FLUID-ROCK INTERACTION DURING FORMATION OF METAMORPHIC QUARTZ VEINS: A REE AND STABLE ISOTOPE STUDY FROM THE RHENISH MASSIF, GERMANY

摘要

We have investigated fluid-rock interaction processes during formation of metamorphic quartz veins that are abundant in the fold-and-thrust belt of the Rhenish Massif, southwest Germany. These veins record two successive assemblages that were formed in a different fluid-rock environment, which are (1) massive vein filling (elongateblocky quartz, chlorite, apatite, albite) and (2) open space filling (euhedral quartz crystals, ankerite/dolomite, calcite, sulfides). Building on previous work that studied the field relationships, mineralogy, vein textures, fluid inclusion and wall rock alteration features, we have performed a detailed REE and stable isotope investigation of vein minerals, altered wall rocks (selvages) and least altered host rock metapelites. The REE and oxygen isotope data of vein quartz and altered wall rocks, in conjunction with mass balance analysis, support the conclusion that local mobilization of material was dominant during formation of the early massive vein filling assemblage, but that contributions from advecting fluids were also important. The pronounced shift in K/Na ratios in altered wall rocks and model fluid temperatures that are substantially higher (350–400 ℃) than estimates for the surrounding host rocks clearly point to substantial fluid advection. Formation of the veins can be essentially explained by a crack-flow-seal model, which involves multiple repetition of vein opening, fluid advection and vein sealing events (consistent with the elongate-blocky textures of massive vein quartz). Each cycle was initiated with vein opening, resulting in enhanced permeability and considerable fluid advection leading to hydrothermal alteration of wall rocks. Conditions during each cycle then evolved towards a decrease in fluid advection, coupled with substantial diffusional leaching of silica from the wall rocks and precipitation in the veins. The formation of the later open space filling assemblage records a transition from an overall advection- to a diffusion-dominated regime. This is supported by vein mineral and fluid inclusion textures recording conditions of slow and undisturbed mineral growth, fluid inclusion data that point to a thermally equilibrated state (150-200 ℃), and stable isotope data that demonstrate a local source for the vein minerals.