Newly identified Middle-Late Permian mafic-ultramafic intrusions in the southeastern margin of the Central Asian Orogenic Belt: Petrogenesis and its implications

摘要

Due to lack of precise geochronological data, the Late Paleozoic tectonic evolution in the eastern segment of the Central Asian Orogenic Belt (CAOB) is poorly constrained. We present geochronological and geochemical data of newly identified Middle Late Permian mafic-ultramafic intrusions (Hongqiling Nos. 4, 5 and 6) in the central area of Jilin Province, southeastern margin of the CAOB, aiming to provide important information for understanding the tectonic processes during their emplacement. Gabbros from the Hongqiling Nos. 5 and 6 intrusions give SHRIMP zircon U-Pb ages of 272.0 +/- 3.6 Ma and 258.8 +/- 3.4 Ma, respectively. Rocks from the Nos. 4, 5 and 6 intrusions have low SiO2, high TFe2O3 and MgO contents. In addition, they are characterized by enrichment in large ion lithophile elements (Ba, Rb, Sr) relative to high field strength elements (Ta, Nb, Zr), indicating that they were derived from a metasomatized lithospheric mantle. On the other hand, they show E-MORB-like geochemical compositions such as Nb/Yb (1.87 similar to 5.33) and Th/Yb (0.18 similar to 1.67) ratios, suggesting that an asthenospheric mantle was involved. Therefore, the dual geochemical characteristics of the Nos. 4, 5 and 6 intrusions may reflect an extensional environment, in which upwelling asthenospheric mantle decompressionally melted and triggered partial melting of overlying metasomatized lithospheric mantle. Recently, another pulse of mafic-ultramafic magmatism (240-212 Ma) representing by the Nos. 1 and 8 intrusions has been identified in the Hongqiling area. In contrast, the Middle Late Permian (272 similar to 258 Ma) mafic magmas have higher Sm/Yb ratios than those of the Middle Late Triassic (240-212 Ma) mafic magmas, suggesting the first pulse of magmatism (272 similar to 258 Ma) derived from a relatively deeper mantle. Thus, two stages of magmatism that derived from different depth have been proposed, namely, Middle Late Permian (272-258 Ma) magmatism (Hongqiling Nos. 4, 5 and 6 intrusions) with a greater depth, and Middle Late Triassic (240-212 Ma) magmatism (Hongqiling Nos. 1 and 8 intrusions) with a shallow depth. Taking into account the regional tectonic and magmatic data, we correlate the first stage magmatism to a slab break-off regime following the initial collision between the North China Block and the Songnen-Zhangguangcai Range Massif (SZRM), and the second stage of magmatism to a post-orogenic extensional environment associated with delamination. These two stages of magmatism with different geochemical characteristics suggest that the final closure of the Paleo-Asian Ocean (PAO) occurred during Late Permian to Early Triassic.