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 delta O-18 values of 2.0 to 2.9 parts per thousand but the highest epsilon(Hf)(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 delta O-18 values of 4.0 to 5.1 parts per thousand and epsilon(Hf)(t) values of -22.5 to -13.2 with Hf model ages of 2.0 to 2.6 Ga. Group III exhibits the highest delta O-18 values of 5.2 to 7.3 parts per thousand but the lowest epsilon(Hf)(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. (C) 2011 Elsevier B.V. All rights reserved.