ISOTOPIC CONSTRAINTS ON THE TIMING OF CRUSTAL ACCRETION OF THE BAMBLE SECTOR, NORWAY, AS EVIDENCED BY CORONITIC GABBROS

This paper presents the first Sm-Nd data for coronitic gabbros (''hyperites'') from the high-grade Proterozoic Bamble Sector, Norway. Sm-Nd whole-rock isochron ages of the Vestre Dale Gabbro, 1.11+/-0.14 Ga, the Jomasknutene Gabbro, 1.77+/-0.19 Ga, and the Flosta Gabbro, 1.64+/-0.23 Ga indicate that, contrary to earlier ideas, intrusion of these gabbros is not exclusively bound to the Sveconorwegian orogeny (1.2-0.9 Ga). Some of the gabbros were already emplaced at the onset of or during the Gothian Orogeny (1.75-1.5 Ga). Initial epsilon(Nd) values of about +5.5 of Jomasknutene and Flosta Gabbros provide clear evidence of a depleted mantle reservoir beneath this part of the Southwest Scandinavian Domain. This highly positive value differs from the Nd-isotope evolution curve for the Southwest Scandinavian mantle as postulated by Mearns et al. (1986), which assumes that depletion of the mantle beneath this part of Scandinavia started at c. 1.74 Ga. However, the initial epsilon(Nd) value of the Vestre Dale Gabbro plots close to this curve, whereas Nd-143/Nd-144 ratios of the mantle sources of the Tromoy and Arendal Gabbros evolved with approximately chondritic Sm-147/Nd-144 ratios; their epsilon(Nd) (1.1 Ga) values range from +5.1+/-0.3 to +6.0+/-0.3. Gothian metasomatism of a depleted mantle source by LILE- and LREE-bearing fluids provides a mechanism to obtain low Nd-isotope ratios in magmas represented by the Vestre Dale Gabbro. Examples of low Nd-isotope magmas have also been found in the Telemark Sector, and in southern Sweden. The marked depletion in Nb indicates that the metasomatic fluids were possibly related to Gothian subduction.