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A Rubidium-Strontium study of the Twilight Gneiss, West Needle Mountains, Colorado

Contributions to Mineralogy and Petrology

By:
, ,
DOI: 10.1007/BF00371424

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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. ?? 1969 Springer-Verlag.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
A Rubidium-Strontium study of the Twilight Gneiss, West Needle Mountains, Colorado
Series title:
Contributions to Mineralogy and Petrology
DOI:
10.1007/BF00371424
Volume
23
Issue:
4
Year Published:
1969
Language:
English
Publisher:
Springer-Verlag
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Contributions to Mineralogy and Petrology
First page:
271
Last page:
282