TRANSITIONAL (2.1 GA) FE-THOLEIITIC THOLEIITIC MAGMATISM IN THE FENNOSCANDIAN SHIELD SIGNIFYING LITHOSPHERIC THINNING DURING PALAEOPROTEROZOIC EXTENSIONAL TECTONICS

The Palaeoproterozoic sedimentary cover of the Archaean basement compl ex in the Fennoscandian Shield is intruded by several stages of mantle -derived magmas. The 2.1 Ga Fe-tholeiitic magmatism, which is characte rized by hypabyssal intrusions, dykes and volcanic flows intersecting the cover, also forms a widespread dyke swarm within the basement in N orth Karelia, eastern Finland. There are certain differences between t hese age-related sets of dykes, suggesting lithospheric thinning resul ting from extensional tectonics during 2.1 Ga events. REE abundances o f the Fe-tholeiitic dykes within the basement range from 10 to 40 time s chondritic, with gently dipping REE patterns ([La/Yb](N)=1.2 to 3.8) . The dykes intersecting the epicontinental sediments of the cover hav e total REE abundances ranging from 10 to 70 times chondritic, with LR EE-enriched, gently or steeply dipping REE patterns ([La/Yb](N)=2.8 to 4.1), whereas the dykes intersecting the shallow-marine sediments wit hin the cover have total REE abundances from 5 to 55 times chondritic, with almost flat REE patterns ([La/Yb](N)=1.2 to 1.8). On the basis o f the REE patterns and modelling the melting it is suggested that the melt generation occurred in a transitional tectonic setting, where bot h garnet- and spinel-bearing mantle lherzolites were involved due to l ithospheric thinning during Palaeoproterozoic extensional tectonism.