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1 1British Antarctic Survey, c/o NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK (e-mail: ilm@bas.ac.uk)
2 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
3 3Isotope Geoscience Unit, Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, UK
Abstract: Magmatic rocks from the Antarctic Peninsula show marked variations in isotope composition, which reflect changes in the geodynamic evolution of the peninsula through time. Most Antarctic Peninsula granitoids formed as a result of subduction: they fall on well-defined trends on plots of
Nd, 207Pb/204Pb and 
18O against 87Sr/86Sri, between a component derived from subduction-modified mantle or juvenile basaltic underplate (Ndi>6, 207Pb/204Pb=15.61, 18O=5.5
, 87Sr/86Sr<0.704) and an end-member interpreted as a melt of Proterozoic lower crust ( Nd=–7, 207Pb/204Pb=15.67, 18O=10,87Sr/86Sr=0.709). A small group of granitoids, emplaced before or during Gondwana break-up, plot on distinct trends towards high 87Sr/86Sri compositions, reflecting mixing between melts derived from Proterozoic lower crust and melts of middle–upper crustal rocks (Ndi=–9, 207Pb/204Pb=15.64, 18O=10, 87Sr/86Sr=0.726), with little or no input of new material derived from the mantle or from juvenile basaltic underplate. These granitoids are thought to have formed as a result of crustal attenuation during the initial rifting phase of Gondwana break-up. Similar trends are shown by data from granitoids of the adjacent crustal blocks of West Antarctica. The isotope data suggest that an enriched Ferrar/Karoo-type lithosphere was not involved in the genesis of granitoids of the Antarctic Peninsula or of the Ellsworth–Whitmore Mountains crustal block.
Keywords: Antarctica, Gondwana, granites, isotopes, subduction.
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