Kibaran events in the southernmost Mozambique belt

Abstract

An area of ∼ 20 000 km 2 in northern Mozambique has been mapped at 1 : 50 000 scale. The dominant, ∼ 1000 Ma old Lurian event (Kibaran orogeny) produced the “Namama belt” — a SE vergent satellite of the Lurio belt described further to the north by Bureau de Recherches Géologiques et Minières (BRGM) in the 1970's. To the west of Namama belt, the Lurian event resulted in an extensive nappe of granulitic rocks. This is perhaps rooted some 200 km to the WNW near the Malawi border. This crustal doubling has led to local increases in metamorphic grade beneath the nappe. The Lurian event involved both pre-Kibaran basement and “Lurian” cover. Its development was preceded by subsidence and crustal thinning that influenced the deposition of the cover. At the NW margin of the Namama belt is a zone of intense deformation and thrusting, where siliceous metasediments are associated with basic and some ultrabasic rocks of ophiolitic aspect. The development of the Kibaran event was accompanied by the emplacement of batholitic granites. The whole-rock Rb/Sr isochrons give Kibaran isotopic ages and low initial ratios even for the supposedly oldest parts of the basement, suggesting pervasive homogenisation. The subsequent Pan-African event (∼ 500 Ma) resulted in the emplacement of circumscribed, crust-derived granites, and radiometric rejuvenation of the minerals, but no Pan-African orogenic process is recognised. The c. 1000 Ma old events of Mozambique fit well in the frame of a major, Lurian-Irumide (Kibaran) orogeny involving considerable crustal shortening.