U-Pb investigations on zircons from pre-Variscan gneisses—I. A study from the Schwarzwald, West Germany

Abstract

Field observations in the Schwarzwald indicate that pre-Variscan rocks were formed by the following events: 1. (1) Intrusion of granitic to granodioritic magma into sediments. 2. (2) A sequence of two anatectic events with an intervening regional metamorphism and development of a gneissic schistosity. The absolute chronology of these older gneisses and migmatites has not yet been satisfactorily resolved. We collected several samples of a coarse-grained, homogeneous orthogneiss of granodioritic composition and one sample of a metasedimentary source rock related to this orthogneiss body in the Einetwald forest north of Steinach (Kinzig valley). The metasedimentary rock corresponds to the biotitepoor paragneisses in the Schwarzwald. The zircons in the latter sample are detrital and only very small specimens (⩽ 45μm) are euhedral. The zircons from the orthogneiss samples are euhedral. Some are zoned and some have dark inclusions. Analyses of the three coarser fractions of the paragneiss form a linear array on a concordia diagram with upper and lower intercepts of about 2000 Myr and 524 ± 20 Myr, respectively. The 2000 Myr value is interpreted as the average age of the sedimentary source rocks for this material. The 524 Myr intercept defines an event responsible for most of the lead-loss in the zircons as well as new zircon growth. Analyses of the zircons from the orthogneiss samples do not show a linear array on the concordia diagram because of inclusions of older inherited zircons. Stepwise leaching of two zircon fractions shows newly grown U-rich zircon rims around old cores. A 207Pb 206Pb - age of 517 ± 15 Myr was found for the first dissolution steps (about 50% of the zircons dissolved). In subsequent steps with progressive dissolution of the residue. 207Pb 206Pb - age increased to 950 Myr. The U-Pb zircon data for the paragneiss and the orthogneiss establish an age of 520 ± 15 Myr for the magma intrusion.