PHOSPHATE ASSEMBLAGES FROM THE BRISSAGO GRANITIC PEGMATITE, WESTERN SOUTHERN ALPS, SWITZERLAND

Abstract

We describe a first significant occurrence of graftonite–triphylite-bearing LCT-type granitic pegmatite in the Alps. In 1999, in the Brissago area, in the western Southern Alps, Canton Ticino, Switzerland, we found a boulder containing an almost complete section of a phosphate-bearing pegmatite dike. The zoned pegmatite is composed by four main units: 1) an aplite unit, 2) a plagioclase + quartz graphic unit, 3) a coarse-grained plagioclase unit, and 4) a quartz core containing several phosphate masses up to 10 cm in diameter. On the basis of textural features and results of chemical analyses, we distinguish two phosphate associations. Association I, of magmatic origin, is represented by lamellar intergrowths of graftonite and triphylite. The leaching of Li and oxidation of Fe have led to the formation of ferrisicklerite and heterosite in replacement of triphylite, whereas the origin of kryzhanovskite and stanekite remains unclear. Association II, of metamorphic origin, consists of a polycrystalline aggregate of graftonite and triphylite. Exceptionally, primary maghagendorfite grains are present. Non-oxidizing low-temperature processes led to the formation of ludlamite and vivianite in replacement of triphylite. Locally, at the rim of phosphate masses, in close association with apatite-(CaOH), jahnsite and mitridatite formed at the expenses of graftonite. The chemical characteristics of the phosphate assemblages and associated mafic silicates indicate that this phosphate-bearing pegmatite dike belongs to a geochemically primitive system. Our U/Pb dating of zircon (242 ± 2.8 Ma) allows us to relate the formation of this dike to the Mid-Triassic tectonomagmatic cycle, characterized by the evolution of a magmatic arc well represented in the Southern Alps. In this geodynamic environment, a pegmatite-forming melt formed after partial melting of metapelites belonging to the Kinzigitic Complex of the Ivrea–Verbano zone.