Geology and major element geochemistry of anorthositic rocks associated with paleozoic hypabyssal ring complexes, air massif, Niger, West Africa

Abstract

Four Paleozoic subvolcanic ring complexes located in the Air Massif, Republic of Niger, contain a bimodal basic/acidic series of rocks. The alkaline acidic series ranges in composition from syenite to granite while the basic series contains predominately anorthositic material with compositions ranging from iron-titanium oxide ores to anorthosite. Within each complex, field relations suggest that the basic series was intruded prior to the acidic series. The major minerals of the basic series are: (1) plagioclase, An 70-An 40; (2) olivine, Fo 65-FO 45; (3) clinopyroxene, Wo 45En 38Fs 17 (avg.); and (4) titaniferous magnetite and ilmenite. Major element geochemistry suggests that the acidic and basic series were not comagmatic, but rather were derived from independent sources. Rare rocks of intermediate composition are believed to be hybrid in nature resulting from the mixing of acidic liquids with previously intruded basic material. The recombined bulk composition of the basic series indicates parental magmas of highly aluminous basaltic compositions (anorthositic gabbro) which appear to have been transitional to alkaline in nature. In the two smaller complexes the basic series appears to have formed by flow differentiation, while in the two large complexes there is evidence that gravity differentiation formed the various rock types of the basic series.