Thermochronology Of Mesoproterozoic Mantle?derived Magmatic Rocks And Their Tectonic Significance In Siziwang,Inner Mongolia

The study area is located in Jisheng Tai and Dajingpo Township,northwest of Siziwangqi City,Inner Mongolia where are two gabbro bodies.The tectonic relationship is on the northern margin of North China Craton.The two gabbro bodies intruded into the Durahala and Bilute Formations of the Mesoproterozoic Great Wall System of Baiyun Obo Group,respectively.Through field investigations,it was found that the Jisheng Tai gabbro is divided into fine-grain gabbro,Jisheng Tai medium-grain gabbro,and Jisheng coarse-grain gabbro based on contact relationship and particle size.Zircon U-Pb isotope dating was performed on the coarse-grained gabbro in the Jishengtai area.The measured age is 1574.3±8.4 Ma?MSWD=0.64?,which represents the emplacement age of the Jisheng-coarse gabbro.In this paper,three phases of gabbro were studied by rock geochemistry.It is believed that the Jisheng fine-grained gabbro is the source of non-orogenic magma,the coarse-grained gabbro is the source of intraplate basalt,and the Dajingpo gabbro is the source characterized by island arc basalt magma.The formation of gabbro in Jisheng Taihe and Dajingpo is a reflection of the three-stage mesoproterozoic extension events in the North China Craton.The North China Craton stretched and cracked.During this period,a large number of magmatic rock events developed,which were 1.72-1.62 Ga basic magmatic events,about 1.6 Ga magmatic events and1.4-1.2 Ga-based magmatic events.The hornblende ⁴⁰Ar-³⁹Ar dating of the Jishengtai rock body and the plateau age shows that the ⁴⁰Ar-³⁹Ar hornblende age of the Jishengtai rock body is 1258.56±25.43 Ma?MSWD=145.01?.Combined with the zircon age,the cooling rate of the Jishengtai rock mass is calculated to be 0.62?/Ma,which represents the uplift rate of the Jishengtai rock mass under the action of the North China Craton extension.The hornblende Ar isotope plate age of the Dajingpo gabbro body is 535.34±109.28 Ma?MSWD=451.27?,and the Ar isotope age of the Dajingpo gabbro is lower,indicating that it may be modified by subsequent thermal events and represents North China's age of tectonic thermal events in the late period.Regarding the extension of 1.8-1.0 Ga in the Mesoproterozoic of the North China Craton,this article will be divided into four periods based on the previous studies.The first stage is 1.8-1.78 Ga,which is mainly developed in the Yanliao rift trough the basic rock wall group in the Luliang area.The second stage is mainly 1700 Ma-16200Ma basic magmatic event,and the third stage is 1573-1552 Ma The basic-ultrabasic magmatic event,the fourth phase is the 1.43-1.23 Ga magmatic event.The disintegration of the North China Craton represents the disintegration process of the Columbia supercontinent.Combined with the research in this paper,it is shown that the Columbia supercontinent has begun to split before 1.67 Ga.