The chemical evolution of Earth’s crust over time is controlled by first-order processes, including erosion, sediment dispersion on the continent and transport to the continental margin, recycling during subduction and/or continental collision by fluids and melts with return to the convecting mantle or to the crust. The geochemical and isotopic compositions of crustal rocks represent formidable tracers of the source of materials and of the processes forming and/or altering these crustal units, including mineral deposits. The Variscan orogen provides an excellent case study area for material cycling in orogens since unmetamorphosed protoliths are locally preserved and their equivalents can be followed through the prograde metamorphic sequence to the end stage of partial melting and granite formation. Furthermore, the Variscan orogen in Europe hosts a variety of important ore deposits whose formation is a direct result of element cycling and related element enrichment.
The contents and isotopic compositions of the fluid-mobile elements boron and lithium are widely used as tracers of fluid-rock interaction during prograde and retrograde metamorphism and of weathering and hydrothermal alteration. We calibrated the partitioning and isotopic fractionation of Li and B between minerals and fluids using synthetic systems (in cooperation with Section 4.3) and combined the results of these experimental studies with well-constrained field studies to demonstrate (i) mineralogically controlled decoupling of the Li and B isotopic composition; (ii) that prograde metamorphic loss of Li and B is controlled by phase stability rather than by Rayleigh-type loss.