New data on the metamorphism of the Nanga Parbat-Haramosh Massif, and the adjoining Ladakh island arc terrain, northern Pakistan
Abstract
The Nanga Parbat-Haramosh Massif in northern Pakistan records the Tertiary metamorphism and dynamics of the Himalayan collision and subsequent overthrusting of the Asian plate onto the Indian plate. The massif consists of an intercalated sequence of paragneisses derived from the Precambrian Salkhala Series, of orthogneisses from Precambrian and Early-Paleozoic granites, of minor metabsites and calc-silicate rocks, and of post-metamorphic pegmatite dikes. The adjacent part of the Ladakh arc consists of metasediments intruded by younger massive tonalites. The massif and the Ladakh arc are metamorphosed under high pressure, upper amphibolite facies conditions. P-T estimates and P-T paths have been determined for the metapelitic samples from the massif and from the adjacent areas of the Ladakh arc along two cross-strike transects (Indus and Astore rivers) through the massif. Results show that the massif followed a compressional (counter-clockwise) and the Ladakh arc a decompressional (clockwise) P-T path, consistent with the tectonic history of the Himalaya of northern Pakistan.
Geothermobarometry on garnet-biotite and garnet-Plagiocalse pairs from pelites in the massif indicates that the rocks started their metamorphic history at ~5.5 kb and ~625oC. During collision, the Pressure and temperature rose ~9 kb and about 725oC. The Ladakh garnets started to grow at ~10 kb and ~725oC with a subsequent decrease in metamorphic Pressure and temperature to ~8.5 kb and ~625oC After the collision, the massif and the adjacent areas equilibrated at ~8 kb and ~700oC.
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