Paleomagnetic Study Of The Ophiolites In The Aksay Area,Altun Tagh Mt.,and Its Tectonic Implications

The Early Paleozoic was an important period of global plate tectonic evolution.The evolution of the Tarim,Qaidam and North China plates occurred in western China.However,the paleogeographic location and integration process of each plate are not clear,and there are few research data on plate reconstruction and evolution process restoration using quantitative methods such as paleomagnetic.The Altun Tagh MT.,located between the Qaidam and Tarim blocks,and are connected to the Qilian block,experienced the process of oceanic expansion and closure evolution in the Early Paleozoic,which retained a relatively complete Ophiolites suite to study the plate evolution in this area.The AltunTagh MT.is an ideal material.There are two sets of Early Paleozoic Ophiolites in the AltunTagh MT.,Previous studies have shown that they represent the remnant oceanic crust of the Cambrian,and the basalts in it have the properties of mid-ocean ridge basalt or ocean island basalt.Due to the lack of paleomagnetic research,the paleogeographical(the latitude of the mid-ocean ridge and its expansion process)change of the Ophiolites belt is still unclear,which restricts the understanding of the plate tectonic evolution in this area to a certain extent.This paper further clarified the composition and spatial distribution of the Ophiolites by field geological survey of the Aksay Ophiolites Heishitougou area,located in the eastern section of the North Altun Tagh Ophiolites.Paleogeomagnetism and rock magnetism studies of basalt,peridotite and overlying tuff have clarified the paleogeography change of the Aksay Ophiolites in the Early Paleozoic,and obtained the following results:1)The Aksay Ophiolites is composed of Olivine pyroxene,Basalt,and Siliceous rock from bottom to top,perfecting the previous understanding of the composition of the Ophiolites in the area;The Aksay Ophiolites is covered by volcanic tuff and intruded by late Gabbro.Volcanic tuff and Gabbro may represent magmatic activity in the plate subduction stage.2)The rock magnetism and thermal demagnetization experimental results show that the characteristic remanent magnetization is mostly carried by magnetite and titanium-magnetite,and secondary remanent magnetization is carried by goethite.Magnetic mineral particles are mainly MD particles.3)Three stable characteristic remanent magnetic directions of Aksay Ophiolites are: the Basalt(D= 302 °,I = 44 °),Olivine pyroxenite(D = 102 °,I = 39 °)and Tuff(D=90°,I=43°).Paleomagnetic pole position and Paleolatitude are: Basalt(40.3 ° S,187.2 ° E),south latitude 26.2°;Olivine pyroxenite(4.7 ° N,159.3 ° E),north latitude 22 °;Tuff(15.3 ° N,163.6 ° E),north latitude 25.3 °.4)In the Cambrian,the Aksay area was part of the back-arc basin in the southern hemisphere at low latitude,and it was located near the south latitude of 26.2 °.With the extension of back-arc basin,the ridge continued to drift to the north,and it was subducted under the tarim block roughly in the silurian period.Finally,the back-arc basin was closed,and the Altun terrane was connected with the tarim block.