Geology and K/Ar chronology of the Målvika scheelite skarns, Central Norwegian Caledonides.

Abstract

Skarns present on the northern shore of Tosenfjord overprint the S2 fabric of the adjacent gneiss (which formed during peak regional metamorphism). Together with petrographic evidence of the late crystallization of sheelite, this indicates that mineralization was post-metamorphic and epigenetic. K\/Ar apparent ages of mine mineral separates (four post-scheelite amphiboles from skarn, two muscovites and three biotites from intrusions) are concordant at the 1q level, giving an inverse Variance Weighted Mean (VWM) age of 402 +- 2 Ma The close agreement of results from different minerals points to rapid post-orogenic uplift in the Central Scandinavian Caledonides. Two other post-scheelite amphibole skarn samples gave indistinguishable (at 1q) apparent ages whose mean is 472 Ma. If valid, this age would substantially constrain the timing of regional metamorphism and mineralization. However, the possibility that the age are spurious, due to the presence of excess argon, cannot be excluded,