Abstract
40 Ar 39 Ar incremental-release ages have been determined for hornblende and muscovite from several high-level, exotic nappes in northern Troms County, Norway. Within the Nordmannvik Nappe and upper tectonic levels of the Lyngen Nappe Complex (Balsfjord Group) both minerals record plateau and isotope correlation ages between c. 427 and 417 Ma. These indicate relatively rapid post-metamorphic cooling which is interpreted to have occurred during Scandian translation of exotic tectonic elements to higher crustal levels along the Baltoscandian margin. Muscovite within various structural levels of the Tromsø Nappe Complex (Uppermost Allochthon) yeilds plateau ages of 427-410 Ma which also record post-Scandian metamorphic cooling. However, hornblende within the Tromsø Nappe Complex records significantly older isotopic correlation ages which range between c. 481 Ma and 432 Ma. The 40 Ar 39 Ar spectra display no evidence of partial Scandian rejuvenation, and the isotope correlation ages are therefore interpreted to date cooling following pre- Scandian tectonothermal activity. These results indicate that early Paleozoic tectonothermal events affected outboard exotic tectonic elements at approximately the same time as the “early” Caledonian orogenesis previously described for nappe units with initial Baltic palinspastic affinities (e.g., Seve Nappe Complex).
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