Geochronology and geochemistry of Late Pan-African intrusive rocks in the Jiamusi–Khanka Block, NE China: Petrogenesis and geodynamic implications

Abstract

To constrain the early Paleozoic tectonic evolution of the Jiamusi–Khanka Block and its relationship to the Late Pan-African event in Gondwana, we undertook zircon U–Pb dating and geochemical analyses (major and trace elements, and Hf isotopic compositions) of early Paleozoic intrusive rocks in the Jiamusi–Khanka Block, NE China. LA-ICP-MS zircon U–Pb age data demonstrate that these intrusive rocks were emplaced at three stages during the Late Pan-African event, represented by ~ 540 Ma syenogranite, ~ 515 Ma quartz syenite, and ~ 500 Ma monzogranite and gabbro. Geochemically, the ~ 500 Ma gabbros in the Jiamusi–Khanka Block have low SiO2 (50.26–51.21 wt.%), relatively high MgO (4.08–5.67 wt.%), Ni (13.1–14.1 ppm) and Cr (28.4–56.0 ppm), and are slightly enriched in LILEs (e.g., Ba, K) and LREEs, and depleted in Zr, Hf, Nb, Ta and P. The εHf(t) values of zircons in the gabbro range from + 2.6 to + 6.4. All these geochemical features indicate that the gabbros were likely produced by the partial melting of a depleted mantle that had been metasomatized by fluids derived from a subducted slab. In contrast, the ca.540–500 Ma granites and quartz syenites contain high SiO2 (64.49–72.20 wt.%) and low MgO (0.40–0.75 wt.%), Cr (1.69–6.88 ppm) and Ni (1.26–3.26 ppm). They have relatively low 176Hf/177Hf ratios of 0.282247–0.282599 with Hf two-stage model ages of 1173–2280 Ma, and most of the magmatic zircons have positive εHf(t) values varying from + 0.2 − + 4.8, indicating that these granites and quartz syenites were probably derived from a dominantly Paleo–Mesoproterozoic “old” crustal source with possible different degrees of addition of juvenile materials. According to the geochemical data and global geological investigations, we propose that the 541–498 Ma intrusive rocks in the Jiamusi–Khanka Block formed in a post-collisional or post-orogenic extensional setting linked to the collapse of a Late Pan-African orogen associated within the Gondwana.