Rb-Sr isotopic composition of granites in the Western Krun, hory/Erzgebirge pluton, Central Europe: record of variations in source lithologies, mafic magma input and postmagmatic hydrothermal events
Abstrakt
The late Variscan (327-318 Ma) Western Krun, hory/Erzgebirge pluton (Czech Republic and Germany) represents a multiply emplaced intrusive sequence ranging from low-F biotite monzogranites (with rare minor bodies of gabbrodiorites and granodiorites) to high-F topaz-zinnwaldite alkali-feldspar granites. This granite suite is characterized by progressively increasing concentrations of incompatible elements (Li, Rb, F), monotonous decrease in mafic components and compatible elements (FeOtot, MgO, TiO2, CaO, Sr) with increasing silica.
Consequently, this leads to extreme variations in the Rb/Sr ratios (0.52 to 59), which impose highly variable Rb-87/Sr-86 and Sr-87/Sr-86 signatures. The low-F biotite monzogranites represent isotopically heterogeneous mixture with (Sr-87/Sr-86)(323) = 0.707-0.709 between partial melts from the Saxothuringian metasediments and mantle-derived mafic precursors.
The medium-F two-mica microgranites show variable (Sr-87/Sr-86)(323) = 0.708-0.714, indicating involvement of multiple precursors and more mature crustal protoliths. The evolved high-F topaz-zinnwaldite alkali-feldspar granites were derived from a precursor with (Sr-87/Sr-86)(320) = 0.707-0.708 at 324-317 Ma by differentiation, which produced the extreme Rb/Sr enrichment and variations.
The Li/Rb ratios remain nearly constant (similar to 0.5), thus insensitive to the degree of geochemical differentiation. In comparison to terrestrial variations, the high Li/Rb values indicate derivation of granitic magmas from predominantly sedimentary precursors, in accord with Li-7-Li-6 and Nd-143-Nd-144 isotope composition reported previously.
The Rb-Sr element variations in each granite unit are sligthly different and indicate ascent and emplacement of separate magma batches, which do not form a single liquid line of descent.