DISCOVERY AND GEOLOGICAL SIGNIFICANCE OF ADAKITIC ROCKS IN THE LATE PALEOZOIC GEGENAOBAO FORMATION IN ZHALANTUN AREA, MIDDLE DAXINGANLING MOUNTAINS

The Late Paleozoic adakites in Zhalantun area in the middle section of Daxinganling Mountains are composed of mainly andesite, trachyandesite, dacite and trachydacite. One of the andesite samples yields the zircon U-Pb age of (316.9±2.4) Ma by LA-ICP-MS, which represents the eruption age. The andesite has a high Si (54.97%-63.80%) and alkali content, with relatively high Na(Na₂O/K₂O>1). Besides, high Al(Al₂O₃=14.97%-17.69%) and Sr(715.98×10^-6-2100×10^-6)and low Y (12×10^-6-18.41×10^-6)and Yb (1.02×10^-6-1.91×10^-6)are also shown in this sample. The primitive mantle-normalized spider diagram of the andesite shows enriched LREE and depleted HREE, with slight positive Eu anomaly. The Mg^# values range from 0.35 to 0.57, averagely 0.46. Generally speaking, this andesite belongs to the high-K calc-alkaline adakite (C-type adakite), which should originate from the partial melting of thickened basaltic lower crust. The discovery of this high-K calc-alkaline adakite in Zhalantun area provides a new clue for the amalgamation of Xinganling and Songnen massifs, which has a tectonic significance for understanding the regional crustal evolution, and therefore is useful for locating the mineral deposits related to adakite.