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The Precambrian rocks of the Needle Mountains, southwestern Colorado, have undergone two major cycles of sedimentation, metamorphism, and igneous intrusion. The earlier events of the first cycle (eugeosynclinal deposition, formation of the 1780-m.y.-old Twilight granite (L. T. Silver and Fred Barker), and metamorphism to amphibolite facies) are not discussed in this paper. The later events of that cycle include intrusion of the post-kinematic Tenmile granite 1720±50 m.y. ago, granite of Bakers Bridge 1711±80 m.y. ago, and Whitehead granite 1675±74 m.y. ago, with ages determined by the Rb-Sr whole-rock method. Erosion, deposition of the Uncompahgre formation, folding, and low-to-intermediate rank metamorphism took place in the interval 1675–1466 m.y. and were followed by post-kinematic intrusion of the Eolus granite and the granite of Florida River 1466±27 m.y. ago and of the gabbro of Electra Lake 1454±50 m.y. ago. The Eolus and Florida River intrusives yield mineral isochrons of 1430±35, 1410±29, and 1390±60 m.y., and the Electra Lake body yields mineral isochrons in the range 1456±1424 m.y. The last known Precambrian event in the Needle Mountains was the intrusion of the stock of Trimble granite and of rhyolite porphyry dikes 1350±50 m.y. ago.
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