Melt evolution beneath a rifted craton edge: 40Ar/39Ar geochronology and Sr–Nd–Hf–Pb isotope systematics of primitive alkaline basalts and lamprophyres from the SW Baltic Shield

摘要: A new high-precision Ar-40/Ar-39 anorthoclase feldspar age of 176.7 +/- 0.5 Ma (2-sigma) reveals that small-volume alkaline basaltic magmatism occurred at the rifted SW margin Baltic Shield in Scania (southern Sweden), a time global plate reorganization associated with inception Pangea supercontinent break-up. Our combined elemental and Sr-Nd-Hf-Pb isotope dataset for representative basanite nephelinite samples (>8 wt.% MgO) from 16 subvolcanic necks 30 by 40 km large Jurassic volcanic field suggests magma derivation moderately depleted mantle source (Sr-87/(86)Sri = 0.7034-0.7048; epsilon Nd-i +4.4 to +5.2; Hf-i +4.7 +8.1; Pb-206/(204)Pbi 18.8-19.5). The mafic melts segregated mixed peridotite-pyroxenite potential temperature similar 1400 degrees C 2.7-4.2 GPa (similar 90-120 depths), which places ultimate melt generation within convecting upper mantle, provided lithosphere-asthenosphere boundary beneath southern was <= 100 depth during Mesozoic-Cenozoic rifting. Isotopic shifts incompatible element enrichment relative Depleted Mantle reflect involvement least 20% recycled oceanic lithosphere component (i.e., pyroxenite) some minor continent-derived sediment partial melting well-stirred peridotite. Although pargasitic amphibole-rich metasomatized lithospheric is excluded as main magmas Scania, hydrous ultramafic veins hornblendite) may have caused subtle modifications compositions passing sublithospheric melts. For example, modeling more radiogenic Hf (epsilon +6.3 +8.1) Pb (Pb-206/Pb-204(i) 18.9-19.5) isotopic sodic H2O-rich nephelinites, compared relatively homogenous basanites +6.1; Pb-206/Pb-204(i) 18.8-18.9), originate interactions between rising asthenospheric metasomatic components. components were likely introduced extensive Permo-Carboniferous magmatic activity, scenario supported geochemical ca. 286 lamprophyres 0.7040-0.7054; +2.0 +3.1; +6.1 +9.0; 17.8-18.2). Strong variations thickness thermal structure across transient small-scale convection. This resulted fast focused upwellings lateral flow thinned lithosphere, where formed low decompression mantle. Such geodynamic would allow enriched points be preferentially sampled refractory when channeled along destabilizing craton edge. Similar 'lid effect' island provinces, architecture exert strong control on regime, thus offer simple explanation resemblance continental intraplate high Na/K affinity.