Abstract
The Annapurna region in central Nepal Himalayas is underlain by various kinds of metamorphic rocks including metapelites, metabasites and subordinate metamarls. Two geological units, the Higher Himalayan Sequence and the Lesser Himalayan Sequence, are separated by a regional tectonic boundary, the Main Central Thrust (MCT). Geothermometry based on the Fe-Mg distribution between garnet and biotite in the metapelites suggests that the metamorphic temperatures remain nearly constant in the Lesser Himalayan Sequence (400-450°C). However, they increase steeply towards the Higher Himalayan Sequence (up to ca. 750°C) and attain 600-650°C just below the MCT. Metamorphic history of the Lesser Himalayan Sequence and the Higher Himalayan Sequence is distinctive to each other inferred from the zoning profiles of garnets in the metapelites. Garnets from the Lesser Himalayan Sequence preserve normal growth zoning whereas those from the Higher Himalayan Sequence show mostly reverse zoning with little evidence of growth zoning. The origin of the inverted metamorphism is inferred to be due to overthrusting of ‘hot’ Higher Himalayan Sequence onto the ‘cool’ Lesser Himalayan Sequence.
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