ZIRCON U-Pb AGE AND Hf ISOTOPIC CHARACTERISTICS OF THE GNEISSIC GRANITES IN LAOZUOSHAN GOLDFIELD OF HEILONGJIANG PROVINCE: Geological Implication

The Laozuoshan gold deposit in Heilongjiang Province is a large hydrothermal superimposed skarn deposit in the north-central of Jiamusi block. Three groups of age, 697.1±8.0, 703.0±8.8, and 484.7±2.7 Ma respectively, are acquired through the LA-ICP-MS U-Pb dating and Hf isotope analysis of gneissic granites in the east ore belt of orefield, among which the former two are the age of captured zircons, suggesting that there might be Neoproterozoic crystalline basement in the south margin of Jiamusi block, and the latter represents the crystallization ages of gneissic granites. The zircon Hf isotope tests show that the ε_Hf(t) ranges from -2.4 to +3.6, with T_DM2 of 1620-1390 Ma, indicating that the protolith of gneissic granite may be the derived from the remelting of Mesoproterozoic continental crust material. Combined with the regional tectonic evolution, it is believed that the gneissic granite was possibly formed by the late Pan-African-early Caledonian collision-orogeny as the product of anatectic magma crystallization of Mashan Group.