Crystal chemistry and origin of epidote-(Sr) in alkaline rocks of the teschenite association (Silesian Unit, Outer Western Carpathians, Czech Republic)

Abstract

A new occurrence of epidote-(Sr) CaSr(Al₂Fe³⁺)[Si₂O₇][SiO₄]O(OH), Sr-REE-rich epidote and Sr-rich allanite-(Ce) is located in Lower Cretaceous, Sr-rich hydrothermally altered leucocratic dykes penetrating alkaline igneous rocks (teschenites) near the Nový Jičín town (the Silesian Unit, Outer Western Carpathians). The dykes contain phenocrysts of pyroxene, amphibole, biotite, fluorapatite and dominant felsic groundmass consisting mostly of alkali feldspars and zeolites (analcime, natrolite and thomsonite-Ca). Accessory minerals include Ti-rich magnetite, prehnite, chamosite, pyrite, calcite, (OH, F)-rich grossular, epidote-group minerals, HFSE-, REE-rich minerals, Sr-rich baryte and slawsonite. The Sr-rich epidote forms columnar crystals or irregular aggregates, which are mostly spatially related to chamosite-titanite pseudomorphs; it contains 0.15–0.81 apfu Sr, ≤ 0.55 apfu REE; Fe³⁺/(Fe³⁺ + Al) = 0.16–0.48. The Sr²⁺ substitutes Ca²⁺ in the A2 site by a coupled substitution involving other A2 (REE³⁺, Th⁴⁺) or M (Al³⁺, Fe³⁺, Fe²⁺) cations. The Sr-rich epidote crystallized from hydrothermal solutions, probably at temperatures between ~ 250–430 °C, during cooling of the host rock. The dykes show higher Sr contents (5680–7830 ppm) and ⁸⁷Sr/⁸⁶Sr_i(120 Ma) ratios (~ 0.7046–0.7047) compared to host mesocratic teschenites (1310–1470 ppm Sr and ~ 0.7038–0.7045, respectively). The Sr isotopes indicate origin of most Sr from primary magmatic plagioclase in parent teschenite. Nevertheless, there also participated external fluids, derived from the Lower Cretaceous seawater or diagenetic waters related to associated siliciclastic sediments with ⁸⁷Sr/⁸⁶Sr_i(120 Ma) = ~ 0.7073–0.7083. These more radiogenic sources contributed at least 6–21% of the bulk Sr budget of the studied Sr-rich epidote-bearing leucocratic dykes.