Metapelites record two episodes of decompressional metamorphism in the Himalayan orogen

In order to decipher the metamorphic evolution of the Himalayan orogen, a combined study of petrology, geochronology and geochemistry was carried out for three types of metapelites from the upper segment of the Higher Himalayas in the Cona area, eastern Himalaya. The results indicate a change in the thermal state of regional metamorphism and thus provide insights into the tectonic evolution of the collisional orogen during the Cenozoic. Type I Grt-St-Sil mica schists experienced amphibolite facies metamorphism under subsolidus peak conditions of 655–670 °C and 9.5–10 kbar, followed by nearly isothermal decompression to ca. 660 °C and 6.5 kbar at 41–30 Ma, corresponding to an increase of metamorphic thermobaric ratios from 670–689 °C/GPa to 1015 °C/GPa. Type II Grt-Sil mica schists experienced early amphibolite facies metamorphism at 40–30 Ma and then superimposed by late granulite facies metamorphism at peak conditions of 755–775 °C and 10.4–11.2 kbar, followed by nearly isothermal decompression through ca. 755 °C and 9 kbar to ca. 700 °C and 7.5 kbar at 26–18 Ma. Such P–T changes correspond to tortuous changes in thermobaric ratios from 692–726 °C/GPa through 839 °C/GPa to 933 °C/GPa. Type III Grt-Kfs-Sil Bt gneisses experienced amphibolite to granulite facies metamorphism from 700–720 °C and 12–13 kbar to 800–850 °C and 11.5–12.5 kbar at 25–21 Ma, followed by nearly isothermal decompression to ca. 770 °C and 8.6 kbar at 20–15 Ma. These P–T changes also correspond to a progressive increase of metamorphic thermobaric ratios from 554–583 °C/GPa through 696–739 °C/GPa to 895 °C/GPa. The metamorphic P–T–t paths for these three types of metapelites can be categorized into two episodes of decompressional metamorphism at 41–30 Ma and 26–15 Ma, respectively, under subsolidus amphibolite facies and suprasolidus granulite facies conditions. The polymorphic transformation from kyanite to sillimanite provides mineralogical evidence for the decompressional metamorphism at elevated thermobaric ratios from moderate T/P Barrovian type to high T/P Buchan type. In addition, the metapelites are characterized by the top-to-the-north shear sense due to the movement of the South Tibet Detachment System, indicating its earlier initiation in eastern Himalaya. Throughout the Himalayan orogen, the first episode of decompressional metamorphism results in moderate T/P Barrovian facies series with rare magmatic activity, whereas the second episode of decompressional metamorphism gives rise to high T/P Buchan facies series with extensive magmatism. Therefore, the two episodes of decompressional metamorphism are associated with secular changes of both thermal state and dynamic regime in the Himalayan orogen.