Eocene partial melting recorded in peritectic garnets from kyanite-gneiss, Greater Himalayan Sequence, central Nepal

Abstract

Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet\ud of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (GHS) along the Kali\ud Gandaki valley, central Nepal, c. 1 km structurally above the Main Central Thrust (MCT). In\ud situ U–Th–Pb dating of monazite included in garnets, in the same structural positions as melt\ud inclusions, allowed us to constrain partial melting starting at c. 41–36 Ma. Eocene partial\ud melting occurred during prograde metamorphism in the kyanite stability field (Eo-Himalayan\ud event). Sillimanite-bearing mylonitic foliation wraps around garnets showing a top-to-the-SW\ud sense of shear linked to the MCT ductile activity and to the exhumation of the GHS. These findings\ud highlight the occurrence of an older melting event in the GHS during prograde metamorphism in\ud the kyanite stability field before the more diffuse Miocene melting event.\ud The growth of prograde garnet and kyanite at 41–6 Ma in the MCT zone, affecting the bottom of\ud the GHS, suggests that inverted metamorphism in the MCT zone and folded isograds in the GHS\ud should be carefully proved with the aid of geochronology, because not all Barrovian minerals grew\ud during the same time span and they grew in different tectonic settings

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

“…Plagioclase around quartz inclusions in kyanite was sporadically observed (e.g. Carosi et al, 2014a). This feature suggests that also kyanite was present during melting (Carosi et al, 2014a).…”

“…Based on Ar-Ar geochronology, Vannay and Hodges (1996) suggested that the whole GHS rapidly cooled below the white-mica closure temperature (300-430°C) during the Early-Middle Miocene (13-15 Ma). Carosi et al (2014a) identified "nanogranites" as inclusions in garnets from kyanite-bearing rocks (bottom of the upper GHS) testifying that these rocks have experienced melting starting at c. 41-36 Ma, based on U-Pb in situ monazite ages.…”

“…1). In both cases the structurally higher samples experienced melting earlier than the lower ones Vannay and Hodges, 1996;Carosi et al, 2014a). Numbers in the legend: 1 = Tethyan Sedimentary Sequence (TSS); 2 = orthogneiss (unit 3, GHS); 3 = metapelite and calcsilicate (unit 3, GHS); 4 = calc-silicate and marble (unit 2, GHS); 5 = kyanite-garnet gneiss (unit 1, GHS); 6 = quartzite of LHS in Vannay and Hodges, (1996); 7 = alluvial debris; 8 = STDS; 9 = minor normal fault; 10 = Kalopani shear zone (KSZ); 11 = MCT in Vannay and Hodges, (1996) or CT in Parsons et al (2014); 12 = main foliation; 13 = object lineation; 14 = location of study samples.…”

“…1) located in a key structural position (Vannay and Hodges, 1996) due to its closeness to the Main Central Thrust (MCT). Carosi et al (2014a) identified within garnets coming from the same outcrop, the presence of crystallized melt inclusions, referred as "nanogranites" (Carosi et al, 2014a and references therein) showing a peculiar chemical composition of high-Ca melts (tonalites). According to Patiño-Douce and Harris (1998) and Prince et al (2001) this melt types is interpreted as result of high-pressure (HP) melting, possibly in the presence of free water and through K-feldspar-absent reactions (see also King et al, 2011;Palin et al, 2014;Weinberg and Hasalová, 2015).…”

Tectonics of the Himalaya: an introduction

“…U -Th -Pb dating of monazite in kyanitebearing gneiss from the central Nepal Himalaya by Carosi et al (2014) indicates that a tonalite melt was entrapped within the gneiss at 41-36 Ma in the kyanite stability field. They found that the Oligocene age melts are not limited to the South Tibetan Gneiss Domes, but also occur in the frontal part of the Himalayan belt, as well as in the Namche Barwa syntaxis.…”