Zircon U–Pb Age and Tectonic Setting of the Manitu Formation in the Middle–East Inner Mongolia, China

The Manitu Formation widely occurs in the middle–east Inner Mongolia Autonomous region, and was thought to be formed in the late Jurassic together with the regional Manketouebo Formation and the Baiyingaolao Formation. However, the Nestoriapissovi fossils in the andesite interlayer of the Manitu Formation indicate this formation was formed during the Jibei Stage of the early cretaceous by previous studies. In addition, the zircon U–Pb ages for the volcanic rocks in the Manitu Formation have a wide range, and the tectonic setting is still under debate. Therefore, this study carried out the analyses of zircon U–Pb dating and whole–rock compositions to examine the age and tectonic setting of the Manitu Formation. The zircon U–Pb ages for three samples collected from Zarutqi–Bahrainyouqi are 142.4±1.4Ma, 130.0±1.4Ma and 145.0±1.5Ma, respectively, suggesting the Manitu Formation in the Zarutqi–Bahrainyouqi area were formed in the early Cretaceous. Rocks from the Manitu Formation have SiO₂, Al₂O₃, K₂O and Na₂O of 59.43%～64.73%, 15.66%～17.19%, 2.56%～5.03%, 3.59%～4.53%, respectively, with ALK of 6.36%～9.56%. They have high concentration of LILE and LREE, and depletion of Nb, Sr, P and Ti. Total rare earth element (ΣREE) concentration ranges from 118.71×10^–6 to 189.99×10^–6, and LREE and HREE are highly fractionated with (La/Yb)_N of 8.88～9.58, with negative Eu anomalies (δEu=0.66～0.95), showing an island arc–like geochemistry. We propose that the formation age becomes younger from the west to east of the Great Xing’an Mountain based on the comparison of ages of rocks from the west, middle and east segment, and this may ascribed to the closure of the Mongol–Okhotsk ocean and the subsequent post–collision extension.