Rare earth abundances in the eastern Alpine peridotites, Nonsberg area, Northern Italy
Morten, Lauro; Obata, Masaaki
European Journal of Mineralogy Volume 2 Number 5 (1990), p. 643 - 654
published: Oct 4, 1990
manuscript accepted: Apr 11, 1990
manuscript received: Nov 21, 1988
ArtNo. ESP147050205008, Price: 29.00 €
Abstract REE and minor element analyses were carried out on spinel lherzolite, garnet-bearing and garnetfree peridotites, garnet pyroxenites and some separated minerals from the Nonsberg area, Eastern Alps, Northern Italy. Chondrite normalized whole-rock REE patterns for all but one of the coarse-grained spinel lherzolites are very different from previously reported Alpine peridotites. The spinel lherzolites from the Nonsberg are LREE enriched (La/Yb > 1) and show a distinct U-shaped REE pattern. The REE abundances in whole rocks and minerals - as well as the cpx/opx partition coefficient data - are similar to the majority of spinel lherzolite xenoliths from alkaline basalts. The same features (La/Yb from 1.7 to 14.2) are shown by the fine-grained recrystallized peridotites. The close similarity between the Nonsberg peridotites and lherzolite xenoliths suggests that the observed LREE enrichment is due to upper mantle metasomatism. Alternatively, comparison of coarse and fine samples from the same outcrop reveal a very large increase in K, Sr and to a lesser extent, Li in all the fine-grained recrystallized peridotites. Since the fine-grained garnet peridotites are the metamorphic derivatives of coarse-grained spinel lherzolites at lower-crust P-T conditions, metasomatism for the fine-grained peridotites probably occurred within the lower crust. Such an enrichment in REE, K, and Sr has never been reported in high temperature peridotites and this makes the Nonsberg Alpine peridotites somewhat unusual.