Abstract
The Statherian Period (1.8-1.6 Ga), currently the final period of the Paleoproterozoic Era, is named "stable, firm" for the interval following orogenesis (the Orosirian Period) that culminated in the assembly of likely Earth's first supercontinent, Columbia. Thus, the Paleoproterozoic-Mesoproterozoic boundary is supposed to demarcate increased continental "stability", defined as a transition from orogenesis and arc magmatism to unmetamor-phosed shallow-water cover sequences and intraplate magmatism. It has become apparent, however, that (i) the assembly of Columbia appears to be highly diachronous and (ii) many stable cover sequences once mapped as Mesoproterozoic appear to pre-date 1.6 Ga quite significantly. These discrepancies have led to die recent suggestion by a large group of Precambrian geologists that the Statherian Period belongs more to the Mesoproterozoic Era and thus that the end of the Paleoproterozoic Era should be redefined at ca. 1.8 Ga. North China Craton is one such craton that has pre-1.6 Ga stable sedimentary covers, but dating the base of the basins have proven difficult and thus other indicators of continental stability must be sought. Here we present new geochronological and geochemical data from two dyke swarms at 1.70 and 1.62 Ga from western Shandong Province (Luxi area) of the North China Craton that can be used to constrain the tectonic setting at this critical interval in question. Precise U-Pb SIMS analysis yields baddeleyite 207Pb/206Pb ages of 1700 ± 5 Ma (n = 9, MSWD = 0.14) and 1620 ± 4 Ma (n = 14, MSWD = 0.56). The new dated ca. 1.62 Ga Xiaobeizhuang dyke (330° trending, >15 m wide) belongs to the coeval Taishan dyke swarm. The ca. 1.70 Ga swarm, newly discovered in the North China Craton, is referred to as Ximaiyao dyke swarm (340-345° trending, 0.4-13 m wide). Paleogeographic affinities of the North China Craton with the Sao Francisco Craton in super-continent Columbia are considered in light of this newly identified swarm. Given that both dyke swarms pre-date 1.6 Ga and that their geochemistry implies intraplate magmatism, our new data support the suggestion that this time is more Mesoproterozoic than Paleoproterozoic in character. These pre-1.6 Ga intraplate dyke swarms, including the 1.78 Ga, the 1.73 Ga, and later 1.70 Ga and 1.68-1.62 Ga swarms, thus provide local support from North China Craton for redefining the base of the Mesoproterozoic Era as ca. 1.8 Ga and including the Statherian Period, consistent witii the ages of platform cover sequences.
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