Late Paleozoic-Early Mesozoic Tectonic Evolution Of Orogenic Belt Of The Changchun-Kaiyuan Area In The Eastern Segment Of The Northern Margin Of The North China Block

Amidst the backdrop of oceanic plates subducting under continental plates,a series of island arcs,microcontinents,oceanic islands and seamounts continually converge and amalgamate along the continental margin,forming typical accretionary orogens.The Central Asian Orogenic Belt(CAOB),as a classic global accretionary orogenic system,which is characterized by the growth of new crust,has attracted the attention of many geologists.In recent years,the eastern segment of the northern margin of the North China Plate serves as a key area for the Xingmeng orogenic belt,displaying abundant magmatic and stratigraphic records.Based on detailed field investigation,this thesis carried out comprehensive petrological,geochemical,geochronological,and zircon Hf isotopic analyses in order to elucidate the magmatic activity,petrogenesis and sedimentary strata protolith formation during the Late Paleozoic-Early Mesozoic.Combined with various structural styles in geological bodies,this thesis further discusses the calssification of the tectonic units within Changchun-Kaiyuan region,and delineates the tectonic evolution process of CAOB during the Late Paleozoic-Early Mesozoic.The main achievements are as follows.1.The geochronological framework of the Late Paleozoic-Early Mesozoic magmatism in the Changchun-Kaiyuan areaIn this thesis,LA-ICP-MS zircon U-Pb dating of the Late Paleozoic-Early Mesozoic magmatism has been carried out to constrain their accurate formation ages and establish exact geochronological framework,including the Early Carboniferous(348～341 Ma),Early-Middle Permian(288～272 Ma),Middle-Late Permian(～260 Ma),Late Permian to Early-Middle Triassic(252～246 Ma)and Late Triassic(230～221 Ma).2.The petrogenesis and tectonic setting of the Late Paleozoic-Early Mesozoic magmatic rocks in Changchun-Kaiyuan area(1)The Early Carboniferous metamorphic andesite is characterized by enrichments in large ion lithophile elements(LILEs)and light rare earth elements(LREEs),and depletions in high field strength elements(HFSEs)and heavy rare earth elements(HREEs).Its geochemical characteristics are similar to that of A2-type granites,which is indicative of a post-orogenic extensional setting.(2)The Early-Middle Permian andesite display no obvious differentiation of LREEs and HREEs,relatively depleted Nb and Ta,and enriched Sr and Pb elements.It was generated from partial melting of the mantle wedge metasomatized by the subduction-related fluids,which is related to the southward subduction of the Paleo-Asian Ocean(PAO).The Middle-Late Permian adakitic quartz diorite-granite rock assemblage(263～259 Ma)is characterized by high Sr/Y and Na2O/K2O ratios.It is the product of the interaction between different degrees of slab melt and the overlying mantle wedge,which is related to the heterogeneous closure process of the PAO.(3)The Late Permian Jianshanzi volcanic rocks are characterized by high Sr/Y and(La/Yb)N ratios,are typical adakitic rocks within thickened lower crust origin.The Early Triassic A-type granites reveal an extensional tectonic environment associated with the slab break-off,indicating that the residual ocean basins still exist in the eastern part of the PAO during the Early Triassic.(4)The Late Triassic Jianshanzi volcanic rocks are mainly composed of high-K calc-alkaline trachyandesite,displaying enrichments in LILEs and depletions in HREEs,with low Mg#value and no Eu negative anomalies.The geochemical characteristics are similar to those of the high-Mg diorites,which derived from partial melting of the delaminated basic lower crust,indicaing the Late Triassic Jianshanzi volcanic rocks formed in the lithospheric extension and collapse environment during the post-orogenic period.3.The formation age and sedimentary environment of the Late Paleozoic-Early Mesozoic clastic rocks in Changchun-Kaiyuan areaThe sedimentary age of the Shoushangou Formation in the Jingjiatai area is limited after the Middle Permian.The provenance analysis shows that most zircon grains are derived from the Late Paleozoic orogenic belt in the northern margin of the North China Plate,indicating that the magmatic activity of the same period is the mian source of Shoushangou Formation.The study area is in an active continental margin tectonic environment associated with subduction,which may be a local inter-arc basin.The sedimentary age of the clastic rocks in the Xiaertai area is limited after the Late Permian.The provenance analysis shows that the zircon age distribution is consistent with that of the Shoushangou Formation,demonstrating that the magmatic activity of the Carboniferous-Permian period is the mian source of clastic rocks.The geochemical analysis indicates that the clastic rocks in the Xiaertai area have experienced moderate weathering,they were first circulated and rapidly deposited in an active tectonic environment,and formed in the inter-arc basin of the active continental margin.The protolith of Zhaobeishan Formation in Kaiyuan area is mainly a set of carbonate rock construction dominated by limestone,and contains some intermediate-basic volcanic rocks and clastic rocks.Its sedimentary age is after the late Triassic,and the provenance is mainly from the Carboniferous-Triassic island arc magmatic belt.It was formed in a back-arc marginal sea basin during the transformation from subduction compression in orogenic stage to post-orogenic extension stage,indicating that there were residual sea basins in the study area.In addition,the difference of the provenance between the Late Permian and the Late Triassic Zhaobeishan Formation suggests a transition in the tectonic system.4.The structural style and deformation phase of Xiaertai tectonic melange provides structural evidence for the back-arc extension and closure of the PAOThe Xiaertai tectonic melange is mainly divided into two tectonic deformation events:(1)bedding ductile shear sliding(early period);(2)NE-SW trending fold structure and nearly parallel oblique movement ductile shear zone(late period).The early bedding ductile shear sliding includes banded structure,mylonitic foliation,tensile lineation,bedding shear fold,boudinage structure and sheath fold.The timing of this deformation event is limited to 262～260 Ma,which was formed in the back-arc extension environment caused by mantle flow,providing a key structural geological evidence for the back-arc extension related to subduction.The NE-SW trending fold structures are developed in the Late Carboniferous metamorphic carbonate rocks.They mainly composed of tighting folds or vertical folds,as well as deformation bands,structural foliation,replacement bands and fold structures in the nearly parallel ductile shear zones,indicating a NE-SW compression.According to the emplacement age of the geological bodies that involved in the deformation event in the study area,the formation time of the deformation can be limited to 249～244 Ma,which provide a further restriction of the collision closure time of the eastern part of the PAO.5.The tectonic units and tectonic evolution of Late Paleozoic-Early Mesozoic accretionary orogenic belt in Changchun-Kaiyuan areaCombined with regional data,the tectonic units in the study area are divided into Carboniferous-Permian arc-basin system and Late Permain-Triassic back-arc marginal sea basin.Due to the influence of subduction slab rollback in the Early Carboniferous,A2-type rhyolite appeared in the study area,forming back-arc basins.During the Early-Middle Permian,the PAO continued to subduct southward,resulting in the development of the calc-alkaline igneous rocks,forming inter-arc basins and back-arc marginal sea basins.From the Middle-Late Permian to the Early-Middle Triassic,the formation of adakitic quartz diorite-granite assemblage and A-type granites in the study area were accompanied by layer-parallel ductile shear sliding,which was related to the heterogeneous subduction and break-off of slabs in the PAO.Until the PAO was closed in the Middle Triassic,the study area developed(soft)collisional magmatic features—typical thickened lower crust-origin adakitic rocks,accompanied by NE-SW compressional forces.In the Late Triassic,the lithospheric delamination-collapse occurred after the closure of the PAO,accompanied by the generation of Jianshanzi volcanic rocks with petrogenesis of partial melting from delaminated lower crust and the Zhaobeishan Formation with a single provenance.