U-Pb and Ar-40/Ar-39 geochronological constraints on the tectonic evolution of the easternmost part of the Zambezi orogenic belt, northeast Zimbabwe

The Zambezi belt is a key segment of the network of Neoproterozoic/lower Pa leozoic orogenic belts in southern Africa that formed during amalgamation o f central Gondwana. We present new geochronological data from the easternmo st Zambezi belt in northeast Zimbabwe, near the junction between that belt and the Mozambique belt. Allochthonous high-pressure granulite-facies migma titic and mylonitic rocks at the highest exposed structural levels in this part of the Zambezi belt are tectonically juxtaposed against amphibolite-fa cies supracrustal rocks, which are intruded by a regionally extensive, shee t-like composite granitoid batholith having peralkaline affinities. This ba tholith separates the Zambezi supracrustal rocks from a zone of Archean bas ement that wraps the northeast margin of the Zimbabwe craton and shows vari able degrees of structural and thermal overprinting. U-Pb zircon and titani te and Ar-40/Ar-39 hornblende geochronological data from the allochthonous granulites are interpreted to record high-grade migmatization at ca 870 to 850 Ma, with pervasive amphibolite-facies retrogression at ca 535 Ma during tectonic emplacement into mid-crustal levels. U-Pb zircon and titanite dat a from the peralkaline batholith indicate that it crystallized at 805.2+/-1 1.1 Ma; partial thermal resetting of the U-Pb system occurred in the same t ime frame as retrogression of the allochthonous granulites. Hornblende from two samples in thermally overprinted Archean basement farther south yields Ar-40/Ar-39 plateau ages of 507.9+/-2.5 and 491.3+/-2.1 Ma. Together these new data indicate that tectonic elements in the easternmost Zambezi belt h ave a. protracted history, involving early, lower crustal, granulite-facies metamorphism (ca 870-850 Ma) followed by intrusion of a peralkaline granit ic batholith (ca 800 Ma) into supracrustal rocks within the belt. The major tectonostratigraphic units in the easternmost Zambezi belt were juxtaposed under amphibolite-facies conditions at ca 535 Ma, followed by relatively r apid cooling through Ar closure temperatures in hornblende. The 535 Ma even t reflects deformation in the Zambezi belt in the same time frame as widesp read orogenesis that is recorded in other Pan-African belts in southern Afr ica and is related to final stages in Gondwana assembly. (C) 1999 Elsevier Science B.V. All rights reserved.