ZIRCON U-PB EMPLACEMENT AND ND-HF CRUSTAL RESIDENCE AGES OF THE STRAUSSBURG GRANITE AND FRIERSDALE CHARNOCKITE IN THE NAMAQUA-NATAL PROVINCE, SOUTH AFRICA

Abstract

Granitoids of the Kaaien, Areachap and Kakamas Terranes of the Namaqua-Natal Province have now been dated with sufficient precision that they can be divided into three clusters relative to the ~1220 Ma collision events of the Namaqua Orogeny. The 1371 to 1220 Ma pretectonic cluster correspond mainly to the arc-related Areachap and Korannaland Groups. The proposed Augrabies Suite comprises 1203 to 1146 Ma syntectonic granitoids and the lower Koras Group extrusives in the Kaaien Terrane. ‘Stitching granites’ like the Riemvasmaak Granite intrude both the Kakamas and Areachap Terranes, showing that they were adjacent during Augrabies Suite emplacement. The post-tectonic 1113 to 1078 Ma Keimoes Suite, for which a revised definition is proposed, includes the Straussburg Granite and Friersdale Charnockite, which are the main topic of this work as well as the upper Koras bimodal volcanic rocks and intrusions. We used U-Pb ion probe zircon dating, laser ablation ICPMS Hf isotope analysis and electron microanalysis to investigate the geochronology, crustal residence ages and emplacement depths of these two granites. The Straussburg Granite in the Areachap Terrane was previously regarded as a syn- to late-tectonic granite because of its foliated appearance. It is now dated at 1090 ± 9 Ma (2σ error) by a U-Pb zircon discordia upper intercept, with a lower intercept of 322 ± 170 Ma reflecting lead loss during the overlying Karoo sediment deposition and diagenesis. Its foliation is interpreted as having formed during magmatic emplacement. It is thus post-tectonic, apart from discrete shear zones which reflect uplift and transcurrent movements. It is coeval within error with the undeformed upper Koras 1104 to 1092 Ma bimodal volcanic rocks exposed in the adjoining Kaaien Terrane and correlated dykes and intrusions. Despite the overlap in age, the coarse-grained Straussburg Granite is clearly older, because it must have cooled and been uplifted from a depth of 22 km and been juxtaposed with the Kaaien Terrane before it was fractured and intruded by numerous fine-grained Koras quartz porphyry dykes. The Friersdale Charnockite is an undeformed mafic-felsic hybrid rock which occurs in both the Kakamas and Areachap Terranes, thus it was emplaced after their juxtaposition following the ~1220 Ma collision events. Two samples of the charnockite give indistinguishable weighted mean 207 Pb- 206 Pb ages for concordant magmatic zircon. Combining the ages gives 1078 ± 10 Ma for the magmatic crystallisation. The Friersdale Charnockite is thus younger than the 1090 ± 9 Ma Straussburg Granite at the 66% (or 1σ) confidence level although the ages overlap at the 95% (or 2σ) confidence level. A few of the cores are older xenocrysts, two being much older and some around 1100 Ma. Lu-Hf zircon model crustal residence ages for magmatic zircon give a range of 2181 to 1789 Ma for the Friersdale Charnockite and 2370 to 2003 Ma for the Straussburg Granite. Together with petrographic and field data, this suggests that both magmas are hybrids which were assimilating material of different ages at the same time as zircon crystals were forming. The Friersdale Charnockite also contains xenocrysts aged 1725 ± 20 and 1475 ± 15 Ma, confirming the presence of Palaeoproterozoic crust in the lower crust. Published Sm-Nd T DM crustal residence ages support this interpretation. A common feature of most of the 1100 to 1080 Ma felsic rocks is their close association with mafic magma, seen as mafic xenoliths, coeval basalt lavas or mineralogical evidence for hybridization. It is suggested that mantle melts were generated due to late-Namaqua transcurrent movements causing pressure release in mantle already heated by the 1112 to 1106 Ma subcontinental-scale Umkondo plume. Mafic intrusions triggered melting of a crust comprising layers with different crustal residence ages. This had been previously assembled during the collisions as the Mesoproterozoic Kakamas and Areachap Terranes overrode the Archaean to Palaeoproterozoic Kaapvaal and Rehoboth Terranes. The granitic components mixed with each other and with the mafic magma, giving rise to the hybrid granitic magmas.