Monazite And Detrital Zircon Geochronology Of The Chuanlinggou Formation In The Ming Tombs District,Beijing

The Changcheng Group,the first sedimentary cover of the North China Craton,is considered the beginning of the Mesoproterozoic in the regional stratigraphic chart in China.The study of the depositional age and source area are of great geological significance.Monazite grains and detrital zircons from five sedimentary samples,MC1 to MC5,which were collected from the lower part of the Chuanlinggou Formation in the Ming Tombs District,have been dated by the SHRIMP and LA-ICP-MS U-Pb methods.A lot of monazite grains are found in sample MC1 by using energy spectrum analysis.The maximum particle size of the monazites is up to 88 microns.According to lots of back-scattering images,the monazites show jagged edges and irregular shapes,appeared as fishes,birds,flowers,worms and so on.An overwhelming majority of the monazites are distributed in quartz or hematite veins.In the places far away from the veins,however,monazites hardly be found.These monazites with age of 152 ± 11 Ma are dated using SHRIMP.According to the preliminary age data,we suggest that these monazites belong to secondary monazites,which are associated with the late hydrothermal event,not formed in the diagenetic stage.All five samples show a major peak at ca.2.5 Ga and a secondary peak at ca.2.0 Ga in the age spectrum of the detrital zircons,suggesting their provenance is mainly from the North China Craton basement and Paleoproterozoic orogenic belt in the central part of the North China Craton.The youngest zircon has an age of 1673 ± 44 Ma,indicating that the Chuanlinggou Formation was deposited after this age.The age range of detrital zircons in sample MC3 is greater than in sample MC1 and MC2,suggesting that there was a transgressive event when the MC3 layer was deposited,and seawater carried ca.2.0 Ga clastic material to the basin in which the Chuanlinggou Formation was deposited,leading to the addition of ca.2.0 Ga detritus.From sample MC4 to MC5,the lithology changed from clastic to carbonate rocks,suggesting that the deposition basin became progressively deeper because of another transgressive event.Our research indicates that the source area for the sediments became more extensive with time.