Progressively elevated geothermal gradients for Miocene metamorphism in the Himalayan orogen

The property of regional metamorphism at convergent plate margins is substantially related to the change of geothermal gradients, resulting in different types of crustal reworking through metamorphic dehydration and partial melting. The relationship between crustal metamorphism and thermal evolution of collisional orogens can thus be deciphered by investigating the change of metamorphic thermobaric ratios in both space and time. This is illustrated by a combined study of field observation, petrographic observation, whole-rock and mineral major and trace elements, laser ablation−inductively coupled plasma−mass spectrometry zircon and monazite U-Th-Pb isotopes and trace elements, and phase equilibrium modeling of granitic gneisses from the Cona area, southern Tibet, in the Himalayan orogen. The results reveal variations in spatial level, protolith age, petrological composition, and metamorphic pressure−temperature−time path along an N-S transect across the Higher Himalayas. On this basis, three types of granitic gneisses were identified for the Miocene metamorphism. Type I gneiss was formed at 16.2−14.7 Ma under upper amphibolite facies conditions of 720−735 °C and 0.77−0.82 GPa, Type II gneisses were formed at 18.3−14.3 Ma under granulite facies conditions of 765−795 °C and 0.99−1.06 GPa, and Type III gneisses were formed at 21.8−13.0 Ma under high-pressure granulite facies conditions of 850−875 °C and 1.40−1.45 GPa. These rocks underwent anatectic metamorphism at lower to upper crustal depths from 22 to 13 Ma, and their metamorphic thermobaric ratios increase from 586−803 °C/GPa at 22−18 Ma to 878−967 °C/GPa and finally to 1071 °C/GPa at 15−13 Ma. This increase corresponds to a progressive increase in metamorphic geothermal gradients from 16.1−22.0 °C/km to 24.1−26.5 °C/km and finally to 29.4 °C/km. Therefore, the crustal rocks in the Himalayan orogen experienced the three types of regional metamorphism at the three different geothermal gradients in the Miocene. The progressively elevated geothermal gradients are attributed to continental rifting due to asthenospheric upwelling consequential to thinning of the lithospheric mantle in the collisional orogen. This has important implications for the tectonic evolution of collisional orogens in the post-collisional stage.