A strontium isotopic study of the volcanic rocks of the Myoko volcano group,central Japan

Thirty-one selected volcanic rocks from the Myoko volcano group which comprises a volcanic chain of four independent volcanoes of Quaternary to Recent age are analyzed for ⁸⁷Sr/⁸⁶Sr ratios. The rocks of the lizuna volcano, the oldest among the Myoko volcano group, have higher ⁸⁷Sr/⁸⁶Sr ratios and show a larger scatter ranging from 0.7043₇ to 0.7055₆ than those of other volcanoes. The Kurohime volcanic rocks have a restricted range of ⁸⁷Sr/⁸⁶Sr ratios (0.7040₃∼0.7043₅). ⁸⁷Sr/⁸⁶Sr ratios of the Myoko volcanic rocks are almost the same in average to those of the Kurohime volcanic rocks, although somewhat varied ranging from 0.7037₈ to 0.7046₁. A single analysis of the Yakeyama volcanic rock yielded a ⁸⁷Sr/⁸⁶Sr ratio of 0.7042₇. A characteristic pattern in ⁸⁷Sr/⁸⁶Sr ratios is observed through the volcanic activity of the Myoko volcano group; ⁸⁷Sr/⁸⁶Sr ratios are high in the early stage of the volcanic activity and then decrease to low values, the late eruptives being characterized by constant ⁸⁷Sr/⁸⁶Sr ratios. The negative correlation between ⁸⁷Sr/⁸⁶Sr and Rb/Sr, and positive correlation between ⁸⁷Sr/⁸⁶Sr and Sr found in the rocks of the Iizuna volcano are interpreted to show the occurrence of contamination by materials with high ⁸⁷Sr/⁸⁶Sr ratios (>0.7056), low Rb/Sr ratios (<0.01) and high Sr contents (>300 ppm). Sialic crustal contamination may have played only a minor role.     

Pb and Sr isotopes in volcanic rocks from the Aleutian Islands and Pribilof Islands,Alaska

The Pb and Sr isotope ratios of Plio-Pleistocene volcanic rocks from the Aleutian volcanic arc are used as tracers of the lithospheric subduction process at the converging Pacific and Bering plates. Aleutian arc lavas do not have the same Pb isotopic compositions as volcanic rocks of the subducted Pacific ocean crust or the nearby Pribilof Islands, but appear to contain an ‘old continental crustal component’ with high ²⁰⁷Pb/²⁰⁴Pb ratio, as has been found in some other volcanic arcs.⁸⁷Sr/⁸⁶Sr ratios in the Aleutian volcanic arc rocks average 0.70322, slightly higher than fresh volcanic rocks from normal ridge segments, but within the range of values from ‘Icelandic’ ridge segments, oceanic islands and the Pribolof Islands. The Pb and Sr isotopic compositions of Aleutian lavas show a positive correlation and the range of values does not change for volcanoes distributed along strike in the arc, even though the crustal type in the hanging wall of the Benioff zone changes from oceanic in the west to continental in the east. Since the basement of the continental arc segment is older than the basement of the oceanic segment, and probably has a different isotopic character, the constancy of isotopic ratios along the arc argues against contamination by wall rocks of the type exposed in the arc.A sufficient explanation for the isotopic data is the mixture of several per cent of continent-derived sediment with melt derived from the underthrust oceanic crust and overlying mantle. This small amount of contaminant is difficult to document by geophysical observations. Such a model implies extensive recycling of Ba, Pb, K and Rb through volcanism at convergent plate margins like the Aleutians.     

The petrology and geochemistry of the Azores Islands

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

Sr and Nd Isotopic Characteristics and Magma Genesis of Mesozoic Volcanic Rocks Along the Coastal Region of Southeastern China

The Mesozoic volcanic rocks in the coastal region of southeastern China were superimposed on some different basement tectonic elements. The volcanic rocks developed in these different basement tectonic elements have great differences in Sr and Nd isotopic compositions. The rocks in western Zhejiang and northeastern Jiangxi Provinces which belong to the Lower Yangtze subplate have lower initial 87Sr/ 86Sr ratios, but are higher in initial Nd isotopic ratios. The initial 143Nd / 144Nd values of the volcanic rocks developed in the Cathaysian subplate increase clearly from early to late in time, and from the core of the Wuyishan uplift coastwards constantly, but the initial 87Sr/86Sr values tend to decrease. The isotopic characteristics and their spatial variations in Mesozoic volcanic rocks in the study region are, to a great extent, manifestations of the isotopic characteristics in basement metamorphic complexes, and the generation of the Mesozoic acid magma in this region is attributed to the recycling o     

New geochronological data on volcanic rocks from northeast Sudan and their implication for crustal evolution

The basement volcano-sedimentary rocks of northeast Sudan form part of the Nubian Shield of northeast Africa. Volcanic rocks from the Kadawēb area yield Rb—Sr wholerock isochron ages of 718 and 722 Ma and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7027 and 0.7029. In the Homogar area, 150 km to the south, volcanic rocks yield a Rb—Sr whole-rock isochron age of 671 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7034. Although all of these lavas have been altered by a low-grade greenschist facies event, isotopic and geochemical evidence indicates limited open system behaviour. Thus these dates most probably represent extrusive ages indicating two episodes of volcanic activity during the evolution of the Nubian Shield. These results place some important constraints on the nature of crustal evolution in northeast Africa.     

Strontium isotopes in Cenozoic volcanic rocks from southeastern Papua New Guinea

Strontium isotope data are presented from 28 rock samples representing four distinct episodes of late Cenozoic volcanism in southeastern Papua. Eocene tholeiitic basalts have initial ⁸⁷Sr/⁸⁶Sr ratios (0.7037) which are higher than those in chemically comparable basalts and are thought to have been enhanced by rock-sea water interaction. Late Cenozoic are trench type volcanoes in the Papuan islands have initial ⁸⁷Sr/⁸⁶Sr ratios which show little variation (0.7041±2) in contrast to those from the contiguous Papua New Guinea mainland (0.7036-0.7054). This isotopic discontinuity does not appear to be due to contamination by immediately underlying sialic metamorphics. High-K trachytes in the Lusancay Islands north of the late Cenozoic are have comparable initial ⁸⁷Sr/⁸⁶Sr ratios. Wide variation (0.703-0.710) in the initial ratios measured in a group of apparently closely related rocks ranging in composition from transitional basalt to peralkaline rhyolite cannot be explained by differences in age or by late magmatic fractionation and continue to pose an enigma.     

Early Cenozoic volcanism of the Kolyuchin-Mechigmen graben, Chukotka Peninsula

The paper reports new isotope-geochemical data on Late Paleocene-Early Eocene basalts from the central part of the Kolyuchin-Mechigmen graben, eastern Chukotka Peninsula. The distribution of the major and trace elements and trace-element ratios indicates that the basalts were formed in a marginal-continental rift setting. The peculiar feature of the basalts is a combination of depleted within-plate and suprasubduction geochemical signatures, which make the volcanic rocks from Mt. Otdel’naya different from rocks of suprasubduction volcanic belts and from tholeiites and alkaline lavas of continental rifts and oceanic islands. Extremely high ⁸⁷Sr/⁸⁶Sr ratios in the studied basalts as compared to those of similar volcanic rocks from extension zones are probably related to the involvement of carbonate material in the magma generation zone.     

Hydrogeochemistry and strontium isotopes of spring and mineral waters from Monte Vulture volcano,Italy

This paper describes the results of a study that was conducted to determine the relationship between hydrogeochemical composition and ⁸⁷Sr/⁸⁶Sr isotope ratios of the Mt. Vulture spring waters. Forty samples of spring waters were collected from local outcrops of Quaternary volcanites. Physico-chemical parameters were measured in the field and analyses completed for major and minor elements and ⁸⁷Sr/⁸⁶Sr isotopic ratios. A range of water types was distinguished varying from alkaline-earth bicarbonate waters, reflecting less intense water–rock interaction processes to alkali bicarbonate waters, probably representing interaction with volcanic rocks of Mt. Vulture and marine evaporites. The average ⁸⁷Sr/⁸⁶Sr isotope ratios suggest at least 3 different sources. However, some samples have average Sr isotope ratios (0.70704–0.70778) well above those of the volcanites. These ratios imply interaction with other rocks having higher ⁸⁷Sr/⁸⁶Sr ratios, probably Triassic evaporites, which is substantiated by their higher content of Na, SO₄ and Cl. The Sr isotope ratios for some samples (e.g. Toka and Traficante) are intermediate between the value for the Vulture volcanites and that for the local Mesozoic rocks. The salt content of these samples also lies between the value for waters interacting solely with the volcanites and the value measured in the more saline samples. These waters are thus assumed to result from the mixing of waters circulating in volcanic rocks with waters presumably interacting with the sedimentary bedrock (marine evaporites).     

Strontium and neodymium isotopic study of the western Mogollon-Datil volcanic region,New Mexico,USA

The Sr- and Nd-isotopic compositions of large mid-Cenozoic caldera-forming eruptions, and related rocks, from the western portion of the Mogollon-Datil volcanic field have been determined. The average initial ⁸⁷Sr/⁸⁶Sr ratios of 27 samples from felsic flows range from 0.70629 to 0.72872; however, all but two flows are 0.71337 or less. Ten analyses of intermediate and mafic rocks showed a tendency towards lower initial ⁸⁷Sr/⁸⁶Sr ranging from 0.70363 to 0.70968. Initial ¹⁴³Nd/¹⁴⁴Nd ratios of II felsic and intermediate rocks range from 0.51216 to 0.51231. Two basalts analyzed for ¹⁴³Nd/¹⁴⁴Nd have ratios of 0.51250 and 0.51291. During the course of the volcanic activity from 34 Ma to the present, there was a shift towards lower initial ⁸⁷Sr/⁸⁶Sr ratios, and lower SiO₂ contents. A number of models of crustal melting, fractionation, mixing, and assimilation and fractional crystallization (AFC), using a variety of possible endmembers, were tested, to see if they could explain the isotopic and geochemical characteristics of the Mogollon-Datil volcanic rocks. The best fit was an AFC model using two components, one a mantle-sourced primary magma, with isotopic ratios of the Kilbourne Hole, N. M., basanite, and the other an upper crust with average continental isotopic ratios, and Sr and Nd abundances similar to the Texas Canyon pluton of Arizona.     

Strontium isotopic composition of oil-field and gas-field waters,Japan

The⁸⁷Sr/⁸⁶Sr ratios of 10 formation waters in petroleum and natural gas reservoirs along the western coast of northeastern Honshu, Japan, ranged from 0.7052 to 0.7084, and are distinctly lower than the ratio in seawater. This forms a marked contrast to oil-field brines from the U.S.A. which have higher⁸⁷Sr/⁸⁶Sr ratios. Taking into consideration that petroleum and natural gas in Japan accumulate in volcanic rocks derived from intense submarine volcanism in the Middle Miocene, the⁸⁷Sr/⁸⁶Sr ratios of the water samples are explained in terms of the isotope exchange between waters containing seawater Sr and reservoir rocks with lower ratios.     

High-Nb hawaiite–mugearite and high-Mg calc-alkaline lavas from northeastern Iran: Oligo-Miocene melts from modified mantle wedge

Tertiary volcanic rocks in northwestern Firoozeh, Iran (the Meshkan triangular structural unit), constitute vast outcrops (up to 250 km²) of high-Mg basaltic andesites to dacites that are associated with high-Nb hawaiites and mugearites. Whole-rock ⁴⁰Ar/³⁹Ar ages show a restricted range of 24.1 ± 0.4–22.9 ± 0.5 Ma for the volcanic rocks. The initial ratios of ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd vary from 0.703800 to 0.704256 and 0.512681 to 0.512877, respectively, in the high-Mg basaltic andesites–dacites. High-Th contents (up to 11 ppm) and Sr/Y values (27–100) and the isotopic composition of the subalkaline high-Mg basaltic andesites–dacites indicate derivation from a mantle modified by slab and sediment partial melts. Evidence such as reverse zoning and resorbed textures and high Ni and Cr contents in the evolved samples indicate that magma mixing with mafic melts and concurrent fractional crystallization lead to the compositional evolution of this series. The high-Nb hawaiites and mugearites, by contrast, have a sodic alkaline affinity and are silica undersaturated; they are also enriched in Nb (up to 47 ppm) and a wide range of incompatible trace elements, including LILE, LREE, and HFSE. Geochemistry and Sr–Nd isotopic compositions of the high-Nb hawaiites and mugearites suggest derivation from a mantle source affected by lower degrees of slab melts. Post-orogenic slab break-off is suggested to have prompted the asthenospheric upwelling that triggered partial melting in mantle metasomatized by slab-derived melts.     

Strontium isotope composition of marine carbonates of Middle Triassic to Early Jurassic age,Lombardic Alps,Italy*

The ⁸⁷Sr/⁸⁶Sr ratios and strontium concentrations for thirty-three samples of marine carbonate rocks of Middle Triassic to Early Jurassic age have been determined. The samples were collected from four measured sections in the areas of Val Camonica in northern Italy. The strontium concentrations vary from 40 to 7000 ppm. Most of the samples are calcitic limestones containing less than 10% of non-carbonate residues. Dolomitic samples and those containing appreciable non-carbonate residues have significantly diminished strontium concentrations. ⁸⁷Sr/⁸⁶Sr ratios of the carbonate phases of these rocks appear to be unaffected by dolomitization and by the presence of non-carbonate minerals. The average ⁸⁷Sr/⁸⁶Sr ratios of the formations vary systematically in a stratigraphic sense. The ratio increased from Early Anisian to Early-Middle Ladinian, declined during Late Ladinian and Carnian, rose again during the Norian and then declined throughout the Late Norian (Rhaetian), Hettangian, Sinemurian and Pliens-bachian ages. The average ⁸⁷Sr/⁸⁶Sr ratios, relative to 0.7080 for the Eimer and Amend standard, are: Anisian: 0.70805 ± 00019; Early Ladinian: 0.7085 ± 0.00038; Late Ladinian: 0.70791 ± 0.00013; Carnian: 0.70776 ± 0.00015; Norian and Rhaetian: 0.70791 ± 0.00014; Hettangian: 0.70762 ± 0.00021; Sinemurian: 0.7070 ± 0.00038; Pliensbachian: 0.7070 ± 0.00015. These variations reflect changes in the isotopic composition of Sr entering the oceans in early Mesozoic time due to varying rates of weathering and erosion of young volcanic rocks (low ⁸⁷Sr/⁸⁶Sr) and old granitic rocks (high ⁸⁷Sr/⁸⁶Sr). The data presented in this report contribute to a growing body of information regarding the changes that have occurred in the ⁸⁷Sr/⁸⁶Sr ratio of the oceans in Phanerozoic time.     

Strontium isotopic geochemistry of the volcanic rocks and associated megacrysts and inclusions from Ross Island and vicinity,Antarctica

Twelve whole-rock samples of volcanic rocks and a composite of 11 basanitoid samples from Ross Island and vicinity, Antarctica show a narrow range of ⁸⁷Sr/⁸⁶Sr ratios from 0.7030₅ to 0.7033₉. This range is consistent with a model of differentiation from a single parent magma, but the data allow a 30% variation in the ⁸⁷Rb/⁸⁶Sr ratio in the source region if the average ratio is less than 0.057 and if the source region has existed as a closed system for 1.5 b.y. Megacrysts of titaniferous augite, kaersutite, and anorthoclase are isotopically indistinguishable from the host volcanic rocks and therefore are probably cogenetic with the volcanic sequence. A single trachyte sample is isotopically distinct from the rest of the volcanic rocks and probably was contaminated with crustal strontium.Ultramafic and mafic nodules found in association with basanitoids and trachybasalts have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7027₅ to 0.7057₅. Several of these nodules exhibit evidence of reaction with the melt and are isotopically indistinguishable from their hosts, but data for seven granulite-facies nodules show an apparent isochronal relationship. Although this isochron may be fortuitous, the resulting age of 158±22 m.y. is similar to ages reported for the voluminous Ferrar Dolerites, and suggests isotopic re-equilibration within the lower crust and upper mantle. These nodules are not genetically related to the Ferrar Dolerites, as evidenced by their lower initial ⁸⁷Sr/⁸⁶Sr ratios.Three ultramafic nodules are texturally and isotopically distinct from the rest of the analyzed nodules. These are friable, have larger ⁸⁷Sr/⁸⁶Sr ratios, and may represent a deeper sampling of mantle rock than the granulite-facies nodules. They were, however, derived at a shallower depth than the alkalic magma. Thus they are not genetically related to either the magma or the granulite-facies nodules.     

Crustal contamination in early Basin-and-Range hawaiites of the Los Encinos Volcanic Field,central México

The Los Encinos Volcanic Field (LEVF) consists of Miocene (10.6–13.6 Ma) hawaiitic volcanic necks and lava-capped mesas that crop out sparsely over an area of 11,500 km² at the eastern margin of the Mexican Basin and Range Province (BRP). The LEVF rocks are similar to other early extensional hawaiites from the southern BRP, and provide numerous contrasts with younger basanites and alkali basalts that erupted during the Quaternary at the Ventura and Santo Domingo Volcanic Fields about 100 km to the south. A suite of 18 LEVF hawaiites was studied in thin section, and analyzed for mineral compositions, whole-rock major and trace element compositions, and Sr, Nd, and Pb isotopic ratios. All samples contain the stable minerals plagioclase (An_53–64), olivine (Fo_61–88), clinopyroxene, titanomagnetite, and minor biotite. Most samples also contain a complex assemblage of resorbed and reacted xenocrysts and megacrysts. Some of these minerals appear to have crystallized slowly from related, but more differentiated magmas, but other xenocrysts were clearly derived from lower-crustal, high-grade orthogneisses and paragneisses that are found as large xenoliths in the nearby Quaternary volcanic fields. Quartz xenocrysts are especially common in many hawaiites (up to 3.9 vol.%) and show a wide range of reaction styles. One sample contains microxenoliths of sillimanite- and quartz-bearing paragneiss with fine-grained domains that are interpreted as coronal structures related to original garnet xenocrysts. The geochemical effects of crustal contamination in the LEVF hawaiites vary widely. Five samples appear to be essentially uncontaminated (type U). Aside from being somewhat differentiated from a more primitive parent, the type-U samples can be used to infer the geochemistry of the mantle that was melting during the early stages of basin-and-range rifting. Type-U samples range up to ε_Nd=+7.6 and down to ⁸⁷Sr/⁸⁶Sr = 0.70286 and ²⁰⁶Pb/²⁰⁴Pb = 18.74, compositions that are more extreme than any of the nearby Quaternary volcanic rocks. The other 13 samples are divided into two contamination types, A and B. Both types show trends toward higher ⁸⁷Sr/⁸⁶Sr (to 0.7040) and ²⁰⁶Pb/²⁰⁴Pb (to 18.98), lower ε_Nd (to +3.1), and elevated Yb, which appear to reflect bulk or AFC-style contamination by granulites, particularly garnet-bearing paragneisses. Type-A hawaiites also show selective enrichments in Cs, Rb, Th, Sb, U, Pb, K, and Si. These elements were probably transferred into the type-A hawaiitic magmas through mixing with low-degree partial melts from deep-crustal granulites. The enrichments of these elements in type-A hawaiites complement the depletions of many of these same elements in high-grade granulites worldwide and provide insight into the origin of those depletions. Mixing models between type-U hawaiites and paragneiss xenoliths indicate that up to 45% of the Pb found in type-A hawaiites is crustally derived. In comparison with more mafic Quaternary basanitic rocks from the volcanic fields to the south, which carried large peridotite and granulite xenoliths to the surface, the LEVF hawaiites are relatively differentiated and megacryst rich, but free of large xenoliths, and show a wide variety of petrographic and geochemical evidence for crustal contamination. These differences probably reflect the slow and interrupted ascent of the LEVF hawaiites during early stages of basin-and-range extension in the Miocene, when the crust had a somewhat lower density and the entire lithosphere was relatively thick and cool. We argue that Quaternary basanites were able to ascend significantly faster through the thinner, hotter, and more fractured and extended lithosphere, whose crust was made denser by mafic intrusions during the preceding magmatic episode. Consequently the Quaternary basanites rose without stagnating and interacting with crustal lithologies, and without losing their entrained peridotite xenoliths. Received: 29 September 1993 / Accepted: 16 May 1994     

Petrogenesis of a basalt-comendite-pantellerite rock suite: the Boseti Volcanic Complex (Main Ethiopian Rift)

Petrological and geochemical data for basic (alkali basalts and hawaiites) and silicic peralkaline rocks, plus rare intermediate products (mugearites and benmoreites) from the Pleistocene Boseti volcanic complex (Main Ethiopian Rift, East Africa) are reported in this work. The basalts are slightly alkaline or transitional, have peaks at Ba and Nb in the mantle-normalized diagrams and relatively low ⁸⁷Sr/⁸⁶Sr (0.7039–0.7044). The silicic rocks (pantellerites and comendites) are rich in sanidine and anorthoclase, with mafic phases being represented by fayalite-rich olivine, opaque oxides, aenigmatite and slightly Na-rich ferroaugite (ferrohedenbergite). These rocks were generated after prolonged fractional crystallization process (up to 90–95 %) starting from basaltic parent magmas at shallow depths and fO₂ conditions near the QFM buffer. The apparent Daly Gap between mafic and evolved Boseti rocks is explained with a model involving the silicic products filling upper crustal magma chambers and erupted preferentially with respect to basic and intermediate products. Evolved liquids could have been the only magmas which filled the uppermost magma reservoirs in the crust, thus giving time to evolve towards Rb-, Zr- and Nb-rich peralkaline rhyolites in broadly closed systems.     

Intraplate volcanism in New Zealand: the role of fossil plume material and variable lithospheric properties

The geologic evolution of the New Zealand microcontinent was characterised by intermittent Cretaceous to Quaternary episodes of intraplate volcanism. To evaluate the corresponding mantle evolution beneath New Zealand with a specific focus on the tectonic evolution, we performed a combined major and trace element and Hf, Nd, Pb, Sr isotope investigation on a suite of representative intraplate volcanic rocks from both main islands and the Chatham Islands. Isotopically, the data set covers a range between “HIMU-like” end member compositions (²⁰⁶Pb/²⁰⁴Pb: 20.57, ²⁰⁷Pb/²⁰⁴Pb: 15.77, ⁸⁷Sr/⁸⁶Sr: 0.7030, εHf: + 3.8, εNd: + 4.2), compositions tending towards MORB (²⁰⁶Pb/²⁰⁴Pb: 19.01, ²⁰⁷Pb/²⁰⁴Pb: 15.62, ⁸⁷Sr/⁸⁶Sr: 0.7028, εHf: + 9.9, εNd: + 7.0) and compositions reflecting the influence of subducted sediments (²⁰⁶Pb/²⁰⁴Pb: 18.99, ²⁰⁷Pb/²⁰⁴Pb: 15.67, ⁸⁷Sr/⁸⁶Sr: 0.7037, εHf: + 4.4, εNd: + 3.9). Whereas volcanism on the Chatham Islands constitutes the HIMU end member of our data set, intraplate volcanic rocks from the North Island are dominated by MORB-like compositions with relatively radiogenic ²⁰⁶Pb/²⁰⁴Pb signatures. Volcanic rocks from the South Island form a trend between the three end members. Assuming a polybaric melting column model, the primary melt compositions reflect variations in the degree of melting, coupled to variable average melting depths. As the three isotope and trace element end members occur throughout the volcanic episodes, the “HIMU-like” and the sediment influenced signatures most likely originate from a heterogeneous subcontinental lithospheric mantle, whereas an asthenospheric origin is inferred for the MORB-like component. For the South Island, affinities to HIMU wane with decreasing average melting depths whereas MORB and sediment-like signatures become more distinct. We therefore propose a polybaric melting model involving upper asthenospheric mantle and a lithospheric mantle source that has been modified by subduction components and veins of fossil “HIMU-like” asthenospheric melts. The proportion of asthenospheric versus lithospheric source components is controlled by variations in lithospheric thickness and heat flow, reflecting the different tectonic settings and rates of extension. Generally, low degree melts preferentially tap enriched vein material with HIMU signatures. The widespread occurrence of old Gondwana-derived lithospheric mantle beneath intraplate volcanic fields in East Gondwana is suggested by overall similarities between New Zealand intraplate volcanic rocks and volcanic rocks in East Australia and Antarctica. The petrogenetic model proposed here may therefore serve as a general model for the petrogenesis of Cretaceous to Recent intraplate volcanic rocks in former East Gondwana. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A study of Sr isotopic characteristics and ore-search indicators of Gejiu Sn-bearing granites

Whole-rock Rb-Sr isochron dating of Sn-bearing granites and alkaline rocks from Gejiu, Yunnan Province has been conducted for their emplacement ages and initial⁸⁷Sr/⁸⁶Sr ratios. The Sr isotopic compositions of apatites from some basic rocks in Jiasha, and granite bodies also have been studied in detail. The genesis and evolution of Gejiu Sn-bearing granites as well as ore-search indicators are discussed on the basis of the available data in conjunction with the geochemical data on trace elements (such as Rb and Sr), Sr isotopic characteristics of the volcanic rocks, meta-diabase and host rocks and the isotopic features of ore leads.     

Association of Late Paleozoic Adakitic Rocks and Shoshonitic Volcanic Rocks in the Western Tianshan, China

The late Paleozoic adakitic rocks are closely associated with the shoshonitic volcanic rocks in the western Tianshan Mountains, China, both spatially and temporally. The magmatic rocks were formed during the period from the middle to the late Permian with isotopic ages of 248-268 Ma. The 87Sr/86Sr initial ratios of the rocks are low in a narrow variation range (-0.7050). The 143Nd/144Nd initial ratios are high (-0.51240) with positive εND(t) values (+1.28-+4.92). In the εNd(t)-(87Sr/86Sr)i diagram they fall in the first quadrant. The association of the shoshonitic and adakitic rocks can be interpreted by a two-stage model: the shoshonitic volcanic rocks were formed through long-term fractional crystallization of underplated basaltic magma, while the following partial melting of the residual phases formed the adakitic rocks.     

Strontium isotope composition of alpine-type ultramafic rocks in the Lake Chatuge District,Georgia—North Carolina

The ⁸⁷Sr/⁸⁶Sr ratios for a series of ultramafic rocks from the Lake Chatuge region range from 0.7023 to 0.7047, suggesting a direct upper mantle source and precluding a multiple differentiation origin for these alpine-type rocks. Higher ⁸⁷Sr/⁸⁶Sr ratios (0.7058–0.7068) for serpentinized rocks from this suite apparently reflect the influx of radiogenic ⁸⁷Sr from the surrounding gneisses and schists during serpentinization.     

Geochemistry of the Adamello massif (northern Italy)

The Tertiary Adamello massif, outcropping over an area of more than 550 km² in the southern Alps (northern Italy) is composed mainly of granitoid rocks (granodiorite, tonalite, quartz diorite) with minor amounts of diorite and gabbro. The major and trace element composition of these rocks is comparable to calc-alkaline volcanic rocks of continental margins. The granitoid rocks display spatial and temporal variations in their composition, particularly in Na, P, Sr, La, Nb and Y contents and ⁸⁷Sr/⁸⁶Sr ratios. The variations were probably produced by concurrent contamination/wall-rock assimilation and fractional crystallization of high-alumina basaltic magma.