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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 thosethat involve partial melting. Their involvement in melting reactions means that the presence ofaluminosilicates in migmatite mineral assemblages can help to (broadly) constrain the P-T conditionsof melt formation, which then has implications for evaluating models of orogenic tectonics.Xenocrystic grains could lead to spurious tectonic interpretations, so being able to distinguishbetween different petrogenetic sources is important. Petrological and geochemical investigation ofmigmatite-hosted kyanite from Eastern Bhutan shows that kyanite petrogenesis may be constrainedby combining information from morphology, cathodoluminescence response, microtextural positionand geochemical zoning patterns. Mg, Ti, Ca, Fe, Cr and Ge concentrations provide diagnostic cluesthat distinguish sub-solidus kyanite from kyanite that crystallised directly from melt, or grewperitectically during muscovite dehydration reactions. The abundance of these elements in kyanite isalso strongly controlled by protolith composition, with considerable inter-sample variation observedin this sample set. LA-ICP-MS maps, especially of Cr/V, provide additional information aboutchanging geochemical environments during kyanite growth. These data and observations show thatmost kyanite is of xenocrystic origin in the analysed samples, and therefore that its presence does notnecessarily constrain the P-T conditions of the melt reaction(s). This finding has significantimplications for the interpretation of kyanite-bearing migmatites as representing early stages ofmelting during Himalayan evolution. |
