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 FeMg-bearing silicate (chlorite) ; (b) in narrow domains of the Brianconnais and Piemontais 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 Brianconnais andpiemontais 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, 0, 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: 6r80 = + 28%0 (SMOW) in the northern Paris Basin, + 26%0 in the Brianconnais zone, + 22%0 in the Piemontais zone and + 18%0 in the Dauphinois zone. Within each studied region, calcite G'*O-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. G'*O/GD-values of analysed hydrated minerals and whole rocks ( Si80 = + 15 to -t-25%0 and 6D = -60%0 SMOW)' lie within the field of Alpine cover rocks and of fluids in equilibrium with these rocks: interaction with an external '*O-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 within 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.

Trace element geochemistry during metamorphic dehydration: A case study from the Xingzi Group of Lushan, southeast China.

“…This last observation combined with the fact that some sheets of muscovite occur both in the veins and their immediate wallrocks, indicates that the veins are mineralogically equilibrated with the wallrocks, and hence the vein-forming materials are locally derived from the wallrocks. These field and petrographic features provide strong evidence that the veins formed during metamorphism, and hence can be regarded as synmetamorphic (Cesare, 1994;Henry et al, 1996). This conclusion is further confirmed by geochemical data presented in this paper.…”

“…Consequently, we argue that these consistent characteristics resulted from fractionation of these trace elements, which is strongly influenced by electron configuration and the nature of complexing ligands of aqueous fluids (Bau, 1996). In the data of Henry et al (1996), the fractionations of Nb-Ta and LuHf are also apparent (Kamber and Collerson, 2000). Another geochemical pair, Pb and Nd with the similar incompatibility and generally no significant fractionation in silicate-melt systems, is fractionated during metamorphic dehydration.…”

“…From these case studies, it can be inferred that fractionation of trace elements is most likely a commonly occurring process during metamorphic dehydration. Considering the various geochemical fractionations observed in this study (Nb-Ta, Zr-Hf, and Pb-Nd) and in Henry et al (1996) (Nb-Ta and Lu-Hf), we therefore infer that the fractionation of trace elements is mainly controlled by the chemical nature of fluids (closely related to protolith chemistry) and metamorphic conditions. From this consideration, the fact that no the REE tetrad effect is observed in the quartz veins at Xixiansi (Fig.…”

“…A number of studies have shown that quartz veins in metamorphic terrains must be precipitated from silica-rich fluids produced by dehydration during metamorphism (e.g., Yardley, 1983;Yardley and Bottrell, 1992;Ague, 1991;Henry et al, 1996;Wilkinson et al, 1996). In addition, some feldspar-bearing veins (Vidale, 1974), quartz-Al 2 SiO 5 veins (Putlitz et al, 2002), and high-pressure veins (Becker et al, 1999) in metamorphic rocks also been formed from fluids with the same origin as that of quartz veins.…”

“…Hence the unusual REE patterns of some minerals as well as their host rocks cannot be explained by the ionic radius effect, and can only be understood by referring to the electronic configuration of these elements, which plays an important role in the complexes they form in fluid-rich systems. Based on the data of Henry et al (1996), Kamber and Collerson (2000) recently showed that the REE fractionation (tetrad effect) also exists in synmetamorphic veins formed during metamorphic dehydration in the western Alps. Thus, it appears that the cases with the tetrad effect are closely related to fluids, and usually accompanied by the fractionation of trace elements with identical charge and similar radius (Liu et al, 1993;Bau, 1996;Irber, 1999;Minami et al, 1998).…”