Contrasting P-T paths in the Eastern Himalaya, Nepal: Inverted isograds ina 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 a nd Into the Greater Himalayan Sequences reveal a Paired Metamorphic :Mounta in Belt (PMLB) composed of two thrust-bound metamorphic terranes of contras ting metamorphic style. At the higher structural level the Greater Himalaya n Sequences experienced high-T/moderate-P metamorphism, with an anticlockwi se P-T path. Low-P inclusion assemblages of quartz + hercynitic spinel + si llimanite have been overgrown by peak metamorphic games + cordierite + sill imanite assemblages that equilibrated at 837 +/- 59 degreesC and 6.7 +/- 1. 0 kbar. Matrix minn als are overprinted by numerous metamorphic reaction te xtures that document isobaric cooling anti re-equilibrated samples presence evidence of cooling cooling to 600 +/- 45 degreesC at 5.7 +/-1.1 kbar. Bel ow the,Main Central Thrust, the Lesser Himalayan Sequences are a continuous . (though inverted) Barrovian sequence of high-P/moderate-T metamorphic roc ks. Metamorphic zones upwards from the lowest structural levels in the sout h 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 + ky anite. 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 d egreesC, and peak-metamorphic conditions from rims average 8.8 +/- 1.1 kbar and 609 +/- 42 degreesC in Zones D-F. Matrix assemblages are overprinted b y decompression reaction textures, and in Zones F and G progress into the s illimanite field. The two terranes were brought into juxtaposition during f ormation of sillinzanite-biotite +/- gedrite foliation seams (S-3) formed a t conditions of 674 +/- 33 degreesC and 5.7 +/- 1.1 kbar: The contrasting a verage geothermal gradients and P-T paths of these two metamorphic terranes suggest they make up a PMMB. The upper plate position of the Greater Himal ayan Sequences produced an anticlockwise P-T path, with the high average ge othermal-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 i sograds as a result progressive ductile overthrusting of the hob Greater Hi malayan Sequences during prograde metamorphism.