Deciphering the origin of a basanite-alkali basalt-tholeiite suite using Zn isotopes

Abstract

Basanite-alkali basalt-tholeiite suites have unique chemical and isotopic signatures associated with recycled crustal materials. However, the relative contribution of recycled carbonates and silicates is unclear owing to the inability of traditional isotopic approaches (e.g., Sr, Nd, Pb, and O isotopes) to fully identify trace amounts of recycled crustal materials. On the other hand, the identification of the types of recycled components would help decipher origin of these magmatic suites. Since the zinc isotopic ratios (reported as the per mil deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard, δ66Zn) are distinct between marine carbonates and silicate reservoirs (e.g., mantle, oceanic mafic crust, and siliciclastic sediments), they could be used to identify the different source endmembers in magmatic rocks. In this work, we used the Zn and Sr-Nd isotopic ratios of Cenozoic continental basalts from the Central North China Craton (also known as Trans-North China Orogen, TNCO) to identify the nature of the recycled materials present in the mantle source of these basanite-alkali basalt-tholeiite suite. We found that, basanites and alkali basalts have high δ66Zn values (ranging from 0.32‰ to 0.46‰), while tholeiites have low δ66Zn values (0.28 ± 0.04‰, 2sd) similar to mid-oceanic ridge basalts (MORB) (δ66Zn = 0.27 ± 0.05‰, 2sd). Given the limited extent of Zn isotopic fractionation (<0.1‰) during crystallization and <20% partial melting, the elevated δ66Zn values observed in the basanites and alkali basalts reflect the involvement of recycled marine carbonates (which have an average δ66Zn of ∼0.91‰) in the mantle sources. Moreover, the δ66Zn values of these basalts show clear correlations with carbonate metasomatism indexes (such as, Ti/Eu and Zr/Hf ratios). Mixing models for δ66Zn values vs. 87Sr/86Sr ratios and εNd(t) values demonstrate that the mantle sources of basanites, alkali basalts, and tholeiites were metasomatized by different proportions of carbonates and siliceous sediments. Furthermore, both isotopic and geochemical compositions exhibit symmetrical variations from the central zone towards the south and the north along the TNCO, that is, δ66Zn values and 143Nd/144Nd and Ce/Pb ratios decrease, while SiO2 content and 87Sr/86Sr ratios increase. We suggest that more carbonates and fewer siliceous sediments are released from the subducted slab, with an increase in the depth of subduction. We further propose that this process is one of the most important causes of compositional variations in the basanite-alkali basalt-tholeiite suite.