ALBERT

Beschreibung: Combined in situ U–Pb and Hf–O isotope analyses for zircon are often used to date igneous events precisely and to gain insights into the origin of the magma from which the zircon crystallized. In conjunction with its resistance to weathering, zircon can therefore be considered a unique crystal toolbox and an ideal crustal archive. This concept, however, relies on the basic assumption that each zircon crystal is in Hf isotope equilibrium with its host magma. Here we test this hypothesis for zircon crystals from mafic–ultramafic layered intrusions and show that this assumption may not always be correct. We find Hf isotope disequilibrium between zircon crystals and their host-rocks in three Neoproterozoic mafic–ultramafic layered intrusions from the northwestern margin of the Yangtze Block, Central China, formed as part of convergent margin magmatism along the Hannan–Panxi subduction zone. Zircon crystals separated from diorite samples from these three intrusions confirm prolonged magmatism for over 90 Myr for the Beiba (869 ± 5 Ma), Wangjiangshan (822 ± 4 Ma) and Bijigou (785 ± 5 Ma) intrusions, with a chronologically progressive decrease in 18 O values from 7·4 to 6·3 and 6·0, respectively. We interpret the transition from an isotopically evolved (high 18 O) towards a progressively more primitive mantle source (lower 18 O) as the fading influence of subducted sediment-derived melts in a subduction zone, consistent with a reconstructed change in subducting plate motions from the northern to the western margin of the Yangtze Block. Unlike the coherent O isotopes, the Hf ( t ) values of zircon populations from each intrusion show a range of several Hf units (Beiba: –1·0 to+3·0; Wangjiangshan: +2·7 to +8·3; Bijigou: +2·3 to +7·8), outside analytical uncertainty and inconsistent with an origin from a single magma batch. Whole-rock Hf isotope analyses obtained by high-pressure dissolution indicate that the diorite samples from the Beiba, Wangjiangshan and Bijigou intrusions have Hf ( t ) of +8·2, +7·5 and +9·3, respectively. In contrast, table-top dissolutions for the same samples yield Hf ( t ) of +9·7, +10·0 and +11·7, respectively. The apparent systemic offset in Hf ( t ) values towards more crustal compositions in high-pressure dissolutions is interpreted here to reflect mixing of zircon-hosted Hf isotopes with less evolved Hf isotopes in associated mineral phases. The more crustal character of in situ Hf isotope determinations in zircons and their range of several Hf units are interpreted here to reflect progressive crustal contamination in magma chambers at the time of zircon saturation. This implies that Hf isotope compositions of zircon crystals can be biased towards crustal signatures, particularly in mafic–ultramafic intrusions that are more susceptible to crustal contamination. In such cases, source interpretation as well as Hf model ages calculated from these isotopic mixing pools are geologically meaningless. Inevitably, contaminated igneous suites, mafic–ultramafic complexes in particular, and detrital zircon populations derived therefrom may have a complex Hf isotopic history that cannot be resolved by fast in situ analyses of Hf isotopes alone. This history may be revealed only by an atypical range of Hf isotope compositions within single magmatic suites and, if unidentified, can lead to biased geological interpretations.