Zircon U-Pb age and geochemistry of the bojite in Dong Ujimqi, Inner Mongolia and its constrains on the Late Paleozoic tectonic evolution

The research area is located in the east of the Xing'anling-Mongolian Orogenic Belt,Dong Ujimqi of Inner Mongolia. The complex tectonomagmatic activity in this area is the key to solve the tectonic evolution of Xing'anling-Mongolian Orogenic Belt.This study carried out zircon U-Pb dating analyses and petrographical,geochemical for the bojites firstly recognized in the Dong Ujimqi,so as to understand the characteristics of the mantle nature and structure.The bojite generally extends north-east and presents like rock plant.In these rocks,main rock forming minerals are plagioclase,hornblende and pyroxene. The dating results indicate that the bojite was formed in the Late Carboniferous(299±3 Ma).The geochemical data of the samples show that: SiO₂(49.88%~51.98%),TiO₂(1.2%~1.86%),MgO(4.42%~7.41%),Al₂O₃(15.55%~16.84%),MnO(0.12%~0.17%),CaO(5.67%~6.52%),Na₂O+K₂O(3.35%~4.63%),m/f(0.41~0.49),Mg^#(52.56~60.82),TFeO(7.12~9.18).The standardized distribution pattern of REE chondrites is right-leaning with relatively enriched LREE.The rocks are enriched in LILE(Rb,Ba,K),and depleted in HFSE(Nb,Ta,Ti),LREE/HREE is stable,with negative Eu anomalies(δEu=0.87~0.98).The ε_Hf(t)values of zircons from the bojite vary between 5.1~11.25,and their Hf one-stage model ages vary from 1039 Ma to 604 Ma.Combined with regional tectonic evolution,it may be due to the subduction of oceanic crust,the thermal dehydration of subducted plate resulted in the partial melting of overlying depleted mantle and the generation of calc alkaline magma.In the process of ascending and emplacement,it was contaminated by crustal materials in a weak degree,which formed in the subduction environment of Paleo Asian Ocean.There was a small amount of basic magmatism in Dong Ujimqi in the Late Carboniferous—Early Permian,which indicated that the tectonic setting changed from subduction orogeny to post orogenic extension.