Geochemical Characteristics And Geological Significance Of Late Jurassic-Early Cretaceous Intrusive Rocks In Chejiayingzi Region,Inner Mongolia

This thesis studies on the widely distributed Late Jurassic-Early Cretaceous intrusive rocks in Chejiayingzi area of Inner Mongolia.Based on the field investigation,we studied zircon U-Pb geochronology and geochemistry of the Late Jurassic-Early Cretaceous intrusive rocks in this area,in order to determine their emplacement age,petrogenesis and tectonic evolution of the region..The results of this study will not only improve the geological data of Chejiayingzi area,but also provide new scientific data for the activity time and tectonic evolution history of the Late Jurassic-Early Cretaceous intrusive rocks in the central and southern Daxing'anling.Lithologically,the Late Jurassic-Early Cretaceous intrusive rocks of Chejiayingzi area is composed mainly of fine-grained diorite,granodiorite,biotite granite,monzonite porphyry,granodiorite porphyry and granite porphyry.The LA-ICP-MS zircon U-Pb data indicate that the granodiorite from Chejiayingzi area formed during the Early Cretaceous with the age of 138±10Ma.Geochemically,the Late Jurassic-Early Cretaceous intrusive rocks of Chejiayingzi area show similar major and trace elements compositions.They are characterized by high silicon [w?SiO₂?= 60.34%⁷7.43%] and alkalis,and low calcium and magnesium,which belongs to acidic rocks;The w?Na₂O?/w?K₂O?ratios ranged from 0.75 to 1.31,A/CNK values are ranged from 0.75 to 1.31 and with metaluminous to peraluminous rock series features;In the Harker diagram,with the increase of SiO₂ content,Al₂O₃,Fe₂O₃,MgO,CaO,TiO₂,and P₂O₅ showed a significant negative correlation,K₂O showed a positive correlation;The rare earth element patterns show that these granites are enriched in LREEs and depleted in HREEs with different degree negative Eu anomalies;They are enriched in Rb,Th and other large ion-lithophile elements,depleted in high field strength elements such as Ba?Nb and Ta;They have positive correlations between Rb and Th;The studied samples commonly contain amphibole but carry no typical perluminous minerals such as muscovite and garnet,indicating that they are similar to I-type granite.Geochemical characteristics and tectonic discrimination show that the Late Jurassic-Early Cretaceous intrusive rocks of Chejiayingzi area occured in an extensional tectonic environment,formed by partial melting of the crust,which may be related to the subduction of the Paleo-Pacific Plate.