The Rhyolite Dike at Medicine Park, Oklahoma – a Natural Quenched Experiment in the System Q-Ab-Or-H2O

Investigation of a porphyritic rhyolite provides insights into the ascent history and early development of granitic textures. Phenocrysts (18 vol.%) of Qtz (40 vol.%) and perthitic Ksp (60 vol.%) occur in a fine grained matrix as individual crystals and monomineralic glomerocrysts. Qtz phenocrysts (0.05 – 1.7 mm) are embayed and alkali-feldspar phenocrysts (0.05 – 3.25 mm) are euhedral to subhedral. The melt composition just prior to ascent was calculated using reintegrated Ksp compositions, phenocrysts percentages, a bulk composition of Q₃₅Ab₃₉Or₂₆ and the lever rule in the system Q-Ab-Or-H₂O. This melt composition plots on the 200 MPa cotectic indicating a source depth of ~8 km. The normative composition of the matrix Q₃₉Ab₄₀Or₂₁ projects onto the 50 MPa cotectic indicating an emplacement depth of ~2 km. Crystal Size Distribution analysis of Qtz and Ksp phenocrysts yield complex nonlinear curves. After correction for glomerocrysts CSD curves for Qtz indicate a paucity of crystals at sizes less than 0.75 mm and linear relationships for larger sizes with a slope of 1.31 and intercept of -1.91 (n= 695). Corrected CSD curves for Ksp are linear for all sizes with a slope of -0.72 and intercept -0.686 (n=398). This indicates formation of glomerocrysts occurred during magma ascent, as a result of resorption for Qtz and flow sorting for Ksp. Using a Qtz growth rate of 3 x 10^-8 cm/s (Swanson and Fenn, 1986) the residence time for the largest Qtz phenocryst in the magma prior to ascent is ~27 days. A magma ascent rate from the source depth to the emplacement level of ~0.02 m/s is calculated based upon the time to dissolve the smallest preserved Qtz phenocryst and the crystallization rate as an upper limit for the rate of Qtz dissolution.