Zircon U-Pb age and geochemistry of the stratotype section of the Meiletu Formation in middle southern Da Hinggan Mountains and its geological significance

The Meiletu Formation is the highest strata volcanic rocks in Mesozoic in the southern central part of the Da Hinggan Mountains. The study of the layer section is of great significance for the division and regional correlation. In this paper, the authors carried out the study of the chronology and geochemistry of the Meiletu Formation layer section volcanic rocks, analyzed zircon UPb isotope dating for 4 pieces of andesite samples by using LA-ICP-MS technique, and yielded the ages of 127.7 ±1.4~133.4 ±1.5Ma, suggesting Early Cretaceous. A comparison with the similar volcanic rocks in China shows that the Meiletu Formation volcanic rocks have high content of SiO₂(57.95%~64.25%), TiO₂(0.69%~1.25%) and K₂O(0.55%~4.97%), low content of Fe₂O₃ (2.03%~4.32%), FeO(1.05%~3.77%), MgO(0.52%~2.44%) and CaO(2.22%~6.07%), and consistent MnO(0.05%~1.00%), Al₂O₃ (14.97%~17.61%)and Na₂O(2.61%~4.16%)content, suggesting aluminous-over-aluminous rock of high potassium Ca-alkali series. Lower rare earth (ΣREE) values are between 133.43×10^-6 and 181.7×10^-6, and there exists obvious regional difference in light and heavy rare earth elements(LREE/HREE=7.51~8.14, (La/Yb)_N=7.75~8.78), showing strong fractionation between the light rare earth and the heavy rare earth, with negative EU anomaly(δEu=0.78~0.95). The rocks are enriched in large ion lithophile elements (Rb, Ba, K, Pb, Sr) and LREE and depleted in high field-strength elements (Nb. Ta, Ti and P) and HREE. Magma came for the transitional mantle. The rocks were formed in a continental edge arc environment, and were related to the continental extension after the Pacific Plate subduction.