Petrogenesis Of Early Cretaceous Volcanic Rocks Of The Southern Jilin Province And Liaodong Peninsula

This thesis presents zircon U-Pb ages,whole-rock major and trace element data,and Sr-Nd-Pb isotope data for the early Cretaceous volcanic rocks from the southern Jilin Province and Liaodong Peninsula in the northeastern North China Craton(NCC),combined with geological survey and systematic studies on petrography,which will not only clarify the rock types and distribution of the early Cretaceous volcanic rocks,but will also reveal the nature of magma source,tectonic setting and deep process underneath the northeastern NCC.Combined with the research before in the study area,we establish the geodynamic evolution model of the flat subduction and retreat of the Paleo-Pacific plate during the Mesozoic.The main contents of this thesis are as follows:1.Rock types and distribution of the early Cretaceous volcanic rocksLate Paleozoic to Mesozoic volcanic and intrusive rocks are widely developed in the southern Jilin Province and Liaodong Peninsula,northeastern NCC,and most formed in three periods: the late Triassic(233?210 Ma),Jurassic(180?150 Ma)and early Cretaceous(135?118 Ma).Althogh minor late Triassic to Jurassic volcanic rocks is recorded,but early Cretaceous volcanic rocks are widely distributed.The rock types of early Cretaceous volcanic rocks from different locations in the southern Jilin Province and Liaodong Peninsula are different.Early Cretaceous volcanic rocks from the Tonghua Basin and Guosong Basin within the southern Jilin Province consist of a series of alkaline volcanic rocks.The rock types of the volcanic rocks in the Tonghua Basin include basalt,basaltic trachyandesite and trachyandesite;the volcanic rocks in the Guosong Basin are mainly basalt trachyandesite,trachyandesite and trachyte.Both of them were formed at 129 Ma.A series of subalkaline volcanic rocks are developed in the Liaodong Peninsula.The rock types are mainly andesites(some of which are adakitic andesites),and occasionally basaltic andesites,which formed at 131?129 Ma.Early Cretaceous volcanic rocks from the Huanren area,which located in the transition between the southern Jilin Province and the Liaodong Peninsula,were formed at 129?122Ma.The rock types in Huanren area include basalt,basaltic andesite,andesite,basaltic trachyandesite,trachyandesite and rhyolite.2.Magma source and evolution of the early Cretaceous volcanic rocksMost of the early Cretaceous volcanic rocks in the northeastern NCC underwent assimilation and fractional crystallization(AFC process).Among them,mantle-derived volcanic rocks with low evolution degree and not affected by crustal contamination are enriched in light rare earth elements(LREEs)and large ion lithophile elements(LILEs),depleted in heavy rare earth elements(HREEs)and high field strength elements(HFSEs).These mantle-derived volcanic rocks are characterized by “arc-type” trace element features and “weakly enriched” whole-rock Sr-Nd and highly radiogenic Pb isotopic compositions,which were derived from the partial melting of the asthenosphere mantle which metasomatized by the subducted sediment-derived melt.Intermediate rocks of Xiaoling Formation in Huangren and Xiuyan areas,have mainly undergone fractional crystallization but minor crustal contamination,and their isotopic compositions indicate that their were likely produced by melting of the ancient lithospheric mantle.The rhyolite of Xiaoling Formation in Huanren area and andesite in Dandong area,which show highly enriched Sr-Nd isotopic compositions,suggests that they were derived from the the continental crust.The adakitic trachytes in Guosong Basin of the southern Jilin Province and the adakitic andesites in Guiyunhua Formation of the Liaodong Peninsula resulted from the partial melting of a delaminated region of the lower continental crust,which the magmas subsequently interacting with mantle peridotite upon ascent.The magma source of early Cretaceous volcanic rocks in the southern Jilin Province and Liaodong Peninsula were diverse,suggests a strong mantl-crust interaction process related to decratonization of the NCC.3.The relationship between the formation of East Asia big mantle wedge(BMW)and the evolution of lithosphere in the northeastern NCCEarly Cretaceous igneous rocks are widely developed in the southern Jilin Province and Liaodong Peninsula,northeastern NCC.Identification of four components,i.e.,depleted and enrichen mantle and juvenile and ancient crust in the early Cretaceous magmatism in the region.Early Cretaceous mantle-derived volcanic rocks were derived from partial melting of asthenosphere mantle metasomatized by sediment-derived melt overlying subduction slab,and high-Mg adakitic volcanic rocks resulted from the partial melting of a delaminated region of the lower continental crust,which the magmas subsequently interacting with mantle peridotite upon ascent.These data,together with the exhumation of metamorphic core complex,a large number of half grabens,and detachment faults in the study area,demonstrate that the crust of the region was relatively thin and experienced exceptionally intense extension during early Cretaceous and large-scale retreat of the Paleo-Pacific plate.At the same time,addition of subducted sediment-derived melt may be an important mechanism for mantle enrichment,indicating that the subduction slab does not sink into the lower mantle,but gradually stagnated in the mantle transition zone.Combined with the previously published geochronological,petrological and geochemical data of Mesozoic igneous from the northeastern NCC suggests that the BMW structure most likely already existed in the early Cretaceous.The formation of this BMW structure has greatly influenced many aspects of the geological evolution of East Asia.The basal continental mantle lithosphere of the northeastern NCC was probably disturbed by flat-slab subduction in the Jurassic.The flattening slab may have acted as a barrier for whole mantle convection,which caused a hiatus in mantle-derived melting in the late Jurassic.With the retreat of the Paleo-Pacific plate,the release fluid and melt of the Paleo-Pacific plate gradually stagnated in the mantle transition zone changed the rheological and dynamic properties of the overlying mantle,which not only significantly improved the efficiency of thermo mechanical-chemical erosion at the bottom of the lithosphere,but also caused the delamination due to the gravitational instability of the lithosphere,resulting in the destruction of NCC.