Extreme fractionation in rare-element granitic pegmatites; selected examples of data and mechanisms

Abstract

- Granitic pegmatites of the rare_element (and miarolitic) class display extreme fractionation and accumulation.of rare lithophile elements, beyond the limits observed in other igneous and postmagmatic assemblages. Enrichment in Li and -Cs leads to precipitation of rock-forming spodumene, petalite, Li-phosphates or lepidolite, and pollucite. These gqeoes-can be interpreted as crystallochemically conditioned Li- and-Csbased feldspathoids, crystallizing along with (and after) the last generations of primary K- and Nalfeldspars. Extreme fractionation is encountered in K-feldspar gi,znb !,9.rK/Qs 22.4, K/Tt 236, Rb,zCs 6.5, Rb,/Tt 130 io 35, K,/Ba 18,0@, BalRb <0.002); muscovite and lepidolite q/R! !.4, K,/Cs 4.0, K/Tl3ZO, Rb,/Cs 4.4, nb,ril SO ro 3.6, N/Ga239): pollucite (KAl 10, Rb/Tl 60io l3); microute, srmpsonite and other late oxide minerals of Ta (Nb/Ta < 0.001); manganoranialire (Fe,/Mn 0.@3): spes_ sartine @e,/Mn < 0.@3); lithiophilite (FeZVn < O.oii: iraf_ yoy (ZrlHf 0.014), and hawteyite and dernjite (in/Cd, 0.067)- Classic crystallochemical concepts of Goldichmictr and Ringwood alone are not adequate tb explain the above values. Selective extraction and transport in a volatile phase (by diffusion or in an exsolved hydrous fluid), complex_ ing stable to relatively low late- to postmagmaiic temieratures, and crystallochemical for specific mineral structures operate in conjunction with crystal,/melt fractionation; the environment evolves from that of a volatile_ saturated residual granitic melt, through that ofa melt coex_ isting with exsolved supercritical fluid, to that of an aque_ ous solution and possibly a gas phase, in a closed oirestricted system.