Age determination by µ-PIXE analysis of cheralite-(Ce) from emerald-bearing Pegmatites of Vigezzo Valley (Western Alps, Italy).

A green transparent homogenous crystal of cheralite-(Ce) was separated from an albitized pegmatitic dike outcropping at the summit of Mount Pizzo Marcio (Vigezzo valley, Western Alps, Verbania, Italy) and analysed with a proton-induced X-ray emission microprobe (μ-PIXE) for total U-Th-Pb age determination. 9 spot analyses have been performed, obtaining ages in the range 30.4-34.3 Ma, with errors on single analysis of ±1.6 Ma. These ages are not statistically different and do not show systematic distributions within the analysed crystal (i.e. either in the core or in the rim). Therefore an average age of 32.7 Ma was calculated, with a propagated error of ± 3.2 Ma, representing the youngest total U-Th-Pb age ever obtained on a mineral of the monazite group. Such an age indicates that during the Alpine event two potential sources of pegmatitic magmas producing the Vigezzo valley pegmatitic field can be taken into consideration. A first potential magmatic source is represented by the granodioritic-tonalitic Masino-Bregaglia pluton, aged from 32.9 to 28 Ma., associated with the peraluminous two micas granitic stock of S. Fedelino 25 Ma old (MOTICSKA, 1970, BERGER et al., 1996, HANSMANN, 1996). A second potential source could be related to the Barrovian metamorphism responsible for the development of the so called Lepontine Gneiss Dome (WINTER, 2001), that affected the Central Western Alps during Oligocene. Such metamorphism was concomitant with dextral strike-slip movements along the Insubric Line. Subsequently, the main thermal updoming producing migmatites, which started about 32-30 Ma ago with the intrusion of the Masino-Bregaglia pluton, migrated towards west reaching the metamorphic peak in the Simplon Alpine region, about 20 Ma ago (ENGI et al., 1995, BOSQUET et al., 1997 ).