Evolution of δ11B on tourmaline during prograde contact metamorphism in the thermal aureole of the Adamello batholith (Southern Alps, Italy)

The Permian continental deposits of the Verrucano Lombardo formation (Southern Alps, Italy) are conglomeratic to pelitic redbeds deriving from the disgregation of vulcanites and of the Variscan basement. In the Forcel Rosso area, this formation underwent high-grade metamorphism and partial melting caused by the emplacement of the Adamello batholith during the Eocene. Tourmaline is ubiquitous in the pelitic and metapelitic layers up to migmatitic conditions and this allowed the investigation of the δ11B evolution during the prograde metamorphic history. Tourmalines δ11B values in the unmetamorphosed pelites show a wide difference between the extremely negative cores, and the positive rims. We interpret the cores to be detrital, while the rims were crystallized at low temperature in equilibrium with high δ11B diagenetic or hydrothermal fluids. This δ11B core-rim variation persists up to the greenschist facies while, from lower amphibolitic facies, tourmaline grains recrystallize and become more homogeneous with δ11B values intermediate between cores and rims. Above the solidus, two types of metatexites were recognized based on the tourmaline δ11B signatures. Type-I metatexites are characterized by euhedral tourmalines showing a very narrow range of positive δ11B values. In Type-II metatexites euhedral tourmalines show slightly negative δ11B composition. In the Forcel Rosso area, Type-I metatexites were formed in an isochemical closed system while Type-II metatexites required the influx of external fluids. Preliminar calculations suggest that the δ11B signature of such external fluids was negative. Based on melt-fluid fractionation factors, if these fluids had been exsolved from a magmatic source, the coexisting melt would have had negative δ11B signature outside the range of I-type granitoids. This advocates for a metamorphic source while excluding the influx of magmatic fluids from the nearby I-type Adamello pluton.