Detrital Zircon Geochronology Of The Late Paleozoic Sedimentary Rocks From The Gongshan-Baoshan Blocks In Yunnan Province And Its Tectonic Significance

Gongshan-Baoshan block is located in the Sanjiang area of Yunnan Province,on the southeast side of the Qinghai-Tibet Plateau.It is the northern part of the Sibumasu block.It is bounded by the Gaoligong shear zone in the west.Its east side is the Lancangjiang(Changning-Menglian)suture zone,and further east are the Indosinian block and the Honghe-Ailaoshan shear zone.This region was generally considered as the escape passage in corresponding to the collision and compression between the Indian and Eurasian plates,with a very complicated tectonics.This paper took the Permian-Carboniferous Dingjiazhai Formation quartz sandstone in Baoshan Block and the Triassic Chawalong Group quartz sandstone in Gongshan Block as main research objects.Through studies of field geological observation,petrology,zircon U-Pb geochronology,as well as age comparison to this paper adjacent blocks,this paper gave a full discussion of the tectonic evolution of Gongshan-Baoshan Block and its provenance.The detrital zircon ages in three quartz sandstone samples(NJ68,NJ70,NJ71)of the Permian-Carboniferous Dingjiazhai Formation in Baoshan Block range from 3427 Ma to 322 Ma.The main age peaks are 1200-900 Ma and 600-500 Ma,and the sub-peaks are 2400-2300 Ma,1800-1300 Ma and ~328 Ma.There are a few age records of the Palaeo to Neoarchaean ancient basement.The detrital zircon ages in quartz sandstone samples(NJ66 and NJ67)from the Triassic Chawalong Group in Gongshan Block range from 2894 Ma to 249 Ma.The main peak ages are 1200-900 Ma and 600-500 Ma,and the sub-peak ages are 2400 Ma and 1800-1300 Ma,with a small amount of Neoarchaean zircon records.The youngest zircon age of sedimentary rocks from the Chawalong group in the Gongshan Block is 249±4 Ma,which belongs to the early Triassic,and it provides accurate information about the lower limit of the sedimentary age of the Chawalong group.The youngest detrital zircon in the Dingjiazhai formation sandstone in the Baoshan block is 322±5 Ma,which provides information about the lower limit of the Dingjiazhai formation sedimentation.The age spectrum characteristics of detrital zircons are considered to record the information of geological processes such as continental growth and orogeny.By comparing and analyzing the age spectrum of the Baoshan block and the Gongshan block,we can see that the Gongshan block lacks the corresponding age of Archaean detrital zircon compared with Baoshan block,which indicates that its initial crust formation is later than that of Baoshan block.Within the age range of 2700-400 Ma,the age spectrum of detrital zircons in Gongshan and Baoshan blocks has similar characteristics,showing 2600-2400 Ma peaks corresponding to Neo-Archaean crustal growth information,and revealing that the two blocks have been in similar tectonic regimes since ~ 2600 Ma.The two significant age peaks of 1200-900 Ma and 600-500 Ma correspond to the age periods of the Greenville orogeny and the Pan-African event,respectively.They may record the tectonic thermal events during the formation of the Rodinia supercontinent and the Gondwana supercontinent,revealing that the Gongshan block and the Baoshan block in the Neoarchean-Early Paleozoic may belong to the same tectonic block or continent and have experienced the same tectonic thermal events together.The age spectra of detrital zircons in Gongshan-Baoshan block are obviously different from that in South China block.The records of 2000-1800 Ma are weak,suggesting that the Gongshan-Baoshan blocks were away from the Colombian orogenic belt.Combining the previous studies on detrital zircon age spectrum of adjacent blocks and the research progress in stratigraphy,Paleontology and Paleomagnetism of Gongshan-Baoshan block,This paper conclude that the Gongshan-Baoshan block was a discrete marginal block in Gongwana,northwest Australia,in the early Paleozoic,and was located in the northern margin of East Gongwana continent in the Late Paleozoic-Triassic.