Origin of topaz-bearing and related peraluminous granites of the Late Devonian Davis Lake pluton, Nova Scotia, Canada: crystal versus fluid fractionation

The Davis Lake pluton, in the Late Devonian South Mountain Batholith in southwestern Nova Scotia (Canada), is composed of peraluminous leucomonzogranites and subordinate topaz-bearing leucogranites that host greisens and a primary tin deposit. The concentrations of incompatible trace elements (e.g., Rb, Li, U, F, Ta and Sn) and element (e.g., KRb, NbTa and LaSm) and isotopic ratios (e.g., 206Pb204Pb and 87Sr86Sr display a spatial zonation. The granitic rocks were generated by extensive fractional crystallization of major phases (feldspars and biotite) and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber. Fractional crystallization of the evolving F-rich peraluminous granitic magma was accompanied, particularly in later stages, by fluid fractionation involving fluorine complexing. Fluid fractionation was a major differentiation process during the terminal stages of the plutonʹs evolution. It played an important role in the genesis of the evolved rocks which are characterized by low KRb (24–150), NbTa (1–6), LaSm (1.5–4) and EuEu∗ (0.01–0.3) ratios. These ratios as well as the enhanced concentrations of Li, Rb, U, Ta and Sn in these rocks correlate with F. Fluorine-rich fluids modified the RbSr and UPb ratios which produced the distinct variations of 87Sr86Sr and 206Pb204Pb isotopic ratios. The parent magma was formed by dehydration melting of lower-crustal metasedimentary rocks. The melting was related to crustal thickening in association with terrane and collision along the eastern margin of the Appalachians.