U-Pb zircon dating of the Paleoproterozoic khondalite series in the northeastern Helanshan region and its geological significance

Petrography, zircon cathodoluminescence, and laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon dating were performed on the Paleoproterozoic khondalite series and the intrusive altered diabase dykes developed in the northeastern Helanshan region were analyzed. The results showed that most of the zircons in the khondalite series were detrital zircons with oscillatory zoning and a high Th/U ratio, with few metamorphic zircons having a low Th/U ratio. The Pb-207/Pb-206 age of the detrital zircons ranged from 3131-1980 Ma, which constrained the protolith age of the Helanshan khondalite series to after 1980 Ma. The age of the metamorphic zircons indicated two age groups as follows: 1965-1921 Ma and 1876-1820 Ma. Besides, the age of altered diabase dykes was 1865-1850 Ma. In combination with previous studies, these new metamorphic ages indicated that the metamorphic events in the northeastern Helanshan region involved the collision followed by post-collisional extension and exhumation between the Yinshan Block to the north and the Ordos Block to the south. Moreover, the 1965-1921 Ma group represented the period of the collision between the Yinshan Block and the Ordos Block and the subsequent post-collisional extension event, whereas 1876-1820 Ma indicated the period of the exhumation stage.