T17 Hydrothermal Systems and Their Geologic Records

Sunday, 15 October 2023
8:00 AM - 12:00 PM
319

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

Paper number: T17-9
Program: Technical Sessions
Time: 9:55 AM – 10:10 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-394937

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