Compositions of trioctahedral micas in the Cornubian batholith

New major and trace element chemical analyses of trioctahedral micas of the Cornubian batholith emphasize their enrichment in a 'trace-alkali element ' association (Li, Rb, Cs, F, Nb, Mn) and their deficiency in a 'femic element ' association (Zr, Ce, Th as well as Mg and Ti) compared with micas from many other granite suites, although there are similarities with some Hercynian granites and with rare metal pegmatites. The new data demonstrate a continuous series from siderophyllite through zinnwaldite to lepidolite, principally as a result of Li-R 2 § substitution as indicated by Foster (1960), although a more complete replacement is (Li, AI) = (Fe z+, Fe 3 § Ti). It is suggested that the ranges of these micas are based upon the Li content in the unit cell formula and the ratio of Li to R 2+, in effect, a compromise between the ranges proposed by Foster (1960) and Rieder (1970). Microprobe analyses lack Li20, but can be plotted on FeO-SiO 2 and FeO-AlzO3-SiO 2diagrams (wt. or atom %) in order to locate compositions within the trioctahedral L iFe micas and to distinguish between lepidolite and muscovite. An examination of 55 new mica analyses hows that hornfels biotites are richer in Mg and that the Cornubian Type A granite (as classifed by Exley and Stone, 1982) micas are richer in Ti and Fe compared with those of Type B granites. Micas from microgranite dykes appear to be poorer in femic elements and richer in trace alkalis and F compared with their Type B granite hosts, consistent with their differentiation from the latter. Mica chemistry is consistent with the magmatic evolution of the A-B-microgranite sequence in the biotite granites, the transformation of Types B to D upon emplacement of E, the derivation of Types E from B by extreme differentiation or metasomatic transformation a d mobilization, and the in situ differentiation ofTypes G from E. KEYWOROS: micas, Cornubian batholith, granite, Cornwall