An Anorthosite Suite in a Ring-Complex: Crystallization and Emplacement of an Anorogenic Type from Abontorok, A r, Niger

Abstract

Laminated anorthosite grading outwards into leucogabbro, gabbro, and monzogabbro occurs in a 2.6-km-diameter funnel-shaped intrusion, cut by a quartz alkali syenite plug and concentric syenite and granite ring-dykes. The anorthosite-gabbro series is laminated but not modally or otherwise texturally layered. The lamination, defined by large tabular plagioclase crystals, forms a set of inwarddipping cones, the dips of which decrease from 60–45° in the central anorthosite to < 25° in the outer gabbros. Rocks close to the outer contact are medium-grained isotropic gabbros. Plagioclase, forming >80% of the series, generally has homogeneous labradorite cores (An62–58 in the whole series) and thin strongly zoned rims, which follow progressively longer solidus paths from the anorthosites to the gabbros. All rocks contain a late-magmatic alkali feldspar. Plagioclase is the main or only cumulus phase, the anorthosites being ad- to mesocumulates and the gabbros orthocumulates. Olivine (FO49–41) is more abundant than clinopyroxene in most of the series. Depending on quartz content, the syenites and granites are hypersolvus or subsolvus and the depth of crystallization was calculated to be 5 ± 2 km. A Rb/Sr isochron for the syenites and granites gave an age of 399 ± 10 Ma with an initial strontium isotopic ratio of 0.7084 ± 0.0005. Ten samples from the anorthosite-gabbro scries have an average calculated initial ratio of 0.70582 ± 0-00004 at − 400 Ma, showing that the two series are not comagmatic. The anorthosite-gabbro series has parallel REE trends (LaN/YbN ˜ 7–10) with decreasing positive Eu anomalies and increasing total REE contents from anorthosite to gabbro; two monzogabbros have almost no Eu anomaly. The liquid calculated to be in equilibrium with the lowest anorthosite has almost no Eu anomaly and its normalized REE pattern lies just above those for the monzogabbros. The syenites and granites have complementary REE patterns with negative Eu anomalies. The inferred parental magma was alkalic and leucotroctolitic with high TiO2 P2O5, Sr and K/Rb and with low MgO, very similar to parental magmas in the Gardar province, South Greenland. It was probably produced at depth by settling of olivine and clinopyroxene but not of plagioclase, which accumulated by flotation. It is suggested that plagioclase crystals from this lower chamber were progressively entrained (from 0% in the gabbros to 30–40% in the anorthosites), giving rise to the flow lamination in the upper chamber. The magma in the lower chamber may have been layered, because the plagioclase cores in the anorthosite are considerably richer in Or than those in the leucogabbros or gabbros. Overall convection did not occur in the upper chamber, whereas compositional convection occurred in the more slowly cooled central anorthositic adcumulates.