Abstract
Structural and chemical evolution paths of white K-mica and chlorite from low-temperature metamorphic rocks are studied by X-ray powder diffractometry and electron probe micro-analysis in order to determine the effects of lithology expressed by bulk rock major element chemistry on the phyllosilicate reaction progress. The sample set representing three main lithotypes, namely pelitic and marly slates, metatuffites and massive meta-igneous rocks of basic to intermediate compositions, derived from the Graz and Sausal Paleozoic of the Upper Austroalpine unit, Eastern Alps, Austria. Reaction progress, modelled in terms of proportion of swelling mixed-layers, crystallinity indices, mean crystallite size, lattice strain and mineral chemistry, shows grain-size dependent variations in all lithologies. Referring to disequilibrium conditions, the larger authigenic grains reflect more evolved stages than the < 2 μm grain-size, matrix-forming fraction conventionally used for XRPD studies. Illite crystallinity values increase (apparent metamorphic grades decrease) with advancing celadonite content in white K-mica, while increasing dioctahedral (sudoitic) substitution decreases chlorite crystallinity indices, thus providing apparently higher grades. White K-mica and chlorite chemistries are related to bulk rock major element chemistries. Consequently, the more basic a rock, the larger its illite and chlorite crystallinity values will be, implying lower apparent metamorphic grades for basic rocks as compared to peliticmarly metasediments conventionally used for crystallinity studies. Thus, in addition to the proportion of swelling mixed-layers, mean crystallite size and lattice strain values, bulk rock major element compositions and related phyllosilicate chemistries should be considered when illite and chlorite crystallinity indices obtained from various lithologies are to be used for metamorphic petrogenetic purposes.
