PETROLOGY AND GEOCHEMISTRY OF THE MIDWAY SILL, LLANO UPLIFT, CENTRAL TEXAS

The Llano Uplift of central Texas is a gentle structural dome exposing ~1.37 Ga to 1.23 Ga metaigneous and metasedimentary rocks of Grenville affinity that have been intruded by ~1.12 to 1.07 Ga, post- to syntectonic granites formally called the Town Mountain Granite (TMG).

The Midway Sill (MS) is located in the NE corner of the Llano Uplift along the contact between the Packsaddle Schist and Valley Spring Gneiss just east of Lake Buchanan and the Lone Grove batholith. Stenzel (1935) named the sill and described it as being part of the TMG suite. Recent work on the MS shows: 1) the intrusive to be a foliated hornblende-biotite augen gneiss (orthogneiss) with accessory apatite, titanite, and zoned zircon; 2) foliation in the MS strikes NW to WNW and dips moderately to the SW (Reed, 1996); 3) modal composition (vol.%) is granodiorite-monzogranite with normative hy and ab>or; 3) whole rock analyses vary between 64-65.7 wt.% SiO₂ with high alkalies, K₂O/Na₂O=1.05-1.34 and high CaO, MgO, and TiO₂; 4) A/CNK=.90; 5) trace elements in ppm for Rb=87-138, Sr=245-253, and Ba=915-1296; 6) enriched in REE’s, Ba, Zr, Ni, V, Hf, and Eu relative to the Lone Grove batholith; and 7) Fe/(Fe+Mg)=.66-.67 and .72-.73 for biotite and hornblende, respectively. On the basis of the Fe/(Fe+Mg) for hornblende the MS was emplaced at intermediate levels of f_O2 well above QFM. The MS contains an abundance of megacryts of microcline (Or93) with perthic texture, plagioclase (An16-21), biotite, hornblende, and is metaluminous. On discrimination diagrams (e.g., Rb vs Yb+Ta and Rb vs Y+Nb) the MS is located in the “within plate”, but close to the “volcanic-arc” field.

The Midway Sill a subalkalic, high-K, medium- to coarse-grained two feldspar hornblende-biotite augen gneiss, is interpreted to be a deformed granitic intrusive (sill) which intruded between the Valley Spring and Packsaddle domains during the Grenville orogeny.