The East Kemptville topaz–muscovite leucogranite, Nova Scotia. II. Mineral chemistry (original) (raw)

The major-and trace-element chemistry of the mineral constituents of the East Kemptville leucoglanite reflects chemical equilibration at both the magmatic (muscovite, bulk alkali feldspar, some biotite, topaz) and postmagmatic (albitic plagioclase, exsolved alkali feldspar, apatite in albite, most biotite, biotite alteration products) stages. Muscovite is characterized by elevated Fe (to l2wt,t/o FeO) and F lto >4 wt. q0 with low Fe3+/Fd+ ratios (0.14)]; absolute abundances of selected trace elements and ratios indicate compositions similar to white mica from pegmatites. The bulk composition (Ors2Ab17An1) and trace-element chemistry of alkali feldspar are also more typical of pegmatires. Topaz chemistry tF/(F+oH) = 0.80 t 0.051 is consistent witl high temperatures of formation, the inferred mineral assemblage and relatively dry nature of the melt. Biotite compositions vary considerably from freshest lFe/(Fe+Me) > 0.95, low Ti (<1.14 wt.tlo TiO),2.7 wt,9o Fl to chemistries that deviate considerably from ideal triocrahedral chemistry due to postmagmatic alteration. The albitic (Abe) composition of plagioclase containing abundant apatite inclusions reflects a late-magmatic or early postmagmatic albitization of precursor Ca-bearing plagioclase. The mineral chemistry indicates a highly reduced nature for the melt, similar to that in the case of topaz rhyolites. Collectively, (1) the presence of muscovite, (2) feldspar thermometry and (3) covariation of K and Rb between alkali feldspar and muscovite indicate a 7 of 500-700oC, whereas primary muscovite suggests that P was >, 1 kbar. Postmagmatic equilibration continued to < 350'C, on the basis of (l) the presence of chloritic alteration of biotite and (2) two-feldspar thermometry of exsolved phases in alkali feldspar. Postmagmatic alteration proceeded under low "f(Ot, as the secondary biotite is characterized by elevated Fel(Fe+Mg) ratios.