Z. E. Peterman R.A. Hildreth F. Barker 1969 <div id="Abs1-section" class="c-article-section"><div id="Abs1-content" class="c-article-section__content"><p>The Precambrian trondhjemitic Twilight Gneiss (Twilight Granite of<span>&nbsp;</span><span class="u-small-caps">CROSS</span><span>&nbsp;</span>and<span>&nbsp;</span><span class="u-small-caps">HOWE</span>, 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<span>&nbsp;</span>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.</p></div></div> application/pdf 10.1007/BF00371424 en Springer A Rubidium-Strontium study of the Twilight Gneiss, West Needle Mountains, Colorado article