Indicator characteristics of micas and chlorites from the zones of late catagenesis and very low-grade metamorphism: Evidence from terrigenous rocks of the phanerozoic depressions and orogenic structures of northern Siberia and western Kazakhstan

Sandy and silty pelitic rocks from the Lower Mesozoic Koltogorsk-Urengoi ri ft (in the base of western Siberian platform cover), Priverkhoyansk foredee p, northern part of the Mesozoic Verkhoyansk orogenic belt, and the Precamb rian of the Ulutau Uplift (Caledonian orogenic belt of central Kazakhstan) were deposited in similar delta-marine geologic environments but experience d different degrees of postdepositional alteration: from late catagenesis ( anchizone) and epigenesis (epizone) to greenschist-facies metamorphism. New electron microprobe analyses of micas and chlorites from the cement of the se rocks are presented and discussed. Factors and mechanisms of authigenesi s are examined. Parameters of the boundary between lithogenesis and low-gra de metamorphism are presented. In particular, we distinguished alteration g rades corresponding to the leveling of compositions of authigenic micas and chlorites and complete obliteration of relicts of detrital layered silicat es.