Geochronology And Hf Isotope Compositions Of Detrital Zircons From The Early Paleozoic And Early Mesozoic Sedimentary In The Two Sides Of The Ailaoshan Tectonics Zone: Provenance And Their Geological Significance

Constraining the precise opening,closing timing and location of the suture zone of the Ailaoshan Paleotethys is crucial to better understand the tectonic evolution of the whole Paleotethys in Southwestern China.In this paper,we carried out a systematic U-Pb geochronology and Lu-Hf isotope analysis of detrital zircons from the Early Paleozoic to Early Mesozoic sediments from the two sides of the Ailaoshan tectonic zone to provide insights into the provenance of sedimentary materials and their variation with time and decipher their geological significance.The dating results of detrital zircons from the Upper Silurian-Lower Devonian sedimentary sandstones from the two sides of the Ailaoshan tectonic zone show that all the Upper Silurian detrital zircons are characterized by a similar youngest age peak at ~450 Ma and two same subordinate age populations including the Greenville orogenic events(1100~800 Ma)and the 2600~2400 Ma age groups.All the Lower Devonian detrital zircons gave similar age populations with an exception of those from the east side of the Ailaoshan tectonic zone without the same youngest peak to others.In addition,an alternative feature is also obvious that the Lower Devonian detrital zircons display a stronger age peak at 2600~2400 Ma than the Upper Silurian ones.By comparison with the published dating data of detrital zircons,a common characteristic with same age populations has also been revealed.Integration with the regional geological data,we propose that the opening timing of the Ailaoshan Paleotethys Ocean took place during the Late Silurian to Early Devonian and the Ailaoshan fault should represent the location of the Paleotethys suture zone.The geochronological results also demonstrate that the sandstones from the middle Triassic of the east side of the Ailaoshan belt have three age groups at 230~270 Ma,~330 Ma and 400~500 Ma with their corresponding ?Hf(t)values including-5.58~+13.88,-6.20~12.00 and-15.66~+13.36,respectively.In contrast,the middle Triassic samples from the Ailaoshan belt have two different U-Pb age group at ~350 Ma and 480~560 Ma with their corresponding ?Hf(t)values of-11.56~+17.14 and-13.81~+12.63,respectively.The geochronological results from the upper Triassic of the Ailaoshan belt and the east region share the same age spectrum,showing three same age groups at 230~260 Ma,300~330 Ma and 450~520 Ma with the corresponding ?Hf(t)values of-5~+15,-7~+14 and-13~+19,respectively.The age spectrum and their ?Hf(t)values overlap the magmatism events and their corresponding zircon-in Hf values in the Ailaoshan belt.It remarkably suggests that the Middle to Late Triassic sediments from the Ailaoshan belt and the east region sourced mainly from the inner part of the Ailaoshan belt.These results,combined with regional P3-T3 detrital zircon U-Pb age and Hf isotope data and other geological evidence,reflect that the closure of the the Ailaoshan Ocean had been competed before the Early Triassic.In addition,by comparison with the age spectrum of the detrital zircons from the Cambrian-Ordovician quartz arenites in the microcontients of the Southeast Aisa including the Yangtze,Simao-Indochina and Baoshan-Sibumasu blocks,they have dominant Pan-African(510~600 Ma)and early Neoproterozoic(950~980 Ma)age populations,implying that those shared a common provenance in the Early Paleozoic.In this respect,this in turn reflects that the Yangtze and Simao-Indochina blocks were most likely located close to the Tethyan Himalaya block along the northern margin of the India and northwestern margin of Australia in the reconstruction of the Gondwana super-continent.