THE MIDWAY SILL, LLANO UPLIFT, CENTRAL TEXAS: TOWN MOUNTAIN GRANITE? NO

The Llano Uplift of central Texas exposes ~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 TMG is a pink, K₂O-rich, very coarse- to coarse-grained, generally porphyritic granite with associated fine- to medium-grained, gray to pink granites. The TMG has been divided into two groups (Reed, 1996): 1) plutons with solid-state deformation (typically elongate or irregular; e.g., Wolf Mtn., Legion Cr., and Sixmile) and 2) circular to oval shaped plutons consisting of porphyritic TMG (e.g., Lone Grove(LG), Kingsland(KL), Marble Falls(MF), and Enchanted Rock).

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. Stenzel (1935) named the sill and described it as being part of the TMG suite. Recent work on the MS shows: 1) the intrusion to be a foliated hornblende-biotite augen gneiss (orthogneiss) with accessory apatite, sphene, and zoned zircon; 2) foliation strikes NW to WNW and dips moderately to the SW; 3) modal and normative composition is granodiorite (An33) with normative hy and ab>or; 3) whole rock analyses vary between 64-65.7 wt.% SiO₂ with high total alkalis, K₂O/Na₂O=1.05-1.34 and high CaO, MgO, and TiO₂ relative to the LG, KL, and MF plutons; 4) A/CNK=.90; 5) trace elements in (ppm) for Rb=87-138, Sr=245-253, and Ba=915-1296; and 6) Fe/(Fe+Mg)=.66-.67 and .72-.73 for biotite and hornblende, respectively. Whole rock analyses of the more evolved TMG (e.g., LG, KL; and MF plutons) vary continuously from 68-77 wt.% SiO₂ with high alkalis, K₂O/Na₂O=1.13-1.97, but low in CaO, MgO, TiO₂, and Fe/(Fe+Mg)=.71-.92 and .78-.92 for biotite and hornblende, respectively. On tectonic discrimination diagrams (e.g., Rb vs Yb+Ta) the MS as well as the TMG straddle the boundary between "within plate" and "volcanic-arc" fields.