THE EARLY JURASSIC ADAKITE IN SUNWU AREA, NORTHERN XIAOXINGANLING MOUNTAINS: Genetic Connection to the Activation of Nenjiang-Heihe Tectonic Belt

The quartz monzodiorite in Sunwu area, Northern Xiaoxinganling Mountains, is systematically studied on chronology and geochemistry. The LA-ICP-MS zircon dating yields the emplacement age of 174.9±1.2 Ma (Early Jurassic). Petrological and geochemical study indicates that the rock contains high SiO₂ and Al₂O₃ but low MgO, plus relatively enrichment of Na (Na₂O/K₂O in 1.51-2.53). It is high in Sr (>400×10^-6), low in Y (≤ 18×10^-6) and Yb (≤ 1.9×10^-6), with notable fractionation between LREE and HREE, (La/Yb)_N varying between 12.32 and 22.45. Besides, there is not significant negative Eu anomaly in the chondrite-normalized REE pattern. All of the geochemical characteristics above suggest that the quartz monzodiorite in Sunwu area belongs to adakite. The rock is originated from the partially melted thickened lower crust, and subject to the contamination of mantle material. Integrating with regional geology, it is concluded that the rock is formed under the tectonic system of Mongol-Okhotsk orogenic belt, affected by the activation of Nenjiang-Heihe tectonic belt. This area was in an intracontinental extension during the Early Jurassic Epoch.