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

Abstract

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 εNd(780) values of the protolith rocks calculated from the Sm/Nd ratios of the samples deviate from the igneous initial εNd(t)–εHf(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 εHf(780) values calculated from the Lu/Hf ratios of the samples are nearly identical to the initial εHf(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 εNd(220)–εHf(220) compositions of the samples deviate from the εNd(t)–εHf(t) trend of intraplate lavas to high εHf(220) at a given εNd(220) and are within the arc lava field. The protolith εNd(780)–εHf(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.