Geochemical nature of sub-ridge mantle and opening dynamics of the South China Sea

Geochemistry and Geochronology of the Accreted Mafic Rocks From the Hengchun Peninsula, Southern Taiwan: Origin and Tectonic Implications

“…The elements from Nd to Lu show a generally flat pattern, which is indicative of oceanic basalt (Pearce, ). All of these trends are similar to those of the average N‐MORB (Sun & McDonough, ) and the N‐MORBs from the SCS (G. L. Zhang et al, ). The N‐MORB types also have trace element ratios (Zr/Nb = 14.13–47.61, Zr/Hf = 25.71–40.54, Nb/Ta = 10.04–17.73) that resemble those of typical N‐MORB (~30, ~36, and 6–14, respectively; Rollinson, ).…”

“…The N‐MORB types display concordant light rare earth element (LREE)‐depleted chondrite‐normalized patterns (Figure a) parallel to the average N‐MORB pattern (Sun & McDonough, ), thus distinguishing them from OIB and E‐MORB, which are LREE‐enriched relative to heavy rare earth elements (HREEs) and exemplified by ΣLREE/ΣHREE = 1.07–1.86, (La/Yb) N = 0.45–1.18 and (La/Sm) N = 0.48–1.01. These compositions are similar to those of the N‐MORBs (ΣLREE/ΣHREE = 1.25–1.56, (La/Yb) N = 0.57–0.87, (La/Sm) N = 0.53–0.75) recovered at IODP Site U1431 in the SCS (G. L. Zhang et al, ). The nearly flat HREE patterns represent melts that formed at <90 km in the asthenosphere, that is, above the garnet‐peridotite stability field (Said & Kerrich, ).…”

“…The E‐MORB‐type rocks exhibit a compositional transition from subalkaline to alkaline (Figure b), with Nb/Y ratios ranging from 0.19 to 0.84; they thus resemble the basaltic rocks from the Huatung Basin (Nb/Y = 0.14–1.14; Hickey‐Vargas et al, ). In contrast, the E‐MORBs from the SCS (IODP Site U1431 in the east subbasin, Nb/Y ~ 0.33; Sites U1433 and U1434 in the Southwest subbasin, Nb/Y = 0.16–0.29) are all alkaline (Figure a; G. L. Zhang et al, ). The chondrite‐normalized REE patterns of the E‐MORB‐type rocks are shown in Figure c and compared with those of the E‐MORB‐like rocks from the Huatung Basin (Hickey‐Vargas et al, ) and the SCS (G. L. Zhang et al, ).…”

“…In contrast, the E‐MORBs from the SCS (IODP Site U1431 in the east subbasin, Nb/Y ~ 0.33; Sites U1433 and U1434 in the Southwest subbasin, Nb/Y = 0.16–0.29) are all alkaline (Figure a; G. L. Zhang et al, ). The chondrite‐normalized REE patterns of the E‐MORB‐type rocks are shown in Figure c and compared with those of the E‐MORB‐like rocks from the Huatung Basin (Hickey‐Vargas et al, ) and the SCS (G. L. Zhang et al, ). The E‐MORB‐type rocks within the Hengchun accretionary prism exhibit REE patterns that are transitional between those of N‐MORB and OIB (ΣLREE/ΣHREE = 1.97–3.64, (La/Sm) N = 1.05–1.92, (La/Yb) N = 1.22–3.37).…”

“…On the ε Nd‐ ε Hf diagram (Figure ), the alkaline OIBs from the Hengchun Peninsula define a trend below the mantle array that curves inward to the OIB field with radiogenic Nd together with the rocks collected from IODP Site U1431 in the SCS, similar to the case of the Cook‐Austral island chain in the South Pacific (Salters et al, ), indicating the existence of at least three components in this suite: DMM, HIMU, and EMII. The N‐MORBs within the Hengchun accretionary prism are less influenced by enriched components compared with the N‐MORB‐like basalts from the fossil ridge of the SCS (Site U1431; G. L. Zhang et al, ), because the latter represents the final products of the spreading ridge and was formed spatially and temporally close to the emplacement of postspreading magmatism (seamounts). Most likely, the N‐MORB types within the Hengchun accretionary prism were less/not affected by the EMII‐related Hainan Plume (Yan et al, , ), which upwelled the recycled oceanic crustal components from the deep mantle and interacted with the lithospheric mantle, resulting in carbonated silicate melts (G. L. Zhang et al, ) and OIBs with HIMU‐type Hf‐Nd isotopic signatures in the SCS.…”