MAFIC PHENOCRYSTS AND CRYSTALLIZATION OF A SPESSARTITE DIKE, MAGNET COVE, ARKANSAS

An inclusion-charged spessartite dike near Magnet Cove High School, Arkansas, contains phenocrysts/megacrysts of high-Ca pyroxene, kaersutite, and Fe-phlogopite in a fine-grained matrix of pyroxene, amphibole, feldspar, and oxides. The lamprophyre is characterized by relatively high titania (2.8-3.0 wt. % TiO₂), total iron (10.5-12.0 wt. % Fe₂O₃), potash (3.4-3.7 wt. % K₂O), Rb (>90 ppm), Sr (>1000 ppm), Ba (>850 ppm), V (>300 ppm), Y (>35 ppm), Nb (>140 ppm), Zr (>400 ppm), and Nb/Y ratios (>4); low Ni (<30 ppm) and Cr (<50 ppm); and Na₂O/K₂O ratios near 1. Euhedral pyroxene phenocrysts are calcic endiopside to salite in composition (Wo_45-32-En_50-30-Fs_14-25) with 20-24 wt. % CaO, 1-3 wt. % TiO₂ and 4-9 wt. % Al₂O₃. Al and Ti increase with increasing Fe, indicating substitutions as: R⁺² + Si=Al + Al and R⁺² + 2Si=Ti + 2Al. Ti-Al ratios (between 1:5 and 1:10) indicate that alumina substitutes in the pyroxene lattice predominantly as Ca-Tschermaks molecule. Pyroxene phenocryst compositions overlap those in a large (>15 cm) megacryst, which also has exceptionally high Ti and Al contents (up to 2.7 wt. % titania and 7.0 wt. % alumina; Green and Bersch, 1999). Rare two centimeter, euhedral mica megacrysts are Mg-biotite to Fe-phlogopite. Subhedral kaersutite phenocrysts contain 4-5.5 wt. % TiO₂, 13-14.3 wt. % Al₂O₃, > 0.02 wt. % Cr₂O₃, 1.9-2.4 Al^IV atoms, 1-1.2 (Na +K) atoms, 1.8-2.5 octahedral (Al^VI + Fe³⁺ + Cr³⁺ + Ti) atoms, and Na₂O/K₂O ratios near unity.

Similar phase compositions, coupled with little or no reaction rim around the megacrysts, support an interpretation that phenocryst and megacryst minerals crystallized under similar conditions. Applicable thermobarometric techniques (two-feldspar, Fe-Ti oxide, and plagioclase-amphibole thermometry, together with Al-hornblende and clinopyroxene geobarometry) suggest that crystallization of the volatile-rich spessartite magma occurred within the lowermost crust.