U/Pb dating and geochemical characterization of the Brocken and the Ramberg Plutons, Harz Mountain, Germany

Full text: We dated zircons of two granite samples from the border facies of the Brocken and the Ramberg in the Harz Mountains, Germany, using 238U/206Pb-isotopes and 235U/207Pb-isotopes (Laser-ICP-MS). The results show one 238U/206Pb Concordia age of 283.0 ± 2.1 Ma for the Brocken Pluton and two Concordia ages of 283.0 ± 2.8 Ma and 289.3 ± 0.59 Ma, respectively, for the Ramberg Pluton, characterizing the timing of crystallization. The younger age of the Ramberg zircons indicates the occurrence of a second supply of granitoidic melt. The geochemistry of several granite samples, including border facies as well as main facies, document the origin of the intruding melt and the margin in the earth crust. Additionally, thin sections have been analyzed to reveal the degree of alteration and concrescence. We conclude that the granitoidic melt of the Brocken Pluton intruded into the contamination area of the mantle bulge, which formed due to the surface extension during the Rotliegenden. Accordingly, the geochemical composition shows features of primitive mantle rock. On the contrary, the granitoidic melt of the Ramberg Pluton intruded into the upper crust. Following the a-type granite classification by Pearce et al. the Brocken granite samples can be classified as WPG, whereas the analyzed granite samples of the Ramberg are characterized as syn-COLG. Interestingly, the border facies of the Ramberg granite exhibit only alkali feldspars, whereas the Brocken granite also contains plagioclase. Myrmecitic concrescences were observed in the Brocken granite samples corroborating the contamination with penetrating mantle magma. Micrographic texture is typical for intrusions into the upper crust and can be found in Ramberg samples. (author)