The Mesozoic Tectonic Evolution Of Kuaiyuan Area In The Eastern Segment Of Northern Margin Of The North China Block

This paper takes the Mesozoic magmatites and Qinghezhen Group as the research subject,and determinated their formation time by zircon LA—ICP—MS U—Pb dating.Then,the magma source property and tectonic setting of the Mesozoic magmatite have been discussed based on the geochemistry research.Combined with the distribution and geology characteristic of the magmatite and Qinghe tectonic melange in this region,the paper finally discusses the Mesozoic tectonic evolution of Kuaiyuan area.Four periods of Mesozoic magmatic activity have been identified in the Kaiyuan area by zircon LA—ICP—MS U—Pb dating,which are Early Triassic(Jianshanzi pluton),early Late Triassic(Baoxing and Shudetun pluton),late Early Jurassic — early Late Jurassic(Bakeshu,Dagujia,Lijiatai,Chengzishan and Yingchang pluton)and Early Cretaceous(Lazishan,Tongfengtun pluton and volcanics within Deren formation),respectively.The Triassic Jianshanzi monzonitic granite distributed along with the Qinghe Fault,and its' zircon U—Pb age is 251±1.3Ma.This monzonitic granite is inriched in SiO2 and depleted in MgO,and has obvious negative Ce anomaly(?Ce= 0.14~0.83).This monzonitic granite is also enriched in Rb,K and Ba,and is relatively depleted in Ta,Nb and P.In summary,the magma that formed this monzonitic granite was most likely derived from the partial melting of lower-crustal basaltic rocks,and mixed some oceanic material during subduction.The early Late Triassic magmatites contain the Baoxing pluton and Shudetun pluton.The Baoxing granodiorite has a zircon U—Pb age of 235±1.3Ma,and enriched in LREE and LILE,and relatively depleted in Ti,Nb and P.In summary,the magma that formed this granodiorite was most likely derived from the partial melting of lower-crustal basaltic rocks.The Shudetun diorite has a zircon U—Pb age of 224±1.9Ma,it also has a high REE content and enriched in LREE and LILE,but depleted in Nb,Ta,Ti,Hf,which are typical characteristic of magmatite formed in an activity continental margin.The magma that formed the Shudetun diorite was most likely derived from depleted mantle wedge,and mixed the crust material during rising process.The tectonic setting had been transfer into extensional environment at late Late Triassic.The late Early Jurassic—early Late Jurassic magmatites have variety types of rocks,which contain gabbro,diorite,granodiorite,biotite granite and monzonitic granite,respectively.The Zhenxing biotite granite has a zircon U—Pb age of 180 ± 1Ma,and the magma that formed this biotite granite was most likely derived from the partial melting of upper-crustal rocks.The Shuangling monzonitic granite,the Lijiatai granodiorite,the Chengzishan monzonitic granite and the Yingchang monzonitic granite have zircon U—Pb ages of 176 ± 2Ma,173 ± 2Ma,169 ± 1Ma and 156 ± 2Ma respectively.All of them are adakites,and characterized by high Si,low Mg,enriched in Sr and depleted in Y and Yb.The magma that formed these adakites was derived from the partial melting of thickening lower-crustal rocks.The late Early Jurassic-early Late Jurassic adakites represent a thickening crust caused by the subduction of the Paleo-Pacific plate.The transformation of tectonic framework occurred between late Late Triassic and early Early Jurassic.The Early Cretaceous magmatites(the Lazishan,Tongfengtun pluton and volcanics within Deren formation)were formed in extensional environment,which is related to lithospheric delamination.Geological bodies of different ages and tectonic environments have been identified with the Qinghezhen Group through geochronology research.The Qinghezhen Group is actually tectonic mélange,which contains Mesoarchean,Neoarchean,Mesoproterozoic and Late Pariman—Early Triassic rocks.Detrital zircon ages indicate that the Zhaobeishan Formation was formed at Late Pariman to Early Triassic.The provenance of Zhaobeishan Formation should contain the North China Craton and Songliao Block,and the former one seems to be the mainly provenance.The metamorphic ages of 245 Ma represent the timing of final closure of the Paleo-Asian Ocean is early Mid-Triassic.In conclusion,the Kaiyuan area had experienced subduction of the Paleo-Asian Ocean during Early Triassic,and the final closure in early Mid-Triassic(~245Ma).During the Mid-Triassic,the continent-continent collision was still exsited,the transformation of extrusion to extensional environment occurred at the late Late Triassic.Since from the Early Jurassic,the Kaiyuan area had undergone the Paleo-Pacific tectonic framework.The period of late Early Jurassic to early Late Jurassic undergone a thickening crust caused by the subduction of the Paleo-Pacific plate.The Early Cretaceous magmatites were formed in extensional environment,which is related to lithospheric delamination.