Three-phased Middle Permian–Mesozoic magmatism in the Great Xing'an Range: implications for episodic southern subduction of the Mongol-Okhotsk ocean

The Erguna Massif is located in the southwestern portion of the Great Xing'an Range and is adjacent to the Mongol–Okhotsk suture zone. It has not to be determined whether the tectonic evolutionary processes of the Erguna Massif belong to the Mongol–Okhotsk tectonic regime during the Middle Permian–Mesozoic. In this study, a suite of rocks comprising Mesozoic S-type monzogranite (LA–ICP–MS U–Pb zircon age of 248 ± 1.2 Ma), highly fractionated I-type rhyolite (204 ± 1.1 Ma), gabbro (196 ± 1.9 Ma), A2-type volcanic rocks (190 ± 0.9 Ma), A1-type trachydacite (167 ± 0.8 Ma), and Early Cretaceous A1-type alkaline rhyolite are newly identified and geochemically studied. The rhyolite, gabbro, trachydacite, and alkaline rhyolite whole-rock Sr–Nd isotope analyses got the values of initial 87Sr/86Sr ratios ranging from 0.7044 to 0.7058 and εNd(t) values of −0.68–+2.73. These samples show εHf(t) values ranging from +5.3 to +11.2 and TDM2 ranging from 0.48 Ga to 0.90 Ga. The 248 Ma monzogranites were produced by the partial melting of greywackes. The 204 Ma rhyolites were derived from the partial melting of lower mafic crust. The 196 Ma gabbros originated from the partial melting of an enriched mantle metasomatized by subduction-slab released fluids. The 190 Ma volcanic rocks, 167 Ma trachydacite, and Early Cretaceous alkaline rhyolite were mainly formed by the partial melting of the basaltic rocks. They all show enrichment in the large ion lithophile elements (e.g., Rb, Ba, and K) and depletions in the high field strength elements (e.g., Nb, Ta, and Ti), suggesting they formed in an active continental margin setting. The features of these igneous rocks indicate the southward subduction of the Mongol-Okhotsk ocean plate. Based on compiled age data, three phases of middle Mesozoic magmatism were identified in the Erguna Massif at ca. 275–225 Ma, 215–165 Ma, and 150–110 Ma. In addition, three similar magmatic phases were found in the Xing'an Massif. However, a hysteresis about ca. 15–20 Ma exists between the two massifs. These magmatic rocks may record the three stages of the southward subduction of the Mongol–Okhotsk oceanic plate, and two periods of slab rollback occurred during the Middle Permian to Early Cretaceous.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5459285

Crustal growth and reworking: A case study from the Erguna Massif, eastern Central Asian Orogenic Belt

Despite being the largest accretionary orogen on Earth, the record of crustal growth and reworking of individual microcontinental massifs within the Central Asian Orogenic Belt (CAOB) remain poorly constrained. Here, we focus on zircon records from granitoids in the Erguna Massif to discuss its crustal evolution through time. Proterozoic–Mesozoic granitoids are widespread in the Erguna Massif, and spatiotemporal variations in their zircon εHf(t) values and TDM2(Hf) ages reveal the crustal heterogeneity of the massif. Crustal growth curve demonstrates that the initial crust formed in the Mesoarchean, and shows a step-like pattern with three growth periods: 2.9–2.7, 2.1–1.9, and 1.7–0.5 Ga. This suggests that microcontinental massifs in the eastern CAOB have Precambrian basement, contradicting the hypothesis of significant crustal growth during the Phanerozoic. Phases of growth are constrained by multiple tectonic settings related to supercontinent development. Calculated reworked crustal proportions and the reworking curve indicate four reworking periods at 1.86–1.78 Ga, 860–720 Ma, 500–440 Ma, and 300–120 Ma, which limited the growth rate. These periods of reworking account for the crustal heterogeneity of the Erguna Massif.