GEOCHRONOLOGY AND TECTONIC SETTING OF LUOMAHU COMPLEX IN WODUHE AREA, NORTHWESTERN DAXINGANLING MOUNTAINS

Due to the lack of fossils and incomplete roof and bottom, the forming age of Luomahu Complex, one of the important geological formations in Duobaoshan ore concentration area, has been controversial, restricting the cognition of regional tectonic evolution and metallogenic background. The geochronology of detrital zircons in meta-silty mudstones from Luomahu Complex near Wuduhe Township are studied by zircon U-Pb LA-ICP-MS dating method to investigate the formation age and paleotectonic setting. Among the 85 survey points, 68 effective ones range in age from 216 to 2 636 Ma and can be divided into eight age groups: 216-347 Ma, 430-443 Ma, 463-502 Ma, 513-572 Ma, 596-613 Ma, 718-739 Ma, 830-882 Ma and 1 769-2 636 Ma, with the third to seventh groups forming obvious peak values. Based on the comprehensive analysis of zircon crystalline state, CL image, U content characteristics and macro petrographic features of geological body, it is considered that the third age group represents the maximum sedimentary age of sample layers. Combined with the regional research results, it is concluded that Luomahu Complex was formed in Ordovician-Silurian Ludlovian, with the detrital zircons mainly derived from the Erguna-Xing'an block and Ordovician arc volcanic rocks and formed in the tectonic setting of back-arc basin. The Luomahu Complex, together with Duobaoshan arc magmatic belt, formed the Early Paleozoic arc-basin system.