Abstract
The Cufttown Creek pluton is a leucocratic alkali feldspar granite with (F + Mn)-rich biotite, muscovite, garnet, and fluorite as varietal minerals. Bt ± Ms is preserved as inclusion assemblages in quartz; Bt + Ms ± Grt and Ms + Grt are matrix assemblages. The inclusion assemblages are interpreted as early magmatic, and the matrix assemblages as late magmatic with superimposed subsolidus partial reequilibration. In sequence, the AFMMn crystallization reactions are liquid = Bt, liquid = Bt + Ms, liquid + Bt = Ms + Grt, and liquid = Grt + Ms. Inclusion biotite shows FeMg−1, and Mn(Fe,Mg)−1 exchanges, which may reflect both change in melt composition and cooling along an fo2 buffer. Inclusions of white mica are nearly end-member muscovite. Evolution from inclusion to matrix mica compositions shows narrowing of the compositional gap between the dioctahedral and trioctahedral micas, as well as OHF−1 and MnMg−1 exchanges in the biotite and (Fe2+,Mg,Mn)Si[6]Al−1[4]Al−1 (celadonite), NaK−1, OHF−1, and Ti enrichment in the white micas. Garnets are spessartine-almandine.
The liquid line of descent indicates that the melt became increasingly rich in Fel(Fe + Mg) and Mn/(Fe + Mn) with decreasing temperatures, but each at differing rates at differing stages during crystallization. Peraluminosity of the melt increased rapidly at first, then decreased slightly before increasing at a much slower rate. Final crystallization conditions are estimated at 650 °C and 2–3 kbar. Miarolitic cavities indicate that, at the end of crystallization, Pfluid = Ptot, and a separate fluid phase formed that persisted into the subsolidus. The fluid was dominantly H2O, with lesser amounts of F [log(fH2O/fHF) = 3.16–3.54].
Subsolidus replacement textures, mineral compositions, and fluid buffers result from two reactions that are estimated to have occurred at 300–400 °C: (1) plagioclase + iron biotite + muscovite + iron chlorite + fluid = albite + potassium feldspar + magnesium biotite + phengite + magnesium chlorite + fluorite + qtrartz and (2) garnet + potassium feldspar + fluid = muscovite + rhodochrosite + quartz.
