Nd-Hf-O Isotopic Constraints On The Phanerozoic Crustal Growth And Modification In The Yanbian Area, NE China

The Yanbian area is located among the North China Craton, the Xing-Meng Orogenic Belt and the Sikhote-Alin Fold Belt, Far East of Russia. Since the Paleozoic time, the area was influenced by the multiple subduction processes including the subduction of the paleo-Asian Ocean, paleo-Pacific Ocean, and even the Pacific Ocean. These deep-seated processes have caused the extensive crustal growth and modification and even replacement. Granitoids are mainly derived from crustal melting and their geochemical features can provide insights for the geodynamic processes operated at crustal levels. In this dissertation, we conduct a comprehensive study(including petrology, elemental geochemistry, whole-rock Sr-Nd and in situ zircon Hf-O isotope geochemistry) on the Late Paleozoic-Early Cretaceous granitoids from the Yanbian area, NE China. The purposes of this study are focused on the deep crustal growth and modification processes superimposed by multiple subduction processes. The main conclusions are summarized below.(1) The combined geochemical and isotopic comparison among the Phanerozoic granitoids in the area reveals the existence of the Northern(NGZ) and Southern(SGZ) Granite Zones separated by the Fu’erhe-Gudonghe Fault. In the NGZ, the preCretaceous granitoids show moderately to highly radiogenic Nd and Hf isotopic compositions(whole-rock εNd(t)=﹣0.1~﹢3.8 with an average of ﹢2.4, TDM2= 691~976 Ma with an average of 795 Ma; zircon εHf(t)=﹣2.5~﹢14.0 with an average of ﹢8.9, TDM2 = 330~1388 Ma with an average of 666 Ma) and low δ18O values(3.4±0.48 ~ 8.7±0.39‰ with an average of 5.6±0.36‰), suggesting a significant contribution of juvenile crustal material to their origin. In the SGZ, the pre-Cretaceous granitoids show highly nonradiogenic to moderate radiogenic Nd and Hf isotopic compositions(whole-rock εNd(t) =﹣13.6~﹣0.6, TDM2 = 1004~2166 Ma; zircon εHf(t) =﹣25.8~﹢8.5, TDM2 = 663~2906 Ma) as well as a wide δ18O variation(4.9~10.2‰), implying the contribution of the Precambrian recycled crust of North China Craton to the generation of these rocks.(2) The melting sources for the Phanerozoic granitoids in the Yanbian area contained at least three components:(a) the accretionary arc crust related to the Paleozoic subduction of the paleo-Asian Ocean;(b) the accretionary arc crust related to the Mesozoic subduction of the paleo-Pacific Ocean; and(c) the recycled Precambrian crust from the North China Craton. We also identify an endmember component with depleted mantle-like zircon εHf(t)(﹢11.5~﹢14.6) and low d18O(<5.5‰) from the accretionary arc crust of paleo-Asian Ocean. This component likely represents a high-temperature altered oceanic crust, which was tectonically trapped into the deep crust following the closure of the paleo-Asian Ocean.(3) Following the youthening of the emplacement age, the whole-rock εNd(t) and zircon εHf(t) values of the granitoids in the NGZ decrease gradually, while δ18O(zircon) increases gradually. By contrast, the granitoids in the SGZ show the opposite variation trends. Until the Early Cretaceous, the Nd-Hf-O isotopic compositions of the granitoids in both zones became indistinguishable. Such Nd-Hf-O isotopic variation trends reflect that the chemical and isotopic compositions of the melting sources for the Early Cretaceous granitoids in both zones became the same. The new results indicate that the pre-existing deep crustal materials, which had been respectively inherited from the North China Craton and the Central Asian Accretion Orogen, were gradually modified and replaced by the newly accreted arc crust as a result of the paleo-Pacific Oceanic subduction.(4) The Phanerozoic crustal evolution beneath the Yanbian area can be classified into four stages:(a) 290-250 Ma, a period of crustal growth related to the paleo-Asian Oceanic subduction, with the emplacement of high-K calc-alkaline granitoids;(b) 240-210 Ma, a period of intracontinental extension related to breakoff of the subducted paleo-Asian Oceanic slab, with the formation of alkaline granites;(c) 190-150 Ma, a period of the paleo-Pacific Oceanic subduction and accretion, with the extensive emplacement of high-K calc-alkaline granitoids; and(d) 130-110 Ma, a period of withdraw of the subducted paleo-Pacific slab, with the formation of calc-alkaline granitoids and eruptive counterparts.