The Kisii Group of western Kenya: an end-Archaean (2.53 Ga) late orogenic volcano sedimentary sequence

The Kisii Group (western Kenya) is a volcano sedimentary outlier unconforma bly overlying folded and granitised Victorian sequences of the Tanzanian Cr aton (2.8-2.58 Ga). It shows the following succession: (1) lower detrital b eds of siltite and local conglomerate; (2) the Kisii basalt, comprising sub aquatic basalt and andesitic basalt; (3) the Arenitic Formation of mature s hallow water arenitic sediments with minor cinerite and chert; (4) the Ikon ge ignimbrite of rhyolitic ignimbrite and minor andesite; and (5) the upper volcaniclastic beds of lapilli tuff and lahar conglomerate. The Kisii basa lt is geochemically similar to Phanerozoic continental-are tholeiitic suite s derived from a contaminated mantle. The Ikonge ignimbrite, however, is si milar to recent felsic sequences of extensional post-orogenic or anorogenic settings, and could well have derived from underplated mafic sources geoch emically close to the Kisii basalt. Single zircon evaporation Pb/Pb geochro nology of the Ikonge ignimbrite yields a 2531 +/- 2 Ma emplacement age with , in places, indications of 2.6-2.7 Ga contamination. These age data sugges t that the Kisii Group reflects the ultimate stage of the Victorian orogeny (2.75-2.58 Ga) of the northern part of the Tanzanian Craton. No obvious eq uivalent succession to the Kisii Group, either undeformed or caught up in t he East African Proterozoic belts, is known in eastern Africa, although pos sible relationships with the Buganda Group in Uganda are suggested by the l atter's age bracket (2.57-1.87 Ga) and sedimentary types. (C) 2000 Elsevier Science Limited. All rights reserved.