Genesis And Tectonic Significance Of Magmatic Rocks From Late Ordovician To Early Silurian In Zhalantun And Xinlin,Great Xing'an Range

Duobaoshan-Zhalantun arc zone is located on the southern edge of the Xingan block.It is magma arc formed by NW subduction of the paleo oceanic plate,including Xingan and Songnen block ofearly Paleozoic.The genesis and distribution of the island arc belt is the focus of many geological workers in recent years.Althoughpredecessors determined that the island arc extends south-west to Aershan area by the continuous distribution of Ordovician Marine Strata with nature of island arc-back arc basins in the Xingan block.Except the systematic study of the Ordovician island arc magmatic rocks in Duobaoshan area,there are few evidences of magmatic rocks extending southward to the southwest of the Duobaoshan island arc,asa result of which,the cause and nature of the island arc and the division of arc basin system remained unknown.Therefore,geochemistry of Ordovician intrusive rocks in Zhalantun,Southern-Western Duobaoshan area and early Silurian coarse basaltic rocks in Xinlin area is studied in this paper.By comparison with the magmatic rocks of the same epoch in the area,It provides new constraints for the extension of Duobaoshan island arc to the southwest and the division of arc-basin system.Zhalantun area Ercun rock body mainly consists of quartz diorite,tonalite and granodiorite.The zircon U-Pb age of quartz diorite is 444±4.6 Ma,and that of the granodiorite is 441±1.7Ma.Rock shows enrichment of LILE such as Rb,Ba,K,and significant loss of HSFE such as Nb,Ta,P,Ti,etc.,whichindicates the basic characteristics of the arc magma.The rock composition,mineral composition,and geochemical characteristics of Ercun Rock show that it was produced in the island arc environment.Combined with the tectonic location of Zhalantun area,this paper considers that the Ercun rock mass in Zhalantun area is an extension of the southwest of Duobaoshan Island arc.The total content of rare earth elements in volcanic rock in Xinlin area is152×10^-6-205×10^-6,the ratio of light and heavy rare earth is 7.16-9.89,the sample?La/Yb?_N is between?6.01-13.3?,and?Eu?1.06?.-1.14)isof no abnormality and weak positive anomaly.In addition,Nb and Ta is not of obvious loss compared with La and Ce,and there are obvious positive Zr and Hf anomalies.Trace elements show similar geochemical characteristics as OIB-type basalts,and there is a tendency toward transition to arc volcanic rocks.Based on the comprehensive analysis,the source area of volcanic rock in Xinlin area is mainly contributed by the upwelling mantle of deep OIB,which is mainly composed of the fluid components released by the subducting oceanic crust.Combined with the tectonic location of Xinlin area,this paper believes that the volcanic rock in Xinlin area was generated during the initial period of post-arc basin extension.To sum up,the tectonic evolution characteristics of Duobaoshan-Zhalantun Island arc are as follows:500Ma ago,Xinlin-Toudaoqiaoocean is closed,and the Xing'an massif is integrated with the Ergunblock.In the Early Ordovician,after the collision,the continental crust extended eastward.The oceanic subduction zone retreated to the southeast side of Duobaoshan-Zalantun,forming granodiorite with Adakite nature in Duobaoshan area.At the same time,the back arc zone began to stretch and during469⁴75 Ma,volcanic rocks with post-arc extension nature in Aershan area were formed.In Late Ordovician-Early Silurian,magmatic activitieswere enhanced,an island arc volcanic-deep magmatic rock belt in Duobaoshan-Nenjiang-Zalantun areawas formed,and alkaline basalt magma activities with back-arc extension nature in Xinlin areawere developed.