HR: 1330h
AN: V22B-0589    [PDF]
TI: An Oceanic Protolith for the UHP Eclogites and Garnetites from Western Sulu, Northeast of China
AU: * Liu, Y
EM: l4890106@ccmail.ncku.edu.tw
AF: Department of Earth Sciences, National Cheng Kung University, 1, Ta-Hsueh Road, Tainan, 701 Taiwan
AU: Yang, H
EM: hjyang@mail.ncku.edu.tw
AF: Department of Earth Sciences, National Cheng Kung University, 1, Ta-Hsueh Road, Tainan, 701 Taiwan
AU: Yu, S
EM: scyu@mail.ncku.edu.tw
AF: Department of Earth Sciences, National Cheng Kung University, 1, Ta-Hsueh Road, Tainan, 701 Taiwan
AU: Chen, W
EM: dc@earth.ncu.edu.tw
AF: Department of Earth Sciences, National Central University, 300, Jung-Da Road, Jung-Li, 320 Taiwan
AU: Xu, Z
EM: xuzhiqin@ccsd.org.cn
AF: Institute of Geology, Chinese Academy of Geological Science, 26, Baiwanzhuang Road, Beijing, 100037 China
AU: Yang, J
EM: yangjisui@ccsd.org.cn
AF: Institute of Geology, Chinese Academy of Geological Science, 26, Baiwanzhuang Road, Beijing, 100037 China
AB: The occurrence of coesite in the Dabei-Sulu eclogites led to the establishment of the ultra-high pressure origin for the Dabei-Sulu metamorphic terranes. So far, all of the analyzed Dabei-Sulu eclogites are characterized by enrichment in highly incompatible elements and positive $\epsilon$Nd and $\delta$$^{18}$O values. Therefore, a continental protolith was inferred. Although not impossible, subduction of a large volume of continental protolith to ultra-high pressure regime is uncommon for its low density. To overcome this dilemma, it was proposed that the Dabei-Sulu continental protolith was dragged by an oceanic lithosphere in the subduction front. Upon reaching the ultra-high pressure regime, the break-off of the oceanic lithosphere led to the re-bounce of the metamorphic continental lithosphere back to the Earth­Ýs surface. However, the absence of outcropped oceanic protolith weakens this model to some extents. For the first time, we discover eclogitic samples with oceanic signatures from western Sulu terrane. Rutile and kyanite eclogites along with garnetites were sampled from Maobei western Sulu near the site of Chinese Continental Scientific Drilling project. Most of these eclogites show general depletions in LREE with positive eNd values, reflecting characteristics of oceanic lithosphere. In the $^{87}$Sr/$^{86}$Sr - $\epsilon$Nd space, some Maobei eclogites deviate from the mantle array toward higher $^{87}$Sr/$^{86}$Sr values, consistent with seawater alteration. These Maobei eclogites have REE abundances lower than the Pacific MORB but comparable to the Oman ophiolites, which are representatives of cumulates from N-MORB-like melts. The similarities in the major element distributions between Maobei kyanite eclogites and Oman olivine gabbro further support the oceanic affinity of Maobei kyanite eclogites. The rutile eclogites, however, are lower in MgO but higher in TiO$_{2}$ and total iron contents with slight enrichments in La and Ce compared to the Oman ophiolites. We interpret these geochemical signatures as consequences of fluid metasomatism, which may be a major process for rutile formation. Another type of fluid metasomatism caused the development of amphibole veins resulting in the concaved down REE pattern in one of the studied eclogites. Finally, a kyanite eclogite is distinct from other Maobei eclogites for its LREE enriched REE pattern and a lower $\epsilon$Nd of -5 indicating involvement of some proportions of continental materials. In summary, the studied Maobei eclogites are representatives of cumulate assemblages from MORB-like melts with some samples show fingerprints of fluid metasomatism and addition of continental materials. It is, therefore, inferred that there existed an oceanic lithosphere between the Yangtze and Sino-Korean cratons providing supports for the slab break-off and re-bounce model for the formation of Dabei-Sulu UHP terranes.
DE: 1040 Isotopic composition/chemistry
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting