Recycled oceanic crust in the source of 90-40 Ma basalts in N & NE China

Yi-Gang Xu

State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China; yigangxu@gig.ac.cn

The North China Craton (NCC) experienced thermo-tectonic reactivation, during which a > 100 km thick lithospheric keel under its eastern part was removed (Menzies et al., 1993). A low thermal gradient (~40 mw/m²; Griffin et al., 1998; Xu, 2001) in the lithospheric mantle is shown by refractory peridotite xenoliths enclosed in Ordovician diamondiferous kimberlites. This, together with low ¹⁸⁷Os/¹⁸⁸Os ratios in the xenoliths (<0.113), indicates an old, thick, cold cratonic keel beneath the NCC during the Paleozoic. In contrast, a high thermal gradient (~60 mw/m²; Xu, 2001) is shown by fertile, oceanic-type peridotites in Cenozoic basalts (Gao et al., 2002; Hong et al., 2012). This implies considerable lithospheric thinning beneath the NCC during the late Mesozoic. This presents a major problem in the study of the cause and mechanism of continental lithospheric thinning, because cratonic keels are thought to be stable due to their buoyancy and the low density of highly melt-depleted peridotites (3.3-3.35 g/cm³) (O’Reilly et al., 2001; Lee, 2003).

So far, several potential triggers have been proposed as causes for destabilization of the NCC. These include:

1. the India-Eurasia collision (Menzies et al., 1993);
2. mantle plume activity (Deng et al., 2005);
3. the Yangtze-North China collision (Menzies & Xu, 1998; Gao et al., 2002); and
4. subduction of the Pacific plate underneath the east Asian continent (Griffin et al., 1998; Xu, 2001; Niu, 2005; Wu et al., 2008).

Among these, subduction of the Pacific plate is now regarded as one of the principal triggers for the destruction of the NCC (Zhu et al., 2012). It is therefore a challenge for petrologists and geochemists to identify slab-derived materials involved in the sources of the Mesozoic-Cenozoic magmas in this region.

Identification of components contributing to the genesis of mafic magmas is one of the major geochemical challenges in understanding the dynamics of the Earth's mantle (Zindler & Hart, 1986). Despite much work, the origin of intraplate alkaline basalts still remains unclear. Earlier experiments demonstrated that alkali basalts form at higher pressure, via a smaller degree of partial melting of peridotites, compared to tholeiites. However, it has been repeatedly stated in recent years that melts derived from a peridotitic source are not the same, compositionally, as oceanic island basalts (OIB) and alkaline basalts in general. Pyroxenite, eclogite, or carbonated peridotites and eclogites are considered possible candidates instead of peridotites for the source of alkaline basalts (e.g., Kogiso et al., 2003; Dasgupta et al., 2006).

Recent studies have shown the ubiquitous presence of subduction-related components in late Cenozoic basalts in eastern China (Zhang et al., 2009; Xu et al., 2012; Sakuyama et al., 2013). However, it remains unclear whether similar recycled oceanic components are present in earlier basalts. Xu (2014) compiled major, trace element and Sr-Nd-Pb isotopic data of basalts emplaced at 90-40 Ma in the N and NE China, and identified three major components in the magma source, including depleted components I and II, and an enriched component (Figure 1). Depleted component I, which is characterized by relatively low ⁸⁷Sr/⁸⁶Sr (<0.7030), moderate ²⁰⁶Pb/²⁰⁴Pb (18.2), moderately high εNd (~4), high Eu/Eu* (>1.1) and HIMU-like trace element characteristics, is most likely derived from gabbroic cumulates from oceanic crust. Depleted component II, distinguished by high εNd (~8) and moderate ⁸⁷Sr/⁸⁶Sr (~0.7038), is probably derived from sub-lithospheric ambient mantle. The enriched component has low εNd (2-3), high ⁸⁷Sr/⁸⁶Sr (>0.7065), low ²⁰⁶Pb/²⁰⁴Pb (17), excess Sr, Rb, Ba and a deficiency of Zr and Hf relative to the REE. This component is likely from the basaltic portion of oceanic crust, which is variably altered by seawater and contains minor sediments. Comparison with experimental melts and trace-element modeling suggest that these recycled oceanic components may be in the form of garnet pyroxenite/eclogite. They are young (<0.5 Ga) and show an Indian-MORB isotopic character. Given the similarity of these isotopic characteristics to the extinct Izanaghi-Pacific plate, currently stagnated in the mantle transition zone, it is proposed that it ultimately arises from the subducted Pacific slab (Figure 2).

Figure 1: (a) Primitive mantle-normalized trace element diagrams for representative samples of the 90-40 Ma basalts from N and NE China. (b) εNd versus ⁸⁷Sr/⁸⁶Sr.

Figure 2: Cartoon illustrating recycled oceanic crustal components in intraplate basalts, which are likely derived from the stagnant slab within the mantle transition zone (Huang & Zhao, 2006).

Eu/Eu* and ⁸⁷Sr/⁸⁶Sr of the 90-40 Ma magmas increase and decrease, respectively, with decreasing emplacement age (Figure 3), mirroring a change in magma source from upper to lower parts of subducted oceanic crust. Such secular trends are created by dynamic melting of a heterogeneous mantle containing recycled oceanic crust. Due to the different melting temperatures of the upper and lower oceanic crust and progressive thinning of the lithosphere, the more fertile basaltic crustal component is preferentially sampled during the early stage of volcanism, whereas the more depleted gabbroic lower crust and lithospheric mantle components are preferentially sampled during a later stage (Figure 4). This model is consistent with a protracted destruction process for the lithosphere beneath E China. The presence of significant recycled oceanic crust components in the 90-40 Ma basalts highlights the influence of Pacific subduction on deep processes in the NCC, which can be traced back at least to the late Cretaceous. This, along with the conjugation of the crustal deformation pattern in this region with the movement of the Pacific plate, makes Pacific subduction a potential trigger for the destruction of the NCC.

Figure 3: Plots showing increasing Eu/Eu* and decreasing ⁸⁷Sr/⁸⁶Sr in the 90-40 Ma basalts (MgO > 8 wt%) with decreasing age. Since the upper oceanic crust (OC) has lower Eu/Eu* and higher ⁸⁷Sr/⁸⁶Sr than the lower OC, the trends point to a change in magma source from upper OC to lower OC.

Figure 4: Cartoon illustrating dynamic melting of a heterogeneous mantle during lithospheric thinning that can explain the temporal variation in composition of the 90-40 Ma basalts from N and E China. The solidus curves for upper oceanic crust (OC), lower OC and peridotite are indicated schematically. Rectangular boxes denote melting columns which consist of a recycled oceanic crustal (ROC) component and a peridotitic matrix. The melting columns extend from the asthenospheric solidus to the base of the lithosphere. (a) During the late Cretaceous, the lithosphere was relatively thick and the melting column was mostly confined to large depths, where only the upper part of ROC (i.e. eclogite/pyroxenite) with low-melting-point melts generating high-alkaline basalts with low Eu/Eu* and high ⁸⁷Sr/⁸⁶Sr. (b) During the early Cenozoic, the melting column migrated to a relatively shallow level as a result of lithospheric thinning, and the solidus of the lower OC was crossed. The magmas generated during this period represent pooled melts from both lower- and upper OC. Because of the consumption of the upper OC during previous melting events, the magmas are predominantly from the lower OC, showing high Eu/Eu* and low ⁸⁷Sr/⁸⁶Sr.

The NCC is relatively small compared to other cratons. It experienced multiple subductions from different directions, i.e., northward subduction of the Tethys ocean along the Dabie Shan and Song Ma sutures, south-dipping subduction along the Solonker (Permo-Triassic age) and Mongol-Okhotsk suture (Jurassic age), and westward subduction of Pacific plate. Large amounts of water are released from subducting slabs to the upper mantle under the eastern NCC (Windley et al., 2010). This is supported by recent estimates of the water content in the late Mesozoic lithospheric mantle under the eastern NCC of >1000 ppm (Xia et al., 2013). Water would have considerably weakened the lithosphere (Hirth & Kohlstedt, 1996; Li et al., 2008), facilitating its convective removal and leading to cratonic destruction. Pacific subduction underneath eastern Asia appears to have played a determinant role in cratonic destruction, governing the distribution patterns of post-Mesozoic basins and major tectonic configurations, the temporal change of magmatism, and east-west contrast in lithospheric architecture.

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