Zircon U–Pb ages, Hf–O isotopes and trace elements of Mesozoic high Sr/Y porphyries from Ningzhen, eastern China: Constraints on their petrogenesis, tectonic implications and Cu mineralization

Abstract The relationship between high Sr/Y (adakitic) rocks and Cu mineralization has been long recognized but the mechanism remains unclear. The Cretaceous high Sr/Y porphyries in the Ningzhen area host major Cu polymetallic deposits in the Lower Yangtze River Belt (LYRB) of eastern China. These rocks exhibit some geochemical characteristics (e.g., non-radiogenic Pb isotope ratios) that differ from adakitic rocks from adjacent locations in the LYRB. In this study, we present a study of the zircon U–Pb–Hf–O isotope and trace element compositions for five porphyries from Ningzhen to reveal their petrogenesis and how that correlates with Cu–Fe–Mo mineralization. Zircon U–Pb ages of Anjishan (Cu deposit), Tongshan (Cu–Mo deposit) and Xiangshan (Fe deposit) plutons in the Ningzhen area are 108.8 ± 1.5 Ma, 105–107 Ma and 100–105 Ma, respectively, which are significantly younger than the ore-bearing adakites (140 ± 5 Ma) in the western part of the LYRB. Zircon εHf(t) and δ18O values range from − 23.4 to − 10.6 and from 5.7 to 7.0‰, respectively, falling between subduction-related adakites from the other regions in the LYRB and delamination-related adakitic rocks from the adjacent South Tan-Lu Fault Zone. The similarities of Ce4 +/Ce3 + and Eu/Eu* ratios in zircons from Ningzhen and those from the western LYRB indicate higher oxygen fugacity in their magma sources. Ti-in-zircon thermometer yields magma temperatures of 550 to 700 °C (with an average of ~ 650 °C) for the Ningzhen porphyries, which are significantly lower than those of the South Tan-Lu Fault adakites (> 750 °C), but similar to those for the LYRB adakites ( 50), non-radiogenic Pb, enriched Sr–Nd isotopic compositions, negative zircon εHf(t), mantle-like δ18O values, high oxygen fugacities and low magma temperatures. Mafic rocks that co-exist with ore-bearing porphyries or occur as xenoliths in porphyries are widespread. We proposed that the Ningzhen high Sr/Y porphyries originated from mixing of magmas from metasomatized mantle and ancient lower continental crust. The mantle-derived basaltic magmas supplied not only the heat input, but also the elevated fo2 and water for the generation of Cu–Fe–Mo mineralization. The compiled ages of Mesozoic igneous rocks in the LYRB display an eastward decrease from inland towards the coast. This may suggest rolling back of a flat subducted oceanic plateau; this has been argued to account for the generation of igneous rocks and associated Cu–Fe–Mo mineralization in the LYRB.