Reference 1502
Brooks, William E; Snee, Lawrence W (1996) U. S. Geological Survey Bulletin, Report: B 2154, 25 pp., 1996
| Authors | Brooks, William E; Snee, Lawrence W |
|---|---|
| Year | 1996 |
| Source | U. S. Geological Survey Bulletin, Report: B 2154, 25 pp., 1996 |
| ISSN | 8755-531X |
| GeoRef | 1996-041102 |
| Keywords | absolute age; adularia; alkali feldspar; Ar/Ar; ash-flow tuff; biotite; Cenozoic; dates; detachment faults; faults; feldspar group; framework silicates; geochemistry; Grant Range; hydrothermal alteration; igneous rocks; metaigneous rocks; metamorphic rocks; metasomatism; metatuff; metavolcanic rocks; mica group; middle Tertiary; Miocene; Neogene; Nevada; Nye County Nevada; Oligocene; overprinting; Paleogene; pyroclastics; sheet silicates; silicates; southern Nevada; Tertiary; United States; USGS; volcanic rocks; whole rock; Windous Butte Formation |
Abstract
Whole-rock chemical analyses and super 40 Ar/ super 39 Ar geochronologic data on mineral separates from correlative potassium-metasomatized and unaltered ash-flow tuff from the Windous Butte Formation are presented. The effect of potassium metasomatism, which commonly affects upper-plate rocks at detachment faults in the Western United States, on rock chemistry is well known. However, structural implications and conclusions regarding thermal effects associated with this type of alteration on dates from these altered upper-plate rocks was previously hampered by the the absence of data from correlative, unaltered volcanic units. Results of this study indicate that 1 temperatures of the hydrothermal fluids that contributed the excess potassium did not exceed the resetting temperature of biotite 280 degrees C, 2 alteration of feldspars ranges from incipient to pervasive, and 3 the detachment-fault conduit was closed to hydrothermal circulation at 20 Ma in the Grant Range. Detachment in the Grant Range is, therefore, bracketed between 31 Ma, the regional age of the Windous Butte Formation, and 20 Ma, the age of adularia introduced during detachment-related potassium metasomatism.