Corrensite and associated smectites in the Teschenite Association Rocks from the Podbeskydí Area (Czech Republic and Poland)

Abstract

This study reports on the relatively common presence of the clay mineral corrensite in Teschenite Association Rocks of Early Cretaceous age. Significant quantities of corrensite were noted in monchiquite dikes and pillow lavas, pyroxene-rich rocks (teschenites), picrites, and also in the associated tuffs and tuffites including hyalo clastite breccias, and in strongly albitised calcareous shale enveloping small volcanic bodies. In addition to the transformation of primary mafic minerals such as forsteritic olivine, Ca-Fe-Mg clinopyroxenes, and amphiboles into clay associations, secondary zeolitisation (analcimisation), carbonatisation, silicification, and serpentinisa tion were also observed. The main clay mineral within the investigated rocks was smectite close to montmo rillonite and saponite, but chlorite and serpentinite subgroup minerals were also present. The corrensite-bearing rocks were macroscopically indistinguishable from their smectitised or chloritised equivalents. Corrensite itself formed extremely thin (<1 μm) wrinkled sheets, present both in the rock matrix and the amygdaloid cavities, where it was younger than albite but older than calcite. The presence of corrensite was proven through powder X-ray diffraction analyses; a sequence of basal reflections at 28–29 Å, 14.25 Å, 7.15 Å, etc., were recorded, which after the application of ethylene glycol slightly expanded to 31 Å, 15.5 Å, 7.83 Å, etc. According to the interlayer distances, behaviour of corrensite after ethylene glycolation, the chemical classification criteria, and geological setting the analysed mineral was a HC-type corrensite of hydrothermal origin. The results of wavelength dispersive spectroscopy (WDS) microanalyses yielded an empirical average formula (K0.12Na0.01Ca0.15)Σ0.28(Mg4.84Fe2+ 1.65Fe3+ 1.34Mn2+ 0.02Al0.82)Σ8.95(Si5.86Ti0.10Al2.03P0.01)Σ8.00O20(OH9.89,F0.11) Σ10.00 . nH2O. Elevated Ti content can be explained either by presentce of nanosize TiO2 particles or a substitution of Ti (up to 0.33 apfu) for tetravalent silicon. The abundance of clay minerals in the Teschenite Association Rocks revealed that they were overprinted by several metamorphic events. Corrensite occurrences narrowed the metamorphic conditions to a temperature interval of 60 to 300 ◦C, which overlaps both zeolite and prehnite pumpellyite metamorphic facies. The presence of smectites and corrensite in contact and hydrothermally altered sediments, as well as potassium metasomatites, proved that significant hydrothermal activity took place at the contact zones of magmatic/volcanic bodies.