Magmatism And Tectonic Evolution Of The Erlangping Unit In The Qinlng Orogenic Belt:Implications For Arc-back-arc Systems In Accretionary Orogenic Belts

The Qinling-Tongbai-Dabie-Sulu orogenic belt is one of the most important orogenic belts that constitute the tectonic framework of eastern Asia.This composite orogenic belt is formed by Paleozoic ocean-continent subduction in the Qinling-Tongbai belt and subsequent Mesozoic continent-continent collision in Dabie-Sulu belt extending over ca.300 Ma between the South China Block(SCB)and the North China Block(NCB).The formation of accretionary orogenic belts usually includes accretion of continental arcs and/or oceanic arcs and marginal basins,generating multiple stages of igneous and metamorphic rocks.The subduction related igneous rocks within the island arc,fore-arc,or back-arc region can probe the evolution of the subduction system.A detailed study for these plutonic and volcanic rocks can help modulate the history of the orogenic belt,and shed light upon the accretionary orogenic processes.The Erlangping unit is an accretional arc-backarc unit containing igneous rocks and metasediments located in the Qinling Orogenic belt.This unit records the early to middle Paleozoic volcanism related to the subduction of the Shangdan Ocean.However,its emplacement age,property,and tectonic history are still controversial.In this work,we are performing whole-rock geochemistry,whole-rock Sr-Nd isotope,zircon U-Pb geochronology,and zircon Hf-O isotope analysis on plagiogranites and basalts from two sites in the Erlangping unit to study the petrogenesis,mantle source,and tectonic background.The main achievements are as follows:(1)Zircon U-Pb geochronology of the plagiogranite yielded two groups of ages ca.460 Ma and ca,435 Ma.Zircon U-Pb ages of some intrusion in the Erlangping unit agree with these two groups of plagiogranite suggesting two major magmatic events.(2)The plagiogranites formed at ca.460 Ma are typical ocean plagiogranite.They have low Ti O2,high Si O2,the REE-Si O2 show good covariance,suggesting partial melting to be the main controlling processes.Their ?Hf(t)values fall into the trejectory of typical arc igneous rocks.The low ?18O values suggest a parental rock that has experienced hydrothermal circulation.The most plausible assumption is the anatexis of gabbroic oceanic crust.Magma upwelling induced by the subducting Shangdan plate might have triggered this remelting event.The maturation of arc units like this might be a paradigm of continental growth.(3)Basalts take up the majority of rocks in the Erlangping unit.The Erlangping basalts are sub-grouped based on the zircon U-Pb age of Plagiogranites intercalations.The major and trace element of basalt show limited effect of alteration or fractional crystallization.The basaltic samples are low in Si O2,Ti O2,P2O5,and K2 O and high in Mg.The Erlangping basalts are overall enriched in LILEs(Rb,Ba,Pb,and Sr)and depleted in HFSEs(Nb,Ta,and Ti),and depleted Nd isotope,similar to typical arc basalts.The Th/Nd vs.Ba/Th and La/Yb vs.Ba/Th diagram indicate melt to be the major subduction component.The Ordovician basalts have shown more significant subduction input than the Silurian basalts.The Ordovician basalts are likely to be formed in an arc environment.The Silurian basalts have shown some degree of crustal assimilation,likely generated in the rifting stage of a back-arc basin.(4)The Erlangping unit was generated as an intra-oceanic arc before 490 Ma due to the subduction of the Proto-Tethyan oceanic plate.At ca.490 Ma,an arc-continent collision juxtaposed the NQ and the Erlangping units and resulted in the accretion of the intra-oceanic arc and UHP metamorphism.After the arc-continent collision,the subduction zone jumped to the southern margin of the NQ unit and formed a narrow arc at the southern edge of the NQ unit at ca.485 Ma.The subduction front moved northward underneath the Erlangping unit and continue to generate arc magmatic rocks at ca.460 Ma.This arc magmatic event induced the partial melting of the accreted oceanic arc and led to the emplacement of the 470-460 Ma Erlangping plagiogranites,which intruded the basement and interbedded with contemporaneous island-arc magmatic rock.The rollback of the subduction front induced the back-arc extensional force and form a 435 Ma backarc spreading basin.