Sulla variabilità compositiva delle miche nell'ambito dello stesso affioramento di metapeliti di basso grado: un esempio. 1: Le biotiti

Compositional variability of micas within the same outcrop of low-grade metapelites: a casestudy. 1: The biotites. 140 mica flakes (76 muscovites and 64 biotites) have been analyzed in 4 samples of lowgrade metapelites from a large outcrop of the Southalpine basement in Pusteria (Pustertal, Eastern Alps). The analyzed flakes have been selected: (i) in different microtextural sites (hinges vs. limbs of microfolds); (ii) in different chemical microsystems (granoblastic vs. lepidoblastic domains; flakes in contact with different phases); and (iii) in such a way that each flake could be either referred to the synkinematic crystallization stage Cr2 or to the postkinematic stage Cr3. The present paper reports the results obtained from the biotites, while the muscovites are considered in a separate paper, which also proposes some concluding remarks concerning both micas. Biotites from granoblastic domains are substantially identical to those from lepidoblastic domains, as well as biotites flakes located in the hinges of microfolds are identical to the flakes in the limbs. On the contrary, mica flakes crystallized during Cr2 compositionally differ from those crystallized during Cr3; in the latter case, biotite chemistry suggests slightly higher temperatures although the mineral compatibilities are unchanged. Some other chemical peculiarities of biotite flakes have been also detected: they depend on the different mineral phases in contact with the analyzed flake.