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Earth History of Asia- II October 31 - November 3, Niigata, Tapan
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.
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.