# Petrogenesis, Magma Source, and Geodynamics of Paleogene Mafic Rocks, Huimin Sag, Jiyang Depression, Eastern China

Authors: Lihua Gao; Zhigang Song; Chao Han; Mei Han; Chenlin Zhu; Di Xia; Zuozhen Han; Shuangjian Li
Journal: Geofluids
 (2022)

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## Abstract

A suite of Paleogene mafic rocks was collected from boreholes in the Huimin Sag of the Jiyang Depression with the aim of investigating the petrogenesis and nature of mantle source for these rocks and further providing insights into the characteristic of related mantle plume. Whole-rock geochemical data indicate that the mafic rocks have relatively lower SiO2 (42.93%–48.57%) contents and similar characteristics to alkaline basalt and belong to transitional calc-alkaline series. These samples were clearly enriched in LREEs and depleted in HREEs and were also characterized by the enrichment of LILEs, incompatible elements, and HFSEs, similar to those of the Ocean Island Basalt (OIB). In addition, they exhibited Pb enrichment; Y, Pr, and Yb depletion; absence of Nb-Ta anomalies; high Hf and low Zr; and Rb/Yb ratios exceeding 1.0, indicating characteristics of intraplate rift-type alkaline basalt. The samples exhibited (Th/Ta)PM and (La/Nb)PM ratios less than 1 and plotted within the OIB, EMI, and EMII fields, indicating that crustal components had no role in the generation of the rocks. With the exception of individual samples that have a distinctive range of εNd values, the majority of samples have complex εNd values of -1.15 to 5.56, indicating a mixture of different sources, which was also apparent in the δ18O-87Sr/86Sr diagram, in which the samples plot close to the downward nonlinear curve. Based on the isotopic and trace elemental analyses, these igneous rocks are intraplate rift-type alkaline basalt and are of mantle plume origin. The variations in 87Sr/86Sr, 143Nd/144Nd, εNd values, LREEs, and HFSEs were probably due to the different locations of the mantle plume for different samples. The primary magma of the rocks likely originated from the melting of a mantle plume and the further metasomatism of lithospheric mantle, continental, or oceanic crust.


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## Body

## 1. Introduction

The impact of mantle plumes on mineralization involves all stages of the Earth’s evolution. Some geologists have suggested that most intracontinental deposits formed during the Precambrian–Phanerozoic were all related to mantle hotspots [1–3] and proposed mantle plume metallogenic systems [4, 5]. Evidence from P-wave velocity, lithospheric discontinuity, thinning and transformation, a rise of asthenosphere with a mushroom cloud structure, and large igneous province suggests that mineralization and petroleum accumulation in eastern China during the Mesozoic–Cenozoic were tectonically controlled by submantle plumes [6–10].The submantle plume of North China Craton (NCC) consists of Luxi, Jiaodong, Liaoxi, and Jibei mantle branches, which possess different deposits that are distributed spatially or in groups [9]. Over the past three decades, some researchers have proposed that the Bohai Bay Basin (BBB), which is the largest basin in the Eastern Block of the NCC, has close correlation with a mantle plume, on the basis of seismic data and physical simulation [11–14]. Liu et al. [11] concluded that five mantle plume uplifts, including Bozhong, Bohai Bay, Miaoxi, Liaodong Bay, and Kenli uplifts, existed in the BBB, according to studies on gravity and magnetic fields, geothermal heat flow, and high conductivity layers in the upper mantle. Zhu [14] suggested that mantle plume activities have occurred since the Jurassic and controlled the basin’s tectonic evolution during the Late Cretaceous and established a mantle plume evolution model based on tectonic-sedimentation regularity, seismic section, paleostress, and paleoflow direction. Xiao et al. [12] thought that a mantle plume in the Songliao region, formed during the Late Jurassic–Late Cretaceous, moved to the Beijing-Tianjin-Bohai Bay regions and formed the BBB because of the northward movement of the NCC, based on regional seismic and sedimentary data and electron microprobe analysis. Ye and Wang [13] considered that the regular distribution of geological and geophysical characteristics of Jiyang Depression was caused by the difference of subduction angle of the Western Pacific plate from south to north, as well as tectonic migration of mantle plume from south to north.Though some researchers have done research on various aspects (e.g., sedimentary strata, structural patterns, and tectonic evolutions), systematic studies focused on Cenozoic igneous rocks that are well developed in the BBB are still limited [15–20]. As a consequence, the petrogenesis and source of the igneous rocks deduced by those studies are still controversial. Liu et al. [15] suggested that the Tertiary basaltic rocks in the BBB are typical intraplate basalts that have a mantle source that was free from crustal contamination. Shen et al. [17] proposed that Tertiary basalts from the Jiyang Depression were formed by mixing of DMM and EMI, to varying degrees, with a minor contribution from EMII based on Sr-Nd isotopic analysis. Liu and Xie [16] argued that the magmas of Tertiary mafic intrusive and volcanic rocks in the Jiyang Depression were sourced from the upper mantle. The Tertiary basaltic rocks in the Huanghua Depression, investigated by Zhang et al. [19], have Sr-Nd isotopic characteristics similar with OIB, indicating an asthenospheric mantle magma source. Wang [18] argued that the Paleogene basaltic volcanic rocks from eastern depression of Liaohe Basin have OIB-Continental Flood Basalt (CFB) characteristics and probably sourced from a depleted mantle which were mixed by enriched components.In this study, a suite of igneous samples was collected from five drilling wells from the Xia 38, Xia 39, Xia 381, Xia 382, and Shang 745 fault blocks of Huimin Sag, Jiyang Depression. We report petrological observations, whole-rock geochemical and Sr-Nd-O isotopic data for these rocks. These new data provide great significances for the study of the petrogenesis and nature of mantle source for the Cenozoic igneous rocks of BBB.


## 2. Geological Background and Petrological Characteristics

The BBB is a Meso-Cenozoic superimposed petroliferous basin located in the Eastern Block of the NCC. It is surrounded by four uplifted massifs, the Taihang Massif to the west, the Yanshan Massif to the north, the Jiaoliao Massif to the east, and the Luxi Massif to the south. The Huimin Sag is a NEE-trending sub-half-graben tectonic basin, located to the west of the Jiyang Depression in the BBB (Figure1(a)). Its neighbours are Chengning Uplift to the north, Luxi Uplift to the south, Linqing Depression to the west, and Dongying Depression to the east. The Huimin Sag has a complex fracture system and faulted structures that were controlled by the north-dipping Xiakou Fault and south-dipping Linshang Fault. Many Cenozoic igneous rocks are developed in the Huimin Sag, especially near the conjunct parts of faults in the Linnan Sag and Yangxin Sag (Figure 1(b)). The Cenozoic igneous rocks in this region are more than 1000 m thick and were proposed to be divided into several eruption cycles (Figure 2). The collected igneous rock samples belong to the middle section of the third member, the upper section of the third member–lower section of the second member, and the first member of the Shahejie Formation (Figure 2). The collected igneous rocks are mainly mafic intrusive rocks and volcanic rocks of overflow facies; the petrography of which is described as follows.Figure 1
(a) Distribution of Cenozoic igneous rocks in the Bohai Bay Basin [55]. (b) A simplified structural map of the Huimin Sag.Figure 2
Cenozoic strata column of the Bohai Bay Basin with summary of the tectonic evolution and40Ar-39Ar age data for basalts from the Shahejie Formation in the Jiyang Depression (modified after [56]), showing sample locations.


### 2.1. Intrusive Bodies

The intrusive rock samples from Xia 38, Xia 381, and Xia 39 wells in the study area are greyish green and black-green in colour and have a dense massive structure and poikilitic and ophitic textures (Figures3(a)–3(f)). These samples are mainly composed of augite (~45%), plagioclase (~40%), amphibole-biotite-sericite (~10%), and other opaque minerals (~5%). The augites are pink, Ti-enriched, and characterized by positive high protuberance, obvious pleochroism, and the highest interference colour of second order blue-to-green. The plagioclases are mainly euhedral plate-prismatic with polysynthetic twins, positive low protuberance, and the highest interference colour of first order grey-to-white. Amphiboles are mainly hornblende and barkevicite, with some replaced by biotite. The products of alteration of the intrusive rocks are mainly chlorite, sericite, and saponite.Figure 3
Microphotographs for Paleogene mafic rocks in Huimin Sag. (a, b) Intrusive rock from Xia 38 well, 3830.4 m. This sample is mainly composed of plagioclase and augite, with minor amphibole and magnetite and has a poikilitic texture. Corrosion of amphibole and sericitization of plagioclase can be seen. (c, d) Intrusive rock from Xia 38 well, 3668.5 m. This sample is mainly composed of plagioclase and augite and has a poikilitic texture. The augite shows latticed corrosion and obvious chloritization. The plagioclase experienced severe dissolution, and the dissolved pores were filled with asphaltic components. (e, f) Intrusive rock from Xia 381 well, 4006.1 m. This sample is mainly composed of plagioclase, augite, and olivine with minor amphibole and magnetite and has an ophitic texture. The augite and plagioclase were severely dissolved, and the pores were filled with asphaltic components. (g, h) Volcanic rock from Shang 745 well, 3215.2 m. This sample has a porphyritic texture with the matrix showing intergranular and interseptal textures. The phenocrysts are mainly plagioclase and augite, whereas the matrix is composed of subdirectionally distributed fine lath plagioclase, pyroxene, and glass, with a small amount of hornblende. (i, j) Volcanic rock from Shang 745 well, 3200.2 m. This sample has an interseptal texture and is mainly composed of randomly distributed long strip plagioclase, the spaces of which filled with granular pyroxene and glass that experienced devitrification, as well as a small amount of large pyroxene particles, with some of which have been saponified. (k, l) Volcanic rock from Shang 745 well, 3200.2 m. This sample has an interseptal texture, with a locally intersertal texture. It is mainly composed of randomly distributed long strip plagioclase, the spaces of which filled with granular pyroxene and magnetite. Structural fractures that filled with calcite can be seen. Pl—plagioclase; Hbl—hornblende; Ol—olivine; Aug—augite; Mag—magnetite.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)


### 2.2. Volcanic Rocks

The volcanic rock samples from Xia 382 well in this study are greyish green and black-green in colour with tiny holes and cracks. They have a porphyritic texture, and the groundmass has intergranular and intersertal textures (Figures3(g)–3(l)). The phenocrysts are mainly augite and plagioclase. Minor hornblende and pyrite are also observed in the volcanic rocks (Figures 3(g) and 3(i)). The products of alteration of the volcanic rocks are mainly chlorite, sericite, and saponite.


## 2.1. Intrusive Bodies

The intrusive rock samples from Xia 38, Xia 381, and Xia 39 wells in the study area are greyish green and black-green in colour and have a dense massive structure and poikilitic and ophitic textures (Figures3(a)–3(f)). These samples are mainly composed of augite (~45%), plagioclase (~40%), amphibole-biotite-sericite (~10%), and other opaque minerals (~5%). The augites are pink, Ti-enriched, and characterized by positive high protuberance, obvious pleochroism, and the highest interference colour of second order blue-to-green. The plagioclases are mainly euhedral plate-prismatic with polysynthetic twins, positive low protuberance, and the highest interference colour of first order grey-to-white. Amphiboles are mainly hornblende and barkevicite, with some replaced by biotite. The products of alteration of the intrusive rocks are mainly chlorite, sericite, and saponite.Figure 3
Microphotographs for Paleogene mafic rocks in Huimin Sag. (a, b) Intrusive rock from Xia 38 well, 3830.4 m. This sample is mainly composed of plagioclase and augite, with minor amphibole and magnetite and has a poikilitic texture. Corrosion of amphibole and sericitization of plagioclase can be seen. (c, d) Intrusive rock from Xia 38 well, 3668.5 m. This sample is mainly composed of plagioclase and augite and has a poikilitic texture. The augite shows latticed corrosion and obvious chloritization. The plagioclase experienced severe dissolution, and the dissolved pores were filled with asphaltic components. (e, f) Intrusive rock from Xia 381 well, 4006.1 m. This sample is mainly composed of plagioclase, augite, and olivine with minor amphibole and magnetite and has an ophitic texture. The augite and plagioclase were severely dissolved, and the pores were filled with asphaltic components. (g, h) Volcanic rock from Shang 745 well, 3215.2 m. This sample has a porphyritic texture with the matrix showing intergranular and interseptal textures. The phenocrysts are mainly plagioclase and augite, whereas the matrix is composed of subdirectionally distributed fine lath plagioclase, pyroxene, and glass, with a small amount of hornblende. (i, j) Volcanic rock from Shang 745 well, 3200.2 m. This sample has an interseptal texture and is mainly composed of randomly distributed long strip plagioclase, the spaces of which filled with granular pyroxene and glass that experienced devitrification, as well as a small amount of large pyroxene particles, with some of which have been saponified. (k, l) Volcanic rock from Shang 745 well, 3200.2 m. This sample has an interseptal texture, with a locally intersertal texture. It is mainly composed of randomly distributed long strip plagioclase, the spaces of which filled with granular pyroxene and magnetite. Structural fractures that filled with calcite can be seen. Pl—plagioclase; Hbl—hornblende; Ol—olivine; Aug—augite; Mag—magnetite.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)


## 2.2. Volcanic Rocks

The volcanic rock samples from Xia 382 well in this study are greyish green and black-green in colour with tiny holes and cracks. They have a porphyritic texture, and the groundmass has intergranular and intersertal textures (Figures3(g)–3(l)). The phenocrysts are mainly augite and plagioclase. Minor hornblende and pyrite are also observed in the volcanic rocks (Figures 3(g) and 3(i)). The products of alteration of the volcanic rocks are mainly chlorite, sericite, and saponite.


## 3. Analytical Methods

Fifteen magmatic core samples for this study were collected from 5 wells, Xia 38, Xia 39, Xia 381, Xia 382, and Shang 745, including 7 intrusive rock samples (from wells of Xia 38, Xia 39, Xia 381, and Xia 382, collected from upper Es3 submember) and 8 volcanic rock samples (from the Shang 743 and Shang 745 wells, collected from the middle Es3 submember). Samples were first crushed, cleaned, soaked, dried, and then ground to a powder. The major elements, trace elements, rare earth elements, oxygen isotopes, and strontium-neodymium isotopes were analysed at the Geological Analysis and Testing Research Center of the Nuclear Industry. Major elements were analysed by Axios Max X-ray fluorescence spectrometer (Axios mAX) using fused glass disks according to the Chinese national standard (GB/T14506.14-2010). Trace elements were determined using a NexION 300D inductively coupled plasma mass spectrometer (ICP-MS) according to the Chinese national standard (GB/T14506.30-2010). Analytical results for Chinese standards (GBW07104 and GBW07312) indicate an analytical precision of >95% for both major and trace elements. Oxygen isotopes were determined using a Finnigan MAT 253 instrument with an accuracy exceeding 0.2% indicated by the Chinese standards GB04416 and GB04417. Strontium and neodymium isotopes were analysed using an IsoProbe-T thermal surface ionization mass spectrometer. During the collection of isotopic data, repeated analysis of the Nd isotopic standard JNdi-1 [21] gave an average 143Nd/144Nd value of 0.512098±0.000007 (n=3), and the strontium NBS SRM 987 isotopic standard gave an average 87Sr/86Sr value of 0.710248±0.000008 (n=3). Further details of the analytical methods are given in Lu et al. [22].


## 4. Analytical Results

### 4.1. Major and Trace Elements

The presence of a certain amount of secondary minerals and the moderate to high loss on ignition (LOI, 2.55–5.98) imply that the studied rocks have undergone varying degrees of hydrothermal alteration or metamorphism, to which some elements (such as Rb, Ba, Sr, Na, and K) are sensitive [23–25]. Therefore, the elements that are considered immobile during alteration, such as high field strength elements (HFSEs) and some transition metals, are chosen to describe the primary chemical features of the samples in this study. The igneous rocks in the Huimin Sag exhibited relatively concentrated major elements (Table 1), with SiO2 ranging from 42.93% to 48.57%, with an average of 46.11%. They have relatively higher TiO2, P2O5, FeO, and total alkaline (K2O+Na2O) and lower CaO/TiO2 and Al2O3/TiO2 values, with TiO2 contents ranging from 1.62% to 2.79% (average 2.07%, which was significantly higher than the value of 0.85% for active continental margin basalts) (Table 1). All the samples show transitional to alkaline Nb/Y ratios and plot in the alkaline basalt field on a Zr/Ti versus Nb/Y diagram (Figure 4(a), [26]). In addition, they were plotted onto the Y versus Zr diagram exposing characteristics of transitional to calc-alkaline series rocks (Figure 4(b), [27]).Table 1
Major element (%) compositions for Paleogene igneous rocks in Huimin Sag.
Sample no.LithologySiO2AI2O3TFe2O3MgOCaONa2OK2OMnOTiO2P2O5FeOLossX38-2Intrusive rock47.5715.9410.804.529.983.651.020.1452.120.3277.733.88X38-3Intrusive rock47.4514.2712.894.737.674.442.130.2072.790.4563.42.96X39-1Intrusive rock46.9415.5611.976.628.023.471.370.1742.030.32.753.48X381-1Intrusive rock45.0212.2213.4113.166.572.360.9180.181.730.2665.074.07X382-1Volcanic rock46.3814.4712.288.058.433.220.8660.1691.960.2919.033.78X382-5Volcanic rock47.5414.0512.318.375.424.410.6580.1791.860.293.534.85S745-4Intrusive rock47.6415.0711.845.388.393.711.690.1922.030.3024.053.23S745-6Intrusive rock48.5714.0613.254.638.113.941.70.1922.590.4035.272.56S745-8Volcanic rock47.1715.2510.257.526.864.441.280.1351.670.5577.74.85S745-11Intrusive rock48.0813.6313.224.628.914.321.350.1912.680.4379.352.55S745-AVolcanic rock44.615.2710.215.6410.943.771.390.1511.740.5658.155.73S745-BVolcanic rock44.6614.911.787.328.493.541.570.1771.620.5739.485.33S745-CVolcanic rock42.9715.7411.267.0110.643.170.8480.1751.630.588.785.98S745-DVolcanic rock42.9315.6511.046.411.822.771.020.1591.740.5658.695.89S745-EVolcanic rock44.0813.5112.934.511.363.620.5120.1612.460.378.455.49DeccanContinental basalt50.1813.703.726.5811.441.700.730.212.20—9.08SiberiaContinental basalt48.5815.882.806.489.982.101.020.131.230.108.74Mount EmeiContinental basalt44.757.7013.2517.959.001.180.710.182.240.20—Hawaii HaleakalaOIB46.4610.85—17.658.071.930.390.162.060.2212.20Hawaiian IslandsOIB50.2912.921.488.0710.842.260.460.143.030.369.77Figure 4
(a) Nb/Y vs. Zr/Ti and (b) Y vs. Zr discriminations for the Paleogene igneous rocks from the Huimin Sag. The boundary lines in (a) and (b) are from Pearce [26] and Miyashiro [27], respectively.
(a)(b)The basic igneous rocks in this study exhibited total rare earth elements (ΣREE) of 85.27 to 297.31 ppm (average 154.36 ppm) (Table 2). The light rare earth elements (LREEs) ranged from 69.77 to 272 ppm (average 134.25 ppm), whereas the heavy rare earth elements (HREEs) ranged from 15.38 to 27.50 ppm (average 19.77 ppm). The samples were relatively enriched in LREEs and depleted in HREEs (Figures 5(a) and 5(b)). In the primitive mantle-normalized trace element spider diagrams (Figures 5(c) and 5(d)), all samples were relatively enriched in large ion lithophile elements (LILEs, e.g., Ba, Pb, Sr, U, and K), incompatible elements (Rb, Ba, Th, U, and K), and some high field strength elements (HFSEs, e.g., Nb, Ta, Zr, and Hf).Table 2
Trace element (ppm) compositions for Paleogene igneous rocks in Huimin Sag.
Sample no.X38-2X38-3X39-1x381-1x382-1x382-5S745-4S745-6S745-8S745-11S745-AS745-BS745-CS745-DS745-ESr703.00490.00589.00280.00386.00267.001479.00568.00631.00389.00313.00530.00482.00437.00200.00Ba320.00452.00895.00218.00224.00307.00926.00463.00665.00302.00775.00637.00283.00391.0064.10Cr96.8031.30251.00467.00291.00336.00125.0081.70232.0046.50214.00196.00228.00212.0084.60V233.00259.00216.00181.00198.00199.00195.00249.00178.00250.00183.00149.00164.00182.00237.00Li29.0011.7025.1036.5025.3040.0022.3024.9024.8020.8027.9033.5035.9031.4021.40Be1.171.761.100.771.211.680.911.342.451.082.621.853.664.151.84Sc30.0027.0026.6021.9025.1022.9026.2031.7022.6032.0021.6018.8020.4022.5030.00Co33.9032.9042.7067.5044.6054.6035.1034.6041.0031.6036.3031.7038.5037.2033.80Ni41.5026.1091.30314.00127.00185.0046.3024.50104.0020.7088.9087.6099.9091.5025.60Cu92.3075.6068.7065.4067.1076.0059.5076.7063.8074.9039.9042.7034.7038.5071.20Zn118.00155.00130.00137.00142.00139.00129.00161.00112.00154.00142.00109.00131.00125.00151.00Ga20.7022.8020.4016.0019.4023.7016.2019.8018.8018.7023.3018.8025.5025.3020.20Rb24.6037.1026.5017.5016.2012.9033.7027.2025.6025.1030.6025.9016.6021.2012.80Y30.3042.2030.1025.1028.5028.4030.6037.7024.7035.3024.1021.0024.0026.9036.80Zr146.00233.00141.00114.00140.00133.00139.00173.00172.00163.00153.00155.00175.00182.00171.00Nb26.7050.6026.2022.1025.4026.2022.5030.0064.9030.4065.0056.3061.9064.3028.20Mo2.967.593.862.052.602.196.245.226.234.436.425.245.265.615.29Cd0.190.210.150.140.210.090.190.160.110.150.180.180.210.180.27Cs0.776.691.941.850.510.370.950.491.440.591.371.081.221.631.65La18.4026.9016.0013.1015.5015.8015.7019.1039.1020.4041.2036.7041.5039.4018.00Ce36.0051.8032.1026.2030.3031.1031.4039.3069.9039.8073.0063.2070.9068.0036.40Pr4.686.844.303.423.894.154.495.458.255.428.537.278.038.004.99Nd19.9028.8018.3014.5017.0017.6020.4024.9030.4024.3032.1027.2030.4031.0022.90Sm5.207.074.723.654.644.645.326.656.096.506.255.056.026.156.14Eu1.712.071.761.301.631.441.832.021.871.902.201.641.992.191.75Gd4.906.744.783.804.504.474.826.075.275.975.304.745.245.465.42Tb0.981.350.990.760.950.901.001.150.891.160.880.800.890.951.08Dy6.118.425.804.685.675.746.017.325.127.154.994.284.815.226.87Ho1.151.531.170.871.081.071.141.380.881.300.880.790.840.931.36Er3.034.113.072.272.802.783.033.692.283.292.382.052.282.532.44Tm0.530.680.520.400.520.470.520.580.390.550.410.350.380.400.59Yb2.974.083.072.382.932.813.183.752.433.572.502.102.322.633.63Lu0.430.590.440.330.410.400.420.490.340.460.350.280.300.330.49Hf3.956.303.923.203.883.773.944.794.864.674.334.524.764.854.65Ta1.582.911.601.301.551.571.321.804.201.764.123.553.903.961.61W0.420.660.410.210.511.900.490.457.250.541.090.750.790.810.87Pb4.7620.305.306.1322.0015.603.005.518.3914.4017.708.3711.007.784.90Th2.603.802.201.582.092.262.022.214.682.194.764.124.574.532.10U1.341.901.340.791.131.071.261.121.721.412.311.581.561.791.27∑REE112.79172.78104.9285.27100.22101.97101.36126.95207.71127.88213.87185.54207.40206.49117.35LREE92.69145.2885.0869.7781.3683.3381.24102.52190.11104.42196.18170.16190.34188.0495.48HREE20.1027.5019.8415.5018.8618.6420.1224.4317.6023.4617.6915.3817.0618.4521.87LREE/HREE4.615.284.294.504.314.474.044.2010.804.4511.0911.0611.1610.194.37LaN/YbN4.444.733.743.953.794.033.543.6511.544.1011.8212.5412.8310.753.56δEu1.040.921.131.071.090.971.100.971.010.931.171.021.081.160.93δCe0.950.940.950.960.960.940.920.940.950.930.950.950.950.940.94Figure 5
(a, b) Chondrite-normalized REE patterns and (c, d) primitive mantle (PM) normalized trace element diagrams for the Paleogene igneous rocks in Huimin Sag. The values of chondrite and PM are from Sun and McDonough [57].
(a)(b)(c)(d)


### 4.2. Sr-Nd and O Isotopes

The Sr-Nd and O isotopic compositions of Paleogene igneous rocks from the Huimin Sag are given in Tables3 and 4. The 87Sr/86Sr ratios varied from 0.7046 to 0.7098 (average 0.7078), whereas the 143Nd/144Nd ratios varied from 0.5117 to 0.5129 (average 0.5126). The εNd values ranged from -17.71 to 5.56 (average -0.75), while the εSr values ranged from -1.99 to 71.49 (average 41.8822). The δ18O values ranged from 5.4‰ to 10.4‰, with an average of 7.9‰.Table 3
Sr-Nd-O isotopic compositions of Paleogene igneous rocks in Huimin Sag.
Sample no.87Sr/86SrStd err143Nd/144NdStd errεSrεNdδ18O (‰)X38-20.7082090.0000160.5125790.00001549.08-1.159.5X38-30.7060010.0000090.5126700.00001017.750.628.3X39-10.7066800.0000160.5126700.00001327.390.625.5X381-10.7046100.0000160.5126510.000010-1.990.255.4S745-40.7082500.0000130.5126050.00001049.66-0.64—S745-60.7082240.0000110.5126570.00001049.290.37—S745-110.7088230.0000090.5126520.00001657.79-9.23—X382-10.7062500.0000160.5117300.00000821.82-17.718.5X382-50.7097880.0000080.5126200.00000671.49-0.3510.4S745-80.7082940.0000120.5127830.00000950.292.83—S745-A0.7086710.0000120.5128030.00000955.643.22—S745-B0.7083460.0000160.5127300.00000851.031.80—S745-C0.7083080.0000130.5127430.00001050.492.05—S745-D0.7081270.0000080.5127510.00001047.922.20—S745-E0.7083780.0000130.5126220.00000551.48-0.31—Table 4
Comparison of trace element ratios of Paleogene igneous rocks in Huimin Sag and crust and different mantle members (data of different mantle end members are from [22, 38]).
Element ratioPrimitive mantleDepleted mantleCrustHIMUEMIEMIIVolcanic rock in study areaZr/Nb14.83016.22.7~5.53.5~13.14.4~7.82.35~6.18 (4.42)La/Nb0.941.072.20.64~0.820.78~1.320.79~1.190.53~0.70 (0.63)Ba/Nb9.04.3544.7~6.99.1~23.46.4~13.42.27~41.16 (13.48)K/Nb323296134166~187207~523203~378109.21~623.33 (279.96)Th/Nb0.1170.070.440.07~0.120.09~0.130.10~0.170.07~0.10 (0.08)Th/La0.1250.070.20.10~0.160.09~0.150.11~0.180.11~0.14 (0.12)Ba/La9.64256.2~9.3611.3~19.17.3~13.53.56~58.98 (21.39)87Sr/86Sr0.70220.70280.70530.70780.7046~0.7098 (0.7078)143Nd/144Nd0.51330.51280.51240.51260.5117~0.5129 (0.5126)


## 4.1. Major and Trace Elements

The presence of a certain amount of secondary minerals and the moderate to high loss on ignition (LOI, 2.55–5.98) imply that the studied rocks have undergone varying degrees of hydrothermal alteration or metamorphism, to which some elements (such as Rb, Ba, Sr, Na, and K) are sensitive [23–25]. Therefore, the elements that are considered immobile during alteration, such as high field strength elements (HFSEs) and some transition metals, are chosen to describe the primary chemical features of the samples in this study. The igneous rocks in the Huimin Sag exhibited relatively concentrated major elements (Table 1), with SiO2 ranging from 42.93% to 48.57%, with an average of 46.11%. They have relatively higher TiO2, P2O5, FeO, and total alkaline (K2O+Na2O) and lower CaO/TiO2 and Al2O3/TiO2 values, with TiO2 contents ranging from 1.62% to 2.79% (average 2.07%, which was significantly higher than the value of 0.85% for active continental margin basalts) (Table 1). All the samples show transitional to alkaline Nb/Y ratios and plot in the alkaline basalt field on a Zr/Ti versus Nb/Y diagram (Figure 4(a), [26]). In addition, they were plotted onto the Y versus Zr diagram exposing characteristics of transitional to calc-alkaline series rocks (Figure 4(b), [27]).Table 1
Major element (%) compositions for Paleogene igneous rocks in Huimin Sag.
Sample no.LithologySiO2AI2O3TFe2O3MgOCaONa2OK2OMnOTiO2P2O5FeOLossX38-2Intrusive rock47.5715.9410.804.529.983.651.020.1452.120.3277.733.88X38-3Intrusive rock47.4514.2712.894.737.674.442.130.2072.790.4563.42.96X39-1Intrusive rock46.9415.5611.976.628.023.471.370.1742.030.32.753.48X381-1Intrusive rock45.0212.2213.4113.166.572.360.9180.181.730.2665.074.07X382-1Volcanic rock46.3814.4712.288.058.433.220.8660.1691.960.2919.033.78X382-5Volcanic rock47.5414.0512.318.375.424.410.6580.1791.860.293.534.85S745-4Intrusive rock47.6415.0711.845.388.393.711.690.1922.030.3024.053.23S745-6Intrusive rock48.5714.0613.254.638.113.941.70.1922.590.4035.272.56S745-8Volcanic rock47.1715.2510.257.526.864.441.280.1351.670.5577.74.85S745-11Intrusive rock48.0813.6313.224.628.914.321.350.1912.680.4379.352.55S745-AVolcanic rock44.615.2710.215.6410.943.771.390.1511.740.5658.155.73S745-BVolcanic rock44.6614.911.787.328.493.541.570.1771.620.5739.485.33S745-CVolcanic rock42.9715.7411.267.0110.643.170.8480.1751.630.588.785.98S745-DVolcanic rock42.9315.6511.046.411.822.771.020.1591.740.5658.695.89S745-EVolcanic rock44.0813.5112.934.511.363.620.5120.1612.460.378.455.49DeccanContinental basalt50.1813.703.726.5811.441.700.730.212.20—9.08SiberiaContinental basalt48.5815.882.806.489.982.101.020.131.230.108.74Mount EmeiContinental basalt44.757.7013.2517.959.001.180.710.182.240.20—Hawaii HaleakalaOIB46.4610.85—17.658.071.930.390.162.060.2212.20Hawaiian IslandsOIB50.2912.921.488.0710.842.260.460.143.030.369.77Figure 4
(a) Nb/Y vs. Zr/Ti and (b) Y vs. Zr discriminations for the Paleogene igneous rocks from the Huimin Sag. The boundary lines in (a) and (b) are from Pearce [26] and Miyashiro [27], respectively.
(a)(b)The basic igneous rocks in this study exhibited total rare earth elements (ΣREE) of 85.27 to 297.31 ppm (average 154.36 ppm) (Table 2). The light rare earth elements (LREEs) ranged from 69.77 to 272 ppm (average 134.25 ppm), whereas the heavy rare earth elements (HREEs) ranged from 15.38 to 27.50 ppm (average 19.77 ppm). The samples were relatively enriched in LREEs and depleted in HREEs (Figures 5(a) and 5(b)). In the primitive mantle-normalized trace element spider diagrams (Figures 5(c) and 5(d)), all samples were relatively enriched in large ion lithophile elements (LILEs, e.g., Ba, Pb, Sr, U, and K), incompatible elements (Rb, Ba, Th, U, and K), and some high field strength elements (HFSEs, e.g., Nb, Ta, Zr, and Hf).Table 2
Trace element (ppm) compositions for Paleogene igneous rocks in Huimin Sag.
Sample no.X38-2X38-3X39-1x381-1x382-1x382-5S745-4S745-6S745-8S745-11S745-AS745-BS745-CS745-DS745-ESr703.00490.00589.00280.00386.00267.001479.00568.00631.00389.00313.00530.00482.00437.00200.00Ba320.00452.00895.00218.00224.00307.00926.00463.00665.00302.00775.00637.00283.00391.0064.10Cr96.8031.30251.00467.00291.00336.00125.0081.70232.0046.50214.00196.00228.00212.0084.60V233.00259.00216.00181.00198.00199.00195.00249.00178.00250.00183.00149.00164.00182.00237.00Li29.0011.7025.1036.5025.3040.0022.3024.9024.8020.8027.9033.5035.9031.4021.40Be1.171.761.100.771.211.680.911.342.451.082.621.853.664.151.84Sc30.0027.0026.6021.9025.1022.9026.2031.7022.6032.0021.6018.8020.4022.5030.00Co33.9032.9042.7067.5044.6054.6035.1034.6041.0031.6036.3031.7038.5037.2033.80Ni41.5026.1091.30314.00127.00185.0046.3024.50104.0020.7088.9087.6099.9091.5025.60Cu92.3075.6068.7065.4067.1076.0059.5076.7063.8074.9039.9042.7034.7038.5071.20Zn118.00155.00130.00137.00142.00139.00129.00161.00112.00154.00142.00109.00131.00125.00151.00Ga20.7022.8020.4016.0019.4023.7016.2019.8018.8018.7023.3018.8025.5025.3020.20Rb24.6037.1026.5017.5016.2012.9033.7027.2025.6025.1030.6025.9016.6021.2012.80Y30.3042.2030.1025.1028.5028.4030.6037.7024.7035.3024.1021.0024.0026.9036.80Zr146.00233.00141.00114.00140.00133.00139.00173.00172.00163.00153.00155.00175.00182.00171.00Nb26.7050.6026.2022.1025.4026.2022.5030.0064.9030.4065.0056.3061.9064.3028.20Mo2.967.593.862.052.602.196.245.226.234.436.425.245.265.615.29Cd0.190.210.150.140.210.090.190.160.110.150.180.180.210.180.27Cs0.776.691.941.850.510.370.950.491.440.591.371.081.221.631.65La18.4026.9016.0013.1015.5015.8015.7019.1039.1020.4041.2036.7041.5039.4018.00Ce36.0051.8032.1026.2030.3031.1031.4039.3069.9039.8073.0063.2070.9068.0036.40Pr4.686.844.303.423.894.154.495.458.255.428.537.278.038.004.99Nd19.9028.8018.3014.5017.0017.6020.4024.9030.4024.3032.1027.2030.4031.0022.90Sm5.207.074.723.654.644.645.326.656.096.506.255.056.026.156.14Eu1.712.071.761.301.631.441.832.021.871.902.201.641.992.191.75Gd4.906.744.783.804.504.474.826.075.275.975.304.745.245.465.42Tb0.981.350.990.760.950.901.001.150.891.160.880.800.890.951.08Dy6.118.425.804.685.675.746.017.325.127.154.994.284.815.226.87Ho1.151.531.170.871.081.071.141.380.881.300.880.790.840.931.36Er3.034.113.072.272.802.783.033.692.283.292.382.052.282.532.44Tm0.530.680.520.400.520.470.520.580.390.550.410.350.380.400.59Yb2.974.083.072.382.932.813.183.752.433.572.502.102.322.633.63Lu0.430.590.440.330.410.400.420.490.340.460.350.280.300.330.49Hf3.956.303.923.203.883.773.944.794.864.674.334.524.764.854.65Ta1.582.911.601.301.551.571.321.804.201.764.123.553.903.961.61W0.420.660.410.210.511.900.490.457.250.541.090.750.790.810.87Pb4.7620.305.306.1322.0015.603.005.518.3914.4017.708.3711.007.784.90Th2.603.802.201.582.092.262.022.214.682.194.764.124.574.532.10U1.341.901.340.791.131.071.261.121.721.412.311.581.561.791.27∑REE112.79172.78104.9285.27100.22101.97101.36126.95207.71127.88213.87185.54207.40206.49117.35LREE92.69145.2885.0869.7781.3683.3381.24102.52190.11104.42196.18170.16190.34188.0495.48HREE20.1027.5019.8415.5018.8618.6420.1224.4317.6023.4617.6915.3817.0618.4521.87LREE/HREE4.615.284.294.504.314.474.044.2010.804.4511.0911.0611.1610.194.37LaN/YbN4.444.733.743.953.794.033.543.6511.544.1011.8212.5412.8310.753.56δEu1.040.921.131.071.090.971.100.971.010.931.171.021.081.160.93δCe0.950.940.950.960.960.940.920.940.950.930.950.950.950.940.94Figure 5
(a, b) Chondrite-normalized REE patterns and (c, d) primitive mantle (PM) normalized trace element diagrams for the Paleogene igneous rocks in Huimin Sag. The values of chondrite and PM are from Sun and McDonough [57].
(a)(b)(c)(d)


## 4.2. Sr-Nd and O Isotopes

The Sr-Nd and O isotopic compositions of Paleogene igneous rocks from the Huimin Sag are given in Tables3 and 4. The 87Sr/86Sr ratios varied from 0.7046 to 0.7098 (average 0.7078), whereas the 143Nd/144Nd ratios varied from 0.5117 to 0.5129 (average 0.5126). The εNd values ranged from -17.71 to 5.56 (average -0.75), while the εSr values ranged from -1.99 to 71.49 (average 41.8822). The δ18O values ranged from 5.4‰ to 10.4‰, with an average of 7.9‰.Table 3
Sr-Nd-O isotopic compositions of Paleogene igneous rocks in Huimin Sag.
Sample no.87Sr/86SrStd err143Nd/144NdStd errεSrεNdδ18O (‰)X38-20.7082090.0000160.5125790.00001549.08-1.159.5X38-30.7060010.0000090.5126700.00001017.750.628.3X39-10.7066800.0000160.5126700.00001327.390.625.5X381-10.7046100.0000160.5126510.000010-1.990.255.4S745-40.7082500.0000130.5126050.00001049.66-0.64—S745-60.7082240.0000110.5126570.00001049.290.37—S745-110.7088230.0000090.5126520.00001657.79-9.23—X382-10.7062500.0000160.5117300.00000821.82-17.718.5X382-50.7097880.0000080.5126200.00000671.49-0.3510.4S745-80.7082940.0000120.5127830.00000950.292.83—S745-A0.7086710.0000120.5128030.00000955.643.22—S745-B0.7083460.0000160.5127300.00000851.031.80—S745-C0.7083080.0000130.5127430.00001050.492.05—S745-D0.7081270.0000080.5127510.00001047.922.20—S745-E0.7083780.0000130.5126220.00000551.48-0.31—Table 4
Comparison of trace element ratios of Paleogene igneous rocks in Huimin Sag and crust and different mantle members (data of different mantle end members are from [22, 38]).
Element ratioPrimitive mantleDepleted mantleCrustHIMUEMIEMIIVolcanic rock in study areaZr/Nb14.83016.22.7~5.53.5~13.14.4~7.82.35~6.18 (4.42)La/Nb0.941.072.20.64~0.820.78~1.320.79~1.190.53~0.70 (0.63)Ba/Nb9.04.3544.7~6.99.1~23.46.4~13.42.27~41.16 (13.48)K/Nb323296134166~187207~523203~378109.21~623.33 (279.96)Th/Nb0.1170.070.440.07~0.120.09~0.130.10~0.170.07~0.10 (0.08)Th/La0.1250.070.20.10~0.160.09~0.150.11~0.180.11~0.14 (0.12)Ba/La9.64256.2~9.3611.3~19.17.3~13.53.56~58.98 (21.39)87Sr/86Sr0.70220.70280.70530.70780.7046~0.7098 (0.7078)143Nd/144Nd0.51330.51280.51240.51260.5117~0.5129 (0.5126)


## 5. Discussion

### 5.1. Petrogenesis of the Mafic Rocks

#### 5.1.1. Fractional Crystallization

It is noticed that the studied mafic rocks from the Huimin Sag have variable MgO, Cr, and Ni contents. In the variation diagrams of MgO versus selected major and trace elements (Figure6), with the exception of Fe2O3+FeO, which was not correlated with MgO, the SiO2, K2O+Na2O, TiO2, Al2O3, and CaO concentrations were negatively correlated with MgO, whereas Cr and Ni concentrations were positively correlated with MgO. These data suggest that the geochemical differences of different samples may be attributed to the fractional crystallization of olivine and/or clinopyroxene. In addition, the TiO2 (1.62%–2.79%) contents of the samples are relatively consistent, implying that the fractionation of Fe-Ti oxides did not occur. Moreover, no obvious Eu anomalies (0.92–1.17) have been observed in the studied samples, reflecting no significant fractionation of plagioclase.Figure 6
Various oxides and Cr, Ni plotted against MgO for the Paleogene igneous rocks in Huimin Sag with literature data for comparison.


#### 5.1.2. Crustal Contamination

The petrological observation and geochemical analysed results imply a mantle source for the magma of studied rocks from the Huimin Sag. Thus, it is important to understand the effects of crustal contamination on the mafic rocks, which is a potential process during the ascent and evolution of mantle-derived magmas [28, 29]. It is widely accepted that minor crustal contamination could produce positive Zr-Hf anomalies of mantle-sourced rocks [24]. However, the Paleogene igneous rocks from the Huimin Sag have no positive Zr-Hf anomalies (Figures 5(c) and 5(d)), ruling out the possibility of crustal contamination. This hypothesis can be further evidenced by the lower Th (1.58 ppm–4.76 ppm) and U (0.79–2.31) contents of these samples relative to the upper crust (e.g., Th=10.5ppm and U=2.7ppm) [30]. Moreover, the Paleogene igneous rocks from the Huimin Sag had εNd values of -17.71 to 5.56 and were mostly within the range of mantle plume-derived CFB, with the exception of samples Xia 382-1 and Shang 745-11, which had negative εNd values and 143Nd/144Nd of 0.5117 and 0.5122, respectively, close to that of average continental crust (0.5119), reflecting the lithospheric mantle or crustal contamination [31]. Samples Shang 745-A, Shang 745-B, Shang 745-C, Shang 745-D, and Shang 745-8 exhibited high positive 143Nd/144Nd and εNd values of 0.5127 to 0.5128 and 1.80 to 3.22, respectively, implying an upper depleted mantle source. The remaining samples had εNd values of -1.15 to 0.62, consistent with the CHUR evolution curve (Figure 7), implying that the magma was derived from different degrees of mixture of mantle plume and lithosphere.Figure 7
87Sr/86Sr vs. 144Nd/143Nd costing point of Paleogene igneous rocks in Huimin Sag. Data sources in (a): Hannuoba basalts [58–60], MORB, OIB, EMI, EMII, and PREMA [61]; data source in (b): EMI, EMII, and PREMA are after Zindler and Hart [61] and Xia [62]. Symbols are as in Figure 5.
(a)(b)Ratios of O isotopes and incompatible elements have been proposed to identify crustal contamination [32]. In the δ18O-87Sr/86Sr diagram (Figure 8), the studied igneous rock samples plot within or close to the area of source contamination, also ruling out the significant impact of crustal contamination. (Th/Ta)PM was close to 1.0 and (La/Nb)PM exceeded 1.0 when lower crustal materials were involved, whereas the above ratios exceeded 2.0. The (Th/Ta)PM and (La/Nb)PM ratios of the Paleogene igneous rocks from the Huimin Sag were 0.54–0.79 and 0.55–0.72, respectively, suggesting that the upper crustal component had no role in the generation of the rocks.Figure 8
Variation ofδ18O vs. 87Sr/86Sr for the mixing of crustal and mantle components of Paleogene igneous rocks in Huimin Sag [39]. Symbols are as in Figure 5.


#### 5.1.3. Mantle Sources and Lithospheric Interaction

During mantle melting and metasomatism, Ti, P, and K can be easily enriched or depleted, as they are strongly incompatible elements. The high contents of Ti, P, and K of the Paleogene igneous rocks from the Huimin Sag are consistent with those of mantle plume-derived basalts, indicating a mantle plume source. In addition, theΣREE contents (average 154.36 ppm) of these rocks are clearly higher than those of basalts (average 85 ppm) in eastern China [33], showing the characteristics of mantle plume-derived basalt, which are usually enriched in LREEs (e.g., the Emeishan basalts, Kerguelen mantle plume volcanic rocks, and Iceland volcanic rocks) [5, 34, 35]. The igneous rocks of the present study were characterized by enrichment of LILEs (e.g., Ba, Pb, Sr, U, and K), incompatible elements (Rb, Ba, Th, U, and K), and HFSEs, (e.g., Nb, Ta, Zr, and Hf), similar to those of the mantle-plume-related OIB (e.g., Hawaii basalt of Pacific Plate and Kerguelen basalt of Indian Ocean Plate) or CFB (e.g., Emeishan, Siberia, Deccan, and African continental basalts) [5]. Furthermore, the remarkable Pb enrichment and slightly depleted Y, Pr, and Yb indicate that the Cenozoic mafic rocks in the study area are intraplate rift-type alkali basaltic rocks, which display the characteristics of upper mantle sources. This view is further supported by the Rb/Yb ratios (>1), high Hf and low Zr contents, and the absence of Nb-Ta anomalies in the samples [16, 36]. In the primitive mantle-normalized trace element spider diagrams (Figures 5(c) and 5(d)), the samples plot between the OIB and N-MORB, but closer to the OIB. The concentrations of compatible elements of Cr and Ni for the studied samples were 31.3–467 ppm and 20.7–314, respectively, consistent with those of the basaltic magma derived from primary mantle [37].The average87Sr/86Sr and 143Nd/144Nd ratios of the Paleogene igneous rocks from the Huimin Sag were 0.7078 and 0.5126, respectively, consistent with those of EMII (87Sr/86Sr=0.7078, 143Nd/144Nd=0.5126), close to those of EMI (87Sr/86Sr=0.7053, 143Nd/144Nd=0.5124) and HIMU (87Sr/86Sr=0.7028, 143Nd/144Nd=0.5128), and higher than those of depleted mantle (87Sr/86Sr=0.7022, 143Nd/144Nd=0.5133). The average Zr/Nb, La/Nb, Ba/Nb, K/Nb, Th/Nb, Th/La, and Ba/La ratios for the samples were 4.42, 0.63, 13.48, 279.96, 0.08, 0.12, and 21.39, respectively, distributed in the enriched mantle EMII, EMI, and HIMU fields [22, 38]. In the 87Sr/86Sr-143Nd/144Nd diagram, samples Xia 38-3, Xia 381-1, and Xia 39-1 plot in the OIB field, sample Xia 382-1 plots in the EMI field, sample Shang 745-11 plots in the ancient lithospheric mantle field close to the EMII, and the remaining samples plot close to the OIB, indicating a mantle source, which is further supported by the positive εNd values of most samples (Figure 7(a)). Most samples plot in, or close to, the mantle source field, except samples Xia 382-1 and Shang 745-11, which plot in the EMI field and upper crust field, respectively (Figure 7(b)). The 87Sr/86Sr ratios of the igneous rocks in the study area varied from 0.7046 to 0.7098, similar to those of the OIB and CFB (<0.710) formed by mantle plume magmatism (e.g., mantle plume-derived basalts of Emeishan, Hawaii, Iceland, and Siberia), showing characteristics of typical mantle plume-derived basalt and deep mantle source [5].The samples examined in the present study exhibitedδ18O values of 5.4‰ to 10.4‰, with an average of 7.93‰, similar to those of the continental tholeiite (5.0‰ to 7.5‰) and continental mafic lava (4.9‰ to 8.0‰), as well as clinopyroxene in the Emeishan mantle plume-derived basalt (6.2‰ to 7.86‰), but higher than those of the OIB (5.4‰) [39, 40]. However, the δ18O values of the samples were slightly higher than those of the mantle, which was probably caused by contamination of crustal materials and source mixing, as the O isotopes were obtained by whole rock analysis. Enrichment of incompatible elements can be caused by the partial melting of an overlying mantle wedge, if the mantle source was influenced by fluids released by the subduction plate. Some researchers suggested that Sr and other incompatible elements can reflect the characteristics of the subduction plate, whereas O isotopes and other elements represent the characteristics of the overlying mantle wedge [39]. In the δ18O-87Sr/86Sr diagram, the igneous rocks in the study area plot close to the downward nonlinear curve, indicating that the isotopic characteristics resulted from source mixing (Figure 8).To sum up, the Sr-Nd-O isotopic and trace elemental characteristics described above were similar to those of the enriched OIB, indicating a mantle plume source and the existence of components from ancient subduction oceanic crust in the source region (Figures7 and 9) [41]. The characteristic trace element ratios were consistent with those of the HIMU, EMI, and EMII OIB, which are represented by Mangaia, Pitcairn, and Society, respectively. Samples Shang 745-8, Shang 745-A, Shang 745-B, Shang 745-C, and Shang 745-D plot within the HIMU OIB field and the Ontong Java field, represented by oceanic platform basalt, whereas the remaining samples plot within the EMI and EMII fields, represented by Pitcairn and Society, respectively (Figure 9(a)). The enrichment of subduction-related materials suggests that there were heterogeneous source rocks of the magmatic rocks, which probably originated from different positions or stages of the mantle plume. The 143Nd/144Nd ratios of the samples plot within the field close to the Society OIB (EMII), implying that a recycled oceanic crust that contains a small amount (i.e., a few percent) of terrigenous sediments was involved in the source (Figure 9(b)) [42, 43]. They distribute within the continental lithospheric mantle and crustal contamination fields, represented by Society and Siberia (Figure 9(b)).Figure 9
Trace element ratios of Paleogene igneous rocks in Huimin Sag. (a) Zr/Nb vs. La/Nb, (b) Th/Nb vs. Nb/U, (c) Rb/Sr vs. Nd/Sm, (d) La/Sm vs. Sm/Yb, and (e) Nb/Y vs. Zr/Y diagrams for the Paleogene igneous rocks from the Huimin Sag. Data sources are as follows: PM refers to primitive mantle values [63]; UCC, BCC, and CC mark the upper continental crust, whole continental crust, and average compositions of bulk, respectively [64]. HIMU OIB [65]; Pitcairn [66]; Society [43]; Ontong Java [67]; N. Kerguelen [68]; Hawaii [69]. Siberia, Nd1, 2 represent crustal contamination lavas typically [70]. The arrow points to the average MORB values: Zr/Nb=31.76, La/Nb=1.07 [57]. The values for OIB are from Sun and McDonough [57]. Composition of continental lithospheric mantle (CLM) is from McDonough [71]. Parallel lines mark the upper and lower bounds of the Iceland basalt data [72], All the symbols are the same as in Figure 5.
(a)(b)(c)(d)(e)The EMII enriched units have been proposed as being related to crustal derived material or recycled oceanic crust [44], whereas the HIMU are related to subducted ancient metamorphic oceanic crust [45]. The EMI are widely accepted to be formed in the lithosphere mantle, asthenosphere, mantle plume, and metasomatic lithosphere mantle by subducted ancient crust [46]. Based on the isotopic and trace elemental analyses discussed above, we suggest that the Paleogene igneous rocks from the Huimin Sag were sourced from the mantle rather than the crust. The primary magma of the rocks likely originated from the melting of a mantle plume and the further metasomatism of lithospheric mantle and continental or oceanic crust.


### 5.2. Mantle End Member

Based on the Sr-Nd-O isotopic and trace elemental analyses discussed above, three end members in the study area can be proposed. The characteristics of the samples are similar to HIMU, EMI, and EMII, which are represented by those of Mangaia, Pitcairn, and Society, respectively (Figure9). Samples Shang 745-8, Shang 745-A, Shang 745-B, Shang 745-C, and Shang 745-D, which have high 143Nd/144Nd ratios, plot within the HIMU and Ontong Java OIB fields, whereas other samples plot within the EMI and EMII fields close to the Society field, suggesting the involvement of subduction-related materials. The analyses presented above suggest that the three mantle plume end members of the study area are as follows.First is the low143Nd/144Nd region represented by samples Xia 382-1 and Shang 745-11. Samples Xia 382-1 and Shang 745-11 have 143Nd/144Nd, 87Sr/86Sr, and εNd of 0.5117 and 0.5122 (average of 0.5119), 0.7063 and 0.7088 (average of 0.7075), and -17.71 and -9.23, respectively, and plot within the EMI and ancient lithospheric mantle fields (Figure 7). These characteristics, in conjunction with the high Y contents and Rb/Sr ratios and low LREEs and HFSE contents and Sm/Nd ratios, indicate that the primary magma experienced crustal remelting and mixing of multiple sources.Because Zr/Nb and La/Nb ratios can be slightly influenced by fractionation, they are considered to represent those of the mantle-derived rocks [41]. The Zr/Nb and La/Nb ratios of primary mantle and Ontong Java were 15.96 and 0.98 and 17.65 and 1.03, respectively, suggesting that the Ontong Java experienced a higher degree of melting (Figure 9(a)). Samples Xia 382-1 and Shang 745-11 exhibited relatively low Zr/Nb and La/Nb ratios of 5.118 and 0.610 and 5.362 and 0.671, respectively, close to those of Pitcairn and Society (7). They plot in the HIMU field and the boundary between Pitcairn and Society fields, which are not exactly the same as typical OIB, implying the influence of mixing of multiple sources [43]. The Th/Nb and Nb/U ratios of samples Xia 382-1 and Shang 745-11 were 0.082 and 22.488 and 0.072 and 21.56, respectively, and the samples plot within the Ontong Java field; the Th/Nb and Nb/U ratios of which were 0.082 and 35.15 (Figure 9(b)). They exhibited Rb/Sr ratios of 0.042 and 0.065, respectively, close to those of the Ontong Java (0.069) and plot within the OIB (Figure 9(c)). They exhibited La/Sm and Sm/Yb ratios of 3.341 and 1.584 and 3.138 and 1.821, respectively, and plot within the Siberia field, between the CLM and BCC (Figure 9(d)). They exhibited average Nb/Y and Zr/Y ratios of 0.966 and 4.906, respectively, and plot within the OIB field (Figure 9(e)).Second is the high143Nd/144Nd region represented by samples Shang 745-8, Shang 745-A, Shang 745-B, Shang 745-C, and Shang 745-D. These samples displayed 87Sr/86Sr of 0.7081 to 0.7087 (average of 0.7083) and the highest 143Nd/144Nd of 0.5127 to 0.5129 (average of 0.5128); they plot in a cluster close to the OIB (Figure 7). They also exhibited positive εNd values, implying a depleted mantle source, high LREEs, HFSEs, Rb/Sr and Sm/Nd, and low Y contents. These samples had low Zr/Nb and La/Nb ratios of 2.70 and 0.62, respectively, plotting within the HIMU OIB field (Figure 9(a)). They exhibited relatively lower average Th/Nb and higher average Nb/U ratios of 0.074 and 36.493, respectively, close to those of the Ontong Java (0.082 and 5.15) and plot within the Mangaia and Ontong Java fields, which represent the HIMU OIB and oceanic platform basalt (Figure 9(b)). They displayed an average Rb/Sr ratio of 0.057, close to the Ontong Java (0.069) and plot within the BCC field (Figure 9(c)). These samples had the highest average La/Sm and Sm/Yb ratios (6.58 and 2.52, respectively) of all the studied samples, implying that the magma experienced no crustal contamination (Figure 9(d)). They exhibited average Nb/Y and Zr/Y ratios of 0.937 and 4.866, respectively, and plot within the OIB field (Figure 9(e)).Third is the moderate143Nd/144Nd region, represented by samples Xia 381-1-a, Xia 38-3, Xia 39-1, Shang 745-6, Xia 38-2, Shang 745-4, Shang 745-E, and Xia 382-5. These samples exhibited 87Sr/86Sr of 0.7046 to 0.7098 (average of 0.7075) and 143Nd/144Nd of 0.5126 to 0.5127 (average of 0.5126). The samples plot within the area close to the OIB and the CHUR line and had moderate Rb/Sr and Sm/Nd, low LREEs and HFSEs, and high Y contents, implying an ancient rock source. These samples had average Zr/Nb and La/Nb ratios of 6.178 and 0.698, respectively, close to those of Pitcairn and Society (7) and plot in the HIMU field and the boundary between Pitcairn and Society fields (Figure 9(a)), which are not exactly the same as typical OIB, implying the influence of mixing of multiple sources. The average Th/Nb and Nb/U ratios of the samples were 0.097 and 27.975, respectively, consistent with those of the Ontong Java (0.082 and 35.15), and they plot within the Ontong Java, Pitcairn, and Society fields (Figure 9(b)). They had average Rb/Sr, Nb/Y, and Zr/Y ratios of 0.042, 0.795, and 4.649, respectively, and plot within the OIB field (Figures 9(c) and 9(e)). The average La/Sm and Sm/Yb ratios of the samples were 3.073 and 1.722, respectively, and they plot within the Siberia field, between the CLM and BCC (Figure 9(d)).


### 5.3. Characteristics of Mantle Plume Magmatism

Mantle plumes usually have a very large spherical head and a narrow tail, along which fluids can move upward rapidly [5, 47]. However, the nature of the reactions between mantle plume and lithosphere remains controversial. Campbell and Griffiths [47] suggested that the head of a mantle plume is a mixture of source materials and materials from the sides of magma channels. Because of the different sizes and depths of different mantle plumes, the representative magmatic products of a mantle plume head are alkaline rocks, due to crystallization differentiation, metasomatism, and contamination when the spherical head moves up and leads to the melting of the local lithosphere, as well as involvement of mantle mixed sources with different degrees of enrichment [5, 48]. The narrow and long mantle plume tail can provide channels for the ascent of high temperature and low viscosity mantle materials. The melting of a mantle plume tail and agglutination and ascent of magma provide the heat source and volatile components that enhance further mantle rock melting. The rocks formed by the mantle plume tail exhibit OIB-like geochemical characteristics, including low Yb contents and high LREEs and HFSEs (Ti, P, and Nb) contents [5]. The remelting of ascending mantle materials in the mantle plume channels caused by abnormal heat can form picrite and picritic basalt, whereas the magmatic products of a mantle plume tail are ultrabasic rocks. The OIB are extremely geochemically enriched and were usually considered to be related to hotspots or mantle plumes [49–51]. The samples examined in the current study have generally low 87Sr/86Sr ratios (mostly >0.710) and plot in the OIB field (Figures 5–7 and 9(a)–9(e)), showing the characteristics of a deep mantle source, which suggest that the genesis was the mantle plume. The Sr-Nd-O isotopic and geochemical characteristics of the studied samples indicate that the mantle plume in the study area consists of a head, a tail, and a middle section.The mantle plume head: the EMI end member was formed by metasomatism and melting of mantle materials, which corresponds approximately to the MORB, and the subducting oceanic crust, whereas the EMII end member is formed by metasomatism of crust-mantle-derived fluids of crustal components from the descending ancient slab or oceanic crust [45, 52]. The early formed magma of the mantle plume has large proportions of crustal and lithosphere mantle materials because of contamination and involvement of fertile mantle (EMI and EMII) during ascent. The mantle plume head usually has mixed characteristics of two sources, as the magma forms in the magma chamber of shallow crust and experiences contamination and fractional crystallization. Samples from Xia 382-1 and Shang 745-11 wells have the lowest εNd values of -17.71 and -9.23, respectively, significantly lower than those of the OIB (-4.1 to 8.0) and MORB (6.9 to 11.9) [53]. In the 87Sr/86Sr-143Nd/144Nd diagram, the samples plot in the EMI, EMII, and ancient lithospheric mantle fields (Figure 7). The lowest 143Nd/144Nd values of 0.5117 and 0.5122, as well as the depletion of LREEs, HFSEs (Ti, P, and Nb), and Nb-Ta, indicate that the lithospheric mantle that was previously metasomatized by crustal materials has experienced a low degree of homogenization.The mantle plume tail: this section of mantle plume was formed by the eruption of high-energy mantle materials that experienced no crustal contamination. Rocks formed in the mantle plume tail are mainly ultrabasic rocks, which have high143Nd/144Nd ratios, εNd values, LREEs, HFSEs (Ti, P, and Nb), and low Yb contents. Samples Shang 745-A, Shang 745-B, Shang 745-C, Shang 745-D, and Shang 745-8 have high 143Nd/144Nd ratios (0.5127 to 0.5128), LREEs, and HFSEs (except sample Xia 381-1), as well as other geochemical and isotopic characteristics of typical OIB. We therefore consider that they were formed in the mantle plume tail. Furthermore, the samples exhibited the highest positive εNd values of 1.80 to 3.22 consistent with those of the OIB (-4.0 to 8.0), suggesting a mantle source [53].The middle section: rocks formed in the middle section of mantle plume represented by samples Xia 38-2, Xia 38-3, Xia 381-1, Shang 745-4, Shang 745-6, Xia 39-1, and Shang 745-E which are formed by magmas that experienced a low degree of melting [54], crustal contamination, as well as involvement of lithospheric material. They have moderate 143Nd/144Nd ratios, εNd values, LREEs, and HFSEs (Ti, P, and Nb) contents. The moderate 143Nd/144Nd ratios (0.5126 to 0.5127), εNd values (-1.15 to 0.62), LREEs, and HFSEs of these samples are lower than those of the mantle plume tail but higher than those of the mantle plume head. The εNd values are distributed around zero and close to the CHUR line, consistent with those of the OIB (-4.0 to 8.0) [53].


## 5.1. Petrogenesis of the Mafic Rocks

### 5.1.1. Fractional Crystallization

It is noticed that the studied mafic rocks from the Huimin Sag have variable MgO, Cr, and Ni contents. In the variation diagrams of MgO versus selected major and trace elements (Figure6), with the exception of Fe2O3+FeO, which was not correlated with MgO, the SiO2, K2O+Na2O, TiO2, Al2O3, and CaO concentrations were negatively correlated with MgO, whereas Cr and Ni concentrations were positively correlated with MgO. These data suggest that the geochemical differences of different samples may be attributed to the fractional crystallization of olivine and/or clinopyroxene. In addition, the TiO2 (1.62%–2.79%) contents of the samples are relatively consistent, implying that the fractionation of Fe-Ti oxides did not occur. Moreover, no obvious Eu anomalies (0.92–1.17) have been observed in the studied samples, reflecting no significant fractionation of plagioclase.Figure 6
Various oxides and Cr, Ni plotted against MgO for the Paleogene igneous rocks in Huimin Sag with literature data for comparison.


### 5.1.2. Crustal Contamination

The petrological observation and geochemical analysed results imply a mantle source for the magma of studied rocks from the Huimin Sag. Thus, it is important to understand the effects of crustal contamination on the mafic rocks, which is a potential process during the ascent and evolution of mantle-derived magmas [28, 29]. It is widely accepted that minor crustal contamination could produce positive Zr-Hf anomalies of mantle-sourced rocks [24]. However, the Paleogene igneous rocks from the Huimin Sag have no positive Zr-Hf anomalies (Figures 5(c) and 5(d)), ruling out the possibility of crustal contamination. This hypothesis can be further evidenced by the lower Th (1.58 ppm–4.76 ppm) and U (0.79–2.31) contents of these samples relative to the upper crust (e.g., Th=10.5ppm and U=2.7ppm) [30]. Moreover, the Paleogene igneous rocks from the Huimin Sag had εNd values of -17.71 to 5.56 and were mostly within the range of mantle plume-derived CFB, with the exception of samples Xia 382-1 and Shang 745-11, which had negative εNd values and 143Nd/144Nd of 0.5117 and 0.5122, respectively, close to that of average continental crust (0.5119), reflecting the lithospheric mantle or crustal contamination [31]. Samples Shang 745-A, Shang 745-B, Shang 745-C, Shang 745-D, and Shang 745-8 exhibited high positive 143Nd/144Nd and εNd values of 0.5127 to 0.5128 and 1.80 to 3.22, respectively, implying an upper depleted mantle source. The remaining samples had εNd values of -1.15 to 0.62, consistent with the CHUR evolution curve (Figure 7), implying that the magma was derived from different degrees of mixture of mantle plume and lithosphere.Figure 7
87Sr/86Sr vs. 144Nd/143Nd costing point of Paleogene igneous rocks in Huimin Sag. Data sources in (a): Hannuoba basalts [58–60], MORB, OIB, EMI, EMII, and PREMA [61]; data source in (b): EMI, EMII, and PREMA are after Zindler and Hart [61] and Xia [62]. Symbols are as in Figure 5.
(a)(b)Ratios of O isotopes and incompatible elements have been proposed to identify crustal contamination [32]. In the δ18O-87Sr/86Sr diagram (Figure 8), the studied igneous rock samples plot within or close to the area of source contamination, also ruling out the significant impact of crustal contamination. (Th/Ta)PM was close to 1.0 and (La/Nb)PM exceeded 1.0 when lower crustal materials were involved, whereas the above ratios exceeded 2.0. The (Th/Ta)PM and (La/Nb)PM ratios of the Paleogene igneous rocks from the Huimin Sag were 0.54–0.79 and 0.55–0.72, respectively, suggesting that the upper crustal component had no role in the generation of the rocks.Figure 8
Variation ofδ18O vs. 87Sr/86Sr for the mixing of crustal and mantle components of Paleogene igneous rocks in Huimin Sag [39]. Symbols are as in Figure 5.


### 5.1.3. Mantle Sources and Lithospheric Interaction

During mantle melting and metasomatism, Ti, P, and K can be easily enriched or depleted, as they are strongly incompatible elements. The high contents of Ti, P, and K of the Paleogene igneous rocks from the Huimin Sag are consistent with those of mantle plume-derived basalts, indicating a mantle plume source. In addition, theΣREE contents (average 154.36 ppm) of these rocks are clearly higher than those of basalts (average 85 ppm) in eastern China [33], showing the characteristics of mantle plume-derived basalt, which are usually enriched in LREEs (e.g., the Emeishan basalts, Kerguelen mantle plume volcanic rocks, and Iceland volcanic rocks) [5, 34, 35]. The igneous rocks of the present study were characterized by enrichment of LILEs (e.g., Ba, Pb, Sr, U, and K), incompatible elements (Rb, Ba, Th, U, and K), and HFSEs, (e.g., Nb, Ta, Zr, and Hf), similar to those of the mantle-plume-related OIB (e.g., Hawaii basalt of Pacific Plate and Kerguelen basalt of Indian Ocean Plate) or CFB (e.g., Emeishan, Siberia, Deccan, and African continental basalts) [5]. Furthermore, the remarkable Pb enrichment and slightly depleted Y, Pr, and Yb indicate that the Cenozoic mafic rocks in the study area are intraplate rift-type alkali basaltic rocks, which display the characteristics of upper mantle sources. This view is further supported by the Rb/Yb ratios (>1), high Hf and low Zr contents, and the absence of Nb-Ta anomalies in the samples [16, 36]. In the primitive mantle-normalized trace element spider diagrams (Figures 5(c) and 5(d)), the samples plot between the OIB and N-MORB, but closer to the OIB. The concentrations of compatible elements of Cr and Ni for the studied samples were 31.3–467 ppm and 20.7–314, respectively, consistent with those of the basaltic magma derived from primary mantle [37].The average87Sr/86Sr and 143Nd/144Nd ratios of the Paleogene igneous rocks from the Huimin Sag were 0.7078 and 0.5126, respectively, consistent with those of EMII (87Sr/86Sr=0.7078, 143Nd/144Nd=0.5126), close to those of EMI (87Sr/86Sr=0.7053, 143Nd/144Nd=0.5124) and HIMU (87Sr/86Sr=0.7028, 143Nd/144Nd=0.5128), and higher than those of depleted mantle (87Sr/86Sr=0.7022, 143Nd/144Nd=0.5133). The average Zr/Nb, La/Nb, Ba/Nb, K/Nb, Th/Nb, Th/La, and Ba/La ratios for the samples were 4.42, 0.63, 13.48, 279.96, 0.08, 0.12, and 21.39, respectively, distributed in the enriched mantle EMII, EMI, and HIMU fields [22, 38]. In the 87Sr/86Sr-143Nd/144Nd diagram, samples Xia 38-3, Xia 381-1, and Xia 39-1 plot in the OIB field, sample Xia 382-1 plots in the EMI field, sample Shang 745-11 plots in the ancient lithospheric mantle field close to the EMII, and the remaining samples plot close to the OIB, indicating a mantle source, which is further supported by the positive εNd values of most samples (Figure 7(a)). Most samples plot in, or close to, the mantle source field, except samples Xia 382-1 and Shang 745-11, which plot in the EMI field and upper crust field, respectively (Figure 7(b)). The 87Sr/86Sr ratios of the igneous rocks in the study area varied from 0.7046 to 0.7098, similar to those of the OIB and CFB (<0.710) formed by mantle plume magmatism (e.g., mantle plume-derived basalts of Emeishan, Hawaii, Iceland, and Siberia), showing characteristics of typical mantle plume-derived basalt and deep mantle source [5].The samples examined in the present study exhibitedδ18O values of 5.4‰ to 10.4‰, with an average of 7.93‰, similar to those of the continental tholeiite (5.0‰ to 7.5‰) and continental mafic lava (4.9‰ to 8.0‰), as well as clinopyroxene in the Emeishan mantle plume-derived basalt (6.2‰ to 7.86‰), but higher than those of the OIB (5.4‰) [39, 40]. However, the δ18O values of the samples were slightly higher than those of the mantle, which was probably caused by contamination of crustal materials and source mixing, as the O isotopes were obtained by whole rock analysis. Enrichment of incompatible elements can be caused by the partial melting of an overlying mantle wedge, if the mantle source was influenced by fluids released by the subduction plate. Some researchers suggested that Sr and other incompatible elements can reflect the characteristics of the subduction plate, whereas O isotopes and other elements represent the characteristics of the overlying mantle wedge [39]. In the δ18O-87Sr/86Sr diagram, the igneous rocks in the study area plot close to the downward nonlinear curve, indicating that the isotopic characteristics resulted from source mixing (Figure 8).To sum up, the Sr-Nd-O isotopic and trace elemental characteristics described above were similar to those of the enriched OIB, indicating a mantle plume source and the existence of components from ancient subduction oceanic crust in the source region (Figures7 and 9) [41]. The characteristic trace element ratios were consistent with those of the HIMU, EMI, and EMII OIB, which are represented by Mangaia, Pitcairn, and Society, respectively. Samples Shang 745-8, Shang 745-A, Shang 745-B, Shang 745-C, and Shang 745-D plot within the HIMU OIB field and the Ontong Java field, represented by oceanic platform basalt, whereas the remaining samples plot within the EMI and EMII fields, represented by Pitcairn and Society, respectively (Figure 9(a)). The enrichment of subduction-related materials suggests that there were heterogeneous source rocks of the magmatic rocks, which probably originated from different positions or stages of the mantle plume. The 143Nd/144Nd ratios of the samples plot within the field close to the Society OIB (EMII), implying that a recycled oceanic crust that contains a small amount (i.e., a few percent) of terrigenous sediments was involved in the source (Figure 9(b)) [42, 43]. They distribute within the continental lithospheric mantle and crustal contamination fields, represented by Society and Siberia (Figure 9(b)).Figure 9
Trace element ratios of Paleogene igneous rocks in Huimin Sag. (a) Zr/Nb vs. La/Nb, (b) Th/Nb vs. Nb/U, (c) Rb/Sr vs. Nd/Sm, (d) La/Sm vs. Sm/Yb, and (e) Nb/Y vs. Zr/Y diagrams for the Paleogene igneous rocks from the Huimin Sag. Data sources are as follows: PM refers to primitive mantle values [63]; UCC, BCC, and CC mark the upper continental crust, whole continental crust, and average compositions of bulk, respectively [64]. HIMU OIB [65]; Pitcairn [66]; Society [43]; Ontong Java [67]; N. Kerguelen [68]; Hawaii [69]. Siberia, Nd1, 2 represent crustal contamination lavas typically [70]. The arrow points to the average MORB values: Zr/Nb=31.76, La/Nb=1.07 [57]. The values for OIB are from Sun and McDonough [57]. Composition of continental lithospheric mantle (CLM) is from McDonough [71]. Parallel lines mark the upper and lower bounds of the Iceland basalt data [72], All the symbols are the same as in Figure 5.
(a)(b)(c)(d)(e)The EMII enriched units have been proposed as being related to crustal derived material or recycled oceanic crust [44], whereas the HIMU are related to subducted ancient metamorphic oceanic crust [45]. The EMI are widely accepted to be formed in the lithosphere mantle, asthenosphere, mantle plume, and metasomatic lithosphere mantle by subducted ancient crust [46]. Based on the isotopic and trace elemental analyses discussed above, we suggest that the Paleogene igneous rocks from the Huimin Sag were sourced from the mantle rather than the crust. The primary magma of the rocks likely originated from the melting of a mantle plume and the further metasomatism of lithospheric mantle and continental or oceanic crust.


## 5.1.1. Fractional Crystallization

It is noticed that the studied mafic rocks from the Huimin Sag have variable MgO, Cr, and Ni contents. In the variation diagrams of MgO versus selected major and trace elements (Figure6), with the exception of Fe2O3+FeO, which was not correlated with MgO, the SiO2, K2O+Na2O, TiO2, Al2O3, and CaO concentrations were negatively correlated with MgO, whereas Cr and Ni concentrations were positively correlated with MgO. These data suggest that the geochemical differences of different samples may be attributed to the fractional crystallization of olivine and/or clinopyroxene. In addition, the TiO2 (1.62%–2.79%) contents of the samples are relatively consistent, implying that the fractionation of Fe-Ti oxides did not occur. Moreover, no obvious Eu anomalies (0.92–1.17) have been observed in the studied samples, reflecting no significant fractionation of plagioclase.Figure 6
Various oxides and Cr, Ni plotted against MgO for the Paleogene igneous rocks in Huimin Sag with literature data for comparison.


## 5.1.2. Crustal Contamination

The petrological observation and geochemical analysed results imply a mantle source for the magma of studied rocks from the Huimin Sag. Thus, it is important to understand the effects of crustal contamination on the mafic rocks, which is a potential process during the ascent and evolution of mantle-derived magmas [28, 29]. It is widely accepted that minor crustal contamination could produce positive Zr-Hf anomalies of mantle-sourced rocks [24]. However, the Paleogene igneous rocks from the Huimin Sag have no positive Zr-Hf anomalies (Figures 5(c) and 5(d)), ruling out the possibility of crustal contamination. This hypothesis can be further evidenced by the lower Th (1.58 ppm–4.76 ppm) and U (0.79–2.31) contents of these samples relative to the upper crust (e.g., Th=10.5ppm and U=2.7ppm) [30]. Moreover, the Paleogene igneous rocks from the Huimin Sag had εNd values of -17.71 to 5.56 and were mostly within the range of mantle plume-derived CFB, with the exception of samples Xia 382-1 and Shang 745-11, which had negative εNd values and 143Nd/144Nd of 0.5117 and 0.5122, respectively, close to that of average continental crust (0.5119), reflecting the lithospheric mantle or crustal contamination [31]. Samples Shang 745-A, Shang 745-B, Shang 745-C, Shang 745-D, and Shang 745-8 exhibited high positive 143Nd/144Nd and εNd values of 0.5127 to 0.5128 and 1.80 to 3.22, respectively, implying an upper depleted mantle source. The remaining samples had εNd values of -1.15 to 0.62, consistent with the CHUR evolution curve (Figure 7), implying that the magma was derived from different degrees of mixture of mantle plume and lithosphere.Figure 7
87Sr/86Sr vs. 144Nd/143Nd costing point of Paleogene igneous rocks in Huimin Sag. Data sources in (a): Hannuoba basalts [58–60], MORB, OIB, EMI, EMII, and PREMA [61]; data source in (b): EMI, EMII, and PREMA are after Zindler and Hart [61] and Xia [62]. Symbols are as in Figure 5.
(a)(b)Ratios of O isotopes and incompatible elements have been proposed to identify crustal contamination [32]. In the δ18O-87Sr/86Sr diagram (Figure 8), the studied igneous rock samples plot within or close to the area of source contamination, also ruling out the significant impact of crustal contamination. (Th/Ta)PM was close to 1.0 and (La/Nb)PM exceeded 1.0 when lower crustal materials were involved, whereas the above ratios exceeded 2.0. The (Th/Ta)PM and (La/Nb)PM ratios of the Paleogene igneous rocks from the Huimin Sag were 0.54–0.79 and 0.55–0.72, respectively, suggesting that the upper crustal component had no role in the generation of the rocks.Figure 8
Variation ofδ18O vs. 87Sr/86Sr for the mixing of crustal and mantle components of Paleogene igneous rocks in Huimin Sag [39]. Symbols are as in Figure 5.


## 5.1.3. Mantle Sources and Lithospheric Interaction

During mantle melting and metasomatism, Ti, P, and K can be easily enriched or depleted, as they are strongly incompatible elements. The high contents of Ti, P, and K of the Paleogene igneous rocks from the Huimin Sag are consistent with those of mantle plume-derived basalts, indicating a mantle plume source. In addition, theΣREE contents (average 154.36 ppm) of these rocks are clearly higher than those of basalts (average 85 ppm) in eastern China [33], showing the characteristics of mantle plume-derived basalt, which are usually enriched in LREEs (e.g., the Emeishan basalts, Kerguelen mantle plume volcanic rocks, and Iceland volcanic rocks) [5, 34, 35]. The igneous rocks of the present study were characterized by enrichment of LILEs (e.g., Ba, Pb, Sr, U, and K), incompatible elements (Rb, Ba, Th, U, and K), and HFSEs, (e.g., Nb, Ta, Zr, and Hf), similar to those of the mantle-plume-related OIB (e.g., Hawaii basalt of Pacific Plate and Kerguelen basalt of Indian Ocean Plate) or CFB (e.g., Emeishan, Siberia, Deccan, and African continental basalts) [5]. Furthermore, the remarkable Pb enrichment and slightly depleted Y, Pr, and Yb indicate that the Cenozoic mafic rocks in the study area are intraplate rift-type alkali basaltic rocks, which display the characteristics of upper mantle sources. This view is further supported by the Rb/Yb ratios (>1), high Hf and low Zr contents, and the absence of Nb-Ta anomalies in the samples [16, 36]. In the primitive mantle-normalized trace element spider diagrams (Figures 5(c) and 5(d)), the samples plot between the OIB and N-MORB, but closer to the OIB. The concentrations of compatible elements of Cr and Ni for the studied samples were 31.3–467 ppm and 20.7–314, respectively, consistent with those of the basaltic magma derived from primary mantle [37].The average87Sr/86Sr and 143Nd/144Nd ratios of the Paleogene igneous rocks from the Huimin Sag were 0.7078 and 0.5126, respectively, consistent with those of EMII (87Sr/86Sr=0.7078, 143Nd/144Nd=0.5126), close to those of EMI (87Sr/86Sr=0.7053, 143Nd/144Nd=0.5124) and HIMU (87Sr/86Sr=0.7028, 143Nd/144Nd=0.5128), and higher than those of depleted mantle (87Sr/86Sr=0.7022, 143Nd/144Nd=0.5133). The average Zr/Nb, La/Nb, Ba/Nb, K/Nb, Th/Nb, Th/La, and Ba/La ratios for the samples were 4.42, 0.63, 13.48, 279.96, 0.08, 0.12, and 21.39, respectively, distributed in the enriched mantle EMII, EMI, and HIMU fields [22, 38]. In the 87Sr/86Sr-143Nd/144Nd diagram, samples Xia 38-3, Xia 381-1, and Xia 39-1 plot in the OIB field, sample Xia 382-1 plots in the EMI field, sample Shang 745-11 plots in the ancient lithospheric mantle field close to the EMII, and the remaining samples plot close to the OIB, indicating a mantle source, which is further supported by the positive εNd values of most samples (Figure 7(a)). Most samples plot in, or close to, the mantle source field, except samples Xia 382-1 and Shang 745-11, which plot in the EMI field and upper crust field, respectively (Figure 7(b)). The 87Sr/86Sr ratios of the igneous rocks in the study area varied from 0.7046 to 0.7098, similar to those of the OIB and CFB (<0.710) formed by mantle plume magmatism (e.g., mantle plume-derived basalts of Emeishan, Hawaii, Iceland, and Siberia), showing characteristics of typical mantle plume-derived basalt and deep mantle source [5].The samples examined in the present study exhibitedδ18O values of 5.4‰ to 10.4‰, with an average of 7.93‰, similar to those of the continental tholeiite (5.0‰ to 7.5‰) and continental mafic lava (4.9‰ to 8.0‰), as well as clinopyroxene in the Emeishan mantle plume-derived basalt (6.2‰ to 7.86‰), but higher than those of the OIB (5.4‰) [39, 40]. However, the δ18O values of the samples were slightly higher than those of the mantle, which was probably caused by contamination of crustal materials and source mixing, as the O isotopes were obtained by whole rock analysis. Enrichment of incompatible elements can be caused by the partial melting of an overlying mantle wedge, if the mantle source was influenced by fluids released by the subduction plate. Some researchers suggested that Sr and other incompatible elements can reflect the characteristics of the subduction plate, whereas O isotopes and other elements represent the characteristics of the overlying mantle wedge [39]. In the δ18O-87Sr/86Sr diagram, the igneous rocks in the study area plot close to the downward nonlinear curve, indicating that the isotopic characteristics resulted from source mixing (Figure 8).To sum up, the Sr-Nd-O isotopic and trace elemental characteristics described above were similar to those of the enriched OIB, indicating a mantle plume source and the existence of components from ancient subduction oceanic crust in the source region (Figures7 and 9) [41]. The characteristic trace element ratios were consistent with those of the HIMU, EMI, and EMII OIB, which are represented by Mangaia, Pitcairn, and Society, respectively. Samples Shang 745-8, Shang 745-A, Shang 745-B, Shang 745-C, and Shang 745-D plot within the HIMU OIB field and the Ontong Java field, represented by oceanic platform basalt, whereas the remaining samples plot within the EMI and EMII fields, represented by Pitcairn and Society, respectively (Figure 9(a)). The enrichment of subduction-related materials suggests that there were heterogeneous source rocks of the magmatic rocks, which probably originated from different positions or stages of the mantle plume. The 143Nd/144Nd ratios of the samples plot within the field close to the Society OIB (EMII), implying that a recycled oceanic crust that contains a small amount (i.e., a few percent) of terrigenous sediments was involved in the source (Figure 9(b)) [42, 43]. They distribute within the continental lithospheric mantle and crustal contamination fields, represented by Society and Siberia (Figure 9(b)).Figure 9
Trace element ratios of Paleogene igneous rocks in Huimin Sag. (a) Zr/Nb vs. La/Nb, (b) Th/Nb vs. Nb/U, (c) Rb/Sr vs. Nd/Sm, (d) La/Sm vs. Sm/Yb, and (e) Nb/Y vs. Zr/Y diagrams for the Paleogene igneous rocks from the Huimin Sag. Data sources are as follows: PM refers to primitive mantle values [63]; UCC, BCC, and CC mark the upper continental crust, whole continental crust, and average compositions of bulk, respectively [64]. HIMU OIB [65]; Pitcairn [66]; Society [43]; Ontong Java [67]; N. Kerguelen [68]; Hawaii [69]. Siberia, Nd1, 2 represent crustal contamination lavas typically [70]. The arrow points to the average MORB values: Zr/Nb=31.76, La/Nb=1.07 [57]. The values for OIB are from Sun and McDonough [57]. Composition of continental lithospheric mantle (CLM) is from McDonough [71]. Parallel lines mark the upper and lower bounds of the Iceland basalt data [72], All the symbols are the same as in Figure 5.
(a)(b)(c)(d)(e)The EMII enriched units have been proposed as being related to crustal derived material or recycled oceanic crust [44], whereas the HIMU are related to subducted ancient metamorphic oceanic crust [45]. The EMI are widely accepted to be formed in the lithosphere mantle, asthenosphere, mantle plume, and metasomatic lithosphere mantle by subducted ancient crust [46]. Based on the isotopic and trace elemental analyses discussed above, we suggest that the Paleogene igneous rocks from the Huimin Sag were sourced from the mantle rather than the crust. The primary magma of the rocks likely originated from the melting of a mantle plume and the further metasomatism of lithospheric mantle and continental or oceanic crust.


## 5.2. Mantle End Member

Based on the Sr-Nd-O isotopic and trace elemental analyses discussed above, three end members in the study area can be proposed. The characteristics of the samples are similar to HIMU, EMI, and EMII, which are represented by those of Mangaia, Pitcairn, and Society, respectively (Figure9). Samples Shang 745-8, Shang 745-A, Shang 745-B, Shang 745-C, and Shang 745-D, which have high 143Nd/144Nd ratios, plot within the HIMU and Ontong Java OIB fields, whereas other samples plot within the EMI and EMII fields close to the Society field, suggesting the involvement of subduction-related materials. The analyses presented above suggest that the three mantle plume end members of the study area are as follows.First is the low143Nd/144Nd region represented by samples Xia 382-1 and Shang 745-11. Samples Xia 382-1 and Shang 745-11 have 143Nd/144Nd, 87Sr/86Sr, and εNd of 0.5117 and 0.5122 (average of 0.5119), 0.7063 and 0.7088 (average of 0.7075), and -17.71 and -9.23, respectively, and plot within the EMI and ancient lithospheric mantle fields (Figure 7). These characteristics, in conjunction with the high Y contents and Rb/Sr ratios and low LREEs and HFSE contents and Sm/Nd ratios, indicate that the primary magma experienced crustal remelting and mixing of multiple sources.Because Zr/Nb and La/Nb ratios can be slightly influenced by fractionation, they are considered to represent those of the mantle-derived rocks [41]. The Zr/Nb and La/Nb ratios of primary mantle and Ontong Java were 15.96 and 0.98 and 17.65 and 1.03, respectively, suggesting that the Ontong Java experienced a higher degree of melting (Figure 9(a)). Samples Xia 382-1 and Shang 745-11 exhibited relatively low Zr/Nb and La/Nb ratios of 5.118 and 0.610 and 5.362 and 0.671, respectively, close to those of Pitcairn and Society (7). They plot in the HIMU field and the boundary between Pitcairn and Society fields, which are not exactly the same as typical OIB, implying the influence of mixing of multiple sources [43]. The Th/Nb and Nb/U ratios of samples Xia 382-1 and Shang 745-11 were 0.082 and 22.488 and 0.072 and 21.56, respectively, and the samples plot within the Ontong Java field; the Th/Nb and Nb/U ratios of which were 0.082 and 35.15 (Figure 9(b)). They exhibited Rb/Sr ratios of 0.042 and 0.065, respectively, close to those of the Ontong Java (0.069) and plot within the OIB (Figure 9(c)). They exhibited La/Sm and Sm/Yb ratios of 3.341 and 1.584 and 3.138 and 1.821, respectively, and plot within the Siberia field, between the CLM and BCC (Figure 9(d)). They exhibited average Nb/Y and Zr/Y ratios of 0.966 and 4.906, respectively, and plot within the OIB field (Figure 9(e)).Second is the high143Nd/144Nd region represented by samples Shang 745-8, Shang 745-A, Shang 745-B, Shang 745-C, and Shang 745-D. These samples displayed 87Sr/86Sr of 0.7081 to 0.7087 (average of 0.7083) and the highest 143Nd/144Nd of 0.5127 to 0.5129 (average of 0.5128); they plot in a cluster close to the OIB (Figure 7). They also exhibited positive εNd values, implying a depleted mantle source, high LREEs, HFSEs, Rb/Sr and Sm/Nd, and low Y contents. These samples had low Zr/Nb and La/Nb ratios of 2.70 and 0.62, respectively, plotting within the HIMU OIB field (Figure 9(a)). They exhibited relatively lower average Th/Nb and higher average Nb/U ratios of 0.074 and 36.493, respectively, close to those of the Ontong Java (0.082 and 5.15) and plot within the Mangaia and Ontong Java fields, which represent the HIMU OIB and oceanic platform basalt (Figure 9(b)). They displayed an average Rb/Sr ratio of 0.057, close to the Ontong Java (0.069) and plot within the BCC field (Figure 9(c)). These samples had the highest average La/Sm and Sm/Yb ratios (6.58 and 2.52, respectively) of all the studied samples, implying that the magma experienced no crustal contamination (Figure 9(d)). They exhibited average Nb/Y and Zr/Y ratios of 0.937 and 4.866, respectively, and plot within the OIB field (Figure 9(e)).Third is the moderate143Nd/144Nd region, represented by samples Xia 381-1-a, Xia 38-3, Xia 39-1, Shang 745-6, Xia 38-2, Shang 745-4, Shang 745-E, and Xia 382-5. These samples exhibited 87Sr/86Sr of 0.7046 to 0.7098 (average of 0.7075) and 143Nd/144Nd of 0.5126 to 0.5127 (average of 0.5126). The samples plot within the area close to the OIB and the CHUR line and had moderate Rb/Sr and Sm/Nd, low LREEs and HFSEs, and high Y contents, implying an ancient rock source. These samples had average Zr/Nb and La/Nb ratios of 6.178 and 0.698, respectively, close to those of Pitcairn and Society (7) and plot in the HIMU field and the boundary between Pitcairn and Society fields (Figure 9(a)), which are not exactly the same as typical OIB, implying the influence of mixing of multiple sources. The average Th/Nb and Nb/U ratios of the samples were 0.097 and 27.975, respectively, consistent with those of the Ontong Java (0.082 and 35.15), and they plot within the Ontong Java, Pitcairn, and Society fields (Figure 9(b)). They had average Rb/Sr, Nb/Y, and Zr/Y ratios of 0.042, 0.795, and 4.649, respectively, and plot within the OIB field (Figures 9(c) and 9(e)). The average La/Sm and Sm/Yb ratios of the samples were 3.073 and 1.722, respectively, and they plot within the Siberia field, between the CLM and BCC (Figure 9(d)).


## 5.3. Characteristics of Mantle Plume Magmatism

Mantle plumes usually have a very large spherical head and a narrow tail, along which fluids can move upward rapidly [5, 47]. However, the nature of the reactions between mantle plume and lithosphere remains controversial. Campbell and Griffiths [47] suggested that the head of a mantle plume is a mixture of source materials and materials from the sides of magma channels. Because of the different sizes and depths of different mantle plumes, the representative magmatic products of a mantle plume head are alkaline rocks, due to crystallization differentiation, metasomatism, and contamination when the spherical head moves up and leads to the melting of the local lithosphere, as well as involvement of mantle mixed sources with different degrees of enrichment [5, 48]. The narrow and long mantle plume tail can provide channels for the ascent of high temperature and low viscosity mantle materials. The melting of a mantle plume tail and agglutination and ascent of magma provide the heat source and volatile components that enhance further mantle rock melting. The rocks formed by the mantle plume tail exhibit OIB-like geochemical characteristics, including low Yb contents and high LREEs and HFSEs (Ti, P, and Nb) contents [5]. The remelting of ascending mantle materials in the mantle plume channels caused by abnormal heat can form picrite and picritic basalt, whereas the magmatic products of a mantle plume tail are ultrabasic rocks. The OIB are extremely geochemically enriched and were usually considered to be related to hotspots or mantle plumes [49–51]. The samples examined in the current study have generally low 87Sr/86Sr ratios (mostly >0.710) and plot in the OIB field (Figures 5–7 and 9(a)–9(e)), showing the characteristics of a deep mantle source, which suggest that the genesis was the mantle plume. The Sr-Nd-O isotopic and geochemical characteristics of the studied samples indicate that the mantle plume in the study area consists of a head, a tail, and a middle section.The mantle plume head: the EMI end member was formed by metasomatism and melting of mantle materials, which corresponds approximately to the MORB, and the subducting oceanic crust, whereas the EMII end member is formed by metasomatism of crust-mantle-derived fluids of crustal components from the descending ancient slab or oceanic crust [45, 52]. The early formed magma of the mantle plume has large proportions of crustal and lithosphere mantle materials because of contamination and involvement of fertile mantle (EMI and EMII) during ascent. The mantle plume head usually has mixed characteristics of two sources, as the magma forms in the magma chamber of shallow crust and experiences contamination and fractional crystallization. Samples from Xia 382-1 and Shang 745-11 wells have the lowest εNd values of -17.71 and -9.23, respectively, significantly lower than those of the OIB (-4.1 to 8.0) and MORB (6.9 to 11.9) [53]. In the 87Sr/86Sr-143Nd/144Nd diagram, the samples plot in the EMI, EMII, and ancient lithospheric mantle fields (Figure 7). The lowest 143Nd/144Nd values of 0.5117 and 0.5122, as well as the depletion of LREEs, HFSEs (Ti, P, and Nb), and Nb-Ta, indicate that the lithospheric mantle that was previously metasomatized by crustal materials has experienced a low degree of homogenization.The mantle plume tail: this section of mantle plume was formed by the eruption of high-energy mantle materials that experienced no crustal contamination. Rocks formed in the mantle plume tail are mainly ultrabasic rocks, which have high143Nd/144Nd ratios, εNd values, LREEs, HFSEs (Ti, P, and Nb), and low Yb contents. Samples Shang 745-A, Shang 745-B, Shang 745-C, Shang 745-D, and Shang 745-8 have high 143Nd/144Nd ratios (0.5127 to 0.5128), LREEs, and HFSEs (except sample Xia 381-1), as well as other geochemical and isotopic characteristics of typical OIB. We therefore consider that they were formed in the mantle plume tail. Furthermore, the samples exhibited the highest positive εNd values of 1.80 to 3.22 consistent with those of the OIB (-4.0 to 8.0), suggesting a mantle source [53].The middle section: rocks formed in the middle section of mantle plume represented by samples Xia 38-2, Xia 38-3, Xia 381-1, Shang 745-4, Shang 745-6, Xia 39-1, and Shang 745-E which are formed by magmas that experienced a low degree of melting [54], crustal contamination, as well as involvement of lithospheric material. They have moderate 143Nd/144Nd ratios, εNd values, LREEs, and HFSEs (Ti, P, and Nb) contents. The moderate 143Nd/144Nd ratios (0.5126 to 0.5127), εNd values (-1.15 to 0.62), LREEs, and HFSEs of these samples are lower than those of the mantle plume tail but higher than those of the mantle plume head. The εNd values are distributed around zero and close to the CHUR line, consistent with those of the OIB (-4.0 to 8.0) [53].


## 6. Conclusions

The high Ti, P, and K contents of the studied igneous rocks are consistent with those of mantle plume-derived basalts. They are relatively enriched in LREEs and depleted in HREEs and have patterns of REE similar to OIB. The enrichment of LILEs (Ba, Pb, Sr, U, and K), incompatible elements (Rb, Ba, Th, U, and K), and HFSEs (Nb, Ta, Zr, and Hf) is similar to that of the mantle plume-derived OIB. TheεNd values and 87Sr/86Sr ratios of the rocks were -17.71 to 5.56 and 0.7046 to 0.7098 (mostly less than 0.710), respectively, and mostly plot within the OIB field, with a few samples within the ancient lithospheric mantle field close to the EMI and EMII, typical characteristics of mantle plume.The mantle plume in the study area consists of a head, a tail, and a middle section. The mantle plume head was represented by samples Xia 382-1 and Shang 745-11, which had lowεNd, LREEs, HFSEs, Nb, and Ta contents and mixed characteristics of two sources. The mantle plume tail was represented by samples Shang 745-A, Shang 745-B, Shang 745-C, Shang 745-D, and Shang 745-8, which have the highest εNd, LREEs, and HFSEs and typical isotopic and trace elemental characteristics of OIB. The remaining samples represent the middle section of the mantle plume and had εNd values of approximately zero and close to the CHUR line.Trace elemental and isotopic characteristics suggest that the studied rocks were similar to the HIMU, EMI, and EMII OIB, represented by the Mangaia, Pitcairn, and Society, respectively. The samples Shang 745-8, Shang 745-A, Shang 745-B, Shang 745-C, and Shang 745-D of the high143Nd/144Nd region plot within the HIMU and OIB fields, whereas the remaining samples plot within the EMI and EMII fields close to the Society field, indicating enrichment by subduction-related materials.The (Th/Ta)PM and (La/Nb)PM ratios of the samples were both less than 1.0, suggesting that the upper crust had no role in the generation of the rocks. The O isotopes plot close to the downward nonlinear curve, also indicating the absence of crustal contamination. The εNd values of the sample ranged from -17.71 to 5.56, similar to those of the mantle plume-derived CFB. Based on the isotopic and trace elemental analyses discussed herein, we suggest that the Paleogene igneous rocks from the Huimin Sag were sourced from the mantle, rather than the crust. The primary magma of the rocks likely originated from the melting of a mantle plume, followed by further metasomatism of lithospheric mantle and continental or oceanic crust.



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*Source: 1571066-2022-06-08.xml*