Precambrian Geochronology of the Medicine Bow Mountains, Southeastern Wyoming

Abstract

The Medicine Bow Mountains of Wyoming are located in a transitional zone between the 2.5 b.y. old Superior Geochronologic Province to the north and the 1.3 to 1.7 b.y. old Central U.S. Geochronologic Province to the south. The mountains are crossed by a major northeast-trending shear zone, northwest of which is a quartzofeldspathic-gneiss complex that is intruded by the Baggot Rocks granite, by metagabbro, and by pegmatite. The complex is overlain nonconformably by more than 35,000 feet of miogeosynclinal metasedimentary rocks. Rb-Sr whole-rock isochrons indicate that the quartzofelds-pathic gneiss is 2410 ± 50 m.y. old, and the Baggot Rocks granite is 2340 ± 50 m.y. old. The metasedimentary rocks are at least 1550 ± 50 m.y. old, as indicated by a biotite Rb-Sr date, and are younger than 2410 ± 50 m.y. Whole-rock isochrons of 1550 ± 425 m.y. and 1650 ± 60 m.y. for two formations in the metasedimentary sequence may represent times of metamorphism but define at least minimum dates for sedimentation. Pegmatite intruding the gneiss complex is at least 1600 m.y. old; an anomalous initial Sr 87 /Sr 86 (0.745) permits the interpretation that the primary age is closer to 2400 m.y. Gneiss southeast of the shear zone is more calcic and mafic than that to the northwest, and it is intruded by metagabbro, pegmatite, foliated granite, and massive granite thought to be related to the Sherman Granite. The gneiss is unsuitable for direct dating because of low Rb/Sr ratios. Pegmatite yields mineral isochrons ranging from 1455 ± 40 to 1570 ± 40 m.y., and the Sherman-type granite yields a whole-rock isochron of 1335 ± 30 m.y. Analyzed whole-rock samples of foliated granite do not form a single isochron but show no evidence of being older than about 1715 m.y. No evidence has been found to indicate an age of 2.5 b.y. for rocks southeast of the major shear zone in the Medicine Bow Mountains, nor has any been found in nearby areas southeast of the projections of the shear zone. Rubidium-strontium mineral isochrons and individual mineral dates indicate one or more episodes of metamorphism in the Medicine Bow area between 1600 and 1455 ± 40 m.y. ago. We suggest that miogeosynclinal sedimentary rocks in the northern Medicine Bow Mountains, roughly correlative with Animikie and Huronian strata of Minnesota, Michigan, and Ontario, were deposited on the southern edge of a craton consisting of rocks mainly ≥ 2400 m.y. Gneiss of the southern Medicine Bow Mountains and gneiss and graywacke of the Front Range may be the more metamorphosed, eugeosynclinal counterparts of these sedimentary rocks which have been metamorphosed and transformed during the 1600 to 1455 m.y. old, or older, orogeny.