Geological Characteristics And Tectonic Significance Of Early Paleozoic Island Arc Volcanic Rocks In The North Qilian Mountains

The Northern Qilian Mountains is a Paleozoic orogenic belt between the Alashan terraneand the Qilian-Qaidam block.It is one of the most abundantly developedsubduction zones in the orogenic belts and saves one of the most integrated rock recordsin China.In this paper,the volcanic rocks exposed along the Early Paleozoic suture zone in the North QilianMountainsare studied by geology,petrology,mineralogy,elemental geochemistry and LA-ICP-MS zircon chronology measures to reveal theprocess and evolution history of convergence and subduction in ancient Qilian Ocean.The study area is located in the area of the YeniugoutoQingshuigou,Qilian County,Qinghai Province.The types of the early Paleozoic volcanic and sub-volcanic rocks include rhyolite,quartzite,quartz porphyry,andesite,basalt and tuff,volcanic clastic rocks.Intermediate-acidvolcanic rocks and intermediate-basic volcanic rocks are commonly interbedded.Siliceous rocks,carbonate mezzaninecan be found in volcanic rocks.And varying degrees of alteration often occur amongbasalts.The elemental geochemical characteristics of the volcanic rocks in the study area reveal a significant difference in the K2 O content of the rocks.According to the K₂O> 2 wt% of the intermediate-felsic volcanic rocks and K₂O> 1 wt% of the intermediate-basic volcanic rocks in the study area can be divided into the high-K volcanic rocks series,the other（K₂O<1 wt% of intermediate-basic volcanic rocks,K2 O < 2 wt% of the intermediate-felsic volcanic rocks）belongs to the low-K volcanic rocks series.All volcanic rocks show the characteristics of island arc volcanic rocks with different degrees of Nb,Ta,Zr,Hf and Ti loss,revealing that they are formed in the island arc environment.Butrhyolites and basaltsof the low-K volcanic rocks series have the same near-flatREE distribution type and near-flat trace element spiderin general.It shows that the main oceanic lithosphere fused partially to form the crystalline product.Rhyolites and basalts of high-K volcanic rocks series have the same right-leaningREE distribution type and right-leaning trace elements spider in general,where LREE and large ion lithophile elements are significantly enriched with the mixture of continental lithosphere materials.Through the comprehensivelycomparative study,there are multiple mantle-derived magmatic uplifts and eruptionsin the subduction-convergence-collision process of the Qilian Mountains in the early Paleozoic.Due to the differences of the lithosphere structure,the compositions of the mother magma are obviously different.It forms high-K and low-K volcanic rocks,both of which contain the basic and the acid volcanic arc rock series.The two series of volcanic rocks are superimposed along the main suture belt when the eruption occurs.The diagenetic age of the basalts ofhigh-K volcanic rock series is 482±2.7Magiven by LA-ICP-MS zircon U-Pb chronology.The comprehensive study suggests that the volcanic rocks in the Northern Qilian area include two different types.（1）The high-K island arc volcanic series are formed in the subduction zone of the passive continental margin;there are continental lithosphere materialsobviously.And the volcano eruption age is 482 Ma.（2）The scale of Low-K arc volcanic rocks series exposed is relatively small.The diagenetic era is speculated later thanthe formation of Qilian Ocean closed（445Ma⁴28Ma）.The oceanic crust which has subducted under the continental partial is moltenin the continental strata due to the squeezed collision occurred at the end of the late Ordovician.The low-K and high-Na volcanic rocks erupt along the original volcanic arc path,and thickenthe crust of the continental margin.