Elsevier

Chemical Geology

Volume 611, 20 November 2022, 121117
Chemical Geology

Mantle plume-subducted oceanic slab interaction contributes to geochemical heterogeneity of the Emeishan large igneous province

https://doi.org/10.1016/j.chemgeo.2022.121117Get rights and content

Highlights

  • The marginal zone rocks have enriched isotope compositions and depleted Zrsingle bondHf content.

  • Decoupling between Nd and Hf isotopic compositions in the mafic–felsic intrusions of the ELIP.

  • Subducted Paleo-Tethyan oceanic slab in the source led to its geochemical and redox heterogeneity.

Abstract

Mantle-derived rocks in large igneous provinces yield large variations in their chemical and isotopic compositions as well as redox conditions; however, the origin of these heterogeneities remains unclear. We present new geochemical and Sr–Nd–Hf–O isotopic data for the Hongge mafic–ultramafic intrusion and associated syenite in the Emeishan large igneous province (ELIP), and combine them with previously published data from coeval intrusive and volcanic rocks in the ELIP, to constrain the generation of the ELIP and associated world-class Fe–Ti–V oxide deposits. The rocks from the marginal zone of the Hongge intrusion have high-Ti basaltic-like compositions with high initial 87Sr/86Sr ratios (0.7084–0.7096) and unradiogenic εNd (−7.7 to −5.9) and εHf (−6.0 to −2.4) values, and represent the most enriched endmember of the high-Ti basaltic series observed in the ELIP to date. The marginal zone rocks, temporally and spatially associated basalts, and syenites yield depleted Zr and Hf contents with Zr/Sm (7.1–38.3) and Hf/Nd (0.05–0.12) ratios between those of marine sediment and OIB. The Ndsingle bondHf isotopic compositions of the marginal zone rocks plot above the terrestrial array, suggesting decoupling between Nd and Hf isotopic compositions. Mixing between subducted oceanic crusts, marine sediments and mantle peridotite accounts for the enriched Srsingle bondNd isotopic compositions, Zrsingle bondHf depletion, and the decoupling between Nd and Hf isotopic compositions in these samples as well as in other mafic–felsic intrusions of the ELIP. Zircon grains from the syenites yield more radiogenic εHf values (+3.7 − +6.6) than the marginal zone rocks and have δ18O values of 3.9–5.7‰, most of which are much lower than mantle-zircon values, suggesting that oceanic crust altered at high temperature was involved in the mantle source. The integrated Sr–Nd–Hf–O isotopic compositions of these mafic–felsic intrusions can be explained by a model for the generation of the ELIP whereby a mantle plume interacted with the subducted Paleo-Tethyan oceanic lithospheric slab and overlying marine sediment. Our results suggest that the inclusion of the recycled altered oceanic slab in the mantle source of the ELIP led to its geochemical and, potentially, redox heterogeneity.

Introduction

Large igneous provinces (LIPs) are formed by the rapid emplacement of large volumes of intraplate mafic–ultramafic magma and associated silicic magmas in the crust, and thus have geological and environmental impacts that are regional to global in scale (Bryan and Ernst, 2008). Although various mechanisms have been proposed for the origin of LIPs, the basalts found in LIPs often yield OIB-like geochemical compositions and are generally thought to result from decompression melting of a mantle plume rising from the core–mantle boundary (Campbell and Griffiths, 1990). Magmas from LIPs are geochemically and isotopically heterogeneous. In particular, enriched crustal signatures have been identified in these OIB-like mantle-derived magmas, whose origin, however, is not well understood. It has been proposed that the enriched geochemical compositions were inherited from ancient recycled oceanic slab and sediment that had been subducted to the core–mantle boundary and then returned to the surface by a mantle plume (Eiler et al., 1997; Hofmann and White, 1982; Jackson et al., 2007). In addition to the geochemical heterogeneity, redox heterogeneity has also been identified in some LIPs and hotspots (Bai et al., 2019; Moussallam et al., 2019; Wu et al., 2022). Mid-ocean ridge basalts (MORBs), which are formed by the partial melting of the upper mantle uninfluenced by mantle plumes, also display large variations in geochemical and isotopic compositions. The compositional heterogeneity of MORB has been linked to the recycling of oceanic slabs in the upper mantle (Eiler et al., 2000; Rehka¨mper and Hofmann, 1997). A mantle plume rising from depth to the base of the lithospheric mantle might, therefore, interact with recycled oceanic slabs already existing in the upper mantle; however, it is still unclear whether this contributes to the geochemical and redox heterogeneity in LIPs. LIPs are also economically important due to their direct and indirect links to various types of orthomagmatic and hydrothermal mineral deposits (Ernst and Jowitt, 2013). The type and size of ore deposits associated with each LIP are highly variable. Only a few LIPs are known to be associated with giant Fe–Ti–V oxide deposits, and the factors controlling the metallogenic potential of Fe–Ti–V oxide deposits in LIPs are still unclear.

Similar to lavas from other LIPs, the OIB-like mantle-derived lavas of the Permian Emeishan LIP (ELIP) show large variations in geochemical and isotopic compositions. Ancient recycled oceanic crust and its secondary product (pyroxenite) have been suggested to contribute to the geochemical heterogeneity of the magmatism in the ELIP (Hou et al., 2011; Ren et al., 2017; Yu et al., 2017; Liu et al., 2022). Mafic–ultramafic intrusions form part of the ELIP and host the largest Fe–Ti–V oxide deposits in the world. In this paper, we combine new whole-rock chemical and Sr-Nd-Hf isotopic data as well as zircon Hfsingle bondO isotopic data from the Hongge mafic–ultramafic intrusion and temporally and spatially associated syenite in the ELIP with previously published data from coeval intrusive and volcanic rocks from the ELIP, which provide the first robust evidence for the presence of subducted marine sediments in the source of the magmas. Our results demonstrate that the interaction between subducted oceanic slabs and mantle plumes plays an important role in producing chemical and redox heterogeneity in LIPs as well as the formation of world-class Fe–Ti–V oxide deposits.

Section snippets

Geologic setting and sample descriptions

The ELIP is located to the east of the East Paleo-Tethys suture zone and on the western margin of the Yangtze Block, SW China (Fig. 1a). The basement of the Yangtze Block consists of the Paleoproterozoic–Mesoproterozoic low-grade metasedimentary and metavolcanic rocks of the Huili and Kunyang groups and the Neoproterozoic amphibolite to granulite facies metamorphic rocks of the Kangding Complex. The basement is overlain by a thick cover of Neoproterozoic metasedimentary rocks and Paleozoic

Analytical methods

Whole-rock major element compositions were determined using X-ray fluorescence spectrometry at the ALS Laboratory Group, Guangzhou, China, with an analytical precision of <5%. Whole-rock trace element compositions were determined using a PerkinElmer SCIEX ELAN DRC-e quadrupole inductively coupled plasma–mass spectrometer (ICP–MS) housed at the State Key Laboratory of Ore Deposit Geochemistry (SKLODG), Institute of Geochemistry, Chinese Academy of Sciences (CAS), Guiyang, China. Prior to

Major and trace elements

The major and trace element compositions of the samples are listed in Table 1. Loss on ignition (LOI) values of the samples are 0.3–4.2 wt%, and most of the values are <2 wt%, which is consistent with the fact that the samples have only suffered very slight alteration (Fig. 2). Such a weakened degree of alteration would not have a significant impact on the geochemical compositions, especially for the alteration-resistant incompatible trace elements such as high field strength elements and REEs (

Source or process control on magma compositions?

The fine-grained equigranular textures of the Hongge marginal gabbros (Fig. 2b) are consistent with the rapid nucleation and cooling processes that differ from coarse-grained cumulus gabbros. Cumulus gabbros also have significantly lower contents of incompatible elements (e.g. HFSE and REEs) compared to the coeval basalts in ELIP (e.g. Pang et al., 2010). Combining this observation with the undepleted incompatible element contents of these marginal samples (Fig. 4), we suggest that these rocks

Conclusions

The marginal rocks of the Hongge intrusion have the most enriched Sr–Nd–Hf isotopic compositions of the high-Ti basaltic series in the ELIP. The marginal rocks and temporally and spatially associated basalts and syenites have depleted Zr and Hf contents and there is a decoupling between their Nd and Hf isotopic compositions. These features cannot be explained by fractional crystallization or crustal contamination during magma ascent to shallow crustal levels; instead, they were inherited from

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank F. Xiao and YW. Chen for their help in whole-rock Srsingle bondNd and zircon Hf isotope analysis. This study was supported by the National Natural Science Foundation of China (42122024, 42121003 and 41873055), CAS “Light of West China” Program and Youth Innovation Promotion Association (2019391), Chinese Academy of Sciences and Innovation, K.C. Wong Education Foundation (GJTD-2020-13). Helpful and constructive reviews by the Editor Dr. Sonja Aulbach and two anonymous reviewers were greatly

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