The discovery of the Paleoproterozoic metamorphic rocks in Ulanhot, Inner Mongolia, and its geological significance

A set of amphibolite schist, diopside tremolite schist and amphibolite gneiss supracrustal rock assemblage was newly discovered in Dongbaiyinwusu area. The rocks have generally experienced high greenschist facies and low amphibolite facies metamorphism and the multiple tectonic transformation of plagioclase in later stage. The hornblende schist protolith comprised basic volcanic rocks, whose zircon crystal, cathodoluminescence, back scattering image and high Th/U ratio characteristics suggest magmatic zircon. By using zircon U-Pb dating LA-ICP-MS technology, the authors obtained the isotopic age of 1864.1±7.3Ma, suggesting Paleoproterozoic and representing magma formation age, which constrains the formation age of supracrustal rocks and proves the existence of Precambrian crystalline basement in Ulanhot area. A comparison with the Paleoproterozoic magmatic zircon ages of southern Songnen massif shows that the west margin of Songnen massif can extend to the Ulanhot area.