Compositional variability of micas within the same outcrop of low-grade metapelites: a casestudy. 2: The muscovites, and concluding remarks. The present paper reports and discusses the results on compositional variability of muscovites, obtained in the frame of a research project on the chemistry of biotites and muscovites in low-grade metapelites. The results on biotites have been reported in a separate paper, to which readers are deferred also because it gives the statement of the problem, the strategy of research and the analytical procedures. Muscovite flakes from granoblastic domains are substantially identical to those from lepidoblastic domains, as well as muscovite flakes located in the hinges of microfolds in comparison with those in the limbs. On the contrary, muscovite flakes crystallized during Cr2 compositionally differ from those crystallized during Cr3, in the latter case, muscovite and biotite chemistries suggest slightly higher temperatures although the mineral compatibilities are unchanged. The analytical results indicate a similar behaviour for muscovites and biotites. In particular, within rock samples in which Cr3 effects are widespread, the chemistries of Cr2 muscovites and biotites turn out to be changed. This fact, as well as some chemical peculiarities of the mica flakes depending on the different mineral phases in contact, indicates that use of geothermobarometry in low-grade metapelites requires caution: microtexturally identical mica flakes may give different T, P values depending on their size, the adjacent mineral phase, and the extent of chemical re-equilibration during the thermal climax.
Vengono qui riportati e discussi i risultati sulla variabilità compositiva delle muscoviti, ottenuti nell'ambito di una ricerca sul chimismo di biotiti e muscoviti in metapeliti di basso grado. I risultati sulle biotiti sono stati l'oggetto di uno specifico lavoro, al quale si rimanda anche per la definizione del problema, Ia strategia di ricerca e le procedure. Le muscoviti dei letti granoblastici risultano sostanzialmente uguali a quelle dei letti Iepidoblastici, cosi come le muscoviti situate in zone di cerniera rispetto a quelle lungo i fianchi. Invece le muscoviti cristallizzate durante Cr3 sono compositivamente diverse da quelle cristallizzate durante Cr2, e denotano temperature lievemente più alte (pur essendo rimaste invariate le compatibilità mineralogiche). Quindi I risultati analitici indicano comportamenti analoghi per le muscoviti e Ie biotiti. In particolare, nei campioni nei quali gli effetti di Cr3 sono vistosi, il chimismo delle muscoviti Cr2 cosi come quello delle biotiti risulta modificato. Questo fatto, insieme ad alcune peculiarità compositive delle miche in funzione delle fasi a contatto, indica che occorre grande cautela nell'uso dei geotermobarometri in metapeliti di basso grado: lamelle di miche microstrutturahnente identiche possono fornire valori di T o P diversi a seconda delle fasi mineralogiche a contatto, delle dimensioni, e della maggiore o minore efficacia della riequilibrazione chimica durante il culmine termico.
Sulla variabilita compositiva delle miche nell'ambito di uno stesso affioramento di metapeliti di basso grado: un esempio. 2: Le muscoviti e considerazioni conclusive
Peruzzo L;
1992
Abstract
Compositional variability of micas within the same outcrop of low-grade metapelites: a casestudy. 2: The muscovites, and concluding remarks. The present paper reports and discusses the results on compositional variability of muscovites, obtained in the frame of a research project on the chemistry of biotites and muscovites in low-grade metapelites. The results on biotites have been reported in a separate paper, to which readers are deferred also because it gives the statement of the problem, the strategy of research and the analytical procedures. Muscovite flakes from granoblastic domains are substantially identical to those from lepidoblastic domains, as well as muscovite flakes located in the hinges of microfolds in comparison with those in the limbs. On the contrary, muscovite flakes crystallized during Cr2 compositionally differ from those crystallized during Cr3, in the latter case, muscovite and biotite chemistries suggest slightly higher temperatures although the mineral compatibilities are unchanged. The analytical results indicate a similar behaviour for muscovites and biotites. In particular, within rock samples in which Cr3 effects are widespread, the chemistries of Cr2 muscovites and biotites turn out to be changed. This fact, as well as some chemical peculiarities of the mica flakes depending on the different mineral phases in contact, indicates that use of geothermobarometry in low-grade metapelites requires caution: microtexturally identical mica flakes may give different T, P values depending on their size, the adjacent mineral phase, and the extent of chemical re-equilibration during the thermal climax.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
