Comparison of diagenetic and low-grade metamorphic evolution of chlorite in associated metapelites and metabasites: an integrated TEM and XRD study

Chlorite is a common sheet silicate that occurs in various lithologies over a wide grade range involving diagenesis and low-grade metamorphism. Thus, the reaction progress of chlorite offers a unique opportunity for direct co rrelation of zonal classification of metasedimentary rocks based on illite crystallinity with metabasite mineral facies. To provide such correlation, chlorite crystallinity indices, apparent mean crystallite sizes and lattice strains, crystallite size distributions and compositions of chlorite from coexisting metapelites and metabasites were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscop y (TEM), analytical electron microscopy (AEM) and electron microprobe (EMP) methods. Samples were from Palaeozoic and Mesozoic formations of the Bukki um (innermost Western Carpathians, Hungary) that underwent Alpine (Cretaceo us) orogenic metamorphism. Metapelites range in grade from late diagenesis to epizone, whereas metabasites vary from prehnite-pumpellyite through pump ellyite-actinolite to greenschist facies. Despite significant differences in composition, mineral assemblages and tex tures, reaction progress, as measured in part by chlorite crystallinity, in metapelites paralleled that in metabasites. Chlorite crystallinity and mea n crystallite size increase and the proportion of mixed layers in chlorite decreases, whereas the calculated lattice strain does not change significan tly with increasing metamorphic grade. Similar trends, but (especially at h igher grades) significant differences, were found in mean crystallite size values using various methods for XRD line profile analyses. The increase in crystallite size with increasing grade was demonstrated also by direct TEM measurements on ion-milled whole-rock samples, but with a larger scatter o f data at higher grades. In spite of the different kinds of mixed layering in chlorite (Mg-rich smectitic, mostly random, local corrensite-like units in metabasites, and Fe-rich berthierine and dioctahedral smectite in metape lites), XRD-calculated and TEM-measured parameters were found to be reliabl e tools for measuring reaction progress and metamorphic grade of the same d egree in both lithotypes.