MIDDLE JURASSIC GRANITES IN HUOLONGMEN AREA, NORTHEASTERN DAXINGANLING MOUNTAINS: Zircon U-Pb Age, Geochemistry and Tectonic Implications

The Huolongmen area, tectonically located at the junction of Songnen and Xing'an blocks in the eastern Daxinganling-Mongolian orogenic belt, is dominated by the Late Paleozoic granite with a certain amount of Mesozoic granite, lithologically composed of monzogranite, granodiorite, quartz monzonite and alkali-feldspar granite. The zircon U-Pb age of 175.2±2.2 Ma of the monzogranite indicates the Middle Jurassic formation. The major elements of the rock are characterized by high K, rich Na, slightly rich Al, low Mg, and poor P and Ca. The trace elements show obvious enrichment of U, Th, Zr, Ta and Rb, and depletion of Ba, Sr, Nb, P and Ti. The REE content is relatively low, enriched in LREEs and depleted in HREEs with weak negative Eu anomaly and high degree of fractionation. The granites generally belong to high-K calc-alkaline series. The R₁-R₂ and Rb-(Yb+Nb), Rb-(Yb+Ta) diagrams reflect that the Middle Jurassic granite was formed in the tectonic environment from syn-collision to postorogenic stage. Combined with the distribution characteristics and tectonic setting of regional granites, it is thought that the Middle Jurassic granite in the study area was a product of continent-continent collision after the closure of Mongolia-Okhotsk Ocean.