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<p>The metamorphic sole located beneath ophiolites preserves critical information about the formation and subsequent obduction of ophiolites. A typical metamorphic sole consists of an assemblage of high-temperature-low-pressure (high T/P) metamorphic rocks, including amphibolites and garnet pyroxenites. The metamorphic grades range from greenschist facies to high-grade amphibolite and granulite facies, indicating formation under high geothermal gradients (&gt;25°C/km). A synthesis of studies on globally exposed metamorphic soles and their corresponding ophiolites reveals that the protoliths of most mafic metamorphic soles share similar geochemical signatures with the mafic oceanic crust of the overlying ophiolites. Moreover, the crystallization ages of the protoliths and the formation ages of the ophiolitic crust are generally consistent, and the timing of metamorphism is either contemporaneous to or slightly younger than the crystallization age. These features suggest that shortly after the formation of ophiolites (typically within &lt;10 Myr), a rapid geodynamic transition from seafloor spreading to subduction occurred. Therefore, the metamorphic sole serves as a key petrological archive for recording the subduction initiation at former mid-ocean ridges. By summarizing recent research progress on the Yarlung-Tsangpo ophiolites and their metamorphic soles in Tibetan Plateau, we propose a model of “subduction re-initiation at mid-ocean ridges”. In this model, the Neo-Tethyan Ocean experienced ultra-slow spreading between 130 and 120 Ma, leading to the formation of oceanic core complexes along detachment faults. Subsequently, the cessation of earlier subduction to the north triggered new subduction initiation along the detachment fault near the ridge axis. As a result, oceanic lithologies underwent metamorphism under relatively high geothermal gradients (25–30°C/km), forming the metamorphic soles. Eventually, the metamorphic soles and the associated oceanic core complexes were preserved together as the Yarlung-Tsangpo ophiolite-metamorphic sole assemblage. Comparative studies with classic global ophiolites suggest that this “subduction re-initiation” model provides a reasonable explanation for the geological relationship between ophiolites and their metamorphic soles, offering a broadly applicable framework for understanding the formation and emplacement of ophiolites.</p>

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The origin of metamorphic soles and implications for the formation and emplacement of ophiolites

  • 期刊论文

  • 发表日期：

    2025年9月15日

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  • Chang Zhang,
  • Chuanzhou Liu

摘要

The metamorphic sole located beneath ophiolites preserves critical information about the formation and subsequent obduction of ophiolites. A typical metamorphic sole consists of an assemblage of high-temperature-low-pressure (high T/P) metamorphic rocks, including amphibolites and garnet pyroxenites. The metamorphic grades range from greenschist facies to high-grade amphibolite and granulite facies, indicating formation under high geothermal gradients (>25°C/km). A synthesis of studies on globally exposed metamorphic soles and their corresponding ophiolites reveals that the protoliths of most mafic metamorphic soles share similar geochemical signatures with the mafic oceanic crust of the overlying ophiolites. Moreover, the crystallization ages of the protoliths and the formation ages of the ophiolitic crust are generally consistent, and the timing of metamorphism is either contemporaneous to or slightly younger than the crystallization age. These features suggest that shortly after the formation of ophiolites (typically within <10 Myr), a rapid geodynamic transition from seafloor spreading to subduction occurred. Therefore, the metamorphic sole serves as a key petrological archive for recording the subduction initiation at former mid-ocean ridges. By summarizing recent research progress on the Yarlung-Tsangpo ophiolites and their metamorphic soles in Tibetan Plateau, we propose a model of “subduction re-initiation at mid-ocean ridges”. In this model, the Neo-Tethyan Ocean experienced ultra-slow spreading between 130 and 120 Ma, leading to the formation of oceanic core complexes along detachment faults. Subsequently, the cessation of earlier subduction to the north triggered new subduction initiation along the detachment fault near the ridge axis. As a result, oceanic lithologies underwent metamorphism under relatively high geothermal gradients (25–30°C/km), forming the metamorphic soles. Eventually, the metamorphic soles and the associated oceanic core complexes were preserved together as the Yarlung-Tsangpo ophiolite-metamorphic sole assemblage. Comparative studies with classic global ophiolites suggest that this “subduction re-initiation” model provides a reasonable explanation for the geological relationship between ophiolites and their metamorphic soles, offering a broadly applicable framework for understanding the formation and emplacement of ophiolites.