Mineral layering in the Twin Lakes granodiorite, Colorado

GSA Memoirs
Edited by: Leonard H. LarsenMartin Prinz, and Vincent Manson



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.

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Publication type Article
Publication Subtype Journal Article
Title Mineral layering in the Twin Lakes granodiorite, Colorado
Series title GSA Memoirs
DOI 10.1130/MEM115-p235
Issue 115
Year Published 1969
Language English
Publisher Geological Society of America
Description 29 p.
First page 235
Last page 262
Country United States
State Colorado
Other Geospatial Twin Lakes
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