Intracrystalline deformation and slaty cleavage development in very low-grade slates from the Central Pyrenees

Abstract

This thesis presents the results of a study on the role of intracrystalline deformation of phyllosilicates during slaty cleavage development. Chapter one gives an introduction to the problem being investigated. Slaty cleavage has been studied for over 150 years, and much is already known about the cleavage forming processes. Although many authors have shown that significant intracrystalline deformation of phyllosilicates occurs during cleavage development. most studies have concentrated on other processes. Recently new techniques have become available. like transmission electron microscopy (TEMJ, which are particularly suited for the study of intracrystalline deformation phenomena. In the present study TEM has been used to study these phenomena in Cambro-Ordovician slates from the Central Pyrenees. Spain. Chapter two reviews previous research into slaty cleavage development. Slaty cleavage is the product of flattening perpendicular to the cleavage planes. and it is usually formed after lithification of the sediment. The main steps in the development of slaty cleavage are: (1) rough cleavage; (2) disjunctive or crenulation cleavage; (3) truly continuous cleavage. The main cleavage forming processes are: (1) mechanical rotation of existing grains; (2) pressure solution; (3) microfolding of an existing foliation; (4) recrystallisation and (5) intracrystalline deformation. The geology of the Orri Dome is summarized in chapter three. During at least two orogenies (Variscan and Alpine) many deformational events have ocurred. Fortunately the geology of the Orr! Dome is well documented in the literature. and suitable areas for microstructural investigations of the mainphase cleavage can be selected. Because the metamorphic grade of the Seo Slates had not yet been studied in detail. this subject is treated in chapter four. Both mineral paragenesis and illite crystallinity measurements indicate that the slates of the Seo Formation in the Orri Dome have been metamorphosed in the high- grade part of the anchizone. In the Massana Anticline and the Pallaresa Anticlinorium lower greenschist facies metamorphism has occurred.