Disintegration Of Metamorphic Basement And Petergenesis Of Late Paloezoic And Jurassic Granitoids In The Northeastern Xing'an Block

The Precambrian metamorphic rocks and the late Paleozoic to Jurassic granitoids in northeastern Xing'an block were studied in this thesis.Based on systematic field geological survey,studies on the zircon geochronology,petrology and geochemistry were carried out.The composition of provenance compositions and the depositional age of the protolith of the metamorphic series were discussed;the spatial and temporal distribution of the granitoids since the Late Paleozoic in northeastern Xing'an block were built up;petrogenesis and tectonic settings of the granitoids of different periods were analyzed;and further,the tectonic evolution of the Xingan block was discussed.The detrital zircon chronology of three samples of two-mica quartz schist from the Xinghuadukou group and the Luomahu group?Beikuanhe formation?show consistent characteristics of age spectrum.The detrital zircons show that the protolith had complex provenance compositions from Proterozoic to Paleozoic,and the early Paleozoic magmatic rocks were the main sources.The weighted average ages of the youngest detrital zircon groups were 404-417 Ma,which indicates that maximum depositional age of the protolith might be the early Devonian.The late Paleozoic granitoids underwent strongly ductile deformation.6 granites that strongly deformed displayed 295-305 Ma intrusive age.The rock association consisted of monzogranite,synogranite and alkali feldspar granite mainly.Geochemically,these rocks belong to Peraluminous or metaluminous high K calc-alkaline series and have high contents of SiO₂,Na₂O and K₂O,and low contents of CaO,Al2O3 and MgO.All these features show that these granitoid belong to high fractionated I-type or A2-type granite,which usually occurred in post-orogenic setting.It is suggested that these granitoids formed at the beginning of post-orogenic extension setting in the late Carboniferous-early Permian after the collisional orogen between the Xing'an and the Songnen blocks.Mesozoic granitiods are widely distributed in the northeast of Xing'an block and can be divided into two stages: the Early-Middle Jurassic and Late Jurassic.TheEarly-Middle Jurassic granitiods,as the important component of the Mesozoic intrusions of the Great Xing'an Mountain,outcropped from the western Heihe to the southern Huma areas.The zircon LA-ICP-MS U-Pb dating of 11 samples from 6intrusive bodies display similar ages of 169.1Ma-181.2Ma.These granites are composed mainly of tonalite,granodiorite,quartz-diorite,monzogranite and some diotite,belonging to the TTG rock association.Geochemically,the granitoids have high SiO₂,K₂O and N₂O contents and low CaO,MgO and Fe OT contents,belonging to sodic,metaluminous to weak peraluminous series,and calc alkaline and MA series in different classification diagrams.They are rich in the LILEs of Rb,Ba,Sr and Th,U and depleted in HFSEs of Nb,Ta,Zr and Hf,and have no obvious negative Eu anomalies,which is similar to the characteristics of arc and active continental margin magmatic rocks.All these characteristics show that the magma originated from the dehydration and melting of the subducted oceanic crust,and then the magma rose and interacted with the overlying mantle wedge and induced partial melting of the lower crust and incomplete magma mixing.Combined with regional geological data,the J1-2TTGs formed during the Mongol-Okhotsk oceanic plate subducted southward,and the tectonic setting of Erguna and Xing'an blocks belongs to an active continental margin at that time.This thesis confirmed firstly that there was a large amount of Late Jurassic granites in the Northeastern Xing'an block.These granite intruded between151-158 Ma.The rock association is monzogranite,granodiorite and a small amount of two mica granite,which displayed coarse-grained texture,porphyritic texture and partly rapakivi texture.Geochemically,the granitiods is characterized by high SiO₂,K₂O,Na2O and Al2O3,and low CaO,MgO and TiO₂,which generally belong to peraluminous and high-k calc alkaline series.The overall characteristics of these granites belong to C-type adakite.Based on the characteristics of rapakivi texture,peraluminous and C-type Adakite,it is suggested that the late Jurassic granites originated from partial melting of a thickened crust source of high Al2O3 content,which consisted of mainly graywacke and minor pelite.Mixing with basaltic magmaand isothermal decompression might happened during the formation of the granitic magma.Combined with the studies on regional stratigraphic and structure,it is suggested that the late Jurassic magma originated from a thickened crust,which formed under the collision setting between north and south blocks after the closure of the Mongol-Okhotsk ocean.In summary,the so called Xinghuadukou and Luomahu groups in northeast Xing'an block should be disintegrated,with the protolith being early Paleozoic sedimentary rocks and late Paleozoic granites.The tectonic transition of collisional orogeny and post-orogenic extension between the Xing'an and the Songnen blocks occurred during the late Carboniferous-early Permian.During early-middle Jurassic,the northeast Xing'an block was in an active continental setting,which was induced by the southward subduction of the Mongol-Okhotsk ocean plate.At late Jurassic,a strongly collisional orogeny occurred in the study area accompanied with the closure of the Mongol-Okhotsk ocean.