A Rubidium-Strontium study of the Twilight Gneiss,West Needle Mountains,Colorado |
| |
| Authors: | Fred Barker Zell E Peterman Robert A Hildreth |
| |
| Institution: | (1) U.S. Geological Survey, 80225 Denver, Colorado |
| |
| Abstract: | The Precambrian trondhjemitic Twilight Gneiss (Twilight Granite of Cross and Howe, 1905b) of the West Needle Mountains, southwestern Colorado, and its interlayered amphibolite and metarhyodacite yield a Rb-Sr isochron of 1,805±35 m.y. A low initial Sr87/Sr86 ratio of 0.7015 implies that metamorphism of these rocks to amphibolite facies took place soon after their emplacement. The mild metamorphism of Uncompahgran age, prior to 1,460 m.y. ago, and Laramide volcanism did not affect the Rb-Sr system in the Twilight. Rb contents of 26.5 to 108 ppm, Sr contents of 114 to 251 ppm, and K2O percentages of 1.23 to 3.64 in the Twilight Gneiss, in conjunction with high K/Rb ratios and the low initial ratio of Sr87/Sr86, lend support to geologic data that suggest the Twilight originated as volcanic or hypabyssal igneous rocks in a basaltic volcanic pile.Publication authorized by the Director, U.S. Geological Survey. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
