Hf-O isotope systematics of zircons from the Taitao granitoids: Implications for slab-melting material

Slab-melting is considered to have played an important role in the formation of continental crust. The combination of oxygen (O) and hafnium (Hf) isotope signatures can provide key information relating to the melting components during slab-melting. To reveal the melting components, we used SIMS and LA-ICP-MS to determine O isotope ratios at 220 spots and Hf isotope ratios at 61 spots in zircons from five plutons in the Taitao Peninsula, where slab-melting occurred at ca. 4-5 Ma. In addition, we measured whole-rock Hf-O isotope ratios in 11 rocks that were intruded by the granitoids. The zircon delta O-18 values of the Seno Hoppner pluton (5.37 +/- 0.44 parts per thousand) are identical to those of mantle-equilibrated zircons, whereas those of the other four plutons are relatively high (6.09-6.53 parts per thousand). The epsilon Hf(t) values of zircons in the granitoids are negatively correlated with delta O-18, and they fell along the mixing line between a juvenile component and sedimentary rocks. Therefore, the Hf-O profiles can be attributed to mixing of juvenile granitoid magma and the sedimentary rocks, the amount of which was likely minimal in the Seno Hoppner pluton. As such, the delta O-18 values of the Seno Hoppner pluton could best reflect the juvenile component. In view of the delta O-18 variation in whole-rock samples of the Taitao ophiolite, the melting of hydrothermally altered basalt and dolerite is considered responsible for the juvenile magma generation. This further implies that juvenile granitoid magmas can be generated by the melting of the upper half of subducted oceanic crusts. (C) 2020 Elsevier B.V. All rights reserved.