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Kyanite-bearing migmatitic paragneiss of the lower Greater Himalayan Sequence (GHS) in the Kali Gandaki transect (Central Himalaya) was investigated. In spite of the intense shearing, it was still possible to obtain many fundamental information for understanding the processes active during orogenesis. Using a multidis- ciplinary approach, including careful meso- and microstructural observations, pseudosection modelling (with PERPLE_X), trace element thermobarometry and in situ monazite U–Th–Pb geochronology, we constrained the pressure–temperature–time–deformation path of the studied rock, located in a structural key position. The migmatitic gneiss has experienced protracted prograde metamorphism after the India–Asia colli- sion (50–55 Ma) from ~43 Ma to 28 Ma. During the late phase (36–28 Ma) of this metamorphism, the gneiss underwent high-pressure melting at “near peak” conditions (710–720 °C/1.0–1.1 GPa) leading to kyanite- bearing leucosome formation. In the time span of 25–18 Ma, the rock experienced decompression and cooling associated with pervasive shearing reaching P–T conditions of 650–670 °C and 0.7–0.8 GPa, near the sillimanite–kyanite transition. This time span is somewhat older than previously reported for this event in the study area. During this stage, additional, but very little melt was produced. Taking the migmatitic gneiss as representative of the GHS, these data demonstrate that this unit underwent crustal melting at about 1 GPa in the Eocene–Early Oligocene, well before the widely accepted Miocene decompressional melting related to its extrusion. In general, kyanite-bearing migmatite, as reported here, could be linked to the production of the high-Ca granitic melts found along the Himalayan belt

In the Kali Gandaki valley (central Nepal), a ductile, high-temperature, contractional shear zone with a top-to-the-SW sense of shear, known as Kalopani Shear zone (KSZ), is located within the uppermost part of the Greater Himalayan Sequence (GHS). We mapped and investigated this shear zone in in detail, in order to unravel its age and role in the evolution of the GHS. Pseudosection modeling and inverse geothermobarometry reveal that rocks involved in the KSZ experienced pressure-temperature conditions between 0.6-0.85 GPa and 600-660°C. U-Th-Pb in-situ LA-ICP-MS and SHRIMP dating on monazite point to retrograde metamorphism related to the KSZ starting from ~ 41-30 Ma. The kinematics of the KSZ and associated erosion and/or tectonics, caused the Middle-Late Eocene exhumation of the GHS in the hanging wall of the KSZ zone at least nine million years before the activities of the High Himalayan Discontinuity, the Main Central Thrust, and the South Tibetan Detachment. Structural data, metamorphic conditions and geochronology from the KSZ, compared to those of other major tectonic discontinuities active within the GHS in the Kali Gandaki valley, indicate that shear deformation and exhumation were not synchronous but migrated downward and southward at different lower levels within the GHS. These processes caused the exhumation of the hanging-wall rocks of the activated shear zones. The main consequence of this tectonic is that exhumation was driven by an insequence shearing mechanism progressively involving new slices of the Indian crust and not solely by the coupled activity of Main Central Thrust and South Tibetan Detachment.

Minute polyphase inclusions in garnet of quartzofeldspathic rocks (saidenbachite) from the Saxonian Erzgebirge, Germany, contain microdiamond or graphite, phlogopite, quartz, paragonite, phengite and other minerals in minor amounts. These inclusions are interpreted to represent an original dense COH + silicate fluid, trapped in crystallizing garnet at depths of >150 km. Inspection of the inclusion population in a single garnet by SEM reveals two characteristic features: (i) The shape of most inclusions indicates healed radial cracks in the host garnet, and, thus, the buildup of a significant differential pressure DP, i.e. a contrast in pressure between the inclusion (P i ) and the host mineral (P e ). The mineral assemblages sealing the cracks and showing an equilibrated microstructure indicate temperatures of $750 ± 50°C and pressures below 2.5 GPa. (ii) The diverse types of inclusions appear to be randomly distributed in the garnet host. Thus, the variable phase assemblages do not reflect a compositional evolution of the fluid trapped in the garnet. Combining observations (i) and (ii), we propose that the diversity of the phase assemblage in the inclusions is the result of decrepitation at different times, and thus, of distinct histories of P i , as DP at decrepitation is primarily controlled by inclusion size and shape. Applying a flow law for dislocation creep of garnet, a low strength of garnet at 750 ± 50°C for low geological strain rates is predicted. Thus, differential pressure should have been kept low (i.e. P i % P e ) by continuous stretching of the inclusion for typical exhumation rates of metamorphic rocks. To attain the differential pressure (P i >> P e ) required for catastrophic brittle failure of the garnet host, the decompression rate must have been extremely high. As a robust lower bound, a minimum exhumation rate on the order of 100 m year )1 is suggested, which corresponds to ascent rates of magma.

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