Paleozoic Tectonic Evolution In The Helanshan Tectonic Belt: Constraints From U-Pb Geochronology Of Detrital Zircon

The Helanshan Tectonic Belt(HTB),located in the northernmost of North-South Tectonic Belt,is tectonically surrounded triangularly by the Ordos block,the Alxa block and the Qilian Tectonic Belt.There are still controversies about its tectonic settings and evolution processes during Paleozoic,which directly hinder the understanding of the relationship among the three blocks.The deformation structures are relatively simple and magmatic rocks are rearly to be seen in HTB during Paleozoic,but the continuous sedimentary strata from Cambrian to Permian have clastic rocks,from which the detrital zircons can be used to trace the provenance.The Paleozoic regional tectonic evolution can be sketched on the basis of the provenance study.After detailed field investigations,twenty-three sandstone samples from the Xiangshan,Miboshan,Hanxia,Shaliushui,Zhongning,Yanghugou and Taiyuan Formations,as well as Yaogou and Dahuanggou Groups,were collected in order to separate detrital zircons.The detrital zircons were dated by using LA-ICP-MS.Based on the collected zircon U-Pb age data from the publications about the Ordos Block,the Alxa Block,the Khondalite Belt and the Qilian Orogenic Belt.The probability U-Pb age distribution of different blocks can be compared with abtained those of collected samples,which can be used to trace the orogins of clastic materials,from the Ordos Block,the Alxa Block or the Qilian Orogenic Beltin.In this paper,the main achievements are as follows:(1)The youngest zircon age of the Xiangshan Group is 520±4 Ma.Age peaks of the Xiangshan Group are 520 Ma,714Ma,876 Ma,1132Ma,1814 Ma and 2488 Ma.The youngest zircon age of Miboshan Formation is 507±8 Ma.Age peaks of the Miboshan Formation are512 Ma,816Ma,914 Ma,1856Ma and 2452 Ma,which is similar to that of Xiangshan Group.We infer that they have the same provenances.Zircons of ~520Ma and 700-900 Ma are absent in the Ordos Block,thus,we suggest that the Alxa Block may be the main source of the clastic materials from Cambrian to Ordovician.(2)The age peaks of Silurian Hanxia Formation are 432 Ma,942Ma,1156 Ma,1632Ma,1814 Ma and 2480 Ma.The absence of ages in the range from 2000 to 2200 Ma in the Silurian strata suggested that the Ordos Block and the Khondalite Belt may not supply the detritus,but the Alxa Block and the Qilian Orogenic Belt had probably provided the clastic materials.(3)The age spectrum of the Zhongning Formation is similar to that of the ShaliushuiGroup,the age peaks are mainly 416 Ma,930Ma,1832 Ma and 2480 Ma.The age peaks of416 Ma and 930 Ma detrital zircons show that detritus still come from the Alxa Block and the Qilian Orogenic Belt.(4)The zircons from Carboniferous sandstone in northern part of HTB have four age peaks,the two major peaks at 1981 Ma and 2469 Ma and the two minor peaks at 320 Ma and422Ma.The ages of ~1981Ma and ~2469Ma detrital zircons may come from the Ordos Block,the Khondalite Belt and the Alxa Block while 320 Ma zircons come from the Alxa Block.The zircons from Carboniferous sandstone in southern part of HTB have the age peaks of 967 Ma,1625Ma and 2492 Ma,which suggested that The Qilian Orogenic Belt may be the main sources of the sandstone.(5)The zircons from Permian sandstone in northern part of HTB have the major age peaks of 294 Ma,1851Ma,1939 Ma and 2354 Ma,as well as the minor age peak of 417 Ma.The ~1851Ma,~1939Ma and ~2354Ma detrital zircons mainly come from the Ordos Block,the Khondalite Belt and the Alxa Block,while ~294Ma zircons come from Hercynian magmatic rocks in the Alxa Block.The zircons from Permian sandstone in southern part of HTB have the peak ages of 263 Ma,432Ma and 2496 Ma.The ~263Ma zircons mainly come from Hercynian magmatic rocks in the Alxa Block,and the age of ~432Ma and ~2496Ma zircons can come from either the Alxa Block,the Ordos Block or the Qilian Orogenic Belt.(6)Based on the above provenance analysis,from early Cambrian to middle Ordovician,the Qilian Ocean subducted along the south margin of the Alxa Block,which formed a back-arc depression basin in the studied area.The Alxa Block and the arc supplied the main detritus to the back-arc depression basin.In the early Silurian,the central Qilian Block may collided with the Alxa Block.The central Qilian Block and the Alxa Block provided the clastic material for the basin until the late Devonian.Arc igneous detritus appeared in Carboniferous sandstone in the northern part of the studied area due to the subduction of the Paleo-Asian Ocean beneath the northern margin of the Alxa Block while the Qilian Orogenic Belt provided materials for Carboniferous sandstone in the southern part.With the closure,collision and uplift of the Paleo-Asian Ocean,the southern part of basin started to receive detritus from the Qilian Orogenic Belt,and from the Central Asian Orogenic Belt at the same time in Permain.