Kyanite petrogenesis in migmatites: Resolving melting and metamorphic signatures

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Authors

Stacy Elizabeth Phillips , Tom Argles, Clare Warren , Nigel Harris, Barbara Kunz 

Abstract

Aluminosilicates (kyanite, sillimanite and andalusite) are useful pressure-temperature (P-T)
indicators that can form in a range of rock types through different mineral reactions, including those
that involve partial melting. Their involvement in melting reactions means that the presence of
aluminosilicates in migmatite mineral assemblages can help to (broadly) constrain the P-T conditions
of melt formation, which then has implications for evaluating models of orogenic tectonics.
Xenocrystic grains could lead to spurious tectonic interpretations, so being able to distinguish
between different petrogenetic sources is important. Petrological and geochemical investigation of
migmatite-hosted kyanite from Eastern Bhutan shows that kyanite petrogenesis may be constrained
by combining information from morphology, cathodoluminescence response, microtextural position
and geochemical zoning patterns. Mg, Ti, Ca, Fe, Cr and Ge concentrations provide diagnostic clues
that distinguish sub-solidus kyanite from kyanite that crystallised directly from melt, or grew
peritectically during muscovite dehydration reactions. The abundance of these elements in kyanite is
also strongly controlled by protolith composition, with considerable inter-sample variation observed
in this sample set. LA-ICP-MS maps, especially of Cr/V, provide additional information about
changing geochemical environments during kyanite growth. These data and observations show that
most kyanite is of xenocrystic origin in the analysed samples, and therefore that its presence does not
necessarily constrain the P-T conditions of the melt reaction(s). This finding has significant
implications for the interpretation of kyanite-bearing migmatites as representing early stages of
melting during Himalayan evolution.

DOI

https://doi.org/10.31223/X5F90J

Subjects

Earth Sciences, Geochemistry, Geology

Keywords

LA-ICP-MS, Kyanite, Migmatite, Trace elements, LA-ICP-MS, Cathodoluminescence, Cathodoluminescence

Dates

Published: 2021-07-26 12:31

Last Updated: 2021-07-26 16:31

License

CC BY Attribution 4.0 International

Additional Metadata

Conflict of interest statement:
None

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