Occurrences of Slawsonite in rocks of the teschenite association in the Podbeskydí Piedmont area (Czech Republic) and their petrological significance

Abstract

Slawsonite is an extremely scarce natural member of the Feldspar Group, occurring in less than 10 known localities worldwide. We report three new slawsonite occurrences (in Certak, Krmelin, and Repiste.) in rocks of the Cretaceous teschenite association in the Czech Republic. At Certak, a tabular sill of "teschenite'' (in fact analcime syenite, based on its mineral composition) is one of the most alkaline examples of the teschenite association rocks. It shows differentiation to a coarser-grained core enriched in amphiboles, with diffuse transition to the outer zones. Rare thin leucocratic "aplite'' dikes contain slawsonite, celsian, microcline, natrolite, prehnite, analcime, and thomsonite-Ca. Slawsonite forms aggregates exhibiting symplectite-like microstructures up to 2 mm long and is associated with euhedral zoned celsian micrograins up to 100 mu m in size. Wavelength dispersive X-ray analyses indicate that the chemical composition of slawsonite ranges from Sl(90.4)Cn(3.4)An(1.7)Ab(3.1)Or(1.4) to Sl(60.3)Cn(8.7)An(6.9)Ab(18.3)Or(5.8); the composition of celsian ranges from Cn(95.8)An(0.4)Ab(1.0)Or(2.3)Sl(0.5) to Cn(70.1)An(0.5)Ab(2.2)Or(6.3)Sl(20.9); and the Na-rich microcline exhibits a composition of Or(58.3)An(2.8)Ab(29.4)Cn(6.4)Sl(3.1). Slawsonite from Krmelin and Repiste occurs in a similar geological setting, as aggregates up to 0.3 mm in size that are confined to leucocratic dikes at the endo-contacts of teschenite sills or lava pods. We suggest that slawsonite forms pseudomorphs after nepheline or possibly after plagioclase. Whole-rock geochemistry shows that local teschenites have an affinity for alkali basaltoid rocks typical of MORBs, but are significantly enriched in incompatible elements such as Rb, Ba, Th, U, Ta, Nb, and Sr. With regard to the obvious presence of autometamorphic hydrothermal alteration, we propose that bodies of teschenite association rocks should not be described as dikes; instead, a model of lava pods should be applied.