The Formation and Age of Leucogranitic Melt in the Garhwal Himalaya

Oldman, Charlie James (2023). The Formation and Age of Leucogranitic Melt in the Garhwal Himalaya. PhD thesis The Open University.



Metamorphism during orogenic evolution results in the melting of high-grade metamorphic rocks which dramatically decreases mechanical strength, activates shear zones, and may consequently initiate exhumation. The timescales of anatexis, melt amalgamation, migration, and emplacement are the focus of active research and have critical implications for orogenic tectonic development. It is, therefore, important to recognise and distinguish between the geochemical signatures that these processes create and to delineate more precisely the relevant mechanisms and timescales leading to magma genesis. Granitic bodies and source migmatites are exposed across the Himalaya in the Greater Himalayan Sequence (GHS). Many of these granites formed as high-grade metamorphic rocks and were decompressed during the Miocene. However, exact timings of melt formation and melt mechanisms appear to vary laterally across the orogen. This thesis presents constraints on the melt reactions, sources, and timescales of melting episodes that formed the migmatites and leucogranites of the upper GHS using samples from the Alaknanda valley in the Garhwal Himalaya. Detailed petrography and geochemical characterisation of feldspars, micas, and garnet reveal melt reaction systematics through peritectic crystallisation textures and large-ion lithophile element concentrations. Samples are identified with predominately fluid-present, muscovite-dehydration, and biotite-dehydration melting contributions. Combined zircon and monazite geochronology suggests partial melting occurred over an extended period, from 35 to 14 Ma, with melt production sharply increasing around 25 Ma and reaching its peak at 21 Ma, primarily attributed to muscovite-dehydration. Zircon εHf values show little systematic variation with age, suggesting a geochemically-common source of melt. Successive episodes of melt generation around peak metamorphic conditions likely weakened the mid-crustal GHS, priming it for exhumation and subsequent decompression melting.

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