Gahnite from LCT granitic pegmatites of the Comechingones (Blanca Dora, Juan Román, Magdalena, La Ona, and Sin Nombre pegmatites) and Conlara (Nancy pegmatite) pegmatite districts, Argentina, was analyzed to investigate the relationship between its chemistry, pegmatite mineralogy, and pegmatite melt evolution. Based on the molecular proportions of endmembers (gahnite, hercynite, spinel, and galaxite) the composition of gahnite in these pegmatites is defined by the ranges Ghn 77.7-91.0 Hc 7.4-20.0 Spl 0.25-3.37 Glx 0.7-2.4 , which fall within the previously defined field for granitic pegmatites. Gahnite from the Nancy pegmatite has higher Mg and Mn contents than gahnite from the pegmatites of the Comechingones pegmatite district. Chemical variation within gahnite crystals is characterized by higher Zn (∼2.8 wt.% ZnO) and lower Fe (∼2.6 wt.% FeO) contents near the edge compared to inner portions, reflecting the evolution of the pegmatite melt via simple fractional crystallization. Other crystals show a complex zoning pattern of oscillations in Fe and Zn contents within the crystal and higher Zn and lower Fe near the rim compared to the core. A plot of molecular Fe 2+ + Mg versus Zn + Mn yields a strong negative correlation and best displays the substitutions within the gahnite crystal structure and is the best to determine relative pegmatite melt evolution. Without implying a co-magmatic origin of the pegmatites, based on pegmatite mineralogy, Zn contents, Zn/Fe 2+ ratios, and coupled Fe 2+ +Mg and Zn+Mn values in gahnite, the relative degree of evolution of the pegmatites increases in the order: Sin Nombre → Magdalena → Juan Román → Blanca Dora → Nancy → La Ona. Based on the composition of gahnite in worldwide granitic pegmatites reported in the literature and those obtained here, the composition of gahnite in these rocks is defined by the endmember ranges Ghn 50-98 Hc 1-50 Spl 0-7 . This study shows that the major element composition of gahnite in granitic pegmatites can effectively be used to determine the relative degree of evolution of pegmatite-forming melts.
We report the occurrence and composition of gahnite ([Zn,Fe,Mg,Mn]Al 2 O 4 ) in aplites of the Pala Chief and Elizabeth R layered pegmatite-aplite dikes, Pala District, California, and the significance for felsic melt evolution. The aplite is characterized by alternating bands of white, finegrained plagioclase + quartz + muscovite +/− black tourmaline and thin laminations defined by concentrations of reddishbrown garnet (Bline rock^). Accessory gahnite occurs as extremely fine-grained (50-250 μm) green to bluish-green crystals. Gahnite compositions in both occurrences are similar and defined by end-member ranges Ghn 83.5-90.9 Hc 8.7-15 Spl 0-1.4 . Zinc contents in gahnite are high (36.7-39.5 wt.% ZnO for Pala Chief, 37.2-40.2 wt.% ZnO for Elizabeth R), the MgO and MnO contents are negligible (< 0.7 wt.%), and Fe is the main substitution present (< 6.9 wt.% FeO). Gahnite is chemically zoned and characterized by higher Zn (< 2 wt.% ZnO) and lower Fe, Mg, and Mn contents in rims compared to cores, which shows the substitution mechanism, and reflects fast growth during melt evolution via fractional crystallization followed by fast cooling. Compositional variations are greater within individual crystals than among crystals and between laminations, which indicate a homogeneous melt at the aplite scale. Compared with the composition of gahnite from pegmatites worldwide, the studied gahnite reflects a moderate degree of melt evolution. This study confirms the incompatible character of Zn in evolving pegmatite-aplite melts and shows the usefulness of gahnite as a petrogenetic indicator. Because gahnite is a resistant mineral, surficial findings of gahnite with very low Mg contents and Zn contents similar or higher than those measured here may indicate a source of granitic pegmatite-aplite.
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