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

Abstract

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.

A new petro-structural map of the Monte Mucrone metagranitoids (Sesia-Lanzo Zone, Western Alps)

“…The mineralogical support of the new grain-scale planar and linear fabrics is the primary reference to select mineral compatibilities marking each successive foliation (Hobbs, Means, & Williams, 1976;Passchier & Trouw, 2005;Spalla, Siletto, Di Paola, & Gosso, 2000;Spalla, Zucali, Di Paola, & Gosso, 2005;Turner & Weiss, 1963;Williams, 1985). The inferred lithostratigraphic configuration has been represented on the 'drift and solid' map (in which surfacing rocks are separated by drift cover), and the full pattern of the superposed structures has been reported on the 'solid' map and on the cross-section grid.…”

Pre-Alpine contrasting tectono-metamorphic evolutions within the Southern Steep Belt, Central Alps

“…The LCN consists of low to medium grade muscovite-biotiteand staurolite-bearing gneisses and micaschists with interlayered amphibolites, marbles, quartzites and pegmatites. The TS consists of higher grade sillimanite-bearing gneisses and micaschists, garnet-and biotite-bearing amphibolites, marbles and pegmatites (Bonsignore and Ragni, 1966;Venzo et al, 1971;Gazzola et al, 2000;Spalla et al, 2005). Several post-Variscan acidic and basic intrusives (granitoids, diorites and minor gabbros) occur in both units (Knig, 1964;Ragni, 1966, 1968;Venzo et al, 1971;Del Moro et al, 1981;Bockemhl and Pfister, 1985;Tribuzio et al, 1999;Petri et al, 2017).…”

“…Therefore, a working method which enables researchers to recognise distinctive rock units with unique structural and metamorphic histories (the tectono-metamorphic units -TMUs, Spalla et al, 2000;Zucali, 2001;Spalla et al, 2005Spalla et al, , 2010Gosso et al, 2015) is fundamental.…”

Structural and petrographic map of the Sassa gabbro complex (Dent Blanche nappe, Austroalpine tectonic system, Western Alps, Italy)

“…The lower continental crust rocks of the upper Figure 1. Flow chart showing how to delimit tectonic units in poly-deformed and poly-metamorphic terranes; redrawn after Spalla et al (2005). elements are closely similar to those of the Ivrea Zone (Southern Alps; Figure 2). This affinity suggests that Austroalpine and Southalpine domains were continuous (Adria Alpine hinterland) during Alpine convergence (Canepa et al, 1990;Compagnoni et al, 1977;Diehl, Masson, & Stutz, 1952;Stutz & Masson, 1938;Vuichard, 1989).…”

“…The resulting geological maps should synthesize: (i) the grids of superposed foliation trajectories with related lineations; and (ii) the mineral assemblages marking successive fabrics, useful to unravel the metamorphic environment, which assists or overprints each deformation event (e.g., Austrheim, 1990;Connors & Lister, 1995;Gosso et al, 1983;Johnson & Duncan, 1992;Spalla, Siletto, Di Paola, & Gosso, 2000;Spalla, Zanoni, Williams, & Gosso, 2011;Williams, 1985;Zanoni, 2010;Zucali, Spalla, & Gosso, 2002). Such maps represent an effective correlation tool for determining structural and metamorphic regional events to identify shape and size of tectonic units (Figure 1) in polydeformed and polymetamorphic terrains (e.g., Spalla, Gosso, Marotta, Zucali, & Salvi, 2010;Spalla, Zucali, Di Paola, & Gosso, 2005).…”