Petrology and Geochemistry of Cretaceous Mafic and Silicic Dykes and Spatially Associated Lavas in Central-Eastern Coastal Madagascar

Abstract Late Cretaceous dykes and lavas from central-eastern coastal Madagascar (Vatomandry and Mahatsara districts) span a compositional range from basalt to rhyolite. The dykes strike N–S, parallel to the coast, and intrude the Precambrian schists and gneisses of the area. 40Ar/39Ar age determinations on rhyolite from Sakanila massif (western Vatomandry) gave an age of 86.4 ± 0.3 Ma, which is statistically equivalent to the mean age of eastern rifted-margin rocks of Madagascar. Basaltic dykes are generally tholeiitic varying from olivine-normative to quartz-normative, and characterized by relatively low MgO (4.8–8.0 wt%), Cr (61–341 ppm) and Ni (36–128 ppm) contents. The mantle-normalized diagrams have bell-shaped patterns, with the most Mg-rich dykes having <10 times mantle values. The most Fe-rich ferrobasalts have similar patterns displaced at higher element concentrations, but with evident Sr and Ti troughs. Silicic lavas and dykes are dacites and rhyolites, and are characterized by low CaO, MgO, Fe2O3t, TiO2 and P2O5 and high SiO2, K2O, Ba and Rb. The silicic rocks are enriched in light lanthanides (LREE) and have radiogenic initial 87Sr/86Sr ratios (0.70551–0.70833) and negative initial ɛNdi values (–5.6 to –2.5). The magmatic evolution of the mafic dykes is dominated by fractional crystallization of plagioclase and clinopyroxene with minor olivine. The Vatomandry silicic rocks are the result of prolonged fractional crystallization from basalt parental magmas coupled with small amounts of crustal contamination.