Major,trace element and Sr–Nd–Pb–Hf isotopic compositions of Cenozoic basalts in NE China.
We focus on temporal variations in the major element, trace element, and Sr–Nd–Pb–Hf isotopic data of Cenozoic basalts in NE China to better understand deep mantle processes and shed new light on the formation and evolution of the big mantle wedge. The ~52 Ma basalts were derived from partial melting of lithospheric mantle modified by slab-derived fluids and recycled sediments, whereas the ~33 Ma and ~21 Ma basalts were derived from partial melting of asthenospheric mantle. The ~11 Ma basalts were derived from partial melting of residual lithospheric mantle that was the source of the older Cenozoic basalts. However, a larger contribution from asthenospheric mantle is required in the source of ~2 Ma basalts. These findings and published data reveal sustained large-scale upwelling of asthenospheric mantle in NE Asia during the late Eocene to Miocene. The ~33 Ma basalts are interpreted to represent the magmatic response to rollback of the subducting slab and initiation of the big mantle wedge, whereas the ~21 Ma and ~11 Ma basalts record retreat of the trench and opening of the Japan Sea during the final stages of formation of the big mantle wedge. Asthenospheric flow from intracontinental to marginal regions, triggered by rollback of the Pacific slab and retreat of the trench, was the key mechanism that led to opening of the Japan Sea and formation of the big mantle wedge in NE Asia.
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1. Major and trace elements The analysis of whole-rock major and trace elements was completed in the laboratory of Wuhan Shangpu Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The major elements were determined by large-scale X-ray fluorescence spectrometry (XRF); the analytical precision was better than 2%–5%. The analysis of trace elements was performed by Agilent 7700e inductively coupled plasma mass spectrometry (ICP-MS). Four standards (AGV-2, BHVO-2, BCR-2 and RGM-2) were used to monitor the analytical quality; the analytical precision was better than 5%. The detailed sample-preparing procedure is shown in Liu et al. (2008). 2. Sr–Nd–Pb–Hf isotopes Whole-rock Sr–Nd–Pb–Hf isotope data were measured on Neptune Plus MC-ICP-MS at Wuhan Shangpu Solution Analytical Technology Co., Ltd. Mass discrimination correction was carried out via internal normalization using an 88Sr/86Sr ratio of 8.375209 and a 146Nd/144Nd ratio of 0.7219 (Lin et al., 2016). NBS987 and GSB were used as certified reference standard solutions for 87Sr/86Sr and 143Nd/144Nd isotope ratios, with results of 87Sr/86Sr = 0.710241 ± 12 (2σ) and 143Nd/144Nd = 0.512439 ± 1 (2σ). The detailed experimental procedure of Sr–Nd isotopes is shown in Li et al. (2012). All measured Pb isotope ratios were normalized to the established NBS981 values of 208Pb/204Pb = 36.7262 ± 3, 207Pb/204Pb = 15.5000 ± 13, and 206Pb/204Pb = 16.9416 ± 13. The detailed experimental procedure of Pb isotope is shown in Baker et al. (2004). All measured Hf isotope ratios were normalized by the AlfaHf. Analyses of the AlfaHf standard yielded a 176Hf/177Hf ratio of 0.282224 ± 15(2σ). The detailed experimental procedure of Hf isotope is shown in Weis et al. (2007).