Geochronological And Geochemical Studies Of The Mesozoic Volcanic Rocks In The Hailar-Tamtsag Basin

This thesis conducts systematic studies on the petrology,zircon U-Pb geochronology,whole-rock geochemistry,whole-rock Sr-Nd isotopes and zircon Hf isotopes of the Mesozoic volcanic rocks in the Hailar–Tamtsag Basin(HTB),Northeastern(NE)China and Mongolia,and obtains the following achievements: 1)establish a refined chronostratigraphic framework for the Mesozoic volcanic-sedimentary strata in the HTB;2)ascertain the petrogenesis of the Mesozoic volcanic rocks and their tectonic settings;3)reveal the superimposed and reworked geodynamic mechanism of the Mongol–Okhotsk and circum-Pacific tectonic regimes in NE China.The Mesozoic volcanic-sedimentary strata in the HTB is conventionally divided from bottom to top into the Tamulangou,Tongbomiao and Nantun formations based on seismic reflection profiles,rocks assemblages,terrestrial paleontological fossil assemblages,and regional stratigraphic correlation,but there is a lack of high-precision isotope chronological data.This thesis performed systematic LA-ICP-MS zircon U-Pb dating for the Mesozoic volcanic rocks from 32 boreholes in the HTB.These new data have helped us to constrain the formation age of the Mesozoic volcanic-sedimentary strata and establish a refined chronostratigraphic framework: Tamulangou Formation formed in the Middle Jurassic Callovian-Late Jurassic Tithonian(166~145 Ma);Tongbomiao Formation formed in the Early Cretaceous Berriasian-early Valanginian(142~136 Ma);Lower Nantun Member formed in the Early Cretaceous late Valanginian-early Aptian(135~120 Ma);Upper Nantun Member formed in the Early Cretaceous late Aptian-early Albian(119~111 Ma)?This thesis recognized the various Mesozoic volcanic rocks in the HTB,and their formation is closely related to the subduction of the Paleo-Pacific Plate and the closure of the Mongol–Okhotsk Ocean.The Middle Jurassic high-K adakitic volcanic rocks were derived by dehydration melting of thickened garnet-bearing amphibolite-facies lower continental crust,and they were the products of the closure of the Mongol–Okhotsk Ocean.The early Late Jurassic low-K adakitic lavas could be considered as pristine slab-melts that were likely generated by hydrous melting of subducted eclogitic oceanic crust,followed by limited interaction between the slab melt and peridotite as a consequence of flat-slab subduction.The late Late Jurassic Nb-enriched basaltic andesite were derived by low degree(<2%)partial melting of garnet-facies,phlogopite-bearing lherzolite mantle wedge that had been metasomatized by subduction slab-derived melts,and they were the products of the rollback of the subducting Paleo-Pacific Plate.Late Early Cretaceous high-Mg adakitic volcanic rocks resulted from the partial melting of a delaminated region of the lower continental crust,with the magmas subsequently interacting with mantle peridotite upon ascent.Late Jurassic-Early Cretaceous high-silica volcanic rocks can be divided into I-and A-types.The I-type high-silica volcanic rocks formed from magmas generated by partial melting of a dominantly juvenile mica-bearing K-rich basaltic lower crust.The A-type high-silica volcanic rocks originated from the partial melting of a dry mafc–intermediate middle–lower crust that was dehydrated but not melt depleted.Additionally,the A-type high-silica volcanic rocks formed mainly in late Late Jurassic and late Early Cretaceous,corresponding to the rollback of the subducting Paleo-Pacific Plate and delamination of the lithosphere.These data,together with spatial–temporal distribution of Mesozoic magmatism in NE China,led us to constrain the superimposed and reworked geodynamic processes of the circum-Pacific and Mongol–Okhotsk tectonic regimes.Flat-slab subduction of Paleo-Pacific Plate beneath NE China resulted in inland migration of the Jurassic magmatism,whereas a striking magmatic lull gradually occurred in the Songliao Basin and eastern Heilongjiang and Jilin provinces close to the trench during Late Jurassic-early Early Cretaceous.Crustal shortening and thickening clearly occurred in the HTB and Great Xing'an Range as a result of the closure of the Mongol–Okhotsk Ocean.As the flat-slab segment of Paleo-Pacific Plate moved beneath the thickened lithosphere of HTB and Great Xing'an Range,the buoyant oceanic crust was deep enough to undergo sufficient eclogitization to become negatively buoyant once again.Once the buoyant Paleo?Pacific oceanic crust lost its positive buoyancy,the flat slab was no longer stable and began to roll back.During the late Late Jurassic-early Early Cretaceous,low-velocity rollback of the Paleo-Pacific slab resulted in a small-scale and low-velocity upwelling of asthenospheric mantle,and its influence was limited to the front edge of the subduction plate and adjacent regions.In the process of the continued rollback of the Paleo-Pacific Plate,the magmatism gradually resumed in the Songliao Basin and eastern Heilongjiang and Jilin provinces,forming a trenchward-younging(southeastward-younging)trend of the Early Cretaceous magmatism in NE China.Meanwhile,as sinking Paleo-Pacific Plate gradually stagnated in the mantle transition zone,the delamination of the lithosphere and climax of the Early Cretaceous magmatism in NE China migrated southeastward.