Timing of metamorphism in the Paleoproterozoic Jiao-Liao-Ji Belt: New SHRIMP U-Pb zircon dating of granulites, gneisses and marbles of the Jiaobei massif in the North China Craton

Abstract

The Paleoproterozoic Jiao-Liao-Ji Belt lies in the Eastern Block of the North China Craton, with its southern segment extending across the Bohai Sea into the Jiaobei massif. High-pressure pelitic and mafic granulites have been recently recognized in the Paleoproterozoic Jingshan Group (Jiaobei massif). New SHRIMP U-Th-Pb geochronology combined with cathodoluminescence (CL) imaging of zircon has been applied to the determination of the timing of the metamorphism of the high-temperature and high-pressure granulites and associated gneisses and marbles. Metamorphic zircons in these high-pressure granulites, gneisses and marbles occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores. Metamorphic zircons are all characterized by nebulous zoning or being structureless, with high luminescence and relatively low Th/U values. Metamorphic zircons from two high-pressure mafic granulites yielded 207Pb/206Pb ages of 1956±41Ma and 1884±24Ma. One metamorphic zircon from a garnet-sillimanite gneiss also gave an apparent 207Pb/206Pb age of 1939±15Ma. These results are consistent with interval of ages of c. 1.93-1.90Ga already obtained by previous studies for the North and South Liaohe Groups and the Laoling Group in the northern segment of the Jiao-Liao-Ji Belt. Metamorphic zircons from a high-pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207Pb/206Pb ages of 1837±8Ma, 1821±8Ma and 1836±8Ma respectively. Two diopside-olivine-phlogopite marbles yielded weighted mean 207Pb/206Pb ages of 1817±9Ma and 1790±6Ma. These Paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c. 1.85Ga produced from the Ji'an Group in the northern segment of the Jiao-Liao-Ji Belt and c. 1.86-1.80Ga obtained for the high-pressure pelitic granulites from the Jingshan Group in the southern segment. As this metamorphic event was coeval with the emplacement of A-type granites in the Jiao-Liao-Ji Belt and its adjacent areas, it is interpreted as having resulted from a post-orogenic or anorogenic extensional event. © 2010 International Association for Gondwana Research.