Nature and distribution of deep crustal reservoirs in the southwestern part of the Baltic Shield: Evidence from Nd, Sr and Ph isotope data on late Sveconorwegian granites

Late Sveconorwegian ('postorogenic') granites (c. 1.0-0.93 Ga) make up a vo luminous and widespread suite of intrusions across south Norway. From radio genic isotope data, three groups of late Sveconorwegian granites can be dis tinguished: (1) granite with more than 150 ppm Sr, Rb-87/Sr-86 <5,Sr-87/ Sr -86(0.93Ga)<0.710 and epsilon (Nd)<0; (2) granite with less than 150 ppm Sr , Rb-87/Sr-86 >5, Sr-87/S-86(0.93 Ga)>0.710 and epsilon (Nd)<0; (3) juvenil e granite with Sr-87/S-86(0.93 Ga)<0.705 and epsilon (Nd)>0. Granite pluton s belonging to Group 1 ('normal-Sr concentration granite') occur all over s outh Norway and include the largest batholiths (Ostfold, Fla, Herefoss). Gr anite plutons of Group 2 ('low-Sr concentration granites') are restricted t o north-central Telemark (Rjukan rift), and are associated with c. 1.5 Ga R jukan Group rhyolite. Group 3 is represented by one intrusion only, but sti ll suggests input of mantle-derived magma, or the presence of young, mantle derived rocks in the deep crust in the region at c. 0.93 Ga. The Group 1 g ranites are similar in Sr and Nd characteristics to some of the older (1.05 Ga) Sveconorwegian granitic intrusions ('augen gneisses') in the region in terms of radiogenic isotope systematics, but Pb isotopes suggest that the magmas did not form by simple remelting of augen gneiss. The Nd, Sr, and Pb isotopic systematics of the late Sveconornegian granites indicate mixing b etween a depleted-mantle derived component and two or more major components with an extended crustal history. One of the crustal end members is presen t throughout south Norway and has an isotopic signature similar to older gr anitic rocks of the Trans Scandinavian Igneous Belt (TIB). The other crusta l end member is indistinguishable from Rjukan Group rhyolites and slightly younger intrusions associated with these, and is restricted to areas within the mid-Proterozoic Rjukan rift. The available data suggest that the deep continental crust of south Norway, both east and west of the Permian Oslo R ift, is an integral part of the Baltic Shield, with a common history back t o 1.7-1.9 Ga, that is, to the end of the Svecofennian orogeny and the TIE m agmatism.