ALBERT

Notizen: Lower crustal garnet-bearing mafic granulite xenoliths from beneath the cratonic areas of NE Europe (NW Russia, Belarus, Finland) have unradiogenic 143Nd/144Nd ratios that differ strongly from those of xenoliths from beneath Phanerozoic regions of the European plate and worldwide, but closely resemble xenoliths from other cratonic regions of the world. Phanerozoic lower crustal xenoliths worldwide also show a very limited range of Pb isotope compositions whereas most cratonic lower crustal xenoliths have more varied but usually unradiogenic Pb isotope compositions, plotting to the left of the Geochron. However, many of the xenoliths from beneath NE Europe plot on the right-hand side of the Geochron and also have more radiogenic 208Pb/204Pb ratios. Thus, the lower crust of NE Europe shows characteristics of both cratonic lower crust (unradiogenic Nd isotopes) and Phanerozoic lower crust (radiogenic Pb isotopes). Its present-day low U/Pb and Th/Pb ratios indicate that it has been depleted in heat-producing elements, but the radiogenic Pb isotope ratios show that this depletion occurred relatively recently.

Notizen: Abstract  On the island of Mustique, fresh and propylitized olivine–plagioclase–clinopyroxene basalt, plagioclase–clinopyroxene–orthopyroxene and plagioclase–clinopyroxene–amphibole andesite lavas and minor intrusions are interbedded with Oligocene pyroclastic and epiclastic rocks. Chemical data show that two isotopically identical, but chemically different, suites of lava are present: (i) the OPXS (87Sr/86Sr 0.70403–0.70454; 143Nd/144Nd 0.512952–0.512986; δ18Ocpx 5.49 and 5.61), comprising basalts and orthopyroxene-bearing andesites; and (ii) the AMPHS (87Sr/86Sr 0.70401–0.70457; 143Nd/144Nd 0.512981–0.513037; δ18Ocpx 5.54), made up of basalts and amphibole-bearing andesites. The OPXS has higher contents of TiO2, P2O5, light rare earth elements, Sm, Pb, Th, U, Zr, Y and Nb, and higher La/Yb ratios than the AMPHS. The isotopic data suggest that both suites formed from melts derived from the same subduction-modified depleted mantle source as the volcanic rocks of nearby St Vincent and Bequia, and the northern islands of the Lesser Antilles Arc. The immobile trace element contents, and La/Yb ratios, of the OPXS are indicative of ∼10% partial melting of the source, whereas those of the AMPHS are indicative of ∼25% partial melting. The within-suite chemical variation of the OPXS is consistent with ∼45% fractional crystallization of its intratelluric mineral assemblages, and that of the AMPHS is consistent with the removal of ∼65% of its intratelluric assemblages. Experimental evidence suggests that both suites of basalt crystallized at pressures 〈8 kbar from melts containing 1–2 wt% water. After extensive fractional crystallization, the andesites crystallized at pressures between approximately 5 and 2 kbar. The OPXS magmas appear to have lost more of their water content than the AMPHS magmas. Thus, the OPXS andesites formed from melts with an estimated water content of 2–3 wt%, whereas the AMPHS andesites formed from melts containing at least 4.5 wt% water.

Notizen: Abstract  The island of Curaçao in the southern Caribbean Sea is composed mainly of a thick sequence (〉5 km) of pillow lavas, grading upwards from picrites at the base of the exposed section, to basalts nearer the top. Modelling suggests that picrites are related to the basalts by fractional crystallisation. Initial radiogenic isotope ratios of the picrites have a restricted compositional range: ɛNd=+6.1 to +6.6, 87Sr/86Sr=0.70296–0.70319; whereas the basalts display a wider range of compositions: ɛNd=+6.6 to +7.6, 87Sr/86Sr=0.70321–0.70671. This variation in isotope ratios between basalts and picrites may be due to the assimilation of altered oceanic crust (or possibly partial melts of such crust) by a picritic magma along with fractional crystallisation. The relatively narrow range of Nd and Pb isotopic compositions in the Curaçao lavas suggests either that the source region was homogeneous, or that melts from a heterogeneous mantle source were well mixed before eruption. Chondritic to slightly light rare earth element enriched patterns, combined with long-term light rare earth element depletion (positive ɛNd), suggest that the lavas were formed by polybaric melting of spinel lherzolite, with small a contribution from garnet lherzolite melts. High-MgO lavas, the absence of a subduction related chemistry, and the chemical similarity to other oceanic plateaux, suggest a mantle plume origin for the Curaçao lava succession. The Curaçao volcanic sequence is part of an oceanic plateau formed at about 88–90 Ma, fragments of which are dispersed around the Caribbean as well as being obducted onto the western margin of Colombia and Ecuador. The occurrence of high-Mg lavas throughout this Cretaceous Caribbean–Colombian igneous province requires anomalously hot mantle (〉200° C hotter than ambient upper mantle) over a large part of a putative plume head, which is inconsistent with some mantle plume models.

Notizen: Abstract Concentric zoning in the Criffell pluton takes the form of a discontinuous outer margin of metaluminous hornblende granodiorite and an inner core of increasingly peraluminous muscovite granite. Previous investigations using major and selected trace elements have shown the variation to consist of both smooth and abrupt trends. This study of 15 samples for the rare earth elements shows patterns which strongly correlate with Sr and O isotope data. The principal feature of these data is a progressive decrease in total rare earths with approach to the geochemical centre of the pluton, and evolution to more radiogenic Sr and more silicic and peraluminous compositions. No significant europium anomaly is developed. The slope of light to heavy rare earths using La/Yb ratios varies in a complex manner showing no significant correlation with any of the main indices of bulk composition, but with peak values occurring within the inner part of the outer portion of the pluton. A map of Ce/Y variation based on 172 Ce and Y determinations is essentially identical. These data are considered in terms of various petrogenetic models and it is concluded that the data can only be interpreted in terms of a major and progressive involvement of crustally-derived anatectic magma towards the pluton interior. Trace element modelling favours processes of the assimilation-fractional crystallisation (AFC) type for the generation of this example of I-type to S-type granitoid zonation.

Notizen: Abstract  New Sr- Nd- and Pb-isotopic and trace element data are presented on basalts from the Sulu and Celebes Basins, and the submerged Cagayan Ridge Arc (Western Pacific), recently sampled during Ocean Drilling Program Leg 124. Drilling has shown that the Sulu Basin developed about 18 Ma ago as a backarc basin, associated with the now submerged Cagayan Ridge Arc, whereas the Celebes Basin was generated about 43 Ma ago, contemporaneous with a general plate reorganisation in the Western Pacific, subsequently developing as an open ocean receiving pelagic sediments until the middle Miocene. In both basins, a late middle Miocene collision phase and the onset of volcanic activity on adjacent arcs in the late Miocene are recorded. Covariations between 87Sr/86Sr and 143Nd/144Nd show that the seafloor basalts from both the Sulu and Celebes Basins are isotopically similar to depleted Indian mid-ocean ridge basalts (MORB), and distinct from East Pacific Rise MORB, defining a single negative correlation. The Cagayan Arc volcanics are different, in that they have distinctly lower ɛNd(T) for a given ɛSr(T), compared to Sulu and Celebes basalts. In the 207Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, the Celebes, Sulu and Cagayan rocks all plot distinctly above the Northern Hemisphere Reference Line, with high Δ7/4 Pb (5.3–9.3) and D8/4 Pb (46.3–68.1) values. They define a single trend of radiogenic lead enrichment from Celebes through Sulu to Cagayan Ridge, within the Indian Ocean MORB data field. The data suggest that the overall chemical and isotopic features of the Sulu, Cagayan and Celebes rocks may be explained by partial melting of a depleted asthenospheric N-MORB-type (“normal”) mantle source with isotopic characteristics similar to those of the Indian Ocean MORB source. This asthenospheric source was slightly heterogeneous, giving rise to the Sr-Nd isotopic differences between the Celebes and Sulu basalts, and the Cagayan Ridge volcanics. In addition, a probably slab-derived component enriched in LILE and LREE is required to generate the elemental characteristics and low Nd(T) of the Cagayan Ridge island arc tholeiitic and calcalkaline lavas, and to contribute to a small extent in the backarc basalts of the Sulu Sea. The results of this study confirm and extend the widespread Indian Ocean MORB signature in the Western Pacific region. This signature could have been inherited by the Indian Ocean mantle itself during the rupture of Gondwanaland, when fragments of this mantle could have migrated towards the present position of the Celebes, Sulu and Cagayan sources.

Notizen: Abstract Major element, trace element, and Sr isotope data are used to study the temporal variation in the chemistry of the ejecta from the 1979 eruption of Soufriere volcano, St. Vincent, and to compare the compositions of the 1979 and 1971/2 magmas. Both the 1971/2 and 1979 products were basaltic andesites almost identical in petrography. A small temporal variation in chemistry is apparent in the 1979 samples but these cannot be related to the 1971/2 lava by fractional crystallisation of phenocryst phases, and the two eruptions may therefore have sampled different batches of magma. 87Sr/86Sr ratios of the two magmas were identical within analytical error. Microprobe analyses of phenocryst phases and glasses from the 1979 ejecta are presented. Clinopyroxene phenocryst cores with very high Mg/Fe ratios indicate that the basaltic andesites are products of fractionation of magnesian parents. Such magmas are represented by lavas on St. Vincent similar to the microphyric alkali picrites found to the south in Grenada. A common origin for the basaltic andesites of both islands by fractional crystallisation of picritic magmas is suggested. Dacitic glass is abundant in the groundmass of scoria blocks from the eruption. It does not represent the liquid originally in equilibrium with the phenocryst phases, but rather this liquid modified by quench crystallisation. Published interpretations suggesting that dacitic glass compositions in tephra from eruptions of the Soufriere are evidence of mixed-magma eruptions are therefore rejected.