Geological Characteristics And Tectonic Significance Of Duerji Tectonic Melange,Inner Mongolia

Duerji tectonic melange is a subduction–accretion complex composed of“matrix”and“block”.In this paper,systematic petrological,geochemistry and chronological studies have been carried out.The rock assemblage of“Matrix”has the characteristics of flysch,which is mainly composed of metamorphic siltstone,biotite sericite phyllite and quartz-muscovite schist.The minimum LA-ICP-MS U-Pb age of detrital zircon from metamorphic siltstone is 262 Ma,and geochemistry characteristics show that they were formed on the edge of active continent.The“block”rocks types of Duerji tectonic melange comprise pillow basalt,massive basalt,diabase,gabbro,gabbro-diorite,diorite.Zircon LA-ICP-MS U-Pb age of the pillow basalt is 310±10 Ma.Their geochemistry show that they are low-K tholeiitic series and rich in LILE,but loss in HFSE such as Nb,Ta.Massive basalts are also characterized by HFSE loss and LILE enrichment,and more LILE and LREE enrichment than pillow basalts.Both of them were formed in the intra-oceanic arc,reflecting the intra-oceanic initial subduction and the intra-oceanic subduction lasting process.The diorites are high-Mg diorite with the LA-ICP-MS U-Pb age of 296.2±4.8Ma,which are characterized by high Mg#(average 62.16)and obviously enriched subduction zone components,indicating intra-oceanic subduction.Gabbros and gabbro-diorites have the same geochemistry characteristics,and LA-ICP-MS U-Pb age of the gabbro-diorite is 290.6±1.6 Ma.They are characterized by the transition geochemistry from subduction zone type igneous rocks to OIB,formed in the background of the late stage of intra-oceanic subduction in the back-arc region.Based on the study of the material composition and structural attributes of the Duerji tectonic melange,combining with the isotopic age information of its components,the regional geological background and the regional tectonic evolution history,we have come to the conclusion that the Duerji tectonic melange may reflects the process of the intra-oceanic subduction from late Carboniferous to early Permian and the closure of back arc ocean basin in the Middle Permian.The identification of the Duerji tectonic melange indicates not only that there was intra-oceanic subduction from late Carboniferous to early Permian in the eastern part of the Paleo-Asian Ocean,but also that the oceanic crust in the study area was not fully integrated until at least the Middle Permian or even later.