Cheng-Cheng Wang

Zircon Hf-O isotopic fingerprints in accretionary vs. collisional orogenesis

Zircon Hf isotopes are able to identify the relative proportion of mantle and crustal inputs to magma systems, and different Hf isotopic patterns can be expected in different tectonic settings. Coupled analysis of O with Hf isotopes has been proposed as an effective way to identify whether the magma was contaminated by sedimentary material. Combined zircon Hf-O isotopes have been widely used in the studies of magma petrogenesis and source composition. However, regional investigations of O isotopes in well-defined tectonic settings are scarce, so it is unclear if the O isotopic evolution can be linked with geodynamic settings.

We compiled Hf-O isotopic data of magmatic zircons from our current studies in four orogenic systems, including two accretionary orogens (the Gothian-Telemarkian orogen in western Fennoscandia and the Maud Belt in East Antarctica), and two orogens that formed related to continental collision (the East Africa-Antarctic Orogen and the Sveconorwegian belt). Their Hf-O isotopic patterns are compared with those of the arc magmas from the Paleo-Pacific margin and syn- and post-collisional magmas from the Himalaya-Tibet, which are considered as typical accretionary and collisional settings respectively. Zircon εHf(t) and δ¹⁸O values show a clearly negative correlation within subduction-related magmas. The most straightforward interpretation is the involvement of at least two sources, a mantle source with juvenile Hf and mantle-like O isotope values and an older crustal/sedimentary source with evolved Hf and elevated O values. In contrast, collision-related magmatic zircon shows a wide spread of Hf isotopic values, associated with relatively homogeneous O isotopic compositions. This probably reflects a mixed source, namely older lower crust and subordinate enriched mantle, which have similarly moderate O but significantly distinct Hf isotope values, while the sedimentary contribution is significantly less obvious than in arc magmas. This study indicates that the distinct zircon Hf and O isotopic evolution appears to provide a good potential to test accretionary vs. collisional orogenesis, although it remains to be explored further if the observation described here applies universally.