Low-Grade Mineral Facies in Upper Triassic and Lower Jurassic Rocks of the Aldrich Mountains, Oregon

Abstract

ABSTRACT Authigenic mineral assemblages characteristic of the prehnite-pumpellyite facies and of the laumontite stage of the zeolite facies occur in a thick sequence of graywacke, shale, mudstone, tuff, pillow lava, chert, and volcanic graywacke of Late Triassic and Early Jurassic age in the Aldrich Mountains of Oregon. Albite, quartz, chlorite, calcite, and leucoxene occur in most of the rocks, but laumontite, prehnite, pumpellyite, and celadonite are mainly in rocks entirely or partly of volcanic origin. Fine granules of sphene and epidote are also common but possibly are of detrital origin. The minerals grew as a result of increased pressure and temperature due to burial and probably also to tectonic folding penecontemporaneous with deposition. Chemical components in trapped sea water and from decomposition of calcic plagioclase, pyroxene, and volcanic glass in the sinking pile of sediments provided the elements for albitization of plagioclase and growth of the authigenic minerals. Intrusive stocks produced notable contact effects, but regionally had little influence on the authigenic mineral assemblage.