Geochemical and isotopic (Sr, Nd and Pb) evidence on the origin of the anorthosite-bearing anorogenic complexes of the Aïr Province, Niger

Abstract

Abundant leucogabbro and anorthosite associated with other felsic rocks occur in seven high-level ring complexes of Ofoud-type in central Aïr. The leucogabbroic rocks are plagioclase cumulates, whereas troctolites and melatroctolites in the Meugueur-Meugueur ring dyke are olivine cumulates. The plagioclase cumulates are strongly laminated but generally unlayered and have positive europium anomalies. Fine-grained marginal gabbros and monzogabbros without europium anomalies are interpreted as having been close to magmatic liquids, as are the other felsic rocks, which have complementary negative europium anomalies. The fine-grained basic rocks are mildly alkaline with a troctolitic tendency with high Ti, P and incompatible elements and low transition elements. Cumulus mineral compositions ( ∼ An 65 and ∼ Fo 65-55) are moderately differentiated. The felsic syenites and granites are alkaline to rarely peralkaline. Strontium isotopic initial ratios (Sr i) in the leucogabbroic rocks, calculated assuming ages for each intrusion based on whole rock isochrons for felsic rocks, range from 0.7035 to about 0.707, whereas ε Nd( t) varies sympathetically from +2.6 to −4.2. Although there is a spread in values, each intrusion has a distinct isotopic signature, Bous and the marginal facies of Meugueur-Meugueur having the highest ε Nd( t) and lowest Sr i, whereas rocks from the others have lower ε Nd( t) and higher Sr i. Sr i in the felsic rocks obtained from isochrons (ages in the range 400–490 Ma) are considerably larger and vary from 0.7084 for Abontorok and Tagueïto 0.7138 for Bonus; ε Nd( t) varies from −7.5 to −14.4. The initial lead isotopic ratios for the leucogabbroic rocks show a narrow range of values ( 206Pb/ 204Pb: 16.7–17.75; 207Pb/ 204Pb: 15.45–15.53) while the felsic rocks have less ratiogenic ratios, down to 206Pb/ 204Pb: 16.4 and 207Pb/ 204Pb: 15.32. The parental basic magma fractionated olivine and (ortho)pyroxene but not plagioclase, which must have accumulated prior to intrusion at moderately shallow depths. The inferred parental magma probably had characteristics suggesting an origin by minor contamination of a slightly depleted OIB-type source. The other felsic rocks are not comagmatic and could have arisen by crustal contamination of a residual felsic melt. The Ofoud-type complexes are similar to Labrador-type massif anorthosites.