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

Abstract

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., K Rb , Nb Ta and La Sm ) and isotopic ratios (e.g., 206 Pb 204 Pb and 87 Sr 86 Sr 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 pluton's evolution. It played an important role in the genesis of the evolved rocks which are characterized by low K Rb (24–150), Nb Ta (1–6), La Sm (1.5–4) and Eu Eu ∗ (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 Rb Sr and U Pb ratios which produced the distinct variations of 87 Sr 86 Sr and 206 Pb 204 Pb 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.