Lamprophyres are mantle-derived, mafic to ultramafic, volatile-rich (e.g.H2O and CO2) igneous rocks. According to the classification criteria of the International Union of Geological Sciences, lamprophyres can be subdivided into calc-alkaline, alkaline and ultramafic types. Although these rocks are volumetrically insignificant, they are of great economic significance and petrological interest because they:(1) are associated with diamond and hydrothermal gold and gold-copper mineralization worldwide, and (2) provide the prime opportunity to obtain the deep mantle materials, and to understand deep mantle melting and large-scale geodynamic processes. Regardless of their tectonic setting and formation processes, it is generally assumed that lamprophyres are derived from low-degree partial melting of metasomatically enriched mantle sources (Krmíček and Chalapathi Rao 2022; Rock 1991). However, the nature and origin of the enriched mantle sources remain in debate, especially whether they are enriched by hydrous silicate melts or fluids, or carbonate-rich melts.
To address this issue, the "Rocking Mantle Group" conducted high-precision Zn isotope data on the contemporary Mesoproterozoic (~ 1.1 Ga) lamprophyres from three localities of the Eastern Dharwar Craton(EDC). The results indicate that the Mudigubba and Kadiri lamprophyres with island-arc basalt (IAB)-like trace element features, such as positive Pb and negative Nb-Ta, Zr-Hf and Ti anomalies, have mid-ocean ridge basalt (MORB)-like δ66Zn values ranging from 0.22‰ to 0.29‰. In contrast, the Udiripikonda lamprophyre shows higher-than-MORB δ66Zn values of 0.39‰ to 0.48‰ and elemental features of intra-plate magmas with a lack of pronounced Nb-Ta negative anomalies (Figure 1).
Figure 1 A summary of δ66Zn values for the Mudigubba, Kadiri and Udiripikonda lamprophyres
Based on the covariations between Zn isotopes and trace element ratios, we infer that the Mudigubba and Kadiri lamprophyres with MORB-like Zn isotopes and high Ba/La, K/Nb and low Nb/La, Ce/ Pb ratios are inherited from sub-continental lithospheric mantle metasomatized by fluids derived from a subducted slab (Figure 2). Further studies have shown that the Udiripkonda lamprophyres have higher δ66Zn values (0.39‰ to 0.48‰) than the terrestrial mantle and MORB, which cannot be explained by post-magmatic alteration and crustal contamination as well as fractional crystallization and partial mantling processes but indicates the involvement of recycled sedimentary carbonates in their mantle source. There are two ways in which subducted carbonates may contribute to the source of mafic–ultramafic igneous rocks: one is via metasomatism of the sub-continental lithospheric mantle by carbonatitic melts and the other is through subduction of carbonate-bearing sediments into the deep mantle, e.g., the mantle transition zone (MTZ) or even the lower mantle. The Udiripikonda lamprophyres have highly radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd isotopic compositions. Combining with their low Ba/La, K/Nb and high Nb/La and Ce/Pb ratios (Figure 2), we suggest that one possible way in which carbonate sediments could contribute to the source of the Udiripikonda lamprophyre source region is through metasomatism of the sub-continental lithospheric mantle by carbonatite melts. It is notable that in the primitive mantle-normalized incompatible trace element diagram, the Udiripikonda lamprophyres lack subduction-related geochemical signals such as Nb-Ta-negative anomalies but resemble intraplate potassic rocks in northeast China and cratonic lamproites from Gaussberg. Combined with their extremely high K/U and Ba/Th ratios (figure 3), we prefer that they are derived from the mantle transition zone. During the partial melting process of the subducted carbonate-bearing sediments in the mantle transition zone, K, Ba, and Pb are retained in the K-hollandite. The extraction of a small amount of carbonate melt leads to the depletion of Th, U, and light rare earth elements, while K, Ba, Pb, as well as Zr and Hf are preserved in the residual minerals, resulting in the observed Zr-Hf positive anomaly and high K/U and Ba/Th ratio characteristics. Thus, our study suggests that contemporaneous lamprophyres can be derived from different enriched source regions.
Figure 2 Covariations between δ66Zn and (a) Ba/La, (b)K/Nb, (c)Nb/ La and (d) Ce/Pb for the Mesoproterozoic lamprophyres in Eastern Dharwar craton.
Figure 3 Ba/Th versus K/U for the studied Mesoproterozoic lamprophyres in Eastern Dharwar Craton.
This study was recently accepted in publication in《Contributions to Mineralogy and Petrology》. Liu, J.Q.*, Chen, L.H., Wang, X.J., Krmíček, L., Zeng, G., Zhang, X.Y., Murphy, D.T., Dalton, H., Pandey, A., Chalapathi Rao, N. V. 2023. Zinc isotopes reveal disparate enriched sources of contemporary lamprophyres in Eastern Dharwar Craton. Contributions to Mineralogy and Petrology. https://doi.org/10.1007/s00410-023-02073-1.
Associate Professor Jian-Qiang Liu is the first author and corresponding author of the paper, with contributions from Li-Hui Chen, Xiaojun Wang, Lukáš Krmíček, Gang Zeng, and Xiaoyu Zhang. This work was supported by the National Natural Science Foundation of China (42130310 and 41802045).
The paper link is: https://link.springer.com/article/10.1007/s00410-023-02073-1.