Petrogenetic relationships of acid and basic rocks in iceland: Sr-isotopes and rare-earth elements in late and postglacial volcanics

Abstract

Strontium isotope ratios and rare-earth element abundances have been measured in acid, intermediate and basic rocks from three late to postglacial volcanic complexes, and several other postglacial basalts in Iceland. Late and postglacial basalts in Iceland have been generated from a source region which is essentially homogeneous with respect to 87Sr/ 86Sr. The mean 87Sr/ 86Sr ratio for the basalts analysed is 0.70328 and the range is from 0.70317 ± 6to0.70334 ± 5 (2σ). Acid rocks from the Kerlinganfjöll and Namafjall volcanic complexes have 87Sr/ 86Sr ratios which are indistinguishable from analysed basalts from the same complexes. However, intermediate and acid rocks from the Torfajökull complex have significantly higher 87Sr/ 86Sr ratios and could not have been derived by fractional crystallization from basaltic magmas similar to those found in the same complex. These latter rocks have most probably been produced by remelting of Tertiary gabbroic rocks in Layer 3. Most of the basalts analysed have higher total rare-earth element abundances than typical dredged ocean-ridge tholeiites, and show less light rare-earth depletion. Intermediate and acid compositions show overall higher abundances and light rare-earth enrichments. The measured rare-earth abundances are compared with abundances generated by differential partial melting of various model source regions. It is shown that both the tholeiitic and alkali basalt compositions could be generated from the same source material by different degrees of partial melting. Variable partial melting of gabbroic material may account for the rare-earth element abundances of both the rhyolitic rocks (small degrees of melting) and the intermediate rocks (more extensive melting).