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

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 (similar to 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 (similar to 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 delta O-18 values of 9.0 +/- A 2.7aEuro degrees (2SD) and 9.8 +/- A 2.0aEuro degrees for the Kemnay and Cove granites support this model, the Nigg Bay Granite contains zircons with much lower delta O-18 values (6.8 +/- A 2.1aEuro degrees), similar to those found in Scottish I-type granites. This suggests that the Nigg Bay Granite contains low-delta O-18 material representing either altered supracrustal material, or more likely, an infracrustal source component with mantle-like delta O-18. Mixing trends in plots of delta O-18 vs. epsilon Hf 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.