Crustal Growth And Reworking Processes Of The Songnen Massif: Evidence From Granitoids

Continental crust is the archive of Earth history,the processes governing the formation,destruction and preservation of continental crust have been the key issues in Earth science and remain debated today.Petrogenetic models for granitoids that are compositionally similar to the bulk continental crust are important for tracing the growth and evolution of the continental crust.This thesis studies the geochronology,geochemistry,in situ apatite trace element and Nd isotope,as well as zircon Hf isotope of the granitoids widely occurred in the Songnen Massif from the eastern Central Asian Orogenic Belt(CAOB).In this study,66 granitoid samples from different locations with different ages are selected according to their temporal and spatial distribution characteristics.Our aims are to:(1)fingerprint granitoid petrogenesis,magmatic and post-magmatic evolution processes using apatite geochemistry;(2)constrain crustal nature of the Songnen Massif and establish its crustal growth and reworking models using zircon U-Pb-Hf isotopic systems;and(3)reveal the genetic relationship between crustal growth and reworking processes of the eastern CAOB and supercontinent evolution.The main achievements are as follows:1.Ascertaining petrogenesis and evolution processes of granitoids in the Songnen MassifGranitoid samples in the Songnen Massif can be broadly grouped into two types:(1)unaltered granitoids(UG)whose compositions have not been obviously disturbed by post-magmatic metamorphic or metasomatic processes;and(2)variably altered granitoids(AG).Apatites from these two types of granitoids are characterized by markedly different crystal shapes,internal textures,and trace element and Nd isotopic compositions.Apatites from AG commonly have irregular overgrowth rims,complex internal textures,and low light rare earth element(LREE)contents.These metamorphic/metasomatic apatites are characterized by large variations in Sm?Nd isotopic compositions,and the apatite εNd(t)and zircon εHf(t)for most of the granitoid samples exhibit significant decoupling.This is due to the higher compatibility of Sm in the apatite structure and lower mobility of Sm in Cl-bearing fluids as compared with Nd.147Sm/144 Nd ratios would increase and result in an error in the calculation of the εNd(t)and,in turn,misinterpretation of the Sm–Nd isotopic data.Apatites from UG are of igneous origin based on their homogeneous or concentric zoned textures and coupled Nd–Hf isotopic compositions.Trace element variations in igneous apatite are controlled primarily by the geochemical composition of the parental melt,the fractional crystallization of other REE-bearing minerals,and the thermodynamic and compositional effects on apatite?melt partitioning.Apatites from UG can be subdivided into three groups based on REE patterns.Group 1 apatites have right-leaning REE patterns similar to the host granitoids,but are slightly enriched in middle REEs(MREEs).Partition coefficients for REEs between apatite and melt indicate that apatite prefers to incorporate the MREEs relative to the LREEs and heavy REEs(HREEs).As such,the REE patterns of Group 1 apatites were likely controlled by parental melt composition and REE partitioning.Group 2 apatites exhibit strong depletions in LREEs,whereas their host rocks are LREE-enriched.In contrast,Group 3 apatites are depleted in MREEs and HREEs.The REE patterns of these two groups are indicative of the crystallization of epidote-group minerals and hornblende before and/or during apatite crystallization,respectively.Besides,inherited apatites from wall rocks during magma generation and ascent could have some unusual geochemical features,including distinct REE patterns and Nd isotopic compositions,positive Eu anomalies,and anomalously high Sr contents.These characteristics provide a record of small-scale crustal assimilation during magma evolution that is not evident from the whole-rock geochemistry,and also demonstrate that a single intrusion can retain both primary and inherited apatite grains simultaneously.Taken together,it is concluded that apatite geochemistry is an important tool for recording complex petrogenetic information.2.Identifying crustal nature of the Songnen Massif and its heterogeneitySeveral stages of granitic magmatisms have been identified in the Songnen Massif,and the magmatic zircons in these granitoids show four main age peaks of ~919 Ma,~472Ma,~252 Ma,and ~188 Ma.The εHf(t)values of the granitoids in the Songnen Massif increase generally from-8.9 to +13.0 with decreasing emplacement age(917 to 176 Ma),whereas their TDM2 ages decrease gradually from 2295 to 544 Ma.The trend reflects the introduction of greater contributions of relatively juvenile materials in the magma source through time,i.e.,from the melting of Paleoproterozoic accreted continental crust to the melting of Neoproterozoic accreted crust.The zircon U-Pb-Hf isotopic data indicate that the crustal evolution of the Songnen Massif during the Phanerozoic is dominated by crustal reworking rather than crustal growth,which is different from the previously suggested Phanerozic crustal growth.Furthermore,lateral heterogeneity is also identified in the Songnen Massif.The εHf(t)values for the granitoids appear to progressively decrease from south to north,reflecting a northward increase in the proportion of ancient crustal material in the magma source regions.3.Constructing crustal growth and reworking curves of the Songnen Massif and crustal growth model of the eastern CAOBThe timing of crustal growth can be constrained by the TDM2(Hf)ages of the granitoids,whereas zircon U–Pb ages reflect the timing of crustal reworking.Integration of the proportion of juvenile and reworked materials added to the crust at each time slice yields crustal growth and reworking curves of the Songnen Massif,respectively.The resultant growth curve shows a step-like pattern over time with three major periods of development: Paleoproterozoic crustal growth at 2.2~1.8 Ga,Mesoproterozoic growth at1.6~1.0 Ga,and a third pulse of growth at 0.85~0.6 Ga,along with two short pauses at1.8~1.6 Ga and 1.0~0.85 Ga.Episodic crustal reworking of the Songnen Massif occurred during 1000~180 Ma,with a fluctuation at 800~600 Ma.Combined with previous studies on granitoids from several microcontinents and suture zones in the eastern CAOB,it is concluded that the eastern CAOB underwent two tectonic stages of crustal growth:(1)episodic growth occurred primarily during the formation and evolution of microcontinents during the Precambrian(mostly during the Meso–Neoproterozoic),and(2)Neoproterozoic–early Paleozoic lateral accretion occurred primarily in island arcs during orogenies and amalgamation of the microcontinents(e.g.,the Duobaoshan arc terrane).4.Revealing the genetic relationship between crustal grwoth and reworking processes of the eastern CAOB and supercontinent evolutionThe initial crustal growth of the Songnen Massif happened at ca.2.2 Ga,and the volume of continental crust increased steadily until ca.1.8 Ga,which was related to the assembly of the Columbia supercontinent during 2.1~1.8 Ga.The fluctuation of the growth curve at 1.8~1.6 Ga was likely due to the collision and eventual amalgamation of the Columbia supercontinent,resulting in a decrease in growth rate.The breakup of the Columbia supercontinent and the formation of the subsequent Rodinia promoted crustal growth during a second period of enhancement between 1.6 and 1.0 Ga,whereas the curve inflected at 1.0~0.85 Ga in response to the onset of crustal reworking within the Songnen Massif,which is coincident with the collisional assembly of the Rodinia supercontinent.The third period of crustal growth at 850~600 Ma was due to the breakup of the Rodinia supercontinent.Enhanced reworking of pre-existing continental crust gradually delayed the production rate from the late stage of Neoproterozoic onwards,and crustal growth of the Songnen Massif has been minor since the early Paleozoic,from which point the Paleo-Asian oceanic and Paleo-Pacific tectonic regimes controlled the evolution of the region successively.Based on crustal evolution processes of several microcontinents in the eastern CAOB,step-like crustal growth pattern is likely a common feature of microcontinents in the region,the rate of crustal growth varies episodically in response to different stages of supercontinent evolution.