Structural relations and Pb-Pb zircon ages for the Makuti gneisses: evidence for a crustal-scale Pan-African shear zone in the Zambezi Belt, northwest Zimbabwe

The Makuti Group of northwest Zimbabwe is composed of mafic and intermediat e biotite-rich gneisses interlayered with quartzofeldspathic gneisses of gr anitic composition, and minor sedimentary units. The gneisses have experien ced a multi-staged metamorphic history, including an early high temperature -high pressure event and subsequent reworking at upper- to mid-amphibolite- facies conditions. They are positioned along the strongly deformed, souther n margin of the east-west trending Zambezi Belt, and have been correlated w ith supracrustal gneiss units along the northern margin of the Zimbabwe Cra ton. The Makuti Group is characterised by an intensely developed gneissic layeri ng and complex disharmonic folds that resulted from non-coaxial deformation involving repeated stages of transposition. The basal contact of the group coincides with a decrease in strain intensity, but not with a directional change of characteristic structural elements (e.g. lineations, fold axes), nor with a clear change in rock types. Pink quartzofeldspathic gneisses of granitic composition are typical for the Makuti Group, but locally intrude basement gneiss as well. The quartzofeldspathic gneisses occur as porphyrit ic and non-porphyritic varieties that are, invariably, intensely sheared. The age and nature of the basal contact of the Makuti Group and its relatio nship to the quartzofeldspathic gneisses has been investigated. Samples for single zircon Pb-Pb dating were collected from a felsic biotite gneiss jus t below (2704 +/- 0.3 Ma) and above (2510 +/- 0.4 Ma) the lower contact of the Makuti Group at an 'unconformity' 2 km northwest of Vuti. Further sampl es were collected from pink quartzofeldspathic units at the base (737 +/- 0 .9 Ma), central part (764 +/- 0.9 Ma; 797 +/- 0.9 Ma) and top (794 +/- 0.5 Ma; 854 +/- 0.8 Ma) of the Makuti Group. Two samples of Kariba orthogneiss (1920 +/- 0.4 and 1963 +/- 0.4 Ma) underlying the Makuti Group in the north west were also collected. In all samples, long-prismatic, colourless to bro wn, igneous zircon grains were selected. Dates were obtained using a stepwi se single-grain evaporation technique. Although this technique only allows minimum age estimates, the dates are highly reproducible, indicating that t hey approximate emplacement ages. The ages conform with the field observati ons that the basement has been reworked in the Makuti Group and that the qu artzofeldspathic units may have been emplaced as granites. It is proposed that the Makuti Group represents a crustal scale shear zone that partly reworked basement gneisses and acted as a conduit for granite e mplacement. Shearing took place in an extensional setting around 800 Ma ago , and may have resulted in the exhumation of lower crustal rocks. (C) 1999 Elsevier Science Limited. All rights reserved.