Geochronology and geochemistry of Early–Middle Triassic magmatism in the Erguna Massif, NE China: Constraints on the tectonic evolution of the Mongol–Okhotsk Ocean

Abstract

In this paper, we present new zircon U–Pb dating, Hf isotope, major and trace elements, and Sr–Nd isotope data for Early–Middle Triassic intrusive rocks in the Erguna Massif of NE China, in the eastern segment of the Central Asian Orogenic Belt (CAOB). Our aim is to elucidate the tectonic evolution of the Mongol–Okhotsk Ocean and its influence on NE China. Zircons from five representative intrusions in the Erguna Massif are euhedral–subhedral in shape, display oscillatory growth zoning in CL images, and have Th/U ratios of 0.17–1.54, and in combination these features indicate that the zircons are of igneous origin. Zircon U–Pb dating results demonstrate that the Early–Middle Triassic magmatism in the Erguna Massif took place between 241 and 247Ma. The Early–Middle Triassic rocks are composed mainly of a suite of diorites, quartz–diorites, granodiorites, monzogranites, and syenogranites. They have SiO2=57.71–72.86wt.%, Mg#=19–52, Al2O3=14.27–17.23wt.%, and Na2O/K2O=0.67–1.59. Chemically, they are metaluminous to peraluminous and belong to the high-K calc-alkaline series. Moreover, they are enriched in light rare earth elements and large ion lithophile elements, and depleted in heavy rare earth elements and high field strength elements (e.g., Nb, Ta, and Ti). The whole-rock εNd(t) and zircon εHf(t) values of these samples range from −4.68 to −3.42, and from −2.0 to +6.6, respectively, indicative of formation from primary magmas generated by partial melting of juvenile crustal material with a contribution from mantle-derived mafic magmas. These magmas subsequently underwent fractional crystallization, contamination, and magma mixing. Geochemically, these Early–Middle Triassic intrusive rocks have affinities with intrusive rocks from active continental margin settings. We conclude, therefore, that the Early–Middle Triassic magmatism in the Erguna Massif was generated within an active continental margin setting related to the southwards subduction of the Mongol–Okhotsk oceanic plate beneath the Erguna Massif. This conclusion is supported by the occurrence of Early Mesozoic porphyry and skarn deposits along the southeastern side of the Mongol–Okhotsk suture belt.