A U-PB STUDY OF THE MORKHEIA COMPLEX AND ASSOCIATED GNEISSES, SOUTHERN NORWAY - IMPLICATIONS FOR DISTURBED RB-SR SYSTEMS AND FOR THE TEMPORAL EVOLUTION OF MESOPROTEROZOIC MAGMATISM IN LAURENTIA

Precise U-Pb zircon ages obtained for granitoid units in the Morkheia region, Norway, indicate that at least three periods of felsic magmati sm can be discerned: 1.21, 1.19, and 1.13 Ga. The oldest period is rep resented by the Drihveia gneiss (1205 +/- 9 Ma), followed by the Gjers tad augen gneiss (1187 +/- 2 Ma), and three separate plutons within th e Morkheia Complex, from south to north, that yield similar ages of 11 32 +/- 3, 1134 +/- 2, and 1130 +/- 2 Ma, respectively. The Morkheia Co mplex contacts are discordant to the regional foliation in the Telemar k Sector; therefore, the 1.13 Ga age provides a minimum estimate for t he timing of regional high-grade metamorphism and deformation. However , the complex is transected by the crustal-scale, Sector-bounding Pors grunn-Kristiansand Shear Zone, documenting a major phase of Grenvillia n ductile deformation post-1.13 Ga. Additional evidence for Grenvillia n metamorphism in this region includes a 907 +/- 14 Ma lower intercept age for a sample of Drivheia gneiss, that is controlled by a concorda nt titanite analysis. Previously reported Rb-Sr whole-rock results obt ained for multiple samples collected at the same U-Pb sample localitie s yield geologically meaningless apparent ages. Re-evaluation of these data indicate that at least two main mechanisms are responsible for t he disturbance; the anomalously old ages result from the mixing of two distinct isotopic components and the generation of a fictitious isoch ron at the time of emplacement, whereas the anomalously young ages gen erally reflect postcrystallization Rb addition during a pervasive low grade alteration event ca. 1050 Ma. Granitoid magmatism at the southea stern boundary of the Telemark Sector, Norway, may be part of a more w idespread igneous event that occurred throughout Laurentia. Magmatism in the period between 1.21 and 1.13 Ga can be traced over distances of more than 4000 km in the former supercontinent, and may represent the products of mantle melting during an anomalous thermal regime that ex isted prior to Mesoproterozoic continental rifting.