Blueschist facies metamorphism of peralkaline rhyolites from the Tenda crystalline massif (northern Corsica): evidence for involvement in the Alpine subduction event?

Abstract

The Tenda crystalline massif (northern Corsica) is a fragment of the western Corsica basement involved in the Alpine orogeny. Rhyolite dykes crosscutting the gabbroic complex of Bocca di Tenda (southern sector of the Tenda crystalline massif) show an unusual metamorphic mineral assemblage, defined by jadeite‐bearing (up to 46 mol percentage) aegirine, riebeckite, celadonite‐rich phengite (Si=3.50–3.65 apfu), quartz, albite and K‐feldspar. Jadeite‐bearing aegirine and riebeckite mostly occur as coronas around jadeite‐free aegirine and arfvedsonite, respectively, which both are relics of igneous origin. This metamorphic assemblage reflects the peralkaline compositions, which are characterised by anomalously high contents of SiO2 and Na2O, and negligible CaO and MgO. The evolved rocks of the gabbroic sequence (quartz‐diorites to tonalites) and the surrounding granitoids are characterised by the development of riebeckite/ferroglaucophane, epidote, celadonite‐rich phengite and albite, thus pointing to a metamorphic crystallization in the epidote‐blueschist facies. In all the studied rocks, metamorphic reactions were controlled by fluid‐assisted mass‐transfer through grain boundaries and microfractures. The different mineral assemblages allow the peak P–T metamorphic conditions to be constrained to between 0.8 GPa/300 °C and 1.1 GPa/500 °C. These estimates attest to a geothermal gradient (dT/dP) of 10–13 °C km−1 and indicate that the Tenda crystalline massif was buried to a minimum depth of 27 km during the Alpine orogeny. The blueschist facies recrystallization in the Tenda crystalline massif has been related to the cessation of an eastward‐dipping subduction event.

High‐pressure mineral assemblage in granitic rocks from continental units, Alpine Corsica, France

“…In these slices, the metamorphism ranges from low-grade blueschist facies (T ¼ 300-370 C and P ¼ 0.50-0.80 GPa; slices of Corte and Popolasca area) to epidote-blueschist facies (T ¼ 300-500 C and P ¼ 0.80-1.10 GPa; Tenda Massif). This picture supports the hypothesis that large portions of the Corsican continental basement were deformed under HP/LT metamorphic conditions during their involvement in the tectonics connected with Alpine subduction (Amaudric du Chaffault et al 1976;Bézert and Caby 1988;Tribuzio and Giacomini 2002;Molli and Tribuzio 2004) and were subsequently juxtaposed against the metamorphic and non-metamorphic oceanic units during exhumation (e.g. Malavieille et al 1998).…”

“…Amaudric du Chaffault et al 1976), but it has generally been under-evaluated, not only with respect to the pressure-temperature (P-T) conditions of the climax, but also with respect to the extent of the units affected by this metamorphism. Recently, Tribuzio and Giacomini (2002) and Molli and Tribuzio (2004) have demonstrated that the rocks of the Tenda continental massif experienced higher pressures than previously suggested. In this paper, we use detailed geological mapping, structural analysis and thermobarometry to provide further evidence for HP/LT metamorphism in Alpine Corsica.…”

“…However, the lack of phlogopite (or biotite) in the assemblage means that the method yields only minimum pressure estimates (Tribuzio and Giacomini 2002 Figure 6. Composition of sodic amphiboles, using the classification of Leake et al (1997).…”

Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

“…centre of the pole figure) from the host-grain orientation (e.g. Van Daalen et al, 1999;Menegon et al, 2011). This is confirmed by the boundary trace analysis (Prior et al, 2002;Menegon et al, 2010) of the two main sets of straight low-angle boundaries defining localized extinction bands (parallel to bands of recrystallized grains), one running ENE-WSW (subset 1) and one running ca.…”

Alpine metamorphic and tectonic evolution of the Inzecca‐Ghisoni area (southern Alpine Corsica, France)

“…Many recent works carried out on the Alpine Corsica pointed out that Tertiary deformations associated to highpressure/low-temperature (HP/LT) metamorphism occurred not only in the oceanic metamorphic units, that is Schistes Lustrés, as it has long been known (Lahondère and Guerrot 1997;Brunet et al 2000 and references therein), but also in the tectonic slices derived from the continental margin of the European Plate (Amaudric du Chaffaut et al 1976;Amaudric du Chaffaut and Saliot 1979;Bezert andCaby 1988, 1989;Tribuzio and Giacomini 2002;Molli and Tribuzio 2004;Malasoma et al 2006;Molli et al 2006).…”