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

M. P. Mata1, G. Giorgetti2, P. Árkai3 and D. R. Peacor4
1 Department of Geology, The University of Cádiz, Pol. Rio San Pedro, 11510, Pto Real, Cádiz, Spain
2 Department of Earth Sciences, University of Siena, Via Laterina, 8, 53100 Siena, Italy
3 Laboratory for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budapest, Budaoersi út 45, Hungary
4 Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109, USA
E-mail of corresponding author: pilar.mata@uca.es

Abstract: The evolution of texture, structure and chemical composition of chloritic clays in coeval pairs of metabasites and metapelites of a prograde sequence from the Bükk Mountains has been investigated using electron microscopy techniques. Samples are from the Bükkium (innermost Western Carpathians, Hungary) that underwent Alpine metamorphism, ranging from late diagenesis to epizone for pelites and from prehnite-pumpellyite to greenschist facies for the metabasites.

Although bulk-rock compositions, textures and primary minerals are different, chlorite evolved at similar rates in coeval metabasites and metasediments, but along different paths. The principal similarities in the prograde sequence are a decrease in the percentage of interstratified material in both dioctahedral and trioctahedral phyllosilicates and increase in thicknesses of chlorite and illite crystallites. The principal difference is in the type of interstratification in chlorite, with berthierine in metapelites, and smectite (saponite) in metabasites, although smectitic mixed layers also occur in the former. The evolution of trioctahedral phyllosilicates is marked by a decrease in the number of mineral species with increasing grade, chlorite, sensu stricto, being the only trioctahedral mineral at higher grades. This is consistent with the trend in reaction progress where both metastable systems (metabasites and metapelites) tend toward the same end-member, thermodynamically stable chlorite, as well as texture (crystal size), and where all intermediate states are metastable, and determined by the Ostwald step rule.

Key Words: AEM • Berthierine • Bükk Mountains • Chlorite • Low-grade Metamorphism • Reaction Progress • Smectite • TEM • XRD

Clays and Clay Minerals; August 2001 v. 49; no. 4; p. 318-332; DOI: 10.1346/CCMN.2001.0490406
© 2001, The Clay Minerals Society
Clay Minerals Society (www.clays.org)