Pressure–temperature–time–deformation path of kyanite-bearing migmatitic paragneiss in the Kali Gandaki valley (Central Nepal): Investigation of Late Eocene–Early Oligocene melting processes

• Published in: Lithos 2015-08, Vol.231, p.103-121
• Main Authors: Iaccarino, Salvatore, Montomoli, Chiara, Carosi, Rodolfo, Massonne, Hans-Joachim, Langone, Antonio, Visonà, Dario
• Format: Article
• Language: English
• Subjects: Himalaya; In situ geochronology; Kyanite-bearing migmatite; Monazite; Pseudosection modelling; Zr-in-rutile thermometry
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2015.06.005

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 multidisciplinary 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 collision (50–55Ma) from ~43Ma to 28Ma. During the late phase (36–28Ma) of this metamorphism, the gneiss underwent high-pressure melting at “near peak” conditions (710–720°C/1.0–1.1GPa) leading to kyanite-bearing leucosome formation. In the time span of 25–18Ma, the rock experienced decompression and cooling associated with pervasive shearing reaching P–T conditions of 650–670°C and 0.7–0.8GPa, 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 1GPa 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. •A P–T–D–t path is constructed for kyanite-bearing migmatite in the Kali Gandaki.•In situ monazite dating reveal a protracted metamorphic history for these rocks.•High pressure melting occurs in the “Eohimalayan” stage.•Evidences of a complex structural architecture in the GHS of Central Himalaya