SHRIMP U-Pb zircon ages of high-pressure mafic and pelitic granulites and associated rocks in the Jiaobei massif: Constraints on the metamorphic ages of the Paleoproterozoic Jiao-Liao-Ji Belt 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 where high-pressure pelitic and mafic granulites have recently recognized in the Paleoproterozoic Jingshan Group. SHRIMP U-Pb geochronology combined with U-Th chemistry and cathodoluminescence (CL) imaging of zircon has been applied to determination of the timing of the metamorphism of the high-pressure granulites and associated rocks, which is critical to understanding of tectonic setting and evolution of the Paleoproterozoic Jiao-Liao-Ji Belt. U-Th chemistry and CL images reveal the presence of metamorphic zircons in these high-pressure pelitic and mafic granulites and associated rocks, which occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores, and are all characterized by nebulous zoning or being structureless, with high luminescence and very low Th/U ratios. Metamorphic zircons from two high-pressure mafic granulite samples yielded 207Pb/206Pb ages of 1956 ± 41 Ma and 1884 ± 24 Ma. One metamorphic zircon from a garnet-sillimanite gneiss sample (08JB02-1) also gives an apparent 207Pb/206Pb age of 1939 ± 15 Ma. These metamorphic ages are consistent with the metamorphic age group of 1.93-1.90 Ga previously obtained for the North and South Liaohe Groups and the Laoling Group in the northern segment of the Jiao-Liao-Ji Belt, and are considered to approximate the time of the peak metamorphism of the belt. Metamorphic zircons from one high-pressure pelitic granulite sample and two pelitic gneiss samples yielded weighted mean 207Pb/206Pb ages of 1821.2 ± 8.3 Ma, 1835.8 ± 7.8 Ma and 1837 ± 8 Ma, respectively. Two diopside-olivine-phlogopite marble samples contain large amounts of metamorphic zircons that yielded weighted mean 207Pb/206Pb ages of 1816.8 ± 9.2 Ma and 1790 ± 6 Ma. These metamrphic ages are largely in accordance with metamorphic ages of 1850 Ma produced from the Ji’an Group in the northern segment of the Jiao-Liao-Ji Belt and 1865-1800 Ma 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.