In situ zircon U-Pb dating and whole-rock geochemistry of metasedimentary rocks from South Liaohe Group, Jiao-Liao-Ji orogenic belt: Constraints on the depositional and metamorphic ages, and implications for tectonic setting
Abstract The Liaohe Group is an important Paleoproterozoic lithostratigraphic unit of the Jiao-Liao-Ji orogenic belt and traditionally subdivided into the North and South Liaohe Groups. However, their tectonic setting and evolution are still a matter of debate. The precise depositional and metamorphic data of the studied metasedimentary rocks from the South Liaohe Group provide important information on the controversial discussion of the tectonic setting and evolution. Based on cathodoluminescence (CL) imaging and connecting LA-ICP-MS U-Pb geochronology, zircon grains separated from metasedimentary rocks of different formations of the South Liaohe Group were subdivided into detrital and metamorphic zircons or zircon domains. Igneous zircon cores of detrital zircons of the Li’eryu, Gaojiayu, Dashiqiao and Gaixian Formations yield U-Pb age peaks at ∼2130 Ma, ∼2520 Ma, ∼2160 Ma/∼2500 Ma, and ∼2040 Ma, indicating that their sediments were transported from the Liaoji Granitoids, the Archean basement rocks of the adjacent Complex. In addition, deposition age of the sedimentary precursors was investigated which must have taken place at some time after ∼2.05 Ga, ∼2.07 Ga, ∼2.04 Ga and ∼1.92 Ga, respectively. These new geochronological data can be well explained by (1) a continent-arc collision model which suggests that the South Liaohe Group formed an intervening island arc and foreland basin sandwiched between the Longgang and Nangrim Complexes, and (2) a northward subduction of the Nangrim Complex source rocks of the South Liaohe Group; these source rocks were successively deposited along the margin of the two Complexes.