ACCESSORY MINERALS AND MICROSTRUCTURE: CRYSTALLIZATION SEQUENCE IN THE WICHITA GRANITE GROUP, SOUTHERN OKLAHOMA AULACOGEN, USA

The Wichita Granite Group (WGG) consists of Cambrian rift-borne alkali-feldspar granites expressed as tabular plutons. These are exposed in the Wichita Mountains and encountered in basement-penetrating wells along the aulacogen trend. Most hold evidence of early crystallization in the mid-crust, followed by ascent and significant undercooling with emplacement at 2 km and shallower depths. The WGG are dominated by perthite and Qz ± Pl, with glomerocrystic or interstitial mafic silicates and Hem ± Mt ± Ilm. Accessory minerals include Zrn + Ap ± Ttn ± Fl ± Aln ± Mnz with rare bastnäsite.

The WGG are largely similar in bulk composition, expressing only a small amount of variability in accessory-building components. Generally, WGG members are 75 wt.% SiO₂, with whole-rock Zr ≈ 500 ppm, 0.2 wt.% P₂O₅, 0.25 wt.% TiO₂, Y ≈ 75 ppm, La ≈ 65 ppm, and Ce ≈ 170 ppm. Higher-SiO₂ (77 wt.% SiO₂) units tend to have lower Zr, La, and Ce (200, 50, 110 ppm) and TiO₂ (0.10 wt.%); lower-SiO₂ (73 wt.% SiO₂) units contain relatively elevated TiO₂ and P₂O₅ (0.5 and 0.08 wt.%) and Zr, La, and Ce (540, 85, 180 ppm).

Accessory mineral microstructure varies. The grains are generally distributed into the mafic agglomerations; Zrn also occurs as inclusions in and boundary phases between Kfs, Qz, and oxides. Zircon euhedra (~50 um) are typical to all units; however, irregular morphologies are observed along grain boundaries of quartz. WGG may contain euhedral Ap, and Ttn or Fl. Ttn also occurs as epitaxy on Ilm in rare cases. Ttn and Fl are also sub- or anhedral, skeletal, or patchy, as are most occurrences of the other accessory minerals, with the exception of Zrn. Ttn, Aln, and Mnz preserve sharp compositional zoning patterns.

Their microstructure is consistent with near-liquidus crystallization for Zrn, and Ttn or Fl; this is corroborated by thermometry indicating co-crystallization Zrn and Ttn with Kfs ± Hbl near 800 °C. The other accessories’ morphology supports their early crystallization followed by resorption due to decompression and/or increases in melt-volatile concentration. Skeletal crystals and epitaxy are consistent with (re)saturation and growth in the undercooled emplacement environment. These relationships seem consistent across all WGG, suggesting that sequence is not strongly influenced by variability in accessory-building components in these rocks.