Petrogenesis and tectonic setting of the Late Paleozoic Xing’an complex in the northern Great Xing’an Range, NE China: Constraints from geochronology, geochemistry and zircon Hf isotopes

Abstract

To determine the petrogenesis and tectonic setting of the Late Paleozoic Xing’an complex in the northern Great Xing’an Range (GXR), northeastern China, we undertook zircon U–Pb dating and geochemical analyses (major and trace elements, and Hf isotopic compositions) on samples obtained from the complex. The Xing’an complex is composed mainly of the Xinshali (XSL), Ershihao (ESH), Xinnangou (XNG) and Xing’an Station (XAS) plutons. The U–Pb zircon ages measured by secondary ion mass spectrometry (SIMS) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) indicate that the Xing’an complex was emplaced in three stages, represented by the ∼358Ma XSL, ∼308Ma XNG/XAS, and ∼294Ma ESH plutons. The XSL pluton is composed mainly of gabbro diorites (SiO2=53.49–56.81wt.%; MgO=4.60–5.52wt.%) of the mid-K calc-alkaline series. These rocks are weakly enriched in large ion lithophile elements (LILEs) and light rare earth elements (HREEs), and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, and Ti) and heavy rare earth elements (HREEs), with εHf(t) values of +4.07 to +7.59. Based on these geochemical and isotopic features, we propose that the magma of the XSL pluton was derived from partial melting of depleted lithospheric mantle that was metasomatized by subducted slab-derived fluids. The ESH and XSL plutons have similar geochemical compositions and zircon Hf isotopic values, thereby indicating a common petrogenesis. In contrast, the XNG and XAS plutons comprise syenogranites and monzogranites that are geochemically similar to I-type granites based on their high SiO2 (67.93–74.98wt.%) and Na2O+K2O (7.12–9.20wt.%) contents, low MgO (0.33–1.14wt.%) content, enrichment in LILEs (e.g., Rb, Th, U, K) and LREEs, and depletion in Nb, Ta, Ti and P. The positive εHf(t) values (+6.34 to +12.72) of the XNG and XAS plutons and their corresponding Hf two-stage model ages of 1149Ma to 484Ma indicate that the parental magma was derived from partial melting of juvenile crustal material. According to the geochemical data and regional geological observations, we propose that the formation of the Late Paleozoic Xing’an complex in the northern GXR was related to the subduction of the Paleo-Asian oceanic plate beneath the Xing’an terrane. Hence, we suggest that the Xing’an and Songnen terranes were not amalgamated before the Early Permian.