Composition of trioctahedral micas in the Karlovy Vary pluton, Czech Republic and a comparison with those in the Cornubian Batholith, SW England

University of Exeter, Earth Resources Centre, Exeter, United Kingdom

Trioctahedral micas in the Karlovy Vary pluton range in composition from Fe-biotites in the granites of the Older Intrusive Complex (OIC) through siderophyllite and lithian siderophyllite to zinnwaldite in the granites of the Younger Intrusive Complex (YIC). Li+Al ^VI +Si would appear to substitute for Fe (super 2+) +Al ^IV in biotite with a formula similar to that given in Henderson et al. (1989), but Li+Si appears to substitute for Fe (super 2+) Al ^IV in the Li-micas. In mica vs. host rock plots, Rb and F show positive linear covariation except for the Li-mica granites, but femic constituents and tFeO/(tFeO+MgO) have separate trends for OIC and YIC granites and micas. Further differences between OIC and YIC granite micas are seen in their Ti and Mg contents and in plots like V vs. SiO ₂ , Al ^IV vs. Fe/(Fe+Mg) and Li vs. total iron as Fe (super 2+) and in the results of discriminant analysis. These reveal a geochemical hiatus between OIC and YIC granite micas that coincides with a major temporal hiatus. Biotite compositions in the YIC granites are similar to those in the granites of the Cornubian batholith and reveal a similar magmatic evolution and genesis in which later biotites evolve to lithian siderophyllites with some enrichment in trace alkalis and F. It is suggested that the biotite granites in the YIC were derived from the products of partial fusion of the OIC granites. A less well-marked geochemical hiatus exists between YIC biotites and zinnwaldites. In some plots (e.g. Si vs. Li, Li vs. tFe) apparent continuity between biotite and the Li-micas suggests continuous evolution, but in others (e.g. Rb vs. TiO ₂ , Rb(biotite) vs. Rb(rock)), Li-mica data points stand apart from the biotites suggesting, like the whole rock data, a separate evolution. Comparison with the more abundant data for Li-micas of the Cornubian batholith suggests derivation of the Li-mica granites by partial fusion of the OIC/YIC granite residues.

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