Paleoproterozoic postcollisional metamorphic and igneous activities in the Jinan area of the Jiao-Liao-Ji Belt in the North China Craton and their tectonic implications

Abstract

The Jiao-Liao-Ji Belt (JLJB) is a Paleoproterozoic collisional belt between the Longgang Block in the northeastern North China Craton and the Nangrim Massif on the Korean Peninsula. The Jinan area, located in the northeastern JLJB, was intruded by monzogranite at 2151 ± 6 Ma and then intruded again by porphyritic granites at 1842 ± 5 Ma in a postcollisional tectonic setting. The monzogranite and an associated gabbro were metamorphosed into metamonzogranite and metabasite at 1868 ± 11 Ma–1846 ± 3 Ma. Pelitic granulites occur together with the porphyritic granites. The pelitic granulites have peak metamorphic mineral assemblages of garnet + biotite + plagioclase + K-feldspar + sillimanite + quartz + ilmenite and underwent retrograde metamorphism, as evidenced by the presence of cordierite and muscovite. Pseudosection analysis of the granulites yields peak metamorphic conditions of ~8.5–7 kbar and 800–760 °C, and conventional geothermobarometry analysis of garnet rims and adjacent minerals yields retrograde metamorphic conditions of ~5–3.5 kbar and 740–680 °C. Dating of monazite from the two granulites demonstrates that peak metamorphism occurred between 1840 ± 2 Ma and 1847 ± 4 Ma. These results, together with the findings of previous studies, indicate that in the Liaodong and Jinan areas of the JLJB, igneous and metamorphic activities occurred together at ~1.85 Ga in a postcollisional tectonic setting, suggesting that collision between the Longgang Block and the Nangrim Massif occurred before ~1.85 Ga.