Zhuang Li, Bin Chen

a State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing 102249, China

b College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, China

c Department of Earth and Space Sciences, Southern University of Science and Technology, Shenzhen 518055, China

Keywords:

Crustal contamination

OIB-like basalt

Os isotope

Os concentration

Geological survey engineering

Inner Mongolia

North China Craton

China

A B S T R A C T

The Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton are thought to experience minor even no crustal contamination during the magma evolution. The whole-rock Sr-Nd-Pb-Hf isotopes are attributed to a two-component mixing between depleted and enriched mantle sources,while the major element variations are controlled by the fractional crystallization of olivine and clinopyroxene. However, in this study, the new Os isotopic data proposes an opposite model for the Cenozoic basalts in northern marginal region of the North China Craton. In this model, the Jining basalts were contaminated by the Archean mafic rocks during the magma storage and ascent. The crustal contamination process is supported by (1) the highly radiogenic Os isotopic compositions, and (2) the positive correlation between 187Os/188Os and 1/Os of the Jining basalts. By modeling the Os isotopic composition of the basalts, an incorporation of ＜ 10% mafic granulites/amphibolites to the parental magma can successfully explain the initial values of highly radiogenic Os. In contrast, the unradiogenic and uniform Os isotopic compositions of the Chifeng basalts suggest negligible crustal contamination. Os isotopic data acts as an indicator of crustal contamination during magma evolution, providing us a novel insight into the evolution of the intra-continental OIB-like basalts worldwide.

1. Introduction

Ocean island basalt (OIB) is commonly believed as a kind of plume-related magma, with the pronounced positive Nb and Ta anomalies and the enriched light rare earth elements in the primitive mantle-normalized trace element patterns (White WM, 2010). The most popular proposal for its petrogenesis is the partial melting of the mantle source that was previously metasomatized by the recycled ocean crustal melts (Sobolev AV et al., 2007). Intra-continental basalt can have similar trace element abundances and Sr-Nd-Pb-Hf isotopic ratios as the OIB, although they are generated by different magmatic processes (Barry TL et al., 2003; Zou HB et al., 2000). For example, the intra-continental basalts are distributed widely in China from Hainan island in the south to Wudalianchi in the northeast, Mongolia, and the Sea of Japan (Fig. 1; Guo PY et al., 2016), but the small volume of each basaltic province, the absence of obvious hot-spot trail magmatism, and the lack of either geophysical or tectonic evidence argue against a plume origin for the basalts (Barry TL et al., 2003). The whole-rock geochemical and Sr-Nd-Pb-Hf isotopic data of the intracontinental OIB-like basalt in northern marginal region of the North China Craton suggest the mixing of an isotopically depleted mantle component and an enriched component (Guo PY et al., 2016). Guo PY et al. (2020) recently recognized that the first-order observations of these basalts are a straightforward manifestation of the lid effect. The intracontinental OIB-like basalt en route to the surface has the opportunity to be contaminated by crustal materials, which may modify the chemical and isotopic compositions during the magma evolution (Chesley J et al., 2002). However, it should be noted that the crustal contamination of the OIB-like basalt is difficult to be recognized by traditional trace element and Sr-Nd-Pb-Hf isotope analysis (Hart RJ et al., 2004).Comparatively, the Re-Os isotope system is a robust indicator of the crustal contribution during the genesis of basalt, as the enriched mantle and lower continental crust may share a similar pattern in Nd, Sr, Pb, and Hf isotope, but in Os isotope(Chesley J et al., 2002). Os is a highly compatible element during the partial melting of mantle peridotite, whereas Re is moderately incompatible, which leads to a strong Re-Os fractionation, resulting in large Re/Os ratios in the melts(crustal rocks) and very low Re/Os ratios in the residual mantle. As a consequence, crustal rocks evolve rapidly to very radiogenic Os isotopic composition with super-chondritic187Os/188Os ratios (generally above 0.5), whereas the mantle evolves at a rate lower than the chondritic mantle and shows unradiogenic Os isotopic compositions (187Os/188Os below 0.128) (Meisel T et al., 2001). Conspicuously, the Re-Os isotopic data as a way to evaluate crustal contamination are rarely utilized for analyzing the genesis of intra-continental OIB-like basalts (Chesley J et al., 2002). In this contribution,the authors present new Os isotopic data against the negligible crustal contamination model of the Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton, and propose that the Jining basalts were significantly contaminated by crustal materials during the magma ascent,whereas the Chifeng basalts experienced minor crustal contamination. This requires reconsideration of the enriched geochemical signatures observed within the intra-continental OIB-like basalt in northern marginal region of the North China Craton, which previously were thought to reflect isotopic features of the enriched mantle source. The crustal contamination of the intra-continental OIB-like basalts might have been also overlooked in other parts on Earth.

Fig. 1. Sketch map of major tectonic divisions of eastern China and the distribution of the Cenozoic basalts (modified from Guo PY et al., 2016; Zhang WH et al., 2012). The Cenozoic basalts are distributed widely in China from Hainan island in the south to Wudalianchi in the northeast. The samples in this contribution are located in Jining and Chifeng areas within northern marginal region of the North China Craton.

2. Geological background

The Jining and Chifeng areas are situated in central Inner Mongolia, northern marginal region of the North China Craton. The North China Craton is one of the largest and oldest cratonic blocks in China, with the oldest banded trondhjemitic gneiss and zircon well-preserved at Anshan up to 3.85 Ga and 4.17 Ga, respectively (Kang CX et al., 2019;Yang FC et al., 2020; Zhao Y et al., 2017). The northern and southern boundaries of the North China Craton are the Paleozoic Central Asian Orogenic Belt and the Qingling-Dabie-Su-Lu Orogenic Belt (Fig. 1; Feng JP et al., 2020; Li ZS et al., 2020). The North China Craton is subdivided into three parts: The Eastern Block, the Western Block, and the Trans-North China Orogen in between (Meng J et al., 2018).The NNE-trending Trans-North China Orogen is also known as Jinyu Mobile Belt proposed by Zhai MG and Santosh M(2013), which is approximately parallel to the North-South Gravity Lineament. The North-South Gravity Lineament not only marks contrast in Bouguer gravity anomaly and upper mantle seismic velocity structures between the western to the eastern blocks, but also exhibits sharp differences in other aspects, such as topography, crust thickness, lithosphere thickness, and heat flow (Guo PY et al., 2020). The Cenozoic basalts are predominantly distributed along the North-South Gravity Lineament and the Tan-Lu Fault Zone of the North China Craton, such as Jining area of the central Inner Mongolia, Fansi area of the Shanxi, and Changle area of Shandong (Fig. 1; Zhang WH et al., 2012a). Among those outcrops, Jining, Hannuoba, and Chifeng areas in northern marginal region of the North China Craton have the best preservation of the late Eocene to the Pleistocene basalts aging about 36.40-0.11 Ma (Zhao XM et al., 2013). The basalts in northern marginal region of the North China Craton built multiple lava plateaus with the total area of ＞ 20000 km2with a thickness from tens to hundreds of meters (Guo PY et al., 2016), taking into account the Abaga area in the Central Asian Orogenic Belt (Fig. 1). They are composed of multiple flows and unconformably cover the Archean metamorphic basement or Jurassic to Eocene sediments (Zhao XM et al.,2013). The Archean basement consists mainly of about 2.5 Ga granitoid gneisses (tonalite-trondhjemite-granodiorite in large part) and mafic granulites/amphibolites (Zhai MG and Santosh M, 2013). Mantle xenoliths, such as spinel lherzolite and pyroxenite, are hosted by the Jining basalts and Hannuoba basalts (Zhang HF et al., 2012). In addition, the lower crustal xenoliths are recognized from the Hannuoba area (Zhang WH et al., 2012). Detailed geology and petrology the Cenozoic basalts in the Jining and Chifeng areas refer to Guo PY et al.(2020), Han BF et al. (1999), and Zhang WH et al. (2012).

3. Analytical methods

To analyze Osmium isotope and trace element, the entire procedure mainly consists of pre-treatment and analysis. The pre-treatment was performed at the Wuhan Sample Solution Analytical Technology Co., Ltd, Wuhan, China. First, to avoid the effects of weathering, the samples collected in the field were examined under a microscope to ensure the selection of relatively fresh ones. Second, all whole-rock samples were crushed in an agate mill to about 200 mesh after their altered surfaces were removed. Finally, Osmium isotope analyses were performed on a Thermo Finnigan TRITON®at Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan (Liu AK, 2010). Sample dissolution (one to three grams of sample powder with enriched190Os solutions) was carried out using 10 ml inverse aqua regia (HNO3∶HCl = 3∶1) in Carius tubes for at least 24 hours at 240°C. Osmium was extracted by carbon tetrachloride solvent extraction from the aqua regia fractionated, then back-extracted into concentrated HBr, and finally further purification by micro-distillation using chromic acid. Osmium was loaded on high purity platinum filament.The sample was covered with 10 μg Ba using a custom-made 10000×10-6Ba(NO3)2solution. Osmium isotopic compositions were measured on an ion counting detector. The typical ion beam intensities of192Os were over 10000 cps.Instrumental mass fractionation of Osmium was corrected by normalizing the measured192Os/188Os ratio to 3.08271. Oxide corrections were made using17O/16O=0.00037 and18O/16O=0.002047. The above method eliminates the instrumental mass fractionation effect, and obtains higher accuracy and precision. The long-term reproducibility of an in-house Osmium standard solution yields187Os/188Os ratios better than 0.4%. Osmium was corrected for the total blank level 2.42 ± 0.03 (2σ) pg. Whole-rock trace element analyses were performed using inductively coupled plasma-mass spectrometry (ICP-MS) at the Wuhan Sample Solution Analytical Technology Co., Ltd, Wuhan, China. Rock powders (50 mg) were dissolved using mixed acids(HF/HClO4) in capped Teflon bomb at 190°C for two days,and subsequently dried to wet salt and re-dissolved in 0.5 ml HClO4. The solutions were then evaporated to wet salt at 140°C and re-dissolved in 1 ml HNO3and 3 ml water for one day at 190°C. The solutions were diluted in 2% HNO3for analysis. The uncertainties based on the replicate analyses of internal standards are ± 5% for REE and ±5%-10% for trace elements (Zhang JH et al., 2019).

4. Results

The new Os isotopic and trace element (e.g., Ni, Cr, Nd and La/Yb) data coupled with previously published geochemical data from Zhang WH et al. (2012) for the Cenozoic basalts in northern marginal region of the North China Craton are listed in Table 1. The Chifeng basalts have high and variable abundances of Osmium [(59.05-184.10)×10-12, 103×10-12on average], with187Os/188Os ratios ranging from 0.135 to 0.193. In contrast, the Jining basalts have much lower abundances of Osmium [(2.36-112.55) ×10-12,18×10-12on average], with187Os/188Os ratios ranging from 0.144 to 0.720. Conspicuously, the Os abundances of the Cenozoic basalts in northern marginal region of the North China Craton are much lower than those of mantle peridotites(＞3000 ×10-12; Gao S et al., 2002). The Chifeng basalts possess higher Cr [(201-236) ×10-6, 224×10-6on average]and Ni [(156-219) ×10-6, 177×10-6on average] contents in comparison with the Jining basalts (166 ×10-6and 115×10-6on average, respectively). The basalts in northern marginal region of the North China Craton have indistinguishable Nd abundances, Mg#[Mg-=100×molar Mg/(Mg+Fe2+)] values,and (La/Yb)N(N means chondrite-normalized) ratios (Figs. 2,3; Table 1).

Table 1. Chemical data of Cenozoic basalts from Chifeng and Jining areas.

5. Discussion

5.1. First-order control on the osmium abundances

The relatively large variations of Os, Ni and Cr concentrations of the Cenozoic basalts in northern marginal region of the North China Craton (Figs. 2a, b) may be controlled by the magma source heterogeneity, the degree of partial melting, fractional crystallization, crustal contamination, and/or a combination of the aforementioned factors. (1) First, the Cenozoic basalts are products of variable mixing between depleted and enriched mantle sources (Guo PY et al., 2016; Zhang WH et al., 2012). However, as both mantle sources share similar Os, Ni and Cr concentrations, the large variations in those elements can hardly be explained by the magma source heterogeneity. (2) In the Osvs. La/Yb ratio(and Nd concentration) diagram, a considerably positive correlation between Os ratio and La/Yb (and Nd concentration) is observed for the Chifeng basalts with highest Os abundances (＞ 50 ×10-12), showing a partial melting trend (Trend 1; Figs. 2c, d). The decreasing La/Yb ratios and Nd concentrations of these samples correspond to an increasing degree of partial melting, consistent with the highly compatible behavior of Os during partial melting(Baksi AK, 2001). The results above suggest that the influence of Os-rich phase during melt generation diminishes successively and further confirm the role of the degree of partial melting (Burton KW et al., 2002). (3) Major element compositional variations of the Cenozoic basalts in northern marginal region of the North China Craton are largely controlled by varying extent of fractional crystallization of olivine and clinopyroxene (Guo PY et al., 2016), whereas the possibility of crustal contamination cannot be ruled out.Except the samples with highest Os abundances, the remaining with relatively low Os abundance also defines a crustal contamination or fractional crystallization process(Trend 2 or 3; Figs. 2c, d). The lack of linear relation between Os and Ni concentrations (with lowR2＜ 0.2) implies that the behavior of Os-rich phase in the Cenozoic basalts was not influenced by olivine or clinopyroxene crystallization (Fig.2a). That is, olivine crystallization can produce sulfur saturation of the melt, in which Os is highly compatible(Burton KW et al., 2002).

Fig. 2. (a) Os vs. Ni correlation and (b) Os vs. Cr correlation for the Cenozoic basalts in northern marginal region of the North China Craton.Note the lack of linear relation with low R2 ＜ 0.4. (c) Os vs. La/ Yb correlation and (d) Os vs. Nd correlation for the Cenozoic basalts in northern marginal region of the North China Craton. Arrows show the trends of the increasing degree of partial melting (Trend 1), fractional crystallization (Trend 2) and crustal contamination (Trend 3).

5.2. Evidence for crustal contamination

However, it is difficult to distinguish fractional crystallization from crustal contamination in the aforementioned diagrams (e.g., Figs. 2c, d). A diffusely positive correlation of187Os/188Os and 1/Os in the Jining basalts [correlation coefficient (R) = 0.92 when two samples are excluded; Fig. 3a implies the mixing between two isotopically distinct end-members (Burton KW et al., 2002),whereas the Chifeng basalts are plotted in a cluster with both low187Os/188Os and 1/Os values (Fig. 3a). The187Os/188Os ratio of the Chifeng basalts does not change with the Mg#increasing considerably (Fig. 3b), indicating significant fractional crystallization with minor crustal contamination. In contrast, the Jining basalts show large variations of187Os/188Os and Mg#, further demonstrating the possible scenario of the felsic crustal contamination (Os isotopic ratios show an increase with decreasing Mg#values), or mafic crustal contamination (Os isotopic ratios vary at relatively uniform Mg#values) with fractional crystallization (Os isotopic ratios remain unchanged with decreasing uniform Mg#). This is consistent with the observation of lower crustal xenoliths within the basalt in the vicinity (Zhang WH et al.,2012). These large187Os/188Os variations of the Jining basalts also exclude the possibility of the contamination process in the mantle source, but instead support crustal contamination en route to the surface (Fig. 3).

Fig. 3. (a) 187Os/188Os vs. 1/Os for the Cenozoic basalts in northern marginal region of the North China Craton. The linear relation indicates that crustal contamination accounts for the variation of Os isotope and content. (b) 187Os/188Os vs. Mg# for the Cenozoic basalts in northern marginal region of the North China Craton. Arrows show the trends of the increasing degree of fractional crystallization (Trend 1), the felsic crustal contamination (Trend 2), and mafic crustal contamination (Trend 3).

5.3. Nature of the crustal contaminants

To test the validity and further determine the source of the crustal contaminants, viz. felsic or mafic crustal rock, the authors employed Os isotopic mixing model (Fig. 4). Note that an Os concentration of 250×10-12and187Os/188Os ratio of 0.1261 are assumed for the mantle-derived parental magma(Chesley J et al., 2002). These data are referred to from the Phanerozoic mantle peridotite in the North China Craton (Gao S et al., 2002). The authors used the late Neoarchean granitoid gneiss to represent felsic crustal contaminant and the coeval granulites or amphibolites as the mafic one. The authors assumed that the granitoid gneiss contains Os=7×10-12and187Os/188Os=5, while the mafic has Os=50×10-12and187Os/188Os=30 (Hanski E et al., 2001; Hart RJ et al. 2004).These data are based on the Archean basement from other cratons with similar ages and lithology as the North China Craton, such as the Kaapavaal Craton (Hanski E et al., 2001;Hart RJ et al., 2004; Liu AK, 2010). As shown in Fig. 4, the Chifeng and the Jining basalts would require incorporation of up to about 30%-80% felsic crustal contaminant, respectively,due to the low Os content in the granitoid gneiss. Thus, the granitoid gneiss is not the best candidate for the Os isotopic variation, because such high proportions of felsic crustal addition cannot match the high Mg#value of these basalts.For the mafic crustal contaminant model, the Jining basalts evolve with an addition of ＜ 10% granulites or amphibolites,while the Chifeng basalts might experience negligible crustal contamination (Fig. 4). That is to say, the geochemical and isotopic characteristics of the Jining basalts do not necessarily represent the nature of the mantle sources.

Fig. 4. Binary mixing between mantle-derived parental magma and potential crustal contaminants. End members used in the modeling are described in the text, including the mantle-derived parental magma, the Archean granitoid gneisses, and the mafic lower crust.The Os isotopic data of the Chifeng and Jining basalts are shown as the pink and green areas, respectively.

6. Conclusions

The high value of radiogenic Os isotope of the Jining basalts demonstrates that they experienced significant contamination by an old continental crust during magma emplacement. Isotopic modeling suggests that the Os isotopic variation was mainly attributed to the contamination of Archean mafic lower crust, while the varying element composition was mainly due to the fractional crystallization.In contrast, the geochemical trend of the Chifeng basalts was controlled by partial melting and fractional crystallization,exhibiting unradiogenic and uniform Os isotopic compositions. The Os isotopic data of Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton allows a better estimation of crustal contamination during the evolution of mantle-derived magmas, and, in particular, allows a better discrimination for the nature of crustal contaminant. This study also serves as an inspiration for the future research on other intra-continental OIB-like basalts worldwide, where the crustal contamination was used to be overlooked.

CRediT authorship contribution statement

Zhuang Li and Bin Chen wrote the main manuscript text and prepared all the figures. Bin Chen prepared the samples.Both authors reviewed the manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgement

Special thanks are due to editors and four anonymous reviewers for their constructive comments and careful corrections that led to significant improvement to the manuscript. The authors also thank Dr. Yan Zhan of Carnegie Institution for Science, Prof. Chun-jing Wei of Peking University, and Dr. Peng-yuan Guo of Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences for their perceptive suggestions and careful corrections on preparing the manuscript for this paper. This work was supported financially by Beijing Natural Science Foundation(8194073), the Science Foundation of China University of Petroleum, Beijing (2462017YJRC032 and 2462021YXZZ004), the Science Foundation of State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing (PRP/indep-4-1702), and the National Natural Science Foundation of China (41872057 and 42002238).