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Composition of zircons from the Cornubian Batholith of SW England and comparison with zircons from other European Variscan rare-metal granites

Published online by Cambridge University Press:  02 January 2018

Karel Breiter*
Affiliation:
Geological Institute of the Czech Academy of Science, Rozvojová 269, CZ-16500 Praha, Czech Republic
Axel Müller
Affiliation:
Natural History Museum, University of Oslo, Sars' gate 1, 0562 Oslo, Norway Natural History Museum, Cromwell Road, London SW7 5BD, UK
Robin Shail
Affiliation:
Camborne School of Mines, College of Engineering, Mathematics and Physical Science, University of Exeter, Penryn Campus, Penryn, Cornwall TR10 9FE, UK
Beth Simons
Affiliation:
Camborne School of Mines, College of Engineering, Mathematics and Physical Science, University of Exeter, Penryn Campus, Penryn, Cornwall TR10 9FE, UK
*

Abstract

Zircon from 14 representative granite samples of the late-Variscan Cornubian Batholith in SW England was analysed 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 electron probe microanalyses. 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.% Sc2O3 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-lasting 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.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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