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

Abstract

The Eckergneiss Complex (EGC) is a geologically unique medium-to high-grade metamorphic unit within the Rhenohercynian domain of the Mid-European Variscides. A previously, poorly defined conventional 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 constrain 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 (PanAfrican) 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), implying 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) Rhenohercynian in the Harz Mountains, including the direct vicinity of the EGC, along with the high-grade metamorphism, 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 Rhenohercynian domain of the Variscides, which is tectonically overridden and suppressed by the Mid-German Crystalline 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.

Late Carboniferous–early Permian events in the Trans-European Suture Zone: Tectonic and acid magmatic evidence from Poland

“…Silurian metasediments in the Harz Mts. contain detrital zircons dated at 560 Ma and 436-410 Ma (Martin-Gombojav, 2003;Geisler et al, 2005). Similar provenance can be attributed to sedimentary rocks that were deposited along the Baltica (Laurussia) passive margin (Krzemiński and Poprawa, 2006;Poprawa, 2006), also in its Polish sector, and supplied by detritus derived from Neoproterozoic and Palaeozoic sources located further north.…”

“…The 2.5 km thick Brocken granite body and the Harzburg mafic-ultramafic intrusion are separated by a flat, few hundred meters thick body of the Ecker gneisses (Gabriel et al, 1997;Franke, 2007). The gneisses came from Silurian/Devonian metasediments which contained detrital zircons of Proterozoic (1.75-1.57 Ga, 1.2-0.9 Ga, 0.56 Ga) and Palaeozoic (436-410 Ma) ages (Martin-Gombojav, 2003;Geisler et al, 2005), and their protolith was metamorphosed around 395-379 Ma (Rb-Sr; Schoell et al, 1973). Metamorphism occurred coevally with an arc magmatism in the Mid-German Crystalline Rise and the two processes were linked with the subduction of the Rhenohercynian crust = East Avalonian terrane beneath the Saxothuringian terrane (Franke, 2006(Franke, , 2007.…”

“…They thus suggested that the basin basement represents part of Baltica. Some support for this option comes from the inherited zircons, with similar age clusters (1.75-1.57 Ga, 1.2-0.9 Ga), that were found in the Ecker gneiss of the Harz Mts., in the Variscan foreland (Martin-Gombojav, 2003;Geisler et al, 2005). In the Late Ordovician, Baltica was docked by East Avalonia which have had the Cadomian basement (Torsvik et al, 1993) that provided Phanerozoic magmas with Neoproterozoic zircon xenocrysts, unknown form Baltica.…”

Death of super-continents and birth of oceans heralded by discrete A-type granite igneous events: the case of the Variscan-Alpine Europe

“…They are essentially made up of metamorphosed Devonian-Carboniferous sedimentary units, with very few occurrences of metabasalts. Detrital zircon grains yield dominant Precambrian and rare Silurian-Early Devonian igneous, not metamorphic ages, implying that the sedimentation never took place before 410 ± 10 Ma (Geisler et al 2005). Pelagic sediments are associated with turbidites, like the non-metamorphosed successions observed in autochthonous units.…”