Preservation of deep Himalayan PT conditions that formed during multiple events in garnet cores: mylonitization produces erroneous results for rims

Sapkota, J., and Sanislav, I.V. (2013) Preservation of deep Himalayan PT conditions that formed during multiple events in garnet cores: mylonitization produces erroneous results for rims. Tectonophysics, 587. pp. 89-106.

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Abstract

The Kathmandu Thrust Sheet, which overlies the Lesser Himalayas along the southern part of the Main Central Thrust (MCT) and forms the leading edge of the Higher Himalayan crystalline rocks, is folded at a regional scale by the Gorkha-Kathmandu fold couplet in Central Nepal. Garnet porphyroblasts lying close to the MCT within this thrust sheet preserve structural and metamorphic history that predates mylonitization during thrust emplacement. The succession of five FIA sets preserved within these porphyroblasts formed due to changes in the direction of India's motion relative to Asia after they collided. The intersection of Fe, Ca and Mn isopleths for garnet cores reveals that FIA sets 1, 2, 3, 4 and 5 nucleated respectively at 6.2 kbar and 515 °C, 6–7 kbar and 545–550 °C, 6.6 kbar and 530 °C, 5.6–6.2 kbar and 525–550 °C and 6.8–6.9 kbar and 520–560 °C. The average PT mode of thermocalc, which relies on equilibrium being achieved between the garnet rims and the matrix, gives pressures around 11 kbar that do not accord with the lengthy succession of lower core pressures. The many foliations in the matrix, which formed during top to the south thrusting plus subsequent deformations that eventually led to these rocks reaching the surface, truncate all foliations preserved within the porphyroblasts that are defined by inclusion trails. This has resulted in the garnet rims not being in equilibrium with the matrix and the anomalously high pressures. The garnet rims may have been affected by slow dissolution and solution transfer over the period of time that the matrix was deforming plastically at high strain rates as the rocks were uplifted. The assumption of equilibrium between garnet rims and surrounding silicates used by various rim geothermobarometric methods does not hold for these rocks.

Item ID: 31576
Item Type: Article (Research - C1)
ISSN: 0040-1951
Keywords: FIAs; MCT; PT pseudosection; garnet core isopleths; rim geothermobarometry
Date Deposited: 14 Mar 2014 04:48
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 40%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 30%
04 EARTH SCIENCES > 0403 Geology > 040313 Tectonics @ 30%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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