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

Abstract

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

The P–T–t evolution of the exhumed Himalayan metamorphic core in the Likhu Khola region, East Central Nepal

“…The relative timing of growth of different domains within garnet and monazite can be further examined based on the partitioning of trace elements between these minerals. Assuming garnet and monazite as the major sources and sinks of HREE, the partitioning between these two minerals should be a direct representation of their growth histories during metamorphism (Buick et al., ; Iaccarino et al., ; Regis et al., ). Recent studies that have established equilibrium partitioning between garnet and monazite during simultaneous growth have shown a systematic decrease in the partitioning values for HREE, with an almost constant negative slope in a logarithmic scale (Figure ; Buick et al., ; Hermann & Rubatto, ; Rubatto et al., ).…”

“…These strike‐parallel discontinuities, identified at various structural levels within the HMC (e.g. Ambrose et al., ; Carosi, Montomoli, Rubatto, & Visonà, ; Carosi et al., ; Corrie & Kohn, ; Imayama et al., ; Iaccarino et al., ; Larson & Cottle, ; Larson et al., ; Martin, Ganguly, & DeCelles, ; Montomoli et al., ; Rubatto, Chakraborty, & Dasgupta, ; Wang, Rubatto, & Zhang, ; Wang, Zhang, et al., ; Wang et al., ; Warren et al., ; Yakymchuk & Godin, and others), rarely have an associated field expression (Larson & Cottle, ; Warren et al., ) and are typically recognized on the basis of abrupt breaks in P–T–t paths of adjacent rock packages (Ambrose et al., ; Larson et al., ; Rubatto et al., ). Whilst most of these discontinuities have been investigated based on a single type of analysis or the combination of few analytical methods (see Larson et al., and references therein), which range from field‐based structural identification to laboratory‐based techniques such as thermobarometry (e.g.…”

“…Ambrose et al., ; Corrie & Kohn, ; Khanal, Robinson, Kohn, & Mandal, ; Larson, Cottle, & Godin, ; Warren et al., ), P–T–t phase equilibria modelling (e.g. Ambrose et al., ; Iaccarino et al., ; Larson et al., ; Rapa et al., ; Wang et al., ; Warren et al., ), or quartz crystallographic fabric analysis (e.g. Larson & Cottle, ; Yakymchuk & Godin, ), rarely have they been studied in an integrated approach (e.g.…”

“…Larson & Cottle, ; Yakymchuk & Godin, ), rarely have they been studied in an integrated approach (e.g. Carosi et al., ; Iaccarino et al., ; Larson et al., ; Montomoli et al., ). The inconsistency in the methods used to identify and characterize individual discontinuities has hampered efforts to correlate these structures across the orogen (Larson et al., ; Montomoli et al., ).…”

Tectonometamorphic evolution of the tip of the Himalayan metamorphic core in the Jajarkot klippe, west Nepal

“…The GHS records a cryptic phase of early Palaeozoic metamorphism that is rarely preserved in the hinterland and is better documented in the foreland klippen (e.g., Argles et al., ; Cawood et al., ; DeCelles et al., ; Gehrels et al., , 2006a; Godin et al., ). During the Himalayan orogeny, the GHS was metamorphosed at greenschist to granulite facies conditions and was extensively deformed from the Eocene to the Miocene (Carosi et al., ; Godin et al., ; Grujic, Hollister, & Parrish, ; Grujic, Warren, & Wooden, ; Iaccarino, Montomoli, Carosi, Massone, et al., ; Iaccarino et al., ; Inger & Harris, ; Larson & Cottle, ; Larson et al., ; Pêcher, ; Soucy La Roche, Godin, Cottle, et al., ; Streule, Searle, Waters, & Horstwood, ; Vannay & Hodges, ). The base of the GHS is marked by the Main Central thrust (MCT) zone, a several km‐thick top‐to‐the‐SW shear zone that propagated down‐section from the early Oligocene to the late Miocene as slices of footwall rocks were successively accreted to the hangingwall (Gansser, ; Hunter, Weinberg, Wilson, Luzin, & Misra, ; Larson, Ambrose, Webb, Cottle, & Shrestha, ; Martin, 2017b; Mottram et al., ; Searle et al., ).…”