International audienceThis paper highlights the use of synkinematic white mica, biotite and phlogopite for the dating of deformation in ductile shear zones within crystalline rocks under low-grade metamorphic conditions. The Mont Blanc shear zones range from 1 mm to 50 m in width and have localized intense fluid flow, resulting in substantial differences in mineralogy and whole-rock geochemistry. On the basis of their synkinematic alteration assemblages and geographic distribution within the Mont Blanc Massif, three main metamorphic zones are distinguished within the network of shear zones. These are: (i) epidote±white mica-bearing assemblages; (ii) chlorite–phlogopite-bearing assemblages; and (iii) white mica±biotite±calcite±actinolite±epidote- bearing assemblages. 40Ar/39Ar age spectra of biotite and phlogopite are complex, and reflect significant variations in chemical composition. In biotite, this is partly due to inheritance from precursor Variscan magmatic biotite. In contrast, new white mica grew at the expense of feldspar during Alpine deformation and its Ar spectra do not show any excess 40Ar. On the SE side of Mont Blanc, ages of shear zone phengites have a narrow range of 15.8–16.0±0.2 Ma, which is in the same age range as 40Ar/39Ar ages of minerals from kinematically related veins. The top-to-SE sense of shear is consistent with initiation of a Mont Blanc flower-structure within a dextral transpressional system by 16 Ma. On the NW side, mini-plateaux ages of 14.5±0.3 and 23.4±0.4 Ma are preserved in the same sample, suggesting the possibility of two phases of deformation. This is also supported by partly preserved ages of 18–36.6 Ma in biotites and phlogopites. Ages between 36 and 18 Ma might reflect ongoing top-to-NW thrusting, following Penninic Front activation, in a context of nappe stacking and crustal thickening. NW-directed thrusting on the NW side of Mont Blanc continued after 18 Ma, synchronous with SE-directed thrusting on the SE side of the massif. These divergent movements produced the overall pop-up geometry of the Mont Blanc Massif, which may correspond to a positive flower structure developed within a zone of regional dextral transpression extending SW from the Rhone valley into the Mont Blanc area
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,
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