Genesis of Paleozoic Two-Stage Magmatism of Wulagai Pluton in Central Inner Mongolia:Response to Tectonic Switch from Advancing to Retreating Subduction of Paleo-Asian Ocean Plate

Jiang Guanzhe; Li Shan; Zhu Junbin; Guo Donghai; Qiu Zhiyi; Wan Xue

doi:10.3799/dqkx.2024.002

Abstract：Subduction zone is an important place for the interaction between crust and mantle,and is accompanied by magmatism of different periods and nature.Understanding the characteristics of magmatism in different subduction processes(e.g.,advancing subduction and retreating subduction)is of significance for an in-depth understanding of the evolution history of orogenic belts.In this paper,it presents zircon U-Pb ages,whole-rock major-and trace element data,whole-rock Sr-Nd isotope and zircon Hf isotope for Paleozoic intrusions of the Wulagai pluton from East Ujimqin of central Inner Mongolia.Zircon LA-ICP-MS dating for gabbro-diorite yields a weighted mean age of～480 Ma,suggesting the crystallization age is Early Ordovician;the age of granitoids is 348-344 Ma,representing the products of magmatic activities during Early Carboniferous.Early Ordovician gabbro-diorites(SiO2=51.27％-53.39％)are relatively enriched in light rare earth elements and large ion lithophile elements(e.g.,Rb,Th and U),and depleted in high field strength elements(e.g.,Nb and Ta),with positive whole-rock εNd(t)values(+3.3 to+3.5)and zircon εHf(t)values(+7.3 to+13.3).The gabbro-diorites were originated from partial melting of a depleted lithospheric mantle that was metasomatized by slab-derived fluids.The Early Carboniferous granitoids can be divided into two groups according to their geochemical compositions.The first group is mainly syenogranite with high SiO2 content(76.19％-77.52％),high total rare earths content,strong Eu negative anomalies,high 10 000 Ga/Al values(3.90-5.95)and zircon saturation temperature(average 965° C),displaying typical A-type granite affinity.The second group is mainly granite porphyry and biotite monzogranite with SiO2 content(70.67％-76.42％)lower than the first group.They showing metaluminous to weakly peraluminous characteristics,combined with negative correlation between SiO2 and P2O5,and hornblende can be observed,displaying I-type granite affinity.In terms of Nd-Hf isotopic compositions,these two groups of Early Carboniferous granitoids have similar isotopic compositions(first group:εNd(t)=+5.8 to+6.3,εHf(t)=+11.8 to+15.5;second group:εNd(t)=+4.9 to+5.1,εHf(t)=+11.6 to+15.4),indicating that they have similar magmatic source and both derived from partial melting of the juvenile crust,but underwent different evolution processes.Combined with regional magmatism,it proposes an advancing subduction of Paleo-Asian Ocean plate occurred during Early Paleozoic in central Inner Mongolia.Paleo-Asian Ocean plate subducted beneath the South Mongolian microcontinent before Late Cambrian and lead to the formation of gabbro-diorites reported in this study.The advancing subduction caused a series of arc accreted to the southern margin of the South Mongolian microcontinent.Then an arc-continent collision occurred during Late Silurian.The Early Carboniferous A-type and I-type granitoids were formed in a back-arc extensional environment which was resulted from the roll back of Paleo-Asian Ocean plate during Late Devonian to Early Carboniferous.