The biography of a mountain as told by its minerals: The petrogenesis of kyanite migmatites in Eastern Bhutan, Himalaya
Stacy Phillips *1,Tom Argles 1, Nigel Harris 1, Clare Warren 1, Nick Roberts 2, Barbara Kunz 1
1 The Open University, Milton Keynes, United Kingdom; stacy.phillips@open.ac.uk
2 Geochronology and Tracers Facility, British Geological Survey, Nottingham NG12 5GG, UK
Migmatites are rocks that contain evidence of crystallised melt. The minerals that they contain can give us information about how these rocks formed, became molten and how they subsequently cooled and crystallised. The mineral kyanite (Fig. 1) generally indicates that a rock formed at high pressures deep in the earth’s crust. In the Himalaya, kyanite migmatites are interpreted as key evidence in support of the theory of ‘melt weakening’ that allows the deep rocks in the heart of the mountain to ‘flow’ buoyantly towards the surface. Understanding how and when this important mountain-building process occurs requires investigation into the minerals found within the migmatites.
Kyanite migmatites were sampled from across Eastern Bhutan and were analysed using a number of techniques. The geochemical composition of kyanite, muscovite (a mineral involved in the melt-producing chemical reaction) and biotite (a common mineral in migmatites that is not involved in the production of melt) were determined, with results showing that trace elements like Cr, V and Ge were exchanged between the minerals as the migmatite formed. Novel laser-ablation mapping of kyanite revealed variations in chemistry within individual crystals, indicating that kyanite is not a passive mineral, but that it can record details of changing geochemical conditions in the melt. Analysis of U-Pb isotopes in the mineral zircon showed that the migmatites formed over a ~12 million-year period, and at a later time than predicted. Such mineral-scale details provide constraints on the conditions present at the regional-scale, and how they have changed over time.
Fig 1. A kyanite migmatite from Eastern Bhutan. The dark material (schist) remains unmelted whereas the white material (leucosome) is crystallised melt. The prominent distinctive blue mineral is kyanite, but it is unclear from this photo if this kyanite is part of the (dark) schist or the (white) leucosome. Determining how and where the kyanite crystal formed has important implications for the interpretation of these rocks at the mountain-scale.