Polyorogenic 40 Ar/ 39 Ar mineral age record within the Kalak Nappe Complex, Northern Scandinavian Caledonides

Abstract

The Kalak Nappe Complex is composed of variably retrogressed, middle Proterozoic basement and imbricated, polydeformed, metasedimentary cover successions which, at least in part, appear to have originated within the late Proterozoic-early Palaeozoic Baltoscandian miogeocline. Mafic-alkaline plutons (Seiland Igneous Suite) occur within uppermost structural units of the nappe complex. 40 Ar/ 39 Ar incremental-release mineral ages suggest local differences in tectonothermal evolution within the Kalak Nappe Complex. Post-metamorphic cooling dates of c. 490 Ma are locally recorded by hornblende within the structurally highest tectonic unit suggesting significant orogenic activity occurred during and/or immediately prior to the Arenig. A distinctly later metamorphic overprint occurred in the Silurian and was of sufficient magnitude to everywhere rejuvenate Ar systems in muscovite and nepheline and locally in hornblende. This was followed by relatively rapid post-metamorphic cooling through hornblende and muscovite Ar closure temperatures between c. 425 and 415 Ma. The complex tectonic history suggested for the Kalak Nappe Complex is generally similar to that previously outlined for the central Swedish Caledonides. Together, 40 Ar/ 39 Ar mineral ages from these widely separated areas indicate that significant tectonothermal activity occurred along the Baltoscandian margin prior to the Silurian. This activity appears, at least in part, to have been associated with formation of a Late Cambrian-Early Ordovician accretionary wedge over a westward-dipping subduction zone during initial closure of the Iapetus Ocean.