Feeding system and mantle sources of the southern and western sector of the Madagascar flood basalt province, and comparisons with Southwest Indian Ridge “anomalous” basalts

Abstract

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%, Fe2O3t = 14–16 wt%; TiO2 = 3–3.6 wt%, Nb =16–22 ppm, Zr = 236–269 ppm; Lan/Ybn = 7–8) in the southernmost outcrops (Lavanono, Tulear), to incompatible element-poor basaltic dikes (MgO = 4.2–4.9 wt%, Fe2O3t = 12–13 wt%; TiO2 = 1.6 wt%, Nb = 8–9 ppm, Zr = 90–100 ppm; Lan/Ybn = 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/86Sri = 0.7121–0.7123, 208Pb/204Pb = 40.9–41.3 for the Tulear-Lavanono lavas vs 87Sr/86Sri = 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.