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

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Title: Zircon Hf–O isotope evidence for crust–mantle interaction during continental deep subduction
Authors: Dai, Li-Qun1, Zhao, Zi-Fu1 zfzhao@ustc.edu.cn, Zheng, Yong-Fei1, Li, Qiuli2, Yang, Yueheng2, Dai, Mengning3
Source: Earth & Planetary Science Letters. Aug2011, Vol. 308 Issue 1/2, p229-244. 16p.
Subjects: Uranium-lead dating, Radioisotopes in geology, Zircon, Subduction zones, Ultrabasic rocks, Geochronometry, Orogenic belts, Magmas, Crystallization, Earth's mantle, Earth (Planet)
Geographic Terms: China
Abstract: Abstract: 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±1Ma for magma crystallization. Survival of residual zircon cores is identified by CL imaging and U–Pb dating, yielding ages of 697±10 and 770±11Ma 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 1.4Ga. 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 2.6Ga. 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 3.0Ga. 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. [Copyright &y& Elsevier]
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Abstract:Abstract: 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±1Ma for magma crystallization. Survival of residual zircon cores is identified by CL imaging and U–Pb dating, yielding ages of 697±10 and 770±11Ma 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 1.4Ga. 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 2.6Ga. 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 3.0Ga. 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. [Copyright &y& Elsevier]
ISSN:0012821X
DOI:10.1016/j.epsl.2011.06.001