The magmatic evolution and genesis of the Quaternary basanite-trachyphonolite suite of Itasy (Madagascar) as inferred by geochemistry, Sr-Nd-Pb isotopes and trace element distribution in coexisting phases

Abstract The Itasy is a Pleistocene-Holocene volcanic field in central Madagascar, located to the west of the Ankaratra volcanic complex. It comprises scoria cones and lava domes (>120), with associated pyroclastic fall and mafic lava flows, covering an area of ab. 400 km 2 . The last volcanic episodes probably dated ca. 6000–7100 y BP; warm springs and geysers are active. The juvenile samples comprise a peculiar, almost bimodal, rock suite ranging from potassic leucite-kaersutite-bearing basanites, tephrites and phonotephrites, to benmoreites and titanite-hauyne-bearing trachyphonolites (MgO from 9–10 wt% to 0.1 wt%). These rocks show continuous and overlapping variations in the bulk-rock and phase composition (olivine, clinopyroxene, amphibole, feldspar, leucite, hauyne, nepheline, oxides, apatite, titanite, glass and other accessories). The basanites have homogeneous isotopic composition ( 87 Sr/ 86 Sr = 0.70366–0.70378, 143 Nd/ 144 Nd = 0.51274–0.51277, 206 Pb/ 204 Pb = 18.7–18.9, 207 Pb/ 204 Pb = 15.53–15.56; 208 Pb/ 204 Pb = 38.89–39.01), and a marked enrichment in the most incompatible elements (LILE and HFSE reach 100–215 times primitive mantle). These features are consistent with low degrees of partial melting of a volatile-, LILE- and HFSE-rich, amphibole-bearing peridotitic mantle induced by uplift during an E-W-directed extensional regime, as is found in central Madagascar. The marked changes in the geochemical composition, and small variations of the Sr-Nd-Pb isotopes in the trachyphonolites ( 87 Sr/ 86 Sr = 0.70425–0.70446, 143 Nd/ 144 Nd = 0.51266–0.51269, 206 Pb/ 204 Pb = 18.18–18.39, 207 Pb/ 204 Pb = 15.49–15.51; 208 Pb/ 204 Pb = 38.38–39.57) with respect to basanites and tephrites point to a limited amount of crustal contamination by the relatively low- 206 Pb/ 204 Pb, low- 143 Nd/ 144 Nd, high- 87 Sr/ 86 Sr Precambrian basement rocks (of Middle Archean to Late Proterozoic age), and highlight the geochemical effects of titanite and anorthoclase removal on the trace element fractionation trends, a feature also shown in the trace element composition of the phenocrysts in trachyphonolites.