Diagenesis and very low-grade metamorphism of pelitic rocks from the Gaspe Peninsula, Quebec.

Abstract

Transmission and analytical electron microscope study of low-grade pelitic rocks from the Gaspe Peninsula, Quebec demonstrates that diagenesis/metamorphism of chlorite/corrensite and illite follows a sequence of phase transitions, compositional homogenization, and recrystallization approaching a state of equilibrium for which chlorite and muscovite are the stable phases. Detrital biotite was altered to form corrensite and chlorite in the diagenetic zone through layer-by-layer replacement and dissolution-transport-precipitation processes through which coarse-grained chlorite-mica stacks originated during early diagenesis. Matrix illite occurred as packets of coalescing layers intergrown with chloritic minerals, forming thick stacks of layers in all grades of rocks. Crystal sizes and perfection of both chlorite and illite and definition of crystal boundaries increase whereas randomness in orientation decreases with increasing degree of diagenesis/metamorphism. Contrasting deformation features in adjacent crystals imply syn-deformational crystallization and recrystallization. Corrensite decreases significantly in proportion from the diagenetic zone to the anchizone, displaying transitional textures to chlorite with chlorite becoming progressively homogenized. Average crystal thicknesses determined by TEM are $\sim$400 A, $\sim$1200 A, and $\sim$2300 A in the diagenetic, anchizone, and epizone rocks, respectively, consistent with Fourier analysis of X-ray peak profiles but differing significantly from those determined by the illite crystallinity index. Mean-square lattice strain decreases with increasing grade, demonstrating that crystal size and lattice strain are interrelated factors controlling illite crystallinity. Illite crystallinity can be viewed as a kinetically controlled function of many variables relating to crystal growth and recrystallization rather than as an indicator of the degree of diagenesis/metamorphism. The illite crystallinity index can only be used as a crude approximation of the degree of recrystallization on a relative basis. The rocks constitute a part of a regional distribution of trioctahedral phyllosilicate-rich rocks in the northern Appalachians, suggesting an origin pertaining to a specific tectonic regime with sediments largely derived from an ancient andesitic arc system(s). Subsequent alteration of diagenetic tri- and di-octahedral clays originating from the detrital materials occurred through deformation, dissolution, crystallization, and recrystallization, evolving to chlorite and muscovite.