Do S-type granites commonly sample infracrustal sources? New results from an integrated O, U–Pb and Hf isotope study of zircon

• Published in: Contributions to mineralogy and petrology 2010-07, Vol.160 (1), p.115-132
• Main Authors: Appleby, Sarah K., Gillespie, Martin R., Graham, Colin M., Hinton, Richard W., Oliver, Grahame J. H., Kelly, Nigel M.
• Format: Article
• Language: English
• Subjects: Earth and Environmental Science; Earth Sciences; Geochemistry; Geology; Granite; Isotope studies; Isotopes; Magma; Melting; Mineral Resources; Mineralogy; Original Paper; Petrology; Rocks
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-009-0469-3

In contrast to I-type granites, which commonly comprise infracrustal and supracrustal sources, S-type granites typically incorporate predominantly supracrustal sources. The initial aim of this study was to identify the sources of three Scottish Caledonian (~460 Ma) S-type granites (Kemnay, Cove and Nigg Bay) by conducting oxygen, U–Pb and Hf isotope analyses in zircon in order to characterise one potential end-member magma involved in the genesis of the voluminous late Caledonian (~430–400 Ma) I-type granites. Field, whole-rock geochemical and isotopic data are consistent with the generation of the S-type granites by melting their Dalradian Supergroup country rocks. While Hf isotope compositions of magmatic zircon, U–Pb data of inherited zircons, and high mean zircon δ 18 O values of 9.0 ± 2.7‰ (2SD) and 9.8 ± 2.0‰ for the Kemnay and Cove granites support this model, the Nigg Bay Granite contains zircons with much lower δ 18 O values (6.8 ± 2.1‰), similar to those found in Scottish I-type granites. This suggests that the Nigg Bay Granite contains low-δ 18 O material representing either altered supracrustal material, or more likely, an infracrustal source component with mantle-like δ 18 O. Mixing trends in plots of δ 18 O vs. εHf for S-type granite zircons indicate involvement of at least two sources in all three granites. This pilot study of Scottish Caledonian S-type granites demonstrates that, while field and whole-rock geochemical data are consistent with local melting of only supracrustal sources, the oxygen isotopic record stored in zircon reveals a much more complex petrogenetic evolution involving two or more magma sources.