Zircon U-Pb age and Hf isotope evidence for contrasting origin of bimodal protoliths for ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling project

A combined study of zircon morphology, U-Pb ages and Hf isotopes as well as whole-rock major and trace elements was carried out for ultrahigh-pressure (UHP) eclogite and felsic gneiss from the main hole (MH) of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. The results show contrasting Hf isotope compositions for bimodal UHP metaigneous rocks, pointing to contrasting origins for their protoliths (thus dual-bimodal compositions). The samples of interest were from two continuous core segments from CCSD MH at depths of 734.21-737.16 m (I) and 929.67-932.86 m (II) respectively. Zircon U-Pb dating for four samples from the two core segments yields two groups of ages at 784 +/- 17 and 222 +/- 3 Ma, respectively, corresponding to protolith formation during supercontinental rifting and metamorphic growth during continental collision. Although the Triassic UHP metamorphism significantly reset the zircon U-Pb system of UHP rocks, the Hf isotope compositions of igneous zircon can be used to trace their protolith origin. Contrasting types of initial Hf isotope ratios are, respectively, correlated with segments I and II, regardless of their lithochemistry. The first type shows positive epsilon(Hf)(t) values of 7.8 +/- 3.1 to 6.0 +/- 3.0, with young Hf model age of 1.03 and 1.11 Ga. The second type exhibits negative epsilon(Hf)(t) values of -6.9 +/- 1.6 to -9.1 +/- 1.1, with old Hf model ages of 2.11 and 2.25 Ga. It appears that the UHP rocks from the two segments have protoliths of contrasting origin. Consistent results are also obtained from their trace element compositions suggesting that mid-Neoproterozoic protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting at the northern margin of the South China Block. Thus, the first type of bimodal magmatism formed by rapid reworking of juvenile crust, whereas the second type of bimodal magmatism was principally generated by rift anatexis of Paleoproterozoic crust. Melting of orogenic lithosphere has potential to bring about bimodal magmatism with contrasting origins. Because arc-continent collision zones are the best place to accumulate both juvenile and ancient crusts, the contrasting types of bimodal magmatism are proposed to occur in an arc-continent collision orogen during the supercontinental rifting, in response to the attempted breakup of the supercontinent Rodinia at c. 780 Ma.