Halogen Compositions of Biotite in the Ramba Leucogranite, Himalaya: Implications for Magmatic Evolution

The magmatic–hydrothermal evolution of Himalayan leucogranites exerts an important control on their petrogenesis and mineralisation, and the halogen composition of biotite provides a useful mineralogical tracer of this process. Here, we investigate biotite from the main two-mica granite and garnet-bearing leucogranite of the Ramba leucogranite in the Himalaya, based on petrographic observations and electron-probe microanalysis of halogens. Three groups of biotite with distinct textures and halogen compositions are identified in the two lithologies. Group I biotite is relatively euhedral and follows the Mg–F and Fe–Cl preference rule (or, equivalently, the Mg–Cl and Fe–F avoidance rule). Group II biotite is characterised by marginal muscovitisation. Although it still follows the Mg–F and Fe–Cl preference rule, its variations in XF (XF = molar F/(F + Cl + OH)), XFe (XFe = molar Fe/(Fe + Mg)), and XMg (XMg = molar Mg/(Ti + Fe + Mn + Mg + Al(VI))) differ markedly from those of Group I biotite. Group III biotite commonly shows marginal chloritization and does not follow the Mg–F and Fe–Cl preference rule. The two-mica granite contains all three groups, whereas the garnet-bearing leucogranite contains only Groups I and III. We interpret Group I biotite in the two-mica granite as a primary magmatic phase, whereas Group I biotite in the garnet-bearing leucogranite represents primary magmatic biotite entrained by a highly fractionated melt. Group II biotite in the two-mica granite is interpreted as primary magmatic biotite modified by melt metasomatism, whereas Group III biotite in both lithologies formed through late hydrothermal alteration of primary magmatic biotite. These results indicate that the Ramba leucogranitic magma underwent a multistage evolution involving melt extraction, melt self-metasomatism, and late hydrothermal alteration. The results provide new mineralogical constraints on the magmatic–hydrothermal evolution of Himalayan leucogranites and highlight the usefulness of biotite halogen compositions in tracing the evolution of highly evolved granitic systems.