The Isotopic Composition of Strontium in Oceanic and Continental Basalts: Application to the Origin of Igneous Rocks

The isotopic compositions of strontium in 25 basalts and relatedvolcanic rocks from both oceanic and continental localitieshave been determined. The isotopic abundance of Sr⁸⁷, convenientlyexpressed as the Sr⁸⁷/Sr⁸⁶ ratio, was found to vary from 0?7047to 0?7101. The range of variation is outside experimental errorsand is considered to be significant. No consistent differencein the value of the Sr⁸⁷/Sr⁸⁶ ratio was observed for basaltsfrom continental and oceanic localities. The average Sr⁸⁷/Sr⁸⁶ratio for eleven oceanic basalts was found to be 0?7072?0?0003,whereas fourteen continental basalts and related volcanic rocksaveraged Sr⁸⁷/Sr⁸⁶=0?7082?0?0003. The errors are the standarddeviations of the mean. The average Sr⁸⁷/Sr⁸⁶ ratio for all25 basalts is 0?7078 with limits of variation of +0?002 and–0?003. The concentrations of rubidium and strontium in a representativenumber of the basalts were determined by isotope dilution. Fiveolivine basalts averaged Rb=17?6 p.p.m., Sr=390?55 p.p.m., Rb/Sr=0?044?0?018;whereas six tholeiitic basalts were found to contain 30?7 p.p.m.Rb, 504?167 p.p.m. Sr, Rb/Sr=0?060?0?024. On the basis of the most reliable rubidium and strontium analysesof igneous and sedimentary rocks available in the literaturethe Rb/Sr ratio of the near-surface part of the continentalcrust is estimated to be 0?25. If its average age is approximately2 billion years and its initial Sr⁸⁷/Sr⁸⁶ ratio was 0?704, thepresent average value would be 0?725. This estimate is compatiblewith results obtained for two composites of Palaeozoic shalewhich averaged Sr⁸⁷/Sr⁸⁶=0?7215?0?001. The hypothesis is advanced that there is sufficient enrichmentof Sr⁸⁷ in crustal materials so that the value of the Sr⁸⁷/Sr⁸⁶ratio of igneous rocks at the time of crystallization can beused as a criterion for the origin of the material. The initialSr⁸⁷/Sr⁸⁶ ratio of an igneous rock formed by assimilation, remelting,or granitization of old crustal material enriched in rubidiumis expected to be measurably higher than that of igneous rockswhich are products of fractional crystallization of basalt magmaor were derived from undifferentiated basaltic material at depthin the crust. The initial Sr⁸⁷/Sr⁸⁶ ratio, time of crystallization, and possibleco-magmatic relationships of differentiated intrusive igneousrocks or series of lava flows of different compositions canbe indicated by the convergence of the whole-rock strontiumdevelopment lines. This method is illustrated by the intrusivealkaline rocks of the Monteregian hills, Quebec.     

Strontium Isotopic Studies of Alkalic Rocks: The Potassium-rich Lavas of the Birunga and Toro--Ankole Regions, East and Central Equatorial Africa

The present-day Sr⁸⁷/Sr⁸⁶ ratios of 117 representative samplesfrom the Birunga and Toro-Ankole regions vary significantlyand range between 0.7036 and 0.7111. The feldspar-bearing lavashave higher ratios (average = 0.707) than the melilite- andnepheline-bearing varieties (average = 0.705). Samples of carbonatedlavas have slightly lower Sr⁸⁷/Sr⁸⁶ ratios. The Sr⁸⁷/Sr⁸⁶ ratios show a highly significant, positive, linearcorrelation with Rb/Sr ratio, and a negative correlation withSr, Nb, and Zr abundances. Graphs of Sr⁸⁷/Sr⁸⁶ ratios againstelemental abundances in some cases give hyperbolic patterns.Such relationships are true, not only for the volcanic fieldas a whole, but also for lava flows from one extensively sampledvolcano. Hypotheses involving simple fractional crystallization or limestonesyntexis are inconsistent with the isotopic data. The elementaland isotopic abundance patterns are most easily explained bythe mixing of two end members of quite different Sr isotopicand chemical compositions. If mixing is assumed, approximatelimits can be set for the compositions of the two end members.These limits are consistent with the hypothesis of assimilationof sialic material by either a carbonatitic or nepheliniticparent magma.     

Strontium Isotope Geochemistry and Petrogenesis of the Early Tertiary Lava Pile of the Isle of Skye, Scotland, and other Basic Rocks of the British Tertiary Province: an Example of Magma--Crust Interaction

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

The Isotopic Composition of Strontium in the Skaergaard Intrusion, East Greenland

The isotopic composition of strontium in the different partsof the differentiated Skaergaard intrusion has been determined.The average Sr⁸⁷/Sr⁸⁶ ratio for the basic rocks was found tobe 0?7065?0?002. Higher values, between 0?7101 and 0?7303, wererestricted to the late-stage acid granophyres. The Sr⁸⁷/Sr⁸⁶ratios for the basic Skaergaard rocks are similar to those foundby previous workers. The enrichment in Sr⁸⁷ expressed in theSr⁸⁷/Sr⁸⁶ ratio is taken to indicate contamination of the acidgranophyres by a source enriched in Sr⁸⁷. From considerationsbased upon circumstantial evidence the average country rock,composed of old Precambrian acid to intermediate gneiss, isnot sufficiently enriched in.Sr⁸⁷ to account for the Sr⁸⁷ enrichmentobserved in the acid granophyres by a simple assimilation process.At the present stage of the investigation the enrichment ofthe acid granophyres in Sr⁸⁷ is unexplained.     

Recent Volcanism in the Puyehue--Cordon Caulle Region, Southern Andes, Chile (40?5?S): Petrogenesis of Evolved Lavas

Puyehue Volcano (40?5?S) in the southern volcanic zone (33?–46?)of the Andes is a largely basaltic stratovolcano constructedon a highly eroded, dominantly andesitic volcanic center. Duringgrowth of Puyehue Volcano there was a trend from basaltic tomore siliceous lavas, and the most recent eruptions (1921–22,1960) are Cordon Caulle rhyodacites and rhyolites erupted fromfissures northwest of the volcano. These basaltic through rhyoliticlavas define a medium-K₂O suite of tholeiitic affinity withtrace element and Pb-isotopic signatures typical of volcanicrocks associated with subduction zones. Most of the evolved lavas, ranging from andesite to rhyolite,formed by low to moderate pressure ( 5 kb) fractional crystallizationof a plagioclase-dominated anhydrous assemblage. Magma mixingproduced aphyric basaltic andesites with anomalously high incompatibleelement contents and latestage andesites with disequilibriumphenocryst assemblages. The age progression from abundant basaltto younger, less voluminous, more silicic lavas reflects increasinglygreater degrees of fractional crystallization which caused theapparent compositional gap between mixing end members to widen. There is no evidence in the silicic lavas for assimilation ofgeochemically distinctive continental crust. Puyehue basaltsare surprisingly more heterogeneous in ⁸⁷Sr/⁸⁶Sr (0?70378–0?70416)and incompatible element abundance ratios (e.g., La/Sm, Ba/Nb)than the more evolved lavas. This geochemical variability mayreflect subcrustal source heterogeneities or contamination bylower crust. The older basaltic andesites and andesites underlyingthe Puyehue edifice have Sr and Nd isotopic ratios and incompatibleelement abundance ratios within the range of Puyehue basalts.Apparently, similar sources and processes were involved in theirgenesis.     

Isotopic Composition of Strontium in Rocks of the Fen Alkaline Complex, South Norway

The isotopic composition of Sr and the abundances of Rb andSr have been determined in the alkaline rocks of the Fen Complex,South Norway. The ⁸⁷Sr/⁸⁸Sr ratios range from as low as 0.703in carbonatite (s?vite) to as high as 0.710 in rauhaugite. Thewhole rock analyses do not plot on a Nicolaysen diagram as anisochron. Calculation of the initial isotopic composition ofSr in the rocks at the time of intrusion of the complex, approximately550 m.y. ago, shows that the rocks are not simply related asthe differentiation products of a single magma, since they donot possess similar initial ⁸⁷Sr/⁸⁶Sr ratios. The lowest initialratio is seen in the s?vites and it is concluded that this rockrepresents the magma from which the other rocks of the complexwere derived. Production of hybrid rocks by bulk assimilationof granite gneiss in carbonatite is inadequate to account forthe observed Rb/Sr ratios. It is envisaged that selective concentrationof granitic Sr of high ⁸⁷Sr/⁸⁶Sr ratio together with loss orgain of variable amounts of Rb has taken place. The processis closely connected with the fenitization observed at the marginsof the complex and is of a metasomatic nature. Thus, the genesisof rocks of the melteigite-ijolite-urtite series may be consideredto be a rheomorphic process. The ⁸⁷Sr/⁸⁶Sr ratios observed inr?dberg and rauhaugite can be explained in terms of the metasomaticalteration of damtjernite.     

The Age and Origin of the Cooma Granite and its Associated Metamorphic Zones, New South Wales

A detailed analysis has been made of the Rb-Sr geochronologyof an area of regional metamorpnism at Cooma, N.S.W. The areacontains a central granite mass surrounded by meta-morphic zonesdecreasing radially in metamorphic grade. Rb–Sr measurements on total-rock and mineral samples definea high uniform Sr⁸⁷/Sr⁸⁶ ratio, 0?7179?0?0005, in the graniteat 415?12 m.yrs. This indicates either the inheritance of radiogenicSr, if the 415 m.yr age represents the time of formation ofthe granite, or else a re-arrangement of radiogenic Sr withinthe granite after its formation. The values for the indicated age and initial Sr⁸⁷/Sr⁸⁶ of thehigh-grade metasediments are very close to those of the granite,suggesting a genetic relationship between the two. In contrastto this, the more distant greenschist facies metasediments havea significantly greater age, 460?11 m.yrs, and a lower initialSr⁸⁷/Sr⁸⁶, 0?710?0?002. By considering the movement of strontium isotopes necessaryto produce the linear correlations between Sr⁸⁷/Sr⁸⁶ and Rb⁸⁷/Sr⁸⁶observed in the various samples, and the restrictions to suchmovement apparently set by geological evidence, it is concludedthat no extensive rearrangement of isotopes occurred at Coomalater than the high-grade metamorphism. The high initial Sr⁸⁷/Sr⁸⁶in the granite is therefore regarded as an original feature. The age and Sr/Rb ratios of the adjacent metasedimentary rocksyield a range in the Sr⁸⁷/ Sr⁸⁶ values at the times of graniteformation whose mean value would approximate to the primaryvalue observed in the granite. Production of the granite byanatexis of these sediments is, therefore, quite compatiblewith its particular Sr⁸⁷ abundance.     

The isotopic composition of strontium in some South African kimberlites

Strontium isotopic studies of kimberlites reveal no significant differences between the respective whole-rock Sr⁸⁷/Sr⁸⁶ ratios of fissure and pipe kimberlites. Kimberlites from the Swartruggens fissure (calcareous micaceous kimberlite) have Sr⁸⁷/Sr⁸⁶ ratios of from 0.709 to 0.716, whilst those from the Wesselton pipe have Sr⁸⁷/Sr⁸⁶ ratios of from 0.708 to 0.715. Other kimberlites range from 0.706 to 0.715. Samples are considered to be late Cretaceous to early Tertiary and thus the ratios are approximately initial ratios. The Sr⁸⁷/Sr⁸⁶ ratios bear no relation to the Rb or Sr content of individual kimberlite bodies. The high initial ratios are not due to bulk assimilation of granitic material in either a kimberlite or carbonatitic magma. Rb-Sr data for garnet peridotites and eclogite xenoliths in kimberlite are not compatible with production of kimberlite by eclogite fractionation from a melt derived from garnet lherzolite. The Sr isotopic composition of kimberlite is compatible with partial melting of garnet mica peridotite. The isotopic composition of liquids formed by partial melting of this rock can be modified by (i) gross contamination with material of low Sr⁸⁷/Sr⁸⁶ ratio or (ii) selective diffusion of material of high Sr⁸⁷/Sr⁸⁶ ratio into kimberlitic fluids.     

On the age of the Onverwacht Group,Swaziland Sequence,South Africa

Some rocks of the Onverwacht Group, South Africa, have been analyzed for Rb and Sr concentrations and Sr isotopie composition. These rocks include volcanic rocks, layered ultramafic differentiates and cherty sediments. Whole rock data indicate that the Rb-Sr isotopie systems in many samples were open and yield no reasonable isochron relationships. However, the data of mineral separates from a basaltic komatiite define a good isochron of $\text{t = 3.50 ± 0.20}$ (2δ) b.y. with an initial Sr⁸⁷/Sr⁸⁶ ratio of $\text{0.70048 ± 5}$(2δ). The orthodox interpretation of this age is the time of the low grade metamorphism. Since the basaltic komatiite is stratigraphically lower than the Middle Marker Horizon (dated as $\text{3.36 ± 0.07}$ b.y. Hurley et al., 1972), and since it is commonly found that volcanism, sedimentary deposition, metamorphism and igneous intrusion in many Archean greenstone-granite terrain all took place in a relatively short time interval (less than 100 m.y.), it is reasonable to assume that the age of 3.50 b.y. might also represent the time of initial Onverwacht volcanism and deposition. The initial Sr⁸⁷/Sr⁸⁶ ratio obtained above is important to an understanding of the Sr isotopic composition of the Archean upper mantle. If the komatiite represents a large degree of partial melt (40–80 per cent) of the Archean upper mantle material, then the initial ratio obtained from the metamorphic komatiite should define an upper limit for the Sr isotopic composition of the upper mantle under the African crustal segment.     

Rubidium-Strontium Age of some Metamorphic Rocks from the Llano Uplift, Texas1

Four total-rock samples of the Valley Spring Gneiss give a Rb-Srisochron age of 1120?25 million years and an initial Sr⁸⁷/Sr⁸⁶ratio of 0.706?0.002. This age is significantly older than the1,000–1,050 and 920?20 million year dates previously determinedfor the igneous rocks of the Llano Uplift. Mineral isochrons for the individual gneiss samples yield anage of metamorphism of 1,000?15 million years, in good agreementwith the time of major plutonism. The strontium in each of theolder metamorphic rocks was isotopically homogenized at thistime, resulting in Sr⁸⁷/Sr⁸⁶ ratios ranging from 0?716 to 0?775.From these data it is concluded that the plutons, which appearto have formed contemporaneously with a Sr⁸⁷/Sr⁸⁶ ratio of 0?706,could not have obtained their strontium from the surroundingmetamorphic rocks unless some special mechanism operated toremove excess radiogenic strontium. Only if the granite remainedan open system, unable to retain radiogenic strontium duringits early history, could it be as old as the total-rock ageof the gneiss.     

用84Sr作稀释剂时样品中87Sr/86Sr比值的计算

用Rb-Sr法测定地质年龄,对地质样品中的锶往往要进行两种同位素分析:非稀释分析以测定⁸⁷Sr/⁸⁶Sr比值;稀释分析以测定样品中⁸⁶Sr的含量。随着稀释剂⁸⁴Sr丰度的提高,Wasserburg et al.(1964),Van schmus(1965),Wetherill与Bickford(1965)采用了以一代二的办法——用稀释分析既测定含量又测⁸⁷Sr/⁸⁶Sr比值。由于稀释剂中⁸⁴Sr高度富集,⁸⁷Sr/⁸⁶Sr受到的影响小,在当时的技术条件下,这些作者没有对一些数据处理进行讨论。但随着计算技术的应用,大大提高了测量精度,稀释剂对⁸⁷Sr/⁸⁷Sr的影响则必须加以考虑。     

The Arc Lavas of the Shirahama Group, Japan: Sr and Nd Isotopic Data Indicate Mantle-Derived Bimodal Magmatism

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     

Strontium isotope geochemistry of Icelandic geothermal systems and implications for sea water chemistry

Chemical and Sr isotopic analyses have been made of waters from 16 geothermal sites in Iceland with particular reference to the systems at Reykjanes and Svartsengi for which compositions of geothermal sea water and fresh and hydrothermally-altered rocks have been compared. The alkalies display mixing relationships indicating a hydrothermal input of Rb and K to local meteoric and sea waters as do results for Sr and Ca involving high-temperature fluids. ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the geothermal waters of meteoric origin parallel those of associated rocks but are higher. Ratios for geothermal sea waters are 0.7042 (Reykjanes) and 0.7040 (Svartsengi), lower than for normal sea water (0.7092) because of leaching of Sr from rocks followed by partial removal into alteration minerals, of which epidote and chlorite may be most important. Consequently, associated hydrothermally-altered rocks have been subject to significant Sr isotopic contamination by sea water Sr raising ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios from 0.7032 for fresh rock to 0.7038–0.7042 (Reykjanes) and to 0.7039–0.7041 (Svartsengi). Altered basalt is only ~50% equilibrated isotopically with geothermal sea water, at a water/rock ratio of ~2, but is internally equilibrated whereas palagonitized rocks (water/rock ratio of 3 to 4) are close to Sr isotopic equilibrium with associated sea water but show significant internal Sr disequilibrium. Hydrothermal input is unlikely to be important in the oceanic mass balance of Sr but is likely to be highly significant in controlling the strontium isotopic composition of sea water.     

Geochemical Evidence for a Rift-Related Origin of Bimodal Volcanism at Meseta Rio San Juan,North-Central Mexican Volcanic Belt

This study reports new geochemical and Sr and Nd isotope data for 11 samples of hynormative late Miocene (～6.5 Ma) basalt, basaltic andesite, and rhyolitic volcanic rocks from Meseta Rio San Juan, located in the states of Hidalgo and Queretaro, Mexico, in the north-central part of the Mexican Volcanic Belt (MVB). The in situ growth-corrected initial isotopic ratios of these rocks are as follows: ⁸⁷Sr/⁸⁶Sr 0.703400-0.709431 and ¹⁴³Nd/¹⁴⁴Nd 0.512524-0.512835. For comparison, the isotopic ratios of basaltic rocks from this area show very narrow ranges as follows: ⁸⁷Sr/⁸⁶Sr 0.703400-0.703540 and ¹⁴³Nd/¹⁴⁴Nd 0.512794-0.512835. The available geological, geochemical, and isotopic evidence does not support the generation of the basic and intermediate magmas by direct (slab melting), nor by indirect (fluid transport to the mantle) participation of the subducted Cocos plate. The basaltic magmas instead could have been generated by partial melting of the upper mantle. The evolved basaltic andesite magmas could have originated from such basaltic magmas through assimilation coupled with fractional crystallization. Rhyolitic magmas might represent partial melting of different parts of the underlying heterogeneous crust. Their formation and eruption probably was facilitated by extensional tectonics and upwelling of the underlying mantle. The different petrogenetic processes proposed here for basaltic and basaltic andesite magmas on one hand and rhyolitic magmas on the other might explain the bimodal nature of Meseta Rio San Juan volcanism. Finally, predictions by the author about the behavior of Sr and Nd isotopic compositions for subduction-related magmas is confirmed by published data for the Central American Volcanic Arc (CAVA).     

Evolution of Calc-alkaline Magmas in Continental Arc Volcanoes: Evidence from Alicudi, Aeolian Arc (Southern Tyrrhenian Sea, Italy)

Major, trace element, and Sr isotopic data are reported forvolcanic rocks from the island of Alicudi, Aeolian Arc, SouthernTyrrhenian Sea. The island is constructed of basalt, basalticandesite to high-K andesite lavas, and pyroclastites, whichshow a continuum in the variation of many major and trace elements.Total iron, MgO, CaO, Ni, Co, Sc, and Cr decrease with increasingsilica, whereas incompatible elements Rb, Ba, Th, and LREE displaythe opposite tendency. Very significant positive correlationsare defined by incompatible elements on interelemental variationdiagrams. Sr isotopic ratios vary from 0–70352 to 0–70410.Overall, basalts (0–70352–O-70410) and basalticandesltes (0–70356–0–70409) are enriched in⁸⁷Sr compared with high-K andesites (O–70352–O–70367),which display the lowest Sr isotopic ratios within the entireAeolian archipelago. Overall negative relationships exist between⁸⁷Sr/⁸⁶Sr and several incompatible trace element abundancesand ratios, such as Th, U, LREE, Zr, La/Yb, and Th/Hf. Otherelemental ratios such as La/Rb, Ba/Rb, and Sr/Rb show more complexbehaviour, even though negative correlations with Sr isotopicratios are observed in the basalts. The observed compositional variations are best explained interms of a model in which primitive calc-alkaline magmas evolvedby crystal-liquid fractionation to give a series of variouslydifferentiated liquids, which underwent different degrees ofinteraction with crustal material. The more mafic and hotterbasaltic liquids appear to have assimilated higher amounts ofmetamorphic wall rocks than did the cooler late erupted andesiticmagmas. This process produced significant variations of Sr isotopicratios, Rb, Cs, Rb/Sr ratios, and LILE/Rb ratios in mafic magmas,but had only minor effects on the abundances and ratios of otherincompatible elements such as Th, LREE, La/Yb, and Th/Hf. When compared with mafic rocks from other Aeolian islands, theAlicudi basalts are more primitive geochemically and isotopically.Going eastward, there is a decrease in Ni and Cr abundances,mg-number and Nd isotopic ratios which parallels an increaseof Sr isotopic ratios in basaltic rocks along the arc. Thesecompositional variations are typical of volcanic series whichhave undergone interaction with upper-crustal material, andsuggest that this process may have contributed significantlyto the regional geochemical and isotopic trends observed inthe Aeolian arc.     

Petrogenesis of the magmatic complex at Mount Ascutney,Vermont, USA

The Ascutney Mountain igneous complex in eastern Vermont, USA, is composed of three principal units with compositions ranging from gabbro to granite. Sr and O isotopic and major element relationships for mafic rocks, granites, and nearby gneissic and schistose country rock have been investigated in order to describe the petrogenesis of the mafic suite which ranges from gabbro to diorite. The entire complex appears to have been formed within a short interval 122.2±1.2 m.y. ago. The granites with ¹⁸O near +7.8 had an initial ⁸⁷Sr/⁸⁶Sr of 0.70395(±6) which is indistinguishable from the initial ratio of the most primitive gabbro. Initial ⁸⁷Sr/⁸⁶Sr ratios and ¹⁸O values for the mafic rocks range from 0.7039 to 0.7057 and +6.1 to +8.6, respectively. The isotopic ratios are highly correlated with major element trends and reflect considerable crustal contamination of a mantle-derived basaltic parent magma. The likely contaminant was Precambrian gneiss similar to exposed bedrock into which the basic rocks were emplaced. A new approach to modelling of assimilation during the formation of a cogenetic igneous rock suite is illustrated. Chemical and isotopic modelling indicate that the mafic rocks were produced by simultaneous assimilation and fractional crystallization. The relative amounts of fractionation and assimilation varied considerably. The mafic suite was not produced by a single batch of magma undergoing progressive contamination; rather, the various rocks probably were derived from separate batches of magma each of which followed a separate course of evolution. The late stage granite was apparently derived from basaltic magma by fractionation with little or no crustal assimilation. The early intrusive phases are much more highly contaminated than the final one. The observed relationships have important implications for the formation of comagmatic complexes and for isotopic modelling of crustal contamination.     

Strontium Isotope Studies related to Petrogenesis in the Caledonian Basic Igneous Province of NE. Scotland

Rubidium-strontium isotope analyses are reported for 90 whole-rocksfrom the Newer Gabbros and associated rocks of NE. Scotland.The Sr⁸⁷/Sr⁸⁶ ratio of the regional gabbro magma at Insch andBelhelvie underwent a progressive increase, in the former casefrom 0·703 to 0·712 during differentiation tosyenogabbros and syenites. This is ascribed to a process ofisotopic equilibration with country rocks of the Dalradian Series.The wider implications of a process by which the Sr⁸⁷/Sr⁸⁶ ratioof a basic magma can be increased in the crust without any othersigns of petrological contamination are considered. Cordierite-noritesand related rocks of the Haddo House and Arnage masses haveinitial Sr⁸⁷sol;Sr⁸⁶ ratios of 0·720–0·730,essentially the same range as for the country rocks at the timeof intrusion. This is interpreted as support for the hypothesisthat such rocks were formed by the partial fusion of peliticsediments adjacent to the basic igneous sheet.     

安徽伏川蛇绿岩套的Nd-Sr-O同位素研究

安徽歙县伏川的蛇绿岩套形成于中-晚元古宙,其Nd、Sr和O同位素组成是:ε_Nd(T)=+0.7-+3.8,ε_Sr(T)=+30.7-+53.9,δ¹⁸O=3.2-11.0‰。据地质学和同位素地球化学特征,该岩套位于杨子板块南缘、江南古岛弧的弧后小洋盆地轴部。ε_Nd(T)值的变化是由于蛇绿岩形成过程中受到下伏不成熟硅铝质基底地壳的混染引起的;ε_Sr(T)和δ¹⁸O的变化,是在蛇绿岩形成时或形成后不久遭受海水热液蚀变的结果。     

Petrogenesis of Sierra Nevada plutons inferred from the Sr, Nd, and O isotopic signatures of mafic igneous complexes in Yosemite Valley, California

Mafic complexes in the central Sierra Nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust. In the intrusive suite of Yosemite Valley, major and trace element compositions of the hornblende-bearing gabbroic rocks from the Rockslides mafic complex and of the mafic dikes in the North America Wall are compositionally similar to high-alumina basalt. Of these rocks, two samples have higher Ni and Cr abundances as well as higher ε_Nd values than previously recognized for the intrusive suite. Plagioclase crystals in rocks from the North America Wall and the Rockslides have prominent calcic cores and sharply defined sodic rims, a texture commonly associated with mixing of mafic and felsic magmas. In situ analyses of ⁸⁷Sr/⁸⁶Sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains. We propose that the high ⁸⁷Sr/⁸⁶Sr (~0.7067) and low ε_Nd (~?3.4) of bulk rocks, the homogeneity of ⁸⁷Sr/⁸⁶Sr in plagioclase, and the high δ¹⁸O values of bulk rocks (6.6–7.3 ‰) and zircon (Lackey et al. in J Petrol 49:1397–1426, 2008) demonstrate that continental crust was assimilated into the sublithospheric mantle-derived basaltic precursors of the mafic rocks in Yosemite Valley. Contamination (20–40 %) likely occurred in the lower crust as the magma differentiated to high-alumina basalt prior to plagioclase (and zircon) crystallization. As a consequence, the isotopic signatures recorded by whole rocks, plagioclase, and zircon do not represent the composition of the underlying lithospheric mantle. We conclude that the mafic and associated felsic members of the intrusive suite of Yosemite Valley represent 60–80 % new additions to the crust and include significant quantities of recycled ancient crust.     

Ultramafic Xenoliths in Plio-Pleistocene Alkali Basalts from the Eastern Transylvanian Basin: Depleted Mantle Enriched by Vein Metasomatism

Ultramafic xenoliths from alkali basalts in the Perjani Mountainsin the Eastern Transylvanian Basin (ETB) of Romania are mainlyspinel Iherzolites, although spinel harzburgites, websterites,clinopyroxenites and amphibole pyroxenites are also present.Amphibole veins cut some spinel peridotite samples. All arederived from the shallow lithospheric upper mantle. In general,textural variations are restricted to protogranular and porphyroclastictypes, compared with the more varied textures found in mantlexenoliths from the alkali basalts of the neighbouring PannonianBasin. Also, ETB peridotites are richer in amphibole. Thus,the mantle beneath the edge of the ETB is less deformed butmore strongly metasomatized than the mantle closer to the centreof the Pannonian Basin.Mineralogical and bulk-rock geochemicalvariations resemble those of spinel Iherzolites from other sub-continentalshallow mantle xenolith suites. There is no apparent correlationbetween deformation and geochemistry, and much of the majorand trace element variation is due to variable extraction ofpicritic melts. The REE patterns of separated clinopyroxenesfrom the peridotite xenoliths are mostly LREE depleted, althoughclinopyroxenes from regions adjacent to amphibole veins haveexperienced an enrichment in La and Ce and a change in theirSr and Nd isotopic values towards those of the vein, while stillretaining an overall LREE depletion. Clinopyroxenes from thewebsterites and clinopyroxenites are more variable. Amphibolein the hydrous pyroxenites and amphibole veins is strongly LREEenriched and is considered to be metasomatic in origin. ⁸⁷Sr/⁸⁶Srand ¹⁴³Nd/^l44Nd isotopic ratios of the xenoliths vary between07018 and 07044, and 051355 and 0 51275, respectively. Thesevalue are more depleted than those obtained for xenoliths fromthe Pannonian Basin. The lower ^l43Nd/^l44Nd and higher ⁸⁷Sr/Sr⁸⁶values are found in anhydrous pyroxenites, metasomatic amphibolesin veins and amphibole pyroxenites, and in the only exampleof an equigranular spinel Iherzolite in the suite.The ETB xenolithswere brought to the surface in alkaline vokanism which post-dateda period of Miocene to Pliocene subduction-related cak-alkalinevolcanism. However, the effects of the passage of either slab-derivedfluids or cak-alkaline magmas through the ETB lithospheric mantlecannot be discerned in the chemistry of the xenoliths. The metasomaticamphibole has ⁸⁷Sr/Sr⁸⁶ and ¹⁴³Sr/Sr¹⁴⁴ ratios similar to thehost alkali basalts, but the least evoked cak-alkaline magmasalso have similar Sr and Nd isotope compositions. The REE patternsof the amphibole resembk those of amphiboles considered to havecrystallized from alkaline melts. No preferential enrichmentin elements typically associated with slab-derivedfluids (K,Rb and Sr) relative to elements typically depleted in cak-alkalinemagmas (Ti, 2jr and Nb) has been observed in the vein amphiboles,although some interstitial amphibole is depleted in all incompatibletrace elements, including LREE. Thus, despite its position closeto the calc-alkaline volcanic arc of the Eastern Carpathians,we cannot readily detect any interaction between the lithosphericupper mantle beneath the ETB and subduction-related magmas orfluids. Metasomatism in the lithospheric mantle is instead largelyrelated to the passage of a primitive alkaline magma similarto the host alkali basal ^*corresponding author