Anticorrelated Rb?Sr and K?Ar age discordances, Leuchtenberg granite, NE Bavaria, Germany

Abstract

The Leuchtenberg granite (Oberpfalz, NE Bavaria) displays a continuous differentiation trend ranging from mildy peraluminous, coarse-grained, porphyritic biotite granites (BG) to strongly peraluminous, medium- to fine-grained, garnet-bearing muscovite granites (GMG). The Rb−Sr and K−Ar age determinations of whole-rock and mineral samples from the granite and associated intermediate rocks (redwitzites) have revealed two divergent age gradients: Rb−Sr wholerock dates decrease and initial 87Sr/86Sr ratios increase for successively more evolved subsets of the granite. All BG samples (87Rb/86Sr=2–16) yield a date of 326±2 Ma with a low initial 87Sr/86Sr ratio of 0.70778±0.00013 (1ϖ), while all GMG samples (87Rb/86Sr=70 to 1000) yield a younger date of 317±2 Ma with an enhanced initial 87Sr/86Sr ratio of 0.7146±0.0039. The K−Ar measurements on biotites and muscovites give closely concordant dates for the GMG (326–323 Ma) and the southern lobe of the BG (324–320 Ma). The northern lobe of the BG, including the redwitzites, shows a well-defined trend of decreasing K−Ar dates from 320 Ma to 300 Ma towards the northwest. Critical consideration of both isotope systems leads to the conclusion that the Rb−Sr system of the GMG was disturbed by a later hydrothermal event. The ca. 326 Ma whole-rock Rb−Sr date for the BG is not in conflict with any of the K−Ar mineral dates and is taken as approaching the crystallization age of the Leuchtenberg granite. The K−Ar age progression within the northern lobe of the BG indicates that this part either cooled down over a protracted period of some 20 Ma or experienced reheating at ca. 300 Ma. The study highlights the potential of combined Rb−Sr and K−Ar dating in deciphering detailed chronology on the scale of a single igneous intrusion.