METAMORPHISM OF THE EXHUMED HIMALAYAN METAMORPHIC CORE, LIKHU KHOLA REGION, EAST-CENTRAL NEPAL

The High Himalaya are dominated by the exhumed, former midcrustal rocks that comprise the Himalayan Metamorphic Core (HMC). The processes controlling the evolution of this package of metamorphic rocks has been one of the main focuses of much research along the Himalaya over the past 25 years. Recent research has begun to outline a series of strike-parallel tectonometamorphic discontinuities that dissect the HMC. These structures are now being found along the length of the orogen, however, their detailed characteristics and significance to the evolution of the mountain belt is still being established.

One such discontinuity was recently inferred in the Likhu Khola region of east-central Nepal on the basis of regional mapping and preliminary metamorphic pressure-temperature (P-T) trends. New, detailed P-T paths derived from phase equilibria modelling of specimens collected from across the HMC exposed in the same region are consistent with the presence of that inferred structure. P-T paths change abruptly across the lower-middle portion of the HMC. Paths for the rocks below the structure are typically prograde and document a higher pressure and lower temperature evolution. This is in contrast to rocks above the structure, which are characterized by higher temperatures but lower pressures and retain evidence of the retrograde portion of the path. This break in metamorphic P-T paths occurs at the same structural position as the previously mapped tectonometamorphic discontinuity and cryptically occurs within rocks that on either side have experienced temperatures exceeding their solidus and have undergone anatexis associated with the muscovite and biotite dehydration. Similar structures identified in other parts of the orogen have been inferred to mark the transition between large-scale hinterland and foreland-style deformation processes. Understanding these structures is critical to elucidating the evolution of the Himalayan orogen and our understanding of convergence accommodation processes active in continent-continent orogens in general.