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<p>High-pressure metamorphic rock belts serve as critical markers for convergent lithospheric plate boundaries and are instrumental in correlating subduction-collision events across different tectonic units. In this study, we document the identification of medium-pressure to high-pressure (MP-HP) rocks along the south margin of the Central Altyn (SMCA), which constitute a Silurian metamorphic belt within the Proto-Tethyan Altyn Orogen. Detailed petrographic, mineralogical, phase equilibrium modeling, and zircon geochronological studies are presented to reconstruct the metamorphic evolution of the belt. The studied rocks preserve evidence of three distinct metamorphic stages: (1) HP peak metamorphism with <i>P</i> = 15.6–18.5 kbar and <i>T</i> = 654–695 °C at ∼430 Ma, with a low geothermal gradient of 9.2–11.8 °C/km; (2) (HP) granulite-facies overprinting with <i>P</i> = 8.3–14.2 kbar and <i>T</i> = 720–838 °C at ∼400 Ma; and (3) amphibolite-facies retrograde metamorphism with <i>P</i> = 4.3–6.5 kbar and <i>T</i> = 607–725 °C at ∼375 Ma. The reconstructed <i>P</i>-<i>T</i>-<i>t</i> paths exhibit an initial decompression-heating phase followed by clockwise decompression-cooling, indicative of Alpinetype peak metamorphism and consistent with subduction/collision-related processes. Zircon U-Pb dating constrains the protolith formation age to 471–456 Ma. The SMCA MP-HP rocks are spatially and temporally distinct from the South Altyn (SA) HP-ultrahigh-pressure (UHP) rocks, which exhibit higher metamorphic pressures and different protolith and metamorphic ages. These distinctions suggest that the SMCA and SA rocks formed in two separate subduction/collision zones that operated sequentially during the Early Paleozoic evolution of the Altyn Orogen. Additionally, the metamorphic ages (both peak and retrograde) of the SMCA MP-HP rocks closely align with those of continental HP-UHP rocks in the north margin of the Qaidam Basin (NQB). This correlation implies that the SMCA MP-HP rock belt represents the westward extension of the NQB HP-UHP rock belt. These findings provide critical insights into the spatial and temporal relationships among the various metamorphic belts in western China, offering new constraints for reconstructing the tectonic evolution of the region.</p>

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Confirmation of Siluran–Devonian MP-HP Metamorphism in the Proto-Tethys Altyn Orogen: Implications for the Early Paleozoic Tectonic Evolution of Western China

  • 期刊论文

  • 发表日期：

    2026年3月13日

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  • Tuo Ma,
  • Liang Liu,
  • Mingguo Zhai,
  • Yongsheng Gai,
  • Chao Wang,
  • Lei Kang

摘要

High-pressure metamorphic rock belts serve as critical markers for convergent lithospheric plate boundaries and are instrumental in correlating subduction-collision events across different tectonic units. In this study, we document the identification of medium-pressure to high-pressure (MP-HP) rocks along the south margin of the Central Altyn (SMCA), which constitute a Silurian metamorphic belt within the Proto-Tethyan Altyn Orogen. Detailed petrographic, mineralogical, phase equilibrium modeling, and zircon geochronological studies are presented to reconstruct the metamorphic evolution of the belt. The studied rocks preserve evidence of three distinct metamorphic stages: (1) HP peak metamorphism with P = 15.6–18.5 kbar and T = 654–695 °C at ∼430 Ma, with a low geothermal gradient of 9.2–11.8 °C/km; (2) (HP) granulite-facies overprinting with P = 8.3–14.2 kbar and T = 720–838 °C at ∼400 Ma; and (3) amphibolite-facies retrograde metamorphism with P = 4.3–6.5 kbar and T = 607–725 °C at ∼375 Ma. The reconstructed P-T-t paths exhibit an initial decompression-heating phase followed by clockwise decompression-cooling, indicative of Alpinetype peak metamorphism and consistent with subduction/collision-related processes. Zircon U-Pb dating constrains the protolith formation age to 471–456 Ma. The SMCA MP-HP rocks are spatially and temporally distinct from the South Altyn (SA) HP-ultrahigh-pressure (UHP) rocks, which exhibit higher metamorphic pressures and different protolith and metamorphic ages. These distinctions suggest that the SMCA and SA rocks formed in two separate subduction/collision zones that operated sequentially during the Early Paleozoic evolution of the Altyn Orogen. Additionally, the metamorphic ages (both peak and retrograde) of the SMCA MP-HP rocks closely align with those of continental HP-UHP rocks in the north margin of the Qaidam Basin (NQB). This correlation implies that the SMCA MP-HP rock belt represents the westward extension of the NQB HP-UHP rock belt. These findings provide critical insights into the spatial and temporal relationships among the various metamorphic belts in western China, offering new constraints for reconstructing the tectonic evolution of the region.