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, geo-chronology 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 re-gional 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 degrees C and 9.5-10 kbar, followed by nearly isothermal decompression to ca. 660 degrees C and 6.5 kbar at 41-30 Ma, corresponding to an increase of metamorphic thermobaric ratios from 670-689 degrees C/GPa to 1015 degrees 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 degrees C and 10.4-11.2 kbar, followed by nearly isothermal decompression through ca. 755 degrees C and 9 kbar to ca. 700 degrees C and 7.5 kbar at 26-18 Ma. Such P-T changes correspond to tortuous changes in thermobaric ratios from 692-726 degrees C/GPa through 839 degrees C/GPa to 933 degrees C/GPa. Type III Grt-Kfs-Sil Bt gneisses experienced amphibolite to granulite facies metamor-phism from 700-720 degrees C and 12-13 kbar to 800-850 degrees C and 11.5-12.5 kbar at 25-21 Ma, followed by nearly iso-thermal decompression to ca. 770 degrees 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 degrees C/GPa through 696-739 degrees C/GPa to 895 degrees 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 fa-cies 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 metamor-phism 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. There-fore, the two episodes of decompressional metamorphism are associated with secular changes of both thermal state and dynamic regime in the Himalayan orogen. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study reveals different metamorphic paths of metapelites, indicating two decompressional metamorphic events in the Himalayan orogen with changing metamorphic thermobaric ratios over time.