Mantle-like to low oxygen isotopes in zircon from the mid-Cretaceous high-silica granites reveal unweathered basement recycling along the present coastal area of SE China

Abstract

Zircon oxygen isotope compositions can provide potential constraints on the origin of granitic magmas. In this work, we report new data of zircon UPb dating, trace elements and HfO isotopes for two types of mid-Cretaceous high-silica granites from Zhoushan archipelago in Zhejiang Province, SE China. The in-situ zircon data are carefully screened for radiation damage and post-magmatic alteration. The analyzed magmatic zircon grains indicate that the Zhoushan calc-alkaline granites (ZCAG; 101–92 Ma) have mantle-like δ18O values of 5.29–5.88 ‰, in contrast to the Zhoushan peralkaline granites (ZPAG; 91–89 Ma) with relatively low δ18O of 4.42–4.6 ‰. The slight decreases in δ18O and εHf(t) from zircon center to periphery of both groups suggest that they may have consistent oxygen isotopic compositions in their parental magma sources and have gone through assimilations to hydrothermally altered wall rocks in the shallow crust. The mantle-like δ18O values in zircon are widespread in the Cretaceous felsic rocks in the present coastal area of SE China, which, in combination Hf isotopes, indicates the possible existence of a buried unweathered Proterozoic basement in the eastern part of the Cathaysia Block. This work highlights the role of deep crustal magma sources, assimilation and post-magmatic radioactive damage on the formation of mantle-like to low δ18O signals in zircons, and has fundamental implications for understanding the formation of widespread mid-Cretaceous felsic rocks along the present coastal area of SE China.