Reconnaissance geochronology, tectonothermal evolution, and regional significance of the middle Proterozoic choma-kalomo block, Southern Zambia

New U-Pb zircon, ⁴⁰Ar-³⁹Ar, Rb-Sr and Sm-Nd data provide the first numerical chronology for the tectonothermal evolution of the Choma-Kalomo block, an extensive Middle Proterozoic terrane in southern Zambia to the south of the Pan-African Zambezi belt. The Choma-Kalomo batholith, a large plutonic body in the central part of the block, is shown to be a composite intrusion emplaced from c. 1345 to 1200 Ma, based on U-Pb zircon ages and detailed mapping of intrusive relations. Early phases of the batholith were intruded as syntectonic to late-syntectonic plutons during development of a penetrative S₁ fabric in the metasedimentary country rocks of the Choma-Kalomo block. D₂ folding produced the regional NE-SW structural trends in the block and possibly occurred at c. 1230 Ma, based on an Rb-Sr isochron from an early phase of the batholith containing a penetrative S₂ foliation. An U-Pb zircon age of c. 1200 Ma from post-tectonic granite provides a younger limit to the timing of D₂. ⁴⁰Ar-³⁹Ar mineral ages from the posttectonic granite, and Sm-Nd and ⁴⁰Ar-³⁹Ar mineral ages from amphibolites in country-rock paragneisses, range from c. 1200 to 500 Ma, recording post-D₂ cooling followed by subsequent low-grade Pan-African overprinting along the southern margin of the Zambezi belt. D₁ in the Choma-Kalomo block at c. 1345 Ma is substantially younger than Early Proterozoic orogenesis in adjacent terranes to the east in Zimbabwe, indicating that a major Precambrian discontinuity beneath the mid-Zambezi Valley separates these regions. D₁ in the block is similar in age to plutonic and deformational events dated at c. 1350-1400 Ma in the Irumide belt of central and northern Zambia, suggesting that these two terranes on each side of the younger, cross-cutting Zambezi belt are parts of a single, originally continuous Middle Proterozoic mobile belt. Continuity between the Choma-Kalomo block and broadly coeval terranes to the southwest in Namibia and South Africa also appears likely, and all of these areas are considered to form parts of a Middle Proterozoic orogenic province lying west of the Kaapvaal and Zimbabwe cratons.