Geology, petrology and isotope geochemistry of massif-type anorthosites from southwest Madagascar

Abstract

Four massif-type anorthosite bodies 25–100 km² in area occur within high-pressure granulite facies supracrustal gneisses in southwestern Madagascar. Two of these bodies (Ankafotia and Saririaky) appear to have been pulled apart by 40 km in a ductile shear zone, but structural features such as sub-vertical stretching lineations indicate an origin by intense west-directed flattening and pure shear. Country rocks (Graphite Series) include abundant graphite schist (some with >60% graphite), marble, quartzite, and minor amphibolite and leucogneiss. Comagmatic granitoids (e.g. charnockites) are conspicuously absent. The anorthosite bodies are dominated by coarse grained anorthosites and leuconorites (feldspars typically 3–5 cm, up to 1 m); minor norites and oxide-rich ferrogabbros occur near the margins, but ultramafic rocks are absent. Typical mineralogy of the anorthositic rocks is: plagioclase (An_41–54) + orthopyroxene (En_38–66) ± augite (Mg♯ = 32–68) ± ilmenite ± magnetite ± apatite. High-alumina (to 6.1 wt% Al₂O₃) orthopyroxene megacrysts are widespread; most have exsolutions of calcic plagioclase (An_72–85) but some contain garnet lamellae. Metamorphism has produced abundant recrystallization and sporadic coronitic garnet (Mg ♯=12–36) + clinopyroxene assemblages. Rb-Sr isotopic analyses of whole-rocks and minerals reveal no meaningful age relationships. The age of late Neoproterozoic metamorphism is best constrained at 559 ± 50 Ma by a 6-point Sm-Nd mineral isochron (whole rock, plag, pyx, ilm, apat, gar) from a Saririaky oxide-rich gabbro. The igneous crystallization age of the anorthosites is estimated at 660 ± 60 Ma by a 19-point combined whole-rock and mineral Sm-Nd isochron for samples from both the Ankafotia and Saririaky bodies. Initial isotopic ratios calculated at 0.66 Ga among 13 whole rocks are: _Nd=+2.6 to +5.2 (mean=+3.7) and I_Sr=0.70328–0.70407 (mean=0.70347), indicating derivation of the Malagasy anorthosites from a depleted mantle source, and little, if any, contamination with Archean crustal material. One anorthosite sample with _Nd=−1.4 and I_Sr=0.70344 (calculated at 0.66 Ga) probably reflects the effects of assimilation of Early to Middle Proterozoic crustal basement, but typical surrounding graphite schist (_Nd=+0.3, I_Sr=0.70636, both at 0.66 Ga; T_DM= 1131 Ma) represents only a minor potential contaminant for the anorthosite bodies. T_DM model ages of the Malagasy anorthosites (797–1280 Ma; mean of 14 samples=949 Ma), as those of most other massif-type anorthosites, are older than the true crystallization age, because of crustal contamination effects. Our isotopic data, together with recent U-Pb data from the anorthosites and surrounding country rocks, are consistent with emplacement of the Malagasy anorthosite bodies at or before the start of a protracted, high-grade metamorphic event or series of events between about 630 and 550 Ma. This period coincides with the collision between, and amalgamation of, East and West Gondwana.