INTERPRETING CAMBRIAN PALEOISOTOPE HYDROLOGY USING FLUID-ROCK INTERACTION IN THE IGNEOUS ROCKS OF THE SOUTHERN OKLAHOMA AULACOGEN

The purpose of this study is to investigate the rift-related igneous activity and fluid rock interaction associated with the formation of the Cambrian Southern Oklahoma Aulacogen covering an area >250,000 km³ and striking ~N60W. Two levels of exposures of the rift zone provide 49 samples for oxygen and hydrogen isotope analyses. The first: the westernmost exposure of igneous rocks crops out in the Wichita Mountains, consisting of a layered mafic complex with A-type granites overlain by marine sediments. The second: closer to the inferred Cambrian triple junction, is the Mill Creek Quarry where the ~1.4 Ga Troy Granite is intruded by a Cambrian sheeted diabase dike complex nonconformably overlain by Cambrian sandstones. Pooled hydrogen isotope data exhibit a broad distribution centered on the “normal igneous rock range” with tails towards high values of δD >-50‰ with clusters >-35‰ indicative of seawater alteration. Using the “freshest” pyroxene (δ¹⁸O >+6‰) as the primary magmatic value of the layered gabbro, plagioclase-pyroxene pairs have anomalous fractionation Δ<0.5 with most reversely fractionated, <0, and slightly lower in ¹⁸O (whole rocks depleted by 1‰), suggesting that the interacting fluid was either ¹⁸O-shifted or seawater. Plagioclase-water fractionations suggest fluid δ¹⁸O ~+3 consistent with initial seawater or average meteoric water >-5. The Mill Creek Quarry diabases exhibit δ¹⁸O values clustering at +4.5 due to fluid rock interaction with fluids calculated δ¹⁸O from -1.2 to +4.0. The Troy Granite wall rocks have δ¹⁸O values clustering at 3‰, shifted from fresh rocks that cluster at 7.5‰. Fluids in hydrogen isotope equilibrium with secondary epidote from the quarry range from -4.4 to -1.6 and fluid in oxygen isotope equilibrium with chlorite was ~ -0.6. Our data show that the Cambrian hydrosphere was not very isotopically different than present day and suggest that the province was relatively close to sea level when it formed.