Heavy rare-earth element enrichment in granites of the Aar Massif (Central Alps, Switzerland)

Abstract

The paper presents rare-earth element (REE) determinations of whole rocks and minerals of the Central Aar Granite, a Hercynian composite granite intrusion in the Aar Massif (Central Alps), consisting mainly of granodiorites, granites and leucogranites. Their geochemical differentation is characterized by a slight depletion of light REE (LREE) and Eu, and a clear enrichment of heavy REE (HREE). This evolution leads to HREE-enriched leucogranites with typical “wing-shaped” distribution patterns in chondrite-normalized diagrams. These distribution patterns can be explained either by pure fractional crystallization of K-feldspar and/or by crystallization of accessory phases that did not deplete the melt in HREE. Due to their affinity to highly polymerized high-silica melts, the latter stayed in the liquid phase until they were incorporated in solids during the final stage of bulk magma crystallization. The REE contents of analyzed mineral phases (allanite, apatite, biotite, epidote, garnet and zircon) are used to calculate a REE budget of the whole rock. These data suggest that existence of further HREE-rich accessory mineral phases in the felsic rock compositions, which have not been analyzed. Qualitative X-ray and EDS analyses revealed the existence of accessory minerals like thorite, chernovite, an Y-Ca mineral and polycrase. These minerals may play an outstanding role in the distribution of REE, U and Th in high-silica melts.