Decoupling of Mg from Sr-Nd isotopic compositions in Variscan subduction-related plutonic rocks from the Bohemian Massif: implications for mantle enrichment processes and genesis of orogenic ultrapotassic magmatic rocks
Abstrakt
The Moldanubian Zone of the Bohemian Massif was intruded by three Variscan (c. 354-335 Ma) subduction-related plutonic suites. Their mantle sources evolved from CHUR-like (low-K calc-alkaline suite), through slightly enriched (εNd346 ~ - 3; high-K calc-alkaline suite-HKCA) to strongly enriched (εNd337 > clastic metasedimentary material.
Subducted carbonates were of limited importance, as recorded by lowered δ26Mg values in several mafic UK samples and in a glimmerite vein cutting the peridotites (- 0.44%o). The Mg isotopic variation in less magnesian potassic rock types reflects a complex interplay between source heterogeneity, equilibrium fractionation during fractional crystallization, kinetic fractionation due to chemical diffusion during magma mixing and/or AFC-style contamination by carbonate-derived fluids.