Ricerc@Sapienza

Tourmalines from the late-Variscan Arbus pluton (SWSardinia) and its metamorphic aureole were structurally
and chemically characterized by single-crystal X-ray diffraction, electron and nuclear microprobe analysis,
Mössbauer, infrared and optical absorption spectroscopy, to elucidate their origin and relationships with the
magmatic evolution during the pluton cooling stages. The Arbus pluton represents a peculiar shallow magmatic
system, characterized by sekaninaite (Fe-cordierite)-bearing peraluminous granitoids, linked via AFC processes
to gabbroic mantle-derived magmas. The Fe2+-Al-dominant tourmalines occur in: a) pegmatitic layers and
pods, as prismatic crystals; b) greisenized rocks and spotted granophyric dikes, as clots or nests of fine-grained
crystals in small miaroles locally forming orbicules; c) pegmatitic veins and pods close to the contacts within
the metamorphic aureole. Structural formulae indicate that tourmaline in pegmatitic layers is schorl, whereas
in greisenized rocks it ranges fromschorl to fluor-schorl. Tourmalines in thermometamorphosed contact aureole
are schorl, foitite and Mg-rich oxy-schorl. The main substitution is Na+Fe2+↔□+Al, which relates schorl to
foitite. The homovalent substitution (OH)↔F at the O1 crystallographic site relates schorl to fluor-schorl, while
the heterovalent substitution Fe2++(OH, F)↔Al+O relates schorl/fluor-schorl to oxy-schorl.
Tourmaline crystallization in the Arbus pluton was promoted by volatile (B, F and H2O) enrichment, low oxygen
fugacity and Fe2+ activity. The mineralogical evolutive trend is driven by decreasing temperature, as follows:
sekaninaite+quartz → schorl+quartz → fluor-schorl+quartz → foitite+quartz. The schorl→ foitite evolution
represents a distinct trend towards (Al+□) increase and unit-cell volume decrease. These trends are typical
of granitic magmas and consistent with Li-poor granitic melts, as supported by the absence of elbaite and other
Li-minerals in the Arbus pluton. Tourmaline-bearing rocks reflect the petrogenetic significance of contribution
from a metapelitic crustal component during the evolution of magmas in the middle-upper crust.