Combined in situ K/Ca and Rb/Sr geochronology of potassic mica

In situ geochronology using laser ablation is essential for integrating microstructural and chemical records of minerals with their geochronological information. The in situ method is especially important for potassic mica, which readily deforms, often define rock structures and fabrics, and are susceptible to chemical modification by reactive fluids. In situ ⁴⁰Ar/³⁹Ar geochronology of mica has been widely used for decades and in situ Rb/Sr geochronology has recently emerged as a prevalent method. In contrast, in situ K/Ca geochronology has been demonstrated, but the method has not been refined or utilized. Combined in situ K/Ca and Rb/Sr geochronology was developed at the Fipke Laboratory for Trace Element Research (University of British Columbia, Okanagan), allowing direct comparison of the two isotopic systems and exploring the potential of K/Ca dating. The methodology was tested using two-mica leucogranites and migmatitic paragneisses from the central Seve Nappe Complex (SNC), comprising remnants of Baltican continental crust in the Scandinavian Caledonides. The central SNC consists of continental crustal rocks hosting volumetrically-subordinate eclogites, peridotites, and pyroxenites. The crustal lithologies record partial melting beginning at c. 484-480 Ma, while the (ultra)mafic lithologies provide evidence for high-pressure metamorphism at c. 460-454 Ma, altogether reflecting subduction-exhumation cycles of the central SNC. Subsequent collision of the Baltican continent with Laurentia led to widespread deformation and metamorphism in the SNC, starting at c. 430-425 Ma. Continental anatexis produced the studied leucogranites and migmatitic paragneisses; these rocks comprise quartz, white mica, biotite, plagioclase, K-feldspar, and clinozoisite, with accessory apatite, monazite, xenotime, and zircon. The accessory phases were dated via in situ U-Pb geochronology to compare with in situ white mica and biotite K/Ca and Rb/Sr geochronology results. The dates yielded by apatite (424 ± 15 Ma, 428 ± 17 Ma), monazite (427 ± 2 Ma), and xenotime (426 ± 2 Ma) are all within uncertainty (pooled age of 426.5 ± 1.4 Ma) and record the timing of continental collision. Zircon provide a wider range of concordant U-Pb dates (473 ± 8 Ma to 419 ± 5 Ma). White mica and biotite Rb/Sr isochron dates from all four examined rocks (438 ± 5 Ma, 430 ± 4 Ma, 433 ± 4 Ma, 431 ± 5 Ma) are slightly older than the apatite/monazite/xenotime and youngest zircon U-Pb dates, but still record continental collision. The same mica volumes yielded older K/Ca isochron dates (484 ± 21 Ma, 482 ± 14 Ma, 486 ± 15 Ma, 486 ± 12 Ma), similar to the oldest concordant zircon U-Pb dates and the overall timing of partial melting in the broader central SNC crustal rocks. The different K/Ca and Rb/Sr dates indicate that these two isotopic systems are decoupled in the same mica volumes, controlled either by interactions of Ca and Sr with the mica lattice or by diffusion gradients influenced by bulk rock composition. The results presented herein demonstrate that the K/Ca isotopic system has the potential for retrieving older geologic histories in polymetamorphic terranes. Further detailed investigations and continued development of the methodology, including improved reference materials, are required.