but difficulty was encountered in identifying this signature by means of oxygen isotopes because of significant postemplacement hydrothermal alteration. Zircon is expected to preserve this signature because its oxygen is resistant to high-temperature hydrothermal alteration. This effect is illustrated by a combined Sm-Nd and O isotope study of whole-rock and mineral samples from a Mesozoic A-type granite in northeastern China. This A-type granite shows positive initial Nd isotope ratios with young Nd model ages of Neoproterozoic for whole-rock, and lower oxygen isotope ratios of zircon than the normal mantle. These results point to a magma derivation from partial melting of a newly mantle derived material that underwent surface-fluid hydrothermal alteration before magma generation. The combined O and Nd isotopic study argues for the recycling of former ocean crust, hydrated with seawater into the continent. This provides the first geochemical evidence for the recycling of juvenile crust by subduction and thus has important implications for understanding the formation and evolution of granitoids and young continental crust. Furthermore, oxygen isotope analyses of zircon and the common rock-forming minerals reveal that the granite underwent pre- and postmagmatic water-rock interactions in the history of its formation and evolution. The results demonstrate the potential use of O versus Nd isotopes in tracking the magma source and its evolution in granite petrology. This study also demonstrates how important it is to know zircon oxygen isotope compositions for igneous rocks that underwent a secondary alteration.

Beijing inundated by the sea within the past 80 k.y.: Nannofossil evidence.
Wuchang Wei, Scripps Institution of Oceanography, University of California, San Diego, California 92093, USA. Pages 379-381

Examination of published data reveals that a marine bed in Beijing can be dated as 80 Ka or younger on the basis of abundant nannofossils
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Contact: Ann Cairns
acairns@geosociety.org
303-357-1057
Geological Society of America
5-Apr-2002

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