The Vedrette di Ries (rieserferner) plutonic complex: Petrological and geochemical data bearing on its genesis

Data on Co, Sc, Rb, Sr, Th, Zr, Ta, Hf and REE contents are reported for tonalites, granodiorites, granites and one diorite from the Alpine plutonic complex of Vedrette di Ries in the Eastern Alps. Co and Sc range respectively between 19.6-1.2 ppm and 32-3.3 ppm showing a good negative correlation with the Differentiation Index (D.I.). Rb values range between 61–197 ppm displaying a positive correlation with D.I. All the samples have variably fractionated light and heavy REE with La_N/Sm_N=1.7–5.55 and Tb_N/Yb_N=0.58–1.65 and no important europium anomalies. On the basis of the HREE fractionation two groups of rocks can be distinguished: one showing higher Tb_N/Yb_N values which range between 1.27–1.65 and are positively correlated with the D.I. and a second group with Tb_N/Yb_N values less than unity which decrease with the differentiation. The rocks with high Tb_N/Yb_N ratio also display a positive Sr versus D.I. variation whereas the samples with flat or upward concave HREE patterns (i.e. with Tb_N/Yb_N<1) define negative Sr versus D.I. trends. Th, Ta, Hf, Zr and LREE show an overall tendecy to increase and then to decrease with the differentiation, with a large scattering of values.The data obtained fit the hypothesis that the entire rock series under study is the product of a two-stage crystal/liquid fractionation process starting from one parent magma of tonalitic or dioritic composition. During the first stage, which occurred at high pressure, the separation of hornblende+garnet produced the liquids displaying the positve Tb_N/Yb_N and Sr versus D.I. correlation. These liquids during their rise through the crust would have undergone a second stage fractionation with separation of hornblende and plagioclase evolving toward low-Tb_N/Yb_N and Sr-poor composition.