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Rocking Mantle”Group

Representative studies of our group

1, Crustal Recycling & Mantle Heterogeneity

Anfengshan is a Miocene volcano located in the Sulu UHP orogeny belt. Radiogenic Sr-Nd-Pb-Hf isotopes show some highly unusual correlations: εNd correlates positively with 87Sr/86Sr,but negatively with εHf. Moreover, 87Sr/86Sr, 143Nd/144Nd,and 206Pb/204Pb ratios all correlate negatively with ΔεHf (= deviation from the global εHf–εNd correlation). The correlations form two distinct mixing arrays with one common, high-εNd end-member. We suggest that the other two source components are eclogites derived from subducted lower crust: both of these differ from ordinary mantle components by their low 87Sr/86Sr and low εNd and 206Pb/204Pb, but high ΔεHf (Figure 1).

This study is the first attempt to link the geochemical heterogeneity of mantle with the recycling of subducted continental crust.

Fig. 1.Variations of εHf vs. εNd for the Anfengshan basalts and Sulu eclogites.

(Chen L.-H.*, ZengG., Jiang S.-Y., Hofmann A.W., Xu X.-S., Pan M.-B., 2009. Sources of Anfengshanbasalts: Subducted lower crust

in the Sulu UHP belt, China. Earth and Planetary Science Letters,286: 426–435.)

2, Carbonated Mantle Sources for Intraplate Alkaline Basalts

The genesis of intra-plate alkaline basalts remains controversial, and three sources have been proposed: silica-deficient eclogite–pyroxenite, hornblendite, and carbonated peridotite. Here, we assess these models by analyzing Cenozoic intra-continental alkaline basalts from Shandong province, North China. Neither silica-deficient eclogite–pyroxenitemelts nor hornblendite melts can satisfy the key geochemical features, such as negative K, Zr, Hf, and Ti anomalies. Here we prefer a carbonated mantle source because the main characteristics of the strongly alkaline rocks resemble those of carbonatites (Figure 2).

It is the first time that the carbonated mantle source of the intra-plate alkaline basalts in North China to be proposed.

Fig. 2. Variations in Na2O+K2O vs. TiO2 for Cenozoic alkaline basalts from Shandong and experimental alkaline melts.

(ZengG., Chen L.-H.*, Xu X.-S., Jiang S.-Y., Hofmann A.W., 2010. Carbonatedmantle sources for Cenozoic intra-plate alkaline basalts

in Shandong, NorthChina. Chemical Geology. 273: 35–45.)

3, A Lateral Flow Model of Deep Mantle Material

We show major, trace element and Sr–Nd–Hfisotopic compositions for two parallel chains of Cenozoic volcanoes from Shandong Province, North China (Figure 3a), which are free of crustal contamination and show clear evidence for recycling of mafic lower crust. Sr, Nd, and Hf isotopes in the two volcanic chains form separate binary mixing arrays, which converge on the composition of Dashan, an isolated, nephelinitic volcano with the most depleted isotopic signature (Figure3a, b). The two chains have lower CaO contents and significantly diverging isotope enrichment trends from this common endmember. Both trends deviate from the normal Sr–Nd and Hf–Nd mantle array toward lower 87Sr/86Sr and higher εHf values, all features that point to a (recycled) eclogitic source.

This work is the pioneer which discovers the small-scale lateral chemical heterogeneities in mantle through elaborate observations of intraplate basalts.

Fig. 3. (a) Distribution of the Cenozoic chain-forming volcanoes in Shandong.

(b-c) Variations in εNd vs. 87Sr/86Sr and εHf vs. εNd for chain-related basalts in Shandong.

(Zeng G., Chen L.-H.*, Hofmann A.W., Jiang S.-Y., Xu X.-S., 2011.Crust recycling in the sources of two parallel volcanic chains

in Shandong, North China. Earth and Planetary ScienceLetters, 302: 359–368.)

4, Spatial and Temporal Distributions and Genetic Mechanism of Small-scale Continental Flood Basalts

We present a case study from eastern China, combining major and trace element analyses with Ar–Ar and K–Ar dating to show that the spatiotemporal distribution of small-scale flood basalts is controlled by the growth of long-lived magma chambers. Evolved basalts (SiO2> 47.5 wt.%) from Xinchang–Shengzhou, a small-scale Cenozoic flood basalt field in Zhejiang province, eastern China, show a northward younging trend over the period 9.4–3.0 Ma (Figure 4A, B). With northward migration, the magmas evolved only slightly ((Na2O+ K2O)/MgO = 0.40–0.66; TiO2/MgO = 0.23–0.35) during about 6 Myr (9.4–3.3 Ma) (Figure 4C, D). The distribution and compositional evolution of the migrating flood basalts record continuous magma replenishment that buffered against magmatic evolution and induced magma chamber growth. With the existence of trans-lithosphere fault zones in the north (The Lishui–Yuyao Fault), the magma chambers grew asymmetrically in that direction. Therefore, the continuous asymmetrical growth of the magma chambers in the lower crust controlled the northward migration of the Xinchang–Shengzhou flood basalts.

Fig. 4. (A) Plot of SiO2vs. total alkali. 1, 2, and 3 represent the trends of low-SiO2 alkaline basalt, high-SiO2 alkaline basalt, and high-SiO2 tholeiitic basalt, respectively. (B–D): Plots of latitude vs. each of age, (Na2O+ K2O)/MgO, and TiO2/MgO, respectively.

(YuX., Chen L.-H.*, Zeng G., 2015. Growing magma chambers control the distributionof small-scale

flood basalts. ScientificReports, 5, 16824; doi: 10.1038/srep16824.)

5, Lithospheric thickness controlled compositional variations in potassic basalts in NE China

Cenozoic potassic basalts in NE China are strongly enriched in incompatible elements and show EM1-type Sr–Nd–Pb isotopes, suggesting an enriched mantle source. These rocks show good correlations between 87Sr/86Sr and K2O/Na2O and Rb/Nb. Notably, these ratios decrease with increasing lithospheric thickness, which may reflect melt-lithosphere interaction (Figure 5). Phlogopite precipitated when potassic melts passed through the lithospheric mantle, and K and Rb contents of the residual melts decreased over time. The thicker the lithosphere, the greater the loss of K and Rb from the magma, which could be treated as another form of "Lid Effect". These observations indicate that the enriched materials of asthenosphere mantle (recycled lithosphere) influenced the potassic meltsand were their direct source. Therefore, the compositions of potassic basalts were controlled by both their enriched sources and reactions with lithospheric mantle.

Fig. 5. Average (a) 87Sr/86Sr,(b) K2O/Na2O, and (d) Rb/Nb decrease, while (c) MgO increases with increasing lithospheric thickness for the Cenozoic potassic basalts from NE China. N represents the number of samples averaged for geochemical compositions and the error bars correspond to 1 standard error (1 SE) of the mean.

(LiuJ.-Q., Chen L.-H.*, Zeng G., Wang X.-J., Zhong Y., Yu X., 2016. Lithospheric   thickness  controlled   compositional   variations  

in   potassic   basalts  of Northeast China by melt–rock interactions. Geophysical Research Letters, doi: 10.1002/2016GL068332)

6, A link between lithological variations of mantle and subduction process of paleo-Pacific plate

Widespread late Mesozoic volcanic magmatism in southeastern China is generally thought to represent products in response to the subduction of paleo-Pacific plate; however, it remains unclear when this process began to affect the mantle and the related magmatism. Here we present a systematic study on the source lithology of late Mesozoic basalts in this area to highlight a link between lithological variations of mantle and subduction process of paleo-Pacific plate. Late Mesozoic basalts can be subdivided into four groups based on their erupted ages: 178~172 Ma, c.150 Ma, 137~123 Ma and 109~64 Ma. The primary source lithology of these rocks is pyroxenite rather than peridotite, and this mafic lithology can be formed by either ancient or young recycled crustal components (Figure 6). Notably, the source lithology of the c.150 Ma and 137~123 Ma basalts is SiO2-rich pyroxenite, and the former is carbonated. The discovery of carbonated, SiO2-rich pyroxenite reflect the influence of a recently recycling event in the mantle. The subduction of paleo-Pacific plate is the most appropriate candidate, and can be responsible for the mantle-derived magmatism after c.150 Ma in southeastern China. Therefore, we suggest a paleo-Pacific slab rollback with increased dip angle as a possible model to control the lithological variations of late Mesozoic mantle beneath southeastern China.

Fig. 6. Identification of mantle source lithologies for Late Mesozoic basalts from southeastern China

(Zeng, G., Z.-Y. He, Z. Li, X.-S. Xu, and L.-H. Chen (2016), Geodynamics of paleo-Pacific plate subduction constrained by the source lithologies

of Late Mesozoic basalts in southeastern China, Geophys. Res. Lett., 43, doi:10.1002/2016GL070346. )