Plio-Pleistocene basanitic and melilititic series of the Bohemian Massif: K-Ar ages, major/trace element and Sr–Nd isotopic data
Abstrakt
The Plio-Pleistocene volcanic rocks of the Bohemian Massif comprise a compositional spectrum involving two series: an older basanitic series (6.0–0.8 Ma) and a younger, melilititic series (1.0–0.26 Ma). The former consists of relatively undifferentiated basaltic rocks, slightly silica-undersaturated, with Mg# ranging from 62 to almost primitive mantle-type values of 74.
The major and trace element characteristics correspond to those of primitive intra-plate alkaline volcanic rocks from a common sub-lithospheric mantle source (European Asthenospheric Reservoir – EAR) including positive Nb, and negative K and Pb anomalies. 87Sr/86Sr ratios of 0.7032–0.7034 and 143Nd/144Nd of 0.51285–0.51288 indicate a moderately depleted mantle source as for other mafic rocks of the central European volcanic province with signs of HIMU-like characteristics commonly attributed to recycling of subducted oceanic crust in the upper mantle during the Variscan orogeny. The melilititic series is characterized by higher degrees of silica-undersaturation, and high Mg# of 68–72 values, compatible with primitive-mantle-derived compositions.
The high OIB-like Ce/Pb (19–47) and Nb/U (32–53) ratios indicate that assimilation of crustal material was negligible. In both series, concentrations of incompatible elements are mildly elevated and 87Sr/86Sr ratios (0.7034–0.7036) and 143Nd/144Nd ratios (0.51285–0.51288) overlap.
Variations in incompatible element concentrations and isotopic compositions in the basanitic series and melilititic series can be explained by a lower degree of mantle melting for the latter with preferential melting of enriched mantle domains. The Sr and Nd isotopic compositions of both rock series are similar to those of the EAR.