ORIGINAL PAPER Thorsten Geisler Æ Roland Vinx Nergui Martin-Gombojav Æ Robert T. Pidgeon Ion microprobe (SHRIMP) dating of detrital zircon grains from quartzites of the Eckergneiss Complex, Harz Mountains (Germany): implications for the provenance and the geological history Received: 18 May 2004 / Accepted: 5 November 2004 / Published online: 13 April 2005 Ó Springer-Verlag 2005 Abstract The Eckergneiss Complex (EGC) is a geolog- ically unique medium- to high-grade metamorphic unit within the Rhenohercynian domain of the Mid-Euro- pean Variscides. A previously, poorly defined conven- tional lower U–Pb intercept age of about 560 Ma from detrital zircons of metasedimentary rocks has led to speculations about an East Avalonian affinity of the EGC. In order to unravel the provenance and to con- strain the age of the sediment protolith, we carried out sensitive high-resolution ion microprobe U–Pb analyses on detrital zircons from five different EGC quartzite occurrences. The obtained age spectrum indicates a SW Baltica provenance of the detritus. Sveconorwegian ages between 0.9–1.2 Ga are particularly well represented by analyses from metamorphic recrystallization/alteration zones penetrating into igneous zircon. Cadomian (Pan- African) ages, which might reflect a metamorphic event, could not be substantiated. Instead, zircons of igneous origin yielded concordant Lower Devonian and Silurian ages of 410±10, 419±10, and 436±6 Ma (1r), imply- ing that sedimentation of the EG protolith must have taken place after 410±10 Ma. The lower age limit of the EGC metamorphism is constrained by 295 Ma intrusion ages of the adjacent, nonmetamorphosed Harzburg Gabbronorite and Brocken Granite. Sedimentation and metamorphism must thus have taken place between about 410 Ma and 295 Ma. Given that this time span coincides with most of the sedimentation within the virtually nonmetamorphosed (lowest grade) Rhenoh- ercynian in the Harz Mountains, including the direct vicinity of the EGC, along with the high-grade meta- morphism, the EGC can hardly be seen as uplifted local basement. A possible candidate for the root region is an easterly, concealed marginal segment of the Rhenoh- ercynian domain of the Variscides, which is tectonically overridden and suppressed by the Mid-German Crys- talline Rise during continent collision. However, based on the concept of strike-slip movement of Variscan terranes with different P–T–t histories as a result of postaccretion intraplate deformation, the EGC could also represent a fault-bounded complex with an origin located far east or south east of the present location. Keywords Eckergneiss Æ Variscides Æ Harz Æ SHRIMP Æ Zircon Æ Provenance Æ Baltica Introduction The Eckergneiss Complex (EGC) in the Harz Mountains (Germany) is a high-grade polymetamorphic crustal segment within the Rhenohercynian domain of the Mid- European Variscides (Fig. 1). The high-grade meta- morphism of the EGC is a matchless feature within the low-grade Rhenohercynian domain, raising questions about its geological history, which, in turn is important to our understanding of the geodynamic evolution of the Variscan orogenic belt. A conventional lower intercept U–Pb age of about 560 Ma from detrital zircon grains, interpreted to reflect the timing of the main metamor- phism (Bauman et al. 1991), suggests that the EGC represents the oldest geological unit of the Harz Mountains. This age has led to controversial specula- tions about the tectonic setting of the EGC. Bankwitz (1995) suggested that the EGC represents a Cadomian– Caledonian tectonic flake formed during the collision of T. Geisler (&) Mineralogisches Institut, Universita¨t Mu¨nster, Corrensstr. 24, 48149 Mu¨nster, Germany E-mail: tgeisler@nwz.uni-muenster.de R. Vinx Æ N. Martin-Gombojav Mineralogisch-Petrographisches Institut der Universita¨t Hamburg, Grindelallee 48, 20146 Hamburg, Germany R. T. Pidgeon School of Applied Geology, Curtin University of Technology, GPO Box U 1987, Perth, 6845, WA, Australia Int J Earth Sci (Geol Rundsch) (2005) 94: 369–384 DOI 10.1007/s00531-004-0460-1