Initial 87Sr/86Sr ratios of plutonic rocks from Japan

Initial ⁸⁷Sr/⁸⁶Sr rations were determined for more than 80 plutonic rocks in Japan. The ⁸⁷Sr/⁸⁶Sr ratios of gabbroic and granitic rocks show no significant difference in plutonic terranes where both rocks occur closely associated, implying a genetic relationship between them (e.g., Green Tuff belt) or reequilibration at deep level (e.g., Ryoke belt). Wherever granitic rocks occur independently from gabbroic rocks, the granites have higher ratios than the gabbros.Initial ⁸⁷Sr/⁸⁶Sr ratios of the granitic rocks are low (<0.706) in Northeast Japan but high (<0.706) in Southwest Japan, the boundary being the Tanakura Tectonic Line. Within Southwest Japan, the ratios are low along the Japan Sea side of the southernmost area. This regional variation is generally correlated with thickness of the continental crust as deduced from the Bouguer anomaly.Initial ⁸⁷Sr/⁸⁶Sr ratios of the granitic rocks vary from 0.7037 to 0.7124. The low group (<0.706) is considered to consist of essentially mantle-derived magmas contaminated by crustal material in lesser but varying degree, because of its geological setting and initial ⁸⁷Sr/⁸⁶Sr values. The high group may have been formed by contamination of a deep-seated magmas by crustal material or by generation of the main part of the magmas within the continental crust. The ratios of individual belts reflect their own history depending upon age and Rb/Sr ratio of the crustal material.Initial ⁸⁷Sr/⁸⁶Sr ratios of granitic rocks are generally low for the magnetite-series but high for the ilmenite-series. Thus, a negative correlation is observed between initial ratios and ³⁴S for most Cretaceous-Paleogene granites. However, Neogene ilmenite-series granites are low in both initial ⁸⁷Sr/⁸⁶Sr and ³⁴S indicating interaction of the granitic magma with young sedimentary rocks enriched in ³²S.     

Strontium isotope composition and petrogenesis of the Kirkpatrick basalt,Queen Alexandra Range,Antarctica

The initial ⁸⁷Sr/⁸⁶Sr ratios of twelve basalt flows of Jurassic age on Storm Peak in the Queen Alexandra Range are anomalously high and range from 0.7094–0.7133. The average value is 0.7112±0.0013 (1). The concentrations of rubidium and strontium have arithmetic means of 60.6±19.4 ppm and 128.8±11.9 ppm, respectively. The corresponding average Rb/Sr ratio is 0.47 which is also anomalously high for rocks of basaltic composition. In addition, these rocks have high concentrations of SiO₂ (56.50%) and K₂O (1.29%) and are depleted in Al₂O₃ (12.92%), MgO (3.44%) and CaO (7.91%) compared to average continental tholeiites. They are nevertheless classified as basalts on the basis of the composition of microphenocrysts.The initial ⁸⁷Sr/⁸⁶Sr ratios and all of the chemical parameters of the flows exhibit systematic stratigraphic variations. These are interpreted as indicating the occurrence of four eruptive cycles. In a typical cycle the initial ⁸⁷Sr/⁸⁶Sr ratios of successive flows and their concentrations of SiO₂, FeO (total iron), Na₂O, K₂O, P₂O₅, Rb and Sr decrease in ascending stratigraphic sequence while the concentrations of TiO₂, Al₂O₃, MgO, CaO and MnO increase upward. The initial ⁸⁷Sr/⁸⁶Sr ratios of the flows show a strong positive correlation with the strontium concentration. Similar correlations are observed between the initial ⁸⁷Sr/⁸⁶Sr ratios and all of the major oxide components. These relationships are incompatible with the hypothesis that these flows are the products of crystal fractionation of a-34 magma at depth under closed-system conditions. It is suggested that the flows resulted from the hybridization of a normal tholeiite basalt magma by assimilation of varying amounts of granitic rocks in the Precambrian basement which underlies the entire Transantarctic Mountain chain.Mixtures of two components having different ⁸⁷Sr/⁸⁶Sr ratios and differing strontium concentrations are related to each other by hyperbolic mixing equation. Such an equation was fitted by least squares regression of data points to a straight line in coordinates of initial ⁸⁷Sr/⁸⁶Sr and the reciprocals of the concentrations of strontium. This equation and plots of strontium versus other oxides were then used to estimate the chemical composition of the parent basalt magma and of the granitic contaminant by substituting reasonable estimates of their ⁸⁷Sr/⁸⁶Sr ratios. The chemical composition of the parent basalt (⁸⁷Sr/⁸⁶Sr=0.706) is generally compatible with that of average continental tholeiite, but is distinctive by having a low concentration of strontium (117 ppm). The chemical composition of the contaminant (⁸⁷Sr/⁸⁶Sr=0.720) is enriched in strontium (173 ppm), SiO₂, FeO (total iron) and the alkalies but is depleted in Al₂O₃, MgO and CaO. The data for strontium indicate that the lava flows on Storm Peak contain between 20 and 40% of this granitic contaminant. The contamination of basalt magma is not a local event but is characteristic of the Jurassic basalt flows and diabase sills throughout the Transantarctic Mountains and in Tasmania.Laboratory for Isotope Geology and Geochemistry, Contribution No. 33.     

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.     

Geochemistry and petrogenesis of tertiary granitic rocks from the Island of Mull,Northwest Scotland

The anorogenic igneous rocks of Mull consist essentially of a lava pile of predominantly basaltic composition, cut by an intrusive complex. The basement consists of Precambrian metamorphic rocks of the Moine Series underlain by Lewisian gneiss. The intrusive complex contains a significant proportion of granitic intrusions which can be ascribed to three successive centres of activity, Centres 1–3. We report new major and trace element, including rare earth element analyses, ⁸⁷Sr/⁸⁶Sr ratios and ¹⁸O values for a comprehensive collection of granitic rocks from the 3 centres. The ¹⁸O values range from +4 to –6 indicating variable extent of interaction between the rocks and heated meteoric groundwater. However, correlations of ¹⁸O with other major and trace element data and ⁸⁷Sr/⁸⁶Sr ratios are uniformly low, apart from Fe₂O₃. It is thus unlikely that the interaction of the rocks with meteoric water has systematically altered the chemical (including Sr isotope) characteristics. The chemical and Sr isotope data reflect magmatic values and can therefore be used to comment on the petrogenesis of the granitic rocks.These data indicate that there are important differences between granitic rocks of the centres with Centre 1 forming one distinct group and Centres 2 and 3 a different group. For a given SiO₂ value, the Centre 1 granites have higher Na₂O, MgO, P₂O₅, TiO₂, Sr, and V and lower Al₂O₃, MnO, Zn, Zr, and Y than those of Centres 2 and 3. In addition, the Centre 1 granites have lower REE contents and higher Ce_N/Yb_N ratios than those of Centres 2 and 3. Granites from all three centres have Eu anomalies, those of Centre 3 being generally greater (Eu/Eu* = 0.66–0.10). Finally, there are important Sr isotope distinctions between the three Centres; calculated initial ⁸⁷Sr/⁸⁶Sr ratios for the Centre 1 granites (using 58.2Ma) range between 0.71366–0.71646 (average 0.71530) and have a general correlation of ⁸⁷Sr/⁸⁶Sr with ⁸⁷Rb/⁸⁶Sr. The initial ⁸⁷Sr/⁸⁶Sr ratios of the Centre 2 granites range from 0.70663 to 0.70868, but the ⁸⁷Sr/⁸⁶Sr data do not define an isochron. Finally, data for the Centre 3 granites define a Rb-Sr wholerock isochron with an age of 58.2±2.5 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71003 ±36.Both the chemical trends and isotopic data for the Mull granites can be interpreted in terms of contrasted origins for the granitic rocks of the two groups. The relatively primitive chemical composition and high initial ⁸⁷Sr/⁸⁶Sr ratios of the Centre 1 granites indicate a substantial crustal contribution and we consider that these granites formed by a combination of partial melting of Lewisian basement together with some magma derived by fractional crystallization of basaltic magma. In contrast, the chemical and isotope data for the Centre 2 and 3 granites are consistent with formation dominantly by fractional crystallization of basaltic magma, together with a relatively small proportion of crustal contamination. A model is proposed which emphasises that acid magmatism in Mull is a consequence of the rise and crystallization of basic magma into continental crust. Granite magma has formed both by partial melting and by fractional crystallization and both of these events probably occurred under open system conditions.With oxygen isotope analyses by J.J. Durham, Geochemical Division, Institute of Geological Sciences, 64–78 Grays Inn Road, London, WC1X 8NG, England     

Development of heterogeneities in the subcontinental Mantle: Evidence from the Ferrar Group,Antarctica

Tholeiitic lava flows (Kirkpatrick Basalts) and dolerite sills and dikes (Ferrar Dolerites) of the Jurassic Ferrar Group from Antarctica and dolerite sills from Tasmania, Australia are characterised by initial strontium isotope ratios ranging from 0.7089 to 0.7153. The mean and standard deviation of 85 analyses is 0.7115±0.0012. Some of the scatter in the initial ⁸⁷Sr/⁸⁶Sr ratios can be attributed to sample inhomogeneity, analytical uncertainties and sample alteration. The published major element data show well-defined trends that are consistent with an evolution by fractional crystallization. Recognition of a parental magma is difficult due to the fractionated nature of the rocks. Trace element analyses, particularly the rare earth elements (REE) support a differentiation model. Compared to mid-ocean ridge basalts, Ferrar Group rocks are enriched in light REE. Kirkpatrick Basalts from the central Transantarctic Mountains show significant correlations between initial ⁸⁷Sr/⁸⁶Sr ratios and major elements only for SiO₂ and CaO. The general lack of strong correlation is the basis for rejecting the possibility of wholesale contamination by sialic material as a possible cause of the high ⁸⁷Sr/⁸⁶Sr ratios. Selective contamination of the basaltic magmas is a possibility and cannot be completely discounted. It would probably involve a fluid phase in order to transport and mix the light REE, Rb, ⁸⁷Sr, and other elements. By analogy with selective contamination of ocean ridge basalts by sea water it is difficult to envisage a similar process acting on magma emplaced in a non-marine environment. Because of the elevated values of the initial ⁸⁷Sr/⁸⁶Sr ratios, their similar average value over 2,500 km and the large volume of magma involved (4× 10⁵ km³) a mantle origin for the high Sr ratios is preferred. Models to account for the enrichment of Rb and light REE in the Antarctic mantle during or prior to the Jurassic include:

1. addition of continental material from a Palezoic Mesozoic subduction zone;
2. metasomatism of volatile elements from the lower mantle; and
3. evolution of a mantle with a high Rb/Sr ratio.

     

Interaction between crustal-derived felsic and mantle-derived mafic magmas in the Oberkirch Pluton (European Variscides, Schwarzwald, Germany)

The composite Oberkirch pluton consists of three compositionally different units of peraluminous biotite granite. The northern unit is relatively mafic (SiO₂∼64%) and lacks cordierite. The more felsic central and southern units (SiO₂=67.8 to 70.4%) can only be distinguished from each other by the occurrence of cordierite in the former. Mafic microgranular enclaves of variable composition, texture and size occur in each of these units and are concentrated in their central domains. Most abundant are large (dm to m) hornblende-bearing enclaves with dioritic to tonalitic compositions (SiO₂=50.8 to 56.3 wt%; Mg#=63 to 41) and fine grained doleritic textures that suggest chilling against the host granite magma. Some of these enclaves are mantled by hybrid zones. Less common are microtonalitic enclaves containing biotite as the only primary mafic phase (SiO₂=53.7 to 64.4%) and small hybrid tonalitic to granodioritic enclaves and schlieren. Synplutonic dioritic dikes (up to 6 m thick) with hybrid transition zones to the host granite occur in the southern unit of the pluton. In chemical variation diagrams, samples from unmodified hornblende-bearing mafic enclaves and dikes form continuous trends that are compatible with an origin by fractionation of olivine, clinopyroxene, hornblende and plagioclase. Chemical and initial isotopic signatures (e.g. high Mg#, low Na₂O, ɛ_Nd=−1.2 to −5.1, ⁸⁷Sr/⁸⁶Sr=0.7055 to 0.7080, δ¹⁸O=8.0 to 8.8‰) exclude an origin by partial melting from a mafic meta-igneous source but favour derivation from a heterogeneous enriched lithospheric mantle. Samples from the granitic host rocks do not follow the chemical variation trends defined by the diorites but display large scatter. In addition, their initial isotopic characteristics (ɛ_Nd=−4.5 to −6.8, ⁸⁷Sr/⁸⁶Sr=0.7071 to 0.7115, δ¹⁸O=9.9 to 11.9‰) show little overlap with those of the diorites. Most probably, the granitic magmas were derived from metapelitic sources characterized by variable amounts of garnet and plagioclase. This is suggested by relatively high molar ratios of Al₂O₃/(MgO+FeO_tot) and K₂O/Na₂O, in combination with low ratios of CaO/(MgO+FeO_tot), variable values of Sr/Nd, Eu/Eu*[=Eu_cn/(Sm_cn × Gd_cn)^0.5] and (Tb/Yb)_cn (cn=chondrite-normalized) as well as variable abundances of Sc and Y. Whole-rock initial isotopic signatures of mafic microtonalitic enclaves (ɛ_Nd=−4.6 to −5.2; ⁸⁷Sr/⁸⁶Sr=0.7060 to 0.7073; δ¹⁸O ∼8.1‰) are similar to those of the low ɛ_Nd diorites. Plagioclase concentrates from a granite sample and a mafic microtonalitic enclave are characterized by initial ⁸⁷Sr/⁸⁶Sr ratios that are significantly higher than those of their bulk rock systems suggesting incorporation of high ⁸⁷Sr/⁸⁶Sr crustal material into the magmas. Field relationships and petrographic evidence suggest that the Oberkirch pluton originated by at least three pulses of granitic magma containing mafic magma globules. In-situ hybridization between the different magmas was limited. Late injection of dioritic magma into the almost solidified granitic southern unit resulted in the formation of more or less continuous synplutonic dikes surrounded by relatively thin hybrid zones. Received: 30 April 1999 / Accepted: 6 August 1999     

Hydrogeochemical Characterization and Groundwater Quality of Jamshedpur Urban Agglomeration in Precambrian Terrain,Eastern India

This paper reports petrography, geochemistry and Rb-Sr age data on the rare metal bearing Neoarchean fertile (Nb-Ta) granite at Allapatna and elucidates its petrogenesis and role in Nb-Ta-Li-Be mineralization. The Allapatna granite (AG) intrudes the Tonalitic-Trondhjemitic - Granodioritic (TTG) Peninsular Gneiss and analysed SiO₂ (72.3-75.6 wt%), K₂O (4.0-5.7wt%), Na2O (3.0-4.4wt%), CaO (0.7-1wt%), MgO (0.13-0.25wt%) and K₂O/Na₂O (>1) indicating evolved nature. The presence of muscovite, biotite and garnet in the mode, peraluminous nature and high initial ⁸⁷Sr/⁸⁶Sr ratio (0.7284±0.0083) attest to their S-type characteristics. Varying Nb/Ta ratio and high Li with moderate abundance of Cs further indicate affinity to Li-Cs-Ta (LCT) type granite-pegmatite system. TheAG showing whole rock Rb-Sr isochron age of 2803± 68 Ma, is the oldest reported fertile granite in India parental to rare metal pegmatites hosting Nb-Ta, Be, and Li resources. Partial melting of a mixed source consisting of both basement TTG rocks and metapelites has generated such type of granitic magma. Fractionation of such granitic magma possibly has given rise to the rare metal (Ta-Nb-Li-Be) bearing pegmatites intruding the nearby schist belt.     

The Kalka layered intrusion,central Australia

The Kalka Intrusion, central Australia has a 5000 m-thick layered sequence comprising Pyroxenite, Norite and Anorthosite Zones; an Olivine Gabbro Zone is enclosed by, and has a facies relationship with, the Norite Zone. Contamination is evidenced by high initial ⁸⁷Sr/⁸⁶Sr ratios ( 0.708) in the lower four-fifths of the intrusion, and resulted in pyroxenite rather than peridotite as a basal crystal accumulate. After an early phase of erratic buildup in contamination due to assimilation of ragged granulite wall-rock, armouring of the walls and mixing produced an homogeneous Norite Zone (threshold) magma body crystallising opx-cpx-plag. Within the succeeding 3500 m of section plagioclase An values have a general decline up sequence (An_74-60-feldspar threshold) with superimposed short term digressions to more calcic compositions. Initial ⁸⁷Sr/⁸⁶Sr ratios also fall very gradually (0.7081-0.7078 — isotopic threshold) with transient fluctuations to distinctly lower values. Maxima in plagioclase An contents and ⁸⁷Sr/ ⁸⁶Sr minima may be correlated with the spasmodic appearance of olivine and pronounced lithological variation. Such features are explained by the ponding of fresh bodies of uncontaminated magma on the floor of the chamber; these formed a hybrid with threshold magma that temporarily dominated crystallisation thereby perturbing plagioclase and isotopic compositions; eventually, mixing of the hybrid into the overlying threshold magma returned crystallisation to its initial state. The facies-controlled Olivine Gabbro Zone is the physical expression of ponded basal hybrid magmas. The onset of the Anorthosite Zone is marked by a pronounced decline in ⁸⁷Sr/⁸⁶Sr ratios to around 0.705 registering a major influx of new magma. In this instance the new magma dominated the system and a change to plagioclase as the dominant liquidus phase indicates a drastic change in input magma composition. The evolution of Kalka was determined by an interplay of crystal fractionation, fresh magma pulses, and magma mixing, with the latter effects producing both increases and decreases in plagioclase An contents; the complete process can only be revealed by combined mineral composition and isotopic data.     

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.     

A petrogenetic model for the igneous complex in the Spanish Peaks region,Colorado

Twenty representative rocks ranging from lamprophyric to granitic composition, from the Spanish Peaks igneous Complex, south-central Colorado, were analyzed for Sr isotopic compositions and their concentrations of K, Rb, Sr and Ba. The various igneous rocks from this Cenozoic complex do not have a comagmatic relationship from the evidence of their Sr isotopic compositions. Due to the generally low Sr⁸⁷/Sr⁸⁶ isotopic ratios, the possibility of the highly radiogenic underlying Precambrian basement as the source of magma generation can be ruled out. The sources for the magmas of this igneous complex must be in the upper mantle or the lower crust. Model calculations using elemental distribution coefficients and assumed mantle materials suggest that the abundant lamprophyric magmas in this region could be derived from a phlogopite-bearing hornblende peridotite by a small degree of partial melting (<5%) at lower pressure environment (<50 km). Other possibilities for lamprophyric magma generation were also examined. The slightly higher Sr⁸⁷/Sr⁸⁶ ratios observed in the granitic rocks are interpreted as reflecting the nature of this source-the lower crust. Alternatively, they may suggest a limited contamination of the original liquid by upper crustal material. For the entire igneous complex, mixing of two independent magmas, lymprophyric and granitic, is suggested to be the mechanism responsible for the complicated and diverse chemical characteristics.     

Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic–Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt, NW China

The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and granodiorites.An age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The ε_(Nd)(t) values in the rock units vary from +6.70 to +9.64, and initial ~(87)Sr/~(86)Sr ratios range between 0.7035 and0.7042. Initial ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)_(PM) values between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)_(PM)ratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust.Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10 vol%contamination from a granitic melt. This was followed by about 5 vol% assimilation of upper crustal rocks. Thus, the high-Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break-off of a subducted slab.     

Evolution of the Proto-Tethys in the Baoshan block along the East Gondwana margin: constraints from early Palaeozoic magmatism

Zircon U–Pb ages and geochemical and isotopic data for Late Ordovician granites in the Baoshan Block reveal the early Palaeozoic tectonic evolution of the margin of East Gondwana. The granites are high-K, calc-alkaline, metaluminous to strongly peraluminous rocks with A/CNK values of 0.93–1.18, are enriched in SiO₂, K₂O, and Rb, and depleted in Nb, P, Ti, Eu, and heavy rare earth elements, which indicates the crystallization fractionation of the granitic magma. Zircon U–Pb dating indicates that they formed at ca. 445 Ma. High initial ⁸⁷Sr/⁸⁶Sr ratios of 0.719761–0.726754, negative ?_Nd(t) values of –6.6 to –8.3, and two-stage model ages of 1.52–1.64 Ga suggest a crustal origin, with the magmas derived from the partial melting of ancient metagreywacke at high temperature. A synthesis of data for the early Palaeozoic igneous rocks in the Baoshan Block and adjacent Tengchong Block indicates two stages of flare-up of granitic and mafic magmatism caused by different tectonic settings along the East Gondwana margin. Late Cambrian to Early Ordovician granitic rocks (ca. 490 Ma) were produced when underplated mafic magmas induced crustal melting along the margin of East Gondwana related to the break-off of subducted Proto-Tethyan oceanic slab. In addition, the cession of the mafic magmatism between late Cambrian-Early Ordovician and Late Ordovician could have been caused by the collision of the Baoshan Block and outward micro-continent along the margin of East Gondwana and crust and lithosphere thickening. The Late Ordovician granites in the Baoshan Block were produced in an extensional setting resulting from the delamination of an already thickened crust and lithospheric mantle followed by the injection of synchronous mafic magma.     

Input of Sr/Sr ratios and Sr geochemical signatures to update knowledge on thermal and mineral waters flow paths in fractured rocks (N-Portugal)

Strontium isotopes and other geochemical signatures are used to determine the relationships between CO₂-rich thermal (Chaves: 76 °C) and mineral (Vilarelho da Raia, Vidago and Pedras Salgadas: 17 °C) waters discharging along one of the major NNE–SSW trending faults in the northern part of mainland Portugal. The regional geology consists of Hercynian granites (syn-tectonic-310 Ma and post-tectonic-290 Ma) intruding Silurian metasediments (quartzites, phyllites and carbonaceous slates). Thermal and mineral waters have ⁸⁷Sr/⁸⁶Sr isotopic ratios between 0.716713 and 0.728035. ⁸⁷Sr/⁸⁶Sr vs. 1/Sr define three end-members (Vilarelho da Raia/Chaves, Vidago and Pedras Salgadas thermal and mineral waters) trending from rainfall composition towards that of the CO₂-rich thermal and mineral waters, indicating different underground flow paths. Local granitic rocks have ⁸⁷Sr/⁸⁶Sr ratios of 0.735697–0.789683. There is no indication that equilibrium was reached between the CO₂-rich thermal and mineral waters and the granitic rocks. The mean ⁸⁷Sr/⁸⁶Sr ratio of the thermal and mineral waters (0.722419) is similar to the Sr isotopic ratios of the plagioclases of the granitic rocks (0.71261–0.72087). The spatial distribution of Sr isotope and geochemical signatures of waters and the host rocks suggests that the thermal and mineral waters circulate in similar but not the same hydrogeological system. Results from this study could be used to evaluate the applicability of this isotope approach in other hydrogeologic investigations.     

THE PETROCHEMISTRY OF ANCIENT GRANITOIDS OF THE SOUTHERN SIBERIAN CRATON AND ITS MARGINAL FOLD BELTS

The geochemistry of Laramide granitic rocks from central Sonora was studied to relate possible compositional variations to the assimilation of different crustal sources. Most of the studied rocks are granodiorites collected near the southern boundary of the Paleozoic North American continent. North of this boundary, the Laramide plutons intruded a thick section of Upper Proterozoic and Paleozoic miogeoclinal strata, whereas south of it, the intrusives were emplaced in Lower Ordovician to Permian eugeoclinal rocks accreted to the continental margin between Late Permian and Middle Triassic times. Whole-rock Na₂O, TiO₂, and P₂O₅ contents are slightly higher in plutons intruded in cratonic and miogeoclinal rocks, whereas MgO and CaO seem to be higher in plutons emplaced in eugeoclinal rocks. The samples located in the north are characterized by steeper chondrite-normal-ized REE slopes and generally well developed negative europium anomalies, whereas the group of granitoids in the south have flatter REE slopes and generally smaller negative europium anomalies. Available isotope data show ⁸⁷Sr/⁸⁶Sr initial ratios above 0.7070, and ε_Nd initial values between ?4.6 and ?4.2 in plutons to the north. In a narrow E-W region just south of the Paleozoic continental margin, the samples yielded similar ⁸⁷Sr/⁸⁶Sr ratios but unexpectedly low ε_Nd values between ?5.4 and ?5.1. South of this region, the isotope signatures are slightly more primitive, with ⁸⁷Sr/⁸⁶Sr initial ratios between 0.7067 and 0.7057 and initial ε_Nd values between ?3.9 and ?3.7. The results of this study suggest that the nature of the assimilated crust may have influenced the final composition of the Laramide granitic rocks of central Sonora.     

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.     

Geochemical and isotopic characteristics of the Cretaceous Orikabe Plutonic Complex,Kitakami Mountains,Japan: magmatic evolution in a zoned pluton and significance of a subduction-related mafic parental magma

The Orikabe Plutonic Complex, northeast Japan, is a zoned pluton and one of the Cretaceous intrusions in the Circum-Pacific area. In the Main body, K-rich calc-alkaline rocks composed of marginal gabbro and a large amount of monzodiorite–quartz monzonite–monzogranite are intruded successively by innermost calc-alkaline rocks of granodiorite. The gabbro and monzodiorite–monzogranite have a continuous chemical variation, while the granodiorite has lower concentrations of K, Rb, Y, Zr, Nb and F at the same SiO₂ content. The gabbro and monzodiorite–quartz monzonite have a Rb-Sr whole-rock age of 119±12 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70392±0.00007. The initial ⁸⁷Sr/⁸⁶Sr ratio of the innermost granodiorite is estimated to be about 0.7042. The ¹⁸O values of fresh rocks range from +6.7 to +8.3, indicating a positive correlation with SiO₂ contents. The K-rich calc-alkaline rocks were derived through fractional crystallization from a mafic parental magma with a slightly high ¹⁸O value, implying a major contribution of a sub-arc mantle at a continental margin. Trace element modeling indicates that the source could have been a fertile lherzolite enriched in LILE and depleted in HFSE. The innermost granodiorite was the differentiation product of a distinct parental magma, suggesting the involvement of a small amount of crustal component in the source and partial melting under a more hydrous condition.Editorial responsibility: J. Hoefs     

Geochronology and geochemistry of Cretaceous Nanshanping alkaline rocks from the Zijinshan district in Fujian Province,South China: Implications for crust–mantle interaction and lithospheric extension

In situ zircon U–Pb ages and Hf isotopic data, major and trace elements, and Sr–Nd–Pb isotopic compositions are reported for Nanshanping alkaline rocks from the Zijingshan district in southwestern Fujian Province (the Interior or Western Cathaysia Block) of South China. The Nanshanping alkaline rocks, which consist of porphyritic quartz monzonite, porphyritic syenite, and syenite, revealed a Late Cretaceous age of 100–93 Ma. All of the rocks show high SiO₂, K₂O + Na₂O, and LREE but low CaO, Fe₂O₃^T, MgO, and HFSE (Nb, Ta, P, and Ti) concentrations. These rocks also exhibit uniform initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7078 to 0.7087 and ε_Nd(t) values of −4.1 to −7.2, thus falling within the compositional field of Cretaceous basalts and mafic dikes occurring in the Cathaysia Block. Additionally, these rocks display initial Pb isotopic compositions with a ²⁰⁶Pb/²⁰⁴Pb_i ratio of 18.25 to 18.45, a ²⁰⁷Pb/²⁰⁴Pb_i ratio of 15.63 to 15.67, and a ²⁰⁸Pb/²⁰⁴Pb_i ratio of 38.45 to 38.88. Combined with the zircon Hf isotopic compositions (ε_Hf(t) = −11.7 to −3.2), which are different from those of the basement rocks, we suggest that Nanshanping alkaline rocks were primarily derived from a subduction-related enriched mantle source. High Rb/Sr (0.29–0.65) and Zr/Hf (37.5–49.2) but relatively low Ba/Rb (4.4–8.1) ratios suggest that the parental magmas of these rocks were most likely formed via partial melting of a phlogopite-bearing mantle source with carbonate metasomatism. The relatively high SiO₂ (62.35–70.79 wt.%) and low Nb/Ta (10.0–15.3) ratios, positive correlation between SiO₂ and (⁸⁷Sr/⁸⁶Sr)_I, and negative correlation between SiO₂ and ε_Nd(t) of these rocks suggest that the crustal materials were also involved in formation of the Nanshanping alkaline rocks. Combined with geochemical and isotopic features, we infer magmatic processes similar to AFC (assimilation and fractional crystallization) involving early fractionation of clinopyroxene and olivine and subsequent fractionation of biotite-dominated assemblages coupled with a lesser amount of crustal contamination, thereby forming the Nanshanping alkaline rocks. The Nanshanping alkaline rocks appear to be associated with an extensional environment in the Cathaysia Block. This extensional regime could have resulted in the slab break-off and rollback of the subducting paleo-Pacific plate and the upwelling of the asthenospheric mantle, which induced partial melting of the enriched lithospheric mantle in the Cretaceous.     

A Paleoproterozoic A—type Rhyolite

The rhyolites in the upper Lueliang Group of Shanxi,China,are Paleoproterozoic weakly alkaline volcanic rocks.They are characterized by high,SiO2,NaO K2O,Zr,Nd,Ga,Y and REE contents and large FeO^*/MgO,Rb/Sr and Ga/Al ratios,and low CaO,Sr and Eu contents,and share much in common with the A-type granitic rocks.They erupted in the rift setting at the continental margin.Chemical features and isotope data,as well as high Nd and low initial Sr ratios,suggest that the original granitic magma was derived from partial melting of Late Archean metamorphic rocks in the lower crust due to the influence of basaltic magma and hot fluid in response to rifting.The A-type rhyolites were finally formed after the fractional crystallization of the dominant mineral feldspar.     

Xiba Granitic Pluton in the Qinling Orogenic Belt,Central China: Its Petrogenesis and Tectonic Implications

Xiba granitic pluton is located in South Qinling tectonic domain of the Qinling orogenic belt and consists mainly of granodiorite and monzogranite with significant number of microgranular quartz dioritic enclaves. SHRIMP zircon U–Pb isotopic dating reveals that the quartz dioritic enclaves formed at 214±3 Ma, which is similar to the age of their host monzogranite (218±1 Ma). The granitoids belong to high-K calc-alkaline series, and are characterized by enriched LILEs relative to HFSEs with negative Nb, Ta and Ti anomalies, and right-declined REE patterns with (La/Yb)N ratios ranging from 15.83 to 26.47 and δEu values from 0.78 to 1.22 (mean= 0.97). Most of these samples from Xiba granitic pluton exhibit εNd(t) values of ?8.79 to ?5.38, depleted mantle Nd model ages (TDM) between 1.1 Ga and 1.7 Ga, and initial Sr isotopic ratios (87Sr/86Sr)i from 0.7061 to 0.7082, indicating a possible Meso- to Paleoproterozoic lower crust source region, with exception of samples XB01-2-1 and XB10-1 displaying higher (87Sr/86Sr)i values of 0.779 and 0.735, respectively, which suggests a contamination of the upper crustal materials. Quartz dioritic enclaves are interpreted as the result of rapid crystallization fractionation during the parent magmatic emplacement, as evidenced by similar age, texture, geochemical, and Sr-Nd isotopic features with their host rocks. Characteristics of the petrological and geochemical data reveal that the parent magma of Xiba granitoids was produced by a magma mingling process. The upwelling asthenosphere caused a high heat flow and the mafic magma was underplated into the bottom of the lower continent crust, which caused the partial melting of the lower continent crustal materials. This geodynamic process generated the mixing parent magma between mafic magma from depleted mantle and felsic magma derived from the lower continent crust. Integrated petrogenesis and tectonic discrimination with regional tectonic evolution of the Qinling orogen, it is suggested that the granitoids are most likely products in a post-collision tectonic setting.     

Assimilation-fractional crystallization of Polvadera Group rocks in the northwestern Jemez Volcanic Field,New Mexico

Pliocene Polvadera Group rocks in the northwestern Rio Grande rift-marginal portion of the Jemez Volcanic Field record the rapid transition from weakly alkaline Lobato Basalt magmatism (48–52% SiO₂; 7.9 Ma) through calc-alkaline Lobato andesite and dacite (53–64% SiO₂) and Tschicoma dacite-rhyodacite magmatism (63–69% SiO₂; 7.4 Ma). Petrologically, Lobato andesite and dacite and Tschicoma dacite-rhyodacite represent a cogenetic suite of differentiates (the La Grulla Plateau or LGP suite) distinctive from the bulk of Polvadera Group rocks including Tschicoma andesite. Increasing (⁸⁷Sr/⁸⁶Sr)_O ratios with differentiation within the LGP suite from 0.7051 (54% SiO₂) to 0.7064 (68% SiO₂), trace element variations, and disequilibrium mineral assemblages suggest open system differentiation involving ⁸⁷Sr-enriched upper crust. A likely parental magma is the voluminous Lobato Basalt ((⁸⁷Sr/⁸⁶)_O= 0.7043–0.7050) which was erupted predominantly earlier and to the east toward the rift axis. The best model for petrogenesis involves bulk assimilation of locally wide-spread Proterozoic (1.4–1.6 Ga) upper crustal granite by fractionally crystallizing Lobato Basalt. Assimilation-fractional crystallization (AFC) modeling of Sr-isotope and trace element variation (DePaolo 1981) indicates that 40% crystallization of Lobato Basalt accompanied by 10% addition of granite reproduces the observed geochemical and Sr-isotopic diversity. Neither magma mixing, nor mafic recharge have complicated the magmatic evolution of the LGP suite. Crustal thickness and/or retarded tectonism could have facilitated conditions necessary for evolution by AFC to occur within the upper crust.