Contrasting P-T Paths in the Eastern Himalaya, Nepal: Inverted Isograds in a Paired Metamorphic Mountain Belt

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 upper-plate 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.