The geochemistry of Dinantian volcanism in south Kintyre and the evidence for provincialism in the southern Scottish mantle     

Pauline L. Smedley 《Contributions to Mineralogy and Petrology》1988,99(3):374-384

Scottish Dinantian transitional to mildly alkaline volcanism is represented by abundant outcrops in the Midland Valley, Southern Uplands and Highlands provinces. Dinantian volcanic rocks from Kintyre in the Scottish Highlands range in composition from basalt through basaltic hawaiite, hawaiite, mugearite and benmoreite to trachyte, the compositions of the evolved types being largely due to differentiation from the basaltic parents.Recent geochemical investigations of Scottish Caledonian granitoids, Siluro-Devonian Old Red Sandstone (ORS) lavas and xenolith suites from numerous vents and dykes of Permo-Carboniferous to Tertiary age have revealed that the Scottish crust and upper mantle both increase in age and are increasingly enriched in incompatible elements towards the north and northwest. The upper mantle and lower crust below the Highlands province are therefore largely considered to be more enriched and in parts older than those below the Midland Valley and Southern Uplands. Dinantian alkali basalts from these latter two provinces have Nd values predominantly in the range +3 to +6, initial ⁸⁷Sr/⁸⁶Sr values of 0.7029–0.7041 and ²⁰⁷Pb/ ²⁰⁴Pb values of 15.48–15.60. However, similar basalts from Kintyre and Arran in the Highlands have lower Nd (+0.1 to +3.4) and ²⁰⁷Pb/²⁰⁴Pb (for given ²⁰⁶Pb/²⁰⁴Pb ratios; 15.49–15.51) and slightly higher ⁸⁷Sr/⁸⁶Sr (0.7033–0.7046). This regional variation correlates well with the differences seen between Midland Valley and Highland magmas in the ORS calc-alkaline suite (Thirlwall 1986) and it is suggested that both the ORS and Dinantian basic rocks are derived from similar types of mantle, although no lithospheric slab component is present in the later Dinantian suites. Isotopically-distinct portions of mantle therefore appear to have been present below the Highland and Midland Valley-Southern Upland provinces from at least Caledonian to Carboniferous times. The combined incompatible element and Sr-Nd-Pd isotopic evidence from Kintyre and Arran basaltic rocks does not allow unequivocal distinction between a lithospheric mantle and a sublithospheric mantle source. The observed correlation between isotopic composition of Dinantian basalts and the chemical composition of the lithosphere, together with the apparent involvement of long-term separated source reservoirs suggests that Kintyre and Arran lavas were derived largely from a lithospheric mantle source. On the other hand, the isotopic enrichment of Kintyre basaltic rocks is not extreme; trace element and isotopic compositions are still comparable to modem OIB. Sublithospheric mantle could therefore also be a viable source for Kintyre and Arran Dinantian volcanism.  相似文献   

Initial Pb–Sr(–Nd) isotopic heterogeneity in a single allanite–epidote crystal: implications of reaction history for the dating of minerals with low parent-to-daughter ratios     

Rolf?L.?RomerEmail author  Yilin?Xiao 《Contributions to Mineralogy and Petrology》2005,148(6):662-674

We analyzed 17 fragments from a zoned allanite–epidote crystal (ca 2.2 mm × 4.0 mm), which had formed during different prograde and retrograde stages of ultra high pressure (UHP) and amphibolite facies metamorphism (240–230 Ma, Sulu Belt, E China), for the isotopic composition of Pb, Nd, and Sr and contents of Pb, U, and Th, Sr and Rb, and Nd and Sm. Since most fragments had ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb values less than 1, corrections for in situ Pb growth are small and uncertainties in the recalculation of the Pb isotopic compositions to 240 Ma are insignificant. The recalculated Pb falls on a linear trend in the ²⁰⁶Pb/²⁰⁴Pb vs ²⁰⁷Pb/²⁰⁴Pb diagram with the allanite defining the low–²⁰⁶Pb/²⁰⁴Pb end (17.07) of this trend and the epidote defining its high–²⁰⁶Pb/²⁰⁴Pb end (17.56). The recalculated data scatter in the ²⁰⁶Pb/²⁰⁴Pb vs ²⁰⁸Pb/²⁰⁴Pb diagram, which implies that the initial Pb isotopic variation reflects the involvement of at least three different Pb sources. The low ⁸⁷Rb/⁸⁶Sr values account for a change in ⁸⁷Sr/⁸⁶Sr by in situ ⁸⁷Sr growth of less than 0.0007, which implies that the isotopic heterogeneity of ⁸⁷Sr/⁸⁶Sr (0.70601–0.7200) is a primary feature. The Pb and Sr isotope data unequivocally demonstrate that contributions from different precursor minerals result in initial isotopic heterogeneity in the metamorphic reaction product. It is likely that such an initial isotopic heterogeneity also exists for Nd, but it could not be resolved in the present study. Initially heterogeneous Pb and Sr isotope compositions imply that age differences between core and rim of large crystals may result in the determination of highly arbitrary geological rates, especially for minerals with relatively low parent-to-daughter ratios.  相似文献   

Permian basaltic rocks in the Tarim basin, NW China: Implications for plume–lithosphere interaction     

Yutao Zhang  Jiaqi Liu  Zhengfu Guo 《Gondwana Research》2010,18(4):596-610

There are large areas of Permian basaltic rocks in the Tarim basin (PBRT) in northwestern China. Precise Ar–Ar dating of these rocks revealed an eruption age span of 262 to 285 Ma. Most of the PBRT is composed of alkaline basaltic rocks with high TiO₂ (2.43%–4.59%, weight percent), high Fe₂O₃ + FeO (12.63%–17.83%) and P₂O₅ (0.32%–1.38%) contents. Trace elements of these rocks have affinities with oceanic island basalts (OIB), as shown in chondrite normalized rare earth elements (REE) diagrams and primitive mantle normalized incompatible elements diagrams. The rocks show complex Sr–Nd isotopic character based on which they can be subdivided into two distinct groups: group 1 has relatively small initial (t = 280 Ma)⁸⁷Sr/⁸⁶Sr ratio ( 0.7048) and positive εNd(t) (3.42–4.66) values. Group 2 has relatively large initial ⁸⁷Sr/⁸⁶Sr ratio (0.7060–0.7083) and negative εNd(t) (from − 2.79 to − 2.16) values. Lead isotopes are even more complex with variations of (²⁰⁶Pb/²⁰⁴Pb)t, (²⁰⁷Pb/²⁰⁴Pb)t and (²⁰⁸Pb/²⁰⁴Pb)t ranging from 17.9265 to 18.5778, 15.4789 to 15.6067 and 37.2922 to 38.1437, respectively. Moreover, these two groups have different trace elements ratios such as Nb/La, Ba/Nb, Zr/Nb, Nb/Ta and Zr/Hf, implying different magmatic processes. Based on the geochemistry of basaltic rocks and an evaluation of the tectonics, deformation, and the compositions of crust and lithospheric mantle in Tarim, we conclude that these basaltic rocks resulted from plume–lithosphere interaction. Permian mantle plume caused an upwelling of the Tarim lithosphere leading to melting of the asthenospheric mantle by decompression. The magma ascended rapidly to the base of lower crust, where different degrees of assimilation of OIB-like materials and fractionation occurred. Group 1 rocks formed where the upwelling is most pronounced and the assimilation was negligible. In other places, different degrees of assimilation and fractionation account for the geochemical traits of group 2.  相似文献   

Compilation of radiogenic isotope data in Mexico and their petrogenetic implications     

Ignacio S. Torres-Alvarado  Surendra P. Verma  Gerardo Carrasco-Núñez 《Journal of Earth System Science》2000,109(1):67-78

Seven hundred and twenty-five Sr, two hundred and forty-three Nd and one hundred and fifty-one Pb isotopic ratios from seven different Mexican magmatic provinces were compiled in an extensive geochemical database. Data were arranged according to the Mexican geological provinces, indicating for each province total number of analyses, range and mean of values and two times standard deviation (2σ). Data from seven provinces were included in the database: Mexican Volcanic Belt (MVB), Sierra Madre Occidental (SMO), Baja California (BC), Pacific Ocean (PacOc), Altiplano (AP), Sierra Madre del Sur (SMS), and Sierra Madre Oriental (SMOr). Isotopic values from upper mantle and lower crustal xenoliths, basement outcrops and sediments from the Cocos Plate were also compiled. In the MVB the isotopic ratios range as follows:⁸⁷Sr/⁸⁶Sr 0.703003-0.70841;¹⁴³Nd/¹⁴⁴Nd 0.512496-0.513098;²⁰⁶Pb/²⁰⁴Pb 18.567-19.580;²⁰⁷Pb/²⁰⁴Pb 15.466-15.647;²⁰⁸Pb/²⁰⁴Pb 38.065-38.632. The SMO shows a large variation in⁸⁷Sr/⁸⁶Sr ranging from ∼0.7033 to 0.71387.¹⁴³Nd/¹⁴⁴Nd ratios are relatively less variable with values from 0.51191 to 0.51286. Pb isotope ratios in the SMO are as follows:²⁰⁶Pb/²⁰⁴Pb 18.060-18.860;²⁰⁷Pb/²⁰⁴Pb 15.558-15.636;²⁰⁸Pb/²⁰⁴Pb 37.945-38.625. PacOc rocks show the most depleted Sr and Nd isotopic ratios (0.70232-0.70567 for Sr and 0.512631-0.513261 for Nd). Pb isotopes for PacOc show the following range:²⁰⁶Pb/²⁰⁴Pb 18.049-19.910;²⁰⁷Pb/²⁰⁴⁷Pb 15.425-15.734;²⁰⁸Pb/²⁰⁴Pb 37.449-39.404. The isotopic ratios of the AP rocks seem to be within the range of those from the PacOc. Most samples with reported Sr and Nd isotopic data are spread within and around the “mantle array”. The SMO seems to have been formed by a mixing process between mantle derived magmas and continental crust. The MVB appears to have a larger mantle component, with AFC as the dominant petrogenetic process for the evolved rocks. There is still a need for Pb isotopic data in all Mexican magmatic provinces and of Nd isotopes in BC, AP, SMS, and SMOr.  相似文献   

Sr,Nd and Pb isotopic compositions of calc-alkaline and peralkaline silicic volcanics from the D'Entrecasteaux Islands,Papua New Guinea,and their tectonic significance     

A. J. Stolz  G. R. Davies  A. J. Crawford  I. E. M. Smith 《Mineralogy and Petrology》1993,47(2-4):103-126

Summary Sr, Nd and Pb isotope data are presented for a transitional basalt-peralkaline rhyolite suite, and spatially associated calc-alkaline rhyolites from the D'Entrecasteaux Islands, eastern Papua New Guinea. Both suites have a typical convergent margin geochemical signature (i.e. high Zr/Nb, La/Nb, and low Ta/Yb compared with OIB). The transitional basalt-peralkaline rhyolite suite and calc-alkaline rhyolites have a restricted range of²⁰⁶Pb/²⁰⁴Pb (18.522–18.661),²⁰⁷Pb/²⁰⁴Pb (15.543–15.631), and²⁰⁸Pb/²⁰⁴Pb (38.31–38.63) values which overlap the fields of volcanics from Tonga, Fiji, and Pacific MORB and sediments. The transitional basalt-peralkaline rhyolite suite also displays a restricted range of¹⁴³Nd/¹⁴⁴Nd values (0.513053–0.512984), but a much broader range of⁸⁷Sr/⁸⁶Sr (0.703989–0.70585) values. The latter reflects uncertainties in the ages of samples with very high⁸⁷Rb/⁸⁶Sr values and the effects of Sr exchange with seawater. The calc-alkaline rhyolites have consistently lower¹⁴³Nd/¹⁴⁴Nd values (0.512923—0.512867), and a more restricted range of⁸⁷Sr/⁸⁶Sr values (0.703864–0.704028) compared with the transitional basalt-peralkaline rhyolite suite.The isotopic and trace element data are consistent with the interpretation that the calc-alkaline rhyolites were produced by partial melting of a young arc protocrust, whereas associated calc-alkaline basic and intermediate magmas were derived from a depleted mantle source which previously had been modified by subduction along the Trobriand Trough. The transitional basalt-peralkaline rhyolite suite was produced by extensive magmatic differentiation of a parental transitional basalt magma in a relatively shallow magma chamber. The parent magma was produced by partial melting of either a depleted MORB-source mantle or a less-depleted OIB-type source which previously had been modified by subduction processes. The hy- and ol-normative transitional basalt magmas were probably generated in response to lithospheric thinning at somewhat higher pressures than qz-normative calc-alkaline magmas.The close spatial and temporal relationship between the transitional basalt-peralkaline rhyolite suite and the tale-alkaline volcanics reflects the complex and dynamic tectonic setting of eastern Papua and the D'Entrecasteaux Islands. In particular, the change from calc-alkaline to alkaline magmatism appears to have occurred following a change from compressional to extensional tectonics resulting from the westward propogation of the Woodlark spreading ridge into eastern Papua.

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Sr, Nd und Pb Isotopenzusammensetzung saurer kalkalkalischer und peralkalischer Vulkanite der D' Entrecasteaux Inseln, Papua Neuguinea, und ihre tektonische Bedeutung

Zusammenfassung In dieser Arbeit werden Sr, Nd und Pb Isotopendaten transitionaler Basalt-peralkalischer Rhyolithabfolgen und räumlich mit diesen assoziierter kalkalkalischer Rhyolithe der D' Entrecasteaux Inseln im östlichen Papua Neuguinea vorgestellt. Beide Abfolgen zeigen für konvergiernde Plattenränder typische geochemische Signaturen (z.B. hohe Zr/Nb, La/Nb und niedrige Ta/Yb Verhältnisse im Vergleich mit OIB). Die transitionale Basalt-peralkalische Rhyolithabfolge und die kalkalkalischen Rhyolithe zeigen eine geringe Variation in ihren²⁰⁶Pb/²⁰⁴Pb (18.522–18.661),²⁰⁷/²⁰⁴Pb (15.543–15.631) und²⁰⁸Pb/²⁰⁴Pb (38.31–38.63) Verhältnissen, die mit denen von Vulkaniten aus Tonga, Fiji und den pazifischen MOR-Basalten und Sedimenten übereinstimmen. Die transitionale Basalt-peralkalische Rhyolithabfolge zeigt weiters eine geringe Variationsbreite der¹⁴³Nd/¹⁴⁴Nd Verhältnisse (0.513053–0.512984), aber eine größere Streuung der⁸⁷Sr/⁸⁶Sr Verhältnisse (0.703989–0.70585). Letztere reflektiern die Unsicherheiten der Alterseinstufung der Proben mit sehr hohen⁸⁷Rb/⁸⁶Sr Verhältnissen und die Effekte des Sr-Austausches mit Meerwasser. Die⁸⁷Sr/⁸⁶Sr Verhältnisse kalkalkalischer Rhyolithe variieren im Vergleich dazu gerigfügiger (0.703864–0.704028).Isotopen und Spurenelementdaten sind mit der Interpretation, daß die kalkalkalischen Rhyolithe durch partielle Anatexis junger Inselbogen-Protkruste entstanden sind, kompatibel. Die mit ihnen vergesellschafteten basischen bis intermediären klakalkalischen Magmen entstammen einer abgereicherten Mantelquelle, die zuvor durch Subduktionsprozesse entlang des Trobriandgrabens modifiziert wurde. Die transitionale Basalt-peralkalische Rhyolithabfolge entstand durch extensive magmatische Differentiation eines transitionalen basaltischen Stammagmas in einer relativ seicht liegenden Magmenkammer. Das Stammagma wurde durch partielle Aufschmelzung entweder eines abgereicherten MORB-Mantels oder einer weniger stark abgereicherten OIBQuelle, die zuvor durch Subduktionsprozesse modifiziert wurde, gebildet. Die Hyund O1-normativen transitionalen Basaltmagmen wurden wahrscheinlich im Zuge der Ausdünnung von Lithosphäre unter etwas höheren Drucken gebildet als die Quarznormativen kalkalkaischen Magmen.Die enge räumliche und zeitliche Beziehung der transitionalen Basalt-peralkalischen Rhyolithabfolge mit kalkalkalischen Vulkaniten spiegelt die komplexen und dynamischen geotektonischen Vorgänge im östlichen Papua Neuguinea wieder. Der Wechsel von kalkalkalischem zu alkalischem Magmatismus könnte durch die Umstellung von Kompressions- auf Extensionstektonik, die mit der Verlagerung des westlich gelegenen Woodlark Spreadingzenrums in das östliche Papua Neugiunea in Beziehung gebracht wird, erklärt werden.

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With 13 Figures  相似文献   

Sr-Nd-Pb isotope data for ultramafic xenoliths from Hierro,Canary Islands: Melt infiltration processes in the upper mantle     

M. J. Whitehouse  E. -R. Neumann 《Contributions to Mineralogy and Petrology》1995,119(2-3):239-246

We present here Sr, Nd, and Pb-isotopic data from harzburgite (group I) and dunite-pyroxenite (group II) suite mantle xenoliths from the island of Hierro, one of the youngest and westernmost of the Canary Islands. A progressive leaching technique has been developed and applied to the whole-rock powder samples in order to identify and remove as far as possible any recent additions (host basalt and/or sea-water). Isotopic analyses of the leached residues show significant systematic differences between these two suites. Dunite-pyroxenite suite xenoliths (olivine pyroxenites, dunites and wehrlites) exhibit a relatively small range of isotopic compositions (⁸⁷Sr/⁸⁶Sr from 0.70292 to 0.70315; ¹⁴³Nd/¹⁴⁴Nd from 0.51295 to 0.51302; ²⁰⁶Pb/²⁰⁴Pb from 19.18 to 19.40) compared to the harzburgite suite (⁸⁷Sr/⁸⁶Sr from 0.70295 to 0.70320; ¹⁴³Nd/¹⁴⁴Nd from 0.51285 to 0.51296; ²⁰⁶Pb/²⁰⁴Pb from 18.85 to 19.41). In all isotope correlation diagrams the leached dunite-pyroxenite suite xenoliths plot between the Hierro basalt field and a hypothetical depleted mantle suggesting that these xenoliths may have been strongly infiltrated by Hierro-type basalt. Progressive leaching of this suite of samples showed removal of a component with more enriched Sr (higher ⁸⁷Sr/⁸⁶Sr relative to depleted mantle) and Nd (lower ¹⁴³Nd/¹⁴⁴Nd) isotopic compositions that is probably host basalt glass. The leached harzburgite suite xenoliths extend to more enriched Sr and Nd isotopic compositions than Hierro-type basalt but always have more depleted Pb. This relationship can best be explained if this suite has been subject to infiltration by earlier magmas of the Canary Island suite (in particular, those from Gran Canaria show appropriate compositional ranges), although additional infiltration by Hierro basalt cannot be ruled out. The leaching experiments for this suite mostly show removal of a radiogenic Sr component only (? seawater) which supports the interpretation of early infiltration and subsequent recrystallisation and equilibration prior to the Hierro event. Isotopic data presented in this study show that complex interaction with percolating basaltic melts of varying composition was occurring in the upper mantle beneath Hierro prior to and during the volcanic event and was probably related to the generation of earlier Canary Island magmas.  相似文献   

Effects of basement composition and age on silicic magmas across an accreted terrane-Precambrian crust boundary,Sierra Madre Occidental,Mexico     

《Journal of South American Earth Sciences》2000,13(3):255-273

Trace element and isotopic compositions of mid-Tertiary siliceous magma sequences from two localities of the Sierra Madre Occidental, northern Mexico, display differences that reflect the composition and age of the basement through which they erupted. The crust beneath the section at San Buenaventura is thicker and more evolved and forms part of the North American basement, while that under El Divisadero consists of allochthonous terranes of island arc/oceanic? crust accreted during the Mesozoic.The volcanics are highly differentiated and range in composition from basalt to rhyolite (SiO₂=50–76%). Those erupted through the accreted terranes display a small range of isotope ratios and have lowest initial (age-corrected) Sr isotope ratios (>0.7044) and the highest Nd (<0.5126) and Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb ∼18.9). Isotope ratios of the continental suite are more variable and form an array which trends away from that of the accreted terrane suite toward compositions more typical of old crust (to ⁸⁷Sr/⁸⁶Sr ∼0.710 and ¹⁴³Nd/¹⁴⁴Nd ∼0.5123). The volcanics in the continental zone are relatively more enriched in moderately incompatible elements compared with those within the accreted terranes (Ce/Yb=25–45 vs. 13–33, respectively), but are depleted in some highly incompatible elements such as U and Rb (e.g., Th/U=3.8–7.5 vs. 2.5–4.0, respectively). Those higher in the stratigraphic sections have higher ⁸⁷Sr/⁸⁶Sr, ²⁰⁸Pb/²⁰⁴Pb, and Th/U ratios, and lower ¹⁴³Nd/¹⁴⁴Nd ratios than those lower in the sections.The data have implications for the nature of the sources and the petrogenesis of these volcanics. The isotope ratios of both suites fall between those of mafic magma compositions from the Sierra Madre Occidental, and intermediate and felsic lower crustal xenoliths in northern Mexico and the southwestern USA. The relationship between the isotope ratios of the sequences and the age of the basement, combined with the fact that the overall data set forms well-defined isotopic arrays, demonstrates the strong effects of the crust on the chemistry of the silicic magmas. In the continental suite, isotope ratios covary with Th/Pb and U/Pb ratios, approaching the compositions found in the intermediate and felsic granulite facies xenoliths, strongly indicating that they are not anatectic melts of the lower crust but rather reflect interaction between mantle-derived basaltic parental magmas and the crust. Crustal contributions appear to be large, on the order of 20–70%. The small range of isotope ratios in the accreted terrane suite appears to reflect interaction of the basaltic parent with relatively juvenile crust whose isotopic composition is similar to the mantle-derived magmas. High Th/U and Th/Rb ratios indicate that the crustal contamination occurs in the lower crust. Moreover, the less radiogenic ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios in the continental suite indicate that the depletion in highly incompatible elements in the continental lower crust is an old feature. The secular changes in the isotope ratios within the stratigraphic sections indicate increasingly shallow crustal contributions with time, initially by predominantly mafic deep lower crust and later by more felsic middle crust. Using lavas from outside of the two heavily sampled stratigraphic sections, the differences in the isotopic compositions between volcanics erupted through the accreted terranes and the continental basement help to delineate the location of the boundary.  相似文献   

Geochemistry of Cretaceous mafic rocks from the Lower Yangtze region, eastern China: Characteristics and evolution of the lithospheric mantle   总被引：14，自引：0，他引：14  

Jun Yan  Jiang-Feng Chen  Xi-Sheng Xu   《Journal of Asian Earth Sciences》2008,33(3-4):177-193

Geochemical and isotopic data for Cretaceous mafic rocks (basalt, gabbro, and diorite) from the Lower Yangtze region, northern Yangtze block, constrain the evolution of the lithospheric mantle. The mafic rocks, separated into the northeast and southwest groups, are alkaline and evolved, with low Mg# values (44–58) and variable SiO₂ contents (47.6–57.4 wt%). Enriched LREEs, LILEs, and Pb, together with depleted Nb, Zr, and Ti, suggest that the mantle sources were metasomatized by slab-derived fluid/melt. All samples show high radiogenic ²⁰⁷Pb/²⁰⁴Pb(t) (15.41–15.65) and ²⁰⁸Pb/²⁰⁴Pb(t) (37.66–38.51) ratios at given ²⁰⁶Pb/²⁰⁴Pb(t) (17.65–19.00) ratios, consistent with the mantle sources having been metasomatized by ancient slab-derived material. Mafic rocks of the southwest group show enriched Sr–Nd isotopic characteristics, with ⁸⁷Sr/⁸⁶Sr(t) ranging from 0.7056 to 0.7071 and ε_Nd(t) ranging from −5.3 to −8.3, indicating an origin from enriched lithospheric mantle. Mafic rocks of the northeast group, which record ⁸⁷Sr/⁸⁶Sr(t) ratios of between 0.7044 and 0.7050 and ε_Nd(t) of −2.8 to −0.7, possibly formed by the mixing of melts from isotopically enriched lithospheric mantle and isotopically depleted asthenospheric mantle. Taking into consideration the geochemical and isotopic characteristics of Cretaceous mafic rocks, Cenozoic basalts, and basalt-hosted peridotite xenoliths from the Lower Yangtze region, we propose that an isotopically enriched, subduction-modified lithospheric mantle was replaced by or transformed into an isotopically depleted “oceanic-type” mantle. Such a process appears to have occurred in the eastern North China Craton as well as the eastern Yangtze block, probably in response to subduction of the paleo-Pacific plate beneath East Asia.  相似文献   

Sr isotopic heterogeneity in primitive basaltic rocks,southeastern Australia: correlation with mantle metasomatism     

Suzanne Y. O'Reilly  W. L. Griffin 《Contributions to Mineralogy and Petrology》1984,87(3):220-230

Rb/Sr data for seven basaltic provinces (K-Ar ages 50-0 Ma) in southeastern Australia imply isotopic heterogeneities in the mantle sources. The total range of ⁸⁷Sr/ ⁸⁶Sr is 0.7031–0.7054. Effects of crustal contamination are negligible, since the rocks analyzed represent primary or primitive magma compositions. The inferred scales of heterogeneity range from <1 km for small intraprovince variations, to in the order of 100 km for the larger differences between provinces.Correlation of regional high ⁸⁷Sr/⁸⁶Sr in basaltic rocks with the presence of amphibole-bearing upper mantle xenoliths suggests that the degree of metasomatic activity in the underlying mantle is a major control on the Rb/Sr and ⁸⁷Sr/⁸⁶Sr values of mantle source volumes and partial melts derived from these. Xenolith data also indicate that both pervasive metasomatism and the presence of crystallized melts or cumulates as veins and dykes in mantle wall rock are possible mechanisms for metasomatic additions.Mantle isochrons can be constructed both within some provinces and between provinces. However, episodic metasomatism in the mantle source regions, with correlated enrichment in Rb/Sr and ⁸⁷Sr/⁸⁶Sr, can produce artificial isochrons which may have no relevance to mantle differentiation events.  相似文献   

Petrogenesis and mantle source characteristics of the late Cenozoic Baekdusan (Changbaishan) basalts,North China Craton     

《Gondwana Research》2020

Late Cenozoic intraplate basaltic rocks in northeastern China have been interpreted as being derived from a mantle source composed of DMM and EM1 components. To constrain the origin of the enriched mantle component, we have now determined the geochemical compositions of basaltic rocks from the active Baekdusan volcano on the border of China and North Korea. The samples show LREE-enriched patterns, with positive Eu and negative Ce anomalies. On a trace element distribution diagram, they show typical oceanic island basalt (OIB)-like LILE enrichments without significant Nb or Ta depletions. However, compared with OIB, they show enrichments in Ba, Rb, K, Pb, Sr, and P. The Nb/U ratios are generally within the range of OIB, but the Ce/Pb ratios are lower than those of OIB. Olivine phenocrysts are characterized by low Ca and high Ni contents. The radiogenic isotopic characteristics (⁸⁷Sr/⁸⁶Sr = 0.70449 to 0.70554; ε_Nd = −2.0 to +1.8; ε_Hf = −1.7 to +6.1; ²⁰⁶Pb/²⁰⁴Pb = 17.26 to 18.12) suggest derivation from an EM1-like source together with an Indian MORB-like depleted mantle. The Mg isotopic compositions (δ²⁶Mg = −0.39 ± 0.17‰) are generally lower than the average upper mantle, indicating carbonates in the source. The ⁸⁷Sr/⁸⁶Sr ratios decrease with decreasing δ²⁶Mg values whereas the ¹⁴³Nd/¹⁴⁴Nd and (Nb/La)_N ratios increase. These observations suggest the mantle source of the Baekdusan basalts contained at least two components that resided in the mantle transition zone (MTZ): (1) recycled subducted ancient (∼2.2–1.6 Ga) terrigenous silicate sediments, possessing EM1-like Sr–Nd–Pb–Hf isotopic signatures and relatively high values of δ²⁶Mg; and (2) carbonated eclogites with relatively MORB-like radiogenic isotopic compositions and low values of δ²⁶Mg. These components might have acted as metasomatizing agents in refertilizing the asthenosphere, eventually influencing the composition of the MTZ-derived plume that produced the Baekdusan volcanism.  相似文献   

Sr-Nd-Pb isotopes from the Radicofani Volcano, Central Italy: constraints on heterogeneities in a veined mantle responsible for the shift from ultrapotassic shoshonite to basaltic andesite magmas in a post-collisional setting     

Sandro Conticelli  Riccardo Avanzinelli  Sara Marchionni  Simone Tommasini  Leone Melluso 《Mineralogy and Petrology》2011,103(1-4):123-148

The Radicofani Volcano is characterised by few lava flows, a cinder cone and a denudated neck, and is part of the Tuscan Magmatic Province, the northernmost volcanic region of the Italian peninsula. In spite of the short time span of activity, a large time-dependant chemical and isotopic variability is observed. Most of the rocks of the Radicofani volcano are ultrapotassic shoshonites associated to younger basaltic andesites, found at the bottom of the neck. K₂O contents are positively correlated with trace element and isotopic variations. Shoshonitic and high-K calc-alkaline rocks of the Radicofani volcano are significantly different from shoshonites occurring in association with leucite-bearing ultrapotassic rocks in the southernmost portion of the Roman Magmatic Province. The studied rocks are characterised by high, but variable, levels of incompatible trace elements with a subduction-related signature, with troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of ⁸⁷Sr/⁸⁶Sr range from 0.71333 to 0.71588, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.512050 to 0.512183, while the lead isotope ratios vary between 18.672 and 18.716 for ²⁰⁶Pb/²⁰⁴Pb, 15.665 to 15.696 for ²⁰⁷Pb/²⁰⁴Pb, and 39.981 to 39.081 for ²⁰⁸Pb/²⁰⁴Pb. Ultrapotassic shoshonites show the highest incompatible trace element contents coupled with the highest ⁸⁷Sr/⁸⁶Sr and the lowest ¹⁴³Nd/¹⁴⁴Nd. On the basis of geochemical and isotopic signatures it is argued that magmas were generated in a modified lithospheric peridotitic source containing metasomatic veins generated by K-rich melts from recycled sediments within the mantle via subduction. A further metasomatic event generated by slab-derived fluids pervasively enriched the peridotitic source. Partial melting of the veins produced leucite-free ultrapotassic magmas (i.e. lamproite), and was triggered by rising of the isotherms after the orogenic front migrated eastward in the Italian Peninsula. Further rise of the isotherms induced larger degrees of partial melting inducing melting of the surrounding wall peridotite. The variation of the degree of partial melting of such a heterogeneous peridotitic source produced a wide spectrum of magma compositions, which mimic a mixing line between two components: ultrapotassic magma from partial melting of the metasomatic vein and a basaltic andesitic magma from partial melting of the surrounding peridotite.  相似文献   

Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr–Nd–Pb isotope data on the Roccamonfina volcanic rocks,Roman Magmatic Province,Southern Italy     

Sandro Conticelli  Sara Marchionni  Davide Rosa  Guido Giordano  Elena Boari  Riccardo Avanzinelli 《Contributions to Mineralogy and Petrology》2009,157(1):41-63

The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera Roccamonfina volcano. On the basis of new trace element and Sr–Nd–Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of ⁸⁷Sr/⁸⁶Sr range from 0.70926 to 0.70999, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788–18.851 for ²⁰⁶Pb/²⁰⁴Pb, 15.685–15.701 for ²⁰⁷Pb/²⁰⁴Pb, and 39.048–39.076 for ²⁰⁸Pb/²⁰⁴Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. ⁸⁷Sr/⁸⁶Sr ranges from 0.70665 to 0.70745, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.51234 to 0.51238, ²⁰⁶Pb/²⁰⁴Pb ranges from 18.924 to 19.153, ²⁰⁷Pb/²⁰⁴Pb ranges from 15.661 to 15.694, and ²⁰⁸Pb/²⁰⁴Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone produced pre-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post-collisional isotherms relaxation or increasing T°C due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature, of the same metasomatic assemblage with addition 10–20% of OIB-like astenospheric mantle material. We suggest that addition of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

Post-collisional lamprophyric event in Sierra Norte, Córdoba, Argentina: Mineralogical, geochemical and isotopic characteristics     

M.S. O&#x;Leary  R. Lira  M.J. Dorais  C.C.G. Tassinari 《Journal of South American Earth Sciences》2009,28(3):277-287

Over 20 lamprophyre dykes, varying in width between a few centimeters and several meters, have been identified in central Sierra Norte – Eastern Pampean Ranges, Córdoba, Argentina. Their mineralogy and chemistry indicate that they are part of the calc-alkaline lamprophyres clan (CAL). They contain phenocrysts of magnesiohornblende ± augite set in a groundmass of magnesiohornblende, calcic-plagioclase, alkali feldspar, and opaque minerals, which designate them as spessartite-type lamprophyres. Alteration products include chlorite, calcite and iron oxides after mafic phenocrysts, though some are partially replaced by actinolite. Feldspars are replaced by carbonate and clay minerals.The dykes are relatively primitive, and show restricted major element variation (SiO₂ 51.1–55.3 wt.%, Al₂O₃ 12–16.6 wt.%, total alkalies 1.5–4.7 wt.%), high Mg# (55–77), high Cr contents (27–988 ppm) and moderate to high Ni contents (60–190 ppm). Lamprophyre LILE (e.g. Rb averages 110 ppm, Sr 211–387 ppm, Ba 203–452 ppm) are high relative to HFSE (e.g., Ta 0.2–1.6 ppm, Nb 4–11 ppm, Y 17–21 ppm), and are enriched in LREE (30–70 times chondrite). They are characterized by relatively high ²⁰⁸Pb/²⁰⁴Pb (38.8–39.9), ²⁰⁷Pb/²⁰⁴Pb (15.7), and ²⁰⁶Pb/²⁰⁴Pb (18.7–20.1), combined with low (epsilon)ε_Nd (−4.69 to −1.52) and a relative moderately high (⁸⁷Sr/⁸⁶Sr)_i of 0.7055–0.7074. The Rb–Sr whole rock isochron indicates an Early Ordovician age of 485 ± 25 Ma. The calculated T_DM (1.7 Ga) suggests that these rocks appear to have originated from a reservoir that was created during a mantle metasomatism event related to the Pampean orogeny.The Sierra Norte lamprophyres show affinities with a subduction-related magma in an active continental margin. Their geochemical and isotopic features suggest a multicomponent source, composed of enriched mantle material variably contaminated by crustal components. The lamprophyric suite emplacement occurred at the dawning stage of the Pampean orogeny, in a regional post-collisional extensional setting developed in the Sierra Norte-Ambargasta batholith (SNAB) in Early Ordovician times.  相似文献   

K-Ar ages and Pb,Sr isotopic characteristics of Cenozoic volcanic rocks in Shandong Province,China     

Chen Daogong  Peng Zicheng  Marvin A. Lanphere  Robert E. Zartman 《中国地球化学学报》1985,4(4):311-324

28 samples of Cenozoic volcanic rocks collected from Shandong Province have been dated by K-Ar method. They are mainly Neogene with an age range of 4–19 m. y. The basalts from Linqu and Yishui in west Shandong Province are Miocene and those from Penglai and Qixia in east Shandong Province are Miocene and Pliocene in age. The basalts from Wudi in north Shandong Province are Middle-Early Pleistocene in age. In each area the duration of volcanic eruption was estimated at about 2–3 m. y. Pb and Sr isotopic compositions and U, Th, Pb, Rb, Sr, and major elements in most of the samples were determined. The isotopic compositions are:²⁰⁶Pb/²⁰⁴Pb—16.92-18.48,²⁰⁷Pb/²⁰⁴Pb—15.30-15.59,²⁰⁸Pb/²⁰⁴Pb—37.83-38.54, and (⁸⁷Sr/⁸⁶Sr)_i—0.70327-0.70632. There are some positive or negative linear correlations between²⁰⁶Pb/²⁰⁴Pb and²⁰⁷Pb/²⁰⁴Pb, Pb isotopes and Pb content, Pb isotopes and Sr isotopes, and Sr isotopes and other elements. The basaltic rocks from east and west Shandong Province have somewhat differences in isotopic composition and element content. The basalts probably are products of multi-stage evolution of the mantle. They have preserved the primary features of the source, although they were influenced, to some extent, by the contamination of crustal materials.  相似文献   

Geochemistry of lamprophyres from the Western Alps,Italy: implications for the origin of an enriched isotopic component in the Italian mantle   总被引：3，自引：0，他引：3  

Jennifer Paige Owen 《Contributions to Mineralogy and Petrology》2008,155(3):341-362

Cenozoic lamprophyres (minettes, spessartites, kersantite) from the Western Alps, northern Italy, represent small volume, mafic melts with high Mg#s and high Ni and Cr contents. All the lamprophyres show light REE enrichment, high incompatible element contents, and Ta, Ti and Nb troughs on chondrite-normalized diagrams. Age-corrected ⁸⁷Sr/⁸⁶Sr isotopic ratios (assuming t = 30 Ma) are highly variable and range from 0.70590 to 0.71884; ¹⁴³Nd/¹⁴⁴Nd ratios range from 0.51203 to 0.51242. Pb isotopic ratios are: ²⁰⁶Pb/²⁰⁴Pb = 18.669–18.895, ²⁰⁷Pb/²⁰⁴Pb = 15.605–15.689 and ²⁰⁸Pb/²⁰⁴Pb = 38.224–39.134. ⁸⁷Sr/⁸⁶Sr ratios show a negative correlation with ¹⁴³Nd/¹⁴⁴Nd, and a positive correlation with K, Ba, and Rb as well as with Ti, Th, Ta, Nb and Zr abundances. The primitive nature of the lamprophyres, coupled with their enriched incompatible trace element and isotopic signatures, suggest derivation from a metasomatized upper mantle source. Linear arrays in isotope space and elemental data plots suggest mixing between two distinct end-members in the Italian mantle; an enriched end-member that is isotopically similar to pelagic sediments, and a significantly less enriched end-member that approaches Bulk Earth values. New isotopic data indicate that the mantle source(s) of the lamprophyres from the Western Alps contain a very high proportion of the enriched end-member. The geochemical signature of the enriched end-member is attributed to fluids or melts derived from pelagic sediments subducted during the closure of the Tethyan Ocean in the late Cretaceous to early Tertiary.  相似文献   

Chemical and isotopic variations along the superfast spreading East Pacific Rise from 6 to 30°S     

W. Bach  E. Hegner  J. Erzinger  M. Satir 《Contributions to Mineralogy and Petrology》1994,116(4):365-380

Chemical data of 39 fresh basaltic glasses from the East Pacific Rise (EPR) between 6 and 30°S and Pb, Sr, and Nd isotopic compositions of 12 basalt glasses are presented. Major and trace element data indicate a wide compositional range, including primitive basalts (Mg#=0.67) and highly evolved FeTi-basalts (Mg#=0.34) [molMg/(Mg+Fe²⁺)]. The compositional range can be attributed to low-pressure fractional crystallization. Fractionation-corrected major element concentrations provide evidence for varying mantle melting conditions. Calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar, at temperatures between 1430 and 1280°C, and total degrees of partial melting between 17 and 20% by weight. Leached and hand-picked basalt glasses display large variations in ⁸⁷Sr/⁸⁶Sr (0.70235–0.70270), ¹⁴³Nd/¹⁴⁴Nd (0.51312–0.51323), and ²⁰⁶Pb/²⁰⁴Pb (18.064–18.665), but are similar to other N-type MORB from the EPR. The isotopic ratios of basalts from 13 to 23°S show strong correlations and delineate two systematic trends. From 23 to 17°S, ⁸⁷Sr/⁸⁶Sr and Pb isotope ratios increase and ¹⁴³Nd/¹⁴⁴Nd decrease in agreement with previous results (Mahoney et al. 1989). A reverse trend is indicated by basalts from 17 to 13°S. However, K/Ti and (La/Sm)_N continuously increase from 23 to 13°S. This opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17°S. North of 13.5°S (Garrett Fracture Zone), isotopic data show no systematic variation with ridge location and display an overall weaker covariation. The results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions (P, T, F) (F, degrees of melting). There is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation. Our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30°S. We interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components. In contrast, variations in trace elements are attributed to young mantle differentiation processes.  相似文献   

Trace-element characteristics of east–west mantle geochemical hemispheres     

Hikaru Iwamori  Hitomi Nakamura  Masaki Yoshida  Takashi Nakagawa  Kenta Ueki  Atsushi Nakao  Tatsuji Nishizawa  Satoru Haraguchi 《Comptes Rendus Geoscience》2019,351(2-3):209-220

Recent statistical analyses on the isotopic compositions of oceanic, arc, and continental basalts have revealed that the Earth's mantle is broadly divided into eastern and western hemispheres. The present study aimed to characterize the isotopically defined east–west geochemical hemispheres using trace-element concentrations. Basalt data with Rb, Sr, Nd, Sm, Pb, Th, and U in addition to the isotopic ratios ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb were selected mostly from the GEOROC and PetDB databases. A total of 4787 samples were used to investigate the global geochemical variations. The results show that the wide trace-element variations are broadly explained by the melting of melt-metasomatized and fluid-metasomatized mantle sources. The larger amount of the fluid component derived from subducted plates in the eastern hemisphere than that in the western hemisphere is inferred from the basalts. These characteristics support the hypothesis that focused subduction towards the supercontinent created the mantle geochemical hemispheres.  相似文献   

Petrogenesis of isotopically unusual Pliocene olivine leucitites from Deep Springs Valley, California   总被引：2，自引：0，他引：2  

Brian L. Beard  Allen F. Glazner 《Contributions to Mineralogy and Petrology》1998,133(4):402-417

High-K mafic alkalic lavas (5.4 to 3.2 wt% K₂O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; ⁸⁷Sr/⁸⁶Sr ratios decrease and ɛ_Nd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; ⁸⁷Sr/⁸⁶Sr ratios range from 0.7121 to 0.7105 and ɛ_Nd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, ²⁰⁶Pb/²⁰⁴Pb ∼17.2, ²⁰⁷Pb/²⁰⁴Pb ∼15.5, and ²⁰⁸Pb/²⁰⁴Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛ_Nd values and unradiogenic ²⁰⁶Pb/²⁰⁴Pb ratios have relatively low ⁸⁷Sr/⁸⁶Sr ratios (the EM I array), whereas basalts with low ɛ_Nd values and high ⁸⁷Sr/⁸⁶Sr ratios have radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios and low ɛ_Nd values and ²⁰⁶Pb/²⁰⁴Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998  相似文献   

Shear zone-related syenites in the Damara belt (Namibia): the role of crustal contamination and source composition     

S.?JungEmail author  K.?Mezger  S.?Hoernes 《Contributions to Mineralogy and Petrology》2004,148(1):104-121

Geochemical and Nd-Sr-Pb-O isotope data for a suite of syn-collisional (ca. 520 Ma) syenites associated with a major shear zone in the Proterozoic Damara orogen (Namibia) constrain their sources and petrogenesis. Major rock types from within and outside the shear zone range from highly potassic nepheline syenites to quartz syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Even the most primitive samples show pronounced depletion in Nb, Ti, Sr and P on a primitive mantle-normalized diagram, indicating the involvement of a recycled crustal component in the source. Extrapolation of the Sr-Nd-Pb-O isotope composition of the syenites from within the shear zone back to a hypothetical parental melt with 10 wt% MgO suggests derivation from a moderately enriched lithospheric upper mantle (⁸⁷Sr/⁸⁶Sr: 0.705, Nd: –2, ¹⁸O: 6, ²⁰⁶Pb/²⁰⁴Pb: 19.40, ²⁰⁷Pb/²⁰⁴Pb: 15.82). More evolved quartz syenites show increasing ⁸⁷Sr/⁸⁶Sr ratios, increasing ¹⁸O values but less radiogenic Nd values and Pb isotopes with decreasing MgO, indicating assimilation of ca. 10% Archaean to Proterozoic local lower crust with unradiogenic Nd, high ⁸⁷Sr/⁸⁶Sr and low U/Pb. For samples from outside the shear zone a hypothetical parental melt with 10 wt% MgO has distinctly more radiogenic Sr but less radiogenic Nd isotopic composition (⁸⁷Sr/⁸⁶Sr: 0.712, Nd: –13), with strongly unradiogenic Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb: 17.40, ²⁰⁷Pb/²⁰⁴Pb: 15.50), suggesting another strongly enriched lithospheric mantle source for these rocks. Differentiated syenites from outside the shear zone show decreasing ⁸⁷Sr/⁸⁶Sr, increasing ¹⁸O values, more radiogenic Nd values and Pb isotope ratios with decreasing MgO indicating interaction with a lithospheric component with low Rb/Sr but high Sm/Nd and U/Pb.  相似文献   

Magma source components in an arc-continent collision zone: the Flores-Lembata sector,Sunda arc,Indonesia   总被引：3，自引：0，他引：3  

A. J. Stolz  R. Varne  G. R. Davies  G. E. Wheller  J. D. Foden 《Contributions to Mineralogy and Petrology》1990,105(5):585-601

Major, trace-element, and Sr-, Nd-and Pbisotope data are presented for volcanics from 12 active or recently active volcanoes from the islands of Flores, Adonara, Lembata and Batu Tara in the eastern Sunda are. The volcanics vary in composition from low-K tholeiite, through medium-and high-K calcalkaline types to the K-rich leucite basanites of Batu Tara. From the tholeiites to the leucite basanites there are marked increases in the concentrations of LILE (K, Rb, Ba, Sr), LREE and La/Yb, and all the volcanics have high Ba/ Nb, La/Nb and Ba/La compared with mid-ocean ridge and intraplate eruptives. K/Cs values are generally lower than OIB values, and overlap those of other arc volcanics and northeast Indian Ocean sediments. The volcanics exhibit a broad range of ⁸⁷Sr/⁸⁶Sr (0.70468–0.70706), ¹⁴³Nd/¹⁴⁴Nd (0.512946–0.512447), and a moderate range in ²⁰⁶Pb/²⁰⁴Pb (18.825–19.143), ²⁰⁷Pb/ ²⁰⁴Pb (15.643–15.760) and ²⁰⁸Pb/²⁰⁴Pb (38.97–39.51). Trace-element and isotopic data suggest that the mantle beneath the eastern Sunda arc is a complex heterogeneous mixture of 3 or 4 major source components: MORB-source or depleted MORB-source, OIB-source and subducted Indian Ocean sediment. The low-K tholeiites were probably formed by relatively large degrees of melting of depleted MORB-source mantle, modified by subduction-related fluids, whereas the trace-element and isotopic characteristics of the K-rich volcanics suggest that they were derived from an OIB source which and been modified by a subduction-related melt component. The source components of the medium-to high-K calcalkaline rocks are more difficult to determine, and probably include mixtures of MORB-source or OIB-source, and melt/fluid derived from subducted oceanic sediment. Minor-and trace-element modelling calculations indicate substantial difficulties in producing the relatively low Ti-contents of arc volcanics by melting OIB-source mantle. Where OIB mantle is considered to be an important component of arc magmas it is suggested that the HFSE are buffered to relatively low concentration by a residual Ti-rich accessory phase.  相似文献