Event geochronology of the Pan-African Kaoko Belt, Namibia

Abstract

Zircon and monazite U-Pb dates, garnet Sm-Nd dates and hornblende 40Ar/39Ar data from the transpressional Kaoko Belt of the late Neoproterozoic Pan-African Orogenic system confirm three distinct tectono-metamorphic cycles: M1 (655-645 Ma), M2 (580-550 Ma) and M3 (535-505 Ma). The high-grade M1 metamorphic cycle and associated intrusive complexes are evident only within the westernmost Coastal Terrane. The isotopic data record a progressive and protracted history for the M2 metamorphic cycle that is initiated by collision and terrane docking, but with three distinct tectono-thermal periods including (1) peak metamorphic parageneses and voluminous granitoid emplacement at 580-570 Ma, (2) overlapping whole-scale transpressional orogenesis and reworking dominated by crustal-scale shear zones, throughout the period 575-550 Ma, and (3) cessation of transpressional strain before 530-508 Ma, the age of late-kinematic pegmatite dykes that cross cut the major shear zones. M1 metamorphism of the exotic Coastal Terrane at 650 Ma must have occurred out-board from the Kaoko Belt passive margin, where M1 intrusives and metamorphic mineral parageneses have not been recognised. Accretion of the Coastal Terrane to the Kaoko Belt proper must have occurred between 645 Ma (M1) and 580 Ma, prior to the peak of M2 metamorphism accompanying transpressional orogenesis. Low-grade buckling of the Kaoko Belt, minor post-kinematic granite and pegmatite intrusions and post-metamorphic cooling occurred between 535 and 505 Ma during the M3 metamorphic cycle accompanying NNE-SSW directed, high-angle convergence between the Congo and Kalahari Cratons.