Tertiary LCT and NYF pegmatites of the central Alps

A large field (about 100 km in E-W length and 15 km in N-S thickness) of Oligocene pegmatites extends in the
central Alps from the Bergell pluton (to the east) to the Ossola Valley (to the west) within the Alpine nappes north
of the Periadriatic Lineament. The pegmatite field geographically overlaps (i) the highest temperature domain
of the Lepontine Barrovian metamorphic dome, and (ii) the zone of Alpine migmatization. Most pegmatites
have a simple mineralogy consisting of K-feldspar, quartz and muscovite, but a minor amount (< 5%) includes
Sn-Nb-Ta-Y-REE-U oxides, Y-REE phosphates, Mn-Fe-phosphates, Ti-Zr-silicates, Be-Y-REE-silicates, garnet
(almandine-spessartine), and schorl-dravite-fluorelbaite tourmaline.
Major and trace elements geochemistry of pegmatite bulk rock, rock-forming and accessory minerals al-
lows the distinction of different pegmatite populations ranging from NYF (niobium, yttrium, fluorine) to LCT
(lithium, cesium, tantalum) pegmatites, or mixed LCT-NYF ones. Actually, LCT pegmatites of the Central
Alps did not reach a high degree of geochemical evolution. In the Codera Valley (on the western side of the
Bergell pluton) LCT and NYF pegmatites are respectively hosted in tonalites and granodiorites; these pegmatites
include the most evolved types which contain Mn-fluorelbaite, Mn-phosphates, pink-beryl and Cs-Rb-rich feldspar.
From the structural point of view 2 main types of pegmatites can be distinguished: (i) pegmatites that were
involved in ductile deformation, and (ii) pegmatite crosscutting the ductile structures of the SSB. Many peg-
matites from Codera Valley belong to the first structural type: they were at emplaced at relatively high ambient
temperatures (> 450
◦
C) and locally show pervasive recrystallization of quartz. More to the east (Mesolcina and
Bodengo Valleys) the main set of pegmatites crosscut the ductile deformation structures of the SSB, but the area
also includes an earlier generation of boudinaged and folded dykes. The undeformed pegmatites from this area
may contain large miarolitic pockets. There is no systematic difference in mineralogy and geochemistry between
the 2 structural types of pegmatites.
Structural data and the few existing radiometric ages suggest that pegmatites were emplaced over a time
span between 29 and 25 Ma with the youngest dykes postdating the ductile deformations of the Alpine nappes.
The present work presents a first comprehensive geochemical and mineralogical classification of the Oligocene
pegmatite field of the central Alps. In order to constrain the timing of pegmatite formation monazite and xenotime
have been sampled from the different generations of pegmatite.