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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


L. Melluso1, R.D. Tucker2, C. Cucciniello1, A.P. le Roex3, V. Morra1, A. Zanetti4, R.L. Rakotoson5

1DISTAR, Università di Napoli Federico II, Napoli, Italy
2BRGM, Orleans, France
3University of Cape Town, Rondebosch, Republic of South Africa
4IGG-CNR, Pavia, Italy
5Analamaitsy, Antananarivo, Madagascar
Lithos, 310-311, 50-64
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 km2. 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-haüyne-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, haüyne, nepheline, oxides, apatite, titanite, glass and other accessories). The basanites have homogeneous isotopic composition (87Sr/86Sr=0.70366-0.70378, 143Nd/144Nd=0.51274-0.51277, 206Pb/204Pb=18.7-18.9, 207Pb/204Pb=15.53-15.56; 208Pb/204Pb=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 (87Sr/86Sr=0.70425-0.70446, 143Nd/144Nd=0.51266-0.51269, 206Pb/204Pb=18.18-18.39, 207Pb/204Pb=15.49-15.51; 208Pb/204Pb=38.38-39.57) with respect to basanites and tephrites point to a limited amount of crustal contamination by the relatively low-206Pb/204Pb, low-143Nd/144Nd, high-87Sr/86Sr 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.