HR: 14:48h
AN: V13C-06    [Abstracts]
TI: HP/UHP Eclogites From the Hong'an Block, China: Isotope Disequilibrium and Geochronological Controversy
AU: * Jahn, B
EM: jahn@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128, Section 2, Academia Road, Taipei, 115 Taiwan
AU: * Jahn, B
EM: jahn@earth.sinica.edu.tw
AF: Dept. of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei, 106 Taiwan
AU: Liu, X
EM: liuxcqw@yahoo.com.cn
AF: Dept. of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei, 106 Taiwan
AU: Liu, X
EM: liuxcqw@yahoo.com.cn
AF: Institute of Geomechanics, CAGS, 11, Minzhu Daxue Nanlu, Haidianqu, Beijing, 100081 China
AU: Yui, T
EM: tfyui@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128, Section 2, Academia Road, Taipei, 115 Taiwan
AB: The Hong'an Block (western Dabieshan) exposes a series of HP/UHP metamorphic rocks, with a S-to-N distribution from blueschist-greenschist, kyanite-free, to kyanite- and coesite-bearing eclogites. The Block appears to have preserved excellent archives of tectonic evolution of the Dabie Orogen. Geochronology plays a crucial role in our understanding of the processes but the published age data have shown much complexity as a result of the lack of isotopic equilibrium during the metamorphic recrystallization (ca. 700 to 500§C). In comparison with the Sulu and Dabie terranes, the metamorphic temperatures in the Hong'an Block are lower by 50 to 150§C. New trace element and Rb-Sr, Sm-Nd and O isotopic analyses on minerals provide much insight into the geochronological complexity. The present results indicate that: (1) Trace element distribution patterns suggest that garnet and omphacite in many cases are out of chemical equilibrium; and the presence of high-temperature LREE-rich mineral inclusions (e.g. epidote) in garnet and omphacite has further contributed to the isotope disequilibrium. (2) Sm-Nd isotope analyses yielded no isochron ages for the Hong'an eclogites. (3) Rb-Sr isotope analyses gave mixed results; in some cases, coexisting minerals are completely out of isotope equilibrium, and in others, isochron relationship is established. However, the pattern of Rb-Sr isotope disequilibrium is random and independent of the petrological and O-isotope temperatures. The established isochrons yield ages from 210 to 225 Ma. (4) In contrast to the disequilibrated Sm-Nd isotopic systems, oxygen isotopes of the eclogite minerals seem to have attained isotope equilibrium or near-equilibrium. Oxygen isotope temperatures are comparable with petrological temperatures. However, this is an apparent feature from the mass balance consideration. (5) Whole-rock d18O values show a large variation from +10% to -8%, suggesting that their protoliths have undergone very different processes of water-rock interaction. (6) In view of the overall geochronological information, we conclude that the HP/UHP metamorphism in the Hong'an Block took place in the Triassic at about 220-230 Ma. This is identical to that observed in the Dabie and Sulu terranes. A variety of Paleozoic dates (450 to 300 Ma) reported for the Xiongdian eclogite are still difficult to understand. However, any hypotheses advocating two periods of UHP metamorphic events for the same tectonic unit or in the same locality are beyond our comprehension.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting