Decoupling of the Lu-Hf, Sm-Nd, and Rb-Sr isotope systems in eclogites and a garnetite from the Sulu ultra-high pressure metamorphic terrane: Causes and implications

The whole-rock Hf, Sr and Nd isotope data of five high-Fe-Ti eclogites, nine high-Al eclogites, and a garnetite from the Sulu ultrahigh pressure (UHP) metamorphic terrane at eastern China were analyzed to resolve the causes for the decoupling of the Lu-Hf, Sm-Nd, and Rb-Sr isotope systems in these UHP rocks and to infer their protolith characteristics. Seven of the nine high-Al eclogites define an ⁸⁷Rb/⁸⁶Sr-⁸⁷Sr/⁸⁶Sr errorchron age of 192±43Ma (MSWD=2.8), which is within the time span of retrograde metamorphism despite the large uncertainty. The high-Fe-Ti eclogites and garnetite, however, have low ⁸⁷Rb/⁸⁶Sr ratios of <0.031 with scattered ⁸⁷Sr/⁸⁶Sr ratios of 0.7042-0.7058. Accordingly, it is inferred that the Rb-Sr isotope system in the samples reflects the effects of processes postdating the UHP metamorphism. Despite having different constituent mineral assemblages and whole rock geochemistry, the samples, however, define a ¹⁴⁷Sm/¹⁴⁴Nd-¹⁴³Nd/¹⁴⁴Nd errorchron age of 232±36Ma (MSWD=4.6). Although the uncertainty of ±36Ma implies incomplete Nd isotope equilibrium among the samples, the large overlap between this errorchron age span and the mineral isochron ages of 245-210Ma for the UHP metamorphism indicates the control of peak metamorphism on the Sm-Nd isotope system. The incomplete Nd isotope re-equilibration was accompanied by metamorphic modification on the Sm/Nd ratios as indicated by the U-shaped LREE patterns. The initial ε_Nd(780) values of the protolith rocks calculated from the Sm/Nd ratios of the samples deviate from the igneous initial ε_Nd(t)-ε_Hf(t) trend to significantly lower values, consistent with the metamorphic increase in the Sm/Nd ratios. In contrast, the Lu/Hf ratios are generally within the range for basalts and do not vary systematically with the ¹⁷⁶Hf/¹⁷⁷Hf ratios. The protolith ε_Hf(780) values calculated from the Lu/Hf ratios of the samples are nearly identical to the initial ε_Hf(t) values of the ~780Ma magmatic zircon cores from the Yangtze craton, suggesting the dominance of protolith characteristics on the Lu-Hf isotope system. Being controlled by different processes, the Rb-Sr, Sm-Nd, and Lu-Hf isotope systems of the investigated samples are therefore decoupled. The ε_Nd(220)-ε_Hf(220) compositions of the samples deviate from the ε_Nd(t)-ε_Hf(t) trend of intraplate lavas to high ε_Hf(220) at a given ε_Nd(220) and are within the arc lava field. The protolith ε_Nd(780)-ε_Hf(780) compositions calculated from metamorphic initials using igneous Sm/Nd and Lu/Hf ratios show a similar distribution pattern, strengthening the arc affinity of the eclogites. These arc signatures together with the felsic-mafic bimodal geochemical features of the UHP rocks from the Sulu terrane are explained as the characteristics of protoliths generated by backarc rifting.