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Title: Data report: major and trace element and Nd-Pb-Hf isotope composition of the Site U1504 metamorphic basement in the South China Sea (IODP Expedition 367/368/368X)
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.  more » « less
Award ID(s):
1326927
NSF-PAR ID:
10320094
Author(s) / Creator(s):
; ; ;
Date Published:
Journal Name:
Proceedings of the International Ocean Discovery Program
Volume:
367/368
ISSN:
2377-3189
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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  1. null (Ed.)
    International Ocean Discovery Program Expedition 368 is the second of two consecutive cruises that form the South China Sea Rifted Margin program. Expeditions 367 and 368 share the common key objectives of testing scientific hypotheses of breakup of the northern South China Sea (SCS) margin and comparing its rifting style and history to other nonvolcanic or magma-poor rifted margins. Four primary sites were selected for the overall program: one in the outer margin high (OMH) and three seaward of the OMH on distinct, margin-parallel basement ridges. These three ridges are informally labeled A, B, and C. They are located within the continent–ocean transition (COT) zone ranging from the OMH to the interpreted steady-state oceanic crust (Ridge C) of the SCS. The main scientific objectives include 1. Determining the nature of the basement within crustal units across the COT of the SCS that are critical to constrain style of rifting, 2. Constraining the time interval from initial crustal extension and plate rupture to the initial generation of igneous ocean crust, 3. Constraining vertical crustal movements during breakup, and 4. Examining the nature of igneous activity from rifting to seafloor spreading. In addition, the sediment cores from the drill sites targeting primarily tectonic and basement objectives will provide information on the Cenozoic regional environmental development of the Southeast Asia margin. Expedition 368 was planned to drill at two primary sites (U1501 and U1503) at the OMH and Ridge C, respectively. However, based on drilling results from Expedition 367, Expedition 368 chose to insert an alternate site on Ridge A (Site U1502). In total, the expedition completed operations at four sites (U1501, U1502, U1504, and U1505). Site U1503, however, was not completed beyond casing to 990 m because of mechanical problems with the drilling equipment that limited the expedition from 25 May 2017 to the end of the expedition to operate with a drill string not longer than 3400 m. New alternate Site U1504 proposed during Expedition 367 met this condition. Site U1505 also met the operational constraints of the 3400 m drill string (total) and was an alternate site for the already drilled Site U1501. At Site U1501, we cored to 697.1 m in 9.4 days, with 78.5% recovery. We also drilled ahead for 433.5 m in Hole U1501D and then logged downhole data from 78.3 to 399.3 m. In 19.3 days at Site U1502, we penetrated 1679.0 m, set 723.7 m of casing and cored a total of 576.3 m with 53.5% recovery, and collected downhole log data from 785.3 to 875.3 m and seismic data through the 10¾ inch casing. At Site U1503, we penetrated 995.1 m, setting 991.5 m of 10¾ inch casing, but no cores were taken. 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Efforts were made at every drill site to correlate the core with the seismic data and seismic stratigraphic unconformities interpreted within the Eocene to Plio–Pleistocene sedimentary sequence prior to drilling. The predrilling interpretation of ages of these unconformities was in general confirmed by drilling results. As a result of the constraints on the length of drill string that could be deployed during the later part of Expedition 368, the secondary expedition objectives addressing the environmental history of the SCS and Southeast Asia received more focus than planned because these sites are located in shallower water depths and required less penetration depth. This forced change in emphasis, however, was without fatal consequences for the primary tectonic objectives. The two expeditions together provided solid evidence for a process of breakup that included vigorous synrift magmatism as opposed to the often-favored interpretation of the SCS margin as a magma-starved margin. 
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The main scientific objectives include the following: Determining the nature of the basement in crustal units across the COT of the SCS that are critical to constrain style of rifting, Constraining the time interval from initial crustal extension and plate rupture to the initial generation of igneous ocean crust, Constraining vertical crustal movements during breakup, and Examining the nature of igneous activity from rifting to seafloor spreading. In addition, the sediment cores from the drill sites targeting primarily tectonic and basement objectives will provide information on the Cenozoic regional environmental development of the Southeast Asia margin. Site U1499 on Ridge A and Site U1500 on Ridge B were drilled during Expedition 367. Expedition 368 was planned to drill at two primary sites (U1501 and U1503) at the OMH and Ridge C, respectively, but based on drilling results from Expedition 367, Expedition 368 chose to insert an alternate site on Ridge A (Site U1502). 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