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Importance of Tourmaline Gneiss and Vein near Main Central Thrust in Sikkim Darjeeling Himalaya

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Document pages: 13 pages

Abstract: Lingtse gneiss (LGn) and Higher Himalayan crystallines (HHC) comprise parts of main central thrust (MCT) in the Darjeeling Sikkim Himalaya. Tourmaline bearing gneiss and quartz tourmaline veins are reported in immediate contact with the LGn and some lesser Himalayan rocks in this study. Their importance is inferred via their comparative occurrence, micro-texture and chemistry. Flow of ductile crust was proposed to expose deep crustal rocks in the Himalayas in form of these gneissic rocks. Generation of paragneissic HHC from the protolith like the lesser Himalayan rocks like biotite-muscovite schist was proposed and documented in previous studies. The main central thrust where the principal motion is reported to date at circa 20 Ma appears in the contact regions of the HHC and lesser Himalayan rocks. Whether the tourmaline bearing gneiss or veins is a product during the episode of generation of the Higher Himalayan crystallines, which is taken as a component of the higher Himalayan crystallines episode remains a question as both concordant and discordant tourmaline bearing gneiss and or quartzo-feldspahic veins appear respectively. The mm-cm scale tourmaline in the occasionally discordant quartz tourmaline veins shows strong zonation and less effects of shearing. Those are strongly zoned indicating magmatic hydrothermal character. The matrix tourmaline shows a separate composition. However, evidences of a less prominent shearing in them might signify rejuvenation. Lower temperature activity and fluid movement in this thrust zone are signified from the microstructure signifying that the high grade main central thrust was probably rejuvenated during or after the veining.

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