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The flood basalts of Cretaceous age in southwestern Madagascar range from moderately incompatible-element-enriched, tholeiitic, high-Ti ferrobasalt/basaltic andesite lavas (MgO = 4–5 wt%, Fe 2 O 3t = 14–16 wt%; TiO 2 = 3–3.6 wt%, Nb =16–22 ppm, Zr = 236–269 ppm; La n /Yb n = 7–8) in the southernmost outcrops (Lavanono, Tulear), to incompatible element-poor basaltic dikes (MgO = 4.2–4.9 wt%, Fe 2 O 3t = 12–13 wt%; TiO 2 = 1.6 wt%, Nb = 8–9 ppm, Zr = 90–100 ppm; La n /Yb n = 2.3) around Morondava. The two magma types, at the same degree of magmatic evolution, have markedly different Sr-Nd-Pb isotopic composition (e.g., 87Sr/86Sr i = 0.7121–0.7123, 208Pb/204Pb = 40.9–41.3 for the Tulear-Lavanono lavas vs 87Sr/86Sr i = 0.7033–0.7040, 208Pb/204Pb = 38.7 for the Morondava dikes), that suggest markedly different sources, magmatic history, and variable interplay between Madagascan Precambrian crust and mantle-derived melts, some with a marked enriched mid-ocean ridge basalt-like composition. The geochemical and isotopic data show no involvement of ancient basement crust in the genesis of the Morondava dikes. The Tulear-Lavanono lavas have high Ba/Nb and La/Nb ratios and Sr-Nd-Pb isotopic characteristic of magmas that have assimilated crustal material. The chemical and isotopic composition of the contaminated tholeiites of southern Madagascar matches some Late Cretaceous group B1 basalts of the Androy volcanic complex and tholeiites of the Ejeda-Bekily dike swarm, and therefore they could derive from the same feeding system or were fed by similar parental magmas. Basalts from the Southwest Indian Ridge between 39 and 41°E section (anomalous magmatic sectors) with low 206Pb/204Pb and 143Nd/144Nd have been interpreted to contain a component of either recycled lower continental crust and/or subcontinental lithospheric mantle. A detailed review of the isotopic and geochemical compositions of the Mesozoic -and Cenozoic- magmatism in Madagascar, and of the lower and upper crustal domains point out that the sources of the anomalous magmatic sectors of the Southwest Indian Ridge do not contain lower crustal (or lithospheric mantle) components of Madagascan origin. Comparison between the samples from the northern and southern parts of the Madagascar flood basalts province indicates that several different parental magmas and mantle sources were involved in the petrogenesis of the Madagascan basalts, and that chemical contributions from a "Marion" hotspot (as a magma source or source component) to the Madagascar primary magmas is not identifiable. • Flood basalts and dikes of Cretaceous age in southern and western sector of Madagascar • Contamination of magmas by ancient Precambrian crust • Flood basalts modelled by assimilation and fractional crystallisation (AFC) • Mantle sources of magmas markedly different • Marked provinciality of sources and feeding systems of Madagascar CFB [ABSTRACT FROM AUTHOR]

Cenozoic (≥ 43 Ma) silica undersaturated (potassic) trachybasalts and trachyandesites in southwestern Madagascar (Tsianihy-Manja, southern Morondava Basin) form a small monogenetic volcanic field emplaced above Paleogene detritic sedimentary rocks, along a NE-SW-trending fault system. These olivine-chromite±clinopyroxene-phyric primitive lavas (Mg# = 69; MgO = 10–11 wt%; Cr = 450 ppm, Ni = 200 ppm; K 2 O = 3–4 wt%) have highly peculiar trace element and isotopic composition (e.g., Ba/Nb = 18.4; 87Sr/86Sr i = 0.70529–0.70555, 143Nd/144Nd i = 0.51262–0.51263, 206Pb/204Pb m = 18.415–18.424, 207Pb/204Pb m = 15.576–15.579, 208Pb/204Pb m = 38.799–38.813). A hitherto undescribed plug of primitive (sodic) basanite of the 11–12 Ma-old Ankililoaka district south of Tsianihy-Manja (hosting spinel lherzolite mantle xenoliths) has noticeable different geochemistry (Ba/Nb = 8–9.2; 87Sr/86Sr i = 0.70346–0.7036, 143Nd/144Nd i = 0.51281–0.51282, 206Pb/204Pb m = 19.079–19.374, 207Pb/204Pb m = 15.621–15.645, 208Pb/204Pb m = 39.115–39.424). The relatively low CaO, Sc, V, Fe 2 O 3t , MnO at high MgO, Cr and Ni, and the potassic affinity of the Tsianihy-Manja trachybasalts, all indicate that the mantle source is relatively clinopyroxene-poor (i.e., depleted by previous melt extractions), in the same way as the source of lamproitic (or boninitic) magmas, but the primitive nature, the concentration of high field strength elements, the incompatible element patterns and their isotopic ratios indicate their unequivocal within-plate setting and indicate a derivation by low-degree partial melting of an incompatible element-enriched mantle and insignificant role for crustal contamination. In terms of incompatible element concentrations, and thus also Sr-Nd-Pb-isotopic composition, we find no evidence in favour of a mid-ocean ridge basalt (MORB)-mantle component, or for a MORB-mantle strongly enriched by ocean island basalt-like components, to form the mantle source regions of the Tsianihy-Manja and Ankililoaka mafic alkaline rocks. The significant isotopic change from the northernmost Cenozoic volcanic rocks of Madagascar and those in the central and southern part of the island (which range in composition from sodic to potassic, and from tholeiitic basalt to olivine melilitite) implicates a distinct source heterogeneity, and ultimately assess the role of the old continental lithospheric mantle as source region. • Cenozoic potassic and sodic mafic rocks in southwestern Madagascar • Mantle sources of potassic magmas is relatively clinopyroxene-poor. • No crustal contamination during magma ascent • Significant role of heterogeneous lithospheric mantle in generating alkaline melts [ABSTRACT FROM AUTHOR]