Abstract

Systematic variations in major elements, U and Th contents, and Sr isotope ratios in graywackes of the Flournoy Formation near Agness, Oregon, are interpreted to indicate a multilithologic provenance. At the time of deposition, Rb-Sr systems in the graywackes defined a pseudo-isochron corresponding to an age of 110 m.y. Calculated 87Sr/86Sr ratios range from 0.7050 to 0.7074, and compositional variations relate to these values in simple mixing patterns. Rb, U, Th, and K2O show a positive correlation with increasing (87Sr/86Sr)O and decreasing MgO, CaO, and Sr. Average values of Rb/Sr and 87Sr/86Sr (at 50 m.y. ago) for Mesozoic graywackes—Otter Point, Dothan, Galice, and Colebrooke Formations–are coincident with the mixing line defined by the Rb-Sr systems of the Flournoy graywackes, suggesting that some combination of the Mesozoic graywackes comprised part of the felsic component of the source terrane. A mafic source is also required by chemical, isotopic, and mineralogic features of the Flournoy graywackes. This was probably equivalent to basalts in the Paleocene and Eocene Roseburg Formation beneath the Flournoy.

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