Provenance, deposition and age of gneiss units from the KTB drill hole (Germany): evidence from SHRIMP and conventional U–Pb zircon age determinations

Abstract

In order to investigate the provenance, deposition age and metamorphism of metasedimentary gneisses from the pilot and the main holes of the German Continental Deep Drilling Program (KTB), zircons from volcano-metasedimentary (v-unit) and monotonous greywacke (g-unit) gneisses were dated by ion microprobe (SHRIMP) and conventional U-Pb zircon methods. The zircons were found to consist of anhedral detrital or subhedral cores surrounded by one or two metamorphic rims. Analysis of growth zones in 37 zircons indicated ages for the detrital cores between 2.25 and 2.05 Ga in a v-unit gneiss (sample K52), and 1.8-2.35 Ga in two g-unit gneisses (K747 and K798). Neo-Proterozoic (650-545 Ma) subhedral igneous cores were identified in all investigated KTB gneiss samples, and an additional two zircon growth episodes in the source area of the sediments at 712$15 and 525$11 Ma were also recognised. Zircon overgrowths dated at 487$13 Ma are interpreted to have formed during an early metamorphic episode following incorporation of the zircons into their host metasediments. A second metamorphic overprint indicated by a date of 381#22/-11 Ma was determined for zircon overgrowth in the g-unit gneisses. The dates determined for zircon growth phases suggest that the KTB metasediments were derived from northern Gondwana. Hypothetical provenance areas and a new model of terrane positions that is consistent with the geochronological, geodynamic and palaeo-geographic preconditions at F. So¨llner ( ) · H. Miller

SHRIMP dating of zircons from high-grade metasediments of the Schwarzwald/SW-Germany and implications for the evolution of the Moldanubian basement

“…This fits well to the concept of a Neoproterozoic active margin setting along the entire length of the future Palaeozoic microcontinents at the northern margin of Gondwana (e.g. Tait et al 1997Tait et al , 2000So¨llner et al 1997;Matte 2001;von Raumer et al 2002;Stampfli et al 2002 and references therein). In Fig.…”

“…typical of Morocco/ Mauretanian crustal areas (e.g. So¨llner et al 1997). The Neoproterozoic detrital age distributions of the investigated samples show significant differences, in particular due to the presence of ca.…”

“…In the Variscan fold belt detritus ages have been successfully used to detect ancient terranes and to reconstruct the geodynamic and geotectonic evolution of Europe during the Palaeozoic (e.g. Franke 2000;Hegner and Kro¨ner 2000;Linnemann et al 2000;So¨llner et al 1997;Tichomirova et al 2001;von Raumer et al 2002von Raumer et al , 2003. The contribution of detrital zircon dating is most important in the understanding of the internal parts of orogens where complex internal nappe structures and high-grade metamorphism often impede the application of palaeomagnetic, stratigraphic and palaeontological methods.…”

U-Pb SHRIMP and Nd isotopic data from the western Bohemian Massif (Bayerischer Wald, Germany): Implications for Upper Vendian and Lower Ordovician magmatism

“…A coeval, convergent setting (island arc -continent collision) is also proposed for the Habach terrane (protoAlps) at~480 Ma . In contrast, extensional settings, coeval to rift-related magmatism in Avalonia at~485 Ma (van Staal et al, 1998), are known from most other Variscan massifs in central Europe: in the Saxothuringian at 510-500 Ma and at~480 Ma (Kemnitz et al, 2002;Kröner and Willner, 1998;Linnemann et al, 2000;Mingram, 1998;Schätz et al, 2002), in the French Massif Central at~480 Ma (Pin and Lancelot, 1982;Pin and Marini, 1993), in the ZEV at~495-480 Ma (Söllner et al, 1997;von Quadt, 1997); back arc or continental rifting is assumed at 490-470 Ma in the Schwarzwald ; intracontinental rifting is suggested in the Northern Greywacke Zone (Alps) at 475 Ma Schauder, 2002). Successive rifting and drifting of Gondwana-derived terranes started in the Lower Ordovician (Tait et al, 1997) and probably occurred diachronously -starting in the "western" terranes (West-and East-Avalonia) -due to ridge-trench-collision in the West-Avalonia area (Fernµndez-Suµrez et al, 2002;Nance et al, 2002).…”

“…Nd model ages T DM and two-stage Nd model ages T DM(2) for samples with 147 Sm/ 144 Nd >0.14 are calculated according to Liew and Hofmann (1988) 2.0, 2.4, 2.6-2.7 and 3.8 Ga (Gebauer et al, 1989;Grauert et al, 1973;Kröner et al, 1988;Propach et al, 2000;Teufel, 1988); they are also consistent with direct evidence of Palaeoproterozoic basement in the Bohemian Massif provided by 2104-2048 Ma old orthogneisses (Wendt et al, 1993). Possible source rocks for the inherited Palaeoproterozoic-Archaean zircons are located at the northern Gondwana margin (compilations in Murphy, 1996 andSöllner et al,1997). Lack of Mesoproterozoic zircon ages suggests affinities to the West African craton (Fernµndez-Suµrez et al, 2002;Nance and Murphy, 1996).…”