THE DISTRIBUTION OF RARE-EARTH ELEMENTS IN K-FELDSPAR AS AN INDICATOR OF PETROGENETIC PROCESSES IN GRANITIC PEGMATITES: EXAMPLES FROM TWO PEGMATITE FIELDS IN SOUTHERN NORWAY

Abstract

The K-feldspar from strongly zoned gadolinite-type REE–Nb–Ta-rich granitic pegmatites from the Evje–Iveland and Froland pegmatite fields in southern Norway was analyzed for the entire spectrum of rare-earth elements, as well as for a suite of majo r and trace elements, with the HR–LA–ICP–MS technique. Earlier results imply that the two pegmatite fields are nearly indistinguishable in terms of their origin and igneous evolution. An evaluation of the distribution of K, Rb, Sr and Ba in Kfeldspar, elements that are strongly sensitive to igneous processes in felsic rocks, confirm this conclusion. However, the distribution of the ultra-trace elements, and particularly the REE, in K-feldspar is incompatible with a common origin of the two fields. The REE distribution in K-feldspar is buffered by the coexisting REE-phases. When compared to Evje–Iveland, the Froland suite of pegmatites was derived from a relatively LREE-deficient and HREE-enriched parent melt. Finally, the distribution of trace elements in feldspar implies that the Evje–Iveland pegmatites formed from progressively more differentiated melts that were emplaced in a southward-propagating system of vertical and horizontal faults and fractures.