Original paper

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

Bigi, Simona; Brigatti, Maria Franca

Image of first page of:

Neues Jahrbuch für Mineralogie - Abhandlungen Band 161 Heft 2 (1989), p. 227 - 240

25 references

published: Nov 2, 1989

DOI: 10.1127/njma/161/1989/227

BibTeX file

ArtNo. ESP154016102005, Price: 29.00 €

Download preview PDF Buy as PDF

Abstract

Biotites and amphiboles in metabasites from Ivrea-Verbano Zone were studied to illustrate possible correlations between their chemical composition and the metamorphic grade (from lower to upper amphibolite and granulite facies) and paragenesis. The biotites examined are intermediate in composition between phlogopite and annite; with increasing metamorphic grade, the phlogopite end member and Ti content increase, whereas AlVI decreases. The crystal chemical variations of biotites in metabasites are compared with those in metapelites: Ti-tschermak type substitution mechanism prevails in upper amphibolite facies - especially in metabasites - and in granulite facies, while Al - tschermak substitution mechanism prevails - especially in metapelites - in lower amphibolite facies. Statistical analysis (multivariate analysis of variance) has shown that all variables, except those relative to interlayer cations, are highly significant in terms of metamorphic grade, whereas interlayer cations, other than AlIV and AlVI, were the most significant variables in the distinction between biotites of metabasites and those of metapelites. Amphiboles in the amphibolite facies rocks are mainly magnesium-hornblendes, while those in granulite facies are classed as pargasitic hornblendes. A-site alkali content and Ti and Al octahedral contents are generally balanced by AlIV- Si substitution in tetrahedral position. Significant variations in metabasite amphiboles from amphibolite to granulite facies were found: Ti and alkalis 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 analysis shows less marked distinction than for biotites.

Keywords

Biotite chemistry • Amphibole chemistry • Statistical analysis • Ivrea Zone • Metapelites • Metabasites • Amphibolite and granulite facies