Zinc isotopic systematics of deep crustal xenoliths from the southeastern North China craton and implications for intra-crustal differentiation.

Intra-crustal differentiation of primary arc magmas has been suggested to play an important role in continental crust growth. Here we use the zinc (Zn) isotopic systematics of deep continental crustal rocks and island arc basalts to further assess this differentiation model. We present high-precision Zn isotope data for late Archaean deep crustal xenoliths from the southeastern North China craton and early Cretaceous island arc basalts from the Central Philippines. These xenoliths have chemical compositions broadly similar to the globally average middle and lower continental crust and display trace elemental patterns typical of arc magmas (e.g., enrichment in LILEs and depletion in HFSEs). We obtain average δ66Zn JMC-Lyon values of 0.31 ± 0.03‰ (2se, n = 28) and 0.27 ± 0.04‰ (2se, n = 7) for the lower and middle continental crust, respectively, both of which are slightly but statistically higher than the global island arc magma average (δ66Zn = 0.22 ± 0.01‰, 2se, n = 64). This difference cannot be attributed to diffusion, metamorphism, metasomatism and fractionation of clinopyroxene or Fe Ti oxide of the lower crustal rocks or their igneous protoliths. Instead, the gradual increase of δ66Zn with decreasing (Dy/Yb) N (subscript N represents chondrite-normalization) in the studied lower crustal xenoliths points to fractionation of isotopically light amphibole during intra-crustal differentiation of their igneous protoliths. In addition, we find that island arc magmas generated in thicker arcs have systematically higher δ66Zn than those in thinner arcs, and (La/Yb) N and (Dy/Yb) N ratios display negative and positive correlations with MgO contents in global island arc lavas, respectively. These observations collectively suggest that amphibole fractionation plays an important role in shifting the chemical compositions of arc lavas and making the andesitic bulk continental crust. • A new estimate of average δ66Zn of the lower continental crust is 0.31 ± 0.03‰ (2se). • The average δ66Zn of global island arc lavas (0.22 ± 0.01‰, 2se) is lower than that of the average lower continental crust. • δ66Zn values of island arc lavas positively correlate with arc thickness. • Amphibole fractionation exerts an important control on arc magma differentiation and continental crustal formation. [ABSTRACT FROM AUTHOR]