Abstract
Biotite and felspar were chemically analyzed from three plutonic rocks from the Okanogan Highlands, WA, using the EPMA at the Florida Center for Analytical Electron Microscopy, Florida International University. These minerals are from an Eocene tonalite from the Swawilla Granodiorite, a Cretaceous monzogranite from the Midnight Mine pluton, and a Cretaceous granodiorite from the Conconully pluton. Iron in the biotite is calculated as total Fe (super 2+) . EPMA analysis included 7 biotite and 8 feldspar from the Swawilla Granodiorite, 6 biotite and 6 feldspar from the Midnight Mine pluton, and 7 biotite and 2 feldspar from the Conconully pluton. Biotite Fe# and Sigma Al range from 0.49 to 0.57 and 2.61 to 2.69 apfu in the Swawilla Granodiorite sample, 0.53 to 0.61 and 2.80 to 2.89 apfu in the Midnight Mine pluton sample, and 0.47 to 0.60 and 2.60 to 2.84 apfu in the Conconully pluton sample. The strong brown pleochroism, textures, and chemistry of the biotite from all samples suggest they are primary. Plagioclase from the Swawilla Granodiorite is primarily andesine (An (sub 34-30) ), and the Midnight Mine pluton is primarily oligoclase (An (sub 31-25) ). One analyzed alkali feldspar from the Midnight Mine pluton sample is Or (sub 92) and two analyzed alkali feldspar from the Conconully pluton sample are Or (sub 94) and Or (sub 91) . The biotite analyses plot within calc-alkaline fields on discrimination diagrams. Biotite thermometry of Luhr et al. (1984) and barometry of Uchida et al. (2007) where used to estimate crystallization temperatures and pressures. The biotite yields temperatures and pressures of 600 degrees + or - 6 C and 1.53 + or - 0.07 kb for the Swawilla Granodiorite sample, 596 degrees + or - 2 C and 2.08 + or - 0.09 kb for the Midnight Mine pluton sample, and 679 degrees + or - 31 C and 1.64 + or - 0.20 kb for the Conconully pluton sample. Oxygen fugacity using the equilibrium expression of Wones (1989) was calculated using the biotite temperatures and pressures. These yield log of -20.45 + or - 0.24 for the Swawilla Granodiorite sample, -20.62 + or - 0.07 for the Midnight Mine pluton sample, and -17.53 + or - 1.14 for the Conconully pluton sample. This data suggests that these samples crystalized in arc settings at relatively shallow depths. These estimated pressures suggest no appreciable uplift occurred between the Cretaceous plutons and the Eocene Swawilla Granodiorite crystallization.