Abstract
Hyper-solidus fabrics of the early Permian Brocken granite pluton, exposed over an area of c. 160 km² and elongated in NNE–SSW direction, were determined by measuring the anisotropy of magnetic susceptibility (AMS). The Brocken granite pluton consists of five petrographically distinguishable and mappable granitic varieties characterized by a generally paramagnetic behavior with primary biotite, partly altered to chlorite, and secondary tourmaline as the major carrier of the susceptibility. Whereas biotite modal content isolines roughly follow petrographic trends, the tourmaline modal contents do not, hence implying secondary, post-emplacement processes that controlled tourmaline distribution. A few samples with primary and secondary magnetite show ferrimagnetic behavior and are co-axial with the paramagnetic fabrics. The generally low magnetic anisotropies in all granitic varieties are a distinct and challenging feature for determining magmatic flow directions. AMS data indicate the predominance of sub-horizontal fabrics outlining an NNE–SSW trending dome axis interpreted to have resulted from granite sheets stacked syn-magmatically in a high crustal level building up a laccolithic pluton. AMS fabrics together with age data and geophysical indications suggest that the emplacement and distribution of the Brocken granite pluton is structurally controlled by NNE–SSW striking deep fault structures, and only weakly influenced by WNW/NW-trending structures. Late Carboniferous-to-early Permian plutonism in the Harz Mountains evolved in an E–W directed extensional regime, and was accompanied by volcanic activity and development of NW- and NE-trending sedimentary basins.
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Acknowledgements
Constructive reviews of H. de Wall and J. Žák as well as editorial handling of W. Dullo are gratefully acknowledged. We thank I. Klisch for an informative field-trip along the eastern part of the Brocken granite pluton and A. Kontny for fruitful discussions. We also acknowledge T. Güldner, T. Redtmann, and some other former students of the Martin-Luther University Halle-Wittenberg for their help during sampling.
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Zundel, M., Friedel, CH. & Grimmer, J.C. Magnetic fabric constraints for syn-magmatic doming of the laccolithic Brocken granite pluton (Harz Mountains, northern Germany). Int J Earth Sci (Geol Rundsch) 108, 799–816 (2019). https://doi.org/10.1007/s00531-019-01679-w
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DOI: https://doi.org/10.1007/s00531-019-01679-w