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Journal of Petrology Volume 42 Number 7 Pages 1249-1278 2001
© Oxford University Press 2001
The Cretaceous Igneous Province of Madagascar: Geochemistry and Petrogenesis of Lavas and Dykes from the Central–Western Sector
1DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI NAPOLI FEDERICO II, VIA MEZZOCANNONE 8, 80134 NAPOLI, ITALY
2SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY, UNIVERSITY OF HAWAII AT MANOA, 1680 EAST–WEST ROAD, HONOLULU, HI 96822, USA
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 
Nd 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 Nd(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.
KEY WORDS: picrite basalts, rhyodacites, fractional crystallization, crustal contamination, mantle sources, Madagascan igneous province
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