Geochronology, geochemistry and zircon Hf isotopes of the Dongfanghong gabbroic complex at the eastern margin of the Jiamusi Massif, NE China: Petrogensis and tectonic implications

Abstract

This paper reports LA–ICP–MS zircon U–Pb ages, geochemical data and zircon Hf isotope compositions of gabbroic rocks, plagiogranites, and granite porphyries exposed at the eastern margin of the Jiamusi Massif in the easternmost segment of the Central Asian Orogenic Belt (CAOB). LA–ICP–MS zircon U–Pb age data indicate that these rocks were emplaced at 290–274, 277, and 110Ma, respectively. The gabbroic rocks have low- and medium-K tholeiitic compositions, with variable MgO (3.92–12.59%) and low K2O (0.12–0.77%) and TiO2 (0.59–1.37%) contents. They are enriched in large ion lithophile elements (e.g., Rb, Sr), depleted in high field-strength elements (e.g., Nb, Ta), and exhibit positive zircon εHf(t) values (3.1–13.8). It is inferred that the primary magmas of these rocks were derived from a depleted lithospheric mantle source that was metasomatized by subducted-slab-derived fluids shortly before their generation. In addition, the Dongfanghong gabbroic rocks have low Sr/Y (7.32–26.2) and (La/Yb)N (0.99–3.02) ratios, and high HREEs, Y, and Sc contents, indicating that the partial melting of lithospheric mantle occurred under relatively low-pressure conditions. The Early Cretaceous granite porphyries are metaluminous medium-K calc-alkaline I-type granites, where the primary magma originated from the partial melting of crustal material. Based on the new geochemical data and regional geological investigations, we propose that the Dongfanghong gabbroic complex and granite porphyry were both formed in an active continental margin setting, the former most likely related to westward subduction of the paleo-oceanic plate beneath the Jiamusi Massif, and the latter associated with westward subduction of the Paleo-Pacific plate beneath the Eurasian continent. The gabbros and the granite porphyries clearly represent two subduction-related crustal accretion events in the eastern part of the Jiamusi Massif since the late Paleozoic.