Zircon Zirconium isotope fractionation during the arc magmatic differentiation and crustal reworking: Implications for the differentiation of the continental crust

Abstract

The upper continental crust is isotopically enriched in heavy stable zirconium (Zr) isotopes compared to the mantle, indicating significant Zr isotope fractionation during crustal differentiation, though the mechanisms remain elusive. Here, we present zircon in-situ Zr isotope data for two magmatic suites of distinct origins (the 91–88 Ma Lilong Complex and the 83–74 Ma Wolong granites) from the Gangdese arc lower crust in southern Tibet. The Lilong Complex, comprising garnet gabbro, quartz diorite, and tonalite, represents a continuous differentiation sequence of arc magma, whereas the Wolong granites were derived from partial melting of the migmatitic garnet gabbro in the Lilong Complex. Zircon from both suites exhibit consistent core-to-rim increases in δ^94/90Zr values, coupled with the negative correlation between δ^94/90Zr and Zr/Hf ratios, indicating that zircon preferentially incorporates light Zr isotopes during crystallization. In the Lilong Complex, zircon δ^94/90Zr increases with rising whole-rock SiO₂, suggesting gradual enrichment of heavy Zr isotopes during arc magma differentiation. Rayleigh modeling indicates that zircon crystallization primarily controls the Zr isotopic evolution of melts. The Wolong granites have higher zircon δ^94/90Zr than their garnet gabbro source, reflecting Zr isotope fractionation between melt and residue during partial melting. These observations indicate that enrichment in the heavy Zr isotopes in felsic magmas results from both fractional crystallization of isotopically light zircon and retention of light Zr isotopes in restites during partial melting. Our findings highlight the roles of magmatic differentiation and crustal reworking in arc settings in shaping the Zr isotopic composition of the continental crust.