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Oxygen and neodymium isotope evidence for recycling of juvenile crust in northeast China

¹ Department of Earth and Space Sciences, Laboratory for Chemical Geodynamics, University of Science and Technology of China, Hefei 230026, China
² Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA

It has been long recognized from Nd and Sr isotopes that depleted mantle sources consist of recycled oceanic materials, 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 it is resistant to high- temperature hydrothermal alteration. This effect is illustrated by a combined Sm-Nd and oxygen isotope study of whole-rock and mineral samples from a Mesozoic A-type granite at Nianzishan in northeastern China. The Sm-Nd isotope results show positive _Nd(t) values of +0.86 to +4.27 with young Nd model ages of 569–846 Ma, manifesting a significant input of newly mantle derived material. The zircon ¹⁸O values of 3.12–4.19 are significantly lower than the ¹⁸O value of 5.3 ± 0.3 for the normal mantle zircon and thus appear to require remelting of hydrothermally altered oceanic crust. The combined Nd-O isotope studies not only provide compelling evidence for geochemical recycling of young juvenile crust by plate subduction, but also demonstrate that the granitic magmas can result from partial melting of mantle-derived rocks that were subjected to seawater- hydrothermal alteration before magma generation. Disequilibrium oxygen isotope fractionations are observed between common rock-forming minerals with significantly lower ¹⁸O values for alkali feldspar than seawater, corresponding to meteoric-hydrothermal alteration after magma crystallization.

Key Words: oxygen isotopes • neodymium isotopes • zircon • hydrothermal alteration • A-type granite • China

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