U-Pb ages and Nd isotope characteristics of the lateorogenic, migmatizing microcline granites in southwestern Finland

U-Pb ages and whole-rock Nd isotope data have been obtained from the Paleoproterozoic lateorogenic migmatizing microcline granites of southwestern Finland. Isotope dilution and ion microprobe U-Pb data on zircons and monazites show that the age spectrum of these granites is at least 1.85-1.82 Ga. Commonly, zircons and monazites record the same ages. The age variation in the Veikkola granite area is of the order of 25 Ma and indicates that this seemingly homogeneous granite consists of two separate intrusions. The zircons of the lateorogenic granites are pervasively altered and conventional U-Pb results are commonly discordant. The ion microprobe studies reveal that the granites contain very few inherited zircons with preserved original U-Pb isotope ratios, with the exception of the Oripaa granite. Initial e Nd values, mostly in the range of -0.5 to -1.0, imply a moderate input of older crustal material into most of the lateorogenic granites. A shift from more juvenile to less radiogenic Nd isotope composition is observed from north to south, and the variation pattern of e Nd values of the lateorogenic granites is thus similar to that of the surrounding synorogenic granitoid rocks.