Earth History of Asia- II October 31 - November 3, Niigata, Tapan ll An evaluation on using the Nd-Hf isotopic systematic as an indicator for the protolith characteristics of eclogites YANG, Huai-Jen, LIU, Yung-Hsin Department of Earth Sciences, National Cheng-Kung University, Tainan, Taiwan, 70101 , R.O.C. hj [email protected] edu.tw The nature ofprotohth of eclogites is one of the critical elements for re-constructing regional tectonic evolutions. However, identifying protohth characteristics of eclogites has been subjected to complications from metamorphic modifications on the protohth compositions, since a large number of constituent elements of eclogitic protohths are mobile during metamorphism at high P-T conditions. Jahn (1999) postulated that the positive and negative metamorphic initial eNd values of eclogites can be protohth indicators of oceanic and continental affinities, respectively. However, the metamorphic initial gNd values alone cannot determine whether the protohths were generated by subduction-related or intra-plate magmatism, handicapping investigations on tectonic evolution. In contrast to the eNd(t) values that evolve with rates depending on the abundance ratios of two REE (Sm/ the eHf(t) values are sensitive to REEHFSE (Lu-Hf) fractionation, providing additional constraints on protohth characteristics. This contribution presents an evaluation on using the Nd-Hf isotopic systematic to characterize the nature of the protohths of eclogites from the Sulu terrane at eastern China. Based on major and trace element compositions, the analyzed Sulu ultra-high pressure (UHP) eclogites are classified as high-Fe-Ti eclogites, group I high-Al eclogites, group II high-Al eclogites, and garnetites. Despite the compositional differences, these eclogites all deviate from the terrestrial eNd-sHf array, implying metamorphic modifications on the Sm-Nd or Lu-Hf or both of these two isotope systems. All these UHP rocks de丘ne an 143Nd! 44Nd-147Sm/144Nd errorchron of 232土 36 Ma (MSWD - 4.6), which largely overlaps with the peak metamorphism age of 220-245 Ma. The U-shaped LREE patterns of the high-Fe-Ti eclogites and group I high-Al eclogites provide additional evidence for the metamorphic modifications on the Sm-Nd isotope system of the protohths. In contrast, the Lu/Hi ratios of these eclogites are generally within the range for basalts and do not vary systematically with the 7 Hf/ 77Hf ratios. The range for the protolith initial Enf(780) values calculated from the Lu/Hf ratios of the samples is nearly identical to that for the -780 Ma magmatic zircon grains from the Yangtze craton, suggesting the dominance of protohth * Earth History of Asia- II October 31 - November 3, Niigata, Japan characteristics on the Lu-Hf isotope system. Being controlled by different processes, the Sm-Nd, and Lu-Hf isotope systems of the samples therefore are decoupled. Since the Sm/Nd ratios of the eclogites were modified by metamorphism, the protohth initial calculated from these values, eNd(780)meta, are not protohth indicators. The protohth initials, ENd(780)ig, calculated from the metamorphic initials using the Sm/Nd ratios inferred from the Sm/Nd-Lu/Hf trend of igneous rocks better characterize the protoliths. In the ENd(t)-」Hf(t) plot, the protohth initials and metamorphic initials of the eclogites distribute similarly with respect to the trend defined by the rocks formed at 0-1382 Ma by continental magmatism; specifically, higher eHf values at a given ENd and within the field for arc lavas. Metamorphosed from protohths generated by backarc rifting is finally proposed for the eclogite samples to account for their arc signatures and the felsic-mafic bimodal compositions of the Sum UHP rocks. References Jahn, B.-M., 1999. Sm-Nd isotope tracer study of UHP metamorphic rocks: implications for continental subduction and colhsional tectonics. International Geology Review 41, 859-885.
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