Highly evolved miaskitic syenites deciphering the origin and nature of enriched mantle source of ultrapotassic magmatism in the Variscan orogenic root (Bohemian Massif, Moldanubian Zone)

A detailed study of alkali-rich syenites from the Gföhl unit Moldanubian Zone in Bohemian Massif provides constraints on exact origin and character mantle source ultrapotassic magmatism Variscan orogenic root Central Europe. The are characterized by highly alkaline composition (K2O 11.3–12.7 wt%; K2O/Na2O 6.8–7.5), crust-like isotopic signatures (87Sr/86Sr335 ∼ 0.7116; εNd335 ≤ −7.8), significant trace element enrichment (Th, U, Zr, Hf, LILE, LREE). They host an extraordinary amount zircon (0.5–5 vol%) along with subordinate titanite apatite, reflecting their miaskitic affinity. Mineral chemistry suggests that formed through fractional crystallization accumulation reduced, metaluminous to slightly peraluminous mantle-derived magmas. Whole-rock geochemistry Sr-Nd isotopes provide direct evidence they were produced partial melting metasomatic phlogopite-bearing vein network (glimmerites) lithospheric mantle, generated interaction (U)HP fluids/melts derived deeply subducted crustal material (Moldanubian granulites) wall-rock peridotites. Low-degree pure component unusual geochemical syenites, compared common rocks Massif, relatively higher degrees mixed glimmerite-peridotite source. emplacement was almost contemporaneous closely followed initial fast cooling dated 40Ar/39Ar amphibole-biotite ages ranging between 329.8 ± 1.6 331.4 0.7 Ma, corresponding rapid exhumation U-Pb apatite age 305.9 5.3 Ma likely reflects further lower temperatures. syenite linked early impulse associated Andean-type subduction Saxothuringian domain beneath block. close temporal association K-rich is indicated identical other Saxonian rocks, as revealed dates around 330 Ma.