Low-18 O Suzhou A-type granite revisited: Uranium influences in-situ zircon Oxygen-isotope analyses

A large number of Mesozoic A-type granites occur along the E. coast of China; their origin and petrogenesis are controversial. A-type granites with depleted zircon oxygen isotopes (3.15.4) raise the most interest: are they really derived from "low 18 O" magma? Does the 18 O-depletion reflect pre-, syn-, or post-magmatic processes? We carried out in-situ SIMS analyses of oxygen isotopes in zircon-quartz pairs from the xSuzhou A-type granite. Zircons and quartz with zoned CL-images and cleartransmitted light images have homogeneous oxygen isotope compositions, with 18 O = +5.4+ or -0.3 and +7.5+ or -0.3, respectively, indicating that high-temperature (up to 840 C) isotopic equilibrium between quartz and zircon is well preserved. In contrast, zircons with high Uranium contents (>700 ppm) and "dir ty" quartz yield different oxygen isotope compositions, 4.5 and 8.6. A key observation is that the high-U zircons show a negative correlation between U content and 18 O, probably reflecting destruction of the zircon lattice by radiation damage and subsequent water-rock interaction during post-magmatic processes. Apparently, the Suzhou pluton is not a "low 18 O granite". Compared to zircon of high U content, quartz can better preserve the O-isotope ratios of magmas. We suggest that the Suzhou A-type granite was produced by re-melting of ancient mafic-intermediate igneous rocks, triggered by underplating of depleted-mantle-derived magmas that provided both the heat source and material.