Geochemistry and Rb-Sr isochron age of trondhjemite dykes from the Gula Complex, near Snøan, Central Norway.
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Trondhjemite dykes trending c.ENE-WSW and transecting early metamorphic fabrics in rocks of the Gula Complex south-southwest of Støren, are themselves weakly metamorphosed and foliate. Geochemical data show that the dykes are characteristics of high-Al2O3 trondhjemites, and typical of those reported from continental margin environments worldwide.\rRb-Sr isotopic analysis of a series of samples from one particular dyke did not yield any isochron. However, samples lying farthest from the regression line are exclusively from the dyke margins; and if these samples are excluded, then a best-fit isochron defines an age of 465+-11Ma (MSWD 2.2 i.r. 0.70794+-9) This is taken as the minimum age of intrusion. i.e. basal Llanvirn. and is interpreted as reflecting a post-emplacement thermal resetting age. Dyke margin samples are likely to have been affected by an influx of fluids containing abundant ions of radiogenic Sr, and comcentrated along the dyke margins by the decay of 87Rb during thermal resetting.\rThe isotope age of the trondhjemite also provides a minimuim age for the Early Ordovician deformation and metamorphism of the Juxtaposed Støren ophiolite and Gula Complex succession. The gentle folding of the dykes and their weak foliate fabric are thus likely to be of Scandian (Late Silurian-Early Devonian) age.