Lithium isotopic compositions of post-collisional mafic–ultramafic rocks from Dabieshan, China: Implications for recycling of deeply subducted continental crust

Abstract

Lithium (Li) isotopes are potential tracers of processes related to crust–mantle interaction and recycled crustal materials. Previous studies have reported that Li isotopic compositions of island arc lavas are similar to those of fresh mid-ocean-ridge basalts (MORBs), although the former are expected to record high slab-derived δ7Li values caused by fluid metasomatism. However, it is unclear whether Li isotopic signatures of subducted continental crust were transported to mantle depths and preserved there. For clarification, we determined whole-rock major and trace element, and Li isotopic compositions of post-collisional mafic–ultramafic rocks from northern Dabieshan, China. The analyzed samples have high MgO (up to 20.67 wt%) and low SiO2 (41.06–56.20 wt%) contents, and arc-like trace element distribution patterns, suggesting derivation by partial melting of fertile mantle sources. The samples have δ7Li values of −2.9 to +7.2‰ (average = +2.7‰, n = 42) and Li concentrations of 2.50 to 28.1 μg/g (average = 9.20 μg/g), both of which are beyond the relatively limited ranges of mantle values (δ7Li = +2.9 to +4.3‰, average = +3.5 ± 1.0‰, and Li = 1.20 ± 0.10 μg/g). Excluding possible effects of wall-rock assimilation and contamination, magmatic crystallization, and Li isotopic fractionation during subduction–exhumation processes, the observed Li isotopic heterogeneity is most likely related to the recycling of crustal materials. Based on a comparison with Li isotopic compositions of ultrahigh-pressure (UHP) metamorphic rocks from the Dabie–Sulu orogenic belt and terrestrial sediments, we suggest that the recycled continental crustal materials likely comprised gneisses, sediments, eclogites, and/or minor carbonate rocks (i.e., marbles). Our study demonstrates that Li isotopes may provide a novel means of tracing deeply subducted continental materials.