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Identification of 3.5 Ga detrital zircons from Yangtze craton in south china and the implication for Archean crust evolution

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Progress in Natural Science:Materials International 2006年第6期16卷 663-666页

作者： liu xiaoming, gao shan, ling wenli, yuan honglin and HU Zhaochu (National key laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, china university of Geosciences, Wuhan 430074, china key laboratory of continental dynamics, Department of Geology, northwest university, xi' an 710069, china) National key {1. of Geological Processes and Mineral Resources Faculty of Earth Sciences China University of Geosciences Wuhan 430074 China key {1. of continental dynamics Department of Geology Northwest University Xi'an 710069 China

The LA-ICP-MS U-Pb dating of hundreds of detrital zircon grains from the Sinian sandstones of Liantuo formation and tillites of Nantuo formation at Sanxia area in Yichang identified 331.-3508 Ma zircon grains. Their 207Pb/206Pb and 206Pb/238U ages show excellent agreement (concordia degree 99%-1.0% ). Their CL images exhibit well-developed oscillatory zoning and the Th/U ratios are within 0.46-0.76, implying that they are igneous zircons which formed during middle-early Archean. These zircons are the oldest ones discovered in Yangtze craton until now. However, the detrital zircons with ages older than 3. 3 Ga in the metamorphic rocks of Kongling group were not found by further investigation, which suggests the presence of crust older than high-grade metamorphic Kongling terrain in Yangtze craton.

关键词： early Archean detrital zircon zircon dating Yangtze craton.

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Geochemistry and U-Pb zircon geochronology of Late-Mesozoic lavas from xishan, Beijing

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Science china Earth Sciences 2006年第1期 50-67页

作者： yuan honglin1,{1., liu yongsheng2, gao shan1,2 & ling wenli2 1. key laboratory of continental dynamics, northwest university, xi’an 710069, china 2. Faculty of Earth Sciences, china university of Geosciences, Wuhan 430074, china

Zircon U-Pb dating by both SHRIMP and LA-ICP-MS and geochemical study of the Tiaojishan Formation and the Donglintai Formation from xishan, Beijing, reveal that ages of upper lavas of Tiaojishan Formation and Middle of Donglintai Formation are 137.1±4.5 Ma(2σ ) and 130-134 Ma, respectively. The fomer is slightly older than the latter and the age difference between these two formations is less than 5 Ma. These lines of evidence prove that the two volcanoes erupted within a short time. The age of the Tiaojishan Formation from xishan, Beijing is distinctively different from that of the Chende Basin. This indicated that the ages of Tiaojishan lavas varied in different regions. The Tiaojishan Formation consists of typical adakite (SiO2=56%, Na2O = 3.99-6.17, Na2O/K2O = 2.2-3.1, Sr = 680-1074×10?6, Y = 13.2-16.3×10?6, Yb = 1.13-1.52×10?6, Sr/Y = 43-66), high-Mg adakite and high-Mg andesite (Mg# = 54-55). Features of continental crust of adakite from the Tiao-jishan Formation and its syngeneric middle silicic vocanic rocks, such as typical Nd-Ta negative ab-normality and Pb possive abnormality, indicate that these lavas are originated from partial melts of continental crust. These results suggest that the adakite from the Tiaojishan Formation of xishan, Beijing derived from thickened eclogitic lower crust and lithosphere beneath the North china craton at mesozoic that was foundered into the aesthenosphere, and subsequenctly partially melted and in-teracted with mantle olivine during melts upward migration. The age of lavas from the Tiaojishan Formation restrained the foundation which should last at least until 137 Ma. Lavas of the Donglintai Formation are rhyolith and andesite with normal Mg# and thus they did not interact with the mantle. These lavas represent remobilized melts of lower crust material caused by mantle aesthenosphere upwelling migration induced by foundation.

关键词： adakite eclogite geochemistry North china craton lower crust foundation.

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