Crystal chemical characterization of biotites and amphiboles from metabasites (Ivrea-Verbano Zone, Italian Western Alps).

Biotites and amphiboles in metabasites from Ivrea-Verbano Zone were studied to illustrate possible correlations between their chem. compn. and the metamorphic grade (from lower to upper amphibolite- and granulite-facies) and paragenesis. The biotites examd. are intermediate in compn. between phlogopite and annite; with increasing metamorphic grade, the phlogopite end member and Ti content increase, where AlVI decreases. The crystal chem. variations of biotites in metabasites are compared with those in metapelites: Ti-tschermak type substitution mechanism prevails in upper amphibolite facies (esp. in metabasites) and in granulite-facies; the Al-tschermak substitution mechanism prevails (esp. in metapelites) in the lower amphibolite facies. Statistical anal. (multivariate anal. of variance) indicated that all variables, except those relative to interlayer cations, are highly significant in terms of metamorphic grade; however, interlayer cations other than AlIV and AlVI were the most significant variables in distinguishing between biotites of metabasites and those of metapelites. Amphiboles, in the amphibolite-facies rocks, are mainly magnesiohornblendes; those in granulite facies are pargasitic hornblendes. The A-site alkali metals and the octahedral Ti and Al are generally compensated by AlIV-Si substitution in tetrahedral position. Significant variations in metabasite amphiboles from amphibolite to granulite facies were found: Ti and alkali metals increase and Mn decreases, which is the same general pattern seen in biotites from the same area. In the case of lower and upper amphibolite facies, statistical anal. shows less marked distinction than for biotites.