Rb-Sr systematics on paragneiss series from the Bavarian Moldanubicum,Germany

The Nicolaysen-diagram is a means to present mineral or whole rock isochrons. The age values derived from mineral isochrons are normally interpreted as cooling ages, those given by whole rock isochrons as the age of an intrusion or a metamorphic event. Mineral isochrons reflect samples in the order of few mm to cm, whole rock isochrons those of a few decimeter to meter.The investigation presented here deals with samples representing regions of about 100 m to some kilometers in diameter. These regions we are going to name areals.Five areals of the Moldanubicum of Eastern Bavaria (West Germany) consisting of similar paragneisses are yielding mineral ages of about 315 Ma and whole rock ages of about 450 Ma. We calculated mean ⁸⁷Sr/ ⁸⁶Sr- and ⁸⁷Rb/⁸⁶Sr-values of these areals and displayed them in a Nicolaysen-diagram. The areal values define a straight line, yielding an age of 544±29 Ma with a ⁸⁷Sr/⁸⁶Sr-intercept at 0.7048±0.0014.Discussion arises whether or not this line can be interpreted as an isochron. We favour the interpretation of it as an isochron reflecting the possible age of sedimentation or of a metamorphic event which the paragneiss series has undergone. At present it can not be unequivocally decided which of the two possibilities will prove right. The areal isochron, however, appears to indicate a petrogenetic event which is older than the last Sr-isotope equilibration in the whole rocks within an individual areal of this Moldanuvian polymetamorphic region.