40Ar/39Ar Dating Reveals Multiple Generations of Mafic Dikes in Southeastern Alaska

Seven ⁴⁰Ar/³⁹Ar whole-rock incremental heating dates of mafic dikes from the Wrangell transect in southeastern Alaska yield six concordant plateau dates of 14.7 ± 1.0 Ma, 24.7 ± 1.1 Ma, 30.1 ± 0.2 Ma, 41.8 ± 1.1 Ma, 107.5 ± 0.5 Ma, and 113.0 ± 0.7 Ma. One sample is discordant with most of the steps yielding dates >220 Ma. We also obtained a well-defined plateau date of 115.6 ± 0.3 Ma from a hornblende separate from the sample that yielded the 113.0 ± 0.7 Ma whole-rock date. The concordance of these two dates suggests that the whole-rock dates probably reflect a close approximation of the emplacement and crystallization ages of the dikes. The mafic dikes sampled for this study have distinct chilled margins and are 0.5-5 meter thick alkali-rich basalts and basaltic andesite. The dikes crosscut all ductile fabrics and define two main sets in orientation. One set strikes NNW and dips steeply to the east and the other set strikes NE with steep SE dips. Based on crosscutting relationships, the NNW dikes appear to be older and are mostly found west of Clarence Straight. The mid-Cretaceous dikes dated in this study intrude Silurian age rocks of the Alexander terrane and are confined to the western part of the Wrangell transect. The younger (14.7 - 41.8 Ma) dikes are found throughout the study area including within variably metamorphosed Jurassic-Cretaceous meta-sedimentary rocks, mid-Cretaceous plutons, and Tertiary volcanic rocks. These new dates show that mafic dikes intruded the Alexander terrane during mid-Cretaceous shortening in SE Alaska. This suggests that basaltic magmatism occurred in SE Alaska during the final accretion of the Alexander/Wrangellia terrane and during subsequent Cenozoic deformation along a transpressive to transtensional plate boundary.