Anorthosites in the Eastern Granulites of Tanzania—New SIMS zircon U–Pb age data, petrography and geochemistry

Several occurrences of anorthosites are known in the Neoproterozoic Mozambique Belt in Tanzania. These are tectonically incorporated into a suite of enderbitic rocks and migmatitic orthogneisses of the Eastern Granulites. Two larger anorthosite bodies and associated rocks from the Pare Mountains and the Uluguru Mountains have been chosen for a comparative study regarding their formation age, age of subsequent metamorphic processes, mineral chemistry and rock chemistry. All the investigated magmatic bodies were overprinted by Neoproterozoic high-pressure granulite facies metamorphism and deformation, documented by similar petrography and mineralogy, metamorphic textures and deformational characteristics. However, mineral chemistry, geochemistry and geochronology reveal major differences between the two occurrences. The Pare anorthosite contains igneous zircons that yield Archean formation ages of ca. 2.64 Ga and a more calcic mineral and rock chemistry that is typical for Archean-type anorthosites. Extremely narrow metamorphic rims around zircons could not be dated but may have grown during the Neoproterozoic metamorphic overprint. In contrast, the Uluguru anorthosite contains igneous zircon cores with U/Pb ages of 880–820 Ma. A broad metamorphic rim was dated at ca. 640–650 Ma. The adjacent migmatitic basement gneisses yield formation ages of ca. 986 Ma followed by Pb-loss. Early Neoproterozoic events can also be inferred from both anorthosite suite and basement samples of the Mahenge Mountains. There U/Pb ages from zircon cores cluster around 730–800 Ma. Well-developed metamorphic rims yield Neoproterozoic ages of 650 Ma. The basement gneisses are related to a destructive plate margin setting. The anorthosites may have formed when parts of Central Madagascar rifted off East Africa at the time considered.