LA-ICP-MS ZIRCON U-Pb DATING AND GEOCHEMISTRY OF THE VOLCANIC ROCKS FROM MANITU FORMATION IN ULANHOT REGION OF SOUTHERN DAXINGANLING

The Manitu Formation volcanic rocks in Ulanhot region of southern Daxinganling belong to an acid, acid intermediate and intermediate rock assemblage of basaltic andesite, andesite, dacite and trachydacite. The LA-ICP-MS zircon U-Pb dating results show that the volcanic rocks are (142.2±1.0) Ma in age, formed at the beginning of Early Cretaceous, belonging to high-K calc-alkaline series with 53.93%-68.44% of SiO₂ and 15.03%-17.82% of Al₂O₃, low total rare earth elements(ΣREE=103.48×10^-6-147.09×10^-6), distinct LREE and HREE fractionations with (La/Yb)_N=6.68-9.12 and weak positive Eu anomalies(δEu=1.04-1.25). They are enriched in large ion lithophile elements(LILEs, such as Rb, Ba, Th, U and K) and light rare earth elements(LREE), whereas depleted in high field strength elements (HFSEs, such as Nb, P and Ti). The study shows that the volcanic rocks of Manitu Formation were formed in the closing stage of MongoliaOkhotsk Ocean under orogenic collisional tectonic setting, and might be the partial melting product of different residual mineral phases in thickened lower crust.