Exhumation of the high-pressure Richarddalen Complex in NW Svalbard: Insights from 40Ar/39Ar geochronology

Abstract

40Ar/39Ar dating of white mica from foliated metamorphic rocks of the Caledonian orogen of NW Svalbard was performed using the single-grain fusion technique. Two samples, an augen orthogneiss and a mica schist, were collected from the high-pressure Richarddalen Complex. Three additional mica schist samples were collected from the tectonically underlying Montblanc Unit. 40Ar/39Ar analysis of white mica separates yielded weighted average 40Ar/39Ar dates of 446.1 ± 1.1, 438.4 ± 1.1, 439.4 ± 1.2, 439.8 ± 1.5 and 439.3 ± 1.0 Ma, with the oldest date provided by the Richarddalen mica schist. The single-grain fusion 40Ar/39Ar dating was accompanied by geochemical mapping of white mica using a microprobe. We interpret the oldest date as the age of cooling after a Late Ordovician high-pressure event and the four younger dates to represent the timing of subsequent deformation associated with assembly of a Caledonian nappe stack.