K ? Ar age determinations on phengites from the internal part of the Sesia Zone, Western Alps, Italy

Abstract

K-Ar age determinations have been carried out on various, well-defined phengite populations from a small area of the internal part of the Sesia Zone, lower Aosta valley. There, the rocks have suffered high-pressure metamorphism, attributed to early Alpine subduction, in the stability field of jadeite + quartz (P≥15±1 kbar at T=550±50° C). Coarse-grained phengites from well-preserved high-pressure parageneses, and phengites (re)crystallized early during decompression at still elevated temperatures in the stability field of albite, yield identical ages close to 80 Ma. In the most external sample high-pressure phengites yield 72 Ma. The ages around 80 Ma are interpeted as essentially undisturbed cooling ages; they are not notably influenced by paragenesis, chemical composition, polymorphism, grain-size, deformation, and recrystallization at higher temperatures. This part of the Sesia Zone has cooled down about 80 Ma ago to below the blocking temperature for the K-Ar system of white mica. Some of the samples show pronounced retrogression of the high-pressure assemblages, especially formation of albite and fine-grained phengite at the expense of jadeite, and are affected by intense late deformation at temperatures of about 300 to 350° C, estimated from the small grain-size of dynamically recrystallized quartz. The ages obtained from extremely kinked or sheared coarse high-pressure phengites scatter considerably, being partly higher, partly lower than 80 Ma. The fine-grained phengite fractions from these samples yield lowered ages down to 70 Ma. These values are interpreted as mixed ages resulting from variable mixtures of fragmented high-pressure phengites and new phengite replacing jadeite; the K-Ar data suggest that the new phengites have crystallized between 60 and 70 Ma b.p. All phengites formed at low temperatures at the expense of jadeite reveal high Si-contents; these range up to 6.9 for the coarser-grained earlier generations, and up to 6.7 for the very fine-grained last generation. Relatively high pressures are required to stabilize phengites so high in Si. This implies that cooling had not been achieved by uplift and erosion; obviously the thermal structure of the crust was still strongly perturbed by continuing subduction and thrusting during the late Cretaceous.