40Ar/39Ar dates controlled by white mica deformation and strain localization: Insights from comparing in situ laser ablation and single-grain fusion techniques

Abstract

In situ laser ablation and single-grain fusion ⁴⁰Ar/³⁹Ar geochronological techniques were directly compared using white mica from nine metasedimentary rocks from the Vaimok Lens of the Seve Nappe Complex (SNC) in the Scandinavian Caledonides. Seven of the rocks are from the eclogite-bearing Grapesvare nappe within the lens that is defined by D2 structures (S2 and F2), which were formed during exhumation following late Cambrian/Early Ordovician ultra-high pressure metamorphism. Two other rocks were obtained from ‘Scandian’ shear zones that delimit the nappes within the lens. The shear zones were active during terminal collision of Baltica and Laurentia in the Silurian to Devonian. The rocks exhibit variable deformation intensities and degrees of strain localization, expressed in particular by white mica. The in situ laser ablation and single-grain fusion ⁴⁰Ar/³⁹Ar dates both span from the late Cambrian to Middle Devonian. Results of both techniques generally show decreasing dates with increasing bulk deformation intensity and successive structural generations (i.e., D2 then Scandian structures). Furthermore, several discrepancies are evident when comparing the results of the two techniques for the same rocks, indicating the ⁴⁰Ar/³⁹Ar dates are not solely governed by bulk deformation intensities and structural generations. Instead, the discrepancies demonstrate the additional influence of white mica strain localization, which is illuminated by the different analytical volumes of the techniques. Thus, the ⁴⁰Ar/³⁹Ar datasets are altogether deciphered as a function of bulk deformation intensity and degree of strain localization that affected the overall white mica volume. The former controls the gross ⁴⁰Ar loss from the overall volume and the latter dictates the variability of ⁴⁰Ar loss within the volume. Exploiting the interplay of these two phenomena for the Vaimok Lens rocks with in situ laser ablation allows for the broad span of ⁴⁰Ar/³⁹Ar dates to be contextualized into a sequence of tectonic events: (1) cooling at 474 ± 3 Ma, (2) pre-collision deformation at 447 ± 2 Ma and (3) activation of crustal-scale shear zones in the SNC related to continental collision at 431 ± 3 Ma and 411 ± 3 Ma.