Melt evolution in felsic dikes inferred from the composition of gahnite in two new occurrences, Pala Chief and Elizabeth R composite dikes, California

Abstract

We report the occurrence and composition of gahnite ([Zn,Fe,Mg,Mn]Al2O4) 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, fine-grained plagioclase + quartz + muscovite +/− black tourmaline and thin laminations defined by concentrations of reddish-brown garnet (“line 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 Ghn83.5–90.9Hc8.7–15Spl0–1.4. 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.