Miocene high-grade metamorphism of the metapelites in the Cona region, eastern Himalaya, and tectonic implications for Himalayan orogeny

Abstract

Metamorphic thermobaric ratios are used to interpret the evolution of convergent plate margins and to understand the evolution of the Higher Himalayan Crystalline Sequence. To better understand changes in pressure (P) and temperature (T) with time (t), we examine the metamorphic petrology, mineral chemistry, phase equilibrium modeling, and geochronology of the metapelites from the Cona area in the eastern Himalayan orogen. Based on this integrated approach, three metapelites were selected to study the Miocene metamorphism. Our results reveal that the metapelites preserve a peak garnet-kyanite-K-feldspar-bearing high-grade metamorphic mineral assemblage, and a post-peak sillimanite-bearing assemblage, and underwent the granulite-facies metamorphism and associated partial melting under P-T conditions of ca. 10 kbar and 725–775 °C, followed by isothermal decompression and isobaric cooling. Zircon dating indicates that these rocks experienced metamorphism between 24 and 14 Ma, accompanied by an increase in geothermal gradients from 21 °C/km (780 °C/GPa) to 29 °C/km (1190 °C/GPa), and finally to 32 °C/km (1250 °C/GPa). Thus, there is an increase in the thermobaric ratio and geothermal gradient along the north-south transect, concurrent with a decrease in elevation and age. We argue that these extreme gradients are a consequence of the active tectonic processes and ongoing deep crustal magmatism in the eastern Himalaya, whereas the decreasing trend of P-T conditions of peak metamorphism and younging of exhumation ages southward from the upper to lower Higher Himalayan Crystalline Sequence is well matched with a critical taper model.