Sources of primitive alkaline volcanic rocks from the central European volcanic province (Rhon, Germany) inferred from Hf, Os and Pb isotopes

摘要: Basanites and nephelinites from the Tertiary Rhon area (Germany), which are part of Central European Volcanic Province (CEVP), have high MgO, Ni Cr contents prominent garnet signatures indicating that they represent near-primary magmas formed by melting a CO2-bearing peridotitic mantle source at pressure. The Pb Hf isotope (and previously published Nd Sr isotope) ratios lavas rather uniform, whereas Os composition is highly variable. For most primitive basanites, Pb, compositions fall within range enriched MORB some OIB. Other basanites with low concentrations distinctly more radiogenic (187Os/188Os: 0.160–0.469) compositions, inferred to originate crustal contamination. samples highest lowest (187Os/188Os(23 Ma): 0.132–0.135), likely remain unaffected Together their fairly depleted Sr, ratios, isotopic suggests derivation an asthenospheric source. Prominent negative K Rb anomalies, however, argue for amphibole or phlogopite-bearing sources, can only be stable in cold lithosphere. We therefore propose melts precipitated asthenosphere-lithosphere thermal boundary as veins lithospheric were remelted incorporated after short storage times (about 10–100 million years) ascending melts. Due residence time incorporation vein material imposes phlogopite/amphibole signature alkaline basalts but does not lead shift signatures. Melting cannot strictly excluded, has subordinate due lack respective signatures, good agreement thin lithosphere observed area. expected enriched, temperature portions upper (asthenospheric) do require upwelling deep-seated sources many other continental similar