Petrogenesis and geological significance of Neoproterozoic amphibolite and granite in Bowuleshan area, Erguna massif, Northeast China

In this paper, the authors discuss LA-ICP-MS zircon U-Pb ages, major and trace element analyses for Neoproterozoic Bowuleshan amphibolite and gneissic granite on the southeastern margin of Erguna massif, northern Da Hinggan Mountains. The purpose is to elucidate the tectonic history of the Erguna massif and its relationship to the assemblage of the Rodinia supercontinent. Zircons collected from amphibolite exhibit core-rim structure in CL images, the U-Pb dating yielded the age of 904±4Ma for magmatic core and 889~915Ma for metamorphic rim. Zircons collected from gneissic granite are euhedral-subhedral in form, and display fine-scale oscillatory growth zoning in CL images, implying a magmatic origin. The dating age is 915±3Ma. Zircon U-Pb dating demonstrates that these rocks were emplaced during the Neoproterozoic. The Neoproterozoic gneissic granite in the Erguna massif has SiO₂=61.85%~67.63%, Mg^#=36.9~47.9, Na₂O+K₂O=4.21%~9.29%, and A/CNK=0.89~1.01, suggesting metaluminous characteristics, Moreover, the Neoproterozoic granitoids are enriched in LREEs and LILEs, and depleted in HREEs and high field strength elements (HFSEs, Nb, Ta, and Ti), with Eu negative anomalies (δEu=0.77~0.80) and low initial Sr isotope ratios and positive values for the ε_Nd(t) value (3.52), which implies that their primary magmas were derived from partial melting of an original crust. In contrast, the Neoproterozoic amphibolite has low SiO₂ (45.22%~49.16%), relatively high Mg#, high Ni, Cr, low Zr/Hf, Nb/Ta and Th/ U ratios, and Co content. The flat REE patterns are analogous to features of N-type MORB from the depleted mantle source, but are characterized by enrichment of LILEs (Rb, Ba, K, Sr and Pb) and depletion of HFSEs such as Nb, Ta and Ti; it records magmatic processes of subduction zone, and indicates that amphibolite was derived from partial melting of depleted mantle wedge with igneous activity in continental arcs, on the active continental margin. On such a basis, in combination with the regional characteristics of Neoproterozoic magmatic events, the authors have reached the conclusion that the Early Neoproterozoic magmatic events of the Erguna massif occurred in the context of the assembly of the Rodinia supercontinent, and later metamorphic events might correspond to the breakup of the Rodinia supercontinent.