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During International Ocean Discovery Program Expeditions 367/368/368X, Hole U1501D was cored on the continental shelf (2846 meters below sea level) in the northern South China Sea (SCS). In Hole U1501D, sediments were recovered from 433.5 to 644.3 meters below seafloor (mbsf) and the acoustic basement was penetrated at 598.91 mbsf. The acoustic basement is a stratigraphic boundary at which late Eocene Cenozoic sediments likely unconformably overlay heterolithic Mesozoic sandstones that are intercalated with rare siltstones and subordinate conglomerate with pebble- and cobble-sized igneous clasts of proximal provenance. Here, we present major and trace elements and Sr-Nd-Pb-Hf isotope data of a fine-grained granite pebble, a medium-grained granite cobble, and a porphyritic volcanic pebble. The data show that these clasts are relics of the Mesozoic subduction-related magmatism that was active along the southeast Asian margin prior to the Cenozoic rifting. The Pb isotope composition of the clasts partially overlaps with the enriched Cenozoic mid-ocean-ridge basalt type and intraplate basalts of the SCS. However, the clasts are distinct from the Cenozoic basalt volcanism in Sr-Nd-Hf isotope space. Thus, Sr-Nd-Hf isotope systematics of the Cenozoic basalts might be useful in detecting traces of crustal contamination in the earliest rift basalts of the SCS that may have erupted through the Mesozoic continental basement. 

During International Ocean Discovery Program (IODP) Expedition 367/368/368X, Holes U1504A and U1504B were cored on the continental shelf (2817–2843 meters below sea level) in the northern South China Sea (SCS). A total of 106 m of metamorphic basement was penetrated that consists of greenish gray, deformed mylonitic, epidote-chlorite to calc-silicate schists containing granofels clasts ("greenschist"). Here we report bulk-rock major and trace element data from 17 greenschist samples, from which a subset of 9 samples was additionally analyzed for Pb-Nd-Hf isotope ratios. Fluid-mobile elements (U, Li, Rb, K, and Cs) behave somewhat erratically, yet tectonic discrimination and primitive mantle–normalized multielement diagrams reveal signatures that are typical for enriched intraplate basalts. These include a negative Pb anomaly (Ce/Pb = 34 ± 10), relative enrichments of Nb and Ta (Nb/La = 1.5 ± 0.3; Th/Nb = 0.07 ± 0.01), and a steep rare earth element pattern (La/Sm = 3.7 ± 0.7; Ho/Lu = 2.9 ± 0.2). The high values of the uranogenic 206Pb/204Pb (21.2–25.9) and 207Pb/204Pb (15.7–16.0) and their strong correlation point to a postformation "U addition event" that took place at 329 Ma ± 2 My (late Carboniferous). 143Nd/144Nd and 176Hf/177Hf data are consistent with the origin from an enriched Paleozoic age mantle source. In summary, our data suggest that the protolith of the Site U1504 metamorphic basement was an ocean-island basalt–type igneous rock that deformed during the late Paleozoic and was part of the prerift crustal basement of the SCS Basin.