Early Cretaceous Tectono-Magmatic Activity And Regional Tectonic Implications In Junan,Eastern Jiaodong

The tectonic extension of the eastern Eurasian continent during the Early Cretaceous resulted in widespread occurrence of metamorphic core complexes,stretch basin and related magmatic emplacement.The Dashan pluton in Junan,eastern Shandong,located in the southeast of the North China Craton,is a typical example.The pluton is a complex massif of several types of rocks.The mylonitized augen biotite monzonitic granite and biotite-hornblende quartz monzonite along the margins of the complex possess crystallization ages of 129.8±1.1Ma and 132.7±2.8Ma,respectively.The foliations along the northwestern margin of the complex dip to NW at with medium dip angles,and along the southwestern and northeastern margins to SE with medium dip angles.Stretching lineations are constantly plunging WNW-ESE with medium pitch angles.A transition zone is composed of porphyritic biotite-hornblende monzonite with weakly orientated K-feldspar phenocryst and mafic microgranular enclave.The porphyritic biotite-hornblende monzonite and mafic microgranular enclave are dated of 126.2±2.1Ma and128.0±1.5Ma,respectively.Meanwhile,massive porphry-bearing biotite-hornblende granitoid without any evidence of ductile deformation consist of the core of the complex with crystallization ages of 123.8±2.0Ma.The similarities in ages of crystallization of the mylonitized augen biotite monzonitic granite and MME's implies the existence of magmatic mixing processes.The petrographical zonation,relationship of tectono-magmatic and the systematical zircon U-Pb geochronology of the pluton may suggest that the Dashan pluton has experienced multistage emplacement under the same tectonic extension setting.Shear zone of surrounding rock of the initial activity time no later than 133 Ma,end time earlier than 124 Ma.Although with complex tectonic setting,Junan region and the eastern North China craton suffered significantly improvement of unified event during early cretaceous.The interaction between the Pacific plate and Eurasian plate trigger a deep lithosphere mantle and crust shallow detachment,led to regional extension of the east Asian lithosphere.During the formation of the Dashan pluton,the detachment of the crust was gave rise to partial melting of the lower crust and possibly local addition of materials with mantle signatures.