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Metamorphic zone mapping and P-T path of the High Himalayan Crystalline nappe in Dhankuta, Eastern Nepal

Kudo, Shumpei*; Kawakami, Tetsuo*; Nakajima, Toru   ; Sakai, Harutaka*

In Himalaya, how high-grade metamorphic rocks, or High Himalayan Crystallines (HHC), exhumed up to the surface has long been studied and various models for the exhumation mechanism have been proposed (e.g., Jamieson et al., 2004). Estimating pressure-temperature-time (P-T-t) paths of the metamorphic rocks is indispensable to discuss the exhumation mechanism. In this study, we performed metamorphic zone mapping using metapelites collected from the north of Dhankuta, eastern Nepal and estimated a P-T path of a garnet-biotite-kyanite gneiss of HHC. Based on the field survey and microscopic observation, we newly defined the kyanite-in isograd in the study area. We also confirmed sillimanite-in and muscovite-out isograds consistent with those reported in Groppo et al. (2009). The studied sample was collected from the north of the muscovite-out isograd where Ms+Qtz is unstable. The garnet is about 3 mm in diameter and consists of the core with abundant inclusions and the rim with a few inclusions. The core encloses kyanite, plagioclase, quartz, rutile, ilmenite and zircon, whereas the rim encloses plagioclase, rutile, ilmenite, zircon and nanogranitoid inclusions. Geothemobarometry and micro-texture indicate that this sample have experienced the P-T evolution starting from the kyanite stability field (ca. 8 kbar, ca. 700 degree: garnet core stage) to the sillimanite stability field (ca. 4-6 kbar, ca. 600 degree: garnet rim stage), and replacement structure of Bt+Ky+Qtz+Pl was formed in between these P-T conditions. The studied path is similar to that of lower HHC reported in Imayama et al. (2012). Therefore, we confirmed that lower HHC rocks in the HHC nappe share similar P-T path with those from the root zone.

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