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Zircon U-Pb dating for the earliest Neoproterozoic mafic magmatism in the southern margin of the North China Block [查看] LIUYongqing、GAOLinzhi、LIUYanxue、SONGBiao、WANGZongxiu

Huge diabase occurs within the Pro-terozoic strata in the southern margin of the North China Block, geographically in the northern parts of Anhui and Jiangsu provinces, where no strata younger than Neoproterozoic have been found. Zir-cons from the diabase show three different structures from cathodoluminesecence imaging, but they are all of magmatic origin. SHRIMP U-Pb dating yields a range of 206Pb/238U ages from 2600 to 124 Ma, but two geological events can clearly be identified from both concordant and discordant ages. One gives two weighted mean 207Pb/206Pb ages of 976 ± 24 and 1038 ± 26 Ma which are interpreted as timing of dia-base crystallization, and the other has a weighted mean 207Pb/206Pb age of 2523 ± 52 Ma that repre-sents formation time of inherited zircon from the old crust. The dating results indicate that the mafic magmatic event occurred in a transition period from the latest Mesoproterozoic to the earliest Neopro-terozoic, consistent with global subduction of Grenvill ocean crust to result in the assembly of superconti-nent Rodinia. Therefore, the North China Block needs to be taken into account when reconstructing the position of China continents within the framework of Rodinia supercontinent.

U-Pb zircon age for the Daohugou Biota at Ningcheng of Inner Mongolia and comments on related issues [查看] LIUYongqing、LIUYanxue、JIShu’an、YANGZhiqing

SHRIMP U-Pb zircon dating was carried out for the Daohugou Biota near Ningcheng of Inner Mongolia and for lavas overlying or underlying sala-mander-bearing strata at Reshuitang in Lingyuan of West Liaoning. The results suggest that the Dao-hugou Biota occurred at an interval from 168 Ma to 164-152 Ma. Both the Daohugou Biota and the salamander-bearing fossil assemblage are the same biota and thus developed from 168 to 152 Ma, i.e. from late Middle Jurassic to the early Late Jurassic. The Daohugou Biota-bearing rocks, resting on the Jiulongshan Formation in disconformity and being overlain in unconformity by Late Jurassic Tuchengzi Formation and Early Cretaceous rocks containing the Jehol Biota, are mainly composed of volcanic-sedi-mentary rocks in a normal sequence. It is recom-mended that the Daohugou Biota and the related stratigraphy should be correlated with the Tiaojishan Formation (Lanqi Formation in West Liaoning) or its synchronous rocks. It is suggested that the Dao-hugou Biota and the Jehol Biota would be neither taken into one biota nor considered as the earliest elements of the Jehol Biota. The Daohugou Biota and the related rocks and the Yixian Formation were re-spectively formed in different periods of vol-canic-sedimentary tectonics.

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