STRONTIUM AND NEODYMIUM ISOTOPE STUDY OF BOHEMIAN BASALTS

The Cenozoic alkaline basalts of northern and western Bohemia are part of the Central European Volcanic Province (CEVP) which extends from t he Rhineland (Eifel, Germany) to Moravia (Czechoslovakia) and the Lowe r Silesia (Poland). Seven samples from locations within Czechoslovakia have been analyzed isotopically for the Rb-Sr and the Sm-Nd systems. Present-day, normalised 87Sr/86Sr ratios range from 0.7031 to 0.7036, and the corresponding 143Nd/144Nd ratios range from 0.51279 to 0.51286 . An eigth sample from the Silurian basalts of the 'St. Jan' type (K-A r age: 420 Ma) occuring in the Barrandian basin in Central Bohemia is also analysed. Its present-day, normalised, 87Sr/86Sr ratio is 0.7031, and the corresponding 143Nd/144Nd ratio 0.51288. The Nd ratios of the Cenozoic basalts are similar, but more restricted than those from Ger many, but are lower than those from Lower Silesia; a trend which is co nverse for the Sr ratios. Comparison of the results with the systemati cs of Zindler and Hart (1986), suggests that the mantle reservoir sour ce of the Bohemian Cenozoic CEVP basalts is similar to the HIMU (High- mu ocean island basalts), with transition to PUM (primitive upper mant le) or BSE (bulk silicate earth). The reservoir for the Silurian Barra ndian basin basalts suggest some affinity to MORB (mid-ocean ridge bas alts) or HIMU.