Zircon U-Pb dating and study on geochemistry of volcanic rocks in Manitu Formation from southern Manchuria, Inner Mongolia

Petrographically, the volcanic rocks in Manitu Formation from southern Manchuria are composed of andesite, trachyandesite and trachyte, but they are mainly trachyte with minor rhyolite based on chemical composition. Zircon U-Pb dating results show that they formed at Late Jurassic from 146 to 158Ma. These volcanic rocks are alkaline series in composition and rich in alkali and potassium; they are high in total rare earth elements (Sigma REE = 328. 2 x 10(-6) similar to 419. 2 x 10(-6)), highly fractionated LREE from HREE ((La/Yb)(N) = 12. 87 similar to 23. 10) and moderately negative Eu anomalies (delta Eu = 0. 41 similar to 0.70). They contain high LILE (e. g., Rb and K) and LREE, high ratios of Rb/Sr (0. 34 similar to 1. 88) and positive values of epsilon(Hf)(t) (3.68 similar to 8. 65), but low HFSE (e. g., Nb, Ta, P and Ti). The initial Hf-176/Hf-177 ratios for magmatic zircons from these volcanic rocks vary in the range of 0.282780 similar to 0. 282922, and the two-stage model ages of 652 similar to 972Ma. The data suggest that the magmas for these volcanic rocks derived from the newly accreted Neoproterozoic basic igneous crusts, and formed during the extensional setting of lithosphere after the closure of Mongol-Okhotsk Ocean.