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1.
Manam volcano consists of relatively mafic and compositionallysimilar tholeiitic basalts and low-SiO2 andesites that are characterizedby notably low (mainly 0?3–0?35 weight per cent) TiO2contents. These rocks provide an ideal opportunity to investigateboth the extent of depletion in their peridotite magma-sourceregions (which are evidently similar in many respects to thehighly depleted sources of boninitic magmas), and the interplayof the high-level processes of magma mixing, crystal fractionation,and upper crustal contamination, in an island-are volcano. Manamrocks have pronounced enrichments in Rb, Ba, K, and Sr relativeto the light rare-earth elements and, especially, to the high-field-strengthelements (Sr/Ti values are exceptionally high). However, thereis no compelling evidence that these enrichments were causedby addition of a hydrous, slab-derived component to the peridotitesource region. Nd and Sr-isotope ratios plot within the oceanicmantle array; 207Pb/204Pb values are only slightly higher thanthose for oceanic rocks; and the absence of hydrous minerals,the early crystallization and modal preponderance of plagioclaseover pyroxene, high estimated quenching temperatures, and lowwater contents in the Manam rocks, are all evidence that themagmas crystallized under markedly water-undersaturated conditions.Unusually anorthite-rich plagioclase phenoerysts in the morediffrentiated rocks may correspond to crystallization underhigher water-vapour pressures, possibly caused by influxes ofgroundwater, or they may be accidental xenocrysts. Fractionationof olivine, clinopyroxene, and spinel (early chromite followedby magnetite) has dominated the evolution of the magma series.However, clear correlations between incompatible trace-elementratios, 87Sr/86Sr, and 100 Mg/(Mg ? Fe2?) values are convincingevidence for an accompanying mixing process—either of(1) two basaltic magma types (one more fractionated and lowerin 87Sr/86Sr than the other), or (2) pristine magmas and contaminantfrom basaltic conduit and reservoir wall rocks. Wall-rock contaminationis the less likely process, and is the more difficult one toidentify, particularly if it accompanied magma mixing.  相似文献   

2.
K-feldspar megacrysts (Kfm) are used to investigate the magmaticevolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba,Italy). Dissolution and regrowth of Kfm during magma mixingor mingling events produce indented resorption surfaces associatedwith high Ba contents. Diffusion calculations demonstrate thatKfm chemical zoning is primary. Core-to-rim variations in Ba,Rb, Sr, Li and P support magma mixing (i.e. high Ba and P andlow Rb/Sr at rims), but more complex variations require othermechanisms. In particular, we show that disequilibrium growth(related to variations in diffusion rates in the melt) may haveoccurred as a result of thermal disturbance following influxof mafic magma in the magma chamber. Initial 87Sr/86Sr ratios(ISr) (obtained by microdrilling) decrease from core to rim.Inner core analyses define a mixing trend extending towardsa high ISr–Rb/Sr melt component, whereas the outer coresand rims display a more restricted range of ISr, but a largerrange of Rb/Sr. Lower ISr at the rim of one megacryst suggestsmixing with high-K calc-alkaline mantle-derived volcanics ofsimilar age on Capraia. Trace element and isotopic profilessuggest (1) early megacryst growth in magmas contaminated bycrust and refreshed by high ISr silicic melts (as seen in theinner cores) and (2) later recharge with mafic magmas (as seenin the outer cores) followed by (3) crystal fractionation, withpossible interaction with hydrothermal fluids (as seen in therim). The model is compatible with the field occurrence of maficenclaves and xenoliths. KEY WORDS: Elba; monzogranite; K-feldspar megacrysts; zoning; magma mixing; trace element; Sr isotopes; petrogenesis  相似文献   

3.
A detailed study of the pyroclastic deposits of the AD 79 ‘Pompei’Plinian eruption of Vesuvius has allowed: (1) reconstructionof the thermal, compositional and isotopic (87Sr/86Sr) pre-eruptivelayering of the shallow magma chamber; (2) quantitative definitionof the syn-eruptive mixing between the different magmas occupyingthe chamber, and its relationships with eruption dynamics; (3)recognition of the variability of mafic magma batches supplyingthe chamber. During the different phases of the eruption 25–30%of the magma was ejected as white K-phonolitic pumice, and 70–75%as grey K-tephri-phonolitic pumice. The white pumice resultsfrom the tapping of progressively deeper magma from a body (T= 850–900%C) consisting of two distinct layers mainlyformed by crystal fractionation. The grey pumice results fromsyn-eruptive mixing involving three main end-members: the phonolitic‘white’ magmas (salic end-member, SEM), mafic cumulates(cumulate end-member, CEM) and a crystal-poor ‘grey’phono-tephritic magma (mafic end-member, MEM), which was nevererupted without first being mixed with ‘white’ magma.Evidence is provided that mixing occurred within the chamberand was characterized by a transition with time from physicalmixing at a microscopic scale to chemical hybridization. TheMEM magma had a homogeneous composition and constant 87Sr86Srisotopic ratio, possibly as a result of sustained convection.No unambiguous liquidus phases were found, suggesting that theMEM magma was superheated (T = 1000–1100C); its verylow viscosity was a main cause in the establishment of a physicaldiscontinuity separating the white and the grey magmas. Thewhite-grey boundary layer possibly consisted of a multiply diffusiveinterface, periodically broken and recreated, supplying thephonolitic body through mixing of moderate amounts of fractionatedgrey melts with the overlying white magma. The presence of alarge overheated mass indicates the young, growing stage ofthe AD 79 chamber, whose main engine was the periodic arrivalof hot mafic magma batches. These were characterized by K-tephriticto K-basanitic compositions, high temperatures (>1150C),high volatile contents (20–25% H2O +Cl+F+S), low viscosities[(1+2 102 poises)] and relatively low densities (2500–2600kg/m3). The birth of the Pompei chamber followed the repeatedarrival of these batches (on average characterized by 87Sr/86Sr070729)into a reservoir containing a tephriticphonolitic, crystal-enriched,magma, a residue from the preceding ‘Avellino’ Plinianeruption (3400 BP).In fact, about half of magma ejected duringthe AD 79 eruption could have been inherited from pre-Avellinotimes. KEY WORDS: Vesuvius; magma chamber; magma mixing; compositional layering phonolites; magma supply; potassic magmas *Correponding author  相似文献   

4.
TAMURA  Y.; NAKAMURA  E. 《Journal of Petrology》1996,37(6):1307-1319
New Sr and Nd isotopic data are presented and integrated withprevious data for the Shirahama Group Mio-Pliocene medium-Kvolcanic are suite of south-central Honshu, Japan. Main resultsare: (1) The Shirahama lavas range in 87Sr/86Sr from 0.70315to 0.70337 and in 143Nd/144Nd from 0.51298 to 0.51306; the Srand Nd isotopic data cluster tightly within the mantle array,and all lie within an overlapping field of mid-ocean ridge basaltand ocean island basalt; (2) small differences exist among theShirahama tholeiitic series, calc-alkaline series and mixedlavas. The present isotopic data are consistent with a previouslypublished model, which proposes that chemical variations inmagmas of coexisting tholeiitic and calc-alkaline series areproduced through crystal fractionation from mantle-derived magmasof basalt and magnesian andesite, respectively. Moreover, thetholeiitic series and the calc-alkaline series are isotopicallyidentical. Thus, both magma series can be derived from a sourcemantle with the same isotopic composition, supporting the hypothesisof simultaneous generation of basalt and magnesian andesitemagmas from a single diapir rising through the mantle wedgeabove the subduction zone. The differences of water contentand temperature within the diapir are again thought to havebeen produced through dehydration and heating of an isotopicallyhomogeneous hydrous diapir. The isotopic data show that thehigh-SiO2 lavas have the same isotopic compositions as moremafic lavas. These data and liquid lines of descent of the Shirahamamagmas suggest that even rhyolites can be produced by differentiationfrom mantle-derived magmas without crustal contamination. Analysesfrom 38 other arc volcanoes have been compiled to investigatethe intravolcano variability of 87Sr/86Sr. Twelve of these displayno intravolcano strontium isotopic variability, as is the casewith the Shirahama Group, but others show greater variationof 87Sr/86Sr from individual volcanic centers, presumably reflectingcrustal contamination. Most of the latter volcanoes are underlainby thick continental crust. It is noteworthy, however, thatthe greater variations of 87Sr/86Sr correlate with SiO2 content;andesites or dacites, not basalts, from the same volcano havethe lowest 87Sr/86Sr, and these rocks are calc-alkaline in termsof FeO*/MgO and SiO2 Theoretically, assimilation of continentalcrust by the isotopically uniform Shirahama magmas could producethese relationships. Given that mantle-derived basalt and magnesianandesite both encounter continental crust on their ascent tothe surface, the hotter basalt magma would assimilate more crustalwallrocks than the cooler andesite, resulting in the basaltbeing more radiogenic. Fractional crystallization, magma mixing,and/or assimilation-fractional crystallization of these magmasin crustal magma chambers could produce large compositionalvariations, but the derivatives of the hotter basaltic magmas(tholeiitic series in the broad sense) would display greatercontamination than those derived from the cooler andesitic magmas(calc-alkaline series). *Telephone: 81-858-43-1215. Fax: 81-858-43-2184. e-mail: tamura{at}misasa.okayam-u.ac.jp  相似文献   

5.
Chemical and Sr isotopic zoning patterns in plagioclase megacrysts from gabbroic dykes in the Gardar Province can be used to elucidate magma-chamber and emplacement processes. The megacrysts occur either as single crystals or assembled as anorthosite xenoliths. The size of the megacrysts varies from <1 cm to 1 m. They consist of a large core with variable zonation (An58-39) and a relatively small (<600 µm), normally zoned rim (An62-27). The contact between core and rim is sharp and marked by a sharp increase in anorthite content which can reach 11 mol% An. This gap is interpreted as having formed during dyke emplacement due to a sudden pressure release. Some of the megacryst cores show a fairly constant composition whereas others exhibit an unusual wavy-oscillatory zoning which has not been reported elsewhere to our knowledge. The oscillatory zoning has wavelengths of up to 2,500 µm and a maximum amplitude of 7 mol% An. It is interpreted as reflecting movements of the crystals in the magma reservoir. The Sr isotopic composition of one crystal shows a radiogenic inner core ((87Sr/86Sr)i=0.7044) and a less radiogenic outer core ((87Sr/86Sr)i=0.7039-0.7036). The lack of a significant change between outer core and rim ((87Sr/86Sr)i=0.7037) is consistent with formation of the more An-rich rim due to pressure release. Variations in the core may be related to movements of the crystal and/or magma mixing. A trace-element profile across a megacryst shows a small increase in Sr and small decreases in Ba and La contents of the recalculated melt composition across the core-rim boundary, whereas P, Ce, Nd and Eu remain constant. Melt compositional changes upon emplacement are therefore considered to be of minor importance. Constant ratios of incompatible trace elements in the megacryst cores indicate a dominant influence of a lower crustal source on trace-element budgets.  相似文献   

6.
Mafic tholeiitic basalts from the Nejapa and Granada (NG) cindercone alignments provide new insights into the origin and evolutionof magmas at convergent plate margins. In comparison to otherbasalts from the Central American volcanic front, these marietholeiitic basalts are high in MgO and CaO and low in Al2Op,K2O1, Ba and Sr. They also differ from other Central Americanbasalts, in having clinopyroxene phenocrysts with higher MgO,CaO and Cr2O3 concentrations and olivine phenocrysts with higherMgO contents. Except for significantly higher concentrationsof Ba, Sr and 87Sr/86Sr, most of the tholeiites are indistinguishable in compositionfrom mid-ocean ridge basalts. In general, phenocryst mineralcompositions are also very similar between NG tholeiites andmid-ocean ridge basalts. The basalts as a whole can be dividedinto two groups based on relative TiO2-K2O concentrations. Thehigh-Ti basalts always have the lowest K2O and Ba and usuallyhave the highest Ni and Cr. All of the basalts have experienced some fractional crystallizationof olivine, plagioclase and clinopyroxene. Relative to otherCentral American basalts, the Nejapa-Granada basalts appearto have fractionated at low PT and PH2O. The source of primarymagmas for these basalts is the mantle wedge. Fluids and/ormelts may have been added to the mantle wedge from hydrothermally-altered,subducting oceanic crust in order to enrich the mantle in Sr,Ba and 87Sr/86Sr, but not in K and Rb. The role of lower crustaicontamination in causing the observed enrichments in Sr, Baand 87Sr/86Sr of NG basalts in comparison to mid-ocean ridgebasalts, however, is unclear. Rutile or a similar high-Ti accessoryphase may have been stable in the mantle source of the low-TiNG basalts, but not in that of the high-Ti basalts. Mafic tholeiiticbasalts, similar to those from Nejapa and Granada, may representmagmatic compositions parental to high-Al basalts, the mostmafic basalts at most Central American volcanoes. The characterof the residual high-Al basalts after this fractionation stepdepends critically on PH2O Both high and low-Ti andesites are also present at Nejapa. Likethe high-Ti basalts, the high-Ti andesites have lower K2O andBa and higher Ni and Cr in comparison to the low-Ti group. Thehigh-Ti andesites appear to be unrelated to any of the otherrocks and their exact origin is unknown. The low-Ti andesitesare the products of fractional crystallization of plagioclase,clinopyroxene, olivine (or orthopyroxene) and magnetite fromthe low-Ti basalts. The eruption that deposited a lapilli sectionat Cuesta del Plomo involved the explosive mixing of 3 components:high-Ti basaltic magma, low-Ti andesitic magma and high-Ti andesiticlava.  相似文献   

7.
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   

8.
Recent basaltic andesite lavas from Merapi volcano contain abundant,complexly zoned, plagioclase phenocrysts, analysed here fortheir petrographic textures, major element composition and Srisotope composition. Anorthite (An) content in individual crystalscan vary by as much as 55 mol% (An40–95) across internalresorption surfaces with a negative correlation between highAn mol% (>70), MgO wt% and FeO wt%. In situ Sr isotope analysesof zoned plagioclase phenocrysts show that the 87Sr/86Sr ratiosof individual zones range from 0·70568 to 0·70627.The upper end of this range is notably more radiogenic thanthe host basaltic andesite whole-rocks (< 0·70574).Crystal zones with the highest An content have the highest 87Sr/86Srvalues, requiring a source or melt with elevated radiogenicSr, rich in Ca and with lower Mg and Fe. Recent Merapi eruptiverocks contain abundant xenoliths, including metamorphosed volcanoclasticsediment and carbonate country rock (calc-silicate skarns) analysedhere for petrographic textures, mineralogy, major element compositionand Sr isotope composition. The xenoliths contain extremelycalcic plagioclase (up to An100) and have whole-rock 87Sr/86Srratios of 0·70584 to 0·70786. The presence ofthese xenoliths and their mineralogy and geochemistry, coupledwith the 87Sr/86Sr ratios observed in different zones of individualphenocrysts, indicate that magma–crust interaction atMerapi is potentially more significant than previously thought,as numerous crystal cores in the phenocrysts appear to be inheritedfrom a metamorphosed sedimentary crustal source. This has potentiallysignificant consequences for geochemical mass-balance calculations,volatile saturation and flux and eruptive behaviour at Merapiand similar island arc volcanic systems elsewhere. KEY WORDS: assimilation; isotopes; Merapi; xenolith; calc-silicate  相似文献   

9.
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  相似文献   

10.
Over the last several hundred years, Stromboli has been characterizedby steady-state Strombolian activity. The volcanic productsare dominated by degassed and highly porphyritic (HP-magma)black scoria bombs, lapilli and lava flows of basaltic shoshoniticcomposition. Periodically (about one to three events per year),more energetic explosive eruptions also eject light colouredvolatile-rich pumices with low phenocryst content (LP-magma)that have more mafic compositions than the HP-magma. An in situmajor and trace element and Sr isotope microanalysis study ispresented on four samples chosen to characterize the differentmodes of activity at Stromboli: a lava flow (1985–1986effusive event), a scoria bomb from the ‘normal’present-day activity of Stromboli (April 1984), and a scoriaand coeval pumice sample from a recent more explosive eruption(September 1996). Plagioclase (An62–90) and clinopyroxene(Mg-number between 0·69 and 0·91) phenocrystsin all samples record marked major element variations. Largeand comparable Sr isotope variations have been detected in plagioclaseand clinopyroxene. HP-magma crystals have resorbed cores, witheither high 87Sr/86Sr (0·70635–0·70630)or low 87Sr/86Sr (0·70614–0·70608); thelatter values are similar to the values of the outer cores.Mineral rims and glassy groundmasses generally have intermediate87Sr/86Sr (0·70628–0·70613). Similarly,mineral growth zones with three groups of 87Sr/86Sr values characterizeminerals from the LP-pumice, with the lowest values presentin mineral rims and groundmass glass. These results define amixing process between HP- and LP-magmas, plus crystallizationof clinopyroxene, plagioclase and olivine, occurring in a shallowmagma reservoir that feeds the present-day magmatic activityof Stromboli. An important observation is the presence of athird component (high 87Sr/86Sr in mineral cores) consideredto represent a pre-AD 1900 cumulus crystal mush reservoir situatedjust below the shallow magma chamber. These cumulus phases areincorporated by the LP-magma arriving from depth and transportedinto the shallow reservoir. A rapid decrease of 87Sr/86Sr inthe replenishing LP-magma immediately prior to eruption of theAD 1985 lava flow is associated with an increased volume ofLP-magma in the shallow magma chamber. The HP-magma in the shallowreservoir is not fully degassed when it interacts with the LP-magma,making efficient mixing possible that ultimately produces awell overturned homogeneous magma. Further degassing and crystallizationoccur at shallower levels as the HP-magma moves through a conduitto the surface. KEY WORDS: isotopic microsampling; mineral recycling; mixing; Sr isotope disequilibria; Stromboli  相似文献   

11.
Petrology of the Caribou Mountain Pluton, Klamath Mountains, California   总被引：2，自引：1，他引：1  
The Caribou Mountain pluton is a small trondhjemitic body thatintruded semipelitic schist of the Stuart Fork terrane in lateMiddle Jurassic to Early Cretaceous time. Its emplacement followedthe intrusion of an adjoining body of hornblende quartz dioritecalled the Middle Fork pluton and the mode of its emplacementwas as an asymmetric ballooning diapir (Davis, 1963), as shownby concentric foliation, radial late-stage dikes, foliated enclaves,and folded blocks of schlieren-banded tonalite. Coarse-grainedhornblende-bearing trondhjemite is the dominant rock type inthe Caribou Mountain pluton, and it is called the ‘maintrondhjemite’. It was followed by medium-grained ‘latetrondhjemite’ and by late-stage trondhjemitic and granodioriticdikes. All the trondhjemitic rock types are characterized bylow alkali contents, high light rare earth elements, low initial87Sr/86Sr, and low 18O. However, the late trondhjemite has higherNa2O and a higher initial 87Sr/86Sr value than the main trondhjemite,and the two units cannot be related by fractional crystallization.The late granodioritic dikes are richer in Ba, Rb, Y, and Scthan the late trondhjemite and probably reflect assimilationof Stuart Fork metasedimentary rocks by late-stage trondhjemiticmagma. Mafic enclaves in the main trondhjemite contain xenocrysts ofquartz and plagioclase derived from the host by magma mixing.The enclaves have K2O, Ba, and Rb contents similar to, or higherthan those of the host rocks. Their rare earth element (REE)patterns display strong middle REE enrichment caused by accumulationof hornblende, probably as the result of filter pressing. The main trondhjemite cannot be derived from Middle Fork magmabecause the initial 87Sr/86Sr of the Middle Fork pluton is lowerthan that of the trondhjemite. The absence of parental maficmagmas of appropriate composition suggests that the CaribouMountain trondhjemitic magmas formed by partial melting of anamphibolitic source rock compositionally similar to low-K tholeiite.  相似文献   

12.
Most of the flows in the Palaeocene lava pile remnant of Skyeare members of the Skye Main Lava Series (SMLS), comprisingtransitional basalts and two associated suites of evolved lavas.The first suite evolves through Fe-rich hawaiites and mugearitesto benmoreites, and the second suite evolves through Fe-poorintermediates to trachytes. Ca-rich, alkali-poor olivine tholeiites(the Preshal Mhor magma type) occur as sparse flows in the stratigraphicallyhighest parts of the lava pile remnant and are abundant in thedyke swarm transecting it. Initial 87Sr/86Sr ratios rangingfrom 0.70308 to 0.70571 in 45 SMLS samples show no significantcorrelation with degree of zeolitization (H2O+), silica saturation,or 87Rb/86Sr. A moderately good negative correlation with totalSr confirms published Pb-isotope evidence of interaction withancient, sialic crust. Details of the (87Sr/86Sr)l versus Srpattern are consistent with previous hypotheses that the SMLSbasalt-benmoreite suite evolved at a depth near the Moho, whilstthe low-Fe trend to trachyte resulted from near-surface basaltfractionation. (87Sr/86Sr)l values ranging from 0.70307 to 0.70621 for PreshalMhor basalts show a strong positive correlation with total Sr,consistent with a model of extensive fractionation within theupper crust of a mantle-derived low 87Sr/86Sr-low Sr magma,which became progressively contaminated with comparatively radiogeniccrustal Sr. The lowest measured (87Sr/86Sr)l values of 0.70307and 0.70308, for a Preshal Mhor basalt and for an SMLS basaltrespectively, are consistent with the hypothesis that thesetwo magma types were produced by successive phases of partialmelting from a single volume of upper mantle. (87Sr/86Sr)l values for additional miscellaneous basaltic lavas,dykes and major intrusives from Skye and from nearby Isle ofMull exhibit considerable variability within the range 0.7038to 0.7072, whilst three basaltic dykes from Northern Englandare in the range 0.7089 to 0.7123. The latter values overlapwith published (87Sr/86Sr)l values for some of the granitesin the Tertiary Province of northwest Scotland and indirectlyremove objections based on Sr-isotopic arguments to the genesisof the granites by fractionation of basalt contaminated withcrustal Sr, but neither prove this nor disprove large-scalecrustal partial fusion.  相似文献   

13.
A Complex Petrogenesis for an Arc Magmatic Suite, St Kitts, Lesser Antilles   总被引：2，自引：0，他引：2  
St Kitts forms one of the northern group of volcanic islandsin the Lesser Antilles arc. Eruptive products from the Mt Liamuigacentre are predominantly olivine + hypersthene-normative, low-Kbasalts through basaltic andesites to quartz-normative, low-Kandesites. Higher-Al and lower-Al groups can be distinguishedin the suite. Mineral assemblages include olivine, clinopyroxene,orthopyroxene, plagioclase and titanomagnetite with rarer amphibole,ilmenite and apatite. Eruptive temperatures of the andesitesare estimated as 963–950°C at fO2 NNO + 1 (whereNNO is the nickel–nickel oxide buffer). Field and mineralchemical data provide evidence for magma mixing. Glass (melt)inclusions in the phenocrysts range in composition from andesiteto high-silica rhyolite. Compositional variations are broadlyconsistent with the evolution of more evolved magmas by crystalfractionation of basaltic parental magmas. The absence of anycovariation between 87Sr/86Sr or 143Nd/144Nd and SiO2 rulesout assimilation of older silicic crust. However, positive correlationsbetween Ba/La, La/Sm and 208Pb/204Pb and between 208Pb/204Pband SiO2 are consistent with assimilation of small amounts (<10%)of biogenic sediments. Trace element and Sr–Nd–Pbisotope data suggest derivation from a normal mid-ocean ridgebasalt (N-MORB)-type mantle source metasomatized by subductedsediment or sediment melt and fluid. The eruptive rocks arecharacterized by 238U excesses that indicate that fluid additionof U occurred <350 kyr ago; U–Th isotope data for mineralseparates are dominated by melt inclusions but would allow crystallizationages of 13–68 ka. However, plagioclase is consistentlydisplaced above these ‘isochrons’, with apparentages of 39–236 ka, and plagioclase crystal size distributionsare concave-upwards. These observations suggest that mixingprocesses are important. The presence of 226Ra excesses in twosamples indicates some fluid addition <8 kyr ago and thatthe magma residence times must also have been less than 8 kyr. KEY WORDS: Sr–Nd–Pb isotopes; U-series isotopes; crystal size distribution; petrogenesis  相似文献   

14.
The Edgecumbe volcanics, which range from basalt through rhyodacite,have Sr contents between 125 and 370 p.p.m., Rb contents of1 to 70 p.p.m., Ba ranging from 50–550 p.p.m. and initial87Sr/86Sr ratios between 0.70291 and 0.70404. No simple correlationexists between these components and silica. The highest 87Sr/86Srvalues occur in a group of intermediate lavas (55–60 wt.per cent SiO2) while the rhyodacites have initial ratios between0.7035 and 0.7038. With increasing silica, Sr increases to amaximum in the andesites and then steadily decreases; Ba andRb increase over the same compositional range. The highest 87Sr/86Srlavas have major and trace element concentrations which departfrom trends defined by most of the lavas. The variation in strontiumisotopic compositions suggests interaction between parentalbasaltic magma and crustal material. Attempts to model the assimilationprocess using fixed end-member assimilation and assimilation-fractionalcrystallization models have failed to produce the observed chemicaltrends. Because the parental basaltic liquid underwent littlefractionation, the variability in hybrid lavas is attributedto variation in contaminant composition. Initial melts werelow in CaO, A12O3, MgO and Sr and enriched in SiO2. K2O, Na2O,Rb and Ba. As melting progressed, melts became enriched in themore refractory components. Because hybrid strontium isotopiccomposition is a function of Sr concentration as well as isotopiccompositions, the Sr content of the assimilant strongly influencesresultant isotopic systematics. The development of the assimilantssuggests plagioclase was a residual phase during early melting.This model of crustal assimilation represents one end-memberin the spectrum of processes responsible for the generationof continental volcanic suites.  相似文献   

15.
The 1.3 Ma Purico complex is part of an extensive Neogene-Pleistocene ignimbrite province in the central Andes. Like most other silicic complexes in the province, Purico is dominated by monotonous intermediate ash-flow sheets and has volumetrically minor lava domes. The Purico ignimbrites (total volume 80-100 km3) are divided into a Lower Purico Ignimbrite (LPI) with two extensive flow units, LPI I and LPI II; and a smaller Upper Purico Ignimbrite (UPI) unit. Crystal-rich dacite is the dominant lithology in all the Purico ignimbrites and in the lava domes. It is essentially the only lithology present in the first LPI flow unit (LPI I) and in the Upper Purico Ignimbrite, but the LPI II flow unit is unusual for its compositional diversity. It constitutes a stratigraphic sequence with a basal fall-out deposit containing rhyolitic pumice (68-74 wt% SiO2) overlain by ignimbrite with dominant crystal-rich dacitic pumice (64-66 wt% SiO2). Rare andesitic and banded pumice (60-61 wt% SiO2) are also present in the uppermost part of the flow unit. The different compositional groups of pumice in LPI II flow unit (rhyolite, andesite, dacite) have initial Nd and Sr isotopic compositions that are indistinguishable from each other and from the dominant dacitic pumice ()Nd=-6.7 to -7.2 and 87Sr/86Sr=0.7085-0.7090). However, two lines of evidence show that the andesite, dacite and rhyolite pumices do not represent a simple fractionation series. First, melt inclusions trapped in sequential growth zones of zoned plagioclase grains in the rhyolite record fractionation trends in the melt that diverge from those shown by dacite samples. Second, mineral equilibrium geothermometry reveals that dacites from all ignimbrite flow units and from the domes had relatively uniform and moderate pre-eruptive temperatures (780-800 °C), whereas the rhyolites and andesites yield consistently higher temperatures (850-950 °C). Hornblende geobarometry and pressure constraints from H2O and CO2 contents in melt inclusions indicate upper crustal (4-8 km) magma storage conditions. The petrologic evidence from the LPI II system thus indicates an anomalously zoned magma chamber with a rhyolitic cap that was hotter than, and chemically unrelated to, the underlying dacite. We suggest that the hotter rhyolite and andesite magmas are both related to an episode of replenishment in the dacitic Purico magma chamber. Rapid and effective crystal fractionation of the fresh andesite produced a hot rhyolitic melt whose low density and viscosity permitted ascent through the chamber without significant thermal and chemical equilibration with the resident dacite. Isotopic and compositional variations in the Purico system are typical of those seen throughout the Neogene ignimbrite complexes of the Central Andes. These characteristics were generated at moderate crustal depths (<30 km) by crustal melting, mixing and homogenization involving mantle-derived basalts. For the Purico system, assimilation of at least 30% mantle-derived material is required.  相似文献   

16.
Chemical and Sr-Nd isotopic data are presented for 14 Proterozoicdolerite intrusions from Sweden. The dolerites are unmetamorphosedand belong to two age groups at 1.26 and 0.95 Ga. The analysedrocks were selected from over 80 samples characterized for initial87Sr/86Sr ratio, and represent those from each intrusion thathave little to no crustal contamination effect on Sr isolopes.In these tholeiitic to mildly alkaline basalts, MgO varies from4 to 14%, and TiO2 from 1.0 to 4.7%. Incompatible-element abundancepatterns show similar shapes at widely varying overall enrichmentlevels; La varies from eight to 64 times primitive mantle. Manysamples have apparent Sr anomalies; positive for the less incompatible-elementenriched rocks, negative for those with the highest abundances,Hence Sr varies by a factor of only two, whereas neighboringelements vary by a factor of seven. Small Eu anomalies correlatewith the larger Sr variations. All samples have negative Nbanomalies, correlated with Nd 1 values that vary from –0.5to + 3.5 for both dolerite age groups. 87Sr/86Sr1 values aresystematically different in the two groups: 1.26-Ga doleritesvary from 0.7026 to 0.7035, and 0.95-Ga dolerites from 0.7041to 0.7051. Rather than urging any particular model of subcontinental mantleevolution, we use the dolerite results to deconvolute the effectsof crustal contamination, differentiation including crypticdifferentiation effects, and mantle-lithosphere interactionson possibly asthenosphere-derived magmas. Problems related toaddition of Rb and 87Sr giving false isochrons, as well as raised87Sr/86Sr from syn- or post-crystallization fluid effects, wereeliminated by the sample selection criteria. In the selecteddolerites, crustal contamination is not a major control on geochemistry,because all incompatible elements, including those of both highand low crustal abundance, are positively and similarly correlated.Overall abundances have been affected by migrations of residualmagma after much olivine and plagioclase crystallization. OnlySr is an approximate indication of the parent magma's enrichment,whereas all other incompatible elements may be depleted or enrichedby factors greater than two. On entering the crust, the magmasvaried by a factor of only two in overall incompatible-elementenrichment. They had Nb anomalies correlated with Nd1, and the0.95-Ga dolerites possessed higher 87Sr/86Sr than had characterizedthe 1.26-Ga dolerites. All mantle effects could be interpretedas resulting from a lithospheric magma source that experiencedaddition of subduction-related components 1.9–1.7 Ga ago,and maybe locally 1.6–1.0 Ga ago. We prefer a model whereasthenosphere-derived melts interact with lithospheric wallrocks, acquiring variable Nd down to –0.5 and Nb anomalies.Up to two-thirds of the budget of incompatible elements, butnot much Nb, were added to the magmas. Untimately, the contributionof any mantle-source is unconstrained, except that significantlithosphere has to have been involved. The mantle-related geochemicaleffects, which result in only weak constraints on process andsource, are very difficult to isolate in an objective approachto intracontinental basalts, because of superimposed differentiationand contamination effects.  相似文献   

17.
Okmok volcano is situated on oceanic crust in the central Aleutianarc and experienced large (15 km3) caldera-forming eruptionsat 12 000 years BP and 2050 years BP. Each caldera-forming eruptionbegan with a small Plinian rhyodacite event followed by theemplacement of a dominantly andesitic ash-flow unit, whereaseffusive inter- and post-caldera lavas have been more basaltic.Phenocryst assemblages are composed of olivine + pyroxene +plagioclase ± Fe–Ti oxides and indicate crystallizationat 1000–1100°C at 0·1–0·2 GPain the presence of 0–4% H2O. The erupted products followa tholeiitic evolutionary trend and calculated liquid compositionsrange from 52 to 68 wt % SiO2 with 0·8–3·3wt % K2O. Major and trace element models suggest that the moreevolved magmas were produced by 50–60% in situ fractionalcrystallization around the margins of the shallow magma chamber.Oxygen and strontium isotope data (18O 4·4–4·9,87Sr/ 86Sr 0·7032–0·7034) indicate interactionwith a hydrothermally altered crustal component, which led toelevated thorium isotope ratios in some caldera-forming magmas.This compromises the use of uranium–thorium disequilibria[(230Th/ 238U) = 0·849–0·964] to constrainthe time scales of magma differentiation but instead suggeststhat the age of the hydrothermal system is 100 ka. Modellingof the diffusion of strontium in plagioclase indicates thatmany evolved crystal rims formed less than 200 years prior toeruption. This addition of rim material probably reflects theremobilization of crystals from the chamber margins followingreplenishment. Basaltic recharge led to the expansion of themagma chamber, which was responsible for the most recent caldera-formingevent. KEY WORDS: Okmok; caldera; U-series isotopes; Sr-diffusion; time scales; Aleutian arc  相似文献   

18.
The caldera-forming 26·5 ka Oruanui eruption (Taupo,New Zealand) erupted 530 km3 of magma, >99% rhyolitic, <1%mafic. The rhyolite varies from 71·8 to 76·7 wt% SiO2 and 76 to 112 ppm Rb but is dominantly 74–76 wt% SiO2. Average rhyolite compositions at each stratigraphiclevel do not change significantly through the eruption sequence.Oxide geothermometry, phase equilibria and volatile contentsimply magma storage at 830–760°C, and 100–200MPa. Most rhyolite compositional variations are explicable by28% crystal fractionation involving the phenocryst and accessoryphases (plagioclase, orthopyroxene, hornblende, quartz, magnetite,ilmenite, apatite and zircon). However, scatter in some elementconcentrations and 87Sr/86Sr ratios, and the presence of non-equilibriumcrystal compositions imply that mixing of liquids, phenocrystsand inherited crystals was also important in assembling thecompositional spectrum of rhyolite. Mafic compositions comprisea tholeiitic group (52·3–63·3 wt % SiO2)formed by fractionation and crustal contamination of a contaminatedtholeiitic basalt, and a calc-alkaline group (56·7–60·5wt % SiO2) formed by mixing of a primitive olivine–plagioclasebasalt with rhyolitic and tholeiitic mafic magmas. Both maficgroups are distinct from other Taupo Volcanic Zone eruptivesof comparable SiO2 content. Development and destruction by eruptionof the Oruanui magma body occurred within 40 kyr and Oruanuicompositions have not been replicated in vigorous younger activity.The Oruanui rhyolite did not form in a single stage of evolutionfrom a more primitive forerunner but by rapid rejuvenation ofa longer-lived polygenetic, multi-age ‘stockpile’of silicic plutonic components in the Taupo magmatic system. KEY WORDS: Taupo Volcanic Zone; Taupo volcano; Oruanui eruption; rhyolite, zoned magma chamber; juvenile mafic compositions; eruption withdrawal systematics  相似文献   

19.
The volcanic rocks of the Bearpaw Mountains are part of theMontana high-potassium province, emplaced through Archaean rocksof the Wyoming Craton between 54 and 50 Ma ago. Extrusive rocks,dominated by minettes and latites, have a volume of 825 km3.The minettes range in composition from 20 to 6% MgO. The moremagnesian varieties contain the phenocryst assemblage forsterite+ Cr-spinel + diopside phlogopite. More evolved rocks areolivine-free, with an assemblage of either salite + phlogopite+ pseudoleucite or salite + phlogopite + analcime. The analcimeis thought to be secondary after leucite, produced by loss ofpotassium from the minettes. Mineral chemistry and textures,especially of clinopyroxenes, indicate that mixing between minettemagmas of varying degrees of evolution was commonplace. Compositionalvariation was further extended by accumulation of olivine +spinel + clinopyroxene phenocrysts, and by the preservationof mantle xenocrysts in the minettes. The primary minette magmasare inferred to have had 12–14% MgO and to have been generatedat 30 kb from an olivine + diopside + phlogopite-bearing source.The primary magmas evolved dominantly by fractionation of olivine+ diopside. The minettes have high contents of large ion lithophile elements(LILE) and light rare earth elements (LREE), with K2O up to6.18%, Ba 5491 ppm, Sr 2291 ppm, and Ce 99 ppm. (87Sr/86Sr)0ranges from 0.707 to 0.710 and Nd varies from –10 to–16. These data, plus high LILE/HFSE (high field strengthelements) values, are interpreted to show that the minettescontain at least three different mantle components. The lithospherewas initially depleted in Archaean times, but was metasomaticallyenriched in the Proterozoic and in the late Cretaceous and earlyTertiary. The latites have many chemical features in common with the minettes,such as potassic character and high LILE/HFSE values. They formedby fractional crystallization of minette magma in combinationwith assimilation of crustal rocks; this process enriched themagmas in SiO2 and raised Na2O/K2O and 87Sr/86Sr values. Chemicaldata for phenocrysts and bulk rocks in minettes suggest mixingbetween minette and latite magmas.  相似文献   

20.
The Kalka Intrusion, central Australia has a 5000 m-thick layered sequence comprising Pyroxenite, Norite and Anorthosite Zones; an Olivine Gabbro Zone is enclosed by, and has a facies relationship with, the Norite Zone. Contamination is evidenced by high initial 87Sr/86Sr ratios ( 0.708) in the lower four-fifths of the intrusion, and resulted in pyroxenite rather than peridotite as a basal crystal accumulate. After an early phase of erratic buildup in contamination due to assimilation of ragged granulite wall-rock, armouring of the walls and mixing produced an homogeneous Norite Zone (threshold) magma body crystallising opx-cpx-plag. Within the succeeding 3500 m of section plagioclase An values have a general decline up sequence (An74-60-feldspar threshold) with superimposed short term digressions to more calcic compositions. Initial 87Sr/86Sr ratios also fall very gradually (0.7081-0.7078 — isotopic threshold) with transient fluctuations to distinctly lower values. Maxima in plagioclase An contents and 87Sr/ 86Sr minima may be correlated with the spasmodic appearance of olivine and pronounced lithological variation. Such features are explained by the ponding of fresh bodies of uncontaminated magma on the floor of the chamber; these formed a hybrid with threshold magma that temporarily dominated crystallisation thereby perturbing plagioclase and isotopic compositions; eventually, mixing of the hybrid into the overlying threshold magma returned crystallisation to its initial state. The facies-controlled Olivine Gabbro Zone is the physical expression of ponded basal hybrid magmas. The onset of the Anorthosite Zone is marked by a pronounced decline in 87Sr/86Sr ratios to around 0.705 registering a major influx of new magma. In this instance the new magma dominated the system and a change to plagioclase as the dominant liquidus phase indicates a drastic change in input magma composition. The evolution of Kalka was determined by an interplay of crystal fractionation, fresh magma pulses, and magma mixing, with the latter effects producing both increases and decreases in plagioclase An contents; the complete process can only be revealed by combined mineral composition and isotopic data.  相似文献   

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