Formation age and geochemical characteristics of volcanic rocks in Manketou'ebo Formation in Kundu area of Ar Horqin Banner, Inner Mongolia

Volcanic rocks of Late Jurassic Manketou'ebo Formation in Kundu area of Inner Mongolia are mainly composed of a series of acid lava and volcaniclastic rocks. The LA-ICP-MS zircon U-Pb age is 157.7±1.2Ma (MSWD=0.39, n=26), which indicates its eruption in the late Late Jurassic. The geochemical analysis shows that these tuffs are peraluminous alkaline volcanic rocks with high Si, Al, low Ca, Mg, with the REE pattern gently inclined to the right and exhibiting negative Eu anomaly. The characteristics of trace elements suggest that the volcanic rocks are enriched in LILE (such as Rb and K) and HFSE (such as Th and U), and depleted in HFSE such as Nb and Ti, which implies that the source rocks are of crust genetic type. The zircons of the tuffs have ε_Hf(t) values ranging from+2.6 to+8.6, with two-age Hf model age being 590~925Ma, which suggests that the magma for these volcanic rocks was derived from the newly accreted Early Paleozoic-Neoproterozoic juvenile crustal materials. The tuffs of Manketou'ebo Formation possess the characteristics of A2-type granites, indicating that they were formed in a post-orogenic extensional environment, which was possibly related to the lithosphere extension after the closure of the Mongol-Okhotsk Ocean.