Geochemical transition shown by Cretaceous granitoids in southeastern China: Implications for continental crustal reworking and growth

Abstract

Zircon U–Pb ages and in-situ trace elements and Hf–O isotope compositions, together with whole rock geochemical and Sr–Nd–Hf isotopic data, are presented for Cretaceous granitoids in southeastern (SE) China in order to establish their origin and the evolution of the underlying lithosphere during the Late Mesozoic. Two stages of Cretaceous magmatism, with contrasting geochemical features, have been identified: an earlier adakite-like biotite granite as represented by the Shangying pluton and a later enclave-bearing monzogranite as represented by the Zaoshan pluton. The Shangying biotite granites have a zircon U–Pb age of 99±1Ma. They have relatively low Y and Yb contents, with high La/Yb and Sr/Y ratios, showing geochemical features of adakite. Their Sr–Nd–Hf isotope compositions are similar to those of Early Cretaceous mafic rocks in the same area, indicating that they were generated by partial melting of juvenile granulitic crust at a depth of about 40km. The source was formed by underplating of enriched lithosphere mantle-derived magmas. In contrast, the Zaoshan calc-alkaline monzogranites, their enclaves and associated dolerite dykes from the Zaoshan pluton have an emplacement age of ~88±1Ma. The dolerites have high MgO contents, relatively low SiO2 concentrations and low La/Yb ratios, and depleted Hf isotope compositions. All these geochemical features suggest that they were derived from a depleted spinel Iherzolite mantle source. The enclaves have high SiO2 contents, indicating that they were derived from a crustal source. They have variable zircon Hf and O isotope compositions, suggesting that two components, i.e., a high εHf(t) and a low δ18O component and a low εHf(t) and high δ18O component, were involved in their origin. The high zircon εHf(t) values and low δ18O values are similar to those of the dolerites, indicating a common source. Thus, we suggest that the enclaves were generated by partial melting of newly underplated depleted mantle-derived materials. The monzogranites have distinctly different zircon Hf and O isotope compositions from the enclaves, indicating that the parental magmas were mainly derived from ancient crust that interacted with underplated depleted mantle-derived magmas. The monzogranites have relatively high HREE contents, suggesting a garnet-free source (<32km), distinct from the Early Cretaceous adakite-like granites that were generated from a garnet-bearing source. Combined with previously published data, it is evident that Early Cretaceous adakite-like magmatic rocks (107–99Ma) and associated mafic rocks (100–107Ma) were widespread in SE China, indicating a crustal thickening event that was possibly induced by underplating of mantle-derived magma. Subsequently, between 99 and 87Ma, crustal extension and lithospheric thinning induced the later widespread I-type granites with similar geochemical features to the Zaoshan pluton. A-type granites and syenites of this age are also present in SE China. The transition in geochemical and isotopic data, from enriched to depleted Hf isotope compositions as seen in the monzogranites, their enclaves and intrusive dolerites, suggest that depleted mantle-derived materials were involved in the generation of the monzogranites, further indicating continental crustal reworking and later crustal growth in SE China during the early Late Cretaceous.