Origin and formation of barren granitic pegmatites in the Ostrobothnia Schist Belt, western Finland: Insights from geochemistry and comparison to experimental data

Rare-metal enrichment in granitic pegmatites is currently not well constrained, with different models invoking extreme fractional crystallization, low-volume melting of supracrustal rocks, and/or fluid saturation and melt-fluid separation. If we want to better understand the rare-metal enrichment of the granitic pegmatites, we must also understand the formation of rare-metal-poor barren pegmatites that lack such enrichments. We present new whole-rock geochemical data from barren granitic pegmatites and their metapelitic country rocks from the Ostrobothnia Schist Belt, western Finland, which are compared with the published data from partial melting experiments. The studied leucogranitic pegmatites are peraluminous, muscovite–tourmaline–garnet-bearing, enriched in SiO2 (70.4–79.2 wt%), Al2O3 (12.2–17.1 wt%), Na2O (median Na2O = 3.8 wt%), and K2O (median K2O = 3.8 wt%), and low in CaO (< 1.6 wt%), TiO2 (< 0.16 wt%), and total of FeOt + MgO (< 2.5 wt%). They further show depletion in REE (ΣREE < 47 ppm) but higher Bi, Sn, Rb, Cs, Be, Ta, P/Ti, Rb/Sr, K/Na, and Sm/Nd than the host rock metapelites. The geochemical characteristics and comparisons suggest that the barren granitic pegmatites in the Ostrobothnia Schist Belt were formed by direct anatexis of local metasedimentary rocks, and that the dominant anatectic mechanism was muscovite dehydration partial melting.