Silurian magmatism on the eastern margin of the Erguna Block, NE China: Evolution of the northern Great Xing'an Range

Abstract

There are few Early Paleozoic volcanics and mafic rocks in the Xing'an-Mongolia Orogenic Belt in spite of the extensive occurrence of granites caused by a synchronous orogenic event. Despite the fact that this was a major orogen, the properties of the lithosphere and its associated orogenic processes are poorly known. In this paper, a comprehensive study of U-Pb zircon ages, Lu-Hf isotopes, and whole-rock elemental analyses was carried out on three intermediate-mafic intrusions, which were recently identified in the northern Great Xing'an Range. Representative samples of the Yikete, Yalu, and Xinglong intrusions yielded the weighted mean 206Pb/238U ages of 435 ± 1 Ma, 430 ± 8 Ma, and 447 ± 4 Ma, respectively. This indicates the presence of Silurian intermediate-mafic rocks in the region. The dioritic rocks exhibit a Na-rich, a calc-alkaline affinity and enrichment in LILEs (Rb, Ba, K, and Sr) and depletion in HSFEs (Ta, Ti, Zr, and Hf), indicating that the Yikete dioritic rocks were a mantle wedge that had previously been subjected to metasomatism by fluids and melts related to a subducted oceanic slab and its overlying sediments. The gabbroic rocks are tholeiitic in composition with variable MgO (3.14–8.31%), and low K2O (0.16–1.0%) and TiO2 (0.25–2.12%) contents. They are characterized by enrichment of LILEs, depletion of HFSEs, and highly positive zircon εHf(t) (+5.8 to +13.3) values. In addition, they possess geochemical features of MORB with subordinate OIB. The geochemical features indicate that the gabbroic rocks were derived from an upwelling of asthenospheric mantle that had been into the supra-subduction wedge facilitated by subduction rollback before their generation. Combined with the coeval arc-type magmatic rocks widely exposed on the eastern margin of the Erguna Block, we link the generation of the dioritic rocks to the northwestern subduction of the Nenjiang oceanic plate during the Early Paleozoic. The generation of the gabbroic rocks might result from rifting of the back-arc basin. Consequently, an arc-back-arc model is proposed to explain the subduction evolution of the Nenjiang Ocean during the Early Paleozoic.