Abstract
Approximately 17,500 ft of marine Jurassic strata exposed along South Fork of John Day River in SW. Grant County, Oregon, are chiefly elastic rocks whose constituent particles were derived dominantly from Jurassic pyroclastic eruptions of augite andesite. During diagenesis, 3 phases of alteration took place in sequence: 1) crystallization of volcanic glass to heulandite(?) and associated minerals, including chlorite and celadonite; 2) local conversion of early-formed zeolites to laumontite; and 3) widespread decomposition of plagioclase to albite plus one or more hydrous Ca-bearing minerals, including pumpellyite, prehnite, and laumontite. Albitization is incomplete on every scale, and the spatial patterns of albitization and laumontitization, which are intricate on every scale, bear no consistent relation to stratigraphic or structural position. The following considerations suggest that the local distribution of interstitial water exerted the most effective control on the progress of the alterations: a) water was a reactant in most of the reactions, b) water probably served as a catalyst enabling the retrogressive reactions of which the alterations consisted to proceed, and c) certain critical components on whose mobility the alteration reactions depended were probably transported in water solution. The arrested condition of the alterations and the common association of seemingly incompatible Ca-bearing phases indicate that equilibrium was never attained, in a bulk sense, as a result of the diagenetic reactions.
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