Geochronological Constraints Of Mesoproterozoic Lower Boundary In Fengzhen Area,Inner Mongolia And Its Tectonic Implication

The North China Craton is one of the oldest cratons,and it is also the important area for the Precambrian geological research.The formation time of the basement and upper sedimentary cover of the North China Craton restricts the demarcation time between the Paleoproterozoic and Mesoproterozoic,and the orogenic movements and extension events of the North China Craton at ~1.8 Ga can also be compared with the formation and breakup of the global Columbia Supercontinent.This thesis provides evidences for the time boundary between the basement and sedimentary cover of the North China Craton by taking the geochronological study of tectonic cross section near the village of Geliugou in the Fengzhen area,Inner Mongolia.The study area lies in the central section of the North China Craton,which is adjacent to the Xing'an-Neimeng Orogen in the north.It is the junction of the Khondalite belt and Trans-North China Orogen.It has undergone many tectonic changes,metamorphisms,and magmatic activities since the Neoarchean.The geological structure is relatively complex.The metamorphic,intrusive,volcanic,and continental clastic rocks have been exposed,and metamorphism of local metamorphic rocks has reached moderate to high degree.The region has extensively developed the high degree metamorphic rocks of the Jining and the Xinghe groups,and few other metamorphic rocks have been reported.In the Geliugou section,it was discovered that the Early Mesoproterozoic metamorphic sedimentary rocks,the plagioclase leptite and plagioclase gneiss,directly overlay the Paleoproterozoic amphibolite intrusions by the angular unconformity.Through the LA-ICP-MS zircon-grain U-Pb dating,the concordia age of the magmatic zircon in amphibolite intrusion is 1795±16 Ma(MSWD = 0.91).The peak ages of the detrital zircon in the plagioclase leptite are divided into three groups,the first group is 1.78~1.82 Ga,the second is 2.02~2.06 Ga and the third is 2.26~2.3 Ga that is major peak ages.And the peak ages of detrital zircon in the plagioclase gneiss are divided into two groups,the first group is 1.7Ga~1.9 Ga with more grains,and the second is 2.25~2.3 Ga.In addition,the zircon concordia age of the gabbro lense is 1756±31 Ma(MSWD = 1.8).Based on the structural relationships and U-Pb age data,it is considered that the age of amphibolite intrusion restricts that the formation of basement in the North China Craton,is earlier than ~1.8 Ga,and that the oldest sedimentary age in the metasedimentary cover is ~1.7 Ga.The above "basement-cover" relationship might provides evidences for that 1.8 Ga should be the boundary age of the Paleoproterozoic/Mesoproterozoic in North China.In the North China Craton,the gabbro lense corresponds to the rift events represented by the synchronous mafic dyke swarms,indicating that the study area experienced extensional movement after the formation of the crystallized basement,which was associated with the breakup of global Columbia Supercontinent.In further,the age histogram of detrital zircons from metasedimentary rocks indicate that the source of the sedimentary cover originates from the Khondalites in the same area,and that there are rift basins at 2.3~2.1 Ga,which facilitates the deposition of the Khondalites.