Rb-Sr and Sm-Nd systematics of granitic and metamorphic rocks in the Namaqualand Metamorphic Complex, South Africa:Implications for evolution of marginal part of Kaapvaal craton

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We analyzed Sr and Nd isotopic compositions of granitic and metamorphic rocks from the Namaqualand area, South Africa. The Concordia Granite, a member of the Spektakel Suite, near Garies, has a Rb-Sr whole rock isochron age of 1017±30Ma with an initial Sr isotopic ratio of 0.7110±0.0029. This age coincides with the SHRIMP U-Pb zircon age reported from the Okiep Copper District (OCD) within an error. Sm-Nd model ages of this granite (TCHUR : 1030-1195Ma, TDM : 1579-1710Ma) are younger than those in the OCD. The Nababeep Granite Gneiss can be divided into two groups, High and Low NdI Groups, in the Sm-Nd system. The former gives a Rb-Sr whole rock isochron age of 1131±62Ma, and the latter indicates a Rb-Sr isochron age of 1246±62Ma. The gneisses of the High NdI Group are distributed in the southern part of the study area. On the other hand, the Low NdI Group are mainly distributed in the northern area. The boundary between the two parts accords closely with the boundary between lower and upper granulite-facies, which suggests that the Nd isotopic compositions of source rocks of these gneisses were not uniform, and that there is a gap between northern and southern areas. This affects the Nd isotopic composition of the latter granitic rocks such as the Concordia Granite.
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