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Metasomatic origin of the Himalayan banded tourmaline leucogranite.

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Title: Metasomatic origin of the Himalayan banded tourmaline leucogranite.
Authors: Dyck, Brendan1 (AUTHOR) brendan.dyck@ubc.ca, Larson, Kyle P.1 (AUTHOR)
Source: Contributions to Mineralogy & Petrology. Jul2023, Vol. 178 Issue 7, p1-20. 20p.
Subjects: Tourmaline, Muscovite, Melt crystallization, Metasomatism, Plagioclase
Geographic Terms: Nepal, Himalaya Mountains
Abstract: Leucogranite bodies are ubiquitous in the upper structural levels of the Himalayan metamorphic slab. Their formation has ramifications for myriad processes including the generation of crustal melts and orogenic heat budgets. One particularly enigmatic variety of the Himalayan leucogranites, abundant in the Langtang region of Nepal, are the banded tourmaline leucogranites; typified by centimetre-scale compositional banding between tourmaline-rich and quartzo-feldspathic domains. Here, we use in-situ Rb–Sr isotopic chemistry and microstructural analysis to show these banded tourmaline leucogranites do not represent a direct product of melt crystallization but instead were formed by metasomatism of psammitic country-rock. This metasomatism was likely driven by the release of boron-rich volatiles during the crystallization of neighboring muscovite–biotite leucogranite bodies. Rb–Sr isochrons based on biotite–plagioclase ± white mica and K-feldspar data define overlapping dates of ca. 17.5 Ma from both the banded tourmaline leucogranite and its paired muscovite–biotite leucogranite. The characteristic banding appearance of these rocks is a product of heteroepitaxial nucleation of tourmaline on biotite folia and the replacement of pre-existing biotite and plagioclase with tourmaline and K-feldspar. The heteroepitaxy relationship of tourmaline on biotite is characterized by the {10–10} face of tourmaline parallel to biotite (001), with the tourmaline c-axis parallel to either the biotite [110] or [010] direction. One of the broader implications of our findings is that field estimates based on the volume of coarse-grained leucocratic outcrop overestimates the amount of melt generated at the top of the Himalayan slab. [ABSTRACT FROM AUTHOR]
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Abstract:Leucogranite bodies are ubiquitous in the upper structural levels of the Himalayan metamorphic slab. Their formation has ramifications for myriad processes including the generation of crustal melts and orogenic heat budgets. One particularly enigmatic variety of the Himalayan leucogranites, abundant in the Langtang region of Nepal, are the banded tourmaline leucogranites; typified by centimetre-scale compositional banding between tourmaline-rich and quartzo-feldspathic domains. Here, we use in-situ Rb–Sr isotopic chemistry and microstructural analysis to show these banded tourmaline leucogranites do not represent a direct product of melt crystallization but instead were formed by metasomatism of psammitic country-rock. This metasomatism was likely driven by the release of boron-rich volatiles during the crystallization of neighboring muscovite–biotite leucogranite bodies. Rb–Sr isochrons based on biotite–plagioclase ± white mica and K-feldspar data define overlapping dates of ca. 17.5 Ma from both the banded tourmaline leucogranite and its paired muscovite–biotite leucogranite. The characteristic banding appearance of these rocks is a product of heteroepitaxial nucleation of tourmaline on biotite folia and the replacement of pre-existing biotite and plagioclase with tourmaline and K-feldspar. The heteroepitaxy relationship of tourmaline on biotite is characterized by the {10–10} face of tourmaline parallel to biotite (001), with the tourmaline c-axis parallel to either the biotite [110] or [010] direction. One of the broader implications of our findings is that field estimates based on the volume of coarse-grained leucocratic outcrop overestimates the amount of melt generated at the top of the Himalayan slab. [ABSTRACT FROM AUTHOR]
ISSN:00107999
DOI:10.1007/s00410-023-02020-0