Geological And Geochemical Characteristics Of The Halengling Ophiolite With Prospecting Significance In The Sunidyouqi,Inner Mongolia

The Xingmeng orogenic belt is the southeastern segment of the Central Asian orogenic belt(CAOB),which is located between the North China plate and the Siberian plate.It is a complex orogenic belt with long history and complex tectonic magmatic activity.It is the key area to solve the tectonic evolution of the CAOB.The closure time,position and ocean-land transition process of the southern margin of the ancient Asian ocean have always been the focus of debate,such as the age and environment of the Paleo-oceanic basin and ophiolite.The late Paleozoic tectonic-magmatic process and the tectonic setting is also controversial.Ophiolite,which represents the remnant of the paleo-oceanic crust,plays an important role in identifying the paleo-suture zone and exploring the evolution history of the paleo-ocean.Based on detailed field geological investigation,petrology,petrogeochemistry and chronology,the early Permian ssz-type Halengling ophiolites have been newly identified in the Soren suture zone of Xingmeng orogenic belt in the Sunita Youqi area,central Inner Mongolia.The field investigation reveals that the Halengling ophiolites are composed of two sub-belts of ophiolites,the Chaganhushu in the north and the Chaganbaixing in the south,each with a bandwidth of about 5 km and a controlled extension of about 100 km,and a relatively complete assemblage of ophiolite rocks has been developed.The metamorphic peridotite is mainly lherzolite.The gabbro complex is mainly composed of layered gabbro and homogeneous gabbro,the diabase is developed.Basic volcanic lava is rich and consists of pillow basalts,massive basalts,spheroidal basalts and spilite.Typical ductile shear zones are developed in the ophiolite belt,and the local deformation of ophiolite is strong,forming mylonite,physic-chemical rocks and tectonic schist.The whole ophiolite belt is characterized by tectonic schist structure,which is located in the Shoushanggou formation and the Dashizhai formation of Lower Permian.The Shoushanggou formation and Dashizhai formation around the ophiolite belt generally occur dense splitting,fragmentation,magnolization and mylonite in a certain range and intensity.The gabbro,diabase and basalt are characterized by lightrare earth depleted and left tilted pattern of REE and have obvious negative Nb-Ta anomaly.The Halengling ophiolites in the Sunita Youqi are formed in the mature back-arc basin environment.The hf isotopic composition of zircon shows ? hf(t)=23.9-28.6,indicating that the zircon origin is mainly depleted mantle.The LA-ICP-MS zircon U-Pb ages of gabbro and spilite from ophiolites are 297 ±2Ma and 281.4 ±5.2Ma,respectively,indicating that the Halengling ophiolites were formed in the early Permian.Combined with other similar ophiolite in the Soren suture zone,it is suggested that there may be a late Paleozoic ocean-arc basin system in the Soren suture zone.This late Paleozoic ocean-basin system is consistent with the Permian paleo-paleogeographic boundary in the southeastern part of the Central Asian orogenic belt(i.e.the cold-water biota on the north side and the warm-water biota on the south side).Based on the above facts,we propose the following tectonic evolution processes for the southeast part of the Paleo-Asian Ocean:(1)from the late Ordovician to the Middle Permian,the Paleo-Asian Ocean continued to subductted southward under the North China plate;(2)from the Carboniferous to the Middle Permian,the Paleo-Asian Ocean continued northward subduction,forming an accretive orogenic belt on the north side,and(3)the southeast segment of the Paleo-Asian Ocean closed in the late Permian.By comparing the geological,geophysical and geochemical characteristics of typical deposits occurring in the Sauron ophiolite belt,it is believed that the potential mineralization of the early Permian SSZ ophiolite in the Halengling consists mainly of podiform chromite in mantle peridotite and magnesite in weathered/altered peridotite.Chromite mineralization is typical magmatic deposit.Magnesite mineralization is related to weathering and alteration in late diagenetic stage.The early Permian ophiolite in the Halengling ophiolite has the potential to explore pordform chromite and weathered magnesite.