Isotopic studies on a Precambrian geochronologic boundary, Sierra Madre Mountains, Wyoming

Abstract

Geochronologic evidence indicates that a profound discontinuity separating two discrete Precambrian provinces parallels the Wyoming-Colorado border. This east-trending boundary transects the Sierra Madre Range in southern Wyoming adjacent to a fault-bounded sequence of folded middle Precambrian sedimentary rocks. Precambrian rocks of the Sierra Madre Mountains reveal a series of igneous and sedimentary events that occurred near the probable edge of the Archean continental nucleus between 2,600 and 1,600 m.y. B.P. North of the boundary, Archean quartz-biotite gneiss, probably 2,800 to 2,900 m.y. old, grades eastward into a northwest-trending antiform of microcline augen gneiss and granite. The Rb-Sr isochron of the granite gneiss complex yields an age of 2,560 m.y. and an initial Sr 87 /Sr 86 intercept of 0.7000. Geochemical evidence suggests that the quartz-biotite gneiss may have evolved from a supracrustal dacitic protolith formed by anatexis of a basaltic amphibolite. Calc-alkalic volcanic flows, tuffs, and volcanogenic sediments and a 4-km-thick sequence of conglomerate, quartzite, and limestone were deposited on the gneiss basement after 2,560 m.y. B.P. A period of igneous activity and tectonism between 1,650 and 1,900 m.y. B.P. resulted in the metamorphism and folding of the sediments into an east-trending anticlinorium of isoclinal folds. The Sierra Madre south of this metasedimentary structure consists principally of a composite batholith of plutons ranging from gabbro to granite. The Encampment River Granodiorite, the oldest intrusion, is a gneissic unit that was probably derived by partial melting and remobilization of the polycyclic quartz-biotite gneiss, as suggested by isotopic and rare-earth-element data. This thermal reactivation occurred between 1,900 and 2,000 m.y. B.P. The majority of the batholith is made up of a distinctive red granite that yields an isochron age of 1,680 m.y. and an initial Sr 87 /Sr 86 ratio of 0.7024. A white quartz monzonite discordantly intrudes the granite and hornblende gneiss country rock. Isochron ages and initial Sr 87 /Sr 86 intercepts of the monzonite and gneiss are 1,680 m.y. and 0.7020 for the quartz monzonite and 1,880 m.y. and 0.7015 for the country rock. At least three distinct generations of mafic dikes are present in the Sierra Madre; field relations with dated units indicate ages of ∼2700 m.y., 1,800 to 2,650 m.y., and ∼1,680 m.y.