Crustal Growth And Reworking Processes Of The Erguna And Xing'an Massifs

The research on the structure,composition,formation time and mechanism of the continental crust has never stopped and has always been the frontier and core in geoscience research.As an important component of the continental crust,granite has the potential to track crustal evolution.In this thesis,the Erguna and Xing'an massifs in the eastern Central Asian Orogenic Belt(CAOB)are selected as the study areas.According to the temporal and spatial distribution of granitoids in the Erguna and Xing'an massifs,71granitoid samples(34 from the Erguna Massif and 37 from the Xing'an Massif)from different locations with different ages are selected.On the basis of perfecting the chronological and geochemical data of these granitoids,the analyses of in situ feldspar Sr-Pb isotopes,apatite Nd isotope,and zircon Hf isotope were conducted on granitoids within the Erguna Massif,and the in situ apatite Nd and zircon Hf isotopic compositions in granitoids from the Xing'an Massif are also studied.According to the spatiotemporal variation of single mineral Sr-Pb-Nd-Hf isotopes and previous data,the crustal nature and structural characteristics of the Erguna and Xing'an massifs are determined.In addition,through the summary of zircon Hf isotope in granitoids,the crustal evolution process and its mechanism of these two massifs are further revealed.The main achievements are as follows:1.Crustal nature and heterogeneity of the Erguna Massif were identified.Zircon and apatite Hf-Nd isotopes in the granitoids from the Erguna Massif show the model age peaks of Neoarchean,Paleoproterozoic,and Meso-to Neoproterozoic.The feldspar Sr-Pb isotopes of the Precambrian granitoids also display high I_Sr values and high??&??values.Combined with the Neoarchean(?2.5 Ga),Paleoproterozoic(?1.8 Ga)and Neoproterozoic geological bodies discovered in the northeastern of the massif,it is revealed that the Erguna Massif should be an ancient microcontinent with Precambrian crystalline basement in the eastern CAOB.In this thesis,the single mineral Sr-Pb-Nd-Hf isotopic compositions of the granitoids in the Erguna Massif show that with decreased formation age of granitoids,the model ages of apatite Nd and zircon Hf isotope gradually decrease,the feldspar I_Sr values gradually decline,as well as the ²⁰⁶Pb/²⁰⁴Pb values of Pb isotope gradually increase,indicating that the formation of granitoids in this area has experienced a change in the magma source from the melting of ancient crust to the melting of more juvenile crust.The Sr-Pb-Nd-Hf isotopic compositions of the granitoid samples in the Erguna Massif are different from that of the typical granitoids representing the Phanerozoic accretion,which suggests the crustal evolution of the Erguna Massif during the Phanerozoic is dominated by crustal reworking rather than crustal accretion.Furthermore,the temporal and spatial variation of single mineral Sr-Nd-Hf isotopic compositions in granitoids reveals that the continental crust within the Erguna Massif has lateral and vertical heterogeneities.2.Crustal nature and heterogeneity of the Xing'an Massif were identified.In this thesis,Neoarchean and Paleoproterozoic geological bodies of?2.5 Ga and?1.8 Ga have been discovered in the Longjiang area,southern Great Xing'an Range.Together with the Archean and Neoproterozoic model ages of zircon and apatite Hf-Nd isotopes in the granitoids in this area,they indicate that the Xing'can Massif is an ancient microcontinent with Precambrian crystalline basement rather than Phanerozoic accretionary terrane.It is also confirmed by a large number of Precambrian ages from detrital zircons in Paleozoic strata.In the interior of the Xing'an Massif,although the Phanerozoic granitoids have highly positive?_Hf/Nd(t)values,they are also characterized by high Si O₂ content and low ^TFe₂O₃,and Cr,Co,Ni contents,as well as Mg^#values,revealing that the reworking of the newly accreted crust should dominate the crustal evolution during the Phanerozoic in this massif.The existence of the Early Carboniferous ophiolite in the Hegenshan area indicates an important crustal accretion event in the Xing'an Massif during the late Paleozoic.The above characteristics indicate that the interior of the Xing'an Massif experienced the“reactivation”of the crust during the Phanerozoic,i.e.,there was not only an extensive reworking of the newly accreted crust but also crustal accretion represented by the collage of oceanic crust.According to the spatio-temporal variation of zircon Hf isotopic composition from granitoids,the continental crust of the Xing'an Massif has lateral and vertical heterogeneities.Zircon Hf isotopic composition of the early Paleozoic granitoids from the Duobaoshan area shows obviously depleted mantle characteristics.Their model ages are very close to the crystallization age,which indicates that the subduction and convergence of the Heihe-Nenjiang Ocean caused the lateral accretion of arc magma during the early Paleozoic.Since then,the zircon Hf isotopic compositions of the granitoids formed in this area all have the model ages of the early Paleozoic,which represents the reworking of the juvenile crust that accreted during the early Paleozoic,and also reveals that the continental crust in this area is relatively homogeneous in composition.3.Crustal growth and reworking of the Erguna Massif were revealed.The crustal growth curve demonstrates that the initial crust formed in the Mesoarchean(3.2?1.9 Ga)and shows a step-like pattern with three growth periods:2.9?2.7 Ga,2.1?1.9 Ga,and 1.7–0.5 Ga.Phases of growth are constrained by multiple tectonic settings related to the lateral crustal growth caused by the supercontinent development and the subduction and convergence of the Paleo-Asian Ocean from the Neoproterozoic to the early Paleozoic.Calculated reworked crustal proportions and the reworking curve indicate four reworking periods at 1.86?1.78 Ga,860?720 Ma,500?440Ma,and 300?120 Ma,which limited the growth rate.The breakup of the supercontinent,the closure of the branch of the Paleo-Asian Ocean(Xinlin-Xiguitu Ocean),and the evolution of the Mongol-Okhotsk tectonic regime are the main reasons for the crustal reworking in the Erguna Massif.The occurrence of spatiotemporally variable crustal reworking might have led to the heterogeneous continental crust beneath the Erguna Massif.4.Crustal growth and reworking of the Xing'an Massif were revealed.The crustal growth curve of the Xing'an Massif reveals that there were two main stages of crustal growth in this area:the first stage occurred in the late Archean,i.e.,3.2?2.7 Ga,which may be related to the onset of plate tectonics at?3.0 Ga,leading to the initial lateral crustal growth in this area;the second stage occurred during the late Mesoproterozoic to the early Paleozoic,i.e.,1200?400 Ma.Unlike the Erguna Massif,the Xing'an Massif has obvious Phanerozoic crustal growth,including arc magma accretion represented by the Duobaoshan island arc and a small amount collage of the Hergenshan oceanic crust in the late Paleozoic.The crustal reworking curve shows that the pre-existing crust of the Xing'an Massif underwent remelting and reworking during the periods of2.56?2.54 Ga,1.90?1.84 Ga,and 500?120 Ma.This thesis holds that the main factors causing crustal reworking of the Xing'an Massif are the breakup of the Precambrian supercontinent,the closure of the Paleo-Asian Ocean,as well as the evolution of Mongol-Okhotsk and Paleo-Pacific tectonic regimes since Phanerozoic.Based on the research in this thesis,the Precambrian crystalline basement exists in microcontinents in the eastern CAOB,and the Precambrian crustal evolution has been ignored in previous studies to a certain extent.The Phanerozoic crustal growth thus needs to be revisited.Note that,it is undeniable that the CAOB is still different from other regions due to its unique Phanerozoic crustal growth.