U-PB ZIRCON AGES FROM THE HOOK GRANITE MASSIF AND MWEMBESHI DISLOCATION - CONSTRAINTS ON PAN-AFRICAN DEFORMATION, PLUTONISM, AND TRANSCURRENT SHEARING IN CENTRAL ZAMBIA

The Hook granite massif in the inner part of the Lufilian are in central Zambia generally has been interpreted as granitized Archean or Paleoproterozoic sialic basement partly remobilized during Pan-African orogenesis. Reconnaissance field studies and U-Pb zircon geochronology reveal no evidence for exposed basement within the massif, which instead is shown to be a large, syn- to post-tectonic, composite batholith intrusive into upper Katangan (Kundelungu) strata. Syntectonic parts of the massif contain a single, moderate- to high-temperature, solid-state foliation that is continuous with the regionally developed S-1 fabric in the adjacent lower-grade Kundelungu metasedimentary rocks. Two separate phases of syntectonic granite yield U-Pb zircon upper intercept ages of 559+/-18 and 566+/-5 Ma, Both samples show normal, linear discordance patterns, and zircon residues from acid leaching plot nearer concordia. An undeformed rhyolite dike intruding a Katangan pendant in the central part of the massif shows more complex zircon isotopic systematics, but a nearly concordant fraction has an age of 538+/-1.5 Ma. Post-tectonic granite yields an upper intercept age of 533+/-3 Ma. These ages constrain Kundelungu deposition in central Zambia as pre-570 Ma and show that regional deformation and voluminous syntectonic granite plutonism occurred in the inner part of the Lufilian are at 570-560 Ma, with the major tectonic activity ending by 540-530 Ma. The Mwembeshi dislocation, a regionally significant Pan-African transcurrent shear zone, runs along the southern margin of the Hook massif. Syntectonic rhyolite intruded in the dislocation yields an upper intercept age of 551+/-19 Ma, showing that transcurrent shearing occurred in the same time frame as batholith emplacement, probably within an overall transpressive regime. Deformation and batholith emplacement in the inner part of the Lufilian are were synchronous with a major pulse of syn- to post-tectonic granite plutonism in the Damara belt of Namibia, supporting a direct link between these regions during Pan-African orogenesis. In contrast, tectonothermal activity in central Zambia is unrelated to main-phase orogenesis in the Zambezi belt to the east, dated at similar to 820 Ma. Models that propose a direct kinematic link between deformation in the Lufilian are, shearing along the Mwembeshi dislocation, and main-phase orogenesis in the Zambezi belt are not supported by the present age data.