THE STEP FROM DIAGENESIS TO METAMORPHISM IN PELITIC ROCKS DURING ALPINE OROGENESIS

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Abstract
Summary: Pelitic rocks were followed from unmetamorphosed clays and marls of Upper Triassic and Lower Liassic to anchimetamorphosed phyllites of the Alpine border region in Switzerland. Phengite and Al‐rich chlorite formed from mixed‐layer illite/montmorillonite; pyrophyllite from kaolinite. For the formation of paragonite, the following sequence is proposed: irregular mixed‐layer illite/montmorillonite → regular mixed‐layer mica/montmorillonite → mixed‐layer paragonite/phengite → paragonite.Besides the formation of new minerals in the transition zone (anchizone) between diagenesis and greenschist facies, other changes with increasing metamorphic grade are: the crystallinity of illite (in the sense of Kubler, 1967) increases, the intensity ratio 002/001 of the illite basal reflexions increases, 1Md illite changes to 2M1 phengite; the slates change colour from red to pink; the mean density increases; and textural changes due to reactions between clastic quartz and clay cement occur.

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