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.
Highlights
The Mongol–Okhotsk orogenic belt stretches across the Russian Transbaikal and Northeastern (NE)Belt (CAOB) that was formed through a gigantic accretionary orogeny from the Neoproterozoic–LateMesozoic [1,2,3,4]
Other studies argue that some Late Jurassic magmatic rocks were generated by subduction-related processes, and the Middle Jurassic tectonic setting of the Erguna Block was controlled by the southward subduction of the Mongol–Okhotsk oceanic plate (MOOP) [13,15,16,17]
We consider that the Mongol–Okhotsk tectonic regime played a dominant role in accounting for the generation of the Early–Middle Jurassic intrusive rocks in the Erguna Block
Summary
The Mongol–Okhotsk orogenic belt stretches across the Russian Transbaikal and Northeastern (NE)Belt (CAOB) that was formed through a gigantic accretionary orogeny from the Neoproterozoic–LateMesozoic [1,2,3,4]. The Erguna Block of NE China is situated immediately to the southeast of the Mongol–Okhotsk orogenic belt (Figure 1a,b), and is characterized by extensive intrusive and volcanic activities during the Early and Late Mesozoic, respectively [7,8,9,10], providing an ideal natural laboratory for exploring the Mesozoic tectonic evolution of the belt in Chinese territory. Jurassic subduction-related igneous rocks are widespread in the Erguna Block and adjacent areas (Figure 1c) [11,12,13], the associated subduction processes remain debatable. Other studies argue that some Late Jurassic magmatic rocks were generated by subduction-related processes, and the Middle Jurassic tectonic setting of the Erguna Block was controlled by the southward subduction of the MOOP [13,15,16,17]
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