Zircon U-Pb geochronology and Pb-isotope composition of the Mailaka picritic basalt-rhyodacite succession, Central-Western Madagascar: Implications for the role of crustal contamination in the Cretaceous volcanic cycle.

In situ U-Pb zircon geochronology for rhyodacites in the Mailaka lava succession, yields ages of 89.4  1.3 Ma and 90.0  1.0 Ma. The age determinations are within analytical error of the U-Pb age of the Analalava gabbro intrusion, northeast Madagascar (91.6  0.3 Ma) and Antampombato-Ambatovy complex, central-eastern Madagascar (90  2 Ma). The results indicate that the capping rhyodacitic unit of the Mailaka lava succession was emplaced ca. 90 Ma, contemporaneously with the underlying continental flood basalts. The presence of inherited older zircons (183-1927 Ma) and the presence of the peraluminous mineral cordierite in the rhyodacites, and the relatively high concentrations of trace elements with crustal affinity (such as Rb, Ba, and the LREE) in the evolved rocks of Mailaka lava succession clearly indicate the involvement of a crustal component in their petrogenesis. However, the Mailaka rhyodacites do not represent anatectic melts. The most reliable petrogenetic process to generate the evolved rocks is prolonged fractional crystallization of tholeiitic basaltic parental magmas, with assimilation of wall rock (AFC). Pb isotopic data help elucidate the nature of the crustal component. The intermediate-evolved rocks show a large range in 206Pb/204Pb, from 16.151 to 19.156, 207Pb/204Pb, from 15.274 to 15.702, and 208Pb/204Pb, from 36.668 to 39.161. The low Pb isotope ratios found in the andesites, found in the middle of the volcanic succession, indicate the involvement of radiogenic Pb-depleted, likely old lower crust. Starting from picritic basalt, the rhyodacites were produced by ~ 85% fractionation coupled with ~20% assimilation of likely upper continental crust. Instead, the andesite with low Pb isotope ratios has assimilated ~ 8% of lower crust with r of 0.1.Therefore, interaction of mantle-derived magmas with different crustal lithologies of the Madagascan crust is expected in this sector of the flood basalt sequence.