ALBERT

Notes: Abstract Major and trace element analyses and strontium isotope ratios are presented for twenty-four samples of lavas and plugs from the early Tertiary lava pile in Mull. The samples were selected on the basis of petrographic freshness from a large collection from outside the hydrothermally altered “zone of pneumatolysis” which occupies the central region of the volcanic complex. Most of the analyses yield normative hypersthene and we argue that these are essentially unaltered magmatic compositions. The analytical data indicate that the samples may be divided into three groups on the basis of major element chemistry, initial 87Sr/86Sr ratios and correlations between lithophile element contents. Group I comprises an alkaline series (basalt-hawaiite-mugearite) with extremely low initial 87Sr/86Sr ratios (≦0.7030) and generally low lithophile element contents. Apart from their alkalinity and high Sr and Zr contents these samples have affinities with abyssal tholeiites. Group II contains hypersthene normative basalts with more tholeiitic characteristics but (as in the case of the Skye Main Lava Series) the more evolved rocks are trachytes. This group is characterized by more normal levels of lithophile element concentrations and relatively high initial 87Sr/86Sr ratios of about 0.7055. Group III is less clearly defined and contains basalts that are generally sparsely olivine-phyric and in most chemical respects fall between Group I and Group II-including initial 87Sr/86Sr ratios (0.7033 to 0.7043). They may represent mixtures of Group I and Group II type sources or magmas. Groups I and II appear to be similar, respectively, to the relatively sodic iron-rich and the relatively potassic ironpoor silica enrichment trends distinguished in the Skye Main Lava Series. In the Group I magma series the behaviour of Y and Sr relative to other incompatible elements can only be explained by differential partial melting of a deep garnet-lherzolite mantle source. Fractional crystallization has undoubtedly occurred at some stage during the ascent of these magmas from the mantle, as indicated by the behaviour of Ni and Cr, but has not been a major factor in the production of evolved magma compositions. The Group II magmas appear to have originated from a source more enriched in lithophile elements, and a relatively shallow (〈 50 km) plagioclase-lherzolite mantle source is suggested for these magmas because they have Sr/Ba ratios between one and two orders of magnitude lower than those characteristic of Group I. Rb-Sr systematics suggest that the vertical heterogeneity of the mantle which was largely responsible for the chemical differences between these three groups may have existed for a very long time prior to Tertiary magmatism.

Notes: [Auszug] The gneisses of the Gorur-Hassan area (Fig. 1) are components of the polyphase Archaean Peninsular Gneisses of the South Indian Craton. They are unconformably overlain by the low grade volcanic-sedimentary association of the Dharwar supergroup, represented locally by the Bababudan and Shigegudda ...

Notes: Abstract Low grade hydrothermally metamorphosed ophiolitic basic rocks from E. Liguria (Italy), Pindos (Greece) and Troodos (Cyprus) are enriched in O18 relative to the oxygen isotope ratio of fresh basalt (6.0±0.5‰). The maximum observed δO18 value of +13.22‰ corresponds to a positive isotope shift of 7‰ Enrichments in Sr87 relative to Sr86 correlate with hydrothermal alteration. The δC13 values of secondary calcite from E. Liguria are positive, and fall in the range from +0.2% to +3.6‰ Since ophiolitic rocks are considered to be fragments of the oceanic crust and upper mantle, and since the secondary metamorphic assemblages were produced before mechanical emplacement, it is considered that the hydrothermal metamorphism which affected these rocks occurred in the sub-sea-floor environment. The isotope data are directly consistent with the hypothesis that the alteration was produced by interaction of the basaltic material with introduced sea water. Water: rock ratios were sufficiently large to produce the observed isotope shifts. In the Troodos ophiolite sequence δO18 values decrease steadily downwards and change to progressively larger depletions in the Sheeted Intrusive Complex. The trend of δO18 decrease correlates with the original direction of increasing temperature. The O18 depletions, which have also been observed for oceanic “greenstones” (Muehlenbachs and Clayton, 1972b), resulted from water/rock interaction at temperatures greater than the particular temperature range above which whole rock-water fractionations became less than the isotopic difference between fresh basalt and sea water. Since this isotope geochemistry indicates that the water responsible for hydrothermal metamorphism was of sea water origin, the data support the more general hypothesis that convection of sea water within the upper 4–5 kms of the oceanic crust is a massive and active process at oceanic ridges. This process may be completely or partially responsible for (a.i.), the local scatter and low mean value of the conductive heat flux measured near ridges, (a.ii), the transfer of considerable quantities of heat from spreading oceanic ridges, (b) hydrothermal metamorphism, metasomatism and mineralization of oceanic crust, (c), the production of metal enriched, relatively reduced brines during sea water/basalt interaction, d), the high degree of scatter and low mean value of the compressional wave velocities of oceanic basement layer 2 and (e), the low natural remanent magnetization (NRM) intensity of the lower part of layer 2 and upper part of layer 3 of oceanic crust.

Notes: Abstract 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 δ18O values for a comprehensive collection of granitic rocks from the 3 centres. The δ18O values range from +4 to −6 indicating variable extent of interaction between the rocks and heated meteoric groundwater. However, correlations of δ18O 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 CeN/YbN 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.

Notes: Abstract The Kangerdlugssuaq intrusion, East Greenland, consists of quartzsyenites, syenites, pulaskites and foyaites. The age and petrogenesis of the intrusion has been investigated by strontium and oxygen isotope analyses of the major rock types (and some separated minerals) and the surrounding country rocks. Crystallization and rapid cooling of the intrusion close to 50 m.y. ago is indicated by concordance of an Rb-Sr mineral isochron (49.9±1.0 m.y.) and an Rb-Sr whole-rock isochron (50.0±1.9 m.y.) with previously published mineral dates. The feldspathoid-bearing rocks of the intrusion, which were the last to crystallize, have uniformly depleted oxygen (δ18O = +3.9‰, SMOW) and homogeneous initial 87Sr/86Sr ratios (0.70450±7). This is ascribed to equilibration of the magma prior to the crystallization of these rocks with about 10% by weight of meteoric ground water. The concommittant increase of $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ to about 1 Kb (the lithostatic load pressure) would depress the liquidus surfaces in the system Ne-Ks-Qz by about 200 ° C, allowing the magma to evolve continuously down temperature from oversatuated to undersaturated compositions. The chemical mechanism responsible for this trend has not been uniquely identified, but probably involved reduction of SiO2 content in an open system. The outer, quartz-normative, rocks of the intrusion have δ18O values ranging up to +5.5‰ and initial 87Sr/86Sr ratios ranging up to 0.7095. This is due to interaction of the solid rocks, down to temperatures approaching 500 ° C, with ground water which had been enriched in 18O and 87Sr by previous exchange with the Precambrian country rocks. Minimum water/rock ratios are lower than in certain other known cases of interaction in the North Atlantic Tertiary Igneous Province.