SERIAL SECTION OF A K-FELDSPAR MEGACRYSTS FROM THE ENCHANTED ROCK PLUTON, TOWN MOUNTAIN INTRUSIVE COMPLEX, CENTRAL TEXAS

The Town Mountain Granite Intrusive Complex (TMGIC) is exposed in the Llano Uplift of central Texas and represents 1.1 to 1.0 Ga magmatism following Rodinian continental assembly. The Enchanted Rock Pluton is one of six plutons of the TMGIC, and is dominated by a pink K₂O rich, porphyritic, coarse-grained granite with relatively large sub- to euhedral alkali-feldspar phenocrysts. These exhibit variable rapakivi texture on rims. Several included phases are distributed throughout many grains. As part of a broader study on the microstructure and phase characterization of the Enchanted Rock megacrysts using SEM-EDS along with X-ray Powder Diffraction (XRPD) and thin-section petrography, we report here preliminary findings from electron characterization of a single, serial sectioned grain.

The K-feldspar is a sampled loose in-tact near-euhedral grain with scant attached matrix, measuring 14.5 mm (a-axis), 13 mm (b-axis), and 18 mm (c-axis). Sections were cut using a low-speed saw at intervals of 1.5 mm (1.0 mm offset with a 0.5 mm blade width). These were ground and polished to mirror finish on both sides. Provisional work has evaluated two sections: one at 1 mm and one at 3 mm. The grain exhibits weak to patchy perthitic intergrowth of near-end-member albite and microcline. The exsolved albite exhibit decreasing Ca concentration from the core (An_2.2) to rim (An₀). Included minerals found within the sectioned megacryst include Fe-rich biotite, allanite, and iron oxide; biotite inclusions are compositionally similar to grains from the surrounding matrix.