Implications of the distribution, age and origins of the granites of the Mesoproterozoic Spektakel Suite for the timing of the Namaqua Orogeny in the Bushmanland Subprovince of the Namaqua-Natal Metamorphic Province, South Africa

摘要

The Spektakel Suite comprises voluminous late- to post-tectonic granitoids that intruded the pre-tectonic gneisses of the Bushmanland Subprovince (BSP) of the Mesoproterozoic Namaqua-Natal Metamorphic Province (NNMP). Based on new U-Pb zircon geochronology and geochemical data, we redefine the suite as consisting of 22 plutons of porphyritic monzogranite, granodiorite and charnockite that intruded the granulitic core of the BSP between 1097 and 1033 Ma. The new data show that some granites previously incorporated into the Spektakel Suite should be excluded: a) garnetiferous leucogranites (Concordia, Hangsfontein and Ibequas Granites) are excluded on petrological grounds and genetic considerations and b) the porphyritic Nuwefontein Granite (similar to 783Ma) is younger and is now placed within the Richtersveld Suite. The Spektakel Suite has geochemical and isotopic compositions suggesting that it was derived from recycled Palaeoproterozoic crust with Mesoproterozoic mantle input. The main Namaqua Orogeny (D-2) in the BSP comprises at least two phases of compressional tectonics at similar to 1200 Ma and similar to 1110 Ma separated by a period of extension. The new ages for the Spektakel Suite tightly constrain the end of D-2 to between 1109 Ma, the age of the youngest pre-tectonic gneiss and 1097 Ma, the age of the oldest Spektakel Suite granite. Peak low-P, high-T metamorphism (similar to 1050-1020 Ma) occurred well after D-2 and its close spatial and temporal relationship to the Spektakel Suite suggests this prolonged period of granite magmatism was responsible for the transfer of the heat into the BSP. Several other late- to post-tectonic megacrystic granitic suites intruded into other tectonic domains across the NNMP coevally with the Spektakel Suite, indicating widespread granite magmatism at this time, probably due to mafic under- and intra-plating in a back-arc setting as previously suggested.