Chemical composition of zircons from the Cornubian Batholith of SW England and comparison with zircons from other European Variscan rare-metal granites
Zircon from 14 representative granite samples of the late-Variscan Cornubian Batholith in SW England was analyzed for W, P, As, Nb, Ta, Si, Ti, Zr, Hf, Th, U, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, Al, Sc, Bi, Mn, Fe, Ca, Pb, Cu, S, and F using EPMA. Zircons from the biotite and tourmaline granites are poor in minor and trace elements, usually containing 1.0–1.5 wt% HfO2, <0.5 wt% UO2 and P2O5, <0.25 wt% Y2O3, <0.2 wt% Sc203 and Bi2O3, and <0.1 wt% ThO2. Zircon from topaz granites from the St. Austell Pluton, Meldon Aplite and Megiliggar Rocks are slightly enriched in Hf (up to 4 wt% HfO2), U (1– 3.5 wt% UO2), and Sc (0.5–1 wt% Sc2O3). Scarce metamictized zircon grains are somewhat enriched in Al, Ca, Fe, and Mn. The decrease of the zircon Zr/Hf ratio, a reliable magma fractionation index, from 110-60 in the biotite granites to 30-10 in the most evolved topaz granites (Meldon Aplite and Megiliggar Rocks), supports a comagmatic origin of the biotite and topaz granites via long fractionation of common peraluminous crustal magma. In comparison with other European rare-metal provinces, the overall contents of trace elements in Cornubian zircons are low and the Zr/Hf- and U/Th-ratios show lower degrees of fractionation of the parental melt.
This contribution was supported by the Czech Science Foundation, project No. GA14-13600S and RVO 67985831. Bernard Bingen and one anonymous member of the Editorial Board are thanked for careful review and inspiring comments.
This is the author accepted manuscript. The final version is available from the Mineralogical Society via the DOI in this record.
Vol. 80 (7), pp. 1273-1289