Abstract

Near the crest of the Shoshone Range in N-central Nevada, 3 breccia pipes of Tertiary age cut deformed chert, quartzite, shale, and greenstone of Paleozoic age. The pipes average about 1 mi. in longest dimension, have slightly flaring sides, and consist of the following units: 1) an outer discontinuous rim of coarse breccia of Paleozoic rock fragments; 2) an inner mass of fine-grained and much hydrothermally altered breccia that contains a thorough mixture of Paleozoic and Tertiary rock fragments and intrudes the marginal coarse breccia; 3) foundered blocks of Tertiary conglomerate and pyroclastic rocks intruded and engulfed by the fine breccia or younger rhyolite; 4) a more or less central plug of quartz monzonite porphyry or rhyolite that intrudes the fine breccia and the conglomerate or pyroclastics; 5) narrow flowbanded dikes of rhyolite or quartz latite porphyry that cut all other rock types in the pipes and extend into the wall rocks. The pipes were formed by an up-and-down pump action impelled by intermittent rise of the magma in the plugs. Brecciation occurred during upward movement of the magma and associated explosive volcanism. Breccia also was formed by subsidence collapse stoping, and possibly rock-bursting during subsidence of the magma. The fine breccia thus formed ahead of the magma and, in turn, became intrusive, eventually working its way to or near the surface.

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