and SiO2content increases (from 68 to 73 wt.%) in the apparent orderof eruption. Phenocrysts (plagioclase, orthopyroxene, augite,and Fe-Ti oxides) are commonly fragmental or form polycrystallineaggregates with interstitial glass. Discrete phenocrysts withcomplete euhedral outlines are rare except for small elongatedcrystals. The abundance of discrete phenocrysts increases withthat of aggregates. The grain-size of minerals in the aggregatescovers the range of discrete phenocrysts (0.24.2 mm).Rim compositions of phenocrysts and the range of chemical zoningare almost uniform among the three rhyodacite groups, regardlessof whether crystals are discrete or in aggregates. However,a small fraction of phenocrysts, especially small elongatedcrystals, have different compositions: plagioclase with Fe-richcores and augite with Wo-poor cores, both of which are characteristicof crystals in undercooled andesite enclaves in the rhyodacites.The majority of phenocrysts were derived by disintegration ofpolycrystalline aggregates; rare, small phenocrysts crystallizedin andesitic magma similar to that represented by the andesiteenclaves. The modal and chemical compositions of the rhyodacites can beexplained by different degrees of admixing of crystals, representedby the aggregates, into magma having 4 vol.% truephenocrysts, mainly plagioclase. The aggregates may be partsof the rind formed by in situ crystallization near the walland roof of the magma chamber. The rind was disrupted duringor just before eruption, and pieces were variably disaggregatedand incorporated into erupting magma. The amount of rind incorporateddeclined during the sequence of eruptions. Owing to vesiculationof interstitial liquid and shearing during flow, crystals inthe aggregates were separated and became phenocrysts. Pre-Mazamarhyodacite was erupted dominantly as lava, as opposed to thecompositionally similar rhyodacite pumice of the Holocene caldera-formingeruption of Mount Mazama, apparently because its source chamberwas crystallizing inward rather than actively growing. 相似文献
10.
D. J. Smith M. G. Petterson A. D. Saunders I. L. Millar G. R. T. Jenkin T. Toba J. Naden J. M. Cook 《Contributions to Mineralogy and Petrology》2009,158(6):785-801
Savo, Solomon Islands, is a historically active volcano dominated by sodic, alkaline lavas, and pyroclastic rocks with up
to 7.5 wt% Na2O, and high Sr, arc-like trace element chemistry. The suite is dominated by mugearites (plagioclase–clinopyroxene–magnetite ± amphibole ± olivine)
and trachytes (plagioclase–amphibole–magnetite ± biotite). The presence of hydrous minerals (amphibole, biotite) indicates
relatively wet magmas. In such melts, plagioclase is relatively unstable relative to iron oxides and ferromagnesian silicates;
it is the latter minerals (particularly hornblende) that dominate cumulate nodules at Savo and drive the chemical differentiation
of the suite, with a limited role for plagioclase. This is potentially occurring in a crustal “hot zone”, with major chemical
differentiation occurring at depth. Batches of magma ascend periodically, where they are subject to decompression, water saturation
and further cooling, resulting in closed-system crystallisation of plagioclase, and ultimately the production of sodic, crystal
and feldspar-rich, high-Sr rocks. The sodic and hydrous nature of the parental magmas is interpreted to be the result of partial
melting of metasomatised mantle, but radiogenic isotope data (Pb, Sr, Nd) cannot uniquely identify the source of the metasomatic
agent. 相似文献
11.
Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas 总被引:4,自引:0,他引:4
PERINI GIULIA; FRANCALANCI LORELLA; DAVIDSON JON P.; CONTICELLI SANDRO 《Journal of Petrology》2004,45(1):139-182
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 = 0·710370·71115)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% FeTi 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 1029%clinopyroxene + 1215% plagioclase + 4467% sanidine+ 24% phlogopite + 13% apatite + 710% FeTioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·708120·70948) were derived from a potassictrachybasalt by assimilationfractional crystallizationwith 2040% of solid removed and r = 0·40·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·710440·71052) 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 veinperidotite/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
12.
Garnet Granulite Xenoliths from the Northern Baltic Shield--the Underplated Lower Crust of a Palaeoproterozoic Large Igneous Province? 总被引:3,自引:0,他引:3
KEMPTON P. D.; DOWNES H.; NEYMARK L. A.; WARTHO J. A.; ZARTMAN R. E.; SHARKOV E. V. 《Journal of Petrology》2001,42(4):731-763
Garnet granulite facies xenoliths hosted in Devonian lamprophyresfrom the Kola Peninsula are interpreted to represent the high-grademetamorphic equivalents of continental flood tholeiites, emplacedinto the Baltic Shield Archaean lower crust in early Proterozoictime. Geochronological data and similarities in major and traceelement geochemistry suggest that the xenoliths formed duringthe same plume-related magmatic event that created a widespreadPalaeoproterozoic large igneous province (LIP) at 2·42·5Ga. They are, thus, the first samples of the lower crust ofa Palaeoproterozoic LIP to be studied in petrological detail.The suite includes mafic granulites (gar + cpx + rutile ±plag ± opx ± phlog ± amph), felsic granulites(plag + gar + cpx + rutile ± qtz ± Kspar ±phlog ± amph) and pyroxenites (± phlog ±amph), but mafic garnet granulites predominate. Although somesamples are restites, there is no evidence for a predominanceof magmatic cumulates, as is common for Phanerozoic lower-crustalxenolith suites. Metasediments are also absent. Phlogopite and/oramphibole occur in xenoliths of all types and are interpretedto be metasomatic in origin. The K-rich metasomatic event occurredat 相似文献
13.
Petrologic and experimental evidence for the movement and heating of the pre-eruptive Minoan rhyodacite (Santorini, Greece) 总被引:1,自引:1,他引:0
Elizabeth Cottrell James E. Gardner Malcolm J. Rutherford 《Contributions to Mineralogy and Petrology》1999,135(4):315-331
Hydrothermal experiments combined with petrologic observations form the basis for a new two-stage model for the evolution
of the pre-eruption Minoan magma chamber at Santorini, Greece. Ninety-nine percent of the erupted volume is two-pyroxene,
rhyodacitic magma that had been stored at a temperature of ∼885 °C, based on magnetite-ilmenite and QUILF geothermometry.
The rest of the volume is basaltic to andesitic magma, which occurs as <10 cm scoria clasts and as small inclusions in rhyodacite
pumices. Petrologic observations show that these magmas mixed at different scales and at different times (i.e., multiple batches
of mafic magma). Hydrothermal experiments were carried out on samples of rhyodacite and a mafic scoria in order to determine
magma storage conditions and the mixing history of the two magmas. At 885 °C, the rhyodacite must have been stored at water-saturated
pressures of ∼50 MPa, based on its phase assemblage, matrix-glass composition, and crystal content. However, glass inclusions
inside rhyodacitic plagioclase phenocrysts contain more than 6 wt% H2O, indicating they formed at pressures >200 MPa. In addition, the composition of the plagioclase hosts (An56 ± 6) of the inclusions require temperatures of 825 ± 25 °C at pressures >200 MPa. This demonstrates that the Minoan rhyodacitic
magma underwent a two-stage evolution, first crystallizing at ∼825∘C and >200 MPa, and then rinsing to a shallow ∼50 MPa storage region with a concomitant rise in temperature to ∼885 °C. We
suggest that the episodic intrusion of mafic magmas provided the necessary heat and perhaps contributed to the ascent of the
magma to shallow crustal depths where it reequilibrated before the cataclysmic eruption. Phase equilibria suggest that much
of the heating of the rhyodacite occurred in the shallow storage region. Thermal budget calculations suggest that the rhyodacite
magma could have been heated by intrusions of basalt rising at reasonable upwelling rates and injected into the storage zone
over several hundred years. Preservation of amphibole in the mafic scoria indicate that injection of mafic magma continued
up until days before the cataclysmic eruption, perhaps triggering the event.
Received: 30 September 1997 / Accepted: 5 October 1998 相似文献
14.
The Petrology and Geochemistry of the Aniakchak Caldera-forming Ignimbrite, Aleutian Arc, Alaska 总被引:3,自引:0,他引:3
Aniakchak caldera, Alaska, produced a compositionally heterogeneousignimbrite 3400 years ago, which changes from rhyodacitic atthe base to andesitic at the top of the eruptive sequence. Interpretationsof compositionally heterogeneous ignimbrites typically includeeither in situ fractional crystallization of mafic magma andgeneration of a stratified magma body or replenishment of asilicic magma chamber by mafic inputs. Another possibility,silicic replenishment of a more mafic chamber, exists. Geochemicalcharacteristics of the caldera-forming rhyodacite and severallate pre-caldera rhyodacites indicate independent origins foreach, within a maximum of 5000 years prior to caldera formation.Isotopic considerations preclude derivation of the caldera-formingrhyodacite from the caldera-forming andesite. However, the caldera-formingrhyodacite can be explained as the residual liquid of a mostlycrystallized basalt, with addition of crustal material. TheAniakchak andesite probably formed in a shallow chamber by successivemixing events involving small volumes of basalt and rhyodacite,together with contamination. The pre-caldera rhyodacites representerupted portions of intruding silicic magma, whereas anotherportion homogenized with the resident mafic magma. The caldera-formingevent reflects a large influx of rhyodacite, which erupted beforesignificant mixing occurred and also triggered draining of muchof the andesitic magma from the chamber. KEY WORDS: Aniakchak; caldera-forming eruption; geochemistry; ignimbrite; silicic replenishment 相似文献
15.
Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene,
clinopyroxene, olivine, titanomagnetite, magnesio-hornblende and rare quartz grains. In this study, we use the results of
mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase,
the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’
or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt
conditions in a dynamic magma chamber. ‘Fritted’ zones (An55) are frequently overgrown by thin calcic (An72) plagioclase rims over well-developed dissolution surfaces. These features have probably resulted from mixing of a more silicic
magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes.
We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes – initial intrusion
of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage. 相似文献
16.
Multiple Isotopic Components in Quaternary Volcanic Rocks of the Cascade Arc near Crater Lake, Oregon 总被引:2,自引:1,他引:1
BACON CHARLES R.; GUNN SUSAN H.; LANPHERE MARVIN A.; WOODEN JOSEPH L. 《Journal of Petrology》1994,35(6):1521-1556
Quaternary lavas and pyroclastic rocks of Mount Mazama, CraterLake caldera, and the surrounding area have variable Sr, Nd,and Pb isotopic compositions. High-alumina olivine tholeiites(HAOT) have 87Sr/86Sr ratios of 0.703460.70364; basalticandesite, 0703490.70372; shoshonitic basalticandesite, 0.703740.70388; and andesite, 0.703240.70383.Dacites of Mount Mazama have 87Sr/86Sr ratios of 0.703480.70373.Most rhyodacites converge on 0.7037. However, rhyodacite ofthe caldera-forming, climactic eruption has 87Sr/86Sr=0.70354because of an admixed low-87Sr/86Sr component. Andesitic tomafic-cumulate scoriae of the climactic eruption, and enclavesin preclimactic rhyodacites, cluster in two groups but shownearly the entire 87Sr/86Sr range of the data set, confirmingpreviously suggested introduction of diverse parental magmasinto the growing climactic chamber. Pb and Nd isotope ratiosdisplay less variation (206Pb/204Pb= 18.83818.967, 207Pb/204Pb=15.55615.616,208Pb/204Pb=38.40538.619; Nd= +3.9 to +6.1) and generallycovary with 87Sr/86 Sr ratios. Radiogenic isotope data fromCrater Lake plot with published data for other Cascade volcanoeson isotope ratio correlation diagrams. The isotopic data for the Crater Lake area require sources ofprimitive magmas to consist of depleted mantle and a subductioncomponent, introduced in variable quantity to the depleted mantlewedge. Variable degrees of melting of this heterogeneous mantle,possibly at different depths, produced the diversity of isotopiccompositions and large-ion lithophile element (LILE) abundancesin primitive magmas. Trace element ratios do not indicate presenceof an ocean island basalt (OIB) source component that has beenreported in lavas of some other Cascade volcanoes. Crustal contamination may have affected isotope ratios and LILEconcentrations in evolved HAOT, where initial LILE concentrationswere low. Contamination is more difficult to detect in the calcalkalinelavas because of their higher LILE concentrations and the smallisotopic contrast with likely contaminants, such as mid- tolower-crustal rocks thought to be equivalents of igneous rocksof the Klamath Mountains and associated lower crust. Crustalassimilation appears to be required for calcalkaline rocks onlyby 18O values, which vary from lows of +5.6 to + 6.0% in HAOTand primitive basaltic andesites to a high of +7.0% in dacite,a range that is too high to be explained by plagioclase-dominatedclosed-system fractional crystallization. Elevated 18O valuesof differentiated lavas may be attributed to interaction withrelatively 18O-rich, 87Sr-poor crustal rocks. Variably fused granitoid blocks ejected in the climactic eruption,and rarely in late Pleistocene eruptive units, have 18Opl of3.4 to +6.5% and 18Oqz of 2.2 to +8.0% but haveSr, Nd, and Pb isotope ratios similar to volcanic rocks (e.g.87Sr/86Sr0.7037). Rb and Sr data for glass separates from granodioritessuggest that the source pluton is Miocene. Glass from granodioritehas 87Sr/86Sr ratios as high as 0.70617. Oxygen isotope fractionationbetween quartz, plagioclase, and glass indicates requilibrationof O isotopes at magmatic temperatures, after 18O/16O had beenlowered by exchange with meteoric hydrothermal fluids. Unmeltedgranodiorite xenoliths from pre-climactic eruptive units have18O values that are consistent with onset of hydrothermal exchangeearly during growth of the climactic magma chamber. Assimilationof such upper-crustal granodiorite apparently lowered 18O valuesof rhyodacites without significantly affecting their magmaticcompositions in other ways. 相似文献
17.
The rhyodacitic magma discharged during the 30–80 km3 DRE (dense rock equivalent) Late Bronze Age (LBA; also called ‘Minoan’) eruption of Santorini caldera is known from previous studies to have had a complex history of polybaric ascent and storage prior to eruption. We refine the timescales of these processes by modelling Mg–Fe diffusion profiles in orthopyroxene and clinopyroxene crystals. The data are integrated with previously published information on the LBA eruption (phase equilibria studies, melt inclusion volatile barometry, Mg-in-plagioclase diffusion chronometry), as well as new plagioclase crystal size distributions and the established pre-LBA history of the volcano, to reconstruct the events that led up to the assembly and discharge of the LBA magma chamber. Orthopyroxene, clinopyroxene and plagioclase crystals in the rhyodacite have compositionally distinct rims, overgrowing relict, probably source-derived, more magnesian (or calcic) cores, and record one or more crystallization (plag???opx?>?cpx) events during the few centuries to years prior to eruption. The crystallization event(s) can be explained by the rapid transfer of rhyodacitic melt from a dioritic/gabbroic region of the subcaldera pluton (mostly in the 8–12 km depth range), followed by injection, cooling and mixing in a large melt lens at 4–6 km depth (the pre-eruptive magma chamber). Since crystals from all eruptive phases yield similar timescales, the melt transfer event(s), the last of which took place less than 2 years before the eruption, must have involved most of the magma that subsequently erupted. The data are consistent with a model in which prolonged generation, storage and segregation of silicic melts were followed by gravitational instability in the subcaldera pluton, causing the rapid interconnection and amalgamation of melt-rich domains. The melts then drained to the top of the pluton, at fluxes of up to 0.1–1 km3 year??1, where steep vertical gradients of density and rheology probably caused them to inject laterally, forming a short-lived holding chamber prior to eruption. This interpretation is consistent with growing evidence that some large silicic magma chambers are transient features on geological timescales. A similar process preceded at least one earlier caldera-forming eruption on Santorini, suggesting that it may be a general feature of this rift-hosted magmatic system. 相似文献
18.
The aim of this study is to quantify the crustal differentiation processes and sources responsible for the origin of basaltic
to dacitic volcanic rocks present on Cordón El Guadal in the Tatara-San Pedro Complex (TSPC). This suite is important for
understanding the origin of evolved magmas in the southern Andes because it exhibits the widest compositional range of any
unconformity-bound sequence of lavas in the TSPC. Major element, trace element, and Sr-isotopic data for the Guadal volcanic
rocks provide evidence for complex crustal magmatic histories involving up to six differentiation mechanisms. The petrogenetic
processes for andesitic and dacitic lavas containing undercooled inclusions of basaltic andesitic and andesitic magma include:
(1) assimilation of garnet-bearing, possibly mafic lower continental crust by primary mantle-derived basaltic magmas; (2)
fractionation of olivine + clinopyroxene + Ca-rich plagioclase + Fe-oxides in present non-modal proportions from basaltic
magmas at ∼4–8 kbar to produce high-Al basalt and basaltic andesitic magmas; (3) vapor-undersaturated (i.e., P
H2O<P
TOTAL) partial melting of gabbroic crustal rocks at ∼3–7 kbar to produce dacitic magmas; (4) crystallization of plagioclase-rich
phenocryst assemblages from dacitic magmas in shallow reservoirs; (5) intrusion of basaltic andesitic magmas into shallow
reservoirs containing crystal-rich dacitic magmas and subsequent mixing to produce hybrid basaltic andesitic and andesitic
magmas; and (6)␣formation and disaggregation of undercooled basaltic andesitic and andesitic inclusions during eruption from
shallow chambers to form commingled, mafic inclusion-bearing andesitic and dacitic lavas flows. Collectively, the geochemical
and petrographic features of the Guadal volcanic rocks are interpreted to reflect the development of shallow silicic reservoirs
within a region characterized by high crustal temperatures due to focused basaltic activity and high magma supply rates. On
the periphery of the silicic system where magma supply rates and crustal temperatures were lower, cooling and crystallization
were more important than bulk crustal melting or assimilation.
Received: 2 July 1997 / Accepted: 25 November 1997 相似文献
19.
Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones 总被引:10,自引:0,他引:10
Huai-Jen Yang Frederick A. Frey David A. Clague Michael O. Garcia 《Contributions to Mineralogy and Petrology》1999,135(4):355-372
The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record
episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo
Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The
compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these
samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each
sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences
between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts
with similar reverse zonation.
The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene)
saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of
plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma
chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene
with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite +
plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene
phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment
with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in
equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg#
of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the
shield building stage, but may commonly form in ephemeral magma pockets in the rift zones.
The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated
at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher
pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of
magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary.
Received: 8 July 1998 / Accepted: 2 January 1999 相似文献
20.
The Edgecumbe volcanic field is a Holocene volcanic province located on Kruzof Island, SE Alaska. Exposed within the 260 km2 field are basalt, andesitic basalt, andesite, dacite and rhyodacite. The rhyodacites were erupted after the basalts and before
the andesites. The volcanics, which are Al-rich (14–18 wt%) and lack an iron enrichment trend, range from tholeiites (47 wt%
SiO2) through rhyodacites (72%), but a compositional gap of approximately 9 wt% separates the dacites and rhyodacites. Initial
87Sr/86Sr ranges from 0.70297 in the basalts to 0.70440 in a pyroxene andesite. δ
18O increases across the suite: 5.8‰ to 7.9‰. Plagioclase (An32–86) is the dominant phenocryst in all but one lava. Olivine (Fo58–86) occurs in the basic lavas (<53 wt% SiO2), but is replaced by orthopyroxene (En43–73) and clinopyroxene (En31Wo41-En48Wo40) in the more siliceous volcanics. In the basalts and rhyodacites, plagioclase is weakly zoned, but extreme zoning (<30 mole%
An) is characteristic of phenocrysts in the intermediate lavas. Fractionation of the observed phenocryst assemblages could
not have produced the more silicous volcanics. Instead they were generated by partial melting of intrusive basement (87Sr/86Sr=0.70487; δ
18O: 8.7–9.3) by basaltic magma and subsequent assimilation. Mass balance calculations show the rhyodacites are almost pure
partial melt (<5% basaltic component) whereas the intermediate lavas contain between 30 and 60% partial melt. 相似文献