The role of sub-continental lithosphere mantle in deep F and Cl cycling: Insight from the Dabeigou basalts, North China Craton

The sub-continental lithosphere mantle (SCLM) represents a mechanical barrier that separate the converting mantle from the exosphere, and possesses the ability to sequester and release volatiles, making it an integral part in deep volatile cycling. Here we explore the SCLM in modulating F and Cl cycling through a case study from the Early Cretaceous Dabeigou basalts in the North China Craton (NCC). These lavas are associated with decratonization of the NCC, a process involving significant removal and replacement of the ancient SCLM. The Dabeigou basalts, characterized by arc magma-like trace element signatures, evolved initial Sr-87/Sr-86 (0.70574-0.70578), epsilon(Nd)(t) (-11.2--11.1), and epsilon(Hf)(t) (-13.7--13.4) values, along with high Fe/Mn ratios, Ni contents, and delta O-18 values (5.8 parts per thousand-6.8 epsilon) of olivine phenocrysts, are compatible with their origination from a veined, pyroxene-rich source formed through the infiltration and interaction of hydrous silicate melts from the subducting Pacific slabs with ambient SCLM. Olivine-hosted melt inclusions of the Dabeigou basalts have compositions similar to the whole rocks, indicating that they record pre-eruptive melt compositions forming the whole rocks. The melt inclusions exhibit high F (1371-2316 ppm) and moderate Cl (688-900 ppm) contents, with high F/Cl ratios of 2.1 +/- 0.3 (1 sigma). The F and Cl signatures of the Dabeigou basalts are different from those of typical arc magmas, which are commonly characterized by high Cl contents (median of similar to 1200 ppm) and low F/Cl ratios (<1). The observed differences can be attributed to chromatographic reaction that fractionates elements of different compatibility in the metasomatic agents, with amphibole as the main mineral phase for fractionating and preserving F and Cl. That is, amphibole crystallization prefers to retain F rather than Cl, making the metasomes in vein enriched in F and exhibiting elevated F/Cl ratios compared to original metasomatic agents. It is further suggested that the scenario inferred from the Dabeigou basalts might be common for crystallization of hydrous minerals during chromatographic metasomatism, which causes significant heterogeneity with respect to F and Cl in the SCLM, and might have played a crucial role in regulating F and Cl cycling within the SCLM environments.