Late Early Cretaceous Magmatism,Migration Of Rifting In NE China And Implictions For Basin Geodynamics

Northeast China and adjacent areas experienced extensive tectonic extension in the Late Mesozoic.Many rift basins such as Songliao,Hailar and Erlian basin and a large number of magmatic rocks were developed.The Late Mesozoic magmatic rocks from the Daxinganling to the eastern Jilin-Heilongjiang showed a new trend of migration.How is the deep kinetics of such a large-scale rifting and magmatism in the northeastern China in the Late Mesozoic?This issue has long been a focus for geoscientists.This paper focuses on Late Mesozoic magmatism and the evolution and migration of the rift basins.Through the systematic study of petrology,geochronology,whole rock element geochemistry and Sr-Nd isotopes,the age,origin,Source characteristic and tectonic setting of the volcanic rocks were determined,revealing the deep process and temporal-spatial differences of the continental margins and inland volcanic rocks in the northeast;combining the analysis and comparison of the stratigraphic framework of the representative rifting basins and the tectonic analysis and evolution history of regional seismic large sections,the study reveals evolution and migration of the early Cretaceous rift basins.And based on the study of this paper,we combine the characteristics of magmatic and the evolution and migration of rift basins and discuss the dynamic mechanism of the Late Mesozoic rift basin in Northeast China.The Late Mesozoic volcanic rocks in the eastern continental margin of northeastern China are the most representative in Yanji area,the volcanic rocks are of many types and are widely developed.Field investigations and petrological studies have shown that the lower volcanic rocks consist of several sets of andesite-dyanite eruption cycles,while the upper part is dominated by basalt.The geochronological results show that the volcanic rocks mainly formed in the late Early Cretaceous(112-105 Ma).Geochemical analysis of the rock shows that the lower intermediate and intermediate-acid volcanic rocks have the characteristics of island-arc magmatic rocks,such as significant negative Nb,Ta,and Ti anomalies,and low Ti O₂ content,high Al₂O₃ content,low(La/Yb)_N ratio,low K₂O content;K₂O wt%<Na₂O wt%;and the characteristics of adakite such as Sr>535 ppm,low Mg O content(3.38-1.3 wt%),low Y and Yb content,high Sr/Y ratio,slight negative Eu anomaly.The initial ⁸⁷Sr/⁸⁶Sr value is 0.7040-0.7050,and the?Nd(t)is 1.48-3.82,showing that the magma source area has the characteristics of depleted mantle.The source rock may come from the partial melting of the lithosphere mantle that is influenced by fluids and melts metasomatism in extensional background.In the later period,it was influenced by the fractional crystallization and the contamination of the crustal material.The upper Luozigou basalt is generally enriched with large-scale ionic lithophilic elements Ba,K,and U,and depleted high field strength elements such as Nb,Ta,and Zr.Compared with ordinary island-arc basalts,the Luozigou basalt has higher Nb content and Ti content,and the ratio of Nb/Ta is high(23.7-24.9),which belongs to niobium-rich basalt The initial ⁸⁷Sr/⁸⁶Sr value is 0.7041-0.7042,and the?Nd(t)is 0.01-3.36,showing that the magma source area has the characteristic of depleted mantle.The source rocks of the Luozigou basalt originate from melting of the metasomatized lithospheric mantle by the ocean plate melt,and are affected by weak crust mixing and a certain degree of separation and crystallization.The combination of adakite and niobium-rich basalt indicate a continental arc environment in the late Early Cretaceous in Yanji area.The intra-continental volcanic rocks in the southern part of Northeast China are represented by the Daxingzhuang Formation in Jinzhou.The Daxingzhuang Formation is characterized by intermediate-acid volcanic rocks,which is mainly composed of trachyandensite-trachyte and rhyolite.The zircon SHRIMP U-Pb dating shows that it was formed in the late Early Cretaceous(121-120 Ma).Geochemical analysis shows that the rock has high Si O₂,Al₂O₃,Sr content,low Mg O,Yb content,high Sr/Y,La/Yb,similar to the characteristics of adakite;and has lower ¹⁴³Nd/¹⁴⁴Nd(0.511712-0.511825,?Nd(t)=-14.38?-18.36)and moderate initial ⁸⁷Sr/⁸⁶Sr ratio(0.705989-0.706705),showing Thecharacteristics of enrichment type endmembers(EMI),indicate that the volcanic rocks of the Daxingzhuang Formation are derived from the partial melting of the thicker eclogite facies lower crust.Some adakites have a higher Mg#value(47.3-50.3),and other samples have a lower Mg#(27.0-43.2),which indicates that the melt-mantle interaction may exist in the magmatic evolution process.The adakite of the Daxingzhuang Formation may be related to the thickened lower crust delamination.Based on the analysis of the chronostratigraphic framework of the basins in the western,central and eastern basins of northeastern China and the study of the structural deformation and evolution history based on the regional seismic profile,we find that the extensive rifting during the Early Cretaceous expanded gradually from west to east.Especially in the central and eastern belts,both the rifting process and the lithologic filling sequence of the rifting basin are very similar,the initial rifting period mainly developed clastic rock and intermediate-basic volcanic rock,and the strong rifting period mainly developed large thick coal-bearing clastic rock,while the late rifting period ended with acidic or intermediate-basic volcanic rocks,but the stratum have the characterof diachronism.Combined with the characteristics of temporal and spatial evolution of Mesozoic magmatism in northeastern China,this thesis suggested that the Early Cretaceous tectonic extension in Northeast China may be controlled by the thickened lower crust delamination and the subduction of the paleo-Pacific Plate,but the gedynamic had obviousdifference in temporal and spatial.In other words,the western belt may be mainly controlled by the thickened lower crust delamination with the roll back of the paleo-Pacific Plate during the early to middle Early Cretaceous,while the central belt was successively affected by the roll back of the paleo-Pacific Plate during the middle to late Early Cretaceous,and the thickened lower crust delamination.The eastern zone may be mainly related to the subduction of the the paleo-Pacific Plate during the late Early Cretaceous.