Qiu, Liang, Li, Xue, Li, Xiaowei, Yan, Dan-Ping, Ren, Minghua, Zhang, Liangliang, and Cheng, Guangsuo
Voluminous magmatism and extensional deformation are the prominent late Mesozoic tectonic features in East Asia, which recorded the evolutionary history of the continental marginal lithosphere. Due to the thick covering of Quaternary and Pliocene sediments in the region, the Early Cretaceous magmatism in northeastern South China is not well constrained. In this study, we collected samples from the boreholes in Shanghai, East China and provided geochronological, elemental and isotopic analyses of andesite and granodiorite. Zircon U Pb age results indicate that andesites and granodiorites were formed at ca. 135 to 102 Ma. The andesites exhibit moderate MgO contents, slightly radiogenic Sr (87Sr/86Sr(t) = 0.706149 to 0.706170) and variable unradiogenic Nd and Hf isotopic compositions (ε Nd (t) = −5.32 to −5.12; ε Hf (t) = −10.8 to −0.6). The granodiorites exhibit arc-like trace-element patterns with high Sr/Y ratios, as well as slightly radiogenic initial Sr (87Sr/86Sr(t) = 0.706754 to 0.706790) and moderately unradiogenic Nd and Hf isotopic compositions (ε Nd (t) = −6.54 to −6.27; ε Hf (t) = − 4.0 to −6.7). Both andesites and granodiorites exhibit similar initial Pb isotopic compositions (andesites: 206Pb/204Pb = 18.232–18.259, 207Pb/204Pb = 15.547–15.548, and 208Pb/204Pb = 38.338–38.350; granodiorites: 206Pb/204Pb = 17.930–17.946, 207Pb/204Pb = 15.542–15.543, and 208Pb/204Pb (38.256–38.293). The results recommend that the ca.130 Ma andesites were produced by the partial melting of an enriched lithospheric mantle, which was previously metasomatized with sediments in the source region in a continental arc setting of the Paleo-Pacific subduction. The ca.102 Ma adakitic granodiorites probably originated from partial melting of the lower crust with the addition of mantle-derived magmas. With the consolidation of previous studies, we argue that the flat slab subduction of the paleo-Pacific Plate led to the formation of these intermediate to felsic magmatism. • The borehole data provide the first line of evidence of Early Cretaceous magmatism in Shanghai. • The andesites originated from an enriched mantle wedge source with source contamination by sediments. • The Early Cretaceous magmatic rocks in Shanghai were formed in response to the flat subduction of the paleo-Pacific Plate. [ABSTRACT FROM AUTHOR]