TERTIARY PEGMATITE DIKES OF THE CENTRAL ALPS

• DOI: 10.3749/canmin.52.2.191
• Publication Date: 2014-08-01T16:04:53Z
• Authors: A. Guastoni; G. Pennacchioni; G. Pozzi; A.M. Fioretti; J.M. Walter
• Keywords: Pegmatite; Dike

The largest field of Alpine Oligocene pegmatite dikes is in the Central Alps within Southern Steep Belt (SSB) nappes; it extends for about 100 km an E-W direction and 15 a N-S north Periadriatic Fault, from Bergell pluton (to east) to Ossola valley west).The geographically overlaps (1) highest temperature domain Lepontine Barrovian metamorphic dome (2) zone migmatization.We have studied pegmatites two areas: Codera area on western border Bodengo between Mera Mesolcina valleys.Most show simple mineral assemblage consisting K-feldspar, quartz, muscovite ± biotite, only minor percentage (< 5%) contains Sn-Nb-Ta-Y-REE-U oxide, Y-REE phosphate, Mn-Fe-phosphate, Ti-Zr-silicate, Be-Y-REE-U-silicate oxide minerals (beryl, chrysoberyl, bertrandite, bavenite, milarite), garnet (almandine-spessartine), tourmaline (schorl rare elbaite), bismuthinite, magnetite, rarely dumortierite helvite.The assemblages, geological context, chemical compositions allow distinction LCT (lithium, cesium, tantalum) mixed LCT-NYF (niobium, yttrium, fluorine) (with one exception NYF dike area).The did not reach high degree geochemical evolution.The most fractionated are found contain Mn-rich elbaite, triplite, pink-beryl, Cs-Rb-rich feldspar.In locally miarolitic cavities evolved correspond berylcolumbite-phosphate type.From structural point view main types can be distinguished: that were involved ductile deformation postdated SSB.Many belong first type: they emplaced at relatively ambient (ca.500 °C) pervasive recrystallization quartz mylonitic structure.The trend 70° steeply dipping.In set (trending approximately NNE-SSW) crosscuts structures SSB, but also includes earlier generation boudinaged folded dikes.The undeformed this may pockets.There no systematic difference pegmatites.However, chemistry minerals, especially garnet, addition data suggests areas represent distinct generations pegmatites.Structural few existing radiometric ages suggest over time span 29 25 Ma (and possibly as young 20 Ma), with youngest postdating deformations nappes.The present work presents comprehensive description -mineralogical characterization Alps.