The synorogenic Krutfjellet gabbro within the upper Koli Nappes in Nordland displays a complex igneous and metamorphic evolution, including deep-seated contact metamorphism, anatexis and extensive interaction between the crystallizing magma and the country rocks. Gabbroic and metabasic rocks show a wide compositional range, some of which reftects wall-rock contarnination, whereas the Nd isotopic signatures of selected gabbro samples indicate an origin from a depleted magma source. The gabbros/metabasics display subalkaline (including calc-alkaline) to mildly alkaline compositions. Sm-Nd whole-rock +mineral isochron dating of olivine gabbro and gabbro-norite give ages of respectively 437 ± 22 Ma, 143Nd/144Nd, = 0.512353 ± 24 and 439 ± 20 Ma, 143Nd/144Nd, = 0.512301 ± 20. Contact metamorphic minerals in adjacent pelitic rocks yield similar ages (437 ± 6 Ma, Gnt +Pl, sample 6B and 435 ± 5 Ma, Gnt +Pl+ Bi, sample 175), interpreted to date the contact metamorphic growth of garnets in anatectic rocks. Small-scale disequilibria between early- and late-formed minerals in gabbros and in contact metamorphic/anatectic rocks may have been induced by contamination and incomplete mixing during the crystallization.

M. B. E. Mørk, IKU Petroleum Research, N-7034 Trondheim, Norway;
B. Sundvoll & A. Stabel, Mineralogical-Geological Museum, University of Oslo, Sars gt.1, N-0562 Oslo 5, Norway.