MESOPROTEROZOIC MAGMATISM IN NORTHEASTERN OKLAHOMA: PETROGENETIC CONSTRAINTS FROM TRACE-ELEMENT GEOCHEMISTRY

Igneous rocks from the subsurface of northeastern Oklahoma are comprised of granite and rhyolite with minor andesite. U-Pb zircon geochronology indicates ages near 1370 Ma. These rocks are traditionally assigned to the Southern Granite-Rhyolite Terrane and have usually been considered as “within-plate granites”, though their tectonic setting of formation is largely unconstrained.

Whole-rock geochemical data for this area is historically lacking. New analyses of major and trace element abundances from eight cores indicate these rocks comprise a subalkaline high-K metaluminous to slightly peraluminous suite. They are consistently enriched in Cs, Rb, Ba, Th, U, and LREE. HREE slopes are flat. Large depletions in Nb, Ta and Ti are similar to those of subduction-related magmas. All samples additionally show depletion in Sr and P along with pronounced Eu anomalies. The most primitive andesitic samples also show elevated Cr (470 ppm) and Ni (110 ppm). Classification schemes based on immobile elements (e.g., Zr/Ti vs. Nb/Y, Co vs. Th) are congruent with major-element classifications.

Flat HREE slopes are inconsistent with the presence of garnet in the source. Growth of negative Eu, Sr, and P anomalies with silica enrichment indicates that plagioclase and apatite were continuously crystallizing phases. The most silica-rich samples also exhibit negative Ba anomalies consistent with alkali feldspar removal. Nb/Ta values decrease from near-chondritic (14-16) in the most primitive samples to 8-10 in the most silicic. Dy/Yb ratios do not change significantly, suggesting this is due to crystallization and removal of mica rather than amphibole.

Use of chemical discrimination diagrams indicate that while the more silicic specimens may plot in the field of A-type granites, they attained this character via fractionation from I-type melts. Comparison of various tectonic discrimination plots suggests that the geochemistry of these rocks is most consistent with a continental arc setting, though a post-collisional or back-arc origin is also plausible. These results show that these rocks are not typical intraplate granitoids, and their chemistry requires either an alternative tectonic setting or a more complex magmatic source.