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1.
Pliocene volcanics on the island of Bequia comprise two interbeddedsuites of basalts and andesites. The isotopically homogeneoussuite (IHS) has a limited range of Sr—Nd—Pb isotopes(87Sr/86Sr 0.7040–0.7046, 143 Nd/144 Nd 0.5130 and 206Pb/204Pb 19.36–19.51), and mantle-like 18O values (5.5in clinopyroxene). The isotopically diverse suite (IDS) is characterizedby much wider ranges of radiogenic isotopes (87 Sr/86Sr 0.7048–0.7077,143 Nd/144 Nd 0.5128–0.5123 and 206 Pb/204 Pb 19.7–20.2),in which all of the Sr and Pb ratios are higher and Nd ratiosare lower than those of the IHS. The IDS is also characterizedby high 18 O values, up to 7.6 in clinopyroxene. The Sr andPb isotope ratios are too high, and the Nd isotope ratios aretoo low in the IDS for any of these lavas to be derived fromunmodified depleted mantle. Both suites are petrologically very similar and their majorelement compositions and phenocryst contents suggest that theywere formed largely by fractional crystallization of a hydroustholeiitic melt at pressures <3 kbar. The isotopic ratiosand enrichments in large ion lithophile elements (LILE), andto some extent light rare earth elements (LREE), as comparedwith mid-ocean ridge basalts (MORB), of the IHS lavas suggestthat they were derived from a depleted mantle source which hadbeen re-enriched by the addition of 1–4% of a subductioncomponent. This component probably comprised a mixture of dehydrationfluids, and perhaps minor siliceous melts, released from subductingsediments and mafic crust. The extreme isotopic ranges, largeenrichments in incompatible elements, more fractionated LREEpatterns and higher 18 O values of the IDS lavas are interpretedas resulting from 10–55% assimilation—fractionalcrystallization of sediments, derived from the Guyana Shield,which are present in the arc crust, by IHS type melts. KEY WORDS: trace elements; radiogenic isotopes; arc lavas; Lesser Antilles *Corresponding author.  相似文献   

2.
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 0.70346–0.70364; basalticandesite, 0–70349–0.70372; shoshonitic basalticandesite, 0.70374–0.70388; and andesite, 0.70324–0.70383.Dacites of Mount Mazama have 87Sr/86Sr ratios of 0.70348–0.70373.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= 18.838–18.967, 207Pb/204Pb=15.556–15.616,208Pb/204Pb=38.405–38.619; 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 of–3.4 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   

3.
Klauea historical summit lavas have a wide range in matrix 18OVSMOWvalues (4·9–5·6) with lower values in rockserupted following a major summit collapse or eruptive hiatus.In contrast, 18O values for olivines in most of these lavasare nearly constant (5·1 ± 0·1). The disequilibriumbetween matrix and olivine 18O values in many samples indicatesthat the lower matrix values were acquired by the magma afterolivine growth, probably just before or during eruption. BothMauna Loa and Klauea basement rocks are the likely sources ofthe contamination, based on O, Pb and Sr isotope data. However,the extent of crustal contamination of Klauea historical magmasis probably minor (< 12%, depending on the assumed contaminant)and it is superimposed on a longer-term, cyclic geochemicalvariation that reflects source heterogeneity. Klauea's heterogeneoussource, which is well represented by the historical summit lavas,probably has magma 18O values within the normal mid-ocean ridgebasalt mantle range (5·4–5·8) based on thenew olivine 18O values. KEY WORDS: Hawaii; Klauea; basalt; oxygen isotopes; crustal contamination  相似文献   

4.
The origin of large-volume Yellowstone ignimbrites and smaller-volumeintra-caldera lavas requires shallow remelting of enormous volumesof variably 18O-depleted volcanic and sub-volcanic rocks alteredby hydrothermal activity. Zircons provide probes of these processesas they preserve older ages and inherited 18O values. This studypresents a high-resolution, oxygen isotope examination of volcanismat Yellowstone using ion microprobe analysis with an averageprecision of ± 0·2 and a 10 µm spot size.We report 357 analyses of cores and rims of zircons, and isotopeprofiles of 142 single zircons in 11 units that represent majorYellowstone ignimbrites, and post-caldera lavas. Many zirconsfrom these samples were previously dated in the same spots bysensitive high-resolution ion microprobe (SHRIMP), and all zirconswere analyzed for oxygen isotope ratios in bulk as a functionof grain size by laser fluorination. We additionally reportoxygen isotope analyses of quartz crystals in three units. Theresults of this work provide the following new observations.(1) Most zircons from post-caldera low-18O lavas are zoned,with higher 18O values and highly variable U–Pb ages inthe cores that suggest inheritance from pre-caldera rocks exposedon the surface. (2) Many of the higher-18O zircon cores in theselavas have U–Pb zircon crystallization ages that postdatecaldera formation, but pre-date the eruption age by 10–20kyr, and represent inheritance of unexposed post-caldera sub-volcanicunits that have 18O similar to the Lava Creek Tuff. (3) Youngand voluminous 0·25–0·1 Ma intra-calderalavas, which represent the latest volcanic activity at Yellowstone,contain zircons with both high-18O and low-18O cores surroundedby an intermediate-18O rim. This implies inheritance of a varietyof rocks from high-18O pre-caldera and low-18O post-calderaunits, followed by residence in a common intermediate-18O meltprior to eruption. (4) Major ignimbrites of Huckleberry Ridge,and to a lesser extent the Lava Creek and Mesa Falls Tuffs,contain zoned zircons with lower-18O zircon cores, suggestingthat melting and zircon inheritance from the low-18O hydrothermallyaltered carapace was an important process during formation ofthese large magma bodies prior to caldera collapse. (5) The18O zoning in the majority of zircon core–rim interfacesis step-like rather than smoothly inflected, suggesting thatprocesses of solution–reprecipitation were more importantthan intra-crystalline oxygen diffusion. Concave-downward zirconcrystal size distributions support dissolution of the smallercrystals and growth of rims on larger crystals. This study suggeststhat silicic magmatism at Yellowstone proceeded via rapid, shallow-levelremelting of earlier erupted and hydrothermally altered Yellowstonesource rocks and that pulses of basaltic magma provided theheat for melting. Each post-caldera Yellowstone lava representsan independent homogenized magma batch that was generated rapidlyby remelting of source rocks of various ages and 18O values.The commonly held model of a single, large-volume, super-solidus,mushy-state magma chamber that is periodically reactivated andproduces rhyolitic offspring is not supported by our data. Rather,the source rocks for the Yellowstone volcanism were cooled belowthe solidus, hydrothermally altered by heated meteoric watersthat caused low 18O values, and then remelted in distinct pocketsby intrusion of basic magmas. Each packet of new melt inheritedzircons that retained older age and 18O values. This interpretationmay have significance for interpreting seismic data for crustallow-velocity zones in which magma mush and solidified areasexperiencing hydrothermal circulation occur side by side. Newbasalt intrusions into this solidifying batholith are requiredto form the youngest volcanic rocks that erupted as independentrhyolitic magmas. We also suggest that the Lava Creek Tuff magmawas already an uneruptable mush by the time of the first post-calderaeruption after 0·1 Myr of the climactic caldera-formingeruption. KEY WORDS: Yellowstone; oxygen isotopes; geochronology; isotope zoning; zircon; U–Pb dating; caldera; rhyolite; ion microprobe  相似文献   

5.
Geochemical and 40 Ar—39 Ar studies of the Malaita OlderSeries and Sigana Basalts, which form the basement of Malaitaand the northern portion of Santa Isabel, confirm the existenceof Ontong Java Plateau (OJP) crust on these islands. Sr, Nd,and Pb isotopic ratios of Malaita Older Series and Sigana lavasfall within limited ranges [(87Sr/86Sr)T= 0.70369–0.70423,ENd(T)= + 3.7 to +6.0, and 206Pb/204Pb = 18.25–18.64]virtually indistinguishable from those found in the three OJPbasement drill sites as far as 1600 km away, indicating a uniformhotspot-like mantle source with a slight ‘Dupal’signature for the world's largest oceanic plateau. Three chemicaltypes of basalts are recognized, two of which are equivalentto two of the three types drilled on the plateau, and one withno counterpart, as yet, on the plateau; the chemical data indicateslightly different, but all high, degrees of melting and slightvariation in source composition. All but one of the 40Ar-39Arplateau ages determined for Malaita Older Series and SiganaBasalt lavas are identical to those found at the distant drillsites: 121.30.9 Ma and 92.01.6 Ma, suggesting that two short-lived,volumetrically important plateau-building episodes took place30 m.y. apart. Aside from OJP lavas, three isotopically distinctsuites of alkalic rocks are present. The Sigana Alkalic Suitein Santa Isabel has an 40 Ar-39 Ar age of 91.70.4 Ma, the sameas that of the younger OJP tholeiites, yet it displays a distinct’HIMU‘ -type isotopic signature [206Pb/204Pb 20.20,(87Sr/86Sr) T 0.7032, Nd(T) 4.4], possibly representing small-degreemelts of a minor, less refractory component in the OJP mantlesource region. The Younger Series in southern Malaita has an40Ar-39Ar age of 44 Ma and isotopic ratios [Nd(T)=-0.5 to +1.0,(87Sr/86Sr)T =0.70404–0.70433, 206Pb/204Pb = 18.57–18.92]partly overlapping those of the ‘PHEM’ end-memberpostulated for Samoa, and those of present-day Rarotonga lavas;one or both of these hotspots may have caused alkalic volcanismon the plateau when it passed over them at 44 Ma. The NorthMalaita Alkalic Suite in northernmost Malaita is probably ofsimilar age, but has isotopic ratios [(87Sr/86Sr) T 0.7037,Nd(T) +4.5, 206pb/204pb 18.8) resembling those of some OJP basementlavas; it may result from a small amount of melting of agedplateau lithosphere during the OJP's passage over these hotspots.Juxtaposed against OJP crust in Santa Isabel is an 62–46-Maophiolitic (sensu lato) assemblage. Isotopic and chemical datareveal Pacific-MORB-like, backarc-basin-like, and arc-like signaturesfor these rocks, and suggest that most formed in an arc—backarcsetting before the Late Tertiary collision of the OJP againstthe old North Solomon Trench. The situation in Santa Isabelappears to provide a modern-day analog for some Precambriangreenstone belts. KEY WORDS: oceanic plateaux; Ontong Java Plateau; Solomon Islands; Sr-Nd-Pb isotopes; age and petrogenesis *Corresponding author.  相似文献   

6.
The Wrangellia terrane of North America contains a large volumeof Middle to Late Triassic oceanic flood basalts which wereemplaced on top of a preexisting island arc. Nd-, Sr-, and Pb-isotopiccompositions reflect derivation from a plume source with Nd(T)+6 to + 7, 87Sr/86Sri0•7034, and 206Pb/204Pbi19•0.Major and trace element compositions suggest the Wrangelliaflood basalts (WFB) formed through relatively small degreesof partial melting at greater depths than estimated for otheroceanic plateaux such as Ontong Java. It appears that the WFBdid not form in a rifting environment, and that preexistingarc lithosphere limited the ascent and decompression meltingof the source plume. Rocks from the preexisting arc are stronglydepleted in high field strength elements (HFSEs) relative tolarge ion lithophile elements (LILEs), but the WFB are not.Assimilation of arc lithospheric mantle or crust was thereforegenerally minor. However, some contamination by arc componentsis evident, particularly in basalts erupted in the early stagesof volcanism. Minor isotopic shifts, to lower Nd(T) and 206Pb/204Pbiand higher 87Sr/86Sri, are accompanied by shifts in trace elementratios towards more arclike signatures, e.g. low Nb/Th and Nb/La.Arc contamination is greatest in the most evolved basalts, indicatingthat assimilation was coupled with fractional crystallization.A comparison of the WFB with other continental and oceanic floodbasalts reveals that continental flood basalts generally formthrough smaller degrees of melting than oceanic flood basaltsand that the contribution of material from the crust and litho-sphericmantle is significantly greater. KEY WORDS: oceanic flood basalts; Wrangellia terrane; petrogenesis; Sr-Nd-Pb isotopes *Corroponding author  相似文献   

7.
The voluminous, bimodal, Silurian Topsails igneous suite consistsmainly of ‘A-type’ peralkaline to slightly peraluminous,hypersohnis to subsolvus granites with subordinate syenite,onzonite and diabase, plus consanguineous basalts and highsilicarhyolites. Nd(T) values from the suite range from –1.5to +5.4; most granitoid components exhibit positive Nd(T) values(+1.1 to +3.9). Granitoid initial 87Sr/86Sr and most 18 O valuesare in the range expected for rocks derived from mantle-likeprotoliths (0.701–0.706 and +6 to +80/). Restricted 207Pb/204Pbvariation is accompanied by significant dispersion of 206Pb/204Pband 208Pb/204Pb. Superficially, petrogenesis by either direct(via fractionation from basalt) or indirect (via melting ofjuvenile crust) derivation from mantle sources appears plausible.Remelting of the granulitic protolith of Ordovician are-typegranitoids can be ruled out, because these rocks exhibit negativeNd(T) and a large range in 207Pb/204Pb. Geochemical and isotopicrelationships are most compatible with remelting of hybridizedlithospheric mantle generated during arc-continent collision.A genetic link is suggested among collision-related delaminationor slab break-off events and emplacement of ‘post-erogenic’granite suites. A-type granites may recycle previously subductedcontinental material, and help explain the mass balance notedfor modern arcs. However, they need not represent net, new,crustal growth. KEY WORDS: A-type granites; juvenile crust; isotopes; Newfoundland *Telephone: (613) 995-4972. Fax: (613) 995-7997. e-mail: jwhalen{at}gsc.emr.ca  相似文献   

8.
The Jozini and Mbuluzi rhyolites and Oribi Beds of the southernLebombo Monocline, southeastern Africa, have geochemical characteristicsthat indicate they were derived by partial melting of a mixtureof high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types.Compositional variations within the different rhyolite typescan largely be explained by subsequent fractional crystallization.The Sr- and Nd-isotope composition of the rhyolites is uniqueamongst Gondwana silicic large igneous provinces, having Ndvalues close to Bulk Earth (–0·94 to 0·35)and low, but more variable, initial 87Sr/86Sr ratios (0·7034–0·7080).Quartz phenocryst 18O values indicate that the rhyolite magmashad 18O values between 5·3 and 6·7, consistentwith derivation from a basaltic protolith with 18O values between4·8 and 6·2. The low-18O rhyolites (< 6·0)come from the same stratigraphic horizon and are overlain andunderlain by rhyolites with more ‘normal’ 18O magmavalues. These low-18O rhyolites cannot have been produced byfractional crystallization or partial melting of mantle-derivedbasaltic material. The rhyolites have low water contents, makingit unlikely that the low 18O values are the result of post-emplacementalteration. Modification of the source by fluid–rock interactionat elevated temperatures is the most plausible mechanism forlowering the 18O magma value. It is proposed that the low-18Orhyolites were derived by melting of earlier altered rhyolitein calderas situated to the east, which were not preserved afterGondwana break-up. KEY WORDS: rhyolite; Lebombo; stable and radiogenic isotopes; low-18O magmas; partial melting  相似文献   

9.
BAKER  A. J. 《Journal of Petrology》1990,31(1):243-260
Stable isotope compositions of Ivrea Zone marbles and associatedlithologies are in general heterogeneous. The oxygen isotopecomposition of quartz in pelites ranges from 18O +9 to + 17(SMOW) and does not vary systematically with metamorphic grade.Peridotites retain oxygen isotope signatures close to mantlevalues. Marble calcites vary in isotopic composition from 13C + 2(PDB),180 +24(SMOW)to 13C –6(PDB), 18O + 13 (SMOW).Depletions in 18O and 13C may be explained dominantly by interactionwith fluids derived from within the observed metasedimentarysequence during prograde metamorphism. 18O and 13C show gradients of greater than 5/m across marblemargins and within marbles. The preservation of such isotopicgradients is not consistent with the long-term presence of grain-boundary-scaleinterconnected fluid films in and around marbles. There is ageneral lowering of 18O within individual marble bodies althoughlarge carbon and oxygen isotopic gradients are present. Calcitein marbles may attain oxygen isotope equilibrium, but rarelycarbon isotope equilibrium, with surrounding metapelites. Infiltrationof marbles must involve a component of channelized fluid flow. The general lack of isotopic equilibration within the sequencerequires channelized fluid flow and limited fluid-rock ratios.Large pervasive mantle to crust fluid fluxes are not consistentwith the observations. *Present address: Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, England  相似文献   

10.
This study reports oxygen isotope ratios determined by laserfluorination of mineral separates (mainly plagioclase) frombasaltic andesitic to rhyolitic composition volcanic rocks eruptedfrom the Lassen Volcanic Center (LVC), northern California.Plagioclase separates from nearly all rocks have 18O values(6·1–8·4) higher than expected for productionof the magmas by partial melting of little evolved basalticlavas erupted in the arc front and back-arc regions of the southernmostCascades during the late Cenozoic. Most LVC magmas must thereforecontain high 18O crustal material. In this regard, the 18O valuesof the volcanic rocks show strong spatial patterns, particularlyfor young rhyodacitic rocks that best represent unmodified partialmelts of the continental crust. Rhyodacitic magmas erupted fromvents located within 3·5 km of the inferred center ofthe LVC have consistently lower 18O values (average 6·3± 0·1) at given SiO2 contents relative to rockserupted from distal vents (>7·0 km; average 7·1± 0.1). Further, magmas erupted from vents situated attransitional distances have intermediate values and span a largerrange (average 6·8 ± 0·2). Basaltic andesiticto andesitic composition rocks show similar spatial variations,although as a group the 18O values of these rocks are more variableand extend to higher values than the rhyodacitic rocks. Thesefeatures are interpreted to reflect assimilation of heterogeneouslower continental crust by mafic magmas, followed by mixingor mingling with silicic magmas formed by partial melting ofinitially high 18O continental crust (9·0) increasinglyhybridized by lower 18O (6·0) mantle-derived basalticmagmas toward the center of the system. Mixing calculationsusing estimated endmember source 18O values imply that LVC magmascontain on a molar oxygen basis approximately 42 to 4% isotopicallyheavy continental crust, with proportions declining in a broadlyregular fashion toward the center of the LVC. Conversely, the18O values of the rhyodacitic rocks suggest that the continentalcrust in the melt generation zones beneath the LVC has beensubstantially modified by intrusion of mantle-derived basalticmagmas, with the degree of hybridization ranging on a molaroxygen basis from approximately 60% at distances up to 12 kmfrom the center of the system to 97% directly beneath the focusregion. These results demonstrate on a relatively small scalethe strong influence that intrusion of mantle-derived maficmagmas can have on modifying the composition of pre-existingcontinental crust in regions of melt production. Given thisresult, similar, but larger-scale, regional trends in magmacompositions may reflect an analogous but more extensive processwherein the continental crust becomes progressively hybridizedbeneath frontal arc localities as a result of protracted intrusionof subduction-related basaltic magmas. KEY WORDS: oxygen isotopes; phenocrysts; continental arc magmatism; Cascades; Lassen  相似文献   

11.
The petrogenesis of pyroxenite layers within the Beni Bouseraperidotite massif is investigated by means of elemental andNd-Sr-Pb-O-S isotope analyses. The light rare earth element(LREE) depleted nature of many of the pyroxenites, their widevariation in composition, and lack of correlation between incompatibleelements and fractionation indices preclude them from representingcrystallized melts from a peridotitic source. The physical characteristicsof the pyroxenites and their large (greater than a factor of20) range in Ni rule out partial melting as the cause of theirpetrological and geochemical diversity. Major and compatibletrace element geochemistry is consistent with formation of mostof the pyroxenite suite via high-pressure crystal segregationin magma conduits intruding the peridotites. These magmas crystallizedclinopyroxene, orthopyroxene, and garnet. The pressure of crystallizationis constrained to be above {small tilde}45 kbar from the presenceof graphitized diamonds in pyroxenite layers. Lack of correlationbetween fractionation indices and highly incompatible elementsand the wide variation in incompatible element abundances suggestthat the suite did not form from genetically related magmas.The presence of positive and negative Eu anomalies (Eu/Eu* =0•54–2•0) in pyroxenites which crystallizedat pressures much greater than the plagioclase stability field({small tilde} 45 kbar) suggests that the parental magmas originatedfrom precursors which formed in the crust. Oxygen isotope compositionsof coexisting minerals in the pyroxenites indicate high-temperatureequilibration but 18O values vary from +4•9 to + 9•3,ruling out their derivation from the host peridotites or othernormal mantle sources. The extreme O-isotope variation, togetherwith 34S values of up to + 13 in sulphides included within CPXstrongly suggests that the melts from which the pyroxenitescrystallized were derived from hydrothermally altered, subductedoceanic lithosphere. Extreme initial radiogenic isotope variationin the pyroxenites (Nd + 26 to –9 , 87Sr/86Sr 0•7025–0•7110,206Pb/204Pb 18•21–19•90) support such an originbut also require a component with ancient, high U/Pb and Th/Pbin their source to explain the high 7/4 and 8/4 values of somepyroxenites. This component may be subducted hemi-pelagic sediment.Further evidence for a sediment component in the pyroxenitesis provided by isotopically light carbon in the graphite pyroxenites(13C–16 to – 28). Parentdaughter isotopes in thepyroxenites are strongly decoupled, making estimation of formationages speculative. The decoupling occurred recently (<200Ma), probably as a result of partial melting associated withdiapiric upwelling and emplacement of the massif into the crustfrom the diamond stability field. This late partial meltingevent further depleted the pyroxenites in incompatible elements.The variably altered nature of the subducted protolith and complexhistory of trace element fractionation of the pyroxenites haslargely obscured geochemical mixing trends. However, Nd–Pbisotope systematics indicate that incorporation of the componentwith high U/Pb–Th/Pb occurred relatively recently (<200Ma) for some pyroxenites. Other pyroxenites do not show evidencefor incorporation of such a component and may be substantiallyolder. Tectonic, geophysical, and isotopic constraints indicateformation of the pyroxenites in the mantle wedge above a subductingslab during the Cretaceous. Physical and chemical evidence forhigh-pressure fractionation seen in most of the pyroxenitesprecludes them from simply representing ancient subducted oceaniclithosphere, thinned by diffusion. However, the petrologicaland isotopic diversity of the massif support the concept ofa ‘marble cake’ mantle capable of producing theobserved geochemical diversity seen in oceanic magmas. *Present Address: Department of Terrestrial Magnetism, 5241 Broad Branch Road, N.W., Washington, DC 20015 Present address: Department of Geological Sciences, 1066 C.C. Little Building, University of Michigan, Ann Arbor, Michigan 48109  相似文献   

12.
Mount Galunggung is a historically active volcano in southwesternJava that has erupted four times in the last two centuries.During the most recent event, which occurred during a 9–monthinterval in 1982– 83, some 305 106 m3 of medium–K,calc–alkaline magma was erupted. This eruption was unusualbecause of its duration, the diversity of eruption dynamicsand products, and the range of lava compositions produced. Thecomposition of juvenile material changed gradually during thecourse of the eruption from initial plagioclase (An60–75)and two–pyrozene bearing andesites with 58% SiO2 to finalplagioclase (An85–90), diopside, and olivine (Fo85–90)bearing primitive magnesium basalts with 47% SiO2 Mineralogicaland compositional relationships indicate a magmatic evolutioninvolving differentitation of high–Mg parental melt. Theeruptive volumes of 35 106 m3 andesite, 120 106 m3 maficandesite, and 150 106 m3 basalt are consistent with the ideathat the 1982– 83 eruption progressively tapped and draineda magma chamber that had become chemically stratified throughextensive crystal fractionation. Separates of plagioclase and pyroxene have 18O( SMO W) rangesof + 5. 6 to + 6.0 and + 5.3 to + 5.6, respectively, with 18Oplag–pxvalues of + 0.4 to + 0.6o, indicating internal O–isotopeequiliburium at temperature of 1100–850 C. The magenesianbasalts have magmatic 18O/ 16O ratios similar to those of mid–oceanridge basalt, and the O–isotope ratios of compositionallyevolved derivative melts show no evidence for contaminationof the galunggung magmas by 18O–rich crust during differentiation.Andesites and transitional mafic and sites have a more variableO–isotope character, with laves and phenocrysts havingboth higher and lower 18O values than observed in the parentalmagnesium basalts. These features are interpreted to reflectintramagma chamber processes affecting the upper portions ofthe differentiating Galunggung magma body before the 1982–83eruption.  相似文献   

13.
Whole-rock geochemical data on basaltic to rhyolitic samplesfrom 12 volcanic centers are used to constrain the role of continentalcrust in the genesis of magmas formed beneath the anomalouslywide subduction-related volcanic arc in Ecuador. Relativelyhomogeneous, mantle-like, isotopic compositions across the arcimply that the parental magmas in Ecuador were produced largelywithin the mantle wedge above the subduction zone and not byextensive melting of crustal rocks similar to those upon whichthe volcanoes were built. Cross-arc changes in 143Nd/144Nd and7/4Pb are interpreted to result from assimilation of geochemicallymature continental crust, especially in the main arc area, 330–360km from the trench. Mixing calculations limit the quantity ofassimilated crust to less than 10%. Most andesites and dacitesin Ecuador have adakite-like trace element characteristics (e.g.Y <18 ppm, Yb <2 ppm, La/Yb >20, Sr/Y >40). Availablewhole-rock data do not provide a clear basis for distinguishingbetween slab-melting and deep crustal fractionation models forthe genesis of Ecuador adakites; published data highlightinggeochemical evolution within individual volcanoes, and in magmaticrocks produced throughout Ecuador since the Eocene, appear tosupport the deep fractionation model for the genesis of mostevolved Ecuadoran lavas. A subset of andesites, which displaya combination of high Sr (>900 ppm), Nd >4·1 and7/4Pb <6·0, appear to be the best candidates amongEcuador lavas for slab-melts associated with the subductionof the relatively young, over-thickened, oceanic crust of theCarnegie Ridge. KEY WORDS: andesite; Ecuador; trace elements; isotopes; adakite  相似文献   

14.
An oxygen and hydrogen isotopic study of minerals and wholerocks from the granites of the Mourne Mountains Tertiary complex,and related rocks, shows that whereas a significant circulationof meteoric water was associated with the complex, it had onlyminor and localized effects on the granites themselves. TheSilurian slate and greywacke country rocks, which would havehad 18O(SMOW) values of +10 to +20 before the Tertiary igneousevents, have been depicted 18O to values of –40 to –05Tertiary acid minor intrusions outside the main granite massesare also 18O depleted. l8O whole-rock data on the granites showa range of +6.0 to +9.5, and include values significantly higherthan most of those obtained for the granites of the Tertiarycentral complexes of Skye, Mull, and Ardnamurchan. Many of thelowest whole-rock 18O values are found in samples where theminerals are not in isotopic equilibrium. The mineral oxygenisotopic data can be explained in terms of localized interactionwith meteoric water, resulting in preferential 18O depletionin feldspar(s) and biotite, with quartz being much less affected.The granites all show low values of D(SMOW) for biotite andamphibole separates (–137 to –104). The lowest valuesoccur close to the margins of the plutons, near internal contactsor near greisen localities, and these probably reflect limitedinteraction with meteoric water. The higher D values are fromsamples which show evidence of chloritization. This processappears to have occurred both during interaction with meteoricwater, and also during autometasomatism by an exsolved magmaticfluid in other parts of the plutons, including central locationswhere there is little or no evidence for the penetration ofmeteoric water. Granite samples which exhibit near-equilibriumoxygen isotope fractionations for constituent minerals are characterizedby magmatic O-isotopic compositions. The G2 granite, the largestpluton of the eastern centre, has a magmatic 18O(SMOW) valueof {small tilde}+95; intrusions G3 (eastern centre) and G4(western centre) both have 18O(SMOW) values of {small tilde}+90.The other two main intrusive phases have distinctly lower 18O(SMOW)values: {small tilde}+75 for Gl (the least fractionated graniteof the Mourne Mountains central complex), and from +75 to +85for G5. The oxygen isotopic data rule out simple partial meltingof the country rocks as the origin of the granites and alsopreclude an origin by closed-system fractional crystallizationof basaltic magma typical of that represented by Tertiary basicigneous rocks of the region. * Present address: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham BG12 5GG, UK Present address: School of Engineering Technology, Georgian College, Barrie, Ontario, L4M 3X9, Canada  相似文献   

15.
SIMONETTI  A.; BELL  K. 《Journal of Petrology》1994,35(6):1597-1621
Initial Nd, Pb, and Sr isotopic data from carbonatites and associatedintrusive silica-undersaturated rocks from the early Jurassic,Chilwa Island complex, located in southern Malawi, central Africa,suggest melt derivation from a Rb/Sr- and Nd/Sm-depleted butTh/Pb- and U/Pb-enriched mantle source. Initial 143Nd/144Nd(0.51265–0.51270) isotope ratios from the Chilwa Islandcarbonatites are relatively constant, but their initial 87Sr/86Sr(0.70319–0.70361) ratios are variable. The 18Osmow (9.53–14.15%0)and 13CPDB (–3.27 to –1.50%0) isotope ratios ofthe carbonates are enriched relative to the range of mantlevalues, and there is a negative correlation between 18O andSr isotope ratios. The variations in Sr, C, and O isotopic ratiosfrom the carbonatites suggest secondary processes, such as interactionwith meteoric groundwater during late-stage carbonatite activity.The initial 143Nd/144Nd (0.51246 0.51269) and initial 87Sr/86Sr(0.70344–0.70383) isotope ratios from the intrusive silicaterocks are more variable, and the Sr more radiogenic than thosefrom the carbonatites. Most of the Pb isotope data from Chilwa Island plot to the rightof the geochron and close to the oceanic regression line definedby MORBs and OIBs. Initial Pb isotopic ratios from both carbonatites(207Pb/204Pb 15.63–15.71; 206Pb/204Pb 19.13–19.78)and silicate rocks (207Pb/204Pb 15.61–15.72; 206Pb/204Pb18.18–20.12) show pronounced variations, and form twogroups in Pb-Pb plots. The isotopic variations shown by Nd, Pb, and Sr for the ChilwaIsland carbonatites and intrusive silicates suggest that thesemelts underwent different evolutionary histories. The chemicaldata, including isotopic ratios, from the carbonatites and olivinenephelinites are consistent with magmatic differentiation ofa carbonated-nephelinite magma. A model is proposed in whichdifferentiation of the carbonatite magma was accompanied byfenitization (metasomatic alteration) of the country rocks bycarbonatite-derived fluids, and subsequent alteration of thecarbonatite by hydrothermal activity. The chemical and isotopicdata from the non-nephelinitic intrusive silicate rocks reveala more complex evolutionary history, involving either selectivebinary mixing of lower-crustal granulites and a nephelinitemagma, or incremental batch melting of a depleted source andsubsequent crustal contamination.  相似文献   

16.
Hafnium isotope and incompatible trace element data are presentedfor a suite of mid-ocean ridge basalts (MORB) from 13 to 47°Eon the Southwest Indian Ridge (SWIR), one of the slowest spreadingand most isotopically heterogeneous mid-ocean ridges. Variationsin Nd–Hf isotope compositions and Lu/Hf ratios clearlydistinguish an Atlantic–Pacific-type MORB source, presentwest of 26°E, characterized by relatively low Hf valuesfor a given Nd relative to the regression line through all Nd–Hfisotope data for oceanic basalts (termed the ‘Nd–Hfmantle array line’; the deviation from this line is termedHf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature,present east of 32°E, characterized by relatively high Hfvalues and Lu/Hf ratios. Additionally, two localized, isotopicallyanomalous areas, at 13–15°E and 39–41°E,are characterized by distinctly low negative and high positiveHf values, respectively. The low Hf MORB from 13 to 15°Eappear to reflect contamination by HIMU-type mantle from thenearby Bouvet mantle plume, whereas the trace element and isotopiccompositions of MORB from 39 to 41°E are most consistentwith contamination by metasomatized Archean continental lithosphericmantle. Relatively small source-melt fractionation of Lu/Hfrelative to Sm/Nd, compared with MORB from faster-spreadingridges, argues against a significant role for garnet pyroxenitein the generation of most central SWIR MORB. Correlations betweenHf and Sr and Pb isotopic and trace element ratios clearly delineatea high-Hf ‘Indian Ocean mantle component’ that canexplain the isotope composition of most Indian Ocean MORB asmixtures between this component and a heterogeneous Atlantic–Pacific-typeMORB source. The Hf, Nd and Sr isotope compositions of IndianOcean MORB appear to be most consistent with the hypothesisthat this component represents fragments of subduction-modifiedlithospheric mantle beneath Proterozoic orogenic belts thatfoundered into the nascent Indian Ocean upper mantle duringthe Mesozoic breakup of Gondwana. KEY WORDS: mid-ocean ridge basalt; isotopes; incompatible elements; Indian Ocean  相似文献   

17.
EWART  A. 《Journal of Petrology》1982,23(3):344-382
The magmas of the Tertiary volcanic province of S. Queenslandare chemically bimodal, and occur in numerous volcanic centres,at least three representing original shield volcanoes. The maficlavas are dominantly hawaiites and tholeiitic andesites, whereasthe silicic magmas comprise mainly trachytes, rhyolites, andcomendites. The silicic rocks exhibit variable trace element abundance patterns.There is a progressive depletion of Sr, Ba, V, Mg, Ni, Cr, Mn,and P, through the trachytes to the rhyolites and comenditeswhile the behaviour of Zr, Nb, LREE, Y and Zn is very variable.Rb, Th, and to a lesser extent Pb exhibit a more regular behaviour,becoming most generally concentrated in the comendites and rhyolites.These trace element patterns are modelled by application ofthe Rayleigh distillation model, using partition coefficientsbased on analysed phenocrysts from the S. Queensland siliciclavas. Trace mineral phases, namely zircon, chevkinite, andallanite, are shown to be important in the probable controlof LREE, Zr, and Th abundances, while Nb and Zn are probablycontrolled during fractionation by magnetite. Trace elementdata for the hawaiites and tholeiitic andesites also indicateextensive although variable levels of fractional crystallizationof these magmas. The Sr and O isotopic compositions of the mafic lavas, trachytes,comendites and rhyolites are as follows: initial 87Sr/86Sr ratios;0.70357–0.70456, 0.70432–0.70589, 0.70495–0.70917,and 0.70708–0.70863 respectively. 18O range between 5.6–7.0(mafic lavas), 4.9–8.7 (trachytes), 5.0–7.6 (comendites)and 8.1–10.4 per mil (rhyolites). Pb isotopic compositionsare variable, showing a variation of 6.7 per cent for 206Pb/204Pbratios through the range of volcanic compositions. The rhyolitesexhibit a much greater divergence in their O, Sr, and Pb isotopiccompositions compared with those of associated mafic lavas,than is found in the trachytes and comendites. Within the silicicvolcanics, positive correlations exist between 18O and initialSr ratios, and between Pb isotopic compositions and initialSr ratios (with one group of trachytes providing a noteworthyexception). These correlations are not so clearly defined forthe mafic lavas, although these do exhibit positive correlationsbetween differentiation index, 18O, and initial Sr isotope ratios. The development of the silicic magmas, excepting two groups,is interpreted in terms of a model in which assimilation andfractional crystallization occur concurrently, involving a basaltor hawaiite magma component and a crustal component (modelledon the analysed Carboniferous basement greywackes outeroppingin the region); the data indicate, however, that differentiationcontinued in isotopically closed systems (i. e. isolated fromthe wallrocks). The highly depleted Sr and Ba abundances ofthe rhyolites and comendites suggest that contamination didnot occur after differentiation had ceased. The rhyolites havethe highest isotopic input of the crustal components and areinterpreted as crustal anatectic melts, produced locally withinthe crust in response to basalt/hawaiite magma intrusion, whereasmost of the trachytes and comendites are interpreted as primarilythe differentiated products from original mafic parental magmas,with variable assimilation of crustal wallrock components. Theisotopic data suggest that only the Minerva Hills trachyticlavas, and a Glass House comendite, have not been significantlymodified by wallrock assimilation processes. The erpted maficmagmas were also evidently modified by isotopic crustal wallrockinteractions, which independent petrological data suggest hasoccurred at intermediate to lower crustal depths.  相似文献   

18.
Compositions of the principal minerals and Pb, Nd, and Sr isotopeanalyses of clinopyroxene (cpx) separates are reported for TypeI spinel peridotite xenoliths from the Peridot Mesa vent ofthe San Carlos Volcanic Field. The principal phases are in chemicalequilibrium within each inclusion. Systematic changes in mineralcomposition accompany lithological changes from fertile lherzolitesto infertile harzburgites. These changes are consistent witha fusion residue origin for the major element component of thexenoliths, as noted previously by Frey & Prinz (1978). ExcessFe is additionally present in some inclusions. Pyroxene equilibrationtemperatures calculated using the Wells (1977) geothermometerfall in the narrow range of 1022?34?C (1 s.d.). Equilibrationpressures poorly limit corresponding depths to anywhere between30 and 65 km within the lithospheric mantle. The geotherm is‘advective’ and elevated by 500?C at the depth ofsampling over a reference conductive shield geotherm. The highheat flow measured at the surface results from a combinationof extension and magmatism, with the temperature perturbationextending into the lithospheric mantle. 143Nd/144Nd ratios (0?51251–0?51367) and 87Sr/86Sr ratios(0?70190–0?70504) in cpx demonstrate gross isotopic heterogeneitybeneath the Peridot Mesa vent. This largely overlaps the oceanicmantle array, although four inclusions have Nd greater thanmid-ocean ridge basalts (MORB). PM-228J with Nd = +20 is themost extreme yet reported for a spinel Iherzolite. Pb abundancesin cpx (generally <0?03ppm) are far lower than previouslyreported values. 206Pb/204Pb ratios (17?5–19?1) overlapoceanic basalts and do not correlate with 87Sr/86Sr ratio. However,some of the inclusions exhibit MORB-like 206Pb/204Pb ratiosbut much higher 87Sr/86Sr ratios, which suggests a possiblegenetic link of detached lithospheric mantle with certain oceanicislands. Metasomatic trace element enrichment processes are most widespreadin the infertile (Al-poor, Cr-rich) inclusions, as noted byFrey & Prinz (1978). This systematic relationship is a localfeature of the mantle and suggests that some degree of meltingoccurs commensurately with incompatible element addition. Inparticular, anhydrous peridotite above its volatile-presentsolidus that was flushed with C-O-H fluids containing incompatibleelements would melt and form an enriched infertile fusion residue.The ascending magmas responsible for forming Type II peridotiteveins are the most probable source of the volatiles and mayin some cases react to produce chemical gradients in the wall-rock.Prior metasomatism is also evident isotopically in some inclusions.Overall, the lithospheric mantle beneath Peridot Mesa has suffereda multi-stage history of enrichment, depletion and melting atvarious times since it became attached to the crust above.  相似文献   

19.
A Stable Isotope Study of Anorogenic Magmatism in East Central Asia   总被引：8，自引：0，他引：8  
A stable isotope study of 168 plutonic igneous rock and fivewater samples from Transbaikalia, East Asia, has been made,including 318 whole-rock and mineral 18O/16O analyses and 14D/H analyses. This represents the first detailed isotopic studyof the enormous Phanerozoic K-rich (mostly anorogenic) granitoidbelts of this region, which are thousands of kilometers in lengthand span an age range of >250Ma. Of the five main intrusivesuites, emplacement of the youngest (Permo-Triassic) was accompaniedby intense meteoric-hydrothermal activity, locally causing extreme18O depletio (18O feldspar<–12). This reflects thevery low 18O of the water involved in these systems, which probablyhad 18O < – 20 and D < – 150, consistent withthe high paleolatitude of Transbaikalia in the early Mesozoic(80N). Despite local post-emplacement, hydrothermal 18O-depletioneffects near Permo-Triassic and younger plutons, the variationof magmatic isotopic composition in the five intrusive suites,in space and time, can be clearly discerned using the 18O/16Ovariation in phases such as quartz and sphene that are resistantto sub-solidus exchange. A procedure for analyzing 18O/16O insphene using a laser fluorination technique is described: analysisof samples as small as 0.4 mg (including single crystals ofsphene from granitoids) is possible and provides an effectiveway to estimate the magmatic 18O value of plutonic igneous rocks.Most sphene and quartz 18O values vary by 1.0–2.0% withineach of the five main intrusive suites in Transbaikalia (rangingin age from mid-Paleozoic to Mesozoic), and are uniform bothwithin individual plutons and among plutons of the same suiteseparated by tens or hundreds of kilometers. However, each suitehas a unique range in 18O/16O, indicating that, on a regionalscale, the magmatic 18O values of these granitoids decreasedprogressively in 1% decrements from +10 in the earliest groupto +6 in the youngest. This progression was accompanied by increasesin the concentration of elements such as K and Zr, and decreasesin the concentration of elements such as Sr and Ba. These systematicsrequire large scale deep crustal melting and mixing processesto generate the compositional uniformity of individual plutonsand groups over such wide areas, and also a progressive hybridizationof the crust with alkalic, mantle-derived magmas to generatesyenites and granites with progressively lower 18O values. Thisprocess may be a hallmark of anorogenic granitoid petrogenesisand the intracontinental cratonization process in general, andalso represents an important (though largely cryptic) crustalgrowth mechanism. KEY WORDS: anorogenic granitoids; crustal growth; hybridization; hydrothermal systems; stable isotopes 1Present address: Galson Sciences Limited, 5 Grosvenor House, Melton Road, Oakham LE15 6AX, UK.  相似文献   

20.
Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite,damtjernite) and subordinate dolomite-bearing carbonatite withU–Pb perovskite emplacement ages of 590–555 Ma occurin the vicinity of Aillik Bay, coastal Labrador. The ultramaficlamprophyres principally consist of olivine and phlogopite phenocrystsin a carbonate- or clinopyroxene-dominated groundmass. Ti-richprimary garnet (kimzeyite and Ti-andradite) typically occursat the aillikite type locality and is considered diagnosticfor ultramafic lamprophyre–carbonatite suites. Titanianaluminous phlogopite and clinopyroxene, as well as comparativelyAl-enriched but Cr–Mg-poor spinel (Cr-number < 0.85),are compositionally distinct from analogous minerals in kimberlites,orangeites and olivine lamproites, indicating different magmageneses. The Aillik Bay ultramafic lamprophyres and carbonatiteshave variable but overlapping 87Sr/86Sri ratios (0·70369–0·70662)and show a narrow range in initial Nd (+0·1 to +1·9)implying that they are related to a common type of parentalmagma with variable isotopic characteristics. Aillikite is closestto this primary magma composition in terms of MgO (15–20wt %) and Ni (200–574 ppm) content; the abundant groundmasscarbonate has 13CPDB between –5·7 and –5,similar to primary mantle-derived carbonates, and 18OSMOW from9·4 to 11·6. Extensive melting of a garnet peridotitesource region containing carbonate- and phlogopite-rich veinsat 4–7 GPa triggered by enhanced lithospheric extensioncan account for the volatile-bearing, potassic, incompatibleelement enriched and MgO-rich nature of the proto-aillikitemagma. It is argued that low-degree potassic silicate to carbonatiticmelts from upwelling asthenosphere infiltrated the cold baseof the stretched lithosphere and solidified as veins, therebycrystallizing calcite and phlogopite that were not in equilibriumwith peridotite. Continued Late Neoproterozoic lithosphericthinning, with progressive upwelling of the asthenosphere beneatha developing rift branch in this part of the North Atlanticcraton, caused further veining and successive remelting of veinsplus volatile-fluxed melting of the host fertile garnet peridotite,giving rise to long-lasting hybrid ultramafic lamprophyre magmaproduction in conjunction with the break-up of the Rodinia supercontinent.Proto-aillikite magma reached the surface only after coatingthe uppermost mantle conduits with glimmeritic material, whichcaused minor alkali loss. At intrusion level, carbonate separationfrom this aillikite magma resulted in fractionated dolomite-bearingcarbonatites (13CPDB –3·7 to –2·7)and carbonate-poor mela-aillikite residues. Damtjernites maybe explained by liquid exsolution from alkali-rich proto-aillikitemagma batches that moved through previously reaction-lined conduitsat uppermost mantle depths. KEY WORDS: liquid immiscibility; mantle-derived magmas; metasomatism, Sr–Nd isotopes; U–Pb geochronology  相似文献   

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