Sr-Nd-Pb Isotope Mapping Of Mesozoic Igneous Rocks In NE China: Constraints On Tectonic Evolution Of The Eastern CAOB

The NE China fold belt is the eastern segment of Central Asian Orogenic belt (CAOB), which is the largest accretion orogen between the North China Block and the Siberian Plate. Based on systematic Sr-Nd-Pb isotopic analyses of Mesozoic volcanic rocks and available data from Mesozoic intrusive plutons by previous studies in NE China, this paper presents a new isotope mapping of region, and discusses the origin of the extensive felsic magmatism and their relationship with the crustal growth and mantle-crust interacion. The results provide robust evidence for understanding the regional tectonic framework and the evolution of the CAOB. Major conclusions are summarized below.1 The Sr-Nd-Pb isotope mapping results of Mesozoic igneous rocks in NE China identify four Nd-Pb isotope provinces: (1) the northern Hinggan Mountains; (2) the southern Hinggan Mountain-Yanji-Liaoyuan region; (3) the Zhangguangcai Range -Jiamusi block; and (4) the Wandashan massif. The Wandashan massif, in which occurred HIMU-type OIBs with highly radiogenic Pb isotope compositions from the Raohe area, is a foreign terrane accreted to the CAOB. Except for HIMU-type OIB with negativeΔ7/4 ratios in the Wandashan massif, all other rocks have positiveΔ7/4 andΔ8/4 values, a feature of the Laurasia continent in the Northern Hemisphere.2 In the southern Hinggan Mountains distributes a Cretaceous felsic igneous rock belt characterized by highly positive-ε_Nd (T_DM = 0.42～0.64Ga,ε_Nd(t) = +4.0～+5.9) and radiogenic Pb (²⁰⁶Pb/²⁰⁴Pb(i) = 18.36 18.56) isotope compositions. These melts were derived from a source comprising mainly juvenile crust with subordinate metasediments. Combined geochemical data and subordinate crustal growth during late Mesozoic time suggest that the tectonically entrained Paleo-Asian oceanic crustal relics were the likely candidatesfor the juvenile components.3 The west Ujimqin andesites in the southern Hinggan Mountains have unradiogenic Nd (ε_Nd(t) = -10.5～-6.3, T_DM = 1.22～1.51Ga) and unradiogenic Pb (²⁰⁶Pb/²⁰⁴Pb(i) = 17.29～17.33) and moderately radiogenic Sr isotopic compositions (⁸⁷Sr/⁸⁶Sr(i) = 0.7063～0.7066). These melts also have positiveΔ7/4 (3.7～6.9) andΔ8/4 (94～104) values, a DUPAL Pb isotope signature as observed in the Indian Oceanic mid-ridge basalts. Such features are typical of ancient lower continental crust, e.g., Archean granulite terranes that experienced extensive U-Th loss and developed time-integrated low radiogenic Pb compositions. The occurrence of the west Ujimqin andesites indicates the existence of ancient continental lower crust beneath the NE China fold belt.4 The Mesozoic granitoids in the Zhangguangcai Range and Jiamusi block have lower Nd (ε_Nd(t) = -3.5～+ 2.1, T_DM = 1.0～1.27Ga) and highly radiogenic Pb and Sr isotopic composition (²⁰⁶Pb/²⁰⁴Pb(i) = 18.4～18.9, ⁸⁷Sr/⁸⁶Sr(i) = 0.705～0.711), suggesting that the pelagic sediments of the Paleo-Asian Ocean or their metamorphosed derivates were an important component in the region.5 The Pb isotope mapping results show that the mafic rocks in the northern Hinggan Mountains have higher and weakly variable ²⁰⁶Pb/²⁰⁴Pb ratios than those from the southern Hinggan Mountains and the Liaoyuan-Yanji area. Such a Pb isotope difference between the two regions supports the previous hypothesis that the Suolun-Hegenshan-Less Hinggan mountain belt represented the collisional suture between the North China and the Mongolian Blocks. On the other hand, the higher radiogenic Pb compositions in the northern Hinggan Mountains mafic rocks were possibly attributed to the recycling of the pelagic sediments into the mantle source, whereas the less radiogenic Pb compositions in the southern Hinggan Mountains and Liaoyuan-Yanji counterparts suggested the involvement of terrigneous sediments derived from the continental lower crust of the North China Block.6 The lack of systematic correlations between emplacement age andε_Nd(t) (or T_DM) and Pb isotope ratios and the absence of abrupt change in Sr-Nd-Pb isotope data at any given time throughout the Mesozoic indicate that the crustal growth was subordinate during Mesozoic time, especially at the peak stage (early Cretaceous) of magmatic activity. Generation of the late Mesozoic voluminous intermediate-felsic magmas in NE China is thus ascribed to remelting, recycling and redistribution of the preexistent crustal components, which had been juxtaposed during the tectonic evolution of the Paleo-Asian Ocean before Mesozoic.