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The Cretaceous lava sequence and associated mafic dyke swarm in central–western Madagascar (Mailaka and Bemaraha areas) range in composition from picrite basalts to cordierite–orthopyroxene-bearing rhyodacites (MgO from 14 to 0·6 wt %). Petrographic and chemical data indicate the presence of both tholeiitic and transitional magma series, with variable degree of rare earth element enrichment [(La/Nd)n = 1–1·4 for tholeiites vs (La/Nd)n = 0·65–1 for transitional rocks]. Initial (at 88 Ma) 87Sr/86Sr and εNd range from 0·7044 to 0·7046 and −1·6 to −3·0 in the tholeiitic picrite basalts and basalts, and from 0·7030 to 0·7043 and +7·6 to +3·7 in the transitional picrite basalts and basalts. The rhyodacites have (87Sr/86Sr)88 = 0·7155 and εNd(88) = −10·6. Fractional crystallization of the observed phenocryst phases, starting from the most primitive tholeiitic basalts, combined with moderate amounts of contamination by peraluminous melts derived from partial melting of metapelitic basement rocks, explains the chemical composition of the rhyodacites reasonably well. The different parental magmas of the two series were probably generated by low degrees of partial melting (2·5–5%) of a depleted source (transitional basalts), and higher degrees of partial melting (5–7%) of a source very slightly enriched with a crustally derived component (tholeiitic basalts). Comparison between the samples from the eastern and northern parts of the province indicates that several different parental magmas and mantle sources were involved in the petrogenesis of the Madagascan basalts, and that contributions from mantle chemically equivalent to the modern Marion hotspot were negligible, overall. [ABSTRACT FROM PUBLISHER]

Tholeiitic lavas forming a flood basalt sequence of 870 m thickness at Toranmal in the northern Deccan Traps have a large range in isotopic ratios [εNd(t) = +2·1 to –15·7, (87Sr/86Sr)t = 0·70467–0·71416, 206Pb/204Pb = 16·699–20·246], similar to that of lavas in the well-studied southwestern part of the province. The basalts with the lowest εNd(t) values display distinctive lows at Nb, Ta, P and Ti, and large positive Pb spikes in their primitive-mantle-normalized element patterns, indicative of significant continental lithospheric influence in their petrogenesis. As in much of the southwestern Deccan, εNd(t) exhibits a rough negative correlation with Mg/Fe and SiO2 and a positive correlation with Fe, consistent with temperature-controlled assimilation. Overall, the Toranmal section appears distinct from sections in the northwestern sector of the province; however, some Toranmal basalts are isotopically and chemically similar to flows in the northeastern Deccan, and a thick pile of lavas resembling the Poladpur Fm of the southwestern Deccan, the closest type-sections of which lie ∼380 km to the south, is present. If it indeed represents a northern remnant of this formation, the Poladpur Fm, which also extends far into the central and southeastern parts of the province, is one of the most widespread of Deccan formations, with a possible original extent ≥3 × 105 km2. The Ambenali Fm, which forms a thick sequence lying above the Poladpur in the southwestern Deccan, is not present at Toranmal. Several flows have broad geochemical affinities with the southwestern Bushe and Mahabaleshwar formations, which respectively lie below the Poladpur and above the Ambenali; however, these flows are not in the southwestern stratigraphic order and are probably of relatively local origin as dikes compositionally very similar to these flows are present at Toranmal and elsewhere in the vicinity. [ABSTRACT FROM PUBLISHER]

Submarine lavas erupted onto the Hawaiian arch 200–400 km north of Oahu show that the areal extent of Hawaiian volcanism is much larger than previously recognized. The North Arch volcanic field comprises 25 000 km2 of ∼0·5–1·15 Ma, volatile-rich, olivine-phyric alkalic lavas (alkalic basalt to nephelinite). These lavas are similar in composition to rejuvenated-stage lavas such as the Koloa Volcanics (Kauai) and Honolulu Volcanics (Oahu). North Arch lavas that encompass the compositional extremes have similar Sr, Nd and Pb isotopic ratios. Olivine accumulation and fractionation was the major post-melting process that affected the compositions of North Arch lavas. After correction for these processes, the inferred primary magma compositions show that they were derived by variable, factor of four, and relatively low extents of melting of garnet peridotite. Garnet and olivine were important residual phases during partial melting; in contrast to the Honolulu Volcanics, there is little evidence for residual hydrous phases, sulfides or Fe–Ti oxides. The mantle source for the North Arch lavas had Sr, Nd and Pb isotopic ratios intermediate between those of Pacific Ocean lithosphere and the inferred range for Hawaiian plume components. These data are consistent with a mixed lithosphere–plume source. Although the plume-derived component was probably from the Hawaiian plume, an alternative hypothesis is that during the middle Cretaceous, South Pacific lithosphere was contaminated by plumes that formed large oceanic plateaux (e.g. Ontong Java). This mixed source was subsequently partially melted as it passed near the Hawaiian plume. [ABSTRACT FROM PUBLISHER]