Genesis of Early–Middle Jurassic Intrusive Rocks in the Erguna Block (NE China) in Response to the Late-Stage Southward Subduction of the Mongol–Okhotsk Oceanic Plate: Constraints from Geochemistry and Zircon U–Pb Geochronology and Lu–Hf Isotopes

Abstract

The subduction processes and geodynamic scenarios of the late-stage southward subduction of the Mongol–Okhotsk oceanic slab since the Early Jurassic are subjects of great debate. This contribution presents new U–Pb zircon dating, trace element geochemistry, Ti-in zircon geothermometry, and Lu–Hf isotopes of zircon, as well as bulk-rock geochemical data for Early–Middle Jurassic intrusive rocks in the Erguna Block, NE China. Approximately 181–198 Ma monzogranites and ca. 162–174 Ma quartz monzonites were identified in the block. The Early Jurassic monzogranites are high-K calc-alkaline I-type granites, which display moderately concave-upward rare earth element (REE) patterns with slightly negative Eu anomalies, and low zircon crystallization temperatures. The Middle Jurassic quartz monzonites have low Yb and Y concentrations, high Sr/Y ratios, and strong high field strength elements (HFSEs) depletions, that are in excellent agreement with adakitic rocks. They exhibit right-sloping REE patterns with negligible Eu anomalies, and a wide range of zircon crystallization temperatures. The intrusions yield εHf(t) values between −4.1 to +4.8 and juvenile two-stage model (TDM2) ages varying from 918–1488 Ma. The geochemical and isotopic signatures suggest that the monzogranites were likely derived by the partial melting of K-rich meta-basalts within the lower part of a juvenile crust that had medium-thickness (≤40 km), with the involvement of minor mantle materials. Whereas, the quartz monzonites were possibly produced by partial melting of a thickened continental lower crust (≥50 km). The Mongol–Okhotsk tectonic regime played a dominant role in accounting for their formation. An Andean-type continental arc setting was developed during the Early–Middle Jurassic, with gradual thickening of the continental crust. The significant crustal thickening may reach its ultimate stage at ca. 162–174 Ma, which marks the tectonic transition from compression to extension. The southward subduction beneath the Erguna Block was continuous and stable during the Early Jurassic. Rollback of the subducted slab occurred at ca. 174–177 Ma, followed by moderate magmatic activities represented by adakitic rocks.

Editorial for Special Issue “U-Pb Dating and Chemistry of Zircon in Metamorphic, Magmatic and Sedimentary Rocks”

“…The metasomatic effect of the continental crust on the lithosphere mantle is revealed by monitoring the chemical and isotopic evolution of magmatic rocks [8,9]. Although the partial melting of younger, thinned continental crust against thickened continental crust in the temporal evolution of subduction [10,11] can be defined using classical geochemical data, isotopic markers in zircon allow more precise and accurate interpretations. The timing of the evolution of the subduction zone can be defined using U-Pb zircon data from magmatic rocks in many contexts [12,13].…”