A Rb/Sr whole-rock isochron date from the lowermost gneiss complex of the Gaular area, west Norway and its regional implications

The Precambrian rocks within the Gaular area can be tectonically divided into two major rock units; a lower complex (Jostedal Complex), which largely consists of migmatites and gneiss-granites, and an overlying largely metasupracrustal sequence. The latter unit, which may be subdivided into the Holsen Gneiss (mainly quartz-rich paragneisses) and the Vevring Complex (mainly amphibolites, paragneisses, and eclogitic rocks), has been interpreted as a thurst sheet, the Sunnfjord nappe. The Jostedal Complex yields a metamorphic Rb/Sr whole-rock isochron date of 1625 ±75 m.y. An initial 87Rb/86Sr ratio of o.70 1 ± 0.002 excludes an extended crustal history prior to the major metamorphism. Two biotite/whole-rock ages of 392 ± 15 m.y. and 396 ± 15 m.y. provide evidence for a Caledonian influence in the area. It is assumed that the emplacement of the Sunnfjord nappe took place late in the Sveconorwegian (Grenvillian) orogeny.