Linking petrographic and geochemical indicators of melt reactions in pelitic migmatites and leucogranites

Anatexis in orogenic systems has wide-reaching implications for the structure, mechanical strength, and later exhumation of the middle crust during orogeny. Previous studies of melt reactions in metapelitic rocks have identified three major melt-producing reactions: fluid-present incongruent melting, fluid-absent muscovite dehydration melting, and fluid-absent biotite dehydration melting. Due to the composition of the reacting phases and the formation of peritectic phases, each of these reactions has implications for element mobilisation. Petrographic and geochemical signatures of melt reactant and product minerals allow different reactions to be identified and distinguished. Migmatites and leucogranites occur across the Himalaya in the Greater Himalayan Sequence (GHS). Using samples from the Alaknanda valley, near Badrinath in the Garhwal Himalaya, we identify petrographic and mineral compositional characteristics that suggest a progressive thermal sequence through the fluid-present, muscovite-dehydration, and later biotite-dehydration melting reaction fields, but a differential record. Samples from a single outcrop only preserve records for one reaction type, despite having experienced the same thermal history. Our observational and in-situ chemical data from feldspars, micas, and garnet reveal melt reaction systematics through peritectic crystallisation textures and large-ion lithophile element concentrations. Our results show that the mineral chemistry and petrographic observations are similar between (source) migmatites and (sink) granites, and that together these datasets provide more powerful insight into migmatite and granite melting reaction history than bulk-rock data alone.