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

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 titanitehauyne-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, haflyne, nepheline, oxides, apatite, titanite, glass and other accessories). The basanites have homogeneous isotopic composition (Sr-87/Sr-86 = 0.70366-0.70378, Nd-143/(144) Nd = 0.51274-0.51277, Pb-206/Pb-204 = 18.7-18.9, (207)pb/(204)pb = 15.53-15.56; Pb-208/Pb-264 = 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 (Sr-87/Sr-86 = 0.70425-0.70446, Nd-143/Nd-144 = 0.51266-0.51269, Pb-206/Pb-204 = 18.18-1839, Pb-207/Pb-264 = 15.49-15.51; Pb-208/Pb-204 = 3838-39.57) with respect to basanites and tephrites point to a limited amount of crustal contamination by the relatively low-Pb-206/Pb-204, low-Nd-143/Nd-144, high-Sr-8.7/Sr-86 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. (C) 2018 Elsevier B.V. All rights reserved.