The Twin Lakes intrusion is composed mainly of coarse-grained porphyritic granodiorite, and is zoned from a felsic core to a slightly more mafic border. Steeply dipping mineral layers, typically a few inches to 5 feet thick and several tens of feet long, occur in discontinuous marginal zones as wide as 5000 feet. Four main types of layers are defined by increased abundances of orthoclase, quartz, plagioclase, and mafic minerals. The characteristic minerals of each type of layer differ markedly in size (orthoclase, average length about 10 cm; quartz, average diameter about 1 cm; plagioclase, average length .45 mm; and mafic minerals, average length .15 mm). Textural evidence from fine-grained granodiorite porphyry and deformed mafic layers indicates that the magma contained 50 to 60 volume percent suspended crystals during emplacement. Structures in the mafic layers such as size and concentration grading normal to the plane of layering, wedge layering, and cross layering superficially resemble sedimentary structures. Inspection of these structures, however, reveals a number of features that are difficult to explain by a process of sedimentation, but which are consistent with a flow sorting process accompanied by deformation. The layering probably formed by size sorting of the suspended crystals in marginal zones of the intrusion by essentially vertical shear flow during emplacement.
