Decoupling between Oxygen and Radiogenic Isotopes: Evidence for Generation of Juvenile Continental Crust by Partial Melting of Subducted Oceanic Crust

There is increasing evidence indicating that melts derived from subducted oceanic crust and sediments may have played a key role in building continental crust. This mechanism predicts that juvenile arc crust should have oxygen isotope characteristics ranging from mantle-like to supracrustal, but consistent mantle-like radiogenic (Nd-Hf) isotopic signatures. Here we present in-situ zircon U-Pb dating, Hf-O isotope analyses, and whole rock major-trace element and Nd isotope analyses of a granitoid from NW India. In-situ secondary ion mass spectrometry (SIMS) zircon U-Pb dating yields a weighted mean Pb-207/Pb-206 age of 873 +/- 6 Ma for the granitoid. It displays mantle-like zircon epsilon(Hf)(epsilon(Hf(873 Ma))=+9.3 to +10.9) and whole-rock Nd (epsilon(Nd(873 Ma))=+3.5) values but supracrustal delta O-18 values, the latter mostly varying between 9 parts per thousand and 10 parts per thousand. The calculated whole-rock delta O-18 value of 11.3 parts per thousand +/- 0.6 parts per thousand matches well with those of hydrothermally-altered pillow lavas and sheeted dykes from ophiolites. The major and trace element composition of the granitoid is similar to petrological experimental melts derived from a mixture of MORB+sediments. Thus, the granitoid most likely represents the product of partial melting of the uppermost oceanic crust (MORB+sediments). We propose that the decoupling between Hf-Nd and O isotopes as observed in this granitoid can be used as a powerful tool for the identification of slab melting contributing to juvenile continental crustal growth. Such isotopic decoupling can also account for high delta O-18 values observed in ancient juvenile continental crust, such as Archean tonalite-trondhjemite-granodiorite suites.

Automated summary

By studying a granitoid from northwest India, it was found that its major and trace element composition is similar to experimental melts derived from a mixture of MORB and sediments, indicating it is a product of partial melting. Additionally, the decoupling of Hf-Nd and oxygen isotopes in this granitoid can serve as a powerful tool for identifying slab melting contributing to juvenile continental crustal growth. Such isotopic decoupling may also explain the high delta O-18 values observed in ancient juvenile continental crust.