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

This paper presents the first Sm-Nd data for coronitic gabbros (''hype rites'') from the high-grade Proterozoic Bamble Sector, Norway. Sm-Nd whole-rock isochron ages of the Vestre Dale Gabbro, 1.11+/-0.14 Ga, th e 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 ga bbros 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 duri ng the Gothian Orogeny (1.75-1.5 Ga). Initial epsilon(Nd) values of ab out +5.5 of Jomasknutene and Flosta Gabbros provide clear evidence of a depleted mantle reservoir beneath this part of the Southwest Scandin avian Domain. This highly positive value differs from the Nd-isotope e volution curve for the Southwest Scandinavian mantle as postulated by Mearns et al. (1986), which assumes that depletion of the mantle benea th this part of Scandinavia started at c. 1.74 Ga. However, the initia l epsilon(Nd) value of the Vestre Dale Gabbro plots close to this curv e, whereas Nd-143/Nd-144 ratios of the mantle sources of the Tromoy an d 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 rat ios in magmas represented by the Vestre Dale Gabbro. Examples of low N d-isotope magmas have also been found in the Telemark Sector, and in s outhern Sweden. The marked depletion in Nb indicates that the metasoma tic fluids were possibly related to Gothian subduction.