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Type: Journal article
Title: Contrasting P-T paths in the Eastern Himalaya, Nepal: inverted isograds in a paired metamorphic mountain belt
Author: Goscombe, B.
Hand, M.
Citation: Journal of Petrology, 2000; 41(12):1673-1720
Publisher: Oxford Univ Press
Issue Date: 2000
ISSN: 0022-3530
Abstract: Petrology and phase equilibria of rocks from two profiles in Eastern Nepal from the Lesser Himalayan Sequences, across the Main Central Thrust Zone and into the Greater Himalayan Sequences reveal a Paired Metamorphic Mountain Belt (PMMB) composed of two thrust-bound metamorphic terranes of contrasting metamorphic style. At the higher structural level, the Greater Himalayan Sequences experienced high-T/ moderate-P metamorphism, with an anticlockwise P-T path. Low-P inclusion assemblages of quartz + hercynitic spinel + sillimanite have been overgrown by peak metamorphic garnet + cordierite + sillimanite assemblages that equilibrated at 837 ± 59°C and 6·7 ± 1·0 kbar. Matrix minerals are overprinted by numerous metamorphic reaction textures that document isobaric cooling and re-equilibrated samples preserve evidence of cooling to 600 ± 45°C at 5·7 ± 1·1 kbar. Below the Main Central Thrust, the Lesser Himalayan Sequences are a continuous (though inverted) Barrovian sequence of high-P/moderate-T metamorphic rocks. Metamorphic zones upwards from the lowest structural levels in the south are: Zone A: albite + chlorite + muscovite ± biotite; Zone B: albite + chlorite + muscovite + biotite + garnet; Zone C: albite + muscovite + biotite + garnet ± chlorite; Zone D: oligoclase + muscovite + biotite + garnet ± kyanite; Zone E: oligoclase + muscovite + biotite + garnet + staurolite + kyanite; Zone F: bytownite + biotite + garnet + K-feldspar + kyanite ± muscovite; Zone G: bytownite + biotite + garnet + K-feldspar + sillimanite + melt ± kyanite. The Lesser Himalayan Sequences show evidence for a clockwise P-T path. Peak-P conditions from mineral cores average 10·0 ± 1·2 kbar and 557 ± 39°C, and peak-metamorphic conditions from rims average 8·8 ± 1·1 kbar and 609 ± 42°C in Zones D-F. Matrix assemblages are overprinted by decompression reaction textures, and in Zones F and G progress into the sillimanite field. The two terranes were brought into juxtaposition during formation of sillimanite-biotite ± gedrite foliation seams (S3) formed at conditions of 674 ± 33°C and 5·7 ± 1·1 kbar. The contrasting average geothermal gradients and P-T paths of these two metamorphic terranes suggest they make up a PMMB. The upperplate position of the Greater Himalayan Sequences produced an anticlockwise P-T path, with the high average geothermal gradient being possibly due to high radiogenic element content in this terrane. In contrast, the lower-plate Lesser Himalayan Sequences were deeply buried, metamorphosed in a clockwise P-T path and display inverted isograds as a result of progressive ductile overthrusting of the hot Greater Himalayan Sequences during prograde metamorphism.
DOI: 10.1093/petrology/41.12.1673
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