Early Mesozoic Southward Subduction of the Eastern Mongol–Okhotsk Oceanic Plate: Evidence from Zircon U–Pb–Hf Isotopes and Whole‐rock Geochemistry of Triassic Granitic Rocks in the Mohe Area, NE China

Abstract

In this paper, we present new U-Pb zircon ages, Hf isotope data and major and trace elements for Early Mesozoic granitic rocks in Mohe area in the Erguna Massif of northeast China to elucidate the southward subduction of the eastern Mongol-Okhotsk Oceanic plate in Early Mesozoic. Zircons from two representative intrusions, syenogranites and monzogranites, in the Mohe area are euhedral-subhedral in shape, display oscillatory growth zoning in cathodoluminescence (CL) images, and have Th/U ratios of 0.10-0.72, and in combination these features indicating that the zircons are of igneous origin. U-Pb zircon dating results demonstrate that the syenogranites formed at 245.1 ± 1.4 Ma and monzogranites formed at 212.2 ± 1.7 Ma. These granitic rocks are characterized by high SiO 2 , Al 2 O 3 and (Na 2 O + K 2 O), low TFeO, MgO, TiO 2 and P 2 O 5 concentrations, belonging to the high-K calc-alkaline series. They are enriched in LREE and large ion lithophile elements (e.g., Rb, K, and Sr), depleted in HREE and high field strength elements (e.g., Nb, Ta, Th, and Ti), as well as very weak negative Eu anomalies (Eu/Eu* = 0.48~1.01). Their zircon εHf(t) values range from À7.9 to À2.0 and range from 0.20 to 0.49, in response to their two-stage Hf model ages (T DM2 ) range from 1.40 Ga to 1.77 Ga range from 0.94 Ga to 1.24 Ga, respectively, indicating that primary magmas of syenogranites were derived from partial melting of newly accreted juvenile crustal material that formed from the enriched mantle during the Mesoproterozoic, monzogranites are generated by partial melting of newly accreted juvenile crustal material that formed from the depleted mantle during the Meso-to Neoproterozoic. We conclude, therefore, that the early Mesozoic granitic rocks of the Mohe area are associated with the continuous southward subduction of the Mongol-Okhotsk oceanic plate rather than the Paleo-Asian and circum-Pacific tectonic regimes.

Deformation of granitic rocks within Derbugan Fault belt, Erguna Massif, Northeast China: Implication of the subduction of Mongol‐Okhotsk oceanic plate

“…Although the Early Triassic magmatism (~250 Ma; this study) possibly coincided with the final closure of the Paleo‐Asian Ocean (Chen et al, ; Guo, Fan, Miao, & Zhao, ; Shi et al, ), the samples are at present ~500 km away from the Solonker‐Xar Moron‐Changchun‐Yanji suture (Figure a). Meanwhile, widespread occurrence of similarly aged magmatism and mineralization on either side of the MOSB (235–260 Ma) and the study area is presently ~100 km south of the MOSB (Arakawa, Naito, Takahashi, Oyungerel, & Amakawa, ; Jahn et al, ; Li et al, ; ; ; Orolmaa et al, ; She et al, ; Tang et al, , ; Wu et al, ; Zhang, Zhu, Shao, & Jin, ; Zhu et al, ). So, the Early Triassic magmatic rocks in Erguna Massif (17CG1‐2 in this study) are most likely formed in an active continental margin setting that was related to the southward subduction of the Mongol‐Okhotsk oceanic plate beneath the Erguna Massif (Figure a; Tang et al, ).…”