Geochemistry of Cenozoic basalts from the Bohai Bay Basin: Implications for a heterogeneous mantle source and lithospheric evolution beneath the eastern North China Craton

Lithos 196–197 (2014) 54–66

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Geochemistry of Cenozoic basalts from the Bohai Bay Basin: Implications for a heterogeneous mantle source and lithospheric evolution beneath the eastern North China Craton Hong-Yan Li, Xiao-Long Huang ⁎, Hua Guo State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 29 September 2013 Accepted 21 February 2014 Available online 11 March 2014 Keywords: Cenozoic Alkali basalts Lithospheric thinning Lithospheric thickening Bohai Bay Basin North China Craton

a b s t r a c t Eocene and Miocene alkali basalts were collected from boreholes in the Jiyang Sag of the Bohai Bay Basin with the aim of investigating lithospheric evolution beneath the eastern North China Craton (NCC). These alkali basalts have oceanic island basalt (OIB)-like trace element characteristics and overall depleted Nd–Hf isotopes (εNd(t) = 2.4–6.3; εHf(t) = 7.0–10.1), consistent with derivation from slightly enriched asthenosphere. The Miocene (b 23 Ma) Guantao basalts, like the Miocene basalts in Shandong, have relatively higher concentrations of incompatible trace elements and are more depleted in heavy rare earth elements (HREEs) than the Eocene Shahejie basalts (47–45 Ma), indicating a larger proportion of melts derived from the garnet peridotite/pyroxenite. We interpret this difference as resulting from lithospheric thickening. Lithospheric thickness, estimated from the basalt geochemistry, is ~65–85 km at ~45 Ma and ~85–95 km after about 23 Ma. This inferred lithospheric thickening beneath the eastern NCC after the end of the Oligocene is consistent with decreasing paleoheat flow in the Bohai Bay Basin from ~23 Ma to the present. The geochemical variations of the Cenozoic basalts in the Jiyang Sag reveal that the lithospheric thinning beneath the eastern NCC was on-going at ~45 Ma, but ceased after the end of the Oligocene. © 2014 Elsevier B.V. All rights reserved.

1. Introduction It is well established that the lithospheric mantle beneath the eastern North China Craton (NCC) had been thinned in the Late Paleozoic– Cenozoic (Fan and Menzies, 1992; Gao et al., 2004; Griffin et al., 1992, 1998; Menzies et al., 1993; Xu, 2001; Xu et al., 2004a; Yang et al., 2008; Zheng et al., 1998). However, the timing and cause of this thinning event remain controversial (e.g., Menzies et al., 2007; Wu et al., 2008; Xu et al., 2009; Zhu et al., 2011). Some studies have advocated that the lithospheric thinning mainly occurred by means of delamination in the Jurassic or Early Cretaceous (e.g., Deng et al., 1994; Gao et al., 2004, 2008, 2009; Liu et al., 2008; F.Y. Wu et al., 2005), whereas Xu et al. (2004a, 2009) suggested a prolonged (N100 Ma) thinning process probably starting from the Late Triassic–Early Jurassic to Late Cretaceous–Early Cenozoic. Extensive tectonic extension and paleotopographic subsidence in the eastern NCC in the Early Cenozoic (Hu et al., 2001) have been interpreted as an isostatic effect of lithospheric thinning (Qi and Yang, 2010), however, this hypothesis has not been tested. The compositional variation of mantle-derived rocks in time and space can help reveal the nature of lithospheric evolution ⁎ Corresponding author. Tel.: +86 20 85290010; fax: +86 20 85291510. E-mail address: [email protected] (X.-L. Huang).

http://dx.doi.org/10.1016/j.lithos.2014.02.026 0024-4937/© 2014 Elsevier B.V. All rights reserved.

(e.g., Bianchini et al., 2008; Gao et al., 2008; Griffin et al., 1992, 1998; Guo et al., 2001; Hong et al., 2013; Huang et al., 2012; Kuang et al., 2012; Liu et al., 2008; Martins et al., 2008; Menzies et al., 1993, 2007; Thirlwall et al., 1994; Xu et al., 2004b; Y.G. Xu et al., 2012; Yang and Li, 2008; Zhang et al., 2002). Cenozoic lithospheric evolution beneath the eastern NCC has not been well recognized in the previous studies, primarily because: (1) Seismic tomography revealed the thinnest lithosphere beneath the Bohai Bay Basin, the largest Cenozoic basin in the eastern NCC (Chen, 2010; Tian et al., 2009), but few geochemical and age data are available on basalts from the Bohai Bay Basin because of limited exposure (Xu et al., 2004a); (2) Most studies have focused on the magmatism in the Early Cretaceous (e.g., Gao et al., 2008; Guo et al., 2001; Huang et al., 2012; Xu et al., 2004b; Yang and Li, 2008; Zhang et al., 2002) and Late Cenozoic (b23 Ma) (e.g., Chen et al., 2007; Fan and Hooper, 1991; Song et al., 1990; Tang et al., 2006; Xu et al., 2004a, 2005; Y.G. Xu et al., 2012; Zeng et al., 2010, 2011; Zhang et al., 2009; Zhi et al., 1990; Zhou and Armstrong, 1982), but there are few studies on the Early Cenozoic (66–23 Ma) basaltic rocks (Dong et al., 2010; Guo et al., 2006; C.Z. Wu et al., 2005). Borehole basalts interlayered in the Eocene–Miocene sediments from the Jiyang Sag of the Bohai Bay Basin (Guo et al., 2006) offer an excellent opportunity to reveal the evolution of mantle source in the region in terms of lithospheric thinning beneath the eastern NCC.

H.-Y. Li et al. / Lithos 196–197 (2014) 54–66

In this study, we present 40Ar/39Ar ages, whole-rock major and trace element analyses and Sr–Nd–Hf isotopic data for the Eocene–Miocene basalts from the Jiyang Sag to constrain the petrogenesis of these basalts, and the nature of mantle source in the context of understanding the overall lithospheric evolution beneath the eastern NCC, particularly its thinning/thickening processes in the Cenozoic. 2. Geological setting and sampling The Bohai Bay Basin covers ~30–40% of the eastern NCC and borders the Yinshan–Yanshan Orogenic Belt in the north, the Taihang Orogen Uplift (Trans-North China Orogen) in the west, the Western Shandong Uplift in the southeast, and the Jiaodong and Liaodong uplifts in the east (Fig. 1a). The basin consists of the Jizhong, Linqing, Huanghua, Jiyang, Bozhong and Linqing Sags separated by uplifted older strata (Fig. 1a). The basement of the Bohai Bay Basin is composed of Archean granitic gneisses, Paleozoic carbonate deposits and terrigenous clastic sediments, and Jurassic–Early Cretaceous terrigenous volcanic and clastic sediments (Li et al., 2012). The cover strata include the Kongdian (Eocene, 56–51 Ma), Shahejie (Eocene–Oligocene, 51–33 Ma), Dongying (Oligocene, 33–23 Ma), Guantao (Miocene, 23–12 Ma), Minghuazheng (Miocene–Pliocene, 12–2.6 Ma) and Pingyuan (Quaternary, 2.6–0 Ma) groups from the bottom upward (Hu et al., 2001; Fig. 1b). The groups all consist of volcanic and/or terrigenous clastic sediments (Dong et al., 2010; Guo et al., 2006). The volcanic layers in the Bohai Bay Basin have the thickness of N 1 km and can be subdivided into several eruption cycles (Fig. 1b; Dong et al., 2010; Guo et al., 2006). The basaltic samples were collected from the industrial boreholes in the Jiyang Sag of the southeastern Bohai Bay Basin (Fig. 1a), and belong to the Shahejie and Guantao groups (Fig. 1b). The basaltic samples are all porphyritic with a highly variable phenocryst assemblage throughout the volcanic sequence. The samples from the Lower section of the Shahejie group (the Lower Shahejie basalts)

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only contain some plagioclase (b5%) phenocrysts, while those from the Upper section of the Shahejie group (the Upper Shahejie basalts) have abundant pyroxene (5–20%) and plagioclase phenocrysts (10–20%) and minor olivine phenocrysts (b2%). The phenocrysts of samples from the Guantao group (the Guantao basalts) are dominated by olivine (b10%) and pyroxene (5–10%). Most olivine phenocrysts have been partially altered to be iddingsite (Fig. 2d). Microlitic clinopyroxene, plagioclase and Fe–Ti oxides are the major groundmass phases (Fig. 2). The samples all have vesicular and amygdaloidal textures filled with carbonate (Fig. 2c). 3. Analytical methods The bulk-rock 40Ar/39Ar age, major and trace elements and Sr–Nd– Hf isotope data were all obtained at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The samples (Y181 and Y2510) for 40Ar/39Ar dating were powdered into 60–80 mesh and then leached with 10% HNO3, washed with deionized water in an ultrasonic bath, and dried out at last. The 40Ar/ 39 Ar dating was carried out using a GVI5400 mass spectrometer following the analytical procedures described by Qiu and Jiang (2007). Argon gas was extracted from the sample by stepheating using a MIR10 CO2 continuing laser. The released gasses were purified by two Zr/Al getter pumps operated for 5 to 8 min at room temperature and ~450 °C respectively. The background of the sample hold is lower than 2 mV pre-experiment and 4–6 mV during experiment (after 5 minute vacuum), while the signal of the sample is mostly controlled within the range of 40–200 mV. The ArArCALC program (v. 2.2; Koppers, 2002) was used for data reduction and age calculation. The J-values for the samples were determined by ZBH-2506 biotite (132 Ma) flux monitors. Relatively fresh samples were selected for bulk-rock analyses. The samples were crushed into small chips (~0.5 cm in maximum length), and the freshest fragments with few amygdaloids and secondary veins

Fig. 1. (a) Simplified tectonic scheme of the North China Craton (modified after Li et al., 2013), showing the location of Jiyang Sag of the Bohai Bay Basin and the sampling locations (solid star: this study; open star: the Miocene basalts in Shandong (Zeng et al., 2011)). NQO: Northern Qinling Orogen; SQO: Southern Qinling Orogen; SSZ: Shangdan suture zone; MSZ: Mianlue suture zone; BBB: Bohai Bay Basin; WSU: Western Shandong Uplift; JYS: Jiyang Sag; LHS: Liaohe Sag; BZS: Bozhong Sag; JZS: Jizhong Sag; LQS: Linqing Sag; DB: Dabie Orogen; SL: Sulu Orogen; TL: Tan-Lu fault zone; NCC: North China Craton. (b) Cenozoic strata column of the Bohai Bay Basin with summary of the tectonic evolution (modified after Hu et al., 2001), showing sample locations.

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Fig. 2. Petrographic characteristics of Jiyang Sag basalts (crossed-polarized light): (a) the Lower Shahejie basalt; (b, c) the Upper Shahejie basalt; and (d) the Guantao basalt. Mineral abbreviations: Clinopyroxene (Cpx), Olivine (Ol) and Calcite (Cc).

Fig. 3. Whole-rock 40Ar–39Ar plateau age spectra and plots of 40Ar/36Ar versus 39Ar/36Ar for the Lower and Upper Shahejie basalts in the Jiyang Sag: (a, b) Sample Y181 of the Lower Shahejie basalt; and (c, d) Sample Y2510 of the Upper Shahejie basalt (see Supplemental Table S3 for data).

H.-Y. Li et al. / Lithos 196–197 (2014) 54–66

were chosen for further cleaning. The rock fragments were leached with 1% HCl, washed with deionized water in an ultrasonic bath and dried out before powdered into 200 mesh in a corundum mill. The samples were prepared as glass disks using a Rigaku desktop fusion machine. Bulk-rock major elements were analyzed using a Rigaku RIX 2000 X-ray fluorescence spectrometer (XRF). Calibration lines used in quantification were produced by bivariate regression of data from 36 reference materials encompassing a wide range of silicate compositions (Li et al., 2005). Calibrations incorporated matrix corrections based on the empirical Traill-Lachance procedure, and the analytical uncertainties are typically better than 5% (Supplemental Table S1). Trace elements were analyzed using a Perkin-Elmer Sciex ELAN 600 inductively coupled plasma mass spectrometry (ICP-MS) after acid digestion of samples in high-pressure Teflon vessels. The US Geological Survey and Chinese National standards GSR-1, GSR-2, GSR-3, G-2, W-2, BT142, AGV-2, MRG-1, and ROA-1 were chosen for calibrating element concentrations of measured samples. Analytical precision of rare earth elements (REE) and other incompatible elements is typically 1 5% (Supplemental Table S2). Guo et al. (2006) measured Sr–Nd isotopes of unleached samples from these sequences using an IsoProbe MC-ICPMS. However, in this study, the sample powder for Sr–Nd–Hf isotopic analyses was leached using distilled 6 N HCl for 20 min to eliminate carbonate infillings and possible alteration products. The acidic solution was then centrifuged, rinsed using deionized water three times and then evaporated and dried. About 100 mg of leached powder was dissolved in a HF-HNO3 mixture in a Teflon beaker at 150 °C for 7 days. HFSEs, Sr and REEs were separated by ion chromatography using AG50-X8 cation exchange resin. Separation of Nd and Hf was carried out on a HDEHP-coated Kef column. The Sr, Nd and Hf isotopic ratios were measured with a Neptune MC-ICPMS, and normalized using the values of 86Sr/88Sr = 0.1194, 146Nd/144Nd = 0.7219 and 179Hf/177Hf = 0.7325, respectively. Reference standard solutions analyzed along with samples gave 87 Sr/86Sr = 0.710251 ± 11 (n = 8; all the errors are given as 2σ) for NISTSRM987, 143Nd/144Nd = 0.512108 ± 6 (n = 8) for Shin Etsu JNdi-1 and 176Hf/177Hf = 0.2821543 ± 10 (n = 8) for JMC475, which are all in agreement with recommended values of NISTSRM987 (87Sr/86Sr = 0.71025 ± 7; Woodhead and Hergt, 2001), Shin Etsu JNdi-1 (143 Nd/144Nd = 0.512115 ± 7; Tanakaa et al., 2000) and JMC475 (176Hf/177Hf = 0.282151 ± 7; Woodhead and Hergt, 2005). Rock standard W-2 was analyzed with samples to monitor leaching method (6 N HCl leaching), and gave 87Sr/86Sr = 0.706893 ± 10, 143 Nd/144Nd = 0.512538 ± 8, and 176Hf/177Hf = 0.282736 ± 6, all in agreement with recommended values of 87Sr/86Sr = 0.706966 ± 31, 143 Nd/144Nd = 0.512516 ± 22, and 176Hf/177Hf = 0.282724 ± 7 (Yang et al., 2010). Rock standards JB-3 and BHVO-2 were used to monitor the ion-exchange chromatographic purification processes. Analysis of JB-3 gave 87Sr/86Sr = 0.703448 ± 10, 143Nd/144Nd = 0.513057 ± 6, and 176Hf/177Hf = 0.283235 ± 6, respectively, being in agreement with recommended values of 87 Sr/ 86 Sr = 0.703396 ± 21, 143 Nd/ 144 Nd = 0.513064 ± 21, and 176Hf/177Hf = 0.283222 ± 7 (Yang et al., 2010). Analysis of BHVO-2 gave 87Sr/86Sr = 0.703500 ± 10, 143Nd/144Nd = 0.512979 ± 6, and 176Hf/177Hf = 0.283089 ± 6, respectively, being in agreement with recommended values of 87 Sr/ 86 Sr = 0.703481 ± 20, 143 Nd/ 144 Nd = 0.512983 ± 10, and 176 Hf/177Hf = 0.283096 ± 20 (Weis et al., 2005). 4. Results 4.1. 40Ar/39Ar geochronology Two samples Y2510 and Y181 from the Upper and Lower Shahejie basalts, respectively, were selected for 40Ar/39Ar dating. The results are given in Supplemental Table S3 and shown graphically in Fig. 3. The initial argon compositions have 40Ar/36Ar ratios (Y181: 295 ± 6; Y2510: 294 ± 5) identical to the present atmospheric value of 295.5 ± 0.5

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within error, indicating that the excess argon is insignificant in the samples (Fig. 3b, d). The plateau ages of two samples are all identical to their isochron ages within error, and interpreted as the eruption ages of the basalts. The dating results of the Shahejie samples (Y181: 46.9 ± 0.8 Ma; Y2510: 45.5 ± 0.8 Ma) are in agreement with the stratigraphic age of the Shahejie group based on apatite fission track ages and K–Ar ages (50.5–32.8 Ma; Dai et al., 1993; Guo et al., 1996; Yao et al., 1994). The Eocene eruption age of the Shahejie basalts in the Jiyang Sag is significant since the dominant volcanic rocks in Shandong are Late Cenozoic basalts (23.0–0.3 Ma) (He et al., 2011; Z. Xu et al., 2012). The eruption ages of the Guantao basalts should be younger than 23 Ma according to the stratigraphic age of the Guantao group (23–12 Ma; Allen et al., 1997; Dai et al., 1993; Guo et al., 1996; Yao et al., 1994). 4.2. Major and trace elements Bulk rock major and trace element analyses are presented in Table 1. All samples have high loss on ignition (LOI) (2.5–6.5 wt.%), which could not have resulted from secondary alteration, such as weathering and/or metasomatism, because the samples are mostly fresh except for the observation of iddingsite altered from olivine phenocrysts in some samples (Fig. 2d). In addition, the secondary alteration would result in CaO depletion and very low concentration of K2O and Na2O (Wang et al., 2010), which is inconsistent with observed high alkali contents (Table 1) and weak positive correlation between LOI and CaO (Fig. 4a). High LOI of the studied samples can mostly be attributed to abundant carbonate in the vesicles of the rock (Fig. 2c). Although we leached the sample fragments with 1% HCl before powdering, some carbonates in vesicles would have been retained because most vesicles are smaller than the size of the fragments (~0.5 cm). The Eocene Shahejie basalts have higher MgO and CaO, but lower SiO2, Fe2OT3, TiO2 and K2O than the Miocene Guantao basalts (Fig. 4; Table 1). The total alkalis (Na2O + K2O) of all samples are high, in range of 4.2–5.1 wt.% (Table 1). Following the nomenclature of volcanic rocks by Le Bas et al. (1986), the Eocene samples are all alkali basalt, and the Miocene samples are dominantly alkali basalt with minor trachybasalt (Fig. 5a). On the 2Nb–Zr/4-Y diagram, the samples are plot in the field of “within plate alkaline basalts” (Fig. 5b). All the samples are enriched in light REEs relative to heavy REEs with somewhat positive Eu anomalies (Eu/Eu* = 1.00–1.12) (Fig. 6a). The Lower Shahejie basalts have slightly lower La/Yb but higher Dy/Yb than the Upper Shahejie basalts (Table 1). The Guantao basalts have the highest La/Yb and Dy/Yb ratios of the studied samples (Table 1). On the primitive-mantle normalized multi-element diagrams (Fig. 6b), all the samples have positive Nb and Ta anomalies, and most samples have a distinct positive Sr anomaly (except for samples S22 and S23). 4.3. Sr–Nd–Hf isotopes Sr–Nd–Hf isotopic compositions of the basaltic samples from the Jiyang Sag are shown in Table 2. The initial Nd isotopes of the Miocene Guantao basalts were calculated at 20 Ma to enable comparison with the Miocene basalts in Shandong (He et al., 2011; Z. Xu et al., 2012). Leached samples have much lower 87Sr/86Sr ratios than unleached ones (Y181, Y183, Y185, Y2507 and Y2509, Table 2), indicating that the Sr isotopes have been significantly affected by the vessicular carbonate, in agreement with the positive correlation between LOI and CaO (Fig. 4a). Unleached samples (except for sample Y185) have slightly higher 143Nd/144Nd than leached ones. However, all the Nd isotope analyses were used in our petrogenetic interpretations because the elevated εNd(t) values of ~0.3–0.6 units as a result of the leaching process are significantly less than the total εNd(t) variation of all samples (Table 2). The Eocene samples have low initial 87Sr/86Sr ratios (0.70328 to 0.70425) and positive εNd(t) (2.4 to 5.2) and εHf(t) values (7.0 to

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Table 1 Major (wt%) and trace element (ppm) concentrations of the Jiyang Sag basalts. Y181

Y182

Y183

Y184

Y185

Y186

Y2507

Y2508

Y2509

Y2510

S21

S22

S23

S24

S25

S26

Standard rock

LSG

LSG

LSG

LSG

LSG

LSG

USG

USG

USG

USG

GTG

GTG

GTG

GTG

GTG

GTG

W-2

W-2

W-2

1st

2nd

3rd

SiO2 TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 LOI Total Mg# Sc Cr Co Ni Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U La/Yb Dy/Yb Eu/Eu*

44.72 1.89 13.85 11.47 0.14 7.92 10.09 2.71 1.47 0.34 5.86 100.46 61.7 23.5 368 50 204 19.8 841 17.2 144 32.0 343 18.9 37.0 4.81 19.1 4.26 1.40 4.20 0.62 3.45 0.64 1.63 0.23 1.39 0.20 2.50 1.50 1.84 0.52 13.60 2.48 1.01

44.21 1.85 13.63 11.59 0.14 8.06 9.98 2.72 1.47 0.34 5.9 99.89 61.8 23.5 375 54 233 21.4 895 17.8 146 32.6 355 20.3 40.2 5.02 20.2 4.29 1.43 4.44 0.65 3.6 0.68 1.62 0.23 1.40 0.23 2.75 1.70 2.00 0.55 14.50 2.57 1.00

45.19 1.87 14.01 11.49 0.13 7.85 9.36 2.91 1.67 0.38 5.28 100.14 61.4 22.0 346 47 202 27.0 833 17.6 157 38.7 430 22.6 45.0 5.40 21.8 4.6 1.46 4.21 0.64 3.71 0.71 1.75 0.23 1.49 0.21 2.65 1.90 2.44 0.67 15.17 2.49 1.01

44.99 1.88 13.95 11.47 0.14 7.89 9.79 2.88 1.58 0.36 5.60 100.53 61.6 22.3 365 49 218 23.2 829 17.7 150 35.4 372 21.2 41.3 5.12 20.9 4.23 1.46 4.26 0.63 3.52 0.67 1.62 0.24 1.48 0.22 2.70 1.80 2.28 0.63 14.32 2.38 1.05

44.46 1.85 13.81 11.48 0.14 7.96 9.68 2.82 1.59 0.37 6.00 100.16 61.8 29.1 346 48 215 25.0 846 17.9 152 37.2 390 21.9 43.1 5.31 21.6 4.57 1.53 4.42 0.68 3.67 0.69 1.78 0.25 1.49 0.21 2.85 1.84 2.48 0.67 14.70 2.46 1.04

44.73 1.86 13.93 11.53 0.14 7.92 9.52 3.14 1.63 0.37 5.65 100.42 61.6 29.7 344 47 210 25.6 842 17.3 155 38.0 430 22.6 45.3 5.59 22.0 4.62 1.55 4.37 0.64 3.61 0.70 1.77 0.26 1.52 0.21 2.86 1.96 2.54 0.71 14.87 2.38 1.05

46.91 1.71 15.04 10.75 0.17 8.09 9.57 2.83 1.33 0.48 3.91 100.79 63.7 24.1 344 39 124 9.6 747 20.3 185 52.7 400 30.6 59.0 6.89 25.5 4.97 1.59 4.73 0.69 3.98 0.75 1.98 0.29 1.77 0.27 3.07 2.74 3.03 0.87 17.29 2.25 1.00

46.02 1.70 15.1 10.67 0.16 7.83 9.46 2.98 1.31 0.48 4.22 99.93 63.1 24.1 343 39 126 9.0 738 20.1 186 53.7 406 32.0 58.7 7.15 27.3 5.19 1.70 4.93 0.74 4.18 0.82 2.03 0.31 1.79 0.25 3.36 2.85 3.28 0.87 17.88 2.34 1.03

46.49 1.72 15.01 10.81 0.15 8.2 9.25 2.83 1.36 0.49 3.79 100.1 63.9 32.7 348 39 123 10.4 724 20.7 209 53.7 393 32.0 57.6 6.94 25.8 5.11 1.70 4.96 0.75 4.09 0.80 1.99 0.32 1.80 0.29 3.60 2.81 3.31 0.87 17.78 2.27 1.03

46.04 1.72 14.95 10.69 0.17 8.08 9.46 2.98 1.34 0.49 4.29 100.21 63.8 30.4 369 42 134 10.0 748 20.5 186 53.0 407 32.5 59.1 7.14 26.2 5.13 1.64 4.77 0.69 4.11 0.79 2.04 0.3 1.75 0.26 3.22 2.81 3.22 0.87 18.57 2.35 1.01

48.00 2.36 15.08 11.99 0.18 6.41 7.97 2.65 2.19 0.73 3.38 100.94 55.5 18.6 119 37 87.3 17.7 626 22.3 285 61.9 390 36.9 66.9 8.36 35.6 7.8 2.67 6.89 1.02 5.43 0.93 2.21 0.3 1.64 0.22 5.20 3.60 4.60 – 22.50 3.31 1.11

46.67 2.29 15.14 12.44 0.14 5.06 7.71 1.71 2.92 0.72 6.23 101.03 48.7 18.9 114 40 88.2 48.6 416 25.8 279 62.8 424 40.1 68.6 9.31 39.3 8.26 2.87 8.15 1.14 6.03 1.08 2.52 0.35 1.92 0.27 5.40 3.80 5.00 – 20.89 3.14 1.07

45.38 2.30 14.83 12.15 0.18 4.78 9.55 2.38 2.35 0.76 6.33 100.99 47.8 18.2 117 37 78.0 32.0 523 23.2 286 63.4 425 37.1 66.5 8.72 36.0 7.74 2.64 7.22 1.03 5.54 0.97 2.28 0.29 1.69 0.23 5.30 4.00 4.60 – 21.95 3.28 1.08

47.23 2.35 15.03 11.61 0.16 6.53 9.02 2.81 1.77 0.80 3.59 100.9 56.7 18.4 114 40 91.7 10.9 760 22.3 285 63.6 412 36.2 67.9 8.25 34.9 7.64 2.62 7.05 1.02 5.45 0.94 2.21 0.29 1.64 0.22 5.70 4.00 4.40 – 22.07 3.32 1.09

47.94 2.39 15.18 11.77 0.14 6.29 8.59 3.19 1.68 0.80 2.92 100.89 55.5 19.1 114 39 88.9 10.9 795 22.4 286 63.3 425 36.4 66.8 8.53 35.6 7.63 2.65 7.22 1.02 5.48 0.95 2.25 0.3 1.70 0.23 5.80 4.20 4.30 – 21.41 3.22 1.09

48.32 2.45 15.35 12.64 0.13 5.49 8.30 3.00 2.1 0.79 2.46 101.03 50.3 19.9 113 37 83.1 10.7 783 23.8 393 64.6 434 37.5 70.8 9.14 36.9 8.02 2.85 7.51 1.08 5.79 0.99 2.37 0.32 1.80 0.25 5.60 4.60 4.50 – 20.83 3.22 1.12

Abbreviations of units: Lower Shehejie group (LSG), Upper Shahejie group (USG) and Guantao group (GTG). Lower and Upper Shahejie are from Guo et al. (2006). Mg# = 100 × Mg/(Mg + 0.85 × Fetot).

52.31 1.09 15.14 10.33 0.17 6.34 10.68 2.16 0.65 0.14

52.32 1.10 15.12 10.33 0.17 6.33 10.69 2.16 0.65 0.14

52.32 1.09 15.12 10.31 0.17 6.35 10.68 2.17 0.65 0.14

36.0 87 44 70.8 21.0 199 22.1 94.3 6.75 169 10.9 23.5 3.08 13.2 3.32 1.13 3.68 0.64 3.92 0.83 2.20 0.32 2.01 0.31 2.35 0.45 2.22 0.50

35.9 108 44 71.6 19.4 179 19.3 88.6 6.52 162 10.8 23.2 3.16 13.3 3.32 1.11 3.62 0.61 3.94 0.85 2.20 0.32 2.02 0.30 2.43 0.47 2.21 0.50

36.0 97 44 73.4 20.0 184 20.9 94.3 6.44 172 10.1 22.2 3.00 13.1 3.20 1.03 3.68 0.62 3.94 0.83 2.24 0.32 2.05 0.32 2.41 0.48 2.21 0.51

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Sample unit cycles

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Fig. 4. Variation diagrams for LOI, representative major element oxides and trace elements: (a) CaO versus LOI; (b, c, d, e, f, g, h) SiO2, Fe2OT3, TiO2, K2O, Al2O3, Cr and Ni versus MgO, respectively. The data of major element oxides were corrected for the volatile components: X⁎ = X × 100/(100-LOI); X and X⁎ are the measured and corrected values, respectively. The Miocene basalts in Shandong (Supplemental Table S4) are from Zeng et al. (2011).

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Fig. 5. (a) Na2O + K2O versus SiO2 (following the nomenclature of Le Bas et al. (1986)); the line separating alkali basalts and tholeiites is from McDonald and Katsura (1964); the data of Na2O, K2O and SiO2 were corrected for the volatile components (detail method as in Fig. 4); (b) 2Nb–Zr/4-Y diagram (after the classification of Meschede, 1986). The Miocene basalts in the Shandong (Supplemental Table S4) are from Zeng et al. (2011).

10.1). The Lower Shahejie basalts have relatively higher 87Sr/86Sr ratios but less positive εNd(t) and εHf(t) values than the Upper Shahejie basalts (Fig. 7). Overall, the Shahejie basalts have similar Nd–Hf–Sr isotope compositions to the Miocene basalts in Shandong (Zeng et al., 2011), but lack significant Hf–Nd isotope decoupling (Fig. 7). The Miocene Guantao basalts in the Jiyang Sag have variable εNd(t) values (3.7 to 6.3; Table 2) similar to the Miocene basalts in Shandong (Zeng et al., 2011).

5. Discussion 5.1. Crustal contamination and fractional crystallization Crustal contamination is a potential process that affects mantlederived melts during their ascent through, and evolution within, magma chambers in the continental environment. Because the continental crust has higher Th/La but lower Nb/La than the mantle (Rudnick and Gao, 2003), positive correlation between Nb/La ratios and εNd values and negative correlation between Th/La ratios and εNd values would be predicted if the crustal contamination plays an important role in the petrogenesis. However, this is not observed in each group of the Jiyang Sag basalts (Fig. 8a, b), which rules out any

significant crustal contamination. Thus, the geochemical and isotopic characteristics of studied basalts should reflect those of the magma sources. The Shahejie basalts have lower MgO and Ni than some Miocene basalts in Shandong (Fig. 4 h), probably due to olivine fractionation. But the Shahejie basalts also have the highest Cr of the Cenozoic basalts. This indicates that the clinopyroxene fractionation in the Shahejie basalts is extremely limited. In addition, the Lower Shahejie basalts have higher Ni than the Upper Shahejie basalts with similar MgO and Cr, suggesting that the geochemical differences between the Lower and Upper Shahejie basalts cannot be attributed to the fractional crystallization of olivine and/or clinopyroxene, but are due to heterogeneity in the mantle source. The Guantao basalts have notably lower MgO, Cr and Ni than the Shahejie basalts (Table 1) and the Miocene basalts in Shandong (Fig. 4 g, f), and this is possibly attributed to olivine and/or clinopyroxene fractionation. However, clinopyroxene crystallization in the Guantao basalts is insignificant due to their nearly constant La/Yb and Dy/Yb ratios (Fig. 8c, d). Olivine has extremely low partition coefficients for the rare earth and other incompatible elements (McKenzie and O'Nions, 1991). Therefore, significant differences of La/Yb and Dy/Yb ratios between the Shahejie and Guantao basalts (Fig. 8c, d) would be mostly inherited from their respective sources rather than caused by the fractional crystallization.

Fig. 6. (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized multi-element patterns of the Jiyang Sag basalts. Chondrite and primitive mantle normalization values are from Sun and McDonough (1989). The OIB and continental crust values are from Niu et al. (2002) and Rudnick and Gao (2003), respectively. The Miocene basalts in Shandong (Supplemental Table S3) are from Zeng et al. (2011).

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Table 2 Sr–Nd–Hf isotope data for the Jiyang Sag basalts. Sample

87

Rb/86Sr

87

Sr/86Sr (±2σ)

(87Sr/86Sr)i

147

143

Sm/144Nd

Nd/144Nd (±2σ)

(143Nd/144Nd)i

εNd(t)

176

Lu/177Hf

176

Hf/177Hf (±2σ)

(176Hf/177Hf)i

εHf(t)

Lower Shahejie group Y181 0.068 Y181* 0.068 Y182 0.069 Y183 0.094 Y183* 0.094 Y184 0.081 Y185 0.085 Y185* 0.085 Y186 0.088

0.704162 0.705968 0.704293 0.704046 0.705924 0.704134 0.704265 0.706199 0.704288

± ± ± ± ± ± ± ± ±

10 17 12 12 17 11 14 18 12

0.704117 0.705923 0.704247 0.703984 0.705862 0.704080 0.704208 0.706142 0.704230

0.135 0.135 0.128 0.128 0.128 0.122 0.128 0.128 0.127

0.512743 0.512768 0.512756 0.512758 0.512801 0.512760 0.512758 0.512769 0.512763

± ± ± ± ± ± ± ± ±

7 11 9 8 12 9 8 11 7

0.512702 0.512727 0.512717 0.512719 0.512762 0.512723 0.512719 0.512730 0.512724

2.4 2.9 2.7 2.8 3.6 2.8 2.8 3.0 2.9

0.0111

0.282951 ± 6

0.282941

7.0

0.0116 0.0110

0.282964 ± 5 0.282962 ± 6

0.282954 0.282952

7.5 7.4

0.0113 0.0102

0.282965 ± 7 0.282969 ± 5

0.282955 0.282960

7.5 7.7

0.0102

0.282970 ± 6

0.282961

7.7

Upper Shahejia group Y2507 0.037 Y2507* 0.037 Y2508 0.035 Y2509 0.042 Y2509* 0.042 Y2510 0.039

0.703327 0.704973 0.703359 0.703310 0.704839 0.703305

± ± ± ± ± ±

11 17 11 13 17 13

0.703303 0.704949 0.703336 0.703283 0.704812 0.703280

0.118 0.118 0.115 0.120 0.120 0.119

0.512876 0.512905 0.512879 0.512872 0.512903 0.512881

± ± ± ± ± ±

6 12 6 8 12 7

0.512841 0.512870 0.512845 0.512836 0.512867 0.512845

5.1 5.7 5.2 5.0 5.6 5.2

0.0122

0.283030 ± 5

0.283019

9.7

0.0103 0.0112

0.283028 ± 5 0.283039 ± 7

0.283019 0.283030

9.7 10.1

0.0112

0.283015 ± 8

0.283005

9.2

0.132 0.127 0.130 0.132 0.130 0.131

0.512821 0.512954 0.512872 0.512912 0.512829 0.512817

± ± ± ± ± ±

12 12 11 10 8 11

0.512804 0.512937 0.512855 0.512895 0.512812 0.512800

3.7 6.3 4.7 5.5 3.9 3.7

Guantao group S21* S22* S23* S24* S25* S26* Standard samples W-2 BHVO-2 JB-3

0.706893 ± 10 0.703500 ± 14 0.703448 ± 10

0.512538 ± 8 0.512979 ± 6 0.513057 ± 6

0.282736 ± 6 0.283089 ± 6 0.283235 ± 6

(1) 87Rb/86Sr, 147Sm/144Nd and 176Lu/177Hf are calculated using whole-rock Rb, Sr, Sm, Nd, Lu and Hf contents in Table 1. (2) In the calculation of εNd(t), (143Nd/144Nd)CHUR = 0.512638, (147Sm/144Nd)CHUR = 0.1967, t = 46.9 Ma (Lower Shahejie group), 45.5 Ma (Upper Shahejie group), 20 Ma (Guantao group). (3) In the calculation of εHf(t), (176Hf/177Hf)CHUR = 0.282785, (176Lu/177Hf)CHUR = 0.0336 (Bouvier et al., 2008), λ(176Lu) = 1.867 × 10−11 yr−1 (Söderlund et al., 2004). (4) *: Unleached samples analyzed with IsoProbe MC-ICPMS. Sr and Nd isotopic data of unleached Eocene Lower and Upper Shahejie samples are from Guo et al. (2006).

5.2. Temporal variation of mantle sources for the Jiyang Sag basalts As discussed above, different isotopic and geochemical characteristics between the three volcanic suites from Jiyang Sag (Figs. 6, 7) mostly reflect their heterogeneous sources rather than magma chamber processes. Therefore, the OIB-like trace element patterns and the variable εNd(t) (2.4–6.3) and εHf(t) values (7.0–10.1) indicate that both an enriched end-member (low εNd(t) and εHf(t) values) and a depleted mantle source contributed to the lavas (Figs. 7, 9). Several models have been proposed for the origin of enriched component in the mantle source, such as delaminated lower continental crust (Chen et al., 2009; Liu et al., 2008; Zeng et al., 2011), subducted oceanic crust and sediment (Hong et al., 2013; Kuang et al., 2012; Tang et al., 2012, 2013; Y.G. Xu et al., 2012) and long-term metasomatized lithospheric mantle (Niu and O'Hara, 2003; Z. Xu et al., 2012; Zhang et al., 2009). The models of delaminated lower continental crust and/or longterm metasomatized lithospheric mantle are obviously inconsistent with the OIB signature of the Cenozoic basalts in the eastern NCC, e.g., positive Nb and Ta anomalies in the primitive mantle-normalized multi-element patterns and moderately depleted Sr–Nd–Hf isotopic signature (Y.G. Xu et al., 2012). A striking characteristic of the Miocene basalts in the Shandong Province is the slight decoupling between Hf and Nd isotopes (Fig. 7a) (Chen et al., 2009; Zeng et al., 2011). For a given εNd value, oceanic crust has a lower while the global subducted sediment (GLOSS) has a higher εHf value than the mantle array (Fig. 7a; Chauvel et al., 2008). Thus the involvement of oceanic crust and/or subducted sediment can generate slight decoupling between Hf and Nd isotopes in the mantle source but preserve the Sr–Nd isotopic correlation. Therefore, subducted oceanic crust with sediment could be a suitable candidate

for enriched components in the mantle source. The Shahejie basalts are similar to the Nd–Hf–Sr isotope compositions of the Miocene basalts in Shandong (Fig. 7). But they apparently lack the Nd–Hf isotope decoupling that might have been diluted by a large proportion of melts derived from the spinel peridotite facies (see discussion below). Overall, the Jiyang Sag basalts are characterized by slightly depleted highly incompatible elements (e.g., Ba, Th, U and LREE) relative to Nb, positive Eu and Sr anomalies in the primitive mantle-normalized multi-element diagram (Fig. 6b) and moderately depleted Sr–Nd–Hf isotopes, which is consistent with the existence of oceanic crust in the magma source (Hofmann, 2004; Kuang et al., 2012; Stracke et al., 2005; Tang et al., 2012, 2013; Y.G. Xu et al., 2012). The Miocene Guantao basalts have similar trace elemental characteristics to the Miocene basalts in Shandong, e.g., high concentrations of incompatible trace elements and steep HREE patterns (Fig. 6a). However, the Eocene Shahejie basalts are different from the Miocene basalts in that they have lower concentrations of incompatible trace elements and less steep HREE patterns (Fig. 6a). La/Yb and Dy/Yb ratios will be strongly fractionated by low-degree partial melting (b10%) of garnet peridotite (Xu, 2001). In contrast, La/Yb is only slightly fractionated and Dy/Yb is nearly unfractionated during the melting of spinel peridotite (e.g., Huang et al., 2013; Kuang et al., 2012; Shaw et al., 2003; Thirlwall et al., 1994). Model calculations show that the partial melting of either garnet or spinel peridotites cannot adequately produce the geochemical characteristics of the Jiyang Sag basalts (Fig. 9). The steep HREE patterns in the Miocene Guantao basalts and contemporary basalts in Shandong suggest that these lavas were derived from partial melting of dominantly garnet peridotite/pyroxenite (Fig. 9). However, the less steep HREE patterns in the Eocene Shahejie basalts are all consistent with derivation from spinel peridotite (Fig. 9). The Upper Shahejie basalts have higher

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Fig. 7. (a-a′) εNd versus εHf, and (b-b′) 143Nd/144Nd versus 87Sr/86Sr for the Eocene Shahejie basalts in the Jiyang Sag and Miocene basalts in Shandong. The Miocene basalts in Shandong (Supplemental Table S4) are from Zeng et al. (2011). The present-day average Sr–Nd–Hf isotopic compositions of MORB (Chauvel et al., 2008; Su and Langmuir, 2003) and Global Subducted Sediment (GLOSS) (Chauvel et al., 2008; Plank and Langmuir, 1998) are also shown in the diagrams. The evolution paths of oceanic crust and GLOSS are shown as dotted and dashed curves, respectively, marked by the ages (Ga). To calculate the initial isotope ratios of oceanic crust and GLOSS at different times in the Earth's history, we assumed: they were formed by similar processes and recycled into the mantle at the same time; there is a linear relationship between their isotopic ratios at present and 4.55 Ga. Then they were modeled to evolve through time using the Rb/Sr, Sm/Nd and Lu/Hf ratios of GLOSS and N-MORB, respectively (Chauvel et al., 2008; Su and Langmuir, 2003). Mixing curves between GLOSS and oceanic crust (MC1) were marked by the GLOSS proportions in percentage. Mixing curves between spinel peridotitic melts (SP DDM; 1% partial melting) and the pyroxenitic melts (20% partial melting) (MC2) were marked by the proportions of spinel facies DMM melts in percentage. The mantle array line is from Chauvel et al. (2008). Sr–Nd–Hf elemental contents were calculated based on the batch melting mode, using the partition coefficients from Zanetti et al. (2004) (Ol), Adam and Green (2006) (Cpx, Opx), Elkins et al. (2008) (Sp) and Pertermann et al. (2004) (Grt in pyroxenite). Assumed trace element concentrations of pyroxenite and peridotite sources are from the mixture of “80% N-MORB (Niu et al., 2002) + 20% GLOSS (Plank and Langmuir, 1998)” and primitive mantle values of Sun and McDonough (1989). The original mineral assemblage of the spinel peridotite source was assumed as “53% Ol + 25% Opx + 20% Cpx + 2% Sp” (Kinzler, 1997), and melting reaction in spinel facies was as “10% Ol + 27% Opx + 50% Cpx + 13% Sp” (Thirlwall et al., 1994). The mineral assemblage of pyroxenite was as “3% Ol + 20% Opx + 60% Cpx + 17% Grt”, and melting reaction was as “5% Opx + 70% Cpx + 25% Grt”.

concentrations of REEs and more positive εNd and εHf values but less steep HREE patterns and lower 87Sr/86Sr ratios than the Lower Shahejie basalts, suggesting less garnet peridotite/pyroxenite melt contributions to the lavas (see discussion in the next section). In contrast, the REE patterns of the Miocene Guantao basalts reflect a large proportion of melts derived from garnet peridotite. 5.3. Lithospheric evolution from thinning to thickening beneath the eastern NCC The geochemical composition of asthenosphere-derived basalts depends on mantle temperature, lithospheric thickness and source composition. Mantle temperature and lithospheric thickness constrain the initial and final melting depths of a melting column of asthenosphere, respectively (Niu and Batiza, 1991). For example, the higher mantle temperature would correspond to the greater initial melting depth, and the mean melting depth under a thick lithosphere would be greater than that under a thin lithosphere (e.g., Fram and Lesher, 1993; Niu and Batiza, 1991; Niu et al., 2011). On the other hand, dynamic melting of a heterogeneous asthenosphere shows temporal shift of geochemical features in the melts. Subducted oceanic crust (with subducted sediment) would initially melt in a deep mantle source (garnet peridotite facies) of heterogeneous asthenosphere (Geldmacher

and Hoernle, 2000; Hirschmann and Stopler, 1996; Kogiso et al., 2003). If the lithosphere is thin enough, the spinel peridotite facies would replace the garnet peridotite facies as the dominant depleted end-member of asthenosphere-derived basalts, corresponding to less garnet signature in the melts due to the dilution effect (Niu et al., 2011). The spinel-peridotite source signature in the Eocene Shahejie basalts reflects a shallow magma source. In contrast, the garnet-peridotite source signature in the Miocene Guantao basalts reflects a deeper magma source than the Eocene Shahejie basalts (Fig. 9). So the lithosphere beneath the eastern NCC might have undergone thinning in the Eocene and thickening after the end of the Oligocene. The tectonic evolution of the Bohai Bay Basin also offers clues in support of this interpretation. The Bohai Bay Basin had undergone significant extension and subsidence in the Eocene when the Shahejie group was deposited (e.g. Allen et al., 1997; Qi and Yang, 2010). Major extension in the Bohai Bay Basin stopped in the Late Oligocene. Subsequently, the basin underwent a period of slow thermal subsidence in the Miocene (Fig. 1b; Hu et al., 2001). Additionally, data from vitrinite reflectance and apatite fission track dating indicates that the paleo-heat flow in the Bohai Bay Basin increased before ~23 Ma and then decreased from ~ 23 Ma (70–90 mW/m2) to the present (53–74 mW/m2) (Hu et al., 2001). In terms of isostatic effects (Ziegler and Cloetingh, 2004) and paleothermal variation of the Bohai Bay Basin, the subsidence in the

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63

Fig. 8. (a) εNd versus Th/La, (b) εNd versus Nb/La; (c, d) La/Yb and Dy/Yb versus Mg# for Jiyang Sag basalts.

Eocene–Oligocene reflects active extension in response to lithospheric thinning. The spinel/garnet facies transition usually occurs at ~ 75–85 km under the subsolidus conditions (e.g., McKenzie and O'Nions, 1991; Robinson and Wood, 1998). The Eocene Shahejie basalts consist of large proportion of spinel peridotite facies melts (Fig. 9), suggesting that the lithosphere thickness at ~ 45 Ma would be less than the depth of spinel/garnet facies transition zone. On the other hand, if the lithosphere thickness is less than 60 km, most of the basalts

would be tholeiitic (DePaolo and Daley, 2000) rather than alkaline as is observed in the Jiyang Sag. Thus the lithosphere thickness beneath the Jiyang Sag is ~ 60–85 km at ca. 45 Ma, less than the present thickness of 85–95 km (Chen, 2010). Therefore, the lithosphere beneath the eastern NCC has undergone thickening during the time between ~ 45 Ma and the present. The paleothermal variation of the Bohai Bay Basin (Hu et al., 2001) further indicates that the thinning lithosphere beneath the Bohai Bay Basin might have begun to thicken at ~ 23 Ma.

Fig. 9. (a) Dy/Yb versus La/Yb, (b) εNd versus Dy/Yb for Jiyang Sag basalts and the Miocene basalts in Shandong. The marked numbers on the partial melting curves (Grt DMM melting; SP DMM melting; Pyroxenite melting) denote the proportions of melts in percentage. Mixing curves (MC-I, MC-II, MC-a, MC-b) are marked by the proportions of pyroxenite melts (MC-1 and MC-II) or the melts of spinel peridotite (MC-a and MC-b) in percentage. Assumed trace element concentrations of pyroxenite and peridotite sources are as that in Fig. 7. The original mineral assemblage of the garnet peridotite source was assumed as “55% Ol + 23% Opx + 12% Cpx + 10% Grt”, and melting reaction in garnet facies was as “5% Ol + 20% Opx + 30% Cpx + 45% Grt” (Thirlwall et al., 1994). The original mineral assemblage and melting reaction of the pyroxenite and spinel peridotite sources are as that in Fig. 7. Partition coefficients are from Zanetti et al. (2004) (Ol), Adam and Green (2006) (Cpx, Opx), Elkins et al. (2008) (Sp), Johnson (1998) (Grt in peridotite) and Pertermann et al. (2004) (Grt in pyroxenite). The Miocene basalts in Shandong (Supplemental Table S4) are from Zeng et al. (2011).

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Late Mesozoic mafic rocks in the eastern NCC have high initial Sr/ 86 Sr ratios and negative ε Nd (t) values (e.g., Menzies et al., 2007; Xu, 2001; Zhang et al., 2002, 2003), while the Cenozoic basalts contain moderately depleted Sr–Nd–Hf isotopes (e.g., Liu et al., 2008; Tang et al., 2006; Xu et al., 2005; Zou et al., 2000). The geochemical contrast between the Late Mesozoic and Cenozoic mafic rocks suggests a temporal source transition from lithospheric mantle to asthenosphere (Xu, 2001; Xu et al., 2009), which has been considered as an indicator of lithospheric thinning (DePaolo and Daley, 2000; Xu, 2001). This transition probably took place between 100 Ma and 80 Ma (Xu et al., 2009), suggesting that the lithospheric thinning of the eastern NCC was ongoing during the Late Cretaceous. However, the most important period of lithospheric thinning in the southeastern Central Asian Orogenic Belt is probably from Late Cretaceous to Early Cenozoic (70–40 Ma; Y.G. Xu et al., 2012). Geochemical variation of the Jiyang Sag basalts and tectonic evolution of the Bohai Bay Basin further suggest that the lithospheric thinning of the eastern NCC was on-going in the Eocene and Oligocene, but ceased in the Miocene before thickening began in the Miocene. 87

6. Conclusions The Cenozoic Jiyang Sag basalts have overall depleted but variable Nd–Hf isotopes, indicating the moderately depleted mantle source involvement. The Eocene basalts have a shallow magma source largely in the spinel peridotite stability field, while the Miocene basalts were derived from a deep source predominantly in the garnet peridotite stability field. The geochemical variation of the Cenozoic basalts in the Bohai Bay Basin reveals that the lithosphere beneath the eastern NCC was thinning in the Eocene but began to thicken in the Miocene. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.lithos.2014.02.026. Acknowledgments We acknowledge the constructive comments of Andrew Kerr, Jingao Liu and an anonymous reviewer, which helped considerably in improving the manuscript. We appreciate Jinlong Ma for analytical assistance. The research was supported by the National Natural Science Foundation of China (NSFC Projects 41273042; 41003017; 91214202; 91014007; 41121002). This is contribution No. IS-1832 to GIG-CAS. References Adam, J., Green, T., 2006. Trace element partitioning between mica- and amphibolebearing garnet lherzolite and hydrous basanitic melt: 1. Experimental results and the investigation of controls on partitioning behavior. Contributions to Mineralogy and Petrology 152 (1), 1–17. Allen, M.B., Macdonald, D.I.M., Zhao, X., Vincent, S.J., Brouet-Menzies, C., 1997. Early Cenozoic two-phase extension and late Cenozoic thermal subsidence and inversion of the Bohai Basin, North China. Marine and Petroleum Geology 14, 951–972. Bianchini, G., Beccaluva, L., Siena, F., 2008. Post-collisional and intraplate Cenozoic volcanism in the rifted Apennines/Adriatic domain. Lithos 101, 125–140. Bouvier, A., Vervoort, J.D., Patchett, P.J., 2008. The Lu–Hf and Sm–Nd isotopic composition of CHUR: constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets. Earth and Planetary Science Letters 273, 48–57. Chauvel, C., Lewin, E., Carpentier, M., Arndt, N.T., Marini, J.C., 2008. Role of recycled oceanic basalt and sediment in generating the Hf–Nd mantle array. Nature Geoscience 1, 64–67. Chen, L., 2010. Concordant structural variations from the surface to the base of the upper mantle in the North China Craton and its tectonic implications. Lithos 120, 96–115. Chen, Y., Zhang, Y.X., Graham, D., Su, S.G., Deng, J.F., 2007. Geochemistry of Cenozoic basalts and mantle xenoliths in Northeast China. Lithos 96, 108–126. Chen, L.H., Zeng, G., Jiang, S.Y., Hofmann, A.W., Xu, X.S., Pan, M.B., 2009. Sources of Anfengshan basalts: subducted lower crust in the Sulu UHP belt, China. Earth and Planetary Science Letters 286, 426–435. Dai, X.Z., Du, Y.H., Wang, Q.L., Wan, J.L., 1993. Fission track dating of authigenic apatite from Tertiary sediments in the Jiyang Depression, Shandong. Scientia Geologica Sinica 28, 283–287 (in Chinese with English abstract). Deng, J.F., Mo, X.X., Zhao, H.L., Luo, Z.H., Du, Y.S., 1994. Lithospere root/de-root and activation of the east China continent. Geoscience 8 (3), 349–356 (in Chinese with English abstract).

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