Geochemical variations and element transfer during shear-zone development and related episyenites at middle crust depths: insights from the Mont Blanc granite (French — Italian Alps)

Abstract

This paper highlights the relationships between the formation of shear zones, associated quartz-rich veins and their quartz-depleted alteration haloes ('episyenites') that have formed in the Mont Blanc Massif during the Alpine orogeny. The shear zones are steeply dipping and formed late (18-13 Ma) during collisional orogeny, at mid-crustal depths (5 + 1 kbar, 400 + 50 ~ during uplift of the Mont Blanc Massif. Between the shear zones, nearly undeformed granite contains widely dispersed, subhorizontal veins with a quartz-dominant quartz + albite + chlorite + adularia assemblage. They do not intersect the shear zones and are surrounded by quartz-depleted alteration haloes up to several metres wide. The compositions of the shear zones and the vein-alteration haloes (episyenites) show substantial departures from the bulk composition of the host rock. Shear zones are characterized by greenschist facies assemblages (epidote-, chlorite-or K-white-micabearing assemblages). Each shear zone type is featured by a specific chemical change: depletions in K20, and enrichments in Fe203 and CaO (epidote-); with depletions in CaO, Na20, K20 and slight SiO2 enrichments (white mica-chlorite-); with depletions in SiO2, CaO, Na20, K20 and enrichments in MgO (phlogopite-chlorite shear zones). Episyenites are characterized by chemically induced porosity enhancement due to dissolution of magmatic quartz and biotite, with subsequent partial infilling of pore spaces by quartz, chlorite, albite and adularia. The vein arrays have accommodated minor vertical stretching in the Mont Blanc Massif, probably at the same time as the adjacent shear zones were accommodating more substantial vertical stretching in the massif. Coupled quartz dissolution in the wallrock alteration haloes and quartz precipitation in veins could be interpreted to reflect local mass transfer between wallrock and veins during essentially closed-system behaviour in the relatively undeformed granite domains between shear zones. In contrast, shear zones probably develop in opened systems due to their kilometric length.Studies of fluid-rock interaction processes during deformation in metamorphic rock provide insights about fluid circulation in the middle-lower continental crust. It also provides constraints to estimate the magnitude of mass transfer during orogenic events. Mass balance calculations during fluid-rock interaction are easier when based on rocks of homogeneous composition at regional scale. This is rather common in many granitic massifs and therefore granitic rocks are good candidates for such studies.In metamorphosed granites, fluid-rock interactions are mainly localized along shear zones,

Miocene to Messinian deformation and hydrothermal activity in a pre-Alpine basement massif of the French western Alps: new U-Th-Pb and argon ages from the Lauzière massif

“…This thrust was active until 4 Ma as shown by fission track data. These ages are discussed by another group of workers [Rolland et al, 2005; who claims that the dense network of shear zones (bearing vertical stretching lineations) that affects the Mont Blanc granite are 15 Ma old (Ar-Ar data) -the shear zones are associated with an intense hydrothermal activity and with the opening of flat-lying quartz veins filled with vertical quartz fibres similar to the situation described in this paper for the Lauzière set.…”

“…15 Ma old shear zones with vertical stretching lineations associated with horizontal veins in the Mont Blanc massif [Rossi, 2005;Rolland et al, 2008]. We did not observe in Lauzière the shear zone pattern described in the Aar massif [Marquer, 1987;Choukroune and Gapais, 1983], and in the Mont Blanc massif [Rolland et al, 2003;Rossi et al, 2005]. An age of 14.2 ± 0.6 Ma (K/Ar on phengite) was obtained from a flat-lying vein oriented N118E, 35 o SW in the Arc-Isère gallery [Gasquet, 1979] in the neighbouring Belledonne massif;…”

Mylonites of the South Armorican Shear Zone: Insights for crustal-scale fluid flow and water–rock interaction processes

“…In both GUI and CAL samples, deformation occurred in the Bt stability field, at temperatures above 400 • C (e.g. Rossi et al, 2005). PLU samples are rather similar to the CAL ones, displaying fish-like Ms embedded in a fine-grained matrix composed by a Qtz + Kfs + Pl + Ms + Bt assemblage ( Fig.…”

The structural history of the Mont Blanc massif with regard to models for its recent exhumation

“…Significant Alpine strain was accommodated along localized mylonitic to phyllonitic shear zones (e.g. Bellière 1956Bellière , 1988Guermani and Pennacchioni 1998;Rossi et al 2005;Rolland et al 2008). Greenschist facies overprint is common close to the overturned basement-cover contact in the Chamonix zone and at the Faille du Midi, which largely follows the granitegneiss boundary and only in the south runs through the preVariscan basement gneiss (von Raumer 1971; Leloup et al 2005).…”