| PDF: | |
| Time: | 2024 |
| Journal: | Acta Petrologica Sinica |
| Volumn: | 40 |
| Issue: | 1 |
| Pages: | 159-177 |
| Type: | SCI |
| Author: | Taiji Yu,Pujun Wang,Youfeng Gao,Yan Zhang,Chongyang Chen |
Abstract: | The southeastward subduction of the Mongol-Okhotsk Ocean, oceanic closure, and continental collision were important regional tectonic events in the Late Mesozoic of Northeast Asia. They are closely related to magmatic activity, metamorphism, basin formation and orogeny in the area. Accurately defining the spatiotemporal range of the interrelated geological processes of the above three events is a prerequisite for understanding the tectonics of the region in the Late Mesozoic. However, it is difficult to conduct this kind of investigation as it is difficult to find suitable geological records related to these events. We discovered peraluminous rhyolites and tonalite porphyrites in the Tuquan area along the western margin of the Songliao Basin. These samples are probably related to the oceanic closure and continental collision. The zircon LA-ICP-MS U-Pb dating results show that their crystallization ages are 156±1Ma and 155±1Ma, respectively, indicating they are the products of Late Jurassic magmatic events. They are calc-alkaline peraluminous rocks with high aluminum saturation index A/CNK (1.32~2.13) and low content of MgO+FeOT (0.96%~3.37%) and low ratio of FeOT/MgO (2.84~5.02). Thin-section work shows that they contain high aluminum minerals such as sericite. Corundum molecules (3.77%~9.65%) appear in CIPW standard mineral calculation, and these rhyolites and tonalite porphyrites are mapped in the geochemical diagrams related to the S-I-M-A classification scheme of granites. These results show the characteristics of S-type granites. The low ratios of Rb/Sr (0.35~0.55) and Rb/Ba (0.08~0.26) of rhyolites and tonalite porphyrites indicate that the primary magma of rhyolites and tonalite porphyrites were originated from poor argillaceous arenaceous rocks. Zircon saturation temperature calculation shows that the magmatic crystallization temperatures of rhyolites and tonalite porphyrites range from 837℃ to 876℃, and the samples have high ratio of Al2O3/TiO2 (38.41~61.36), these temperatures are lower than the formation temperature of A-type granite (900℃). These rhyolites and tonalite porphyrites are richen in large ion lithophile elements such as Rb, Ba, K, and lack high field strength elements such as Nb, Ta, P, Ti. They also show low contents of Y (5.29×10-6~19.75×10-6), Nb (7.44×10-6~8.50×10-6), Sr (60.6×10-6~154.9×10-6) and Yb (0.53×10-6~2.40×10-6), showing arc magmatic properties. In the R1-R2 major element tectonic discrimination diagram, the samples are mainly projected within the range of collisional and orogenic periods. The Nb-Y diagram shows that the samples are dotted in volcanic arcs and syn-collision granites. The Rb/10-Hf-Ta×3 diagram shows that the samples are located in the regions of volcanic arcs and collision type granite. The Sr-Yb diagrams indicate that rhyolites and tonalite porphyrites were formed in the stage of crustal thickening. The tectonic discrimination for the rhyolites and tonalite porphyrites in the Tuquan area is suggested that the Late Jurassic in the area was a period of the volcanic arc and continental crust collision environments. It implies that their formation is related to oceanic crust subduction and continental collision during the closure process of the Mongol-Okhotsk Ocean. The ratio of rhyolites (La/Yb)N range from 6.62 to 8.77, indicating a depth of 40~46km in the source area. The ratio of tonalite porphyrites (La/Yb)N range from 7.93 to 13.39, indicating a depth of 44~55km in the source area. These results indicate that a continuous thickening process of the crust at 156±1Ma to 155±1Ma. These characteristics of rhyolites and tonalite porphyrites provide key igneous rock evidence for evolution from oceanic subduction to continental collision. Combined with the regional geological data, the tectonic-magmatic evolution model of the Mongol-Okhotsk Ocean from the subduction and closure to the continental collision is constructed. In this paper, the influence range of the Mongol-Okhotsk Ocean tectonic system reached the Tuquan area in the western margin of the Songliao Basin, the Mongol-Okhotsk Ocean was closed at 156±1Ma, and the study area at 155±1Ma was at the stage of continental collision and crust thickening after ocean closure. |