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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. 

Models of subduction zone magmatism ascribe the andesitic composition of arc magmas to crustal processes, such as crustal assimilation and/or fractional crystallization, that basaltic mantle melts experience during their ascent through the upper plate crust. However, results from time series study of olivine-phyric high-Nb basalts and basaltic andesites from two monogenetic arc volcanoes (V. Chichinautzin and Texcal Flow) that are constructed on the ~45 km thick continental basement of the central Transmexican Volcanic Belt (TMVB) are inconsistent with this model. Instead, ratios of radiogenic isotope and incompatible trace elements suggest that these volcanoes were constructed through multiple individual melt batches ascending from a progressively changing mantle source. Moreover, the high Ni contents of the olivine phenocrysts, together with their high mantle-like 3He/4Heoliv =7-8 Ra with high crustal δ18O oliv = +5.5 to +6.5‰ (n=12) point to the presence of secondary ‘reaction pyroxenites’ in the mantle source that create primary silicic arc magmas through melt-rock reaction processes in the mantle [1, 2] . Here we present additional trace element concentration of the high-Ni olivines by electron microprobe (Mn, Ca) and laser-ablation ICPMS (Li, Cr and V) analysis in order to test this model. Olivine Li (2-7 ppm) and Mn (1170- 2810 ppm) increase with decreasing fosterite (Fo89 to Fo75), while Cr (29-364 ppm), V (4-11 ppm) and Ca (825-2390 ppm) decrease. Quantitative modeling shows that these trends in their entirety cannot be controlled by fractional crystallization under variable melt water H2O or oxygen fugacity (fO2), or co-crystallization of Cr-spinel. Instead, the variations support the existence of compositionally distinct melt batches during earliest melt evolution. Moreover, the trace element trends are qualitatively consistent with a model of progressive source depletion by serial melting (shown in olivine Ca, V and Cr) that is triggered by the repetitive addition of silicic slab components (shown by olivine Li). These findings suggest mantle source variations are not eliminated despite the thick crust these magmas pass during ascent. [1] Straub et al. (2013) J Petrol 54 (4): 665-701; [2] Straub et al. (2015) Geochim Cosmochim Acta 166: 29-52.