Several layered pegmatite-aplite intrusives exposed at the Little Three mine, Ramona, California, U.S.A., display closely associated fine-grained to giant-textured mineral assemblages which are believed to have co-evolved from a hydrous aluminosilicate residual melt with an exsolved supercritical vapor phase. The asymmetrically zoned intrusive known as the Little Three main dike consists of a basal sodic aplite with overlying quartz-albite-perthite pegmatite and quartz-perthite graphic pegmatite. Muscovite, spessartine, and schorl are subordinate but stable phases distributed through both the aplitic footwall and pegmatitic hanging wall. Although the bulk composition of the intrusive lies near the haplogranite minimum, centrally located pockets concentrate the rarer alkalis (Li, Rb, Cs) and metals (Mn, Nb, Ta, Bi, Ti) of the system, and commonly host a giant-textured suite of minerals including quartz, alkali feldspars, muscovite or F-rich lepidolite, moderately F-rich topaz, and Mn-rich elbaite. Less commonly, pockets contain apatite, microlite-uranmicrolite, and stibio-bismuto-columbite-tantalite. Several of the larger and more richly mineralized pockets of the intrusive, which yield particularly high concentrations of F, B, and Li within the pocket-mineral assemblages, display a marked internal mineral segregation and major alkali partitioning which is curiously inconsistent with the overall alkali partitioning of the system.

Calculations of phase relations between the major pegmatite-aplite mineral assemblages and supercritical aqueous fluid were made assuming equilibrium and closed-system behavior as a first-order model, although these assumptions may not be wholly correct. Isobaric phase diagrams were generated and are used to constrain the observed mineral assemblages in the system K2O-Na2O-Al2O3-SiO2-H2O as a function of temperature and cation-activity ratios of the coexisting fluid phase. Log activity-temperature plots of the stoichiometric assemblage microcline-albite-quartz-muscovite at 2 kbar show that log
a(K+/H+)
of the fluid phase remains essentially constant throughout crystallization, while log
a(Na+/H+)
rises slightly with falling temperatures. Modeling muscovite nonstoichiometry by decreasing activity of the pure component from unity to 0.5 markedly shifts the upper thermal stability limit of the assemblage microcline-albite-quartz-muscovite from approximately 630°C upward to 710°C. In the pegmatite-aplite system exposed at the Little Three main dike, crystallization temperatures are thought to have ranged from approximately 700°C downward toward 540°C at the central pocket zone; muscovite nonstoichiometry caused by F, Li, and Mn substitution may have contributed to the stability of this assemblage at the outer zones of the system, where stoichiometric muscovite is not predicted to be stable.
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First page of Mineralogy and geochemical evolution of the Little Three pegmatite-aplite layered intrusive, Ramona, California
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