A Rubidium-Strontium study of the Twilight Gneiss, West Needle Mountains, Colorado
✍ Scribed by Fred Barker; Zell E. Peterman; Robert A. Hildreth
- Book ID
- 104745434
- Publisher
- Springer
- Year
- 1969
- Tongue
- English
- Weight
- 801 KB
- Volume
- 23
- Category
- Article
- ISSN
- 0010-7999
No coin nor oath required. For personal study only.
✦ Synopsis
The Precambrian trondhjemitic Twilight Gneiss (Twilight Granite of CROSS and HOWE, 1905b) of the West Needle Mountains, southwesteru Colorado, and its interlayered amphibolite and metarhyodacite yield a Rb-Sr isochron of 1,805• m.y. A low initial SrS~/Sr s6 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 K20 percentages of 1.23 to 3.64 in the Twilight Gneiss, in conjunction with high K/Rb ratios and the low initial ratio of SrST/Sr s6, lend support to geologic data that suggest the Twilight originated as volcanic or hypabyssal igneous rocks in a basaltic volcanic pile.