Late Mesozoic Magmatism And Sedimentation In The Tuquan Area Along The Western Margin Of The Songliao Basin:Implications For Regional Tectonics

Northeast（NE）China is located in the eastern of the Central Asian Orogenic Belt,influenced by the Mongol-Okhotsk tectonic regime and the Paleo-Pacific tectonic regime,experienced large-scale magmatism,sedimentation,orogeny and plate tectonics in the Late Mesozoic.The Songliao Basin,located in NE China,is the largest continental oil-bearing basin in the world during the Late Mesozoic era,depositing a Cretaceous volcanic sedimentary sequence with a thickness of 10000meters.The Tuquan Area,located on the western margin of Songliao Basin,mainly composed of Jurassic volcano-sedimentary strata.Their complete volcano-sedimentary sequence from the Jurassic to the Cretaceous provides a basis for the study of tectonic evolution in NE China.On the basis of field geological survey in Tuquan area on the western margin of Songliao basin,this thesis selected the late Jurassic to early Cretaceous lamprophyres,the late Jurassic peraluminous rhyolite and tonalite porphyrite in Tuquan area for petrological,chronological and geochemical studies,and analyzed their petrogenesis and tectonic background.The sedimentary environment,sedimentary facies,composition and structure of sandstone and conglomerate,detrital zircon U-Pb of sandstone were studied to analyze the sedimentary characteristics and source-sink background of Wanbao Formation in Tuquan area.The structural pattern of the structural deformation recorded in the sedimentary strata of Wanbao Formation in Tuquan area are studied,and the tectonic stress attribute and deformation period are analyzed.Based on the magmatism,sedimentary source-sink background and tectonic deformation characteristics of Tuquan area,new understandings of the late Mesozoic tectonic evolution in NE China is proposed,and reveal the superimposed and reworked geodynamic mechanism of the Mongol–Okhotsk tectonic regimes in NE China.The main conclusions of this thesis are as follows:1.Under the background of long period thinning（160-90 Ma）of the Late Jurassic to the Early Cretaceous lithosphere in NE China,there was a short period thickening（156-132 Ma）.（1）A series of lamprophyre dikes have been recently discovered in the Tuquan Area of the western Songliao Basin that have been dated to 156.0±2.3 Ma,132.9±1.2 Ma,and 126.2±2.5 Ma by using the zircon U–Pb technique.The lamprophyres are shoshonite and calc-alkaline in series,are characterized by large amounts of Fe O^T,Ti O₂,Mg O,and Mg#,and are rich in LREEs and LILEs but poor in HREEs and HFSEs.They have high ratios of（La/Yb）_N,La/Ta,La/Nb,Th/Y,Ba/Nb,Ba/Ta,and Ba/Th,and low ratios of Zr/Ba,La/Sm,and Nb/Zr.These features collectively point to the derivation of dike magmas from the partial melting of the enriched lithospheric mantle that had been previously metasomatized by subduction-related fluids.The Late Jurassic lamprophyre（156.0±2.3 Ma）has high ratios of Rb/Sr（0.42）and K/Yb*1000（28.3）,and low ratios of Ba/Rb（13.5）and Dy/Yb（2.35）,suggesting the derivation of magma from a high degree of partial melting of the phlogopite-bearing lherzolite mantle in the spinel–garnet transition zone at a depth of about 60 km.The Early Cretaceous lamprophyre（132.9±1.2 Ma）has low ratios of Rb/Sr（0.02–0.06）and K/Yb*1000（8.13–19.73）,and high ratios of Ba/Rb（17.6–42.6）and Dy/Yb（3.48–4.09）,indicating that the magmas were derived from a low degree of partial melting of the lherzolite mantle in the garnet zone at a depth of ca.85 km.The younger lamprophyres（126.2±2.5 Ma）have a lower Dy/Yb ratio（3.48–3.92）than the older QML（132.9±1.2 Ma）,with a Dy/Yb ratio of 4.09 implying that the younger magma was formed at a shallower depth of the mantle（<85 km）than the older one.These observations indicate that in 156–132 Ma,the lithosphere thickened by approximately 25 km at a rate of approximately 1.0 km/Myr,and in 132–126 Ma the lithosphere thinned.（2）The Late Jurassic peraluminous rhyolites and tonalite porphyrites in the Tuquan Area of the western Songliao Basin have been dated to 156.2±0.82 Ma and155.1±1.4 Ma by using the zircon U–Pb technique.They are calc-alkaline in series,are characterized by high ratios of aluminium saturation index with A/CNK（1.32–2.13）,and low ratios of Fe O^T/Mg O（2.45–5.03）and low amounts of Mg O+Fe O^T（0.96–3.36）.The S-I-M-A classification scheme of granite was used to map in the relevant geochemical diagram.The results showed that the rhyolites and tonalite porphyrites in the study area generally belong to S-type granite.The rhyolites and tonalite porphyrites have low ratios of Rb/Sr（0.35–0.55）and Rb/Ba（0.08–0.26）,suggesting the original magma originated from partial melting of metamorphic sandstones with poor clay.The rhyolites and tonalite porphyrites are rich in large-ion lithophile elements but poor in high field strength elements,have low amounts of Y(5.29–19.75×10^-6),Nb(7.43–8.49×10^-6),Sr(60.6–154.9×10^-6),and Yb(0.53–2.40×10^-6),indicates that they showed the lithologic affiliation to the volcanic arc.In the R1-R2 construction discriminant diagram,the rhyolites and tonalite porphyrites in the study area mainly fall within the range of syn-collision and late orogenic granites.The diagram of Nb-Y,Rb/10-Hf-Ta×3,and Sr-Yb indicates that the rhyolites and tonalite porphyrites in the study area were formed in the background of oceanic crust subduction and continental collision conversion,and are the products of partial melting of crustal materials in the compressive thickening tectonic background.2.The time limit of tectonic compression event at the convergent continental margin margin in NE China was advanced from the Late Jurassic-Early Cretaceous to the Middle-Late Jurassic,and the direction of regional compressive tectonic stress field was determined.（1）The stratigraphic sequence and rock association of the Mid-Upper Jurassic Wanbao formation in the Tuquan Area show that its main lithology is conglomerate,sandstone,siltstone and mudstone.The sedimentary facies from bottom to top are fan delta front subfacies,fan delta plain subfacies,fan delta front subfacies,front fan delta subfacies and semi-deep lake to deep lake subfacies.The sandstone clastic components of Wanbao formation include quartz（40–50%）,feldspar（25–35%）,volcanic rock debris（15–5%）,and sedimentary rock debris（3–8%）.Dickinson’s diagram shows that magmatic arc provenances and recycled orogen provenances are the main source areas of sedimentary rocks.The gravel of the Wanbao formation is mainly composed of granite,rhyolite and quartzite,followed by coarse sandstone,basalt and andesite.The gravel is sub-angular and sub-circular,with a large particle size,ranging from 2.3 to 9.5 cm,they are poorly sorted and have characteristics of near-source deposition.The conglomerate layer is an imbricated and massive structure,with cross-bedding and scour surface developed at the bottom.It is a sedimentary sequence from fan delta plain subfacies to fan delta front subfacies.The detrital zircon age distribution of the Wanbao Formation sandstone has a double peak of 162 Ma and305 Ma,indicating that the maximum deposititional age of the Wanbao formation is156.6±2.4 Ma～161.8±1.9 Ma.The crystallization ages of individual zircon grains present in the sediment are close to the depositional age of the sediment,indicating that the Wanbao Formation in Tuquan Area is in a convergent environment.All the above studies indicate that the Wanbao Formation in Tuquan Area was deposited as a retroarc foreland basin.（2）The Mid-Upper Jurassic Wanbao formation in Tuquan Area experienced strong structural deformation in the stratigraphic epoch,which is manifested by folds,reverse faults,shear joints and angular unconformity.In this paper,38 normal faults and reverse faults recorded in the strata of the three sections of the Wanbao Formation are selected for hemispherical stereographic projection analysis at the same angle as the Wushi network.The formation period of the fault is divided according to the occurrence of the fault layer in the field section,the cutting relationship,the rock sequence of the stratum and the position of the section,so as to determine the stress period of the stress field.The results show that the tectonic stress in Tuquan area has four stages during the Middle to Late Jurassic:（1）NWW-SEE extension,（2）NNW-SSE extrusion,（3）NW-SE extrusion,（4）NW-SE extension.In the early Middle Jurassic,the Wanbao Formation was controlled by normal faults and was in an extensional environment.In the Middle Jurassic to early Late Jurassic,tectonic inversion occurred,and the extensional environment was transformed into a compressive environment.The direction of compression was changed from NNW to NW,and then strike slip occurred,showing NW-SE extension.3.The influence of the Mongol–Okhotsk tectonic regimes（southward subduction of oceanic plate,ocean closure,continent collision,and lithosphere extension after collision）on NE China spatially includes to the Tuquan Area at the western margin of Songliao Basin.The transformation of the lithosphere in NE China by the Mongol–Okhotsk tectonic regime has both thinned and thickened,forming both extensional fault basins and retroarc foreland basins.From the Middle Jurassic to the Early Cretaceous,the influence range of the Mongol–Okhotsk tectonic regime in NE China extended spatially the Tuquan Area on the western margin of the Songliao basin through the Erguna block and Xing’an block.The lithospheric scale of tectonic evolution,ranging from the Siberian craton via the Mongol–Okhotsk Suture Zone to the Tuquan Area during the Kimmeridgian and the Aptian,involved three pulsed stages.The southeastward subduction of the Mongol–Okhotsk oceanic crust that resulted in the suture between the Siberian craton and the Erguna–Xing’an–Songliao Blocks in the Kimmeridgian and Tithonian,the continental collision and lithospheric thickening in the Valanginian and the Hauterivian,and the post-collisional delamination of the thickened lithosphere and its extension during the Barremian up to the Aptian.