UNDERSTANDING CONSOLIDATION OF GRANITIC PEGMATITES: THE FELDSPAR STORY

Subsolvus rock-forming feldspars characterize granitic pegmatites: mostly albitic plagioclase and Na-bearing Kfs, but not necessarily paired. Geochemically primitive, Ca-bearing assemblages locally fit magmatic 2-feldspar sequence and its subsolidus extension, but highly evolved Ca-depleted pegmatites show arrays of uniformly Ca, K-poor albite + Kfs with decreasing Ab component, generating problems in application of 2-fsp geothermometers. Trace elements show extensive fractionation: gradual enrichment in Li, Rb, Cs, Tl, Ga, Eu, P and decreasing Ca, Sr, Ba in Kfs, and decreasing Ca, K in albite. All compositional trends show broad overlaps across (and along) the pegmatite zones, indicative of disequilibrium crystallization. Appreciable □Si₄O₈ substitution and low contents of structural (OH)^- and H₂O are widespread, rarely also NH₄⁺. Subsolidus reactions include exsolution and coarsening of perthite, antiperthite (and peristerite), prominent (micro)porosity, oriented overgrowths by sanidine, orthoclase or microcline in miarolitic cavities, replacement of Kfs by adularia +/- Rb-feldspar, crystallization of adularia (HS to LM) + low albite in leaching cavities and fissures, and migration of radiogenic Sr from Rb-bearing Kfs to plagioclase. Structural state of both rock-forming feldspars is highly disordered at the time of crystallization: albite undergoes very fast, virtually complete ordering (barring Ca-induced complications), but Kfs reacts much slower. Grid-twinned low microcline is mostly closely approached; however, highly ordered but X-ray-monoclinic orthoclase seems to be widespread in rapidly cooled pegmatites. Local advanced ordering is attained even in these cases by action of late residual fluids that also tend to segregate microcline twinning lamellae into large single-crystal units. Overall, feldspars can tell us about the geochemical signature and evolution of pegmatite melts, geological conditions of pegmatite intrusion, rates of nucleation, thermal regime of disequilibrium crystallization, and the extent and nature of hydrothermal overprint, but caution is advised in evaluation of primary magmatic versus subsolidus effects.