A likely Early Ordovician age for the regional penetrative cleavage in the Gaissa Nappe Complex.

Abstract

Pervasively cleaved mudstones and claystones from two different parts of the Gaissa Nappe Complex in Finnmark wre subjected to Rb-Sr whole-rock analytical investigation. Although neither of the two groups of samples yielded a true isochron, the data do support the concept of a probable age at around 480-550 Ma, as can be discerned from the \"isochron fits\" of various combinations of the analytical data. Together with the relatively high 87Sr\/86Sr initial ratios, this results strongly suggests that a Sr-isotopic resetting event occurred at around 480-500 Ma, an event which is considered to reflect an overall, pervasive metamorphic imprint on the rock formations of the Gaissa Nappe Complex. Earlier radiometric dating from this same allochthon has yielded ambiguous results, some pointing to Finnmarkian (Late Cambrian-Early Ordovician) (forkortet)