Evolution of synmetamorphic veins and their wallrocks through a Western Alps transect: no evidence for large-scale fluid flow. Stable isotope, major- and trace-element systematics

Abstract

Quartz-rich synfolial veins and wallrocks from different areas of a Western Alps cross-section are examined in an attempt to constrain the scale and mechanisms of fluid flow through metamorphic terrains. Different portions of this cross-section underwent distinct P-T evolutions as reflected by the mineralogy of veins: (a) in the greenschist external Dauphinois domain, veins are characterized by quartz and calcite, together with an aluminosilicate (pyrophyllite), NaK-bearing phyllosilicates and a FeMg-bearing silicate (chlorite); (b) in narrow domains of the Briançonnais and Piémontais zones, with a non-eclogitic HPLT evolution followed by rapid cooling, veins are characterized by quartz, calcite, Ca- and FeMg-bearing silicates (lawsonite and FeMg-carpholite, respectively); and (c) in the major part of the Briançonnais and Piémontais zones, where low-grade blueschistfacies metamorphism is followed by a greenschist-facies evolution, veins are characterized by the replacement of earlier lawsonite and carpholite by Na- and K-bearing micas. In order to constrain the provenance of fluids involved in vein formation, we determined stable isotope (C, O, H), and major- and trace-element compositions in minerals and whole rocks for a large set of veins and wallrocks. These rocks are compared with some non-metamorphic shales from the northern Paris Basin, considered as protoliths of the Alpine schists. Stable isotope compositions of calcites are regionally distinct: δ 18O ≈ 28% (SMOW) in the northern Paris Basin, +26% in the Briançonnais zone, +22% in the Piémontais zone and + 18% in the Dauphinois zone. Within each studied region, calcite δ 18O-values are quite homogeneous regardless of rock type (vein or wallrock). This feature is explained as an isotopic “homogenization” with the most abundant lithology in each zone (“rock-buffered system”) — where the overall ratio of calcite (of marine origin) to detrital silicate minerals determines the final isotopic composition of the whole rock (w.r.) after equilibration during Alpine metamorphism. δ 18O/ δD-values of analysed hydrated minerals and whole rocks ( δ 18O ≈ + 15 to + 25% and δD ≈ −60% SMOW) lie within the field of Alpine cover rocks and of fluids in equilibrium with these rocks: interaction with an external 18O- or D-depleted fluid, such as hydrothermal waters, meteoric water or “basement fluid”, can be ruled out. Major- and trace-element systematics indicate an enrichment in Si, Ca, P, Sr, Y, Ta and locally Eu in the veins with respect to the wallrock, related to massive crystallization of quartz and carbonates, together with phosphate and oxide. Potential fluid sources, chemical composition of the fluid phase and the mechanisms of fluid flow during vein formation are discussed. Fluid flow takes place which tectonic units, at both hectometric and decimetric scale. Fluid may also have migrated along and across the major tectonic contacts but this process is poorly documented.