The Cretaceous igneous province of Madagascar: Geochemistry and petrogenesis of lavas and dykes from the central-western sector

The Cretaceous lava sequence and associated mafic dyke swann in central-wes tern Madagascar (Mailaka and Bemaraha areas) range in composition from picr ite basalts to cordierite-orthopyroxene-bearirg rhyodacites (MgO from 14 to 0.6 wt %). Petrographic and chemical data indicate the presence of both th oleiitic 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) Sr-87/Sr-86 and epsilon (Nd) range from 0.7044 to 0.7046 and -1.6 to -3.0 in the tholeiitic picrite bas alts and basalts, and from 0.7030 to 0.7043 and + 7.6 to + 3.7 in the trans itional picrite basalts and basalts. The rhyodacites have (Sr-87/Sr-86)(88) = 0.7155 and epsilon (Nd(88)) = -10.6. Fractional crystallization of the o bserved phenocryst phases, starting from the most primitive tholeiitic basa lts, combined with moderate amounts of contamination by peraluminous melts derived from partial melting of metapelitic basement rocks, explains the ch emical composition of the rhyodacites reasonably well. The different parent al magmas of the two series were probably generated by low degrees of parti al melting (2.5-5%) of a depleted source (transitional basalts), and higher degrees of partial melting (5 - 7%) of a source very slightly enriched wit h a crustal 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 p etrogenesis of the Madagascan basalts, and that contributions from mantle c hemically equivalent to the modern Marion hotspot were negligible, overall.