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Geological Society, London, Special Publications; 2005; v. 243; p. 227-247;
DOI: 10.1144/GSL.SP.2005.243.01.16
© 2005 Geological Society of London

A critical assessment of the tectono-thermal memory of rocks and definition of tectono-metamorphic units: evidence from fabric and degree of metamorphic transformations

M. Iole Spalla1,2, Michele Zucali1, Silvia Di Paola1 & Guido Gosso1,2

1 Dipartimento di Scienze della Terra ‘A. Desio’, Università di Milano, Via Mangiagalli 34, 20133 Milano, Italy
2 C.N.R. — IDPA, Via Mangiagalli 34, I-20133 Milano, Italy

(e-mail: iole.spalla{at}unimi.it)

(e-mail: michele.zucali{at}unimi.it)

(e-mail: silvia.dipaola{at}unimi.it)

(e-mail: guido.gosso{at}unimi.it)

A correlation procedure of scattered tectonic and metamorphic imprints in the reactivated crust is elaborated from recent analytical work in three Alpine metamorphic complexes. It consists of: interpretation of the time-sequence of tectonic fabrics and test of their kinematic coherence; determination of paragenetic compatibility among the mineralogical support of mesoscopic fabrics; cross-validation of mineral transformation over-prints; construction of P-T-d-t paths using a time-sequence of parageneses. The representation of structural and metamorphic information conveys the full tectono-metamorphic history on maps displaying combined tectonic and metamorphic effects. Shape and size definition of metamorphic units, now individuated mainly using their lithological homogeneity and dominant metamorphic imprint, is improved. The analysis of interaction between fabric and metamorphic imprint distributions, proposed in three Alpine examples, shows that the dominant metamorphic imprint does not coincide with Tmax-PTmax of each inferred P-T-d-t loop; the dominant metamorphic imprint is that given by the mineralogical support of the most pervasive fabric. Different metamorphic imprints may dominate in adjacent areas of a single tectono-metamorphic unit (TMU), or equivalent metamorphic imprints may dominate in different TMUs. Therefore, lithostratigraphic setting and dominant metamorphic imprint are inefficient to contour TMUs in terrains with polyphase deformation and metamorphism, without considering multiscale heterogeneity of superposed synmetamorphic fabrics.