Crustal Growth And Evlution Of Northeastern China As Revealed By U-Pb Age And Hf Isotopes Of Detrital Zircons From Modern Rivers

Northeastern China (NE China) is an eastern part of the Central Asian Orogenic Belt (CAOB). It is generally believed that the growth of continental crust was largely achieved in the Precambrian time and that little new crust has been added during the Phanerozoic. However, significant productions of juvenile crust in the CAOB have been demonstrated. NE China has probably undergone two stages of tectonic evolution related to the closure of the Paleo-Asian Ocean in the Paleozoic, and the subduction of Paleo-Pacific Ocean in the Mesozoic-Cenozoic.Clastic sediments and sedimentary rocks, are representative samples of the continental crust derived from large areas, and are ideal for studies of formation, evolution, and chemical composition of the continental crust. Detrital zircons from younger sedimentary, or in modern river sediments, may record crustal material that has not been preserved or is no longer exposed.River basin of Nen River, Songhua River, Heilongjiang River, Ussuri River and their tributaries, including Hailar River, Huma River and Hulan River cover the most part of tectonic units (Erguna and Xing'an blocks in the northwest, Songnen block in the middle, Jiamusi blocks in the Southeast) of the NE China. In order to characterize the crustal growth and evolution process of the NE China,1761 concordant detrital zircons in thirteen sand samples from the above mentioned rivers were measured for U-Pb age by excimer laser-ablation ICPMS.1381 zircons from them were measured for Hf isotopic compositions by excimer laser-ablation multi collector ICP-MS.The main understanding of this study made the following:1) In complicated tectonic background, the river catchment flows over several tectonic units inevitably and result in the complicated source of detrital zircons and fallacy of research. There are significant-2.5 Ga age signatures in all samples from the main stream of the Songhua River and its south headstream, the Second Songhua River. However, there is no large scale-2.5 Ga crystalline basement in NE China. The Second Songhua River drains from the northern part of North China Craton (NCC), and bring a mass of zircons from there.-2.5 Ga is one of the characteristic age of NCC.In this study, a series of samples from upper, middle and lower reaches of the main tributaries and the main stream of the Songhua River system were measured for U-Pb age. The proportions of-2.5 Ga in detrital zircon age spectrum decrease with increasing drainage areas represented by sampling points along the Songhua River. The proportion of-2.5 Ga zircons shows significant negative correlation with logarithmic values of the drainage area. Such trends are also significant for detrital zircon source with different times and spaces in NE China. These trends are caused by limited supply of exotic zircons. The exotic zircons deposited during downstram migration but without further supply, and that the increasing drainage area also import other age composition of zircons, it reduce the proportion of exotic zircons further in the lower reaches.In contrast, the native zircons, real from the main magmatic events in basin, will be supplied by surrounding source ceaselessly, and keep their proportions drifting within a certain range, which means that various sampling points distributing in the basin are all representative for the characteristic magmatic events within the basin.2) The-1.8 Ga crystalline basement might exist in the middle (northeastern Songliao basin), the west (west of the middle-northern Great Xing'an Range) and the east (eastern Jiamusi massif) of present NE China. It might originate from reworked Archean crust (2.5-3.2 Ga). A few scattered Meso- and Neo-proterozoic magmatic zircons exist in the internal parts of NE China. A significant production of juvenile crust is demonstrated by two-stage depleted mantle model ages in this period. Voluminous of this crust was reworked in successive tectonic movements in Phanerozoic. In the complicated tectonic background of NE China, it should be pointed out that the source of zircons may undergo several rework and contamination. The model ages, at this time, may record the results mixed by juvenile and reworked or recycled crustal material. The geological significance represented by zircon Hf model ages must be combined with oxygen isotope composition for further restrictions.3) In Paleozoic, the region between Siberian Craton and NCC undergone complicated orogenic movement. Masses of juvenile crust suggested by positiveεHf(t) values were yielded during collision and assemblage between microcontinental blocks and the process of amalgamation between the two cratons. The collision might induce partial melting of asthenospheric or lithospheric mantle, and yielded juvenile crust. Some older crust was reworked during collision and assemblage events, and melted crust and mantle were mixed in various degree.The peak time of collision between the Erguna and Xing'an blocks is -495 Ma. Magmatic events occured at 450-460 Ma and -440 Ma by the collision belt reflected the extensional tectonic environment in post-collision process. The collision also induced the -480 Ma magmatic event happened in centre of present Songnen blocks.Subduction of the oceanic plate between the Songnen and Xing'an blocks induced magmatic events with peak time of-355 Ma. The assemblage of the two blocks might occur between 350 Ma to 300 Ma. The asthenospheric mantle upwelled subsequently and yielded juvenile crust. At post-orogenic extensional tectonic environment, magmatic events occured between 300 Ma to 280 Ma, all in Xing'an blocks.The collision between NCC and the assembled block at margin of Siberian Craton occurred in Late Paleozoic. The asthenospheric mantle under the assembled block upwelled and yielded juvenile crust. The post-orogenic tectonic environment induced several magmatic events in different time and space in NE China. The age peak values were-250 Ma,-240 Ma,-235 Ma and-225 Ma.4) In Middle Mesozoic, the NE China had been controlled by Paleo-Pacific Ocean tectonic domain. The peak time of collision between the Jiamusi and Songnen blocks is -190 Ma. Controlled by the subduction of the Paleo-Pacific plate, magmatism migrated westward from the continental margin to intracontinent, and the peak time of magmatic events became younger from-190 Ma at assemblage belt between the Jiamusi and Songnen blocks to-160 Ma at Erguna and Xing'an blocks. The subduction of oceanic plate induced the upwelling of the asthenospheric mantle, and subsequent productions of juvenile crust. In Late Mesozoic, the delamination of thickened crust also induced the upwelling of the asthenospheric mantle, and the mantle melted partially and yielded juvenile crust.5) Based on zircon U-Pb ages, Hf isotope two-stage model ages (TDM2), U-Pb age and model age groups distinguished by theεHf(t) value, as well as juvenile and older crustal material end-number mixing model, four crustal growth curves were drawn. Curve drawn by U-Pb ages reflected the cumulative distribution of magmatic activity with age. The model ages are difficult to explain for complex magma source. The distinguished juvenile and older crustal source by theεHf(t) value can be used for further reflecting the growth of juvenile crust. This model showed the Meso- and Paleo-proterozoic (1.2-2.2 Ga) and the Phanerozoic (100-500 Ma) are two of the most important periods of juvenile crustal additions in Northeastern China. The mixing model calculated the proportion of juvenile and older crustal material in each Hf isotope datum, and may be more realistic to reflect the growth of juvenile crust. For lack of oxygen isotopes to restrict the source of juvenile crust, the crustal growth curve based on mixing model was incomplete in this study. Compared to the curve drawn byεHf(t) value distinguishing, the curve by mixing model highlighted the episodic crustal growth of Northeastern China in Archean and Proterozoic.