Study On U-Pb Geochronology And Hf Isotopes Of Detrital Zircon Grains Of Nanhua Clastics In Northern Guangxi

The Northern Guangxi region is located in the western section of the Jiangnan orogenic belt and has a complicated tectonic evolutionary history.The region preserves a relatively complete Neoproterozoic stratigraphic record,and is an ideal place to study the history of Neoproterozoic tectonic evolution of the South China Continent.The Nanhua System in the North Guangxi region includes Chang'an Formation,Fulu Formation,and Lijiapo Formation from bottom up.The systematic U-Pb dating of detrital zircon grains shows that the Chang'an Formation has its major peak age range of 703?883 Ma and minor age peak ranges of 662?698 Ma and 874?921 Ma;the Fulu Formation has its major peak age range of 714? 916 Ma and minor peak age ranges of 641?699 Ma,1 840?2 277 Ma,and 2 322?2 747 Ma;and the Lijiapo Formation has its major peak age range of 730?882 Ma and minor peak age ranges of 630?644 Ma,901?1 063 Ma,1 857?2 037 Ma,and 2 484?2 489 Ma.According to the U-Pb detrital zircon geochronological analyses,the deposition ages of the Changan Formation,Fulu Formation,and Lijiapo Formation are 720?660 Ma,660?650 Ma,and 650?635 Ma,respectively.Therefore,the deposition age of the Nanhua System is 720-635 Ma.Based on the detrital zircon age spectra,the Changan Formation clasts were derived mainly from the Neoproterozoic magmatic rocks along the southeastern margin of the Yangtze Block,with some from the Grenville orogenic rocks of the Cathysia Block;The Fulu Formation received clasts also mainly from the Neoproterozoic magmatic rocks along the southeastern margin of the Yangtze Block,with some from the Paleoproterozoic magmatic rocks in the north of the Yangtze Block and from the Grenville orogenic rocks and the Archean magmatic rocks of the Cathay Block.The LijiaPo Formation has its provenance mainly from the Neoproterozoic magmatic rocks along the southeast margin of the Yangtze Block,with some from the Paleoproterozoic magmatic rocks in the north of the Yangtze Block and from the Grenville orogenic rocks and the Archean magmatic rocks of the Cathay Block.The Hf isotopic analysis of the detrital zircon grains collected from the Chang'an Formation shows that 84.26% of the zircon grains have positive ?Hf(t)values,suggesting that they were sourced mainly from a juvenile crust,and that 56.48% of the detrital zircon grains with positive ?Hf(t)values have their second-stage Hf model ages of 980 – 1380 Ma,indicating that most of the Chang'an detrital zircon grains were derived from the Greenvillian juvenile crust.Analysis and comparison of the detrital zircon age spectra of each formation of the Nanhua System demonstrate that,from the Chang'an Formation of the early Nanhua Epoch to the Lijiapo Formation of the late Nanhau Epoch,the Yangtze Block was a major source of clasts and provided even more clasts towards the end of the Nanhua Epoch,whereas the Cathaysia Block contributed much less amount of the clasts;The sedimentary environment of the extension and cracking of the Yangtze and Cathaysian continental blocks continued until onset of the Fulu Formation,and was in the passive continental marginal environment during the Lijiapo Formation period.