Petrogenesis of the paleoproterozoic ultramafites in Wuchuan area, Inner Mongolia: implications for tectonic evolution

Introduction The east-west trending Khondalite Belt, located on the northern margin of the North China Craton, is linked to the Paleoproterozoic Columbia supercontinent’s evolution. However, the relationship between the Khondalite Belt formation and orogenic processes remains unclear. Methods Field investigations, petrology, zircon U-Pb dating, whole-rock geochemistry, and electron probe mineral thermobarometry. Results Garnet-bearing pyroxenite and serpentinite-like dunite in the ultramafites have zircon U-Pb ages of (1947 ± 17) Ma and (1960 ± 25) Ma, respectively. The rocks show characteristics of subalkaline tholeiitic basalt series, with low SiO 2 (35.79%–50.77%), TiO2 (0.01%–0.71%), Al 2 O 3 (0.17%–3.39%), and alkalis (0.02%–2.01%), but high Fe 2 O 3 (12.92%–15.06%). These rocks are enriched in light rare earth elements with slight depletion of Eu, enriched in large ion lithophile elements (Rb and K), and depleted in high field strength elements (P, Zr, and Hf). Discussion These metamorphic environments imply that the conditions for ultramafites formation were insufficient to induce granulite-facies metamorphism in the surrounding rocks, indicating that granulite-facies metamorphism in the Khondalite Belt is not closely related to post-orogenic extension in the Inner Mongolia-Northern Hebei orogenic belt.