Lithium isotopic systematics of A-type granites and their mafic enclaves: Further constraints on the Li isotopic composition of the continental crust

a r t i c l e i n f o Keywords: Lithium isotopes A-type granite Mafic enclave Continental crust Lithium concentrations and isotopic compositions of 39 A-type granites and mafic enclaves from 11 plutons in Northeast China and the North China craton are used to constrain their genesis and to characterize further the average composition of the continental crust. Lithium concentrations (2.8 to 80 ppm) in 29 A-type granites are slightly higher than those of average I-type granites and lower than those of average S-type granites, reflecting source differences. Lithium isotopic compositions (δ 7 Li = − 1.8 to + 6.9) of A-type granites fall within the range of worldwide I-type and S-type granites. By contrast, 10 mafic enclaves have significantly higher Li concentrations (32 to 179 ppm) and less variation in Li isotopic compositions (δ 7 Li = − 3.2 to + 3.1) than their corresponding granites, due to the high modal abundance of amphiboles and biotites. Overall, the weighted mean δ 7 Li for A-type granites, as well as I-type and S-type granites is lighter than that of the mantle. Lithium concentrations in granites vary as a function of fractional crystallization and are mainly controlled by modal mineralogy. Lithium mostly behaves as an incompatible element during granite differentiation, although Li is compatible in hornblende and biotite during crystallization of mafic enclaves. By contrast, Li isotopic compositions of A-type granites and mafic enclaves are not controlled by fractional crystallization, comparable to that for I-and S-type granites, indicating negligible Li isotope fractionation during granite differentiation. Instead, they reflect source heterogeneity produced by mixing of isotopically heterogeneous lower crust and/or variable amounts of re-equilibration between enclaves and host granites. The Li isotopic compositions of A-, I-and S-type granites can be used to derive an independent estimate of the average Li isotopic composition of the continental crust (i.e., δ 7 Li = + 1.7, weighted mean of all granites), comparable to our previous estimate (δ 7 Li = + 1.2) that relies on large-scale sampling of upper, middle and lower continental crust. Compared to the mantle, the continental crust has lower δ 7 Li, indicating the influence of weathering and recycling in controlling the Li isotopic composition of the continents. The composition of the continental crust places constraints on the mechanisms by which the crust has been generated and differentiated through time. However, the continental crust is heterogeneous in chemical and …