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1.
The Palaeocene magnesian transitional basalts of the Main LavaSeries (SMLS) of Skye, NW Scotland, have concentration rangesof K, Ti, P, Rb, Sr, Ba, Nb, Ta, Zr, Hf, Th and light REE varyingby a factor of up to two at a given value of (FeO + Fe2O3)/(FeO+ Fe2O3 + MgO). Their chondrite-normalized REE patterns varywidely in slope and cuvature, with (Ce/Yb)N=2.2–4.7. Theabundances of Ti, P, Zr, Hf, Eu, Gd and Tb correlate negativelywith Si-saturation and are thought to be primary, reflectingvariable localized partial melting (5 per cent) for each magmabatch at about 60 km depth of a spinel-lherzolite upper mantle,leaving a lherzolitic residuum. Y and the heavy REE vary littlewith Si-saturation, due to their partial retention in residualmantle diopside. The large abundance ranges of Rb, Sr, Ba, Nb,Th, La, Ce and Nd in the SMLS basalts, uncorrelated with Si-saturation,may reflect local upper-mantle variability in the concentrationsof the ultra-incompatible elements beneath Skye, caused by thepre-Palaeocene extraction of small quantities of alkalic, incompatible-element-richmagma, such as formed the Permian lamprophyre dykes of westernScotland. The trace element data confirm major-element, least-squaresmodels, which show that fractional crystallization of SMLS magnesianbasalt to less-magnesian basalt residua involved the separationof 10 per cent olivine and 4 per cent plagioclase, whilst thefractionation of SMLS less-magnesian basalt to hawaiite occurredat about 35 km depth by precipitation of 8 per cent olivine,15 per cent plagioclase and 21 per cent aluminous sub-calcicaugite. The variation of Nb and Ta abundances in hawaiites,mugearites and low-Fe intermediate lavas suggests that theseelements partitioned strongly into liquidus titanomagnetitemicrophenocrysts. Zircon fractionation occurred during the finalstages of evolution of benmoreites and trachytes, the latterrepresenting the residuum of at least 90 per cent fractionalcrystallization of SMLS basalt magma. High-Ca, low-alkali olivine tholeiites of the Preshal Mhor magmatype occur near the top of the present lava field erosionalremnant and predominate in the dyke swarm transecting it. Theyhave low incompatible trace-element abundances and REE patternswith (Ce/Yb)N 0.6, similar to those of many mid-ocean ridgebasalts. Models attempting to explain the genesis and relationsof the contrasting SMLS and Preshal Mhor basalts by postulatingseparate mantle sources, arranged in plumes, blobs or layers,fail to account satisfactorily for: (1) the constant incompatible-elementratio ranges of all Skye basalt lavas, (2) the partial interstratificationof SMLS and Preshal Mhor basalts and (3) the appearance of PreshalMhor magmas at the climax of crustal extension in the dyke swarm.The contrasting REE patterns of SMLS transitional basalts andPreshal Mhor tholeiites, and the high Ca/Al, Ca/Na and Ca/Tiof the latter, can be explained if they were produced by furtherlimited partial fusion of the lherzolitic residuum (with a trappedmelt fraction) from SMLS genesis, leaving a final harzburgiticresiduum. The petrogenesis of the Skye lavas provides a localshort-timespan analogue of worldwide processes involved in thegeneration of mid-ocean ridge basalts.  相似文献   

2.
The origin and evolution of the magma(s) involved in the formationof the olivine tholeiite cone-sheets which invade the 60-MaCuillin Igneous Complex, Isle of Skye, have been investigatedusing mineral chemistry data together with whole-rock major-and trace-element and isotope (Sr and Nd) geochemistry data.The most primitive compositions identified are almost identicalto those of the basalts being erupted at the present day alongthe spreading centre in Iceland. Rare examples of lavas fromthe slightly older lava pile of west-central and north Skye,together with a large number of dykes from the axial zone ofthe Skye Main Swarm, are of similar composition and are referredto as being of the Preshal More Basalt type (spelling is accordingto current Ordnance Survey of Great Britain maps). The intra-suitecompositional variation exhibited by the cone-sheets can beexplained in terms of relatively low-pressure fractionationof the three-phase assemblage olivine+clinopyroxene+plagioclasein their cotectic proportions of 10: 35: 55. Modelling of major-and compatible trace-element data indicates that the most evolvedcomposition may be derived by 60% crystallization of the leastevolved composition. Incompatible trace-element modelling impliesgreater degrees of crystal-liquid fractionation: Y and Zr indicate64 and 68% crystallization, respectively, whereas the rare earthelements (Eu, Yb, Gd, and Ce) indicate between 71 and 75% crystallization.This decoupled behaviour of compatible and incompatible elementsis attributed to the cone-sheet magma(s) evolving in a subjacentmagma chamber, before final emplacement in the overlying crust.Sr- and Nd-isotope data indicate that the cone-sheet magma(s)assimilated upper-crustal lithologies during fractional crystallization,most likely involving amphibolite facies gneiss of the LewisianGneiss Complex. This style of simultaneous assimilation andfractional crystallization (AFC) process in the upper crustfor the Skye cone-sheet magma(s) is in direct contrast to theprocesses identified for the magmas which produced the bulkof the lava field of west-central and north Skye, where assimilationoccurred after fractionation. The cone-sheet magmas were generatedby relatively large degrees of partial melting of a depletedmantle source associated with significant lithospheric stretching.The cause of this depletion, together with the temporal relationshipsbetween the cone-sheet magmas and the dominant transitionalmagmas of the Skye lava field and sill complex, are also discussed. * Reprint request to B. R. Bell  相似文献   

3.
Dykes of the East Greenland Tertiary dyke swarm can be dividedinto pre- and syn-break-up tholeiitic dykes, and post-break-uptransitional dykes. Of the pre- and syn-break-up dykes, themost abundant group (Tholeiitic Series; TS) has major elementcompositions similar to the main part of the East Greenlandflood basalts. A group of high-MgO tholeiitic dykes (Picrite–AnkaramiteSeries; PAS) are much less common, and are equivalent to someof the oldest lavas of the East Greenland flood basalts. Isotopiccompositions of the TS and PAS dykes partly overlap with thosefor Iceland, but Pb isotopic compositions extend to less radiogenicvalues than those seen in either Iceland or North Atlantic mid-oceanridge basalt (MORB). The isotopically depleted source requiredto account for this isotopic variation is interpreted as subcontinentallithospheric mantle with low 87Sr/86Sr and 206Pb/204Pb and high  相似文献   

4.
The Koloa Volcanic Suite of Kauai, Hawaii   总被引：3，自引：3，他引：3  
The Koloa lavas of the post-erosional volcanic suite of Kauaivary in composition from melilitite to alkali olivine basalt.87Sr/86Sr varies from {small tilde}0.7030 to 0.7033, the rangebeing similar to that of other Hawaiian post-erosional suites.The chemical composition within single lava flows is variableand forms major-element trends that are different from the mainKoloa trend. The main trend can be related to increased degreesof partial melting during decreasing pressure. The combinedevidence from Sr and Nd isotopes and rare earth elements (REEs)could suggest that an ocean island basalt (OIB) plume was infiltratedby a carbonate-rich melt with a high concentration of Sr andREEs derived from the mid-ocean ridge basalt (MORB) source.However, the Pb-isotope systematics of the Hawaiian islandsdo not suggest that a MORB source was involved during the generationof the posterosional lavas. Instead, we suggest that the Hawaiiantholeiitic and nephelinitic magma suites are derived from azoned plume with a low 87Sr/86Sr core and a high 87Sr/86Sr concentricmargin. The trends of the single flows and their variation inisotope ratios suggest that they formed from magmas accumulatedmainly from the central core of the zoned plume.  相似文献   

5.
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   

6.
The isotopic abundance of strontium has been measured in representativerock-types from the Tertiary igneous suite in the Isle of Skye,north-west Scotland. The isotopic abundance of Sr87, expressedas the Sr87/Sr86 ratio, for twelve basalts, dolerites, gabbros,and peridotites is 0.7058?0.0010. On the other hand, the averageinitial Sr87/Sr86 ratio for one granite from the Eastern RedhillsComplex and for three granites from the Western Redhills Complexis 0.7124?0.0015. The three principal members of the marscoitesuite of the Western Redhills Centre, namely, ferrodiorite,porphyritic felsite, and marscoite, also have significantlyhigher initial Sr87/Sr86 ratios than the basaltic rocks. Itis concluded that the rocks with high initial ratios were derivedfrom a source with significantly higher Rb/Sr ratio than thatof the basaltic rocks. The hypothesis which is most in accordwith the isotopic evidence is that the granitic rocks and themarscoite suite were produced by partial melting of ancientLewisian rocks, which form the underlying basement at no greatdepth in this area. Rb—Sr (and K-Ar) age determinations were carried out onsome representative rocks from the Eastern and Western RedhillsCentres. It was not possible to establish a significant agedifference between any of the rock units from either Centres.The mean age is 54?3 m.y. and suggests that intrusion of thegranitic rocks of Skye occurred during a relatively short periodof time within the Lower or Middle Eocene peroid.  相似文献   

7.
Geochemical data from back-arc volcanic zones in the Manus Basinare used to define five magma types. Closest to the New Britainarc are medium-K lavas of the island arc association and back-arcbasin basalts (BABB). Mid-ocean ridge basalts (MORB), BABB andmildly enriched T-MORB (transitional MORB) occur along the ManusSpreading Center (MSC) and Extensional Transform Zone (ETZ).The MSC also erupted extreme back-arc basin basalts (XBABB),enriched in light rare earth elements, P, and Zr. Compared withnormal MORB, Manus MORB are even more depleted in high fieldstrength elements and slightly enriched in fluid-mobile elements,indicating slight, prior enrichment of their source with subduction-relatedcomponents. Chemical variations and modeling suggest systematic,coupled relationships between extent of mantle melting, priordepletion of the mantle source, and enrichment in subduction-relatedcomponents. Closest to the arc, the greatest addition of subduction-relatedcomponents has occurred in the mantle with the greatest amountof prior depletion, which has melted the most. Variations inK2O/H2O indicate that the subduction-related component is bestdescribed as a phlogopite and/or K-amphibole-bearing hybridizedperidotite. Magmas from the East Manus Rifts are enriched inNa and Zr with radiogenic 87Sr/86Sr, possibly indicating crustalinteraction in a zone of incipient rifting. The source for XBABBand lavas from the Witu Islands requires a mantle componentsimilar to carbonatite melt. KEY WORDS: Manus back-arc basin, mantle metasomatism, magma generation  相似文献   

8.
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   

9.
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   

10.
The petrology and geochemistry of the Azores Islands   总被引：7，自引：0，他引：7  
Forty lavas from the Azores Islands have been analyzed for 87Sr/86Sr ratios, major elements, first transition series metals, and LIL elements. The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria. Differentiated lavas include both typical trachytes and comenditic trachytes and comendites. Major and trace element concentrations define smooth trends on variation diagrams, and these trends can be related to phases crystallizing in the rocks. Systematic interisland differences are also apparent in these variation diagrams. LIL element concentrations in island basalts are roughly twice as high as those in tholeiites from the adjacent Mid-Atlantic Ridge which transects the Azores Plateau. 87Sr/86Sr ratios in lavas from 6 of the 9 islands range from 0.70332 to 0.70354, a range similar to that found in tholeiites from the Mid-Atlantic Ridge transect of the Azores Plateau. This suggests that lavas from these islands and this portion of the Mid-Atlantic Ridge may be derived from a similar source. However, lavas from the islands of Faial and Pico have 87Sr/86Sr ratios up to 0.70394 and ratios in Sao Miguel lavas range up to 0.70525, suggesting basalts from these islands are derived from a chemically distinct source. Differences in the average LIL element concentrations of the least fractionated ridge tholeiites from the Azores Plateau and alkali basalts from the islands result from differences in extent of partial melting and residual mineralogy. The alkali basalts are derived by roughly half as much melting as are the tholeiites. Trace element concentrations in Azores peralkaline lavas preclude their derivation by partial melting of peridotitic mantle or basaltic crust; rather the data suggest they are produced by fractional crystallization of a basaltic parent.  相似文献   

11.
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   

12.
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   

13.
The Younger Giant Dyke Complex consists of a set of massivecoalescing dykes of Proterozoic age (c. 1170 Ma), resultingfrom intrusion of a suite of transitional olivine basaltic/hawaiiticmagmas in a continental rift setting. The suite, compositionallyrelated by low pressure (< 10 kb) olivine-plagioclase fractionation,is believed to have had a deeper level evolution dominated bypyroxene and possibly garnet fractionation. Slow cooling insitu of the interior parts of the dyke complex produced cumuliticsuites. Those exposed range from gabbroic to syenitic; residualbodies of riebeckite granite and, very subordinate, feldspathoidalsyenite were also generated. The basic magmas had notably lowfO2 values, leading to delayed magnetite and clinopyroxene precipitation,relatively iron-rich differentiates and some residual liquidsof pantelleritic composition. The basic magmas had high F/Clvalues and are inferred to have had low H2O contents. They werealso characterized by relatively high K/Rb and low 87Sr/86Srvalues; these characteristics imply a mantle source with highF/Cl but depleted in Rb relative to K and Sr. Basaltic magmasresponsible for (a) the preceding Older Giant Dyke Complex and(b) a suite of anorthositic xenoliths within the Younger GiantDyke Complex, are inferred to have been derived from separateprimary magma batches independent of those that yielded theYounger Giant Dyke Complex. The giant dykes are the highest-levelrepresentatives of a larger basic complex responsible for theextensive linear gravity ‘high’ in the Tugtutôq-Narssaqarea.  相似文献   

14.
Twelve new Sr-isotope analyses and seventeen new rare earthelement distribution patterns are reported for basalts fromIceland and the Reykjanes Ridge, together with Rb, Sr, Na2O,K2O, TiO2, and P2O5 contents. The samples were chosen to representthe widest range of basalt types known from the Iceland-ReykjanesRidge system. 87Sr/86Sr ratios range from 0.70291 ?4 to 0.70325?5 for tholeiitesand up to 0.70341 ?7 for alkali basalt. Rare earth elementsalso show a wide range of both total abundance and degree oflight-REE fractionation (chondrite-normalised Ce/Yb ratios of0.30 to 3.36 for tholeiites and up to 7.07 for alkali basalt).As found in previous studies of either Sr-isotope compositionor REE distribution, the ocean floor basalts from the southernportion of the Reykjanes Ridge have lower 87Sr/86Sr and CeN/YbNratios than most of the Icelandic basalts. However, some highlyMg-rich tholeiites from Theistareykir in northern Iceland andKj?lur in central Iceland also have among the lowest valuesfor these parameters and are indistinguishable in this respectfrom the ridge basalts. There is a very strong positive, linear,correlation between 87Sr/86Sr and CeN/YbN for all the tholeiitesincluding some up to 16 m.y. old, but this relationship doesnot hold for the alkali basalts which have proportionately farhigher CeN/YbN ratios. There is also a positive, linear, correlationbetween 87Sr/86Sr and Sr content, but not between 87Sr/86Srand 1/Sr. These relationships are found to be incompatible with disequilibriummelting of a single mantle source region, whether by variabledegrees of partial melting with different mineral stabilityconditions, or by removal of successive incremental melts. Itis certain that the data reflect relatively gross chemical heterogeneityin the upper mantle beneath Iceland, but the correlation withSr content apparently rules out simple binary mixing models(mantle-plume hypothesis). It is proposed that the heterogeneities result from establishmentof a lithophile element gradient during a single chemical fractionationevent in the upper mantle at least 100–200 m.y. ago. Itis not possible at present to relate this geochemical gradientto known mantle structure.  相似文献   

15.
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   

16.
The Ashland pluton is a calc-alkaline plutonic complex thatintruded the western Paleozoic and Triassic belt of the KlamathMountains in late Middle Jurassic time. The pluton comprisesa series of compositionally distinct magma pulses. The oldestrocks are hornblende gabbro and two-pyroxene quartz gabbro withinitial 87Sr/86Sr = 0{dot}7044, 18O = 8{dot}7%, and REE patternswith chondrite normalized La/Lu = 7. These units were followedby a suite of tonalitic rocks (LaN/LuN = 7) and then by a suiteof K2O- and P2O5 rocks of quartz monzodioritic affinity (LaN/LuN= 13–21; LaN/SmN = 2{dot}4–3{dot}) The quartz monzodioriticrocks were then intruded by biotite granodiorite and granitewith lower REE abundances but more fractionated LREE(LaN/LuN= 13–19; LaN/SmN = 4{dot}3–6 and they, in turn,were host to dikes and bosses of hornblende diorite. The latestintrusive activity consisted of aplitic and granitic dikes.Combined phase equilibria and mineral composition data, indicateemplacement conditions of approximately Ptotal = 2{dot}3kb,PH2O between 1{dot}5 and 2{dot}2 kb, and fO2 between the nickel-nickeloxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO2 togetherwith increasing 18O and decreasing initial 87Sr/86Sr. The isotopicdata are consistent with either (1) combined fractional crystallizationof andesitic magma and concurrent assimilation of crustal materialcharacterized by low Sr1 and high (18O or, more probably, (2)a series of partial melting events in which sources were successivelyless radiogenic but richer in 18O Each intrusive stage displaysevidence for some degree of crystal accumulation and/or fractionalcrystallization but neither process adequately accounts fortheir compositional differences. Consequently, each stage appearsto represent a distinct partial melting or assimilation event. The P2O5-rich nature of the quartz monzodiorite suite suggestsaccumulation of apatite. However, the suite contains abundantmafic microgranitoid enclaves and most apatite in the suiteis acicular. These observations suggest that magma mixing affectedthe compositional variation of the quartz monzodiorite suite.Mass balance calculations are consistent with a simple mixingprocess in which P2O5-rich alkalic basalt magma (representedby the mafic microgranitoid enclaves) was combined with a crystal-poorfelsic magma (represented by the tonalite suite), yielding aquartz monzodioritic magma that then underwent differentiationby crystal fractionation and accumulation.  相似文献   

17.
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   

18.
New Pb isotope analyses are reported for forty-seven basic,intermediate and acid Tertiary igneous rocks from the Isle ofSkye and adjacent regions of northwest Scotland. New Sr isotopeanalyses and two Rb/Sr isochron dates are also reported forthe acid igneous rocks. These results are combined with publishedSr and Nd isotope data for Tertiary igneous rocks from Skyeand adjacent regions in order to examine the genesis and evolutionof Tertiary magmas in the Isle of Skye. Pb isotope analyses form a linear array on a Pb/Pb isochrondiagram, with a slope yielding an apparent age of 2920 ±70m.y. (2), interpreted as a mixing line between ca. 2900 m.y.-oldcrustal Pb and 60 m.y.-old mantle-derived Pb. Combined isotopicdata support a model of selective crustal Pb, Sr and Nd contaminationof basic and intermediate mantle-derived magmas, without significantmodification of major-element composition. This contaminationprocess is probably due to the breakdown of LIL-element-enrichedaccessory mineral phases, and does not necessarily produce linearmixing lines on isotope ratio plots of different elements. SkyeRedhills epigranites contain a predominant fraction of acidmagma which differentiated from mantle-derived basic precursors,and a subsidiary fraction of upper crustal melt. The proportionof this melt is estimated to rise from less than 10 per centin the older granites to around 30 per cent in the younger. Isotopic and geochemical data are used to draw conclusions aboutthe differentiation histories of the four principal magma typesof Skye: the Fe-rich and Fe-poor members of the Skye Main LavaSeries, Preshal Mhor Basalts and Redhills epigranites.  相似文献   

19.
Major and trace element analyses and strontium isotope ratios are presented for twenty-four samples of lavas and plugs from the early Tertiary lava pile in Mull. The samples were selected on the basis of petrographic freshness from a large collection from outside the hydrothermally altered zone of pneumatolysis which occupies the central region of the volcanic complex. Most of the analyses yield normative hypersthene and we argue that these are essentially unaltered magmatic compositions. The analytical data indicate that the samples may be divided into three groups on the basis of major element chemistry, initial 87Sr/86Sr ratios and correlations between lithophile element contents. Group I comprises an alkaline series (basalt-hawaiite-mugearite) with extremely low initial 87Sr/86Sr ratios (0.7030) and generally low lithophile element contents. Apart from their alkalinity and high Sr and Zr contents these samples have affinities with abyssal tholeiites. Group II contains hypersthene normative basalts with more tholeiitic characteristics but (as in the case of the Skye Main Lava Series) the more evolved rocks are trachytes. This group is characterized by more normal levels of lithophile element concentrations and relatively high initial 87Sr/86Sr ratios of about 0.7055. Group III is less clearly defined and contains basalts that are generally sparsely olivine-phyric and in most chemical respects fall between Group I and Group II-including initial 87Sr/86Sr ratios (0.7033 to 0.7043). They may represent mixtures of Group I and Group II type sources or magmas. Groups I and II appear to be similar, respectively, to the relatively sodic iron-rich and the relatively potassic ironpoor silica enrichment trends distinguished in the Skye Main Lava Series. In the Group I magma series the behaviour of Y and Sr relative to other incompatible elements can only be explained by differential partial melting of a deep garnet-lherzolite mantle source. Fractional crystallization has undoubtedly occurred at some stage during the ascent of these magmas from the mantle, as indicated by the behaviour of Ni and Cr, but has not been a major factor in the production of evolved magma compositions. The Group II magmas appear to have originated from a source more enriched in lithophile elements, and a relatively shallow (< 50 km) plagioclase-lherzolite mantle source is suggested for these magmas because they have Sr/Ba ratios between one and two orders of magnitude lower than those characteristic of Group I. Rb-Sr systematics suggest that the vertical heterogeneity of the mantle which was largely responsible for the chemical differences between these three groups may have existed for a very long time prior to Tertiary magmatism.  相似文献   

20.
The Lower Eocene lavas of northern Skye are preserved over anarea of approximately 1500 km2 in a shallow faulted oval basin.Seventy-four new major element chemical analyses have been madeof specimens showing minimal post-consolidation alteration.These demonstrate that the early volcanics vary from hypersthene-tonepheline-normative basalts; the former containing less TiO2and P2O5 but more K2O than the latter. The compositions of thesebasalts straddle the low-pressure thermal divide near the criticalplane of silica under-saturation, the normative ‘join’OI–Pl–Cpx; implying that their variation was causedby high-pressure, upper mantle processes. The unusual incompatibleelement pattern of the lavas suggests that a K-rich mantle phase,tentatively taken to be phlogopite, was involved in their genesis.At subvolcanic pressures the spectrum of basaltic magmas wassplit by the join olivine-plagioclase-augite, producing twodivergent trends; from alkali basalts, via relatively Si- andK-poor and Fe- and Ti-rich hawaiites and mugearites to benmoreite,and from hypersthene-normative basalts, via relatively Si- andK-rich and Fe- and Ti-poor intermediates to trachyte. One-atmospheremelting experiments on 21 lavas are used to supplement the chemicalinvestigation of these low-pressure trends. Two flows of aphyric, low-alkali tholeiitic basalt, with compositionsquite distinct from all other Skye lavas, have been discoveredinterleaved among the alkali-rich rocks near the top of thevolcanic pile, at present exposure level. The almost patternlesschemical variation of all the lavas with time suggests thatmagma chambers, as conventionally envisaged, did not exist beneaththe Skye volcanic field, but rather that fissure eruptions werefed from a sponge-like plexus of conduits and small reservoirs,within which magma, affected to varying extents by upper mantleprocesses, remained for varying periods, pursuing low-pressurefractionation trends. The chemistry of the main Skye basaltsand the low-alkali tholeiites, plus available data on otherpost-lava Skye basic igneous rocks, such as the Cuillin LayeredIntrusion, other gabbros in the central complexes, and late-stagealkali dolerite dykes, are all combined in an attempt to ascribethe variation of the basic magmas which approached or reachedthe surface of Skye to the growth, culmination, and waning stagesof a thermal event in the upper mantle beneath that area.  相似文献   

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