Rare earth and trace element mobility in mid-crustal shear zones: insights from the Mont Blanc Massif (Western Alps)

Abstract

The behaviour of rare earth elements (REE) during fluid-rock interaction in mid-crustal shear zones has received little attention, despite their potential for mass balance calculation and isotopic tracing during deformation. In this study, several cases of large REE mobility during Alpine fluid-driven shear zone development in the pre-Alpine granitic basement of the Mont Blanc Massif are considered. On a regional scale, the undeformed granite compositions range within 5 wt% SiO2 (70.5-75.3 wt%) and magmatic chemical variations are of the order of 10-20%, ascribed to minor effects of crystal fractionation. Major and trace element mobility observed in shear zones largely exceeds these initial variations. Shear zones developed a range of mineral assemblages as a result of shearing at mid-crustal depths (at ∼0.5 GPa, 400°C). Five main shear zone assemblages involve muscovite, chlorite, epidote, actinolite and calcite, respectively, as major phases. In most cases, selective enrichments of light or heavy REE (and Y, Ta, Hf) are observed. REE mobility is unrelated to deformation style (cataclastic, mylonitic), the intensity of strain, and to the shear zone's major metamorphic mineral assemblages. Instead, the changes in REE concentrations are ascribed to the alteration of pre-existing magmatic REE-bearing minerals during deformation-related fluid-rock interaction and to the syntectonic precipitation of metamorphic REE-bearing minerals (mainly monazite, bastnäsite, aeschynite and tombarthite). Minor proportions (<2%) of these accessory phases, with grain sizes mostly <20 μm, account for enrichments of up to 5:1 compared to the initial granite whole-rock REE budget. The stability of the REE phases appears to be largely dependent on the altering fluid composition. REE mobility is ascribed to changes in pH and to the availability of CO32-, PO42-, and SO42-ligands in the fluid. Such processes are likely to influence the mobility of REE, Y, Hf and Ta in shear zones.