Polymetamorphic events in the Jiao-Liao-Ji Belt, North China Craton:Evidence from integrated zircon, xenotime, and monazite LA-ICP-MS U-Pb dating

摘要

Poly-metamorphism has been observed in many Precambrian metamorphic terranes. The metamorphic overprinting in the poly-metamorphic rocks makes it difficult to determine the timing and P - T path of each metamorphic event. In this study, in situ U-Pb dating of zircon, xenotime, and monazite, with simultaneously determination of trace element compositions, is utilized to provide new constraints on the timing of discrete metamorphic events involving reactivation of the Jiao-Liao-Ji Belt, the eastern edge of the North China Craton. Detrital zircons from metasandstones in the Gaixian Formation record a mixed provenance that includes late Neoarchean (ca. 2532 Ma) and middle Paleoproterozoic (ca. 2181 Ma). Xenotimes from the Gaixian pelitic schists yield an upper intercept age of 1906 +/- 25 Ma, and this age is interpreted as the timing of greenschist-facies regional metamorphism. However, monazites from the same pelitic schists yield a distinctly younger age of 242 Ma, with late Paleoproterozoic inherited ages of 1894 and 1809 Ma, which are broadly consistent with the xenotime ages. In addition, magmatic zircons from a granitic mylonite vein yield a similar age of 245 Ma, making it coeval with the monazite. The interpretation of these ages is also reinforced by the mineral assemblage of inclusions in the dated accessory mineral grains. These new geochronological data, combined with published data from the Jiao-Liao-Ji Belt and the related regions, suggest that the Gaixian Formation in the Yanghe area was likely some time deposited at 2181-1900 Ma, and experienced both late Paleoproterozoic and Triassic low-grade metamorphic events related to two distinct orogenic phases in the eastern North China Craton.