Abstract

Granitic pegmatites are present within a small pegmatite field as concordant decimeter- to meter-sized dikes or small stocks emplaced in amphibolite-facies metapelitic gneisses or marbles of the Eastern Alpine crystalline basement in Northern Italy. Five of these pegmatites were investigated and show simple major phase assemblages (quartz-albite-muscovite-paragonite, quartz-plagioclase-muscovite, and quartz-K-feldspar-plagioclase-muscovite) together with minor beryl, garnet, or tourmaline. In addition, accessory phase assemblages of variable complexity are present involving chrysoberyl, cordierite, Nb-Ta-Sn-Ti oxides, zircon, and magmatic phosphate minerals. Two pegmatites have staurolite as inclusions in large beryl crystals and/or as a groundmass phase. Muscovite replacement textures indicate that paragonite is not a primary magmatic phase but a metasomatic phase. All pegmatites show tourmaline-rich reaction zones along their contacts with the country rocks. If present, columbite-group phases (CGP) are the major rare-metal-bearing phases and are present in several textural types. They show a range in XMn and XTa of 0.4–72.9 and 8.5–86.9, respectively. A significant portion of the CGP analyses yields values <20 and many CGP grains show a core-to-rim decrease in XMn, however without any systematic zoning in XTa. Amongst the minor elements in the CGPs, high MgO contents of up to 1.0 wt.% MgO are noteworthy. Four possible causes of the mineralogical diversity of the pegmatites are proposed: (1) the degree of fractionation and ASI of the pegmatite parent melts, (2) a variable Na-metasomatic overprint of the primary pegmatite assemblages, (3) the nature of the Fe-Mn phases and their relative timing of crystallization, and (4) the nature of the pegmatite country rocks. The geochemical signature of the pegmatites is characterized by an enrichment in Be-Nb > Ta-Sn combined with very low Li-F-Y-REE, resulting in the absence of any F- and Li-saturated phases and very low Li- and F-contents in all major pegmatite silicates. Laser ablation ICP-MS U-Pb dating of cassiterite from the most highly fractionated pegmatite yielded a tightly constrained age of 289 ± 5 Ma, whereas U-Pb dating of CGPs from the same pegmatite produced a large age spread in the range 70 ± 4 to 172 ± 3 Ma. Additional SIMS U-Pb dating of Nb-Ta-Sn rich titanites from a reaction zone between a further pegmatite and calcite marble yielded ages in the range 224 ± 12 to 242 ± 8 Ma. The cassiterite ages indicate a pegmatite emplacement during the Permian high-T/low-P metamorphic event, the latter affecting large portions of the Eastern Alpine basement through widespread partial melting of the crust as a result of lithospheric thinning. The range in CGP ages can be explained with a resetting of the U-Pb system during an Eoalpine amphibolite-facies metamorphic overprint at ∼80–100 Ma. The titanite ages are also consistent with an Eoalpine resetting, but a later emplacement of the pegmatite during an extended period of Permo-Triassic pegmatite formation cannot be ruled out. Geochemical enrichment patterns as well as mineral assemblages, the tectono-metamorphic environment of pegmatite emplacement, and a lack of suitable intrusive parent bodies are consistent with an anatectic origin of the investigated pegmatites. We further propose anatectic parent melt formation as the major mechanism for pegmatite genesis in all parts of the Eastern Alpine basement affected by Permo-Triassic metamorphism.

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