Contrasting P-T paths in the Eastern Himalaya, Nepal:: Inverted isograds in a paired metamorphic mountain belt

Petrology and phase equilibria of rocks from two profiles in Eastern,:Nepal from the Lesser Himalaynn Sequences, across the Main Central Thrust Zone and Into the Greater Himalayan Sequences reveal a Paired Metamorphic :Mountain Belt (PMLB) 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 games + cordierite + sillimanite assemblages that equilibrated at 837 +/- 59 degreesC and 6.7 +/- 1.0 kbar. Matrix minn als are overprinted by numerous metamorphic reaction textures that document isobaric cooling anti re-equilibrated samples presence evidence of cooling cooling to 600 +/- 45 degreesC 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: albtie + chlorite + muscovite +/- biotite; Zone B: albite +/- chlorite + muscoviep + biotite + garnet; Zone C: albite + muscovite + biotite + garnet + chlorite; Zone D: oligoclase + muscovite + biotite + garnet +/- kyanite; Zone E: oligoclase + muscovite + biotite + garnet + staurolite + kyranite; 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 cor-es average 10.0 +/- 1.2kbar and 557 +/- 39 degreesC, and peak-metamorphic conditions from rims average 8.8 +/- 1.1 kbar and 609 +/- 42 degreesC 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 sillinzanite-biotite +/- gedrite foliation seams (S-3) formed at conditions of 674 +/- 33 degreesC 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 progressive ductile overthrusting of the hob Greater Himalayan Sequences during prograde metamorphism.