Zircon Hf-O isotope evidence for recycled oceanic and continental crust in the sources of alkaline rocks

Abstract

Post-orogenic and intraplate extensional alkaline rocks are believed to have formed by partial melting of metasomatized ultramafic rocks in the subcontinental lithospheric mantle. Their variable incompatible element abundances and radiogenic isotopic ratios reflect recycling of subducted oceanic or continental crustal material into their source reservoirs. However, whole-rock geochemical and Sr-Nd isotopic data do not well constrain the nature of this recycled crustal material because of complex petrogenetic processes, i.e., crustal assimilation, fractional crystallization, magma mixing, and post-magmatic alteration. Here we present an integrated study of in situ Hf-O isotopes in zircons from two Triassic alkaline complexes in the northeastern Sino-Korean craton, i.e., the Hekanzi and Saima alkaline complexes. The Hekanzi alkaline rocks have zircon eHf(t) values of —2.5 to +0.6 and sub-normal mantle δ18O (+3.8‰ to +5.4‰), indicating a component of high-temperature altered oceanic crust involved in their mantle source prior to magma generation. In contrast, the Saima alkaline rocks have extremely negative zircon eHf(t) (—11 to —14) and elevated δ18O (+7.1‰ to +8.4‰) values, suggesting continental crust recycled into their mantle source prior to magma generation. Our results show that the combined zircon Hf and O isotopes are powerful tracers to distinguish recycled continental crust from recycled oceanic crust and, importantly, provide compelling evidence for recycled continental crust in the sources of alkaline rocks.