Determination And Geological Significance Of Magmatism In The Early Paleozoic Intra-ocean Back-arc Basins In The Western Central Qilian Belt

The central Qilian is held between the North Qilian suture zone and the South Qilian area,Some of the middle-base volcanic rocks and a large number of Caledonian intermediate-acid intrusive rocks were exposed?This paper selects the three geological sections of Heigou,Hamaerdaban and Aergaletai to carry out systematic field geological survey work.Combined with petrology,geochemistry,temperature and pressure calculation and chronology,this paper considers that the volcanic belt is a set of early Paleozoic island arc volcanic rocks,MORP-like basalts,OIB basalts and boninite rocks.The volcanic rock has the following main features:In the field,the north to the south is the rear-arc volcanic rock(the seamount chain in the back-arc basin),MORB,OIB and boninite.The rock combination is similar to the volcanic rocks in the Izu–Bonin arc basin.The characteristics of high calcium gneiss are consistent with those of troodos back arc.The composition of the volcanic belt this study may indicate the location of the extension center of the basin in the back of the arc.The andesite formation is 484±8Ma,and gabbros formation is 477±5Ma,Indicating that they are magmatism in the Early Middle Ordovician,the Lu-Hf isotopic in situ analysis of zircon in andesite shows that ?Hf(t)values are all greater,indicating that the zircon comes from the melting of mantle source region.The temperature and pressure calculations show that the basalt containing plaque has higher temperature and pressure(0.52-0.73GPa;993.06-1003.87?),The temperature and pressure of amphibole in Andesite are relatively small(0.12G-0.47GPa;783-890?),It is shown that the basaltic-containing basalts come from the deep underground source area,and the andesiticmay be formed in the shallow part from the crystal.The arc back volcanic rocks have high potassium content and deep subduction melt composition(such as high Th),which can lead to the formation of flux melting in mantle wedge.The back-arc basin volcanic rocks have N-MORB characteristics,and melt composition characteristics of the subduction zone(higher Rb,Ba,U than N-MORB).The OIB volcanic source area may be formed by partial melting of theper idotite with 0.2%-1%,with the characteristics of the enriched mantle source area,and the addition of no subduction zone components.There are many types of magmatism in Yanchiwan back-arc basin,which is consistent with the characteristics of magmatism in IBM back-arc basin.The genesis of various magmatic types in the back-arc basin is affected by the addition of the mantle source zone and the subduction zone melting/fluid component,and is also controlled by the variation of the subduction plate angle over time in the formation of the post-arc basin.It reflects the magmatic dynamic process of the extension of the back arc basin in Yanchiwan area.However,there are still some problems in the study of the magmatism of the Early Paleozoic arc basin in Yanchiwan,and further work is needed.