REE CHARACTERISTICS IN PEGMATITES AND ADJACENT WALLROCKS OF THE CALC-ALKALINE BERGELL INTRUSION (SOUTHEASTERN CENTRAL ALPS)

Rare-earth element (REE) determinations of pegmatites and adjacent wal lrocks of the calcalkaline Bergell intrusion are presented. The pegmat ites are derived from a granitic magma produced by AFC contamination o f a tonalite with roof pendant material. The geochemical differentiati on of the acid dykes is characterized by a depletion of LREE probably caused by fractional crystallization of allanite and minor sphene. Acc ording to their REE distribution patterns two different pegmatite grou ps can be distinguished: The pegmatites intruded in the high metamorph ic Gruf unit (group B) reveal contaminated REE distribution patterns, strongly related to their adjacent wallrock, whereas the remaining dyk es (group A) have undisturbed patterns. Group A pegmatites can be subd ivided into two categories according to their degree of differentiatio n. A lower fractionated biotite type with K2O > Na2O and a less develo ped negative Eu anomaly can be distinguished from a highly evolved mus covite type with Na2O > K2O and a strongly negative Eu anomaly. Biotit e pegmatites show a differentiation trend towards a lower anorthite co mponent in the feldspars leading to an increase of the Eu anomaly. Mus covite pegmatites are strongly depleted in Eu. The remaining low Eu co ntent correlates with the CaO content. Late in situ fractionation and possible multiple injection led to an internal zonation of large pegma tite bodies and lenses. Strong variations of the REE content in this z ones can be observed, similar to those observed in large scale lateral differentiation processes.