Zircon Hf–O isotope evidence for crust–mantle interaction during continental deep subduction

In-situ SIMS zircon U–Pb dating and O isotope analysis as well as LA-(MC)-ICPMS zircon U–Pb dating and Lu–Hf isotope analysis were carried out for postcollisional mafic–ultramafic rocks in the Dabie orogen, China. The zircon U–Pb dating gave consistent ages of 126 ± 1 to 131 ± 1 Ma for magma crystallization. Survival of residual zircon cores is identified by CL imaging and U–Pb dating, yielding ages of 697 ± 10 and 770 ± 11 Ma that agree with protolith U–Pb ages of UHP metaigneous rocks in the Dabie orogen. The zircon Hf–O isotope compositions show systematic variations that can be categorized into three groups. Group I has the lowest δ18O values of 2.0 to 2.9‰ but the highest εHf(t) values of − 3.5 to − 1.0 with the youngest Hf model ages of 1.2 to 1.4 Ga. Group II displays intermediate δ18O values of 4.0 to 5.1‰ and εHf(t) values of − 22.5 to − 13.2 with Hf model ages of 2.0 to 2.6 Ga. Group III exhibits the highest δ18O values of 5.2 to 7.3‰ but the lowest εHf(t) values of − 29.1 to − 18.6 with the oldest Hf model ages of 2.4 to 3.0 Ga. The three groups of Hf–O isotope compositions correspond to a three-layer Hf–O isotope structure in the subducted continental crust, suggesting their involvement in the mantle source. Along with existing data for whole-rock Sr–Nd isotopes and trace elements, it appears that the mantle source for the postcollisional mafic–ultramafic rocks is characterized by fertile lithochemistry, the arc-like signature of trace elements, the heterogeneous enrichment of radiogenic isotopes, the differential incorporation of supracrustal materials, and the variable concentrations of water. Clearly, such a source is neither the asthenospheric mantle nor the refractory subcontinental lithospheric mantle (SCLM). It is a kind of the orogenic SCLM that would be generated by reaction of the overlying SCLM-wedge peridotite with hydrous silicate melts derived from different layers of the subducted continental crust. Therefore, the postcollisional mafic–ultramafic rocks provide a petrological record of crust–mantle interaction during the continental deep subduction.