ANTICORRELATED RB-SR AND K-AR AGE DISCORDANCES, LEUCHTENBERG GRANITE,NE BAVARIA, GERMANY

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 an d K-Ar age determinations of whole-rock and mineral samples from the g ranite and associated intermediate rocks (redwitzites) have revealed t wo divergent age gradients: Rb-Sr whole-rock dates decrease and initia l Sr-87/Sr-86 ratios increase for successively more evolved subsets of the granite. All BG samples (Rb-87/Sr-86=2-16) yield a date of 326+/- 2 Ma with a low initial Sr-87/Sr-86 ratio of 0.70778+/-0.00013 (1 sigm a), while all GMG samples (Rb-87/Sr-86=70 to 1000) yield a younger dat e of 317+/-2 Ma with an enhanced initial Sr-87/Sr-86 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 th e BG (324-320 Ma). The northern lobe of the BG, including the redwitzi tes, shows a well-defined trend of decreasing K-Ar dates from 320 Ma t o 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 dat es and is taken as approaching the crystallization age of the Leuchten berg granite. The K-Ar age progression within the northern lobe of the BG indicates that this part either cooled down over a protracted peri od of some 20 Ma or experienced reheating at ca. 300 Ma. The study hig hlights the potential of combined Rb-Sr and K-Ar dating in deciphering detailed chronology on the scale of a single igneous intrusion.