Magali Rossi

40 AR/ 39 AR dating of synkinematic white mica; insights from fluid-rock reaction in low-grade shear zones (Mont Blanc Massif) and constraints on timing of deformation in the NW external Alps

Abstract This 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.

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 °C) 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 K2O, and enrichments in Fe2O3 and CaO (epidote-); with depletions in CaO, Na2O, K2O and slight SiO2 enrichments (white mica-chlorite-); with depletions in SiO2, CaO, Na2O, K2O 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 underformed granite domains between shear zones. In contrast, shear zones probably develop in opened systems due to their kilometric length.

Comment on “Alpine thermal and structural evolution of the highest external crystalline massif : The Mont Blanc” by P. H. Leloup, N. Arnaud, E. R. Sobel, and R. Lacassin

In this comment we discuss the approach used and the significance of Ar-Ar dating of synkinematic phengite within low-grade Alpine shear zones, and we comment the geodynamic models that can be derived from this method. The paper by Leloup et al. [2005] is a good step forward in the tectonic comprehension of the Mont Blanc area and provides a good synthesis of preexisting data. Leloup et al. [2005] have proposed a polyphase Alpine history for the Mont Blanc Massif (west Alps) based on a multidisciplinary approach: Ar-Ar on biotite for the higher pressure-temperature events of the Mont Blanc, and Ar-Ar on K-feldspar, fission tracks (FT) on zircon and apatite for its later exhumation stages. However, at this point of our knowledge of Alpine deformation in the Mont Blanc Range, the polyphased tectonic evolution, in particular the timing of thrust and back thrust events are not in agreement with recently obtained Ar-Ar data.

Stable isotope and Ar/Ar evidence of prolonged multi-scale fluid flow during exhumation of orogenic crust: example from the Mont Blanc and Aar massifs (NW Alps)

The spatial and temporal scales and the geometry of fluid pathways in a collisional orogen are investigated using stable isotope analysis (O, C, and H) and 40Ar/39Ar dating of vein minerals formed at circa 11–16 Ma in the Mont Blanc and the Aar External Crystalline Massifs. In both massifs 40Ar/39Ar dating of veins adularia provides evidence for progressive crystallization from 16 to 9 Ma, and mainly at 11–12 Ma following veins opening during shear zone activity. The fluid flow duration thus ranges from 4 to 5 Ma in the two massifs. The δ18O values of vein quartz and calcite are similar to those of undeformed crystalline and sedimentary host rocks, suggesting rock buffering, while carbon isotope ratios of vein calcites fall into three compositional groups. A-type veins have δ13C values that are buffered by the Helvetic metasediments, which suggests that these veins formed in a closed system from a locally derived CO2-rich fluid. The fluid in equilibrium with C-type veins has depleted δ13C values similar to mantle-CO2, while the intermediate δ13C values of B-type veins suggest mixing between the A-type and C-type fluids. These results are in agreement with crustal- to lithosphere-scale upward vertical fluid flow along vertical shear zones related to the strike-slip system bounding the Adriatic block since 16–20 Ma, connecting a deep-seated fluid to some downward flow in the sedimentary cover of External Crystalline Massifs.

Influence of time, temperature, confining pressure and fluid content on the experimental compaction of spherical grains

Theoretical models of compaction processes, such as for example intergranular pressure-solution (IPS), focus on deformation occurring at the contacts between spherical grains that constitute an aggregate. In order to investigate the applicability of such models, and to quantify the deformation of particles within an aggregate, isostatic experiments were performed in cold-sealed vessels on glass sphere aggregates at 200 MPa confining pressure and 350°C with varying amounts of fluid.

The Polymetallic (W–Au and Pb–Zn–Ag) Tighza District (Central Morocco): Ages of Magmatic and Hydrothermal Events

The W–Au, Pb–Zn–Ag, and Sb–Ba deposits of the polymetallic Tighza-Jbel Aouam district (central Meseta, Morocco), hosted in Paleozoic rocks surrounding late Variscan granite stocks, have been considered of magmatic-hydrothermal origin. The spatial distribution of the mineralization was attributed in early studies to zoning around a supposed hidden batholith. New geophysical data (El Dursi 2009) and U/Pb geochronology on zircon and monazite grains (this study) allow revision of this model, giving insights of a more complex setting and history for the Tighza-Jbel Aouam district. The W–Au mineralization formed at 295–280 Ma and is related to a magmatic event visible only in a large hydrothermal biotitic alteration halo, thus suggesting the presence of a hidden batholith. This mineralization cuts the granitic stocks that are dated at 320–300 Ma. From the occurrence of large veins, stockworks, sheeted veins, and disseminations in skarns, the W–Au deposit is considered similar to a porphyry-type deposit. The currently mined Pb–Zn–Ag deposit, which is spatially separated from the W–Au deposit, developed during an epithermal magmatic-hydrothermal episode dated at 254 ± 16 Ma. The polymetallic district of Tighza-Jbel Aouam thus appears to contain Cordilleran-style, porphyry-type mineralization (W–Au) followed by epithermal mineralization (Pb–Zn–Ag), both being related to pulses of calc-alkaline magmatism. Late Variscan and Permo-Triassic transpressive tectonics in the region localized magma emplacement and the generation of genetically associated hydrothermal fluids, with the magmas originating in the mantle and the continental crust.