Provenance Analyses On Detrital Sedimentary Rock From The Longtan Formation In The Ailaoshan Belt: Constraint On The Polarity Of The Ailaoshan Ocean Subduction

The Ailaoshan Ocean,a branch of the eastern Paleotethys,separated the South China and Indochina blocks during the late Paleozoic.Uncertainty remains on whether this ocean was subducted eastward?present-day orientation?beneath the South China Block or westward beneath the Indochina Block.Tectonic reconstruction is complicated by the reactivation and deformation of the late Paleozoic to Mesozoic structures during the Cenozoic India-Eurasian collision.The Longtan formation is widely considered to be deposited before the closure of the ocean,thus may provide some clues to this problem.In this study,we present new detrital zircon U-Pb age,and Hf and O isotope and trace element data for the detrital rocks from the Longtan Formation in order to trace its provenance and reconstruct the tectonic settings from which they were deposited.Based on the detrital zircon U-Pb age and Hf-O isotopes,the sedimentary rocks of the the Longtan Formation from different sides of the Ailaoshan suture can be divided into three types:Type 1:multimodal age distribution.The samples of Type 1 are distributed in the west of the suture and dominate the Longtan Formation in this area,their age spectrum contains a major age peak at 250²90 Ma and minor age peaks at 380⁴30 Ma,600⁷00 Ma,900¹000 Ma and¹850 Ma.Both positive and negative?_Hf?t?values are present in the 250²90 Ma zircons,and the?_Hf?t?and?¹⁸O values are negatively correlated.Their features of trace elements suggest that the detrital zircons came from an igneous arc or orgenic belt.Combined with the previous literatures,we interpret that Type 1 sedimentary rocks were likely deposited in a back-arc basin triggered by western subduction of the Ailaoshan Ocean,and the detritus both from the Indochina Block and a 250-290 Ma arc located near the eastern margin of the block.Type 2:unimodal age distribution with an age peak of²50Ma.The samples with this character are mainly distributed in the west of the Ailaoshan belt.Their age spectrum shows a dominant 250 Ma age peak plus a few older zircons.Their Hf and O isotopic signatures are similar to those of Type 1,so their detrital source should be the same arc with Type 1.Compared with samples of Type 1,those samples of Type 2 show lower textural maturity and the detrital zircons are mostly euhedral,showing that the rocks deposited in a fore-arc basin.Type 3:unimodal age distribution with an age peak of²40Ma.The samples of Type 3 are distributed in the east of the suture and characterized by a distinct single²40 Ma age peak with almost no Precambrian zircons and the?_Hf?t?contains both positive and negative values.These features suggest that sedimentary rocks of Type 3 were likely deposited in a fore-arc basin in the western margin of the South China Block,and the arc developed by the Ailaoshan Ocean's eastward subduction beneath South China block.Based on these results,we argue for a new tectonic modal that the Ailaoshan Ocean may have been bipolar-subducted both westward beneath the Indochina and eastward beneath the South China Blocks,respectively.