The Caledonian tectono-metamorphic evolution of Lindås Nappe: Constraints from U-Pb, Sm-Nd and Rb-Sr ages granitoid dykes
pp. 45-57

Petrological and isotopic (Rb-Sr, Sm-Nd and U-Pb) investigations of garnet bearing pegmatites and trondhjemites in the northernmost Lindås Nappe, Bergen Arcs, Norway are reported. The formation of these pegmatites (sample sites Håkjerringa and Ervik) is dated to around 425 Ma by means of Rb/Sr and Sm/Nd mineral isochrons. U-Pb dating of zircons from a trondhjemitic dyke (sample site Fonnes quay) yields a lower intercept age of 418 ± 9 Ma. Based on field observations, negative ?Nd values ..and a comparatively high initial 87sr/86sr for the Håkjerringa pegmatite, we suggest a crustal-anatectic source for the pegmatite-forming melts. 1'he ages are interpreted to represent the time of crystallization of the granitic melts at mid-crustal levels (8-10 kbar, 650 to 700 °C). The partial melts formed due to the addition of a fluid phase that lowered the solidus of the rocks during a major phase of Scandian deformation in the Lindås Nappe. Fluid mediated edogitization (17 kbar, 700 oC) took place in the western part (Holsnøy), contemporaneously with the formation of the granitoid dykes. This either means very rapid exhumation from depths corresponding to 17 kbars to l O kbars. Or, and more like! y, that the Lindås Nappe represents a part of a former crustal section where the bottom parts (Holsnøy) and the shallower north-eastern parts (Lindås-Austrheim) experienced hydration at the same time.