Early Permian rhyolite volcanism and Jurassic alteration, Northern Schwarzwald, Germany – new evidence from geochemistry and U-Pb dating of zircon, monazite, uraninite, and xenotime

Formation of Early Permian rhyolites in the Schwarzwald postdates the exhumation of Variscan granites and gneisses. Although similar Permian volcanics are recognized across Central Europe, little is known so far about their source(s), precise ages, magmatic evolution, and later alteration. To answer these questions, we investigated 68 rhyolite samples (lavas, tuffs, ignimbrites, diatremes) from five different localities in the Northern Schwarzwald.

Most rhyolite samples contain phenocrysts of quartz, K-feldspar, and biotite set in a fine grained groundmass rich in secondary illite and hematite. Igneous zircon, monazite, Fe-Ti oxides, apatite, and uraninite are common accessories. All samples are peraluminous high-SiO₂ rhyolites with calc-alkaline compositions. Mantle-normalized trace element patterns show negative Ba, Sr, Eu, and Ti anomalies, indicating plagioclase and Fe-Ti oxide fractionation. REE patterns with LREE enrichment and deep negative Eu anomalies characterize them as hot-dry-reduced rhyolites, typically occurring above areas of mantle upwelling. A likely source are lower crustal dry metasediments.

Secondary alteration is correlated with increases in Al₂O₃, LOI, Li, and As. At one locality, a unique secondary REE-P-As-Th-S assemblage, comprising monazite, gasparite, thorite, xenotime, and galenite, is observed. The As-REE-rich phases presumably originated from alteration of primary monazite, zircon, and apatite by As-rich aqueous fluids.

U-Pb La-ICP-MS dating of igneous zircon, monazite, and uraninite from different localities yields consistent crystallization ages of 296.0 ± 1.3, 297.5 ± 2.9 Ma, and 296.2 ± 1.1, respectively. A later Early Jurassic alteration event is evidenced by U-Pb ages of 181.5 ± 2.1 and 173.5 ± 1.6 Ma determined for secondary xenotime.