New Insights into the Geology of the Namaqua Tectonic Province, South Africa, from Ion Probe Dating of Detrital and Metamorphic Zircon

Abstract

An ion microprobe study in the western part of the Namaqua‐Natal Tectonic Province of South Africa provides new insights into ages of sedimentation, magmatism, and metamorphism in the Bushmanland Terrane. Concordant U‐Pb ages and discordia regressions are reported for three samples and, in combination with secondary electron microscope images, Th/U ratios, and rare earth element analyses of different zircon age domains, are used to distinguish several igneous and metamorphic events. Detrital zircon grains from the tectono‐stratigraphic lowermost part of the Kamiesberg Subgroup (Louisrus Formation) yielded ages between 1200 and 1250 Ma and formed before a first tectono‐thermal event at 1187 Ma. In contrast, detrital zircon grains from a metapelitic granulite from the top of the subgroup (Bitterfontein Formation) were deposited during a previously unrecognized sedimentation stage after 1157 Ma and after this first tectono‐thermal event. Therefore, at least parts of the supracrustal rocks in the Bushmanland Terrane are derived from 1250–1200 Ma crustal material and are not Paleo‐ to Mesoproterozoic, as suggested previously. Two major magmatic episodes have previously been recognized: at ∼1.2 Ga (Little Namaqualand Suite) and at 1.03–1.06 Ga (Spektakel and Koperberg Suites). In addition, we have dated another episode of bimodal felsic (Landplaas) and mafic (Oorkraal) plutonism at $$1109+20/ -19$$ Ma. With respect to the metamorphic history, we directly date a stage of early Namaqua metamorphism at $$1187\pm 15$$ Ma that had previously only been envisaged to accompany ∼1.2 Ga plutonism. We also confirm the age and regional significance of the second low‐pressure, high‐temperature granulite facies metamorphic event at 1.02–1.03 Ga and the effects of the low‐grade Pan‐African overprint at ∼0.5 Ga.