西刚果埃迪卡拉纪构造转折OA

Objectives:In order to reveal the Ediacaran tectonic transition process of West Congo orogenic Belt,this paper studies the petrology,zircon U-Pb chronology and geochemistry of the newly discovered quartz diorite and quartz monzodiorite in the northwest of Angola,analyzes the genesis of the rocks and the tectonic environment. Methods:On the basis of field work,the systematic analysis of quartz diorite and quartz monzodiorite is carried out through microscopic observation,rock chemical analysis and LA-ICP-MS zircon U-Pb isotope chronology. Results:The zircon U-Pb ages of quartz diorite and quartz monzodiorite are 578.0±2.3 Ma and 543.9±2.6 Ma respectively,belonging to the middle and late intrusions of Neoproterozoic Ediacaran.Quartz diorite is rich in hornblende,biotite and plagioclase,belonging to the quasi-aluminous potassium-calc-alkaline series.It has the characteristics of island arc magmatic rocks,enriched with large ion lithophile elements such as Ba and K,deficient in Th,U,Ta,Nb,P,Ti and other high field strength elements,and the rare earth partition curve is right-leaning type of LREE enrichment.Quartz monzodiorite is rich in biotite and plagioclase and belongs to high potassium calc alkaline peraluminous rock.Compared with quartz diorite,it is rich in K2O,Na2O,Al2O3,P2O5 and poor in CaO,MgO,Fe2O3T.It enriched large ion lithophilic elements such as Rb,Ba and K(LILE)and high field strength elements such as Hf and Zr,and lost high field strength elements such as Th,U,Ta,Nb and Ti(HFSE).The rare earth partition curve was a right-leaning type with strong LREE enrichment. Conclusions:There are two stages of magmatic activities,namely subduction(quartz diorite,578.0±2.3 Ma)and post-collision(543.9±2.6 Ma),in West Congo belt,confirming the existence of the Pan-African calcium-alkaline magmatic arc.Quartz diorite is the product of crust-mantle mixing under subduction mechanism,which can be compared with the G1 suite in Ara ç uaí belt.Quartz monzoniorite is the product of partial melting of the mafic lower crust under the post-collision background,which can be likened to the G3 suite in Ara ç uaí belt.The northern section of Adamastor Ocean is in a continuous subduction stage at 578.0 Ma,and the closure time is no later than 543.9 Ma.The time when West Congo Belt turned from subduction to collision structure is the middle to late of Ediacaran.