ANATECTIC GRANITIC PEGMATITES FROM THE EASTERN ALPS: A CASE OF VARIABLE RARE METAL ENRICHMENT DURING HIGH-GRADE REGIONAL METAMORPHISM. II: PEGMATITE STAUROLITE AS AN INDICATOR OF ANATECTIC PEGMATITE PARENT MELT FORMATION - A FIELD AND EXPERIMENTAL STUDY

摘要

Staurolite was found in two granitic pegmatites emplaced in crystalline basement rocks of the Eastern Alps in Northern Italy. The pegmatites form decimeter-to meter-sized concordant veins with quartz+albite+muscovitethornparagonite as major phases along with minor garnet + beryl and accessory Nb-Ta-Sn-Ti phases, zircon, and magmatic phosphate minerals. U-Pb dating of cassiterite from one of the pegmatites yielded Permian ages of similar to 290 Ma. The pegmatite country rocks are garnetbiotite-muscovite-plagioclase gneisses with accessory paragonite +/- staurolite +/- kyanite. The pegmatite staurolites are present as an accessory groundmass phase with sizes typically =similar to 200-300 mu m and as tiny (similar to 10-50 mu m) inclusions in large beryl crystals. When enclosed in beryl staurolite is typically intergrown with quartz and is part of a complex solid phase assemblage including tapiolite, chrysoberyl, cordierite, and Zn-Al-spinel. With few exceptions, the staurolites from the pegmatites are perfectly euhedral and free of solid inclusions. They are Fe-rich with X-Mg = 0.02-0.20 and contain 0.6-7.2 wt.% ZnO 0.05-0.4 wt.% TiO2, 0.48-0.69 wt.% Li2O, and similar to 1.8 wt.% H2O. With respect to trace elements, high Be (96-171 mu g/g), Nb (18-49 mu g/g), and Ta (10-72 mu g/g) contents are noteworthy, as is a very low Nb/Ta ratio of 0.6-1.9. Staurolites from the pegmatite country rock show a comparable range in X-Mg of 0.16-0.23 and an identical H2O content but are significantly different in TiO2 (0.49-0.92 wt.%), ZnO (0.1-2.4 wt.%), and Li2O (0.06-0.31 wt.%) contents. Also, Be (13-48 mu g/g), Nb (0.4-4.7 mu g/g), and Ta (0.04-1.2 mu g/g) contents are much lower and Nb/Ta ratios of 4.0-10.9 are higher than in the pegmatite staurolites. In both pegmatites and country rocks staurolite is the major Li carrier and one of the major Ta-fractionating silicate phases. The unusual paragenetic and textural features of the pegmatite staurolites suggest that they are magmatic rather than metamorphic in origin. Partial-melting experiments at 7 kbar and 650-750 degrees C confirm the potential of the pegmatite country rocks to generate strongly peraluminous granitic melts in equilibrium with staurolite and may produce staurolite textures very similar to those found in the pegmatites. Hence, both field and experimental data are consistent with magmatic formation of the pegmatite staurolites and an origin of the pegmatite parent melts by anatexis of metapelites during the Permian high-T/low-P tectonometamorphic event, which affected significant portions of the Eastern Alpine crystalline basement.