Lack of low-grade metamorphism in the Triassic formations of the Argana basin, Morocco: an illite crystallinity re-evaluation

Abstract

Analysis of mineral assemblages and illite crystallinity of the Arganan Triassic formations show that: 1. i) mixed-layers are present across almost all of the section; and 2. ii) values of illite crystallinity are quite dispersed and lie mostly in the diagenetic zone, with no gradient related to burial. It is therefore concluded that, contrary to ideas proposed recently, there is no evidence for very low-grade metamorphism related to crustal thinning during the early rifting of the Central Atlantic. Fluctuations of illite crystallinity values are interpreted as due to complex interactions of detrital heritage and several factors governing illitization and improvement of illite crystallinity. The increase in temperature during burial constitutes only one of them.