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

Abstract

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 correlation 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 microscopy (TEM), analytical electron microscopy (AEM) and electron microprobe (EMP) methods. Samples were from Palaeozoic and Mesozoic formations of the Bükkium (innermost Western Carpathians, Hungary) that underwent Alpine (Cretaceous) orogenic metamorphism. Metapelites range in grade from late diagenesis to epizone, whereas metabasites vary from prehnite–pumpellyite through pumpellyite–actinolite to greenschist facies. Despite significant differences in composition, mineral assemblages and textures, reaction progress, as measured in part by chlorite crystallinity, in metapelites paralleled that in metabasites. Chlorite crystallinity and mean crystallite size increase and the proportion of mixed layers in chlorite decreases, whereas the calculated lattice strain does not change significantly with increasing metamorphic grade. Similar trends, but (especially at higher 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 of 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 metapelites), XRD‐calculated and TEM‐measured parameters were found to be reliable tools for measuring reaction progress and metamorphic grade of the same degree in both lithotypes.

Comparison of Evolution of Trioctahedral Chlorite/Berthierine/Smectite in Coeval Metabasites and Metapelites from Diagenetic to Epizonal Grades

“…The metabasalt (DG-33) and metadiabase (DG-16) from the Szarvask6 complex have a smaller proportion of corrensitic layers, however, as consistent with a higher grade imposed by regional metamorphic overprinting of the normal MORB alteration sequence. The decrease in percentage of expandable layers with advancing grade was also noted by Arkai and Sadek Ghabrial (1997) and Arkai et al (2000) on the basis of the data obtained using the method of Bettison and Schiffman (1988) that determines the proportions of chlorite in chlorite-smectite mixed-layered material. Other studies have demonstrated that the percentage of expandable layers decreases in these systems with increasing T (or grade) also (Alt, 1999, and references therein).…”

“…In the first part, we emphasized comparison of ChC indices, as calculated by XRD, and TEM-and XRD-determined thickness values, as well as the relations between ChC and chlorite chemistry. These relations showed that despite differences in composition, mineral assemblages and textures, ChC values of coeval metapelites and metabasites changed in a regular way with increase in grade, and with nearly identical values in coeval pairs (Arkai et al, 2000). In the present paper we describe the evolution of texture, composition and structure of trioctahedral phyllosilicates in detail, emphasizing the mineralogical relations, as a function of reaction progress in pairs of metasedimentary and metabasic rocks over a broad range of grades.…”

“…Samples were described in detail by Arkai et aL (2000) and thus are only briefly characterized here. All samples came from the innermost tectonic units of the Western Carpathians (Figure la).…”

“…Arkai (1983Arkai ( , 1991, Kov~ics (1989), Downes et al (1990, Arkai et aL (1995), Sadek Ghabrial et al (1996, Kov~ics et al and ,/~rkai et al (2000). The four sequences studied were: Darnd Hill (DH), Szarvsk6 complex (SZ), Eastern Bfikk Mountains (EB) and Uppony Mountains (UM), with metamorphic grades and mineralogy as determined by XRD (Arkai et al, 2000) listed in Table 1. In the Darn6 Hill series, Middle Triassic pillow basalts showing ocean-floor hydrothermal metamorphism with prehnite-pumpellyite facies assemblages are interbedded with pelitic sedimentary rocks that were only diagenetically altered.…”

“…The evolution of crystallite thicknesses of chlorite from lower-grade metabasites is discussed in Arkai et al (2000). The average value is <50 nm but there is considerable scattering of values about the mean.…”