INTERMITTENT 1.53-1.13 GA MAGMATISM IN WESTERN BALTICA - AGE CONSTRAINTS AND CORRELATIONS WITHIN A POSTULATED SUPERCONTINENT

Intermittent, widespread and often bimodal magmatism characterized the Mesoproterozoic development in both western Baltica and eastern Laure ntia, Interorogenic intrusions representing early episodes of post-Got hian and pre-Sveconorwegian/Grenvillian magmatism in SW Sweden, have y ielded U-Pb zircon ages at 1502 +/- 2, 1503 +3/-2 and 1457+/-6 Ma. Int egration of these new ages with well-constrained U-Pb data for other 1 .53-1.13 Ga interorogenic intrusions in western Baltica implies that c rustal addition occurred in discrete magmatic episodes. The occurrence of temporally and petrologically similar rocks in the North Atlantic region supports models of a coherent Laurentia-Baltica supercontinent during the Mesoproterozoic, The prolonged interorogenic magmatism in B altica east of the Oslo Rift was typically manifested by mafic dyke sw arms and gabbro-dolerite-granite complexes. This lithological associat ion, absence of attendant regional deformation and lack of evidence of continental-margin processes, collectively imply an intracratonic pos ition for this segment between 1.50 and 1.20 Ga. It further implies th at some segment of Baltica west of the Oslo Rift was attached prior to 1.50 Ga. These observations also imply that large-scale rifting, now dated at 1.46 Ga in western Baltica, did not lead to full-scale crusta l separation and development of a new continental margin. During the s ame period, the evolution in eastern Laurentia included orogenic condi tions at 1.51-1.45 Ga, and continental margin events between 1.45 and 1.19 Ga. This discrepancy in tectonic settings for eastern Laurentia a nd the area in Baltica east of the Oslo Rift indicates fundamental geo dynamic differences along the southern margin of Mesoproterozoic Laure ntia-Baltica or that the lesser-known segment west of the Oslo Rift ma y have been close enough to the proto-margin to experience an evolutio n more akin to that of eastern Laurentia. (C) 1998 Elsevier Science B. V.