Barium-rich phengite in eclogites from the Voltri Group (northwestern Italy)

A white mica with high barium content has been found for the first time in eclogites from a single body cropping out near the village of Vara Inferiore (Voltri Group,Italy).Petrographic observations reveal that the modal abundance of mica is low and that its crystallisation took place after the climax of the eclogite-facies metamorphism.This study reports data on bulk rock chemistry,obtained by ICP-MS,as well as the results of EMP and LA-ICP-MS analyses on major and selected trace elements carried out in situ on handpicked separate grains of mica.The most significant features of the bulk rock chemistry are the high content of Na2O and the enrichment in LIL elements (Ba and Cs over K and Rb)compared with other eclogites of the area. The white mica is a “phengite ” that contains 3.17-3.30 Si a.p.f.u,and 0.19 -0.31 Mg a.p.f.u.All the analysed grains show marked heterogeneities in both major and trace element distribution.In particular, the content of BaO varies from 1.65 to 4.55 wt% and from 2.46 to 4.59 wt% in two examined samples, thus leading to a range of ˜4-12 % Ba atoms in the cation interlayer (I )of the mica.The high barium content in white mica is coupled with a high abundance in other LILE (Cs,Rb)and light elements such as Li and B.On the contrary,mica has low concentration of REE, Sr, P and HFSE (e.g. Nb, Zr, Ti, Y). Phengite chemistry indicates that its trace element signature is controlled by a fluid activity that also prevented the achievement of complete homogenisation.