Ba- and Ti-enriched dark mica from the UHP metasediments of the Seve Nappe Complex, Swedish Caledonides

We report on the occurrence of peculiar Ba- and Ti-enriched dark mica in metasedimentary rocks that underwent high-pressure metamorphism in the diamond stability field followed by decompression to granulite facies conditions. The mica occurs as well-developed preserved laths in a quartzofeldspathic matrix. The mean concentrations of BaO and TiO₂ in the mica are 11.54 and 7.80wt%, respectively. The maximum amounts of these components are 13.38wt% BaO and 8.45wt% TiO₂. The mean crystallochemical formula can be expressed as (K_0.54Ba_0.39Na_0.02Ca_0.01)_Σ0.96(Fe_1.37Mg_0.85Ti_0.50Al_0.29Mn_0.01Cr_0.01)_Σ3.03(Si_2.59Al_1.41)_Σ4.00O₁₀(OH_1.30O_0.66F_0.02S_0.01)_Σ1.99, with oxyannite, oxy-ferrokinoshitalite and siderophyllite as dominating end-members. Based on the petrographical observations, it is proposed that the dark mica was formed at a rather late stage in the evolution of the parental rock, i.e. under granulite facies conditions.