New 39Ar-40Ar ages and petrogenesis of the Massif d'Ambre volcano, northhern Madagascar

The Massif d'Ambre is the largest stratovolcano (similar to 2500 km(2)) in the Cenozoic igneous province of northern Madagascar. It is broadly elongated in a N-S direction and is formed by hundreds of lava flows, plugs, spatter cones, tuff rings, pyroclastic flows, and pyroclastic fall deposits. New Ar-40-Ar-39 age determinations for lavas of Massif d'Ambre and Bobaomby Peninsula (the northernmost tip of Madagascar) yield ages of 12.1 +/- 0.2 Ma and 10.56 +/- 0.09 Ma. These ages indicate that at least part of the volcanic activity of the Bobaomby Peninsula occurred later than the beginning of the activity of the Massif d'Ambre. The volcanic products of Massif d'Ambre are mildly to strongly alkaline (with sodic affinity) to tholeiitic with very limited amounts of evolved magmas. The mafic rocks have compositions similar to those of primitive mantle-derived magmas (MgO >10 wt%, Cr and Ni >400 and >200 ppm, respectively). The strongly alkaline suite shows a liquid line of descent from basanite to phonolite, dominated by fractional crystallization of clinopyroxene and olivine. The mafic rocks (basanites, alkali basalts, transitional and tholeiitic basalts) have Zr/Nb (2.4-5.8), Ba/Nb (7-24) and La/Nb (0.7-1.1) ratios typical of incompatible element-rich within-plate basalts. The primitive mantle-normalized incompatible element patterns of the Massif d'Ambre mafic rocks are characterized by peaks at Nb and troughs at K, and are identical in shape and absolute abundances to those of the Nosy Be and Bobaomby (Cap d'Ambre) basanites. The range of (La/Yb)(n) ratios (9-24) indicates that the Massif d'Ambre primitive compositions are the product of variable degrees of partial melting (4%-12%) of a broadly similar and slightly incompatible element-enriched mantle source. Initial Sr-87/Sr-86 and Nd-143/Nd-144 ratios of alkali basalts and basanites vary from 0.70326 to 0.70359 and 0.51279 to 0.51286, respectively. Alkali basalts and basanites have little variation in Pb-206/Pb-204 (19.073-19.369), Pb-207/Pb-204 (15.613-15.616), and Pb-208/Pb-204 (39.046-39.257). This range is well within that of Sr-Nd-Pb isotope values of the basanites of the Nosy Be Archipelago, thus again confirming substantially similar source compositions throughout northern Madagascar.