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| Indian Ocean-MORB-type isotopic signature of Yushigou ophiolite in North Qilian Mountains and its implications | |
| 摘 要： | Many researchers have focused on the tectonic evolution of North Qilian Mountains (NQM) since the 1970s1―7]. However, the tectonic affinity of the an- cient oceanic mantle in early Paleozoic remains in de-bate. Three general explanations for it have been pro- posed. The first one suggests that the ancient ocean was a part of Proto-Tethys, and the tectonic evolution of NQM should be regarded as a portion of the562 Science in China: Series D Earth Sciences Tethyan tectonic domain1]. … |
| 收稿时间： | 3 March 2005 |
| 修稿时间： | 17 May 2005 |
Indian Ocean-MORB-type isotopic signature of Yushigou ophiolite in North Qilian Mountains and its implications |
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| Authors: | HOU Qingye ZHAO Zhidan ZHANG Hongfei ZHANG Benren CHEN Yuelong |
| Institution: | 1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China 2. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China 3. School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China |
| Abstract: | In order to explore the disputed issue concerning the tectonic affinity of the ancient ocean mantle of North Qilian Mountains (NQM), geochemical and Sr, Nd, Pb isotopic compositions of pillow basalts of the Yushigou Ophiolite (YSGO) suite from NQM have been analyzed systematically. The pillow basalts exhibit tholeiitic characteristics, with flat chondrite-normalized REE patterns ((La/Yb)N = 0.98–1.27). They display no Nb, Ta, Zr, Hf negative anomalies, and show MORB features in 2Nb-Zr/4-Y and Ti/100-Zr-Y × 3 tectonic discrimination diagrams. These results indicate that the Yushigou ophiolite is most likely to be formed in a mid-ocean ridge or mature back-arc basin. Their isotopic data show a relatively broad and enriched 87Sr/86Sr (0.70509–0.70700), restricted 143Nd/144Nd (0.512955–0.512978). Pb isotopes are in the range of 206Pb/204Pb (18.054–20.562), 207Pb/204Pb (15.537–15.743) and 208Pb/204Pb (38.068–38.530). These isotopic data imply that the basalts originated from the depleted mantle (DMM), with the involvement of enriched mantle components (mainly EMII). Geochemical comparisons between the basalts in YSGO and the MORB-type basalts of ophiolite suites occurring in the known ancient Tethyan tectonic domain indicate that the ancient oceanic mantle represented by YSGO suite forming in early Paleozoic in the North Qilian Mountains is very similar to the Tethyan mantle in both trace elements and isotopic compositions. The North Qilian Mountains should be a part of the Tethyan tectonic domain in early Paleozoic. This further implies that the Tethyan tectonic domain can be deduced to early Paleozoic in the study area, which will be helpful to discussing the tectonic affinity and evolution of the North Qilian Mountains. |
| Keywords: | geochemistry Indian Ocean-MORB-type isotopic signatures Tethyan tectonic domain Yu-shigou ophiolite North Qilian Mountains |
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